Design Challenge: Ultimate Downwind Destroyer

Greetings All!,

I will spare you the long winded intro...time is of the essence. I have experience as a composite boat builder but, am a recent convert to surf skiis. Lots of time in and on the water, mostly racing ultralight sailboats. Love to surf off shore.

I have just acquired use of a shop for two weeks, possibly longer. I am going to shape a foam plug. I am looking for the measurements of a good intermediate boat to use as a baseline to work from or, better yet, an all up original design. Have not figured out how to open downloaded yak. files yet from kfoundry. The program is sure fun to play with though. But, as I said, time is limited. Need to get to work asap.

So, here is my concept. Design a boat that has marginal upwind ability, just enough to keep your six off the rocks in a pinch but, will beat the living bajeezus out of anything downwind in a blow. I am talking fast, accurate and, stable. Drop it in..surf yourself silly...hit the beach 20+ miles down the road.

As I am a man of means by no means, there is no cash prize. Just your name in lights and the honorable glory of a job well done. However, if you live in the PacificNW, I will buy you all the pizza and beer you can handle. LOL

This sport seems to attract a lot of tall gentlemen. However, I am 5'8", and a mere 145lbs.[173cm/66k]. Needless to say, it would be handy if the design worked for someone my size!

Looking forward to your input. And, thank you in advance for any and all help with this project.

Almost forgot, would like suggestions for a lay up schedule when its time to glass.

So, the gauntlet is down. Who will design the Ultimate Downwind Destroyer?

Good Luck!! "Because Gentlemen do not paddle to weather"

Cheers,
Brandon

Downwind yacht are flat bottommed and broad. I wonder if a ski with the usual front end and then broad and flat, about surfboard width at the back. Two smaller rudders on the side and forward and a smaller, central steering rudder. Would get on the plane brilliantly and the 3 rudders would give stability and control

I've been toying with the idea of experimenting with a design to do the downwind stuff as well...I thought of taking the taking the downwind SUP shape...more length...narrower for the catch area....very flat bottom. A 6m longboard in essence...and of course very stable

Some considerations are:

Long boats like the V10/Mako Elite not only catch the runs easily but also pop over the next wave easily. Many boats, like some of the shorter intermediate boats seem to wallow up the next wave so you can't easily put sequences together.

The Mako6 was not as good as the Elite in popping over the next wave.

So it's not just a matter of a flat planing surface, performance also depends on rocker (the more the rocker, the easier to catch the run) or not (the flatter the rocker the easier it is to ramp the next wave); the balance of the boat (ie position of the cockpit); the amount of volume in the nose (small volume means potential nose-diving); the tippiness of the boat (too tippy means you lose critical strokes at the top of the wave)... You also have to avoid making a boat too prone to broaching, while enabling it to turn (again rocker v not rocker plus the position of the rudder).

The key is combining ease of catching a run with the ease of manoeuvrability to position the boat to catch the next run - winners put sequence after sequence of waves together without slowing down much.

There's a whole lot of magic along with the science of designing a downwind (or any other) boat!

Thank you Dicko, Mario and Robin. Great input.

Atta boy Dicko. That is very close to what I have been envisioning. Getting the size of the fins/rudder combo correct might take some fiddling around after the fact. Be good to have them set up for quick change.

As Robin clearly points out, everything is a trade off. That's the magic...where to strike the balance. Every boat/board is a compromise.

I have been wrestling with all these points. Another consideration that has me concerned is the increased drag/wetted surface area of a broader flatter tail section. Will the extra drag, though not so prismatic, cause the boat to be hard to power up/accelerate for the paddler? Particularly on the backside off a wave before the next picks you up and causes a broach because you were to slow.

From what I can tell from the photos, the Vadja Orca seems to have relatively shallow tail sections with a rounded pintail. Looks cool anyway.

One idea I had is to shorten a relatively standard design by 10-15% and add the equivalent volume to a broader flatter tail. Quite possibly with a transom for clean exit.

I finally figured how to open yak. files. Looked at Mojo 24. What a beast! Big boat for big waves? Anyone ever build it?

