Chine Flats

[Wiley Baggins]

I’ve done a bit of reading on these, and they seem to offer performance gains in planing hulls. I’d be interested in the experiences of those that have used them, or operated boats equipped with them.

I’d also like to hear thoughts on the level of complexity added to the construction of a hull equipped with these. I am thinking specifically about their application to a developable hull shape, built in plywood.

[mmd]

Hi, Wiley; good topic. At risk of provoking the ire of those who don’t like long posts, and to possibly explain planing hulls a bit to those who are unfamiliar with how they do what they do, I’ll offer up the following rambling essay:

I assume that what you mean by “chine flats” are the angular ribs that run length-wise down the length of the planing hull bottom and not the “keel pad” that has become a feature on really high-performance speedboats (such as Cigarette, Donzi, etc.). The latter are quite a different critter altogether, and a possible subject for a later discussion. I will limit my comments in this post to chine flats as I have defined them above.

A planing hull is basically a wedge skimming over the surface of the water. A prime example is the photo that was recently posted here on the forum of a guy zipping along on an upside-down table with a wee Mercury outboard clamped to the table skirt. As a matter of fact, that table was probably the most efficient planing powerboat around, as the entire “hull” surface was contributing to vertical lift. However, a flat planing surface isn’t normally acceptable because one pays for this planing efficiency with poor steering, the tendency to “fall off plane” when the balance of the boat is upset, and slamming. Let’s discuss these in a bit more depth.

Poor steering – in naval architecture jargon this is referred to as “directional instability” – is caused by the flat planing surface having little resistance to sideways forces, it being just as happy going sideways as forward. The ultimate free-planing surface example I know of is the round disks called (around here) “boogie boards” that kids at the beach use by running along the surf wash, throw the boogie board down and jump on it, skimming along the water and usually terminating in a gymnastic display by the kid as they get catapulted into the incoming wave. These boards are fully planing surfaces utterly unencumbered by any loss of lift due to steering surfaces such as strakes or even the corners that the table has. The spectacular spins and spills that the kids take are shining examples of just how uncontrollable a planing hull can be. Obviously some form of directional stability has to be built in to the hull. A bit more on this later.

Another nasty characteristic of a pure planing surface such as the table is what is called “falling off plane”, or loss of planing dynamic stability. There are two ways to fall off plane; either the planing hull rises too high in the bow and the stern sinks, presenting too steep a planing surface to the water surface, causing the hull to slow down and stop planing, or – more catastrophic – the bow angles down too far while on plane which increases the surface drag on the planing surface very rapidly causing it to slow down suddenly, bringing the bow down uncontrollably, and the leading edge of the planing surface submarines under the water surface and the hull comes to a crashing halt. Racers call this “stuffing” the boat. Not very pleasant. So the hull must be shaped so that it avoids as much as possible the tendency to fall off plane, and to resist at all costs the tendency to “stuff”.

The last trick that a pure planing hull has up it’s sleeve to make you uncomfortable is it’s inability to negotiate an oncoming wave smoothly – the slamming can just about knock your teeth out. This is caused by the impact energy of the speeding boat meeting the relatively unmoving (and utterly incompressible) wave being transmitted to the hull in a split-second. The faster you go, the bigger the impact energy delivered in a shorter period of time. If you slow down, the energy is less, so the ride is smoother, but there’s no fun in going slow, is there? We can’t do much about the magnitude of the impact, but if only we could distribute the impact over a longer time period, it wouldn’t hurt so much. Well, after fifty or so years of experimentation, we sorta figured this out and came up with a planing hull shape that fixed this and the other two nasties, too.

The vee-section hull with a pointy bow solved a lot of the problems that the planing table had. The vee shape presented a flat planing surface at an acceptable angle to the water surface, so it got up over the bow wave that a displacement hull pushes in front of itself, but also presented a bit of a surface on the side of the hull which resisted being pushed sideways through the water. It sort of cuts its own groove in the water, in which it is easier to go forwards than sideways, so it was more directionally stable. The combination of a pointy bow and a vee’d hull created a planing surface shape that, when viewed from above, was square on the trailing edge, but pointed at the leading edge, much like the outline of a stubby pencil. This shape provides good lift at the stern, so any tendency to lift the bow is met with a strong counter-lift at the stern, preventing the hull from falling off plane by having a too-steep planing angle. The pointy leading edge of the planing surface would gain wetted surface area slowly as the bow fell, preventing a sudden increase in skin friction and thereby minimizing the tendency of the hull to fall forward off plane and stuff the bow. And lastly, the shape of the pointy and vee’d bow allows it to enter an oncoming wave gradually, thereby extending the time duration of the wave impact on the hull and making it less violent. So we now have a planing boat that steers well, stays on plane, and rides softly. Whoopee, right? Well, there are a few niggling problems ….

