Why are double ended row boats more bouyant aft

[mike1]

Junior Member
Join Date: May 2004
Rep: 10 Posts: 30
Location: Cape Town

Hi All,
I've noticed from plans and boats, that many double ended rowing boats and paddle ski's seem to have the maximum beam and max bouancy , aft of halfway,, Why is this so when many of the older boats are true double enders with both ends beign identical?, for instance the Adirondack Guide boat, and the Gardner , Herreshoff 17 ft row boat. Also Paul gartside's designs.
I have often wondered , and now, look forward to finding out why.
Thanx
Mike
http://www.boatdesign.net/forums/images/misc/progress.gif

[boylesboats]

That all depend on its origin..
Some are beamier fore than aft..

All designs have their suppose

[mmd]

IF the design is though out technically and pragmatically...

It could be an attempt to minimize drag and therefore make the boat more efficient underway. The entry angle of the bow has a large impact on the wave-making resistance of the hull, so narrower is better. By moving the maximum waterline beam aft, the bow entry angle is decreased, reducing resistance underway. In a boat the size we are speaking of, the decrease in resistance is so small as to be only pertinent in theory, but it would give bragging rights in the sales pitch: "...the Wonderful Widget has a fine entry resulting in high-efficiency hull form, yada, yada..."

But I also suspect that a lot of the design effort given to a rowboat consists not of hydrodynamic calculation & evaluation, but of intuition, past experience, and following trends...

[kenjamin]

I don't know which rowboats you are talking about but it could be that many designers are realizing that many rowers are wanting to hoist a patch of sailcloth now and then when the winds are right. Fullness aft certainly helps with the sailing ability.

[Andrew Craig-Bennett]

I post this with some trepidation, as MMD, a naval architect, has already commented, but I cautiously suggest that, since a vessel under power tends to draw down aft at speed, a little more fullness aft helps to keep the boat level when under way.

[Antonio Majer]

in some little double enders, you go in one direction when you are rowing alone, in the opposite direction when there are two rowers.
http://img529.imageshack.us/img529/94/01aw5.gif

[Tom Hunter]

A practical consideration, if the rower midships and a passenger is seated aft they can see where the boat is going, tell the rower about hazards and also engage in conversation. A passenger will most often be seated aft because of this. Gear will most often be aft as well, because most of the time people want the boat down by the stern, not down by the bow.

So a bit more bouyancy aft is going to be helpful much of the time.

[Eric Hvalsoe]

Though the question was about true double enders, maximum beam aft of midships is typical of transom sterned boats as well. I like some of the reasons cited above. Of course a transom sterned pulling boat is a double ender on the waterline.

[Chris Ostlind]

I've noticed from plans and boats, that many double ended rowing boats and paddle ski's seem to have the maximum beam and max bouancy , aft of halfway,, Why is this so when many of the older boats are true double enders with both ends being identical?

For this type of asymmetric form, it is hull efficiency as noted above. You can also get a boost in the tracking of the hull with this arrangement. This asymmetric hull shape is common in modern tripping canoes, as well. When designed this way, there is often a bit more rocker in the hull which makes the boat more responsive to turning inputs, when applied.

So, you end up with a boat that wants to track, naturally, but has a high degree of turning responsiveness when you want it.

Seems like a nice set of characteristics for a human powered boat. Ideally, you'd like to spend as little time as possible making course correction strokes and instead, applying that energy to forward propulsion.

[Ben Fuller]

Buoyant ( check topic spelling)

It makes life easier in building if you are doing true double enders.... of which there are very few. St. Lawrence skiffs for example are finer aft which apparently made the boats track better when loaded with a passenger.

Most of these small ones are small enough so that fore and aft trim makes considerable difference in the way the water sees the boat. A little trim by the stern on a true double ender like a ducker fines up the bow and fattens the stern.

Most of what modern designers have done to do truly assymetrical double enders surround performance under sail where some extra after buoyancy makes up for the crew sitting aft and provides some increased stability. Decked sailing canoes are perhaps the extreme with a very long bow and flat stern sections.

