hull shapes for low (paddle) powered ocean-going boat

Hello everyone,

After lurking and reading for the past years I finally decided to get a username and ask a few questions.

As to not inhibit you guys' thinking (this project is probably one of THAT kind), I will - for the moment - not reveal the exact project I am working on, but let this be said: It's stand-up paddle powered, it's rather big (currently 6.50m) and it's rather heavy to the tune of 550kg+ splashing around (if it's going to be less in the end, all the better). It will have a small cabin in the front, and a rather large deck surface in the back. Foot steering like on some of the Unlimited paddleboards.

So over the past weeks that I've not gotten around to 3D modelling the whole thing yet I've pushed around ideas on paper, and coming at it from the sailing angle my first impulse was to go wide in the back and give it "planing" shapes. Now, that is probably not the way to go as I recently learnt that power wise I can expect to harvest 40 to 60W from SUP paddler, and that's pull-your-hairs-out little.

Thus I have two different directions to go in here: Either go for a canoe shaped body like any self-respecting surfski (though this is wider) for better release and less drag, or for a SUP race board shape with a wider aft section giving stability?

Bow should have fine entries, and I am debating wave piercer and axe bow-like shapes. Considering to give it a deep forefoot and raise the keel line continuously to the stern. Given that the cabin will be in the front and most of the back is taken up by an open deck area this should give more volume and buoyancy in the bow. How will that influence the behaviour in waves?

Will it need a daggerboard?

There are a ton of other considerations to be taken into account, but for those I will need to take modelling a step further and evaluate certain concepts first.

 

you should not need a dagger board if you will not have sails. you must have one (or a device that functions like one) if you have sails and intend to go anywhere else besides straight down wind with it.

As to the other issues, I could not help you with since I know little about this kind of water craft. But it would seem to me that the bow and above water line forebody should be designed to keep splash away from the occupant.

 

Hello Petros, and thank you for your answer.

My thinking regarding the daggerboard was that, being on the open ocean, it will be subject to windage (mostly on the cabin forward) in either case, and having a rather flat bottom it will not put any much resistance to being pushed off course. The height of the cabin isn't quite clear yet, but looking at Charlie Pitcher's Rannoch boats (designed by 'Mr Ocean Rowing' Phil Morrison) my guess is that it won't be any much lower. Charlie's boat indeed has a daggerboard, very far forward.



Regarding the bow shape, yes, the idea is to minimize spray and splash as I really don't need wave after wave washing over the deck while paddler does his work. Seeing this I'm considering also drawing a sort of wave barrier rather far aft as is the case in the cuddies of last generation IMOCA (Open) 60s.

 

One might assume the daggerboard well forward in the boat in the picture is because of the cabin situated right forward, which would tend to run off downwind, especially if there isn't much depth of hull underwater up there.

 

That's exactly that, plus it's very much in the way anywhere else on that boat, as is the case in my design as well.

 

Between the rudder and the forward dagger board you have a way to neutralize any tendency for the boat to turn up or downwind. Rudder only might not allow that or require a rediculously large amount of draggy "leeway".

 

John,
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This is very insightful! Thank you for this. Will keep it in mind as a guideline.

 

Hi AED, that is an interesting project.

By reading you OP, I ran into this apparent contradiction: stand-up paddling and foot steering. how do you intend to combine the two?

Given the low available propulsive power, your design will be driven by the need to attain a minimum friction drag, with the constraints of transverse and course stability. Wave drag and displacement will be of secondary importance in your case. Hence, you will probably end up with a nearly-circular cross-sectional shape, a canoe-body hull and a certain amount of ballast to enhance the transverse stability. The trick consists in finding the right mix of these ingredients.

Regarding the course stability, especially with a crosswind or in following seas, the right solution will greatly depend on the distribution of mass and on the shape of the hull and superstructure.

It is interesting to learn that some rules (is it a boat for a competition?) prohibit the use of any "mechanical device to provide guidance". Besides the fact that steering control is a basic safety (and sometimes life-saving) provision on any vessel, I think that the term "device" requires a bit of clarification in this context.

Cheers

 

Hi daiquiri,

The steering part is the easiest of 'em all! On the Unlimited class SUP boards (longer than in races) there often are toe/foot operated steerings where the tiller looks something like this:



On prone paddleboards ("hand paddling") they sometimes take the form of a long tiller (with a bungee in the front, snapping it back into the middle position so it doesn't swing out into an unfortunate position) that can be operated between your feet while kneeling.
In the case of this boat, whenever the paddler isn't available to steer a standard marine autopilot will have to take over, keeping it in the wind, or at least on course.