Planing surfaces are most efficient when they present a flat surface to the water. As the planing surface becomes increasingly vee’d , some of the lifting energy is re-directed from lifting up to pushing out. The deeper the vee, the greater the loss of lifting force on the hull, causing the boat to keep more of itself in the water as opposed to on top of the water, causing more frictional drag, which translates into loss of speed. So what is best for a soft ride is worse for planing speed. Bummer. Another problem is that as the bow slices through the water surface, the surface tension of the water causes it to adhere to the hull surface, and the pressure wave that builds up in front of the advancing hull forces the water up along the bow. The surface tension of the film of water creeping up the bow is strong enough that it will stay adhered to the hull until it finds a change of hull geometry severe enough that it will break the surface tension of the water as it attempts to wrap around the obstacle. In smooth, round surfaces such as those of developed-plate hulls, this obstacle is often not found until the water reaches the sheer. In addition to providing a rather wet ride for the crew as this water blows off the sheer rail, the amount of water clinging to the hull has some substantial weight and even more tensile drag on the hull, both of which combine to – you guessed it – slow the boat down. Double bummer. So now we have a hull that steers well, is stable on plane, rides nicely, but is slow and is drowning the crew in spray from the bow. How can we keep the ride, but get more lift and eliminate the spray?

I believe that it was C. Raymond Hunt who came up with the solution to this. He found that if he fitted wedge-shaped strakes along the hull, parallel to the centreline, that both the problems with the deep-vee hull were alleviated. The flat surfaces increased the average planing surface of the hull and provided better lift than with a smooth vee hull, and the sharp corners of the outer edge broke the surface tension of the parasitic water on the hull causing it to detach from the hull and fall back into the sea. Hot diggity! Now if we can just figure out how many and how big the lifting strakes should be.

Therein, my friends, lies one of the black arts of planing hull design. As with most other things in naval architecture, the best hull for the job is a blending of compromises. Each strake adds wetted surface area, increasing drag on the hull. Each strake adds to the material of the hull, adding weight to the hull. At what point does the advantages of gaining lift and losing parasitic drag get overtaken by the detriments of increased wetted surface and additional weight? If I knew that answer, I’d be rich and you’d have no choice in your hull’s bottom shape. Calculation and model testing can go a long way in determining the best arrangement for a certain hull form in certain conditions, but this is still an area of much experimentation. Through unscientific personal observation I have ascertained that the aggregate width of the strakes is about 25% of the planing surface beam at the transom and spacing between strakes is usually about 8” – 12”. Strake depth is usually no more than about 3”, even on larger hulls. Sometimes the strakes follow the line of the chine and wrap inboard as they approach the stem and sometimes they are parallel to the centreline and taper to nothing as they approach the bow (the European boats seem to favour the former, the US boats the latter). Obviously, determining the performance characteristics between the two configurations is another area suitable for scholarly investigation. Finally, some hulls have the strake’s bottom surface flat and some are angled such that the strake forms an inverted vee with the hull bottom. This permutation is usually explained as an effort to trap air along the hull, which will reduce surface friction as air is “slipperier” than water. This variant must be approached with caution, as they do not produce as much lift as their flat cousins. Another case of compromising between lift and drag.

With the bit of theory and history that I have offered up, hopefully you can now go somewhat boldly to your garage project and consider putting strakes on the hull. If you are fitting them to a ply hull that will have skin of ‘glass, and you feel that you might not get them just right the first time (a reasonable consideration), might I suggest that you skin out the hull without the strakes first, then cut the strakes from closed cell structural foam such as Core-Cel or the stuff that they use for exterior roofing insulation and glue them in place with epoxy. Then ‘glass them over with two layers of cloth and fair the seams in to the bottom. In this way you will be able to cut them off with an angle grinder and grind the hull smooth in preparation for the new, improved model. Wooden strakes are quite as good, but a bit more involved to experiment with due to repairing all the danged screw holes when you move the strake.

To those who have found this of some interest or use, thank you for your patience; to those who don’t like long posts, how did you get all the way here? tongue.gif

Michael

[jlapratt]

What a wonderful engineering discourse that even the uninitiated could follow. Thank you.

What do you know about the rather wide chines used on most v-bottom hulls these days? What attributes do they provide?

Jeff

[Mike Vogdes]

MMD, Thank for the very informative essay, it was definetly worth the time...

[Bruce Taylor]

This is a keeper, Michael. Great stuff.

Is there a formula for determining whether a hull form will plane (at a given speed, or under a given source of power)? I understand that there are many variables -- angle of V, WL beam/length, shape of aft run, amount of rocker...not to mention things like skegs, keels, skids & who knows what -- but are there rules of thumb that might be of use to the layman?

[Roger Stouff]

As always, Michael, you're a valued contributor to this forum. Thanks a bunch.

[Tom Lathrop]

Great discourse Michael,

There are a couple of things I would add in relation to Wiley's question. He was asking about chine flats and I think your story applies mostly to deeper V boats with multiple strakes.

A single flat on the chine does the same job of adding lift for easier planing and also reduces spray by redirecting it downward and provides the sharp edge for breaking the water from the hull. All good. The single chine flat is often used on boats that do not have high deadrise while the lifting strakes are mostly useful on deep V hulls.

As to the method of building a chine flat, one technique that I and many others use on plywood boats is to allow the topsides to extend below the hull bottom to form the vertical side of the flat. Another panel of plywood is then fitted to join the side to the hull bottom. This forms the necessary V shape of the flat. Of course you have to taper the edge of the flat to meet the hull bottom but that is about it. As for the width of the chine flat, even a small one will do the spray deflection job and if you make it too big, you will have a flat bottom boat with all the problems Michael talked about. Sometimes the flat exends all the way to the bow and sometimes it tapers to nothing at the bow.

If you are doing a high powered and fast boat there can be a problem with chine tripping from having too much chine depth at the stern. When put into a violent turn the outside chine can catch the water and throw the boat over. In this case a lot of deep flat extending below the natural intersection of the bottom and the side should be avoided. In some highly maneuverable boats, something called a non-tripping chine is used. That is just the opposite of a chine flat where the last few inches of the bottom near the chine is canted upward at a higher angle than the rest.

There is much more to this but we need to stop and hear whether your question is being addressed.

[Rocky]

I posted that picture of the table, never thinking it would be used to illustrate a technical point. I guess calling it a "redneck boat" doomed it to deletion.

[Rocky]

Third paragraph of mmd's post.

[Tom Lathrop]

Mike,

Your second photo shows an excellent way of adding a flat to an existing hull. Also gets a few extra inches of planing beam in the bargain. Do you do anything to protect what looks like a vulnerable edge on the flat from the usual bangs it might get around docks and such?

[On Vacation]

By the time the rub rail are installed, combined with the flare in the sides, there is no problem with touching anything remotely close to piling. The hull glass comes up and over all of the new surface. The side pieces are a little tricky in the foward section but a huge asset to releasing the hull from the running surface. And yes thats another story and a fineline indeed.

The angle of the bottom wedge is very critical to the bottom amount of vee to allow for proper turning. It is adjusted for different amounts of deadrise. Trial and error as one might say. The intermediate strakes are not as critical in the stern as much as in the bow on a shallower draft boat. They come into play in the following sea conditions.

[Tom Lathrop]

Ah, so you do add a chine rubrail. I do too but glass boats never have such a rail since it would prevent the hull from being removed from the mold. That is why there a lot of spray rails added on some of the local boats to keep them drier. I once owned an Atlantic Skiff (terrible boat) where the water would curl up the sides, over the sheer and into the stern area. Bummer!

The snow is almost all gone and it's warming up outside so I'm going to launch "Liz" and get some chine flats wet.

[ 02-02-2003, 11:01 AM: Message edited by: Tom Lathrop ]

[mmd]

Jeff, the wide chine flats serve several purposes: They provide all the features that I mentioned in my posting above, plus allowing a greater interior volume for accomodations.

Bruce, not as specifically as you would hope for (sorry, no grand unified theory equations here). By that I mean that there is no one formula that you can punch in a set of numbers and get an answer that will tell you that you have a planing hull on your hands that will do X knots. However, there are a series of measurements and equations that can be used in sequence, much like a decision tree, that will provide you with the answer you are looking for. Dave Gerr describes this quite well in his book, "Propeller Handbook". In a nutshell, if a hull has flat after sections with a buttock line that is straight and flat (or nearly so), it is possibly capable of planing. If the after half of the quarter-breadth buttock line has less than 4 degrees of rise when compared to the LWL, it is probable that it will plane. Then you can do the math bits.

It is the common assumption for hulls of moderate length/beam ratios that a speed-to-length ratio of greater than 2.5 - 3.0 is the beginning of the range of planing speeds. (Really long, narrow hulls such as racing shells and catamaran hulls reach S/L ratios of 3.0 while still in displacement mode, but that is another whole topic.) Gerr has derived a formula to determine the speed-to-length ratio (S/L) of a given hull based on the displacement-to-length ratio (D/L) that seems to work quite well. It is:

S/L = 8.26 / (D/L Ratio)^0.311

where D/L = Displacement in tons /(0.01 x waterline length in feet)^3

If, from this calculation, you determine that the hull is capable of planing, you can use one of several methods to determine the approximate speed of the boat. One of the more common such formulae was provided by Crouch, who postulated that:

V = C /(W/SHP)^0.5

where
V = speed in knots
C = a constant dependant on type of boat
W = displacement of the hull in pounds
SHP = horsepower at the propeller shaft

Values for C are:

150 - average runabouts, cruisers, passenger vessels
190 - high-speed runabouts, very light high-speed cruisers
210 - race boat types
220 - three-point hydros, stepped hydroplanes
230 - racing power catamarans and sea sleds

As with most calculations dealing with naval architecture, these are at best a strong indicator of possible performance and you shouldn't place a whole lot of faith in the absoluteness of the hard numbers. Things like the firmness of the bilges, appendage drag, warpage of the planing bottom, etc., will all have a effect on the hull's performance. That caveat being said, use of the above criteria and formulae should give you a pretty good idea whther you can expect the planing performance from your hull that you hope for.

[Bruce Taylor]

Now I have something to do after the kids are in bed! Thanks...that's exactly what I needed.

It's good of you to share your expertise, Michael. If you ever need to know something about instances of litotes in West Saxon texts, or the anagogic phase in the postmodern process poem, I'm at your service! (Strange that nobody ever asks.)

[Rocky]

For you, Oyster. And Scot, this is not a redneck boat, it's a pure planing hull!

D:\JPEGs\Redneck\boat.jpg

[ 02-02-2003, 09:08 PM: Message edited by: Rocky ]

[On Vacation]

Rocky, I am not sure about your humor or desire to take a thread of substance and alter it, but leave me out of your game. If you have something that relates to the topic of chines flats, share it. AS far as Scot in your post, quit pushing the envelope.I chose to post on a decent thread, not to find you here using me as a whipping sitck.

[Wiley Baggins]

First of all, thank you Michael (mmd), for your marvelously understandable explanation of the phenomena of planing, and how chine flats (strake and edge varieties) fit into the larger picture. The rules of thumb are no doubt hard won, and all the more valuable for it. Thanks for sharing that information with us.

Tom, thanks to you as well, for your discussion of the building methods and specific examples of chine flats/spray rails.

Oyster, as always, I appreciate your input, and the photos from your workshop. It's good to see that even you have a moaning chair! ;)

I have more questions (thanks to such good answers). However, it will take me a bit to compose them so that they are as clear as your contributions.

PS Michael, long is good. Perhaps you and Dave Fleming can start a "Boat Talk" program produced jointly by our National Public Radio and your Canadian Broadcasting Corporation. We'll need some catchy names, equivalent to "Click" and "Clack," for the two of you ;)

[Rocky]

From paragraph 3 above:
A planing hull is basically a wedge skimming over the surface of the water. A prime example is the photo that was recently posted here on the forum of a guy zipping along on an upside-down table with a wee Mercury outboard clamped to the table skirt. As a matter of fact, that table was probably the most efficient planing powerboat around, as the entire “hull” surface was contributing to vertical lift. However, a flat planing surface isn’t normally acceptable because one pays for this planing efficiency with poor steering, the tendency to “fall off plane” when the balance of the boat is upset, and slamming. Let’s discuss these in a bit more depth.Now you go to bed.

[Bruce Taylor]

Oyster, Michael mentioned a picture that was posted on the forum a while ago, showing a kitchen table w/ a 2-stroke motor affixed to it, planing admirably. I think Rocky was trying to repost that memorable image.

[AngWood]

Don't worry, Rocky. All the rest of us understand.

[gary porter]

Wiley, the boat I'm currently building and have built uses a 3" chine flat.
The hull is of plywood construction and is 21' +. The original hull design
did not have the flat and was a bit shorter as well. The chine flat is added
by cutting in a spline joint on the botton panel and a noral stitch and glue type
joint on the side panel. This widdens the boat by approx 6" and while I don't
have experience with the older hulls of this type I'm told it does make a significant
difference in performance. The boat is very dry whe breaking through waves etc.
and will do 30mph top with a 70 hp outboard. It comes on step almost instantly without
notice seems quite stable. I wish I had more experience with the old hull that did not
have the flat but those who do have added an external flat much like those above.
You can see a sketch and the boats of this type at www.xyz.net/~mgrt/ (http://www.xyz.net/~mgrt/)

I'll attempt to post an image here of mine to give you some idea.
http://www.imagestation.com/picture/sraid49/p419805722c78af4eb2a420d142e788df/fcb086a0.jpg

They do add some extra work but the add performance, dissplacement, etc, seem well worth it. You'll see in the link above a version (Jumbo) that has an even larger flat.
Gary

