I’ve found many discussions of hull shape in cross section view (flat bottom, round, vee, flare, tumble home, etc.) and some on profile view (rocker, no rocker, forward sweep stem, reverse curve stem), but have not yet found a discussion of the characteristics of hull shape in plan view (from the top - widest amidships, widest toward the bow, widest toward the stern, continuous arc vs. nearly straight gunwale fore and aft with sharper curve amidships, etc.). Where might one find such a discussion?
There have been some discussions on the subject over the years. They have mostly centered around fishform, neutral, and swedeform.
Generalized speculation often pointed towards swedeform having the most efficient glide, fishform making it perhaps marginally easier to manage following seas, and I guess neutral somewhere in between.
It is an interesting topic that I’m sure must be discussed in depth in a few places. I’m guessing most lines would lean more towards a continuous arc for top speed and maneuverability. But like everything, it isn’t going to be the end-all be-all in terms of defining those characteristics. I know there have been many kayaks designed with concave curvature that would lead to arc in both directions along certain lines of that profile. (Sterling kayaks, Romany, Sirius, etc.) I think my NDK Greenlander would probably have some fairly straight sections, but nothing concave. Something like a Gulfstream, or my Tiderace Xtreme, or my Current Designs Extreme/Nomad, I think follow a convex arc pretty much at any depth along the hull from that profile. Not to mention something like my Quest LV, that I think would have a pretty fair constant convex arc from the bow well past amidship, and then from the keel to the chine, have some concave curvature in the stern section.
So there’s a lot of different thoughts going on regarding the subject. As such, I’m not sure how many definitives have been discovered.
Hopefully you’ll get some more input on this one! (Are you contemplating your own design?)
Yes, actually am considering lofting my own design - a tandem with some flare amidships and some tumble home at the paddlers’ locations. But I have to work out the lines to see if it’s possible/practical. If I go skin-on-frame it will need to be all convex. It would be simpler to go neutral (or mirror image), but wondered about asymmetrical designs in both plan and profile. There’s no one design that fits all circumstances, so I’d like to know the trade-offs as much as possible ahead of time.
Thank you for the comments.
I regard Mat Broze of Mariner kayaks (Designer of the brilliant Coaster) as an authority on this subject. Here he is on http://www.paddlewise.com/topics/boatdesign/pro-conswede.html
Nick.
Date: Fri, 21 May 1999 23:27:46 -0700
From: “Matt Broze”
Subject: Re: [Paddlewise] Pro’s and Con’s of the “Swede Form”
Well I’m not named John so this may not be “the gospel” but this subject is
also pet pieve of mine so you sucked me in to giving a response.
The Swede-form shape (greater underwater volume aft of the midpoint) has
less resistance moving at the water’s surface than either a fish-form shape
(its opposite) or a symmetrical hull. The finer bow more gently parts the
water for less wave-making resistance. The longer run of positive pressure
in the forebody also can result in a longer area of the hull being in
laminar flow (laminar flow over a surface creates about four times less drag
than turbulent flow).
Note: A fish-form shape has less resistance underwater or in the air (where
there is no wave drag). This has confused some designers who have consulted
hydrodynamic texts, but not gotten the full picture of what happens at the
water’s surface. Fast ships, canoes and kayaks are Swede-form. Fast
submarines and fast fish are fish-form.)
For a kayak a Swede-form hull has many other advantages over fish-form. They
include:
1)Less pounding in head seas because they are narrower in the area where
pounding occurs (but, bottom shape is a bigger factor in pounding so some
V-bottomed fishforms will be softer than a flat bottomed Swedeform shape),
2)Easier and quicker turning (turns are enhanced by the greater curve to the
stern–and leaning makes this effect even more pronounced),
3)Less weatherhelm (more windage and a longer lever arm in front of the
paddler and less behind)
4)More of its volume is usable storage space behind the paddler and less of
the volume is in the wasted space around the legs.
5)A narrower beam where the paddle enters the water means easier more
efficient paddling (less boat to reach over) and less turning moment
produced with each (less off-center) stroke.
Fishform advocates correctly point out that Swedeform is less directionally
stable (other things being equal). One of them used to even say fish-form
was self-correcting (when he really meant self-stabilizing). I see this as
a disadvantage in a kayak. Directional stability just means you work harder
for each degree you turn rather than being able to translate momentum into
turning by leaning. Course keeping is actually harder with fish-form because
correcting your course after a wave has altered it is more difficult with a
kayak that resists turning. Fishform will be harder to turn and leaning
doesn’t help turning nearly as much as with Swedeform. The less
directionally stable Swedeform shape will track just fine if some keel is
added in the rear, but when leaned it will still retain its superior turning
ability. The best of both, tracks straight and turns readily with a slight
lean.
As far as choosing a kayak goes, trying to figure out what is going to be
best for you intellectually is a hopeless task, you will just get bogged
down in controversy and verbiage. Paddle kayaks that are recommended to you,
and designed for what you want to do. You will feel what you like. Few can
translate the verbiage, dimensions or even the shape of a kayak into the
“feel” that will result so why not go straight for the feel.
Matt Broze
www.marinerkayaks.com
I remember that post from Matt Broze, there’s a lot of excellent stuff in there. It’s also a veritable recipe for their boat designs, particularly the Coaster.
There is also great deal of good information from John Winters in “The Shape of the Canoe,” virtually all is relevant to kayak design: http://www.greenval.com/jwinters.html
To RDavS, Brian Schulz at Cape Falcon is an experienced designer and would knock together SOF hulls with drywall screws and shrink wrap to test subtle design iterations on full sized hulls in the water. Designing on paper/cad is good, but the fine details of performance depend on interactions between all components of the hull which even designers with years of experience cannot always predict. Prototyping is worth consideration.
Many thanks for the responses. Helpful. I agree that a full size is the way to go. One can go only so far with circle arcs and computer generated graduated curves. The computer can give me a starting point for a few control sections, but carefully placing the stringers will determine fair lines. The first boat will be an experimental model and improvements will have to be tweaked for others - unless the first one turns out by pure chance to be “perfect”. I know I’m not an experienced professional, but anticipate enjoying the process. I expect the product will float even if it doesn’t have all the anticipated characteristics. It will be interesting to me to see what characteristics the end product ends up with.
" …interesting to me to see what characteristics the end product ends up with…".
That’s type thing is likely where this came from. https://forums.paddling.com/discussion/2936635/decked-oddity#latest
Enjoy the info thanks