oblique bilge keels vs. leeboards

[shaunbarrymcmillan]

which would give the best performance-especially close to the wind? thanks for imput in advice.

[SchoonerRat]

Bilge keels. Leeboards are easier and cheaper to construct, but do not perform as well.

[Thorne]

Neither.

Seriously, can we have a teensey bit more info? Think about it -- if the hull shape was just right (lots of tumblehome), leeboard angles might be nearly identical to oblique bilge keels as far as the underwater shape goes.

So are we talkin' a 1' model, an 8' dinghy or 50' circumnavigator? What hull shape, leeboard / keel length, angles of same, ????

[Stu Fyfe]

I have bilge keels on my Bolger sloop. Windward performance is not very good. I would think that alot would depend on the surface area of the keel or the leeboard. The more surface area, the better the windward performance. Since large bilge keels are not advantageous on a hull (difficult to ground out and transport)I would think that large leeboards would be easier to use (you can get them up and out of the way). Where is Phil Bolger when you need him?

[dredbob]

It depends, as Thorne has indicated. What is the shape of the hull, how is it ballasted, etc.? If everything else is identical, and the only difference is whether the lateral plane is provided by either bilge keels or leeboards which are of the identical area and aspect ratio and foil shape, weight, etc., then the leeboards would hold a possible theoretical advantage due to the fact that only one is in the water at a time, causing less drag. Against this is the effect of drag due to surface turbulance of the leeboard. Probably only tank testing of scale models will answer the question.

But seldom is it the case that only a single factor will be different. The bilge keels will likely function as ballast, giving some possible advantage there over leeboards, where the ballast will be all in the hull, giving less righting moment. The bilge keeler may be able to carry more sail area, etc.

Neither will go to windward like a boat with a deep ballasted fin keel, unless to windward there is a shoal that the fin keeler can't cross.

You can't ask what will "perform" best, without defining what the criteria will be for measuring said performance. Is it how close to the wind you can sail, or how shallow a depth of water can you sail in, etc.? Do you want to win races, or sail back into the creeks and marshes?

Figure out what your priorities are, and then seek the design that gets you the best balance of the various trade-offs that are inevitable.

Bob

[shaunbarrymcmillan]

When I posted the thread or question I meant that, all else being equal, which is better leeboards or oblique keels?

The design is I want to build is one of two Selway Fischer designs- Tansman or a dory- Pioneer. Tansman has near a vertical hull side but is multi-chined so will cost me more to build. I plan on having the hull professionally built and helping with the construction to defer costs.

Pioneer-the dory- with an inward angled hull sides so I might need Mr. Fischer to make some design changes or I'll just use bilge keels.

As for Tansman if I can afford the greater cost of the multi-chine contsruction - she is 24'7" long and 7'4" wide. Made for trailer sailing with a tabernacle mast. Displacement is 3308 lbs with a 50% ballast ratio (1654 lbs).

If I built Tansman she would not be towed but moored and I would ask that she be re-drawn with another foot of beam.

At this point I may have either boat built stich-n-glue or unpainted aluminum which ever is more cost efficient in terms of the build and maintenance costs after the build. Both construction types have their benefits and weaknesses. If I build stitch-n-glue I might try to save some money but gain more work by using a single part paint versus epoxy two -part paint.

Tansman's existing bilge dseign is rather complex (expensive to build) and there may be maintenance problems as the bilge plates on either side are vertical with a trunk to retract to. I highly respect the designer but would want a simplier underbody which I will ask him to draw - so my question related to oblique blige keels ve lee boards.

[James McMullen]

The only reason people use leeboards is to simplify something other than pure sailing performance, whether it is opening up space in the middle of the boat, avoiding the complication of building and maintaining a CB trunk, or retrofitting an existing boat for sailing quickly and cheaply. Surface piercing foils can never develop the same performance per square inch of wetted surface area and drag as a fully submerged, end-stopped foil--it's the law!*

*Laws of nature not subject to repeal or modification by ordinary mortals. Check your local library for details.

[Lance F. Gunderson]

A well designed leeboarder should out perform a twin bilge keeler; less wetted surface to drag through the water...assuming the weather board is hauled up. Off the wind both boards can be hauled up.

[James McMullen]

shaunbarrymcmillan refers to a "trunk to retract to" so to use the most correct terminology, the items in question are "bilge boards" not "bilge keels", and if they are shaped correctly will certainly outperform leeboards.

I have sailed on board or next to some very well designed leeboarders including designs by Herreshoff and Bolger, and the best that could be said about their windward performance was that at least the best ones weren't quite as bad as you might think--but nobody ever said the reason they chose the leeboard was to improve sailing performance. I repeat, the reason to choose the compromise of a leeboard is for some other design consideration, usually to simplify construction or clear up room in the center of the boat.