We could speculate indefinitely and, probably will. But, time is nigh. Who has the cahones to put the numbers where their heart is? I will scarf up the longitudinal tonight. If no one cares to hang it out there, I will shape the whole thing freehand and pray I can figure the cockpit placement after the fact. Put the finished #'s into Kfoundry and let it decide. I am far better with hand and eye than I am with computers.

So, come on men...no guts, no glory!! This is your chance to stand immortal or, at least die an honorable death for trying.

And, thank you all very much. I really appreciate your replies.

Bx

Having spent many years designing and building SOTA windsurfing boards, I should say that planing hulls are not an easy answer either. There is a huge "hump" of drag to get past before you reach the higher efficiency of a planing hull, and even then, if the speed isn't held, you drop right off and back into high-drag displacement mode. And anywhere below planing speed, your hull drag is many times greater than a long skinny displacement hull, which means that if you aren't planing, you're dog slow. Windsurfers and planing sailboats (A18's, 49ers, etc.) have gobs of leverage and can generate hundreds of pounds of thrust to get past this drag hump and stay there (plus they have the added advantage of generating apparent wind, which is huge), but surfskis are not the same at all.

My guess is that given the relatively limited power of a paddle-based craft, the lack of apparent wind, and the typical downwind speeds of a surfski, you're going to need a planing hull to be VERY wide before it generates enough lift to plane efficiently at these speeds, and even more importantly, to stay on plane between swells. For "typical" swell slopes, think 1m (40") or more. And this width has to be UNDER the CG of the craft to be of any use, in other words, underneath your butt. Such a craft would be so wide as to be almost impossible to paddle efficiently at all.

In the end, I have a feeling you're going to be just wasting a lot of time and effort if you head this way. Better to stick to low-drag skinny displacement hulls, and truth be told, most likely you'll end up very near to what's already being made.

Bugger. Can it at least have 3 fins!

Kestrel wrote: My guess is that given the relatively limited power of a paddle-based craft,

What are you getting at?
Speak for yourself!
wind shmind!

Well, nuts! does this mean the parties over? Just pick up the empties and go home? I think not. By God Dicko, I'll put those fins on the bow for a hood ornament if I have to. Might get a little lift if I angle them right.

Well, the answer was too easy. If it sounds too good to be true then, it probably is.

SO...there will never be a faster ski than what already exists. Is that right? We have reached the limit of technology and creativity? No one has a clue of how to build a better mousetrap? Pity.

I am going to try your patience and tell you a short story.

One night after sailing, several of us were sitting on a boat chatting. For some reason, someone brought up the fact that a Avon Redcrest inflatable could not plane. That, by design, it was virtually impossible. I bet him $100. that I could...with a 4hp motor. He took the bet. I went up to my truck, cut 2 corners off a 4'+8' sheet of plywood and laced it under the boat. Planed all over the harbor without even leaving a wake. Collected $100. Some say I cheated. Matter of opinion. I did it.

There is always a way.

Peace,
Brandon

Sorry, forgot to thank you Kestrel. I hear you loud and clear. That was exactly what was bothering me. I know your right but, if we all gave up so easy, there wouldn't be any surf skiis. We would all be paddling coracles round and round.

Bx

Those of us who used to surf will remember the way that Simon Andersons thrusters (3 fins instead of one) changed surfboard design forever. The boards were more stable, faster, could ride higher on a wave and turned with more power.

If you used a smaller rudder for steering in the usual position and 2 smaller fins forward, there would be no change in drag, but surely the ski would be less likely to broach. If you could improve the angle across the wave this would increase the speed.

The changes that the thrusters effected on surf design are all things that are desirable on a ski. Just a case of getting the placement and size right.

If I had a second ski and the necessary skill I would give it a try......wonder if the wife would mind if I played with her ski.

Like the points you make Dicko.

So the questions on my mind are:
A. Will the smaller rudder have enough authority? Or,
b. Would it be better to make it a twin rudder?
c. Does the Thruster concept work in a boat with the wieght centered as opposed to a big foot mashing down on those fins as a pivot point as another torques?

