Speed versus Length

I think you are being a bit unfair here. Yes, Ralph explores only a fairly small part of the full parameter space of hull design, to study how one factor (displacement) affects the optimal length. While this alone is of course not enough to find the best possible hull, it doesn't mean it isn't interesting.

Of course, it would be interesting to know also the optimal beam, prismatic coefficient, difference between lengthwise symmetric hulls and swede form hulls, different section shapes, amount of rocker, etc. etc - and how all these things interact. And of course, since we are talking about surf skis rather than flat water kayaks, it would be nice to know also how the characteristics of the hull affects behavior in waves. But then it wouldn't be an article anymore, it would be a (rather thick) book. While waiting for somebody to write that book, I'm happy to learn through articles concentrating on more specific topics, like Ralph's article do. (Maybe it only shows how little I know about kayak design, but I feel I did get some new insights from Ralph's work. It didn't teach me everything there is to know about kayak design, granted. But a little bit is still nice.)

Obviously a lot of interest in the science behind the shapes. It would be great to get some of the designers of the current crop of surf skis to come on here and discuss the processes they go through to design the latest and greatest.
As Fredric suggested I'd say it's a blend of experience and art and science.
Obviously wouldn't want it to become a marketing opportunity but having an understanding of the processes involved would be fascinating.
Anyone know any of the designers involved, why don't you encourage some interaction - site admin approved of course.

In simple terms the paper says the following:
- if you are 100-110kg like Oscar then a 6.4m ski (V12 or V10) would be about optimal
- if you are around 80kg then something around 6m (Atom) would be optimal
- if you are around 70kg them something around 5.5m would be optimal

Of course this does not take into account the stability issues so whatever length boat you would need to find a suitably stable hull shape. You will go faster in this than in a non-optimal length boat of the same stability.
However you will almost always go faster in a stable non-optimal length boat than in an unstable optimal length boat.

The paper also says that picking a longer boat than optimal has a very small penalty (<1%) so there is little point in going short. I would guess that a longer narrow boat with more rocker (more stable) would give the best performance in open ocean conditions (Andrew Martin Sharpski or Fenn Elite).

Regardless these speed increases are likely to be small (<5%) compared to the speed increase obtained from a couple more hours training.

Some other interesting boat designs to look at are:
Van Dusen Mohican
Vaaka SK1
Sission Evolution Omega

AR_convert wrote: I am about to jump into an elite ski (yet to choose one) after 16 months in the Vault, it does take time and practise, enjoy the journey.

Hey AR_c - off topic here, but recently purchased SES Excel @ 10.95kg (!!) and loving it in the flat and small bump so far. Next time up in big smoke give me a call if you want to take it out for a thrill. Will be demo-ing a Vault soon (second boat to paddle with mates etc and maybe me for Doctor) so will post feedback. Bum in the seat with gps the way to go, that or out with someone you normally paddle with as a benchmark. Anyway offer there you're welcome to have a blast. Very fast, very light, very stable, very comfortable (but who's to know what the rough will bring?). Text me 0420 998 166 anytime.

Duncan, what's your rational for believing that a boat with more rocker will be inherently more stable?

I would argue the reverse.

A boat with a lot or rocker relies more on the unstable rounded part under your bum than the highly stable deep V section of the front of the boat for it's displacement.

The paper also suggests that V10 sport is just as fast as a V10/12.

｀The paper also suggests that V10 sport is just as fast as a V10/12.｀

I didn`t notice that bit in the study - any chance you could let me know what page you spotted it on RAB (to save me reading through it for a third time!)
Cheers matey,
Owen

Rightarmbad wrote: Duncan, what's your rational for believing that a boat with more rocker will be inherently more stable?

I would argue the reverse.

A boat with a lot or rocker relies more on the unstable rounded part under your bum than the highly stable deep V section of the front of the boat for it's displacement.

The paper also suggests that V10 sport is just as fast as a V10/12.

Not sure why you think the deep v in front is highly, it's highly unstable! Like trying to balance on a pin point rather than abroad flat area.

Ignoring rocker the stability comes from the usually flattish hull under or just behind the seat area. Note that it's easy to prove this using basic centre of gravity / engineering maths (yes I am an engineer) - anyway there are books that can show how to do this. Using your logic one might think a Fenn Elite SL would be more stable because of its super deep V, compared to say an Epic V8. However the Epic 8 is very wide around or behind the seat area - which is where the stabililty comes from.

So for very very skinny boats with little or no flat area under the hull, see any top racing K1 hull, the other way of getting additional stability is from the rocker. However this does depend on the paddlers weight sinking the hull sufficiently such that the upward forces on the ends of the hull provide this stabilizing force. A bit like sitting on a pool noodle - extreme rocker in the U shape, compared to trying to balance on a floating barrel or log.

I used to have a mirage 22S which is a long thin sea kayak - unloaded it was pretty tippy because it was floating too high and relying on the rounded hull below the seat area, fully loaded it was relying on the front and rear ends for stability and was very stable.

Downside of the rocker is increased cross sectional area and resulting increased drag. However a hobbies cat hull is a good example of an extremely narrow hull with a lot of rocker and very low drag....

Not sure if this makes sense - otherwise I suggest digging up some marine engineering or boat design books....

