Hey guys,

over in the topic on flying and foiling amas there’s been a fair bit of talk on what it is and what it takes to actually make a proa go fast anyway. During the day today this rather crazy idea went through my head, and I’ve just spent the last 3 hours or so throwing this thing together.

Essentially this design attempts to achieve a very high DLR by going for really long slender hulls (I decided to just go way over the top and went for an L/B of 35:1, which is probably way too much), with low freeboard to save weight. It has a vertical sides and a semicircular midsection which is carried through most of the boat unchanged, which should make construction dead simple. That will lead to some losses at the transistion from the bow to the straight sections of the hull, but whatever.

Dave also pointed out that in general proas can’t produce anywhere near the amount of righting moment that a catamaran or trimaran can (for a given boat displacement). Maybe he’s right; my idea to fix that was to put all the (very spartan) accommodations into a pod-car, which can travel along the iakos, thus allowing you to adjust RM so that the ama is (hopefully) always skimming, with the vaka carrying the load. With this ultralight boat the crew and accomodations would probably make up half the displacement or more anyway.

The big red thing floating around in the air to leeward, without a connecting structure (like I said, thrown together) is an inflatable hull from a catamaran. You can get it off the shelf, it doesn’t cost much, and it weighs almost nothing—ideal for a safety ama.

In terms of boards I was thinking of opting for performance and simplicity too. I’m thinking side hung daggerboards and rudders for each direction fo travel. You get to use good asymmetric unidirectional foil sections, you can get the daggerboard forward of the centerline slightly, and that at the time of construction you wouldn’t need to know where the boards belong, if they are side-hung you can try them out all over the place until you find the right spot.

LOA is 12m
B_vaka = 0.34m
B_total = 6.4m
displacement hopefully around the 500kg mark
DLR = ~8
Sail area = ~28m^2. Maybe that’s too much, I have no idea—I haven’t run any numbers…
Draft (boards up) <0.2m

Rather than something I’d actually build, I see this thing as a concept to play around with a bit and to spark what will hopefully be a very interesting discussion. The simplicity of it (pod-car problems aside) and the potential for performance is definitely appealing though… 😉 What do you guys think? Totally crazy? Absolute nonsense? Will the performance be utterly miserable—if so why, and what can you do to make this boat give everything else a run for its money?—What would you do differently?

Cheers,
Marco

Why does the ama have to be as long as the vaka?
You can make it shorter to save weight and to take advantage of what has been recognized as an advantage of the proa over the catamaran which is the timing of both hulls hitting waves at the same time (assuming that waves are coming from windward, at an agle, which is true most of the time) rather than the windward hull first and then the leeward hull, generating twist in the structure and “uneasy” motion for the crew.

Cheers,

Laurent

Agreed!

Maybe the boat could also use a bit more freeboard in the immediate vicinity of the bows, to help protect from nosediving…

Marco, I like it a lot!
I should have said that first.

Keep in mind thought that this shape of hull is VERY sensitive to actual weight vs. design weight. If you are the slightest overweight, the hulls will sink significantly deeper than expected and perform not as well as hoped. This is true for all hulls with vertical sides (including my boat…); flared hull sides will be less sensitive to weight increase as the hull will not sink as much for the same amount of overweight.
This is even more so true when there is virtually no extra freeboard like in your design. I do not consider it a flaw, but if you build it, you will have to be absolutely maniacal about your weight estimate and your actual weight during construction.

Weight will be the critical parameter for success, like any proa one may say, but even more so in your case.

Regards,

Laurent

PS: did I say that weight will be critical?....

I like the sliding pod idea.

Marco,

A lot to like in this approach and much preferred to your previous design. I have lots of thoughts and comments. Laurent has already made one in his first post..,.....No time now, but I will come back .

Rob

Having slept over the whole thing, there’s a few things going through my head. First and foremost the appeal of the ease of construction of this design is just enormous. The modularity of it all is great too, you can just try things out, and if you don’t like it, then just build another cabin to put on the iako-car for instance. I also have little doubt that a boat like this would be significantly faster than Firstborne.

What really concerns me though, is the question of seaworthiness; a fast boat that you can’t take out to sea to play, is not worth much to me. Is a half-submerged 12m long torpedo more seaworthy than the 7.5m ‘Firstborne’ for instance? In particular I’m a bit worried about potential nosediving and/or broaching, since it has little freeboard, and no rocker. For a boat this long and light, excessive pitching in waves could be a problem as well.