[BradW]

Don't mean to divert the thread, but if you wanna see chine flats (flaps), HERE'S some chine flaps (flats) for you:

http://www.gdls.com/images/aaav1.jpg

All those flaps and an ungodly amount of horsepower in 2 monster water jets lets 76,000 lbs of armored vehicle plane at 25 knots (prototype) like this:

http://www.usmc.mil/marinelink/image1.nsf/ae82f18a8e1b160b852568ba007e7e5e/7f8c3e99392f0c35852569d1005392b9/$FILE/AAAVlowres1.jpg

It's the Marines next amphib assault vehicle. I have to imagine the ride is brutal. My company is a subcontractor, and the normal (not combat hits) wave shock spectra we have to meet are pretty ugly. Enough area can make a brick plane, but it doesn't have to be happy about it! ;)

[Wiley Baggins]

Gary- Thanks for the additional information in your post, and the link. What I am playing with is not entirely unlike the Tolman skiff, but more like a lobster skiff appearance-wise. Does your boat have a self-bailing deck? Oyster and I kicked the self-bailing deck topic around a bit, and I have come to the conclusion that legitimate self-bailing decks (10”-12” inches above the waterline, per Bolger) with adequate gunwale height (measured from the deck) result in very high sides (and increased weight). Given those criteria, I am not sure when a self-bailing deck becomes practical (with respect to boat length)

mmd- When you say that the aggregate width of the chine flats can be up 25% of the planing surface at the transom, is that effected by the location? I note that with the Tolman skiff (Gary Porter’s post/link) the transom beam is 66” and the flats are each 3” wide. This equates to the chine flats having an aggregate width of ~9% of the (maximum possible) planing surface at the transom. Are the flats more effective if placed at the chine, or is it a matter of 9% being enough for this configuration (weight, power, deadrise, etc.)? I note the following specifications for this boat:

LOA-21’4”
Beam-7’6”
Beam at chine-5’6”
Deadrise at transom-8 degrees
Deadrise amidships-14 degrees
Weight-“less than 900 pounds
Power-70hp-90hp two-stroke outboard

Generally speaking, I am assuming that the greater the deadrise and the greater the weight of the boat, the greater the need (for efficiency) for chine flats. I am also assuming that having them at the chine is more effective in diverting spray, but less convenient when it comes to using trial and error to fine tune performance.

Thanks!

[ 02-04-2003, 05:37 PM: Message edited by: Wiley Baggins ]

[gary porter]

Wiley, Yes my boat has a self bailing deck. Not all Tolmans do but thats how mine and several others are. My fuel tank (80gals) is the center section and is sealed and flush with the rest of the deck. The scuppers , one on each side, run just at the waterline normally depending on loading. The boat also has a spray rail about half way up the side. The sides are about 32" and the deck is about 6" off the bottom at about 15" from the centerline.
Gary

[mmd]

Hi, Wiley.

The 25% rule is just an unscientific observation of typical speedboats in the 18-25 foot range. As with all “rules of thumb”, it is open to wide interpretation. Another ROT: location of the strakes will affect handling; number of strakes will affect lift. A planing strake will have the same amount of lift per immersed foot near the centreline as it will near the chine, but the farther away from the centreline the greater the effect on steering and transverse stability.

In the case of Tolman skiff, where the deadrise is not particularly steep nor does the boat look like it would be particularly heavy in most loading conditions, the 3” chine flat is most likely quite adequate in providing all the lift necessary, and is most likely doing duty as an aid to transverse stability, creating extra interior volume, and knocking spray down more so than adding appreciably to lift. If you can give any credibility to my rule of thumb :rolleyes: , the maximum strake area for the Tolman skiff would be 66” x 0.25 = 16.5”. Divide this by two to get strake area per side, and you get 8.25”. Let’s say we want 3 strakes, so 8.25 divided by 3 equals 2.75”. So we can assume that to fit in with the pack, we can fit a 3” chine flat and two 2.50” strakes. Sounds about right.

The black art of the design is to decide – or calculate, or tank test to find out – whether these strakes are needed. In small boats, it is usually by full-size experimentation; in big expensive boats it’s worth paying for the expertise to calculate a prediction before you commit bags of money :eek: to build the hull on assumptions. Usually one can rely on the presumption that the deeper the vee of the hull, the more strake area is needed to provide lift to counter-act the less efficient hull form. With the Tolman skiff deadrise at only 8 degrees at the transom, the need for extra lifting surface is minimal; a bit of an edge at the chine is desirable to add control in cornering and to knock down spray, but compared to a Cigarette hull with a deadrise of 22 degrees or more, you have all kinds of lifting surface available so strakes are not necessary.