[Woxbox]

Surface piercing foils can never develop the same performance per square inch of wetted surface area and drag as a fully submerged, end-stopped foil--it's the law!*

Well James, as you state it I'd have to agaree. But if you have twin bilgeboards in cases, you have two slots churning up all sorts of turbulance which you don't have with leeboards. Also, as noted, when not in use the leeboard is clear of the water -- retract a bilgeboard and the slot is still there. Plus, as shaunbarrymcmillan notes, the bilgeboards are more trouble/expensive to build and take up cabin space too.

I think the real thing with leeboards is that they're designed along traditional lines where superb windward performance isn't the goal. I'll bet if you had a racing class that required leeboards with no design limitations, you'd see some extremely effective ones.

But back to the original question -- in a small boat I'd generally prefer leeboards just to open up the interior. Neither option being presented is the best if windward ability is high on the list of requirements.

[James McMullen]

Agreed. A properly shaped high-aspect centerboard or daggerboard is unequivocally better than either if you really want to keep up with the Joneses to weather.

[Woxbox]

And here's what it looks like if you pull out all the stops: Two canted daggerboards and a very deep ballasted keel.

http://yachtpals.com/files/userimages/spirit-of-canada.jpg

[ChaseKenyon]

My take on it is based on the designs I saw at Cowes week back in the early 70s.

Properly designed and built trunks of CBs or BBs are not a structural or sailing problem. On a hull that is designed with Bilge Boards in mind from the outset you have a hull with dramatically increasing stiffness as she heels. So the above comment about the windward lee board not affecting performance is just as true for properly designed Bilge boards and trunks. The windward board is raised and the Trunk slot is out of the water as the boat is allowed to heel to a set point of max stiffness/stability. When I sailed A and C scows we always let her heel over so the whole windward board was out of the water even without raising it when tacking to a mark. On a beam reach pulled the windward board up. On a run both up and go aft to get her on a full plane.

At Cowes there were mostly weekender tri keels, but there were some larger offshore capable boats with ingenious bilge boards with integrated raising and lowering on tacks with the sheeting systems. All run easily from the cockpit.

So I would go for the performance and study trunk design and bilge board designs and after achieving some serious knowledge look for a boat design that has them as part of the original design or is fairly ideal for them but did notr have them as a compromise to expected from multiple customers ( hopefully on any design).

As I have said before; in the total cost and maintenance of a boat the cost of two bilge board trunks verses one CB of shallow keel are % wise of little account. The jig for one Bilge trunk works for the other at a big savings in time on the second one. Bilge boards are even easier to work on and repair than centerboards and trunks if properly built. You don't need a second person to be underneath or topside to work with you. Compare that once in awhile maintenance cost and time to replacing an overweight shallow draft solid keel and it's keel bolts and etc. That could easily be beyond your financial ability to keep the boat at some future point. I have seen it happen more than just a few times.

Properly designed bilge keels are can be more effective than a centerboard or full keel when part of the design so they are perpendicular to the surface at the point of max boat stiffness. They can be easier to maintain and build than a massive centerboard or the floors and such needed to support a massive keel bolting on. Properly designed and sized bilge trunks do not appreciable intrude into the cockpit of cabin space.

I would like to see more designs and more folks considering these not common but sensible designs.

Chase

this came up in another thread but i lost track of it when out with back flare up for a few weeks.

[ChaseKenyon]

P.S. I have seen both CB and BB trunks with "cat door" mylar or similar self closing flaps to prevent trunk turbulence when the trunk is raised. Not exactly rocket science here. but I would rather replace if needed and check and clean bilge trunk seling flaps than CB sealing flaps. now you add the size of a centerboard to offset the center of effort and max from the sails at a less than ideeal angle in the water with respect to tha lateral forces to the size of BB to offset the force when the BB is aat 90* to the force and surface and you have a much smaller bilge board times (2 i admit) than the needed centerboard size.

So maintainence is acually easier.

this also brings up whether a wider longer CB trunk open going downwind has more or less drag than two samller shorter bilge trunks that are open but at the side in probably turbulent water that offers much less drag and resistance then the clean water under the center of the boat. Even a semi planing long distance racer would have less resistance with the bilge trunks than with a single large centerboard trunk planing downwind.

[htom]

Best performance close to the wind is neither. You're comparing two rather different craft; I would think that as sketched the Tasman (http://www.selway-fisher.com/Yacht2024.htm#JUN) would go better upwind than the Pioneer (http://www.selway-fisher.com/Yacht2024.htm#ION); neither is a hull form appropriate for leeboards, at least to my amateur eye. Adding a foot of beam to Tasman will not speed her up, and might make her downright pokey. There's a reason she was drawn that width, find out what it is (perhaps some road transport rule.)

[ChaseKenyon]

I agree the Tasman would probably go much better to windward.

