stepped hulls for trimarans

A Farrier tri does plane, but it's the main hull that is planing. They have flat sections with rounded chines. At speed, the main hull is being unloaded and the difference in speed is significant when it starts planing.

The drag of a narrow displacement hull like an ama is pretty low, even at speeds normally associated with planing. A stepped ama would have much more wetted area, not to mention the drag from separation at the step, at lower speeds. When going fast, the ama is getting loaded up, which makes it harder to plane.

Also, the drag due to dynamic lift when planing is inversely proportional to the square of the width of the planing surface, so if you try to plane on a narrow hull form, the drag from the lift is greater than the reduction in drag from reduced wetted area, and the performance can actually be worse.

 

Stepped

Do you mean this ?

 

Aaaack, Tom! You didn't really open that can of worms, did you?

 

Chris- i enjoy your humour!
Manfred.pech yep- sorta..think slighty rounded bottom or flat for planing - not sure but sint there some rule about the larger the surface area the easier it is to get up on plane? anyway designed just as in the transat class but with a step...about 1/3 way from stern not v'd ...eliminate the centerboard..use beamy hulls but with double the sail since the wide beam (12 ft bea, 32 ft loa) could theoretically manage a parallel rig ala- bernd Kohlers designs... the vakas would be small only big enough to stop the heel...its all just guess work but seems to me a stepped hull is the next logical progression for fast monohull/tri(for lack of a better word) even if my design is sub par. one of these days ill scan in my rough sketches...

 

Thanks, Tug... I'll be appearing nightly all this week at your local comedy store.

Truth is, if we can't laugh at this stuff sometimes, then we've very clearly gone down the wrong road.

 

Hey Chris i was going over some of my sketches- i have one that i mentioned that is like a mini transat. at 32 ft. and 12 ft beam....what i did was add two small vakas. if the hull was a planing style -and i used lots of sail area...maybe a parallel rig for example...would it be possible the vessel wouldnt need a centerboard? it would start to heel then the moment would be countered with the small vaka...?? the boat would be very fast since the drag would be less due to not having a centerboard and draught lessened. making it beachable. i have it stepped in the plans as per mentioned earlier...
about 7 ft(give or take) back of the center of effort...and the parallel rigs about 2 ft forward of that.?? i dont plan on building it(right now) but i thought it might be an interesting concept...

 

Step by step

Hi Tug, I`m curious about your plans. The drawings from the multi stepped trimaran are from Michael Spiegel (PhD). A prototype was built and plans of bigger boats were drawn -- but with no significant success. As far as I know the stepped Trimaran was not able to plane. May be too much drag. There were an article and advertisements in "MULTIHULLS" and great hopes (Seperate Reality!). After some time the advertisements were smaller and smaller and stopped. Never again I`ve heard anything about the multistepped Trimaran.

 

HI Manfred, been awhile- but good to be back dreaming of the perfect boat again. ok so to pick up where i left off- couldnt a mini transat type monohull using short amas, work to keep the boat level and planed? I promise ill draw up a couple (very amateurish )sketches for you. kind of a scow /flattie hull but 12 ft wide and 32 ft long, with a stand up cabin, parallel rigs, and using possible anti-vortex panels as well... the amas would be small (short) at about 1/3 rd to 1/4 the length of the hull-just large enough to prevent the boat from heeling. I know this will induce drag(more wetted surface) on contact with the water but wondered if it would be enough to slow the boat down and cause it to pitch outboard slightly? if not then it would do exaclty what id hoped. the idea is the vaka always stays in contact with the water on an even keel level track. it has a step which reduces wetted area when planed off?? this could be quite fast...
is there a way -other than tank tests- to see if this would be feasible to build??.id hate to build it only to find out it was a like sailing a grand banks 40 astern?

 

Stepped planig hull

Hi Tug, this forum has a great tradition with discussions about planing hulls for Trimarans and Catamarans. May be you are interested to have a look at them, for instance here: http://www.boatdesign.net/forums/multihulls/planing-trimaran-bethwaite-hsp-5524.html

 

Tugboat, what you're talking about, with longer amas, is a Farrier/Weta type boat - except the Farriers/Wetas have less main hull beam. I wonder why you're even thinking of putting anti-heeling floats on a wide, planing, main hull monohull anyway .... because you're going to end up with neither one thing, nor the other, both failures. Decide what type of boat you prefer and go along that singular course.
A simple question: remove the floats from a Farrier, will it plane?
No?
If it did, I'd call it a true planing boat .... but it isn't.
No disrespect to the brilliant Tom Speer... but he's all wet on this.

 

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I can't believe that you would say that ,Gary!? Mr. Speer is right as is Ian Farrier, Antrim and others. Heres the whole thread-the one, the only: http://www.boatdesign.net/forums/multihulls/planing-trimarans-14047.html
Gary, can I get you a towel??!

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Pictures, left to right(click on image): 1) two Weta's with main hull PLANING, 2) Parliers proto for a 60' PLANING cat, 3) Bethwaites PLANING trimaran the HSP, 4) Parliers 60' PLANING Cat, 5) Rotatable ama concept model-used in this configuration in light air, 6) Rotatable ama-stepped planing hull configuration-heavy air. Note #1: pictures 2, 3 and 6 show stepped hulls as well.

 

Douglas, I've posted before that a Weta does plane; because it is a small monohull with balancing floats - and it truly planes because it is small and the crew weight is very effective and important. Have you ever seen a Farrier looking like it is planing with no floats in the water like a Weta?
No.
The other three designs you've scraped up - are all failures because of too much drag, can't beat conventional multihulls and provide a horrible ride beating into larger seas.
End of story.

 

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You're dead wrong ,Gary . Stepped hulls w/o some form of variable geometry are a drag in light air. But in heavy air their drag is a fraction of that of a high L/B displacement hull. And as an example of variable geometry, my concept model, above, shows one way to overcome the light air drag of a stepped hull-there are others.

 

We may have the old story of the three blind men looking at an elephant here. Chris is the multihull expert here but I suspect the step in the amas is to help break away from the surface and get up on the foils. That is just what the steps on aircraft floats are for. Not necessarily to go faster but to break surface tension.

 

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On Parliers 60' Hydraplaneur,(and the smaller proto) the steps were to increase planing speed and reduce drag-the boat didn't use foils(except for rudder t-foils). The steps were extremely effective above 20 knots. On Hydroptere the step in the ama is to allow the ama to easily break free in a high speed impact and to help with takeoff-sort of like you suggest.