You mentioned an interesting emphasis on factors there, I will have to keep a close eye on that. Canoe shaped bodies seem to be the way to go but would offer less stability at the same length/width. Apparently, looking at the current stage of iterations, I will have to go longer than 6.50m anyway.
Displacement is actually the starting point of everything, using a calculation of the equipment/supplies/water/boat weights as a base.
Ballast (and ways to re-distribute it) will be a must on this boat, I'm sure. Especially in adverse weather a little additional weight in the right spots will be interesting.
The hull's cross section shape currently alters from a very sharp V in the bow to a wide U in the stern, but as I am looking to model two to three different versions of the hull, and have them battle it out between eachother by use of science (science always wins!) before committing to one, I will put a pronounced U shape on the list.

What John mentioned regarding "mechanical steering devices", I believe, is a singular rule for racing canoes. Of course in canoes there are no rudders, so steering is only allowed by the use of your paddle.

Luckily this is not the case here! In fact the only competition for this boat is waves and wind, and the only race is against dwindling resources on board, cabin/boat fever and, at best, a paddler with a very, very different approach than this.

 

At 550kg and power of 40 to 60w what you have is a raft. The primary propulsion in open ocean will be wind and current. 40 to 60w is hardly enough to push toward favorable wind and current. I presume the silly SUP propulsion is for a "first SUP to ..." attempt so I won't waste my breath telling you smarter ways. Pointy bow to break waves, absolutely. Pointy tail to lower drag is good but your speed is so low it hardly matters. Length hardly matters because you won't go fast enough to make waves. As heavy as your craft is you should minimize wetted surface with an elliptic cross-section with weight as low as possible. A 5M hull can have a lot less surface than a 6.5M hull and you will still not be able to paddle it near hull speed. The limiting factor on width will be the ability to paddle efficiently.

I would strongly recommend an excellent sea-anchor -it will contribute more to your progress than your paddle. I also recommend that your 'cabin' be sturdy fabric that can be taken down or shaped aerodynamically. Small daggerboard and rudders are a must. There will be windy days when standing up will cause more drag than paddling can overcome. On these windy days you can make progress with the right shape of cabin.

 

I would personally consider a flat stern. As far as replacing water is concerned, I do not see a big difference if you work the surface area and displacement curve to similar numbers. I think that the flat stern makes for a much stiffer end in inches of displacement terms. So if you have to go aft to clear flotsam on the rudder, you wouldn't affect trim as much. And as you point out you will have less windage, and more stability.

 

I read somewhere that the majority of oceanic adventure rowers have been English. Mad dogs and Englishmen go out in the midday sun, but also for a mid-ocean row as well !

 

First off, thank you so much for your comment.

I'm always open to hear of smarter ways, but I assume you'd point towards rowing and/or pedal powered propulsion. You are right, though, it's indeed a "first to..." programme in the specs.
Ocean rowers report to put out about 100 to 120W over longer ranges of times. That's at least double as much as I can apparently expect from a SUP paddler, but it's still little and yet move boats. Gabór Rakonczay made his Atlantic trip in a "canoe" boat, paddling, in 2011/2012 in 76 days.



Apart from that all of your points are excellent advice. The cabin idea is inspiring, even if it poses a huge set of problematic points, too. A "cabin" of that sort would have to be sturdy enough to withstand higher wind force when shaped aerodynamically - as I think taking it down will hardly be an option. Having to store it, and allowing for re-assembly will somewhat weaken the structure and might make solutions too complicated. But despite that there are some pretty clever ideas lurking there.

Overall your comment might be the most helpful up to now, putting your fingers into wounds already hurting, but driving home some important points. But where there are issues there also are potential solutions awaiting!

One of the ideas in the back of my mind is that at the length it has there's quite a lot of deck space available. On a SUP board a paddler will always trim by moving back- and forward, changing the length of waterline of the board, and the shape of the submerged hull. While in my idea the hull length itself won't change much (and is, as Skyak pointed out, not important as reaching "hull speed" might not be a point worth considering at all), the shape of the hull submerged might still be interesting. An appropriate hull shape might thus for example have a flat aft body merging into a canoe shape mid-hull.

 

There's your answer: http://www.oceanrowing.com/statistics/rowers_by_country.htm

And it would seem it's in the genes. Not saying this is a project for an Englishman, but... it might just be. *cough*

 

That is quite remarkable, isn't it ? 358 out of a total of 564 ! I wonder what the explanation would be for that ?