[johnw]

It seems to me that most peapods and most whaleboats have the center of buoyancy forward of amidships. In the case of whaleboats, this may be to keep the boat from being dragged under water by a whale. In the case of peapods, I don't know. Some are completely symmetrical.

[mike1]

Hi Folks,
Thanx to all for the great input, can I summerise it as follows.
Boats , even small row boats are designed with a purpose in mind, Some are fuller forward some aft, ie, The St lawrence river skif is fuller forward to accommodate the rower, but settles when loaded with a passeneger
This is an example of where the COB and therfore fineness of the bow, can be changed by shifting weight.
By making the aft section fuller the bow section can be finer , which can reduce the wave making effect of the bow, thus reducing resistance.
The COB aft helps tracking when underway and all the rowers effort can be used to move the boat forward, instead of focussing on stearing.
Rocker can be encreased and this helps with manouverability.
In little boats, much of the design of the Hull shape might have more to do with intuition and experience than hydrodynamic calculations and evaluation.
Sometimes the hull fullness is designed to accomodate other activiies,,a sail (fuller aft), or perhaps loading a fishing nett from the bow ( fuller in the bow)
Thanx to all, If I ever use this info I'll keep all posted
Mike

[Clinton B Chase]

nice summary...add one more...I assume you are talking below the LWL with regards to fullness/fineness, but my Faering Drake is an example of fullness aft only above the LWL...I held the beam out so as to maximize the spread between oarlocks and thus get me the longer oars I wanted. Meanwhile, below the waterline the lines come in much finer. My boat is finer aft than forward in hopes that I can row for very long periods without exhaustion. Sprinting for the boat ramp she'll want to squat a bit.

Cheers,
Clint

[mike1]

Hi Clinton,
I have been following your Faering project, How are you getting on now?
Now its starting to make sense,
Yes I'm talking below the water line.
Directional stability can be achieved with both a skeg or fuller aft sections,
For long. easy rows, a fine aft section would help with the long run between stokes ( ala St Lawrence River Skif), however as the speed increases, a fuller aft section would be help because of the wave generated by the bow , and it's implications.
Mike

[Clinton B Chase]

a fuller aft section would be help because of the wave generated by the bow , and it's implications.

yes...but I always wonder why the sculling boats, obviously super fine on the water, work with all the speed they generate...are their aft sections full? Not by my standards!

Drake is getting along...making floorboards now. I smell like cedar.

Clint

[Daniel Noyes]

Hi all
great discussion
The rowing shells are built to a proportion length x beam that renders hull speed irrelevant, there is no wave making hull speed limit the shells are just fighting skin friction, so there is no bow wave that the stern will be sucked down into...interesting about shells though they do make waves, it's a kind of displacement wave, standing on the banks of the Charles when one of the eights goes by a few seconds later it's like the tide is coming in real fast, water level rises several inches then sucks back out, but you really cant see the wave.
Dan
http://dansdories.googlepages.com

[mike1]

Clinton, Daniel,
I scull a bit in a single, While many say they dont make waves , I can gaurantee they do . The designers story is that 98% of slow is skin resistance, I dont know , some designs are fast out the blocks and slow after , some slow out the blocks and run like crazy once up and running.
and they are fuller aft, I have some plans of a good scull. LCB 52% and a chunk and LCF 53%and a bit. its takes a few strokes to get going , but runs real far between strokes or so I'm told.
Cedar smell niiiice
Mike

[johnw]

Hi Clinton,
I have been following your Faering project, How are you getting on now?
Now its starting to make sense,
Yes I'm talking below the water line.
Directional stability can be achieved with both a skeg or fuller aft sections,
For long. easy rows, a fine aft section would help with the long run between stokes ( ala St Lawrence River Skif), however as the speed increases, a fuller aft section would be help because of the wave generated by the bow , and it's implications.
Mike

Actually, I think they are supposed to track better if they are fine aft, and be faster if they are fuller aft and finer forward.

[neilm]

An airfoil is finer aft. So are fish and submarines. Lots of research goes into submarine shapes so I'm guessing there's a reason for it. Some of the guide boats are finer aft which the curator of the Adirondack Museum claims makes them go faster.

However Chamberlain's Gunning dory is wider aft. I think it has more to do with passengers and weight distribution. The rower sits in the middle and the passenger in the stern. For a rowing only boat it seems to me a symmetrical design is more practical because you can row backwards as mentioned above.

Neil

[Thorne]

Remember that fish and subs are powered from an aft source, whereas rowboats are powered from the center/middle. Can't speak to the physics of sailboats, but suspect they are also powered from the center/middle.

The only pulling/sail boats that I know about that are wider fore than aft are cobles and possibly some Irish designs -- and I think this is because they are sometimes brought into the beach stern-first.

[peterAustralia]

Most boats have a finer bow than stern because of waves

A fine bow allows a boat to go through waves more easily. If a boat was going on perfectly flat water then the bow would not need to be finer than the stern.

n peter evans

[johnw]

I think most of the Norwegian double-ended rowing boats are wider forward.

[Todd Bradshaw]

Touring kayak builders are building both ways. Some, like Current Designs (one of the larger manufacturers of composite and plastic kayaks) build both types. Their design section states " The Swedeform hull (widest aft) has a cleaner longer slender entry giving easy touring speed and maneuverability. A Fishform kayak (widest forward) may have a slightly blunter entry but will have a more slender exit through the water which increases tracking, even in adverse conditions."

Having owned both types in their high end designs, I tend to prefer the Swedeform boats. The Fishform kayaks usually gave me more legroom (which at 6'4" and 210 lbs. is a pretty serious consideration) but I didn't like them in following sea conditions. I often tended to feel like the stern was squatting in the waves and I was paddling up-hill. I also didn't personally care for the feeling that waves were sneaking up on me from behind. By the time they started to lift the hull it seemed like they were almost up to the cockpit. Results on boats wider than kayaks or different preferences for how one's boat should feel might bring other folks to a different conclusion.

It should also be noted that a lot can be done to change the waterline shape and hull volume in various sections without making a boat that looks particularly asymmetrical, especially in plan view. Rather small changes in the chine's roundness and/or bottom flatness in specific areas can make a substantial difference in volume, lift, tracking and other performance characteristics without obvious differences in beam. Back when I used to paddle marathon racing canoes the typical river course might vary from ten or twelve feet deep to fairly long spots where you might be paddling in one foot of water over big sandbars. As you begin to paddle across these shallow spots, you soon notice that the bow goes up and the stern goes down - as does your speed, glide and efficiency. As it was once explained to me by a well-known old canoe designer, the bow wave doesn't just go out to the sides, but also down. If it's hitting the bottom, or bouncing off of it, it actually lifts the front of the boat. Marathon hulls almost always have more volume aft and supposedly this is in part to help keep the boat more level in shallow water. I've never studied bow waves much, so I can't say whether this is true or not, but when you paddle over a shallow spot in a really lean, highly efficient canoe hull you can both feel and see the boat's trim angle change as the bow raises and the boat gets tail-heavy.

[skuthorp]

I note your comments re marathon kayak racing Todd. I raced K1's and K2's in long distance and marathons too, and you could feel the boat speed drop when you hit shallow water and feel the pressure from the wave bounce up from the bottom under the boat. TK's were not so prone but had a bow wave of sorts. In deeper water a K generates a 'rooster tail' behind. I haven't paddled one of the newer designs, mine were from the '80s. I found a wooden hull seemed faster and was certainly more sensitive to the water as are wooden paddles. But I think modern blades are probably more efficient too. I had a Liminat K1 from Denmark.

[Woxbox]

An airfoil is finer aft. So are fish and submarines. Lots of research goes into submarine shapes so I'm guessing there's a reason for it.

It's always been my understanding that the main reason boats are finer forward is to deal with the waves. As soon as you go underwater or work in flat water, the pointed bow does little. Submarines have hemispherical bows because that's the fastest shape. Airplanes don't have pointed noses unless they are designed to break the sound barrier. Likewise, traditional flatwater, which is to say canal, boats have very full bows. Little Key, our replica 17th-century shallop, is a well-researched copy of the type. The bows are far fuller than the stern. Now that doesn't make her fast, because she's still heavy and beamy for her waterline length, but she does carry way very well.

Think of rudder and daggerboard shapes, too. They're wings, with rounded leading edges and finely tapered trailing surfaces. Like a fish, and like boats that aren't driven in rough water.

[johnw]

Checked my copy of "Inshore Craft of Norway" and found that almost all the traditional Norwegian rowing boats, and all the double ended ones shown, are fuller forward than aft. I suspect that if you are more worried about seakeeping than speed, tracking, and keeping the bow from diving, are important factors. In a following sea, keeping the boat from broaching involves both of those things.

[Todd Bradshaw]

....with the possible exception of the Nordfjord Faering shown on page 67 - which is a pretty interesting one if you scope out the waterlines on the plan. It looks like the three waterlines shown are tenths of a meter apart (about 4" for us non-metric types) starting where the garboard meets the keel with waterline "zero". The boat ends up being quite fish-formed up high, but the lower waterlines seem to be somewhat of a toss-up. However, #1, this is one of the older faerings shown in the book and #2, I once lofted parts this boat out using the end view showing the sections as a reference and there are several things that do not agree between the various views, so it's hard to say how accurate the printed waterlines are. In general though it's another example of a double-ender with very fine lines aft and more rounded, voluminous lines forward. I did think this was interesting though when they were writing about these old-style Nordfjord boats:
"As the importance of offshore fishing increased after 1860, the type was altered, speed and ease of rowing being sacrificed in favor of greater seaworthiness, the principal change being a higher freeboard and a fuller hull aft. The old boats, with their fine lines aft, proved dangerous when running before a gale." Reminds me of how my fishform Current Designs "Extreme" kayak felt with big waves coming at me from behind.

[johnw]

Interesting. So they figured the fishform hull was faster? Sort of goes against current theory, doesn't it? I guess being pooped in an open boat would be no fun at all.

[Woxbox]

Here's another thought that seems to me to apply: As you go back in time, ships get more and more "apple cheeked." Come back the other way in time, and they get finer entries. In the olden days, they sailed with or off the wind almost always. When they started to build ships to sail into wind and waves in earnest, they developed what we consider the right kind of bows.

But I don't beleive they were mistaken in them olden days. For the service - wanting to go with the weather safely - the fat bows worked best. On the Kalmar Nyckel, which has very full bows, the shape is only a problem when we motor straight into the waves. Makes bow watch duty more of a bow shower.

[mmd]

Y'know, most of what has been said above is correct, even the conflicting stuff. How a hull moves through the water is a complex and subtle process. It is more complex that how a sub or a fish moves through the water, or how an airplane moves through the skies, because a boat is operating at the interface between two mediums - air and water. So there is hull friction, windage, wave-making, water surface tension, dynamic wave action, weight distribution, and appendage drag to consider, among other things.

In big ships there is budget to evaluate all these forces, even sometimes money to research the forces to better our knowledge of such things. Little boats do not have such luxury of specialists and money, so design usually evolves by intuition and experimentation. Sometimes this leads us down blind alleys and sometimes it results in a breakthrough boat. But the fundamental point that I fixate upon is that for the most part, a good small boat design is a successful compromise of conflicting demands for a specific set of conditions.

In other words, what works well for one area, conditions, and use may not transfer well to a different area, conditions, and use. Therin lies the fascination and frustration of small craft design.

[sailing canoe]

If I can tweek this thread - would anyone like to speculate on the effect of rocker on rowing efficency. Obviously the straight keel makes tracking straighter and the water line some what longer; but since ,I assume ,most row boats are going at less than hull speed most of the time is this the most important point? I'm wondering if some of the lenght factor isn't to prevent hobby horseing - especially with a sliding seat? Is there a hydrodynamic penalty for rocker in a row boat?

[neilm]

I think rocker makes a boat more seaworthy and slightly easier to beach. It also makes it easier to turn. I think those are the primary reasons for rocker. I doubt it makes the boat any faster as seakayaks don't have much rocker. Driftboats have the most rocker so they can change direction quickly.

Neil

[Tom Hunter]

Finished the 1999 Blackburn in 4 hours, 2 minutes, 32 seconds. The boat deserves a lot of the credit for that:

http://farm3.static.flickr.com/2174/2308019853_2cff7aeacd_o.jpg

This dory has slight rocker, and that does make her faster than she would be with none. If she had more rocker it would probably slow her down. To MMD's point that is the magic, where is the perfect amount.

I would not be so certain about going less than hull speed. I like to move right along, and nothing is more frustrating than knowing that a bad hull design is artificially slowing you down. I doubt I am alone on that.

[Todd Bradshaw]

My favorite weird hull shape would have to be my old Mariner II kayak. I'd always wanted one, so I had them build me one. When the trucking company finally found it after three weeks in transit (how the hell do you lose an 18' long package?) I unwrapped it and turned it over to check the bottom for damage and literally said W.T.F.?

It was Swede-formed (widest aft).The front half had a fair amount of rocker. The aft half had almost none. The front half was a narrow, very rounded V-shape with lots of flare above the waterline (good for lift in big waves to keep them from hitting you in the chest). By the time you got to the seat area the bottom was almost flat (beam about 21") and the hull was starting to develop hard chines and plumb sides. The chines continued aft about half-way back to the stern, where they turned upward and tapered-out. The back end of the hull then became sharply V-shaped all the way to the stern. In addition, it had a sliding seat so that you could adjust trim and windage while paddling. It was the stangest combination of differing cross-sections that I'd ever seen on one boat. It turned out to be a great boat in the water - good stability, quite fast, good maneuverability, very seaworthy and you could slide back and ride the chines when surfing. I really loved it - except for one thing...my big feet were jammed against the deck with my heels jammed together on the bottom. I couldn't fit into it with anything heavier-duty than just wet-suit socks. I seriously considered cutting foot holes in the deck with some sort of fiberglass bump installed over them, but didn't want to screw up the resale value of a $3K boat by making it look like it had frog eyes. In any event, it was a very interesting combination of shapes and features that in this case, added up to a hull that did it's job really well.

http://webpages.charter.net/tbradshaw/mariner%20II%20004%20copy.jpg

http://webpages.charter.net/tbradshaw/Mariner1%20copy.jpg
p.s. I'm not really this fat....it was 37 degrees outside that day.:) We literally could have gone iceboating on one lake in town and kayaking on another that afternoon, but we ran out of daylight and energy.

[lagspiller]

When talking about open trad. boats with more beam forward than aft, I immediately thought of the 'Geitbåt' - the workboat/færing from Trondelag area. It is pronounced more beamy 1/4 to 1/3 of the way aft from the bow and tapers almost evenly to the stern from there.
Found this on Wikipedia

Geitbåten skil seg ut frå dei andre tradisjonelle opne båttypane frå Vestlandet (http://nn.wikipedia.org/wiki/Vestlandet) og nordover ved den korte og relativt djupe kjølen, den halvplanande skrogforma og den svært langstrakte overgangen frå kjøl til stamn — eigenskapar som gjer denne båten til ein djerv havseglar og god bidevindsseglar.

The Geitbåt differs from the other traditional boats of western norway and northern norway because of the relatively deep keel, the half-planing hull form and the exceptionally long transition from the keel to the stem - attributes that make this boat a 'brave', seaworthy ocean sailer and a good upwind sailer.

http://mellemverftet.net/album/Sj%C3%B8setting_flytting/sverrejsingeitbtiv.jpg

http://mellemverftet.net/album/Sj%C3%B8setting_flytting/sverrejsingeitbtvi.jpg

[john welsford]

Over recent years it has been found that the location of the center of bouyancy is quite specific to efficiency at a given speed length ratio, and in simple terms this means that the closer to the boats top displacement speed the boat is intended to run at, the further aft the ideal C/b will be. This is not a huge movement in displacement hulls, only about 6 or 8% between an S/l of 0.8 and an S/l of 1.3 but its enough to make a noticeable difference in a very light craft designed for speed.
A hull that has a shape consistent with speed, and having that aft positioned C/b will be noticeably finer forward than aft all else being equal. There are other issues involved as well , many covered in other postings.

Bolger, somewhere in his writings has said something to the effect that with light displacement boats it pays to make to front half "faster" than the back half. I think that he's right, especially for harbour craft that have to deal with a short steep chop as it not only goes faster in flat water but greatly reduces the pitching .

JohnW


Junior Member
Join Date: May 2004
Rep: 10 Posts: 30
Location: Cape Town

Hi All,
I've noticed from plans and boats, that many double ended rowing boats and paddle ski's seem to have the maximum beam and max bouancy , aft of halfway,, Why is this so when many of the older boats are true double enders with both ends beign identical?, for instance the Adirondack Guide boat, and the Gardner , Herreshoff 17 ft row boat. Also Paul gartside's designs.
I have often wondered , and now, look forward to finding out why.
Thanx
Mike
http://www.boatdesign.net/forums/images/misc/progress.gif

[john welsford]

More design theory, ( see my other post) Not only is the ideal centre of bouyancy specific to a given speed length ratio, but so is the prismatic coefficient, and unless the boat is a seriously odd shape you cant achieve the appropriate P/cf unless the boat is rockered the right amount and in the right place.


There are some good references to this and the C/b issue in Cruising Under Power, First ed by Robert P Beebe, long range cruising powerboats are not dissimilar in their requirements to rowing baots in some ways, both have to travel as fast as possible on as little energy as possible.

JohnW

If I can tweek this thread - would anyone like to speculate on the effect of rocker on rowing efficency. Obviously the straight keel makes tracking straighter and the water line some what longer; but since ,I assume ,most row boats are going at less than hull speed most of the time is this the most important point? I'm wondering if some of the lenght factor isn't to prevent hobby horseing - especially with a sliding seat? Is there a hydrodynamic penalty for rocker in a row boat?

[Ben Fuller]

One of the nicest recreational rowing shells out there was designed by Ted Van Dusen who is responsible for some world class racing singles and doubles as well as single ICF racing kayaks and most recently a surf ski. Called the Zephyr it had a long round bow, chines that started amidships or there abouts and a shallow arc between the chines heading to the stern. A lot like the Mariner. Long bow to make open the water if you will, a buoyant stern to keep squatting at speed under control Round sections and some rocker to keep wetted surfact down.

Virtually all the working boats that are fine aft and more buoyant forward are that: working boats generally load carrying, sometimes like whaleboats with forward beam to help keep them from driving under, fine stern to help act as a skeg.

Remember these boats all evolved or ( in modern times) were designed to do different things.

[sailing canoe]

Hi John thanks for responding. I'll go and review the cp in some of my books . I dont have the one you mention -Beebe. Do you care to expand on the relationship between rocker and cp? I was initiall concerned mostly with any negative evects of rocker on rowing efficency; i.e. could a boat with rocker sort of get sucked down between its bow and stern wave - the water line gets longer but??
thanks Nick

[soba]

I was talking to a local naval architect about this thread...his opinion about the swedeform rowing shells is that they evolved from work boats that often needed to go through surf. He says a boat with the widest point amidships or forward of amidships has a greater tendency to broach to in heavy surf. Having a tad finer entry and bit less bouyancy low down in the bow apparently helps a rowed boat keep from "turning down." He also mentioned that many surf rowers turn around and row face first, stern first into the beach...for the same reason, but that putting more weight into the bow in such a scenario makes it easier to beach without broaching.

An interesting point. Discussion?

[Ben Fuller]

I don't think your local naval arch has much experience with beach boats.

There are many reasons to go stern first into the beach that may have nothing to do with where the beam of the boat is located. With a coxed boat it lets the cox see the oncoming waves and gets the crew to slow the boat ( surfing is not desired here). It keeps you from having to turn the boat around on the beach as launching bow on is highly desireable. Working beach boats do not want to surf coming in. One of the most extreme examples of a surf rescue boat is the Mass Humane society boat at the Hull Life Saving Museum with a bucket of displacement forward, which translates to lift as you head backwards into the beach. A second one is the coble design, a transome sterned fishing type that lands on especially steep nasty beaches.

This is similar to landing a heavy kayak through the surf. You want to slow the boat down when overtaken by a wave, not surf it. You go in on the back of a wave.

[Daniel Noyes]

yeah surf boats are very specalized craft. The surfboat Nantasket is really something to see worth the trip out to Hull. Like Ben said the bow is very full to rise and meet the waves while the stern is extremely fine, this also probably made the boat very manuverable when approaching a wreck to pick up survivors. There is a great set of photos of a surf boat and equipment from the Plumisland station where my great uncle Giggy Pickard was a crew member, he is in the breaches in a photo taken during a demonstration of the breaches bouy.

Also lets not even get started on waterlines of sailing craft, all the American centerboarders in Kunhardts Small Yachts have their widest point about 2/3rds or even 3/4 of the way back to the stern, thats in the 1880's! they bassically look like Ausie 18' skiffs with slack bilges. And dont even get me started on the design genius of the waterline manipulation that is the racing scow, incredible!
Dan
http://dansdories.googlepages.com

[Ben Fuller]

Having just been moving some yaks around ( memorial day clean up) I noted that I have a few extreme swede form..... downriver race boats. They are horrible in a following sea. This also noted in a discussion of the evolution of the Swede form kayak design in Heath / Arima's Eastern Artic Kayaks with the shapes coming from low speed semi displacement powerboats of the early years of the 20th century.

There is an interesting discussion of the placement of max waterline beam in Rousmanire's Low Black Schooner discussion of America. Aft placement was coming into vogue in the years before the Civil War, with somewhat sensibly the New York Hudson steam boats leading the way.

[john welsford]

Given a boat with a hull of reasonably conventional shape, you will end up with rocker to achieve the appropriate curve of areas from which you deduce your prismatic C/f. If the boat is to be a reasonably light one and rowed a speed that can be maintained by a normal mortal then you are looking at a speed length ratio of maybe 80% of hull speed, and at that you'll need the center of bouyancy at about 53/55% w/l aft , and a prismatic C/f of maybe 52%.
I note though that there are variations on the theory, and one or two of my boats have been much higher than this and I've not noticed any significant increase in resistance.
But the theory is well proven, and it applies to rowing skiffs as well as motorships and warships, some designers will argue slightly different figures but they dont vary a lot.
Robert P Beebe wrote an excellent book on long range cruising in displacement powerboats, and there is lots of very interesting design theory in there couched in terms that are very easily understood. I much prefer the first edition, well worth reading and I have found the views expressed consistent with my own experiences.

JohnW


Hi John thanks for responding. I'll go and review the cp in some of my books . I dont have the one you mention -Beebe. Do you care to expand on the relationship between rocker and cp? I was initiall concerned mostly with any negative evects of rocker on rowing efficency; i.e. could a boat with rocker sort of get sucked down between its bow and stern wave - the water line gets longer but??
thanks Nick

[Paul Fitzgerald]

Rowboats can be an interesting example of the difference between theory and practice.
Designing a rowboat for maximum speed misses the point that most working boats can't sustain that speed for any amount of time, and are most efficient at a speed of less than the square root of the waterline length. I can row a fixed seat boat all day at that speed.
The most efficient small tender I have rowed is the 8 foot Bolger Nymph, which has excessive rocker, but it is very efficient with a large load at normal rowing power.
On the other hand, a 15 foot boat of similar five chine design but less rocker, a Mertens Scilly Gig, is only good for an extra half knot at normal rowing power and the same load.