Physio: LOL, I like your style. But if you've ever run a HP windsurfer without a harness, just hanging on with your arms, you know very quickly how much force is involved. Your arms are toast in under 5 minutes. Most surfski paddles don't even come close. I suppose the big sprint wings probably do, but speaking for myself, I'm not into that much punishment.

Dicko/Redfrog: Everything that you drag through the water makes... drag. Now if you were losing control often with a single rudder, either from broaching or from partial cavitation, thrusters may be useful. But in terms of keeping the drag down, it's likely better to just use a slightly deeper (single) rudder. Which again is what is already being done....

Also, 99% of rudder effectiveness in craft like these is due to the presence (or lack) of air in the water around the rudder. If the base of the rudder comes clean of the surface, even momentarily, it will cavitate. Again, it's more drag, but one of the more effective solutions to this is to put a small horizontal "plate" or "wing" across the rudder a few inches below the surface, which provides a physical barrier to the cavitation spreading all the way down to the tip of the fin.

Just on the topic of multiple fins, we've experimented with additional fins to try a) to improve stability and b) to reduce the tendency to broach.

Examples:

• The original Fenn XT was VERY prone to broaching and a skeg mounted in line with and about a metre in front of the rudder was a huge help.
• My training partner Dale Lippstreu fitted a skeg to his V10L that, he said, improved stability. As a by-product it apparently tightened the turning circle too.
• I've seen a Fenn Millenium too with long narrow skegs mounted under the cockpit - clearly in an effort to improve stability.

So skegs can have multiple uses on surfskis...

As to planing surfaces , take a look at the Think Legend. The hard chine created by this surface also plays a role in yaw stability (tracking).

Robin: I wonder if anyone has done any time trialing with/without fin(s) underneath the cockpit area. Given that wing paddles do generate some sideways component of force, and given that round-bottom boats don't have much sideways resistance, there would be a (small) tendency for the boat to slip side-to-side under power, which obviously is going to make more drag. I wonder if a small "centerboard" might actually provide more net lift than drag under these circumstances. It certainly would make the rudder more effective and make the boat turn more efficiently, and would reduce any quick-rolling tendencies as well, adding in a way to secondary stability.... The nice thing about a surfski is that you could actually make a retractable daggerboard that came up into the hump between the legs-- it's easy to reach and out of the way at the same time. (I want royalties on the patent....

Alright...now we are having fun!

Robin: That's good info. thx. I knew a forward skeg would likely improve turning radius but, was afraid that it might trip the boat into a broach running diagonal on a wave.

Kestrel:Drag is such a drag! Cavitation plate might be helpful.

Sandy:To chine or, not to chine ...that is the question. Edge control vs drag?

I am grateful to be able to have this conversation with such knowledgeable and experienced folk.

Seems we have pretty well popped all the bright shiny bubbles so, I would like to pose the question to you Robin, as the supremely experienced and intrepid paddler that you are and, you Kestrel, as the resident rocket scientist on this thread: is there a more subtle design concept that you would like to see tried? Or, since the boat will be paddled primarily by a far less than elite paddler, and I would like it to be faster than the average potato, could you please suggest some design parameters. I am trying to present this as an opportunity. What would you like to see?

I should mention that the boat will be made available to the local community for evaluation by far more experienced paddlers than myself, should you desire valid feedback on the outcome of your ideas.

Hard chines have two main effects: (1) higher secondary stability, and (2) much the same effect as centerboards-- they reduce side slip. But they are woefully draggy in accomplishing this, compared to a real foil. That's why I am wondering if a real centerboard/daggerboard might be worth trying.

Re your original question, much will depend on the shape of the swell you plan on riding. Boats with more rocker work better and fit better on short steep swell, whereas long-period swell will be friendlier to a (faster) flatter rocker. Somewhere in there will be your optimum speed.

A fairly thin chord, high aspect elipse? How long would you guess?

As for wave size/shape, I would want the boat to perform on large, long interval swell. But, other than prime days, there is the inevitable wind chop. Worse yet, the intended battlefield would be the Straights of Juan de Fuca. Major wind against tide potential.
Given that a longer hull may be better, do you think a smaller paddler can power it up sufficiently? Or, would you recomend less volume overall (v10l)?