As for fine tuning performance, remember that the higher the velocity, the greater the forces exerted by the planing surfaces. Depending on the shape of the hull, a minor un-symmetry of chine flats is unnoticeable at twenty knots; at forty knots it could cause dangerous instability such as chine walking, porpoising, or momentary loss of steering control. If you are pushing the limits of the hull’s capability at speed, make any experimental changes subtle and precise. Contrary to your comment of strakes at the chine being “less convenient when it comes to using trial and error”, this is the area where you will have the most profound effect on the hull’s handling characteristics, but bold changes may cause you to miss what you were aiming for and create a whole new set of problems. For instance, using the Tolman skiff as the unwitting victim of our experimentation, what would be the effect of changing the chine flat from horizontal to sloping downwards, forming a vee with the hull? I would expect that the speed might be slowed a wee bit, but the straight-line tracking would be better and spray would be slapped down with authority. A reasonable approach to trying this experiment would be to try altering the chine flat angle by two degrees or so, but if a little is good, a lot is better, right? Awww, let’s go whole hog and tip it fifteen degrees, ‘cause we have a setting on our set square at that angle. So we build the boat with the steep chine flat (?) and go for a tear on the lake. Jeez, it is a bit slower, but now it steers like a freight train. Maybe it needs a bit more speed to steer well. So we click the throttle up t’ bust an’ wind her up to warp speed an’ give a mighty yank on the wheel! She carves into her turn just fine ‘n’ dandy until about half-way through, then all of a sudden-like, she pops up out of her bank into the turn, slaps down hard on the other side of the hull, and wildly turns t’other way! Humph! Dat weren’t s’posed to happen. What did happen? :confused:

With the chine strake at such an extreme angle, it acted like a keel when we banked into the turn. Water pressure built up on the inside of the strake until it reached high enough pressure to lift the chine up out of the water. This popped the hull up enough that it lost equilibrium and centrifugal force tipped it to the outside of the turn, where the other chine found “traction” and took over the steering job. Bummer. Spilt yer beer, didn’t ya! :D Two degrees would have been better. ;)

If you are boldly going where no strake-fitter has gone before, remember that the faster you go, the more subtlety is king. Have at it, ‘n’ have fun, but play safe. :D

[Wiley Baggins]

mmd,

With that sort of result, I'd be spilling a lot more than just my beer!

Thanks for the additional insight. Any appendages would be quite conservative, and the deadrise at the transom would not get more than half-way from the Tolman's 8-degrees to the Cigarette's 22-degrees.

In fact, you have highlighted one of my concerns with modifying the lifting surface at the chine. My bias would be to keep it flat, rather than form an inverted 'V.'

I want to keep my beer, and everything else, in their proper containers. ;)

[Tom Lathrop]

Wally & Michael,

Here is a case history. Monohedron hull of 10 degrees deadrise from transom to about 40% of WL forward and then rising to a fine convex entry. WL width at transom is 70 inches and chine width at transom is 11 inches each side. Max WL beam is 78 inches. Chines taper to about 5 1/2 inches at midships. Transverse down angle of chines is 2 1/2 degrees. Displacement is 2500 to 2800 lbs and power is 50 HP outboard.

What would you expect in performance and handling?

Incidentally, I must think that Bolger was misquoted in the 10 - 12 inch height above WL for a self bailing cockpit. That might be the case for a sailboat where heeling is a major concerns but is way more than needed for a powerboat.

I use about 4 1/2 inches above WL at the transom scuppers with an up slope of about 1 inch per 30 - 36 inches forward of the transom. Works fine with no water ever entering the scuppers underway and only once when a gaggle of passengers congregated in the stern at rest. Of course, this depends somewhat on the lbs/inch immersion of the hull and narrow boats might need a bit more clearance.

[mmd]

I'd expect around 18-19 knots and a heel of about 10 degrees in a comfortable turn at that speed, and there should be little sideslip. I'd also predict that you's scrub off about 3 knots completing a 90 degree turn. How'd I do?

[Wiley Baggins]

Tom,

Thanks. I’m guessing she might perform and handle like BLUEJACKET. ;) This must be a good thing, because I have only seen positive comments about that design. Kudos to you.

From you description, I make your chine flats to be a bit over 31-percent at the transom. That indicates to me that the flats, at more than the 25-percent “rule-of-thumb,” are doing a fine job “compensating” for the boat’s low power to weight ratio (compared to the Tolman skiff). Given that you taper the chine flats to approximately 16-percent at midships, do you think that mimics the effect of a warped bottom or a cathedral hull? I am also curious as to how you selected the negative 2-1/2-degrees as the angle for the chine flats, and if there were additional angles tried, what was the relative performance like.

With respect to the Bolger comment, I’ll reproduce it here.

”Scuppered watertight decks or double bottoms are a trap for the ignorant and unwary unless they are on the order of a foot above the normal waterline. Otherwise there’s a “monitor effect,” the boat just barely floating, and she’ll go down on small provocation.”

-from Bolger on Design, Design #472 Plain 18 Sheet-Plywood Outboard Utility, in “Messing About in Boats," May 1, 1999Now this, and a similar comment, were made with respect to open boats, and not a craft with BLUEJACKET's configuration.

[ 02-04-2003, 11:36 PM: Message edited by: Wiley Baggins ]

[ion barnes]

This is a great read. I just want to say that when I was a lad of 16, I rebuilt a Glen-L Squirt. After the first season, my dad suggested that we could improve the performance and make it drier. We cut two yellow cedar triangular strips and wrapped them around at the chine.
At the foreward end, underside of the strip was canted down, midship, it was roughly horizontal. and at the stern it was slightly turned up. We increased the amount, lengthwise that was turned up as well as increasing the angle as you got closer to the transom. Our take on it was that the up turn would reduce the chances of tripping when making hard turns.
The results were very satisfactory. The boat was very much drier, we increased the top speed marginally with some air trapped underneath, and it never had any nasty habits. We also added a large stainless fin about 60% back from the bow which didnt hurt either.

[Tom Lathrop]

Originally posted by mmd:
I'd expect around 18-19 knots and a heel of about 10 degrees in a comfortable turn at that speed, and there should be little sideslip. I'd also predict that you's scrub off about 3 knots completing a 90 degree turn. How'd I do?Hey, this is getting good. Michael, you are pretty close. Highest speed I have seen is 25 mph in with one aboard. She is very insensitive to added weight. On Monday, I had five people (about 900+lbs aboard) and she did 21 mph. When I asked the three larger ones to go to the stern, the speed increased to 22 mph. Wanna make a comment on why that happened?

The heel (bank) in turns is probably a bit less than 10 degrees. As you know, the speed scrub-off is dependent on the sharpness of the turn but is probably about your estimate for a 180 turn and I don't usually make 90 degree turns.

Quote from Wiley:

"From you description, I make your chine flats to be a bit over 31-percent at the transom. That indicates to me that the flats, at more than the 25-percent “rule-of-thumb,” are doing a fine job “compensating” for the boat’s low power to weight ratio (compared to the Tolman skiff). Given that you taper the chine flats to approximately 16-percent at midships, do you think that mimics the effect of a warped bottom or a cathedral hull? I am also curious as to how you selected the negative 2-1/2-degrees as the angle for the chine flats, and if there were additional angles tried, what was the relative performance like."

Wiley, You are close in your comments too. The flats are actually a bit less than 14 per cent midships (transom WL beam is about 86 percent of max WL beam). Yes, the chines may act a bit like a warped bottom but I don't have enough data to support that. It was my thought to get some benefits of a warped bottom without the ill effects or "my perceived ill effects."

The down angle of the flats is the result of something else and I did not select this angle. It is the result of the longitudinal trim angle of the flats. They have a positive one degree angle with respect to the aft monohedron buttocks. Why did I do that?

quote:
--------------------------------------------------------------------------------
”Scuppered watertight decks or double bottoms are a trap for the ignorant and unwary unless they are on the order of a foot above the normal waterline. Otherwise there’s a “monitor effect,” the boat just barely floating, and she’ll go down on small provocation.”

-from Bolger on Design, Design #472 Plain 18 Sheet-Plywood Outboard Utility, in “Messing About in Boats," May 1, 1999
--------------------------------------------------------------------------------

"Now this, and a similar comment, were made with respect to open boats, and not a craft with BLUEJACKET's configuration."

-------------------------------------------------

Without knowing what was behind Bolger's comments, it's difficult to criticize, but I must dissagree on the face of it. Boston Whalers cockpit decks are not as high as mine on their 23 foot Rage series. Sam Devlin's Surf Scoter cockpit decks are about the same height as mine. Some of these boats such as the Sea Ox, with their low transoms are unsafe and subject to swamping by check-up stern waves and I know of one that went under at the dock from wind blown waves from astern.

As a hedge on this, I did put scupper flaps on the transom of the Bluejacket and it has weathered some pretty rough water with no water in the cockpit from either scuppers or over the side.

[mmd]

Tom, I based my speed estimate on 2800 lb displacement, and the program that I have developed to do planing speed estimates assumes more appendage drag (sounders, struts, skegs, etc.) than you boat probably has, so I expected to be a bit low in my estimate. The program has several "fudge factors" built in so that I can tweak it to adjust for such things, but I didn't bother to do so on such short notice. To do a critical analysis I would require a linesplan and weight/C of G estimate.

In response to, "When I asked the three larger ones to go to the stern, the speed increased to 22 mph. Wanna make a comment on why that happened?", I expect that with the extra weight in the stern you a.) changed the trim of the hull so that it presented a more suitable planing angle to the water surface, and b.) by changing the trim and raising the bow, you reduced wetted surface area and thereby lowered frictional drag on the hull.

[gary porter]

Just a couple of comments on this great post.
If you change the angle on the chine flats or for that matter any shape on the hull to form an inverted V you might well compromise comfort of the ride which hasn't been mentioned here yet. while such modification might well allow a boat to come on step quicker and as said slap down the water it might well have the effect of a harder ride. Where your going to operate the boat might be worth plugging in to the equation. I can run 30mph wide open with an old tech 2stroke but never do. With typical water conditions I might run at 20-25mph and sometimes 12.
On the scuppers and self bailing deck, I run scuppers that are made of sewn rubberized canvas and are made up like pump hose, that is they are flat unless you force water through them. I can pull them up when sitting still and if I forget there is a small amount of water that will seep in. They are at and sometime a bit below the water line. They wont let in much water and even so it wont sink. When under way all water is quickly shed. Another owner with the same system took in a large wave and was said to be knee deep in water, his motor did not fail and all the water , though taking a bit of time , did shed off. Not sure what Bolger meant by his statement but I think there is more to the story.
Gary

[Wiley Baggins]

More good stuff. Thanks folks. The rest of the story…

The following are reports of what I assume to be the same incident. The first excerpt is from the above referenced issue of “MAIB.” The second is from Mike O’Brien’s “Boat Design Quarterly.”

from "MAIB," the balance is as recounted above.

These “bowrider” layouts [referring to one of the configurations of design #472] are attractive in many ways, we started suggesting them back in the 60’s, but they can be risky unless there is plenty of height and buoyancy forward. A year or so ago we watched one, overloaded forward, go through a fast tidal stream. Her bow was not high or buoyant enough to clear the crest. She shipped enough water to put her much too low “self-bailing” cockpit sole below the waterline.

She filled through her stern scuppers and proved her manufacturer’s claim that she was unsinkable by stabilizing in a vertical attitude with perhaps a foot of her bow showing above the water. That was due to bad handling, but they would have gotten away with it in this design here, if only because she doesn’t have a “self-bailing cockpit” to make a bad situation worse. From Boat Design Quarterly No. 18; Diablo Grande, by Phil Bolger

Bolger has drawn a weathertight deck higher than some observers might expect: “Its not well enough understood that in these watertight ‘self-bailing’ boats the effective freeboard is the height of the deck-rather than the rail. We saw one of these boats [not a Bolger design] swamp last year. She went through a standing wave and shipped enough water to put the deck scuppers underwater. She wound up floating vertically like a crab net, with only the tip of her bow showing. Spoiled a good weekend, we guess. Diablo Grande’s self-bailing deck is high enough for good reserve buoyancy. If it were any lower, we would be safer to go with a non-draining sole.

[Tom Lathrop]

Michael,

That is exactly my thought too. The chine flats do depress the bow so that the maximum trim angle is 2 degrees in normal load distribution. I was aware of that in the design phase and made the trade off of top end speed for quicker planing and more level trim at all speeds. Unlike other planing powerboats, Bluejacket reaches the maximum trim angle at top speed. The trim actually increases slowly throughout the speed range.

Moving weight around or changing trim of the engine has much less effect on the trim angle of this boat than any other that I am accustomed to. It seems that the combination of dynamic lift of the forward planing surface and the aft chines gives the boat a lot of longitudinal stability in much the same way that a step does. I doubt that the three guys in the stern increased the trim beyond three degrees.

On the self draining thing, I think it needs a thread of its own. I also think that interpreting Bolger requires a lot of context to fully understand what he is saying. The quoted passages don't make a complete picture for me.

Anyway I have enjoyed this exchange of ideas.

[Wiley Baggins]

Originally posted by Tom Lathrop:

On the self draining thing, I think it needs a thread of its own. I also think that interpreting Bolger requires a lot of context to fully understand what he is saying. The quoted passages don't make a complete picture for me.

Anyway I have enjoyed this exchange of ideas.Tom,

I too am enjoying this exchange. Thanks all around.

As to the self-bailing decks, short of your seeing the designs discussed in the excerpts above, there isn't any additional information to add from the citations. I would certainly appreciate any additional input on the self-bailing decks thread which can be found here (http://media5.hypernet.com/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic&f=6&t=000275&p=) .