Her bilge plates are vertical and do have some small advantage beyond the opening up of the center space (traditionally for cargo space). this still is not a performance into the wind craft. I like to cruise but like to get there in a reasonable time also. so I highly value both onto the wind and away from the wind performance.

Once again a proper (designed to heel a bit and then stiffen up) hull design can be very very fast with bilge boards that deploy at 90* to that part of the hull they protrude from, and especially if that puts them at 90* to the surface and the lateral force.

Chase

[Wooden Boat Fittings]

.
These are "oblique bilge keels" in my definition. They're steel, and fixed at right-angles to the outside of the hull at the turn of the bilge.

http://www.woodenboatfittings.com.au/boats/sanderling/starboard-bilge-keel-s.jpg

They allow the vessel, when heeled, to work okay to windward (albeit not brilliantly,) but their main (and four-fold) advantages are that (a) they allow for a reduced draught which in turn allows the vessel to work her way into waters much shallower than available to a fin-keeler, (b) they don't encroach into the internal accommodation at all, (c) they don't need to be 'fiddled with,' either to be rigged or when under way, and (d) they allow the vessel to stand upright in a drying berth, as you see here.

http://www.woodenboatfittings.com.au/public/mud-berth.jpg

If you have a mud berth (or even if you only like gunkholing) you should have a bilge-keeler.

Mike

[shaunbarrymcmillan]

I'll go with oblique bilge keels and contact the designer about their design and use if appropriate. Thanks for the info. As with most (all?) boat designs and boats -there ain't no free lunch

in the end it also boils down to the cost of the build versus the benefits - for us middle classed folks.

As to which I'll build that is uncertain but your info has helped

[Lance F. Gunderson]

Bilge keels complicate launching ( boat haulers hate 'em) and bottom cleaning and painting; they're great for drying out IF the bottom you ground on is relatively flat, but if it's not, you've got a problem. I'd prefer a flat bottomed leeboard sharpie any day.

[wtarzia]

Perhaps I missed it mentioned here, but can't leeboards be fairly efficient if you use two of them, built as asymetric foils, so that the one to leeward (ww one is raised) has a flat(ter) surface to lee and curved(er) surface to ww, for high(er) lift? -- Wade

[Woxbox]

Perhaps I missed it mentioned here, but can't leeboards be fairly efficient if you use two of them, built as asymetric foils, so that the one to leeward (ww one is raised) has a flat(ter) surface to lee and curved(er) surface to ww, for high(er) lift? -- Wade

That's absolutely true, and it has been done for centuries. The boards in the amas of modern racing trimarans are asymetric and provide considerable lift, both to keep the hull from diving and to help keep the whole boat to weather.

But what doesn't seem to be done is to fit a simple boat designed for cruising with high aspect-ratio, asymetric leeboards. Bolger has drawn lots of leeboard boats, and advises that a small amount of toe-in as well as slight asymetry will help noticeably. But he still prefers the practicality of shorter, wider boards. That's what it comes down to after all is said and done, how much hassle do you want to put up with for a bit more performance?

[Kaa]

In some places the bilge keels are very popular for obvious reasons:

http://i296.photobucket.com/albums/mm161/kaa_photobucket/20080528_0023_S1.jpg

My boat (a Eun Mara) has twin retractable bilge boards and I like them a lot. The cabin is much more usable without a centerboard trunk in the middle and they're excellent for gunkholing. One advantage is that when I touch bottom, I touch it with the keel while the slots are off to the sides and are much less likely to get jammed with seaweed, gravel, or anything else. I can also extend the bilge boards when the boat's on a trailer and that makes maintenance so much easier.

Performance-wise, retractable bilge boards give you a bit of additional control. You can deploy the windward one to reduce heeling or you can retract it for less drag. And, of course, the lee one points more or less straight down which is an advantage over a single centerboard.

Kaa

[ChaseKenyon]

My boat (a Eun Mara) has twin retractable bilge boards and I like them a lot. The cabin is much more usable without a centerboard trunk in the middle and they're excellent for gunkholing. One advantage is that when I touch bottom, I touch it with the keel while the slots are off to the sides and are much less likely to get jammed with seaweed, gravel, or anything else. I can also extend the bilge boards when the boat's on a trailer and that makes maintenance so much easier.

Performance-wise, retractable bilge boards give you a bit of additional control. You can deploy the windward one to reduce heeling or you can retract it for less drag. And, of course, the lee one points more or less straight down which is an advantage over a single centerboard.

Kaa


Well said!!!!


Someone else in the hey it really does work camp here!!!!!

Not only that, I have seen a oblique bilge board design where they were quite asymetric and very modern foils to be used just as you have pointed out.

now if I could just rememberr where I saw it!!!!!

lol


getting old I guess.


:rolleyes: