TexasKayakFisherman.com est. 2000

Kayak fishing the Lone Star State...


By CoolfinIE
#1267705
I am looking a various kayak designs for a summer build project. Plan browsing is quite a pleasurable way of passing time.

So as events unfolded I found myself playing with the excellent Carlson hull CAD free software tweaking various designs and then looking at the effects a "tweak" has on performance, stability, max hull speed and draught.

I have always taken at face value that while lateral stability/resistance to heel can be increased by widening the hull, and that adding flare will increase secondary stability, this is just widening the hull when it is at an angle of heel. Both of these are intuitively self explanatory.
However here's my question: I have been told that (in sea kayaks) increasing hull length will also increase lateral stability.
How can this be, considering that a longer kayak will have either the same width, or possibly even a narrower hull, in which case the centre of gravity of kayak and occupant is surely easier placed over a point outside the wetted area, and inducing capsize.

So is this just an old kayakbuilders tall tale?

Norm
User avatar
By gerald
#1267719
Not taking the cross sectional shape of a boat hull into consideration--yet--I believe you can consider the width at the bottom of the boat to determine initial stability and the width (flare) at the sheer to determine secondary stability. You can have a very narrow bottom with a very wide sheer and the boat will be tippy and hard to paddle--and side waves will cause you to pitch badly to the side. There must be a happy medium between the bottom (keel), sheer, and how the boat "heels" to each point, whether the boat is hard chine or soft chine.

So let's assume we have a boat with good initial stability and good secondary stability--maybe just slightly better secondary than first--and the cross sectional shape of the hull allows the transition of the heel of the boat to be easily controlled without any unexpected acceleration of the heel. That's what we are looking for in most cases. If you lengthen that hull by a few feet you'll increase the displacement of the hull by a certain amount. The theory is that the longer hull will increase the "moment" of resistance along the longer hull. The hull MAY also rest higher in the water, and there MAY be a slight increase in stability because of this, but probably not enough that you could detect it. It also may become tippier. There's a point of diminishing returns in this scenario because a boat should be paddled at it's design capacity--more or less. Most boats become more stable with more of a load up to a point--and at a certain point--tippier with less of a load. I have seen boats, usually of the eskimo style design, that won't rest upright on water when unloaded. Only with a passenger does the boat settle into the water. I don't thing that's the kind of design we should go for unless we specifically have that need (I can't imagine why).
So--I think you are basically on target with your thoughts. Make a boat wider for more stability Don't lengthen a boat for more stability. Make the boat longer for other reasons--usually for potential speed.

Many sea kayak paddlers like to point out that a wide, stable, boat is not as seaworthy as a sea kayak because it pitches from side waves because of the greater stability. I would disagree with this in a wide, stable boat that is properly designed for an easily controlled and anticipated heel. It's not just the width but the cross sectional shape that makes the difference. I have not yet determined that perfect shape. I'm working on it though!

Making a boat longer AND narrower is another matter--that's a totally different boat.

clara.net does have some nice information on that site. Another you might look at is http://www.kayakforum.com which is an offshoot of Guillemot Kayaks and Nick Schade. Very good sites. Lots of information.
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By Pogo
#1267720
I don't pretend to be knowledgable on hull design, but I can tell you my personal experiences with a boat called the Cormorant 16, for which free offests are available on the 'net, see: http://www.squeedunk.com/cormorant.htm . Finished boat measures 15'-10" x 22".

As my second building project ever, I built two of them, both as hybrids (S&G hulls w/ strip decks), one for the lady I was seeing at the time, and one for me. Mine came out with a pretty deep vee to the hull, about two inches of deadrise at midships. It turned out to have little primary stability, but tons of secondary stability, which made it a great boat to develop skills in . . . mostly because you were forced to. Hers came second, and I flattened the vee in hers considerably, to about three-quarters of an inch deadrise at mid-point, in an effort to increase primary stability for her. Surprisingly, both boats behaved identically in every regard, I'm convinced there'd be absolutely no telling which was which if you did a blindfold test.

Then, a particularly astute and competent friend in Louisiana built a Cormorant, also as a hybrid, and increased its length to 17'-2" while maintaining the 22" beam. The resulting boat has far more primary stability, hard to believe it's the same design.

For whatever it's worth . . . .
User avatar
By bowgarguide
#1267725
I am going to post to this just from experiences I had personaly.
I built a laker 13 ft, then I built a Laker 15 ft 5 inches,dimension on the width are the same and also the shape of the hull, there is an unbelievable difference in stability that the longer boat has over the shorter one.
I may be crazy ,no comment Gerald, but the last few boats I have built have been asymmetrical and the seem to be more stable that the symmetrical ones. I think your weight is in the widest part of the boat.
Ron
User avatar
By gerald
#1267754
Just for a little thinking exercise regarding pogo's comments I would venture to say that all the changes he made to the second boat never changed the actual width of the bottom section at the water line. It changed the shape IN the water but that is going to have no effect except when the boat is underway--and in that case might have increased drag a bit. The initial stability begins with the width of the hull that is in the water at the water line. The transition from initial to secondary stability (and in truth it is all one thing) is determined by the shape of the hull that is GOING to be exposed to water--not the part that is already in the water. There are limits to this, of course, because as you heel you're going to be bringing part of the boat out of the water. Anyway--S&G hulls with their flat slab sides tend to be more tender than a round chine hull. Round chine hulls are, or can be, more seaworthy. That doesn't mean they always are though.

..and the cormorant IS a 22" S&G boat...isn't it?...
By CoolfinIE
#1267788
gerald wrote:... Anyway--S&G hulls with their flat slab sides tend to be more tender than a round chine hull. Round chine hulls are, or can be, more seaworthy. That doesn't mean they always are though...

Are there any kayaks with railed hulls? Rails have been a highly significant development for small boat designs, if not maybe kayaks.
A stitch & glue flat chine with 2 - 3 strip rails glued onto the flat ply sheets, or a round bilge with added strakes, or possibly even lapstrake built, would achieve the stability benefits I'm thinking of that strakes confer to other small boats.
However I assume everything has already been tried and kept or rejected for good reason, and I don't see any kayak designs with them but still wonder if hull rails would be advantageous.

Norm
Last edited by CoolfinIE on Sat May 30, 2009 5:38 pm, edited 1 time in total.
By Longtooth
#1267806
I have been told that (in sea kayaks) increasing hull length will also increase lateral stability.
How can this be, considering that a longer kayak will have either the same width, or possibly even a narrower hull, in which case the centre of gravity of kayak and occupant is surely easier placed over a point outside the wetted area, and inducing capsize.
So is this just an old kayakbuilders tall tale?
Norm


Hi Norm,

No, it is not a tall "tale", but a true element of kayak(or any watercraft) design.

Think of it this way:

Imagine a rectangular boat(like a barge, or shoebox), say 10 feet long and 4 feet wide.

Put one person aboard, weighing "X" pounds.

As the one person moves toward the gunwale, the boat begins to list (or roll).

As the boat rolls, the immersed shape changes, and the "center of buoyancy"(the point where the water is pushing back AGAINST the roll) moves toward the same gunwale.

Lets say that when this particular X-pounds person sits on the gunwale, the boat has no freeboard left, and is at the point of flooding and sinking.

So, we have a boat that can resist just X-pounds of roll-force, before flooding.

Now, add another boat of the same dimensions, and bolt them together at the ends, making one LONGER boat of the pair.

What you have now is a boat with only the length changed, which is now 20 feet long with a 4 foot beam.

Here is the point: BOTH of the two boats (which are now half boats) can resist "X-pounds' before rolling far enough to flood.

So it will take TWO times "X-pounds" to roll the combination to the flooding point.

But our single person still weighs only ONE times "X-pounds".

Our lucky guy has a safety factor of another "X-pounds" moved to the gunwale, before this longer boat will roll to the same point.

And he did nothing but make his boat longer,...no increase in width or draft.

Differences in hull shape will effect this situation to a degree,...but the basic principle is sound.

A boat which has no changes made except that it is made longer, will be proportionally more "stable" athwartships.

Hope this helps with understanding.

Best wishes!

Longtooth
User avatar
By Pogo
#1267825
gerald wrote:..and the cormorant IS a 22" S&G boat...isn't it?...

Pogo wrote:Finished boat measures 15'-10" x 22".
<snip> <snip> <snip>
. . . . while maintaining the 22" beam.
User avatar
By gerald
#1267870
CoolfinIE wrote:
gerald wrote:... Anyway--S&G hulls with their flat slab sides tend to be more tender than a round chine hull. Round chine hulls are, or can be, more seaworthy. That doesn't mean they always are though...

Are there any kayaks with railed hulls? Rails have been a highly significant development for small boat designs, if not maybe kayaks.
A stitch & glue flat chine with 2 - 3 strip rails glued onto the flat ply sheets, or a round bilge with added strakes, or possibly even lapstrake built, would achieve the stability benefits I'm thinking of that strakes confer to other small boats.
However I assume everything has already been tried and kept or rejected for good reason, and I don't see any kayak designs with them but still wonder if hull rails would be advantageous.

Norm


If I'm understanding you correctly then I'd have to say that there are no advantages for rails, or even a lapstrake hull on a small paddled craft. Rails, lifting strakes, and sometimes lapstrakes give lift to a boat as speed increases, but this is primarily for a planing hull. A paddled displacement hull will never achieve planing speed (there are some differences of opinion here. I can surf with some of my boats--that's planing, but generally you are NOT going to plane). What the rails, lifting strakes, and lapstrakes do is add drag to the boat while under way with no stability benefit while resting. For directional stability you shouldn't need a keel, rails, or any other artificial means other than the shape of the hull EXCEPT in certain hulls designed for single specific tasks with full awareness of the deficiencies in that design.
By the way--lapstrakes sure look good in some designs...
By Longtooth
#1267952
However I assume everything has already been tried and kept or rejected for good reason, and I don't see any kayak designs with them but still wonder if hull rails would be advantageous.
Norm


Hello again :)

Planing:
Gerald is correct in that it is not going to happen in a human-powered craft except an extremely specialized hull with an extremely powerful human engine, and IF then,...only for an extremely brief period(seconds).

The power-to-weight ratio is just wrong ( For a discussion of an optimum displacement/planing "transitional" hull, see "High Speed Sailing, by Frank Bethwaite). Frank gives minimum power to weight figures for planing, and it's not doable in any practical way by we human engines. (Sails give Frank enough power to work with, and he does it very well!)

[Edit - planing trivia):

The only readily-observable demonstration of human-powered planing on the water's surface is the example of kids riding on 'skim-boards' in the shallows at the beach(or similar shallow-water location).

The board(planing surface) is dropped (and then jumped upon) by the already-up-to-speed youngster, and "planes" with it's usually quite-lightweight load for a matter of a few seconds, until the speed drops below what is needed to create the "dynamic" lift.

This type of planing takes place in and is aided by the "ground effect" condition, due to the proximity of the bottom in the shallow water.

Nevertheless, it is definitely "planing" rather than displacement operation, as the board certainly does not displace sufficient water to support the weight of itself and its rider.

"Rails", and "lapstrake":
Rails can be useful as protective rub-rails, and as external hull stiffeners(stringers). A keel-like rail or skeg that increases lateral plane, especially toward the ends of the hull, can act to counter yawing forces from side-to-side power application(paddling, as opposed to rowing) or other yaw-inducing forces(greater windage fore or aft in a cross-wind, for example).

"Lapped"-strakes provide stiffness in a lightweight hull, because each lap is in effect a full-length "stringer" which is thicker than the "skin" between the laps.

Other than these functions, the rails add weight and the rails and lap-strakes add building complexity, and both increase skin(surface)drag, ... none of which aids stability or speed.

Sharp corners(chines) vs. round:
Look at the simplest examples to understand the principles. A square shape shifts the center of buoyancy out-board when a roll begins, to some degree aiding stability. A round form(think log-rolling) does not. (See the B-B method below, for a visual aid).

Either chine-shape is only the begining, however, and the "flare"(concavity), or "flam"(convexity) of the hull which was previously above the waterline comes into play here, and is a whole 'nother discussion.

Here is a method useful to hands-on designers, as opposed to those who prefer to let someone-else's computerized design program provide "answers" to them:

To see what a given form will do when rolled, make a raised outline of a given section, on your slightly-inclined drawing board ( there are flexible rulers you can use for this, or simply cut the shape out of a thick-enough cardboard to contain the "water-molecules ;-), and fill it to the waterline with marbles or B-B's. "Roll" the section, and arrange the B-B's so they are level with the original water surface. This will let you see how an immersed section (of the same displacement) is now shaped, and will give you an idea of how the center-of-buoyancy has shifted with the tilt. ( AFAIK, this "B-B's of displacement" method of determining immersed section shape is original with myself. I have an extensive design library, and have not seen it mentioned elsewhere.)

The actual C. of B. can be determined by finding the C. of G. of a cardboard cut-out of the new immersed shape. When the C.of B. is outside the C. of G. (of the boat and load) the influence willl be to counter the roll, ... i.e., a stabilizing force.

For a better idea of the stability of the whole hull, this procedure needs to be repeated for a greater number(or all) of the hull station-sections.

"Already been tried":
Yes, nearly everything. "Advances", at this late stage of development are nearly all tied to new materials and methods, ...with few really "new" ideas left to discover regarding basic design.

Look to the extreme examples for the limits. A racing shell shape is currently the fastest "displacement" human-powered craft. It has a length and shape determined to be the best compromise for minimizing wave-making and surface drags. It is "dynamically" stabilized by the sculler via the oars. The boat is completely unstable without the oars(sculls) in the water (back to log-rolling again).

Anything done to stabilize this "optimum" hull using "form stability" (which will work equally well at rest or underway,) rather than dynamic stability, has a drag cost, and reduces speed.

The best (least drag-costly) means of stabilizing such a hull is the use of "flying" outriggers - pontoons or floats which are just above the surface of the water, and do not become effective (and thus create no water-drag) unless(untill!) the boat begins to roll.

BUT!, ...these are generally not suitable in a boat where the oar-or-paddle stroke would contact the outrigger!

Form stability: The further "out" to the sides the center of buoyancy can be moved, the more roll resistance it can provide. A catamaran is the prime example. BUT! (again ;-), ... there is no catamaran (or other multi-hull) configuration which can have as little drag as an optimized monohull (physics). Also, a cat is not best suited to oars or a paddle, because of reach requirements, and increased yawing when alternate side power(paddling) is used.

So we have one of those unavoidable "compromises" met with in boat design. When using form stabilization, other-than outriggers , a wider boat (for a given length) is a more stable boat, but is also a slower boat.

So, the best practical compromise between stability and speed for an oarsman or paddler, as determined by thousands of years of trial and error, is a hull which is form-stabilized by it's wider-than-a-log shape, but still as "tippy" as the user is willing to accept.

Users have differing abilities or fears, so the range of "acceptable" stabilities is wide.

Is there a bottom line?

For the non-designer, trying out a number of different boats will soon give one a seat-of-the-pants understanding of what his/her personal requirements are, for a boat in which one can feel safe.

The same is required for the designer, to give a feel-basis for what is "enough" stability, after which he can play with shapes which provide it, while best meeting his other design goals (speed, etc). Success in this activity assumes quite a bit of study :( on the would-be designer's part, of texts in which the tremendous amount of "already done" experimentation, and its results, and lessons-learned, are described.

Best wishes,

Longtooth
Last edited by Longtooth on Sun May 31, 2009 2:18 pm, edited 2 times in total.
User avatar
By gerald
#1268127
Longtooth: Thank you for your comments. Please feel free to expand upon any aspect of boats, boat design, paddling, etc. that you feel will interest us and expand our knowledge of this great sport. I know I'm still alive because I still learn something new everyday. I plan on being around for a long time...
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By gerald
#1268353
We also see what is being referred to as the "ground effect" in shallow water marathon boat racing. Most people don't realize that the "wave" you see radiating away from the bow of the boat is also radiating downward--in all directions actually (in the water). When the boat gets into shallow water this "wave" usually causes a LOT of drag. It feels almost like the brakes are on. Under certain conditions, though, the "wave" can bounce back, provide lift and release drag. The boat feels rocket propelled for a short time. Don't count on that condition though. It's best to avoid the shallows if you want to go fast.
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