Wide-hulled trimaran?

In an attempt to better understand this I came up with the following calculations. Are they correct?

RightingMoment = rm = force * distance

For a sailboat the overturning moment = om, where force is proportional to sail area, and distance = water level to center of sail area
For a trimaran the righting moment = rm, where force = weight of sailboat, and distance = distance between ama and vaka

So for a smaller trimaran, of length=l,

sail height=h
sail area=sa
ama/vaka distance=d
weight=w

So:
rm = w * d
om = sa * h
and we hope rm >= om

Now for a larger trimaran of length=2*l, then:
SA= 4*sa
SH = 2*h
D = 2*d
W = 8*w

So:
RM = 8w * 2d = 16w*d
OM = 4sa * 2h = 8sa*h

And RM is now >= 2*OM
So maybe it’s not necessary to increase the ama/vaka distance at all?

 

Would you mind commenting on the relative advantages/disadvantages for a cruising trimaran to have immersed amas. I assume they sail flatter, but there is more drag when sailing. Apparently higher load capacity? Is the ride smoother when sailing? Is the ride wetter or dryer? Do immersed amas typically have a different shape or volume than non-immersed amas?

 

Pivers had them too....I can't think of too many disadvantages. If not too full on the immersed sections or too deep there really isn't a lot of added drag, certainly less than a cat. On some boats tacking can be slower but a fast spin knocks speed off everything. When adding load there is a better pounds per inch immerse rate. A load that drops me 2 inches would send a similar non immersed ama boat down 3 or so. To carry that load you then need more main hull rocker which doesn't help top speed. The CC35 is built this way, they are a nice boat but out cruising or sailing we have been faster especially in wind that allows higher speeds.
Not talking Pivers here as we fly a ama once angles get over 4-5 degrees so the ride is similar upwind to the high dehydral boats, just less heeling. 10 degrees for us is 15 degrees for a non immersed boat which now has a less functional cabin and deck. Offwind the boat sails flatter which helps the speed as heeling isn't adding asymmetry to the main hull lines.
The ama shape for cruising is a rounded V.Slender ones are faster. While going upwind in waves this allows the ama to slice through and can be drier than a full ama boat heeled over but having the windward ama bang on the big wavetops.. You'll see the Tristars with this sort of ama too. Ed spent his lunchbreaks in Rudy Choy's yard watching how they did things for deep sea. At anchor the immersed boats are more comfortable with less motion like a cat. But they are a cruising approach, a compromise between speed, carrying ability and motion at sea or anchor. Modern tri marketing has been along just speed lines for the most part so these needs have become the things not considered. Immersed or non immersed boats can be good cruisers but there are more things to measure than just speed.

 

Telstar 35 from Tony Smith, Sandwich Marina, Sandwich Kent, GB had amas which were constructed (1974) to be always immersed. About 8 hulls were built but the Trimaran was no success. There was no ability to fold the amas for harbour or trailering.http://www.yachtworld.com/boats/1976/Telstar-35-1644536/-/Greece

As I know Tony Smith raced it Round Britain but I don`t know if the concept was working sufficiently.

 

Always immersed Tris would be like a Piver- very successful despite hull shape issues etc....They didn't fly hulls. If you want an old tri design with always immersed amas but think it needs to fold for success there is the old Oceanbird 30 whose low amas were also wave piercing.

 

Are low-volume amas like this prudent for serious open-ocean sailing?

 

I prefer the calculation that John Shuttleworth carries out in this article he published on multihull considerations for seaworthiness but broadly speaking your on the right track. It's a guide to static stability but the action of wind and waves combined can effect the performance of L/B combinations on trimarans (and catamarans) as well. Wide overall beam has some positives in terms of maximum stability. Low buoyancy float trimarans suffer the most as the energy transfer from the wave impact can cause more float immersion which can potentially provide a fulcrum for an overturning effect.

This article by John Shuttleworth is quite old now but I've not seen a better article on the relative merits of multihulls in heavy weather.

http://www.john-shuttleworth.com/Heavy-weather-article.html

 

The funny thing about those boats is they are great in rough weather and nobody has tipped one over yet. Ask Richard Woods about the waters around England for challenging sailing.
D.H. Clark, production Piver sales force for Cox Marine, had a funny story about opening up the very strict and conservative Japanese market. They wanted to know what building and safety standards trimarans were built to in Britain. Clark replied that if they go out and keep coming back we figure they're all right.

 

Does it absolutely have to be demountable or folding?
This 43' Harris Tri still been for sale for a while...make him a silly offer and put it on a mooring.
https://yachthub.com/list/yachts-for-sale/used/sailing-trimaran/harris-42-sundowner/101112
Been aboard her sistership...heaps of family-friendly room like a big mono and huge aft cabin, will eat up your payload.

 

Thanks for the suggestion, but yes, demountable/folding is a high priority. Wide boats find it much more difficult to find a place large enough to haul out, and are charged more when berthing.

 

I still wonder... If used for coastal sailing it much easier to stay a few extra days in harbor while waiting for a better weather window. An open-ocean sailor will have to manage whatever comes his way. So I'm still wondering how many of these low-volume-ama 30' Oceanbirds actually made serious open-ocean voyages, surviving major storms?

I am interested in the ama pivot arms. Can anyone give more engineering details on the pivots bearings and the arms themselves?

 

Corley, thanks for the heads-up on the Shuttleworth articles/links. I'm working my way through them.

 

I would never have imagined that a 30' tri with low volume folding amas hung on lattice weld frames would be suitable for serious ocean sailing. Esp when those amas are immersed even before you put on that extra almost 2T that you say you have to have.
They certainly would be wave piercing alright!

 

Neville, I have yet to be convinced that those low volume amas are the way I want to go, but the swinging arms are of interest. I'd like to know how they were made and attached, and what sort of problems they may have had.

Here are a couple of views of the Oceanbird 36 pivoting ama system. These have a gang-plank built on top of the arms.

 

I'm not famiiar with the Oceanbird 36 but the 30 was very well tested in all weathers and has a lengthy multi decade track record. I'll see what I can find, some were sailed in the North Sea.....