I think internal ballast tanks in the vaka could partially alleviate these problems. You could have large internal ballast tanks at midships or in the pod, with the purpose of increasing displacement significantly and thus dampening accelerations in the boat. This would make the boat significantly slower, decrease freeboard noticeably, and as such would be intended for use in really rough weather only, but could be of great benefit then. Additional small ballast tanks could be placed near the bows, which may find use even when sailing at top speed in good conditions, just to allow you to tune the moment of inertia about the pitch axis, and thereby give a bit of control over how much you want the boat to have a propensity to go through waves, rather than over them.

Neither of these measures helps with nosediving though; increasing the moment of inertia would make it worse… I’ve been thinking along the lines of anti-dive plates (which Cheers had), though they are counterproductive as soon as the bow does decide to go into submarine mode. An alternative would be to add something like small delta wings, set at a large angle of attack, placed above the normal water line and set back from the bows a bit. If the bow really buries itself, the delta foils are submerged and would produce a powerful bow up moment. Delta foils have relatively low L/D ratios, but can take really high angles of attack before stalling (safety!), and can produce really high lift coefficients. They would produce a fair bit of extra drag in the process, slowing the boat down but, I’d say getting it out of the back of a wave has a higher priority at that point.

What’s your take on this? Anyone have any other idea ideas? Do you guys think that overall the 12m Derlirium concept would be more, or less seaworthy than the 7.5m Firstborne, at the same displacement?

Cheers,
Marco

Many pacific proas had an vertical or near vertical “rod” protuding up from each stem. They will create a very turbulent flow of water mixed with air when punching through a wave. This turbulent water/air mixture is less dense and easier for the stem to rise up through.
James Wharram uses an axe-shaped vedge on his Child of the sea design, for this effect, and reports that it works really well.

...And I in my ignorance always thought that was just decoration... Absolutely mind-blowing!

Manik - 28 May 2014 06:10 AM

...And I in my ignorance always thought that was just decoration... Absolutely mind-blowing!

Likewise, never knew there was another rationale for what I was doing 😉

Skip

By the way, the wetted surface area of this hull at 500kg displacement is ~6.2m^2, compared to Firstborne’s wetted surface area of 4.8m^2 at the same displacement. I’d have thought the increase would be much more pronounced than that.

I just remembered seeing this sometime last year; I think in terms of the cabin / accomodations I could take a lot of inspiration from Jan Gougeon’s catamaran Strings. Overall the ‘Delirium’ concept is actually quite similar to Strings anyway…

Nice design Marco. Are you familiar with Othmar’s ‘Pipemania’?
http://www.multihull.de/down/pipe_mag.pdf

Likewise, never knew there was another rationale for what I was doing

Skip

I take this as positive evidence of a Polynesian muse at work here, Skip! 😊

I wasn’t familiar with ‘Pipemania’, but I’ve had thoughts along similar lines. The problem I have with the concept however is that if you want the hull immersed in such a way as to get a semicircular underwater section, then you’ve only got 50% reserve buoyancy for the hull, which in my opinion is not enough. Also, if you assume you need the full displacement of the hull, so it’s submerged completely, then for reasonable proportions, the round hull will have more wetted surface than a semicircular section with vertical sides, immersed to the same displacement (provided the deck of the latter doesn’t go under).

A circular hull shape which is not half immersed but floats much higher, would be quite a bit wider at the widest point, and may be subject to unpleasant slamming. Having veritcal or slightly flared sides is thus quite advantageous in my opinion, especially for the increased reserve buoyancy. The pipe setup would undoubtedly be faster to build though.

Marco

Those are very valid observations on the usefulness of circular pipes, Marco. I’m not so sure it would be quicker in the end to build with commercially available pipes over a simple ply hull. Though I could easily be wrong there.

I have found it a clever and fascinating concept over the years, never-the-less. Adding the safety ama as you have done is an improvement, to my mind. If Othmar’s cabin was turned 180* so the cockpit was over the lee hull and sheltered by the cabin together with a deck extending to the safety ama (which you may have planned for), it would be a most habitable boat, I think. It would closer resemble a Tepukei proa then, too.

How much reserve bouyancy in the hull(s) would you aim for?

James