Rightarmbad wrote: Duncan, what's your rational for believing that a boat with more rocker will be inherently more stable?

I would argue the reverse.

A boat with a lot or rocker relies more on the unstable rounded part under your bum than the highly stable deep V section of the front of the boat for it's displacement.

The paper also suggests that V10 sport is just as fast as a V10/12.

As for the V10 sport being the same speed as the V10/12 - I don't recall that being stated or implied in the paper. Bear in mind the paper was focused on optimum length and largely ignoring the effects of stability etc.

I used to have a mirage 22S which is a long thin sea kayak - unloaded it was pretty tippy because it was floating too high and relying on the rounded hull below the seat area, fully loaded it was relying on the front and rear ends for stability and was very stable.

You just proved my point.

A V shaped hull is more stable than a flattened round hull.
It introduces more hull underneath the paddler on the side it leans to than a rounded hull, even one with a slightly flattened bottom.
It loses out however by having more wetted area per volume than the rounder profile.

A purely round hull has no stability, a slightly flattened hull introduces some by virtue of the extra volume on either side of the hull that tries to get submerged when you tip it.
A V shape is very stable as it introduces a lot more hull under the paddler as it tips as well as taking away a lot from the other side.

It is nothing like standing or sitting on land, it's all about volume distribution, and the way that changes as the boat rolls.

An SL is tippy by virtue of the very rounded profile in the rear, not the V front.

A V10 sport is almost the same length as the other two boats and shows that the length can be a small portion of the hull speed equation.

A pool noodle is stable because your body is already in a stable position, the noodle contributes nothing but a small lifting force.

A log is a bad analogy as it's stability if it is bent, occurs because of the large amount of displacement at the ends, our pointy at the ends boats are far removed from this.

Although it is applicable at the front if the boat is very narrow, high and contributes significant volume and the rocker moves the volume sideways a lot as it rolls.
But that can really be considered an extension or small supplement to the stability of the V section itself.

RAB wrote: `A V10 sport is almost the same length as the other two boats and shows that the length can be a small portion of the hull speed equation.`

Sorry RAB, still not following you on this bit. As I understood it, the author of the paper was comparing different lengths of boats with different weights of paddlers, all with a 45cm width. I`m just having trouble working out where he was comparing the wider V10S with the other two skinnier boats and saying that they were basically as fast as each other, - which is what you seem to be implying. Let me know if you think I`ve gone troppo, and if you do find that ref., please send it along too.

Happy Paddling,

Owen

V-section more stable? Absurd! I vote for Duncan. Flat (or "flattened round") is more stable for a given volume. Assume a perfect V section hull, the cross section of which is a perfect triangle. Sitting on this hull, we find that there is a lot of buoyancy (displacement) lurking below our CG. If we tip the V, this displacement is now off centerline, on the wrong side. It will continue to push up, further upsetting the hull until your perfectly triangular cross section is now sitting sideways, flat on the surface of the water.

Another thought exercise. Cut a board or log so that it has a perfect triangular cross section. Toss it in the water. Does it float with a flat surface flush to the water surface? Yes. Is the V-section pointing down or up? Up.

RAB's V-section stabiliity argument seems to be describing secondary stability, which only applies to the hull section above waterline. However he is using this argument to refute Duncan, who rightly points out the benefits of added (primary) stability from a flattened section under the seat.

Rightarmbad wrote: You just proved my point.

A V shaped hull is more stable than a flattened round hull.

Try sitting in a half-barrel - perfectly round and perfectly stable... Now put a seat in and raise your height and things become increasingly unstable until some point where you tip over.

stability depends entirely on the position of the Centre of Gravity relative to the position of the Centre of Buoyancy. Check out the following link.

books-for-sail.com/boat-design-and-theor...lity-the-basics.html

If you sit in a half barrel, you will have very little stability at all.
Irregardless of seat height.

Your link only enforces what I have said, the diagram clearly shows the displacement volume shifting to a position that supports the centre of gravity as it moves sideways.

I'll pay you to video yourself sitting in half barrel.

The laughs would truely be worth it.

Good article directly related to our boats

Watto wrote:

AR_convert wrote: I am about to jump into an elite ski (yet to choose one) after 16 months in the Vault, it does take time and practise, enjoy the journey.

Hey AR_c - off topic here, but recently purchased SES Excel @ 10.95kg (!!) and loving it in the flat and small bump so far. Next time up in big smoke give me a call if you want to take it out for a thrill. Will be demo-ing a Vault soon (second boat to paddle with mates etc and maybe me for Doctor) so will post feedback. Bum in the seat with gps the way to go, that or out with someone you normally paddle with as a benchmark. Anyway offer there you're welcome to have a blast. Very fast, very light, very stable, very comfortable (but who's to know what the rough will bring?). Text me 0420 998 166 anytime.

Cheers, will do, was torn between the SES and Vault when shopping last time, from memory the SES was about $500 more and I was still nervous about the Stellars longevity. None of the guys I paddle with have had issues with their stellars so that question has been answered.

The real reason for wanting to upgrade is for length as well, I may be well off the mark but I just think the shorter length needs more inputs to keep it straight, especially when washriding. But it could be my set-up around the rudder peddles too (getting touched as I leg drive, inducing unwanted steering inputs). Anyway I still want to give the SES another try so we should catch up and swap boats for a paddle.