Trimaran Stability - Outrigger displacement

In the begining stage of designing an outrigger of a trimaran; to obtain a rough estimate of the displacement of the outrigger. I know the transverse clearance also as an effect on what the outrigger displacement must be, but I would like to know if the GM isn't known, how can you determine what the ratio of the displacement of the outrigger to the centre hull must be in order to have a stable vessel? Can you determine any values for the displacement of the outrigger if the GM of the centre hull isn't available?

 

more questions..

Seems like there needs to be more info: how big is the boat? Do you intend on the crew playing a role in righting moment; in other words will crew movement be a factor?
Is the boat being built for speed ,daysailing with friends or cruising?
Some trimarans are designed to fly the mainhull in which case the volume of the ama(outside hull) needs to displace two to three times the weight of the whole boat.Or be foil assisted...
Other tri's use relatively low volume ama's and use the ama as a sort of "power limiter" in the sense that the ama will become immersed before the main hull will fly- total ama volume with a displacement of of between 100 and 200% of the total weight of the boat.
I'm most familiar with the design considerations in high speed tri's 20' and under; many of the designers of ocean going tri's and cats have published their theories on seaworthiness borne of many ocean miles and should be researched in the design of that type tri.

 

There's some discussion of trimaran outriggers in either of these locations (two copies of the same article):
John Shuttleworth
Unihedron

 

Anyway, for sailboat total volume of outrigger should not be less than boat's displacement D. In practice, one is (1.7...3)D. If above mentioned condition is matched, maximum GZ of the boat is around distance from centerplane to outrigger. If not matched, than less.

 

Doug thanks for the reply.

That is just the problem I'm sitting with. I don't have the size of the boat, the centre of gravity, metacentre and centre of bouyancy. Just to make it clear, this design is for a powerboat and not a sailing tri. I only want to determine the displacement ratio of the outriggers and the centre hull. This is only for the static stability. The centre hull will be unstable, due to its high slenderness value. Therefore the outriggers will resist the capsizing moment created by the centre hull. Is it possible to determine the dislcement ratio then with this little info? I'm thinking of determining the displacement ratio i.t.o the righting arm GZ.

 

power tri

Seems to me that on a power tri you'd be concernerd with keeping the "ama's" small and so one consideration would be how much the boat would roll when the crew steps on board. Next would be the consideration of where the crew is going to sit-outboard, inboard or both? Depending on size those could be relatively minor considerations or no consideration at all.
Will you consider the use of foils to assist with dynamic roll/pitch stability? I think foils are a natural in this application and they're already being used in power cats in a "foil assist" mode.
You could opt for a design where the foils on the "ama's" absorb X portion of total displacement as the boat goes faster increasing stabilty with speed.
I like the concept of a power tri but the considerations of ama size important to sailing tri's have little or no relevance here in my opinion. I'm interested in following this so let us know what you come up with.

 

The tri will first be tested without foils and later with foils and then the two results will be compared. This infact will determine if the foils have any significant impact on the resistance and the seakeeping of the vessel. I've determined a rough estimate of the position of where the bouyance force will act, taking into account the volume/size and the clearance from the centre hull of the outriggers. Luckily it's still early in the design process and future work with regards to testing will determine if the calculations are correct. Will keep you updated.

 

I think the most useful way to size the outrigger is to consider a footprint plot like this one:

The moments from the rig are converted into equivalent c.g. locations by dividing by the weight (or by the total vertical load if you want to take heel angles into account). These are overlaid on top of the center of buoyancy location to see the relationship between the required vs actual Gz.

The value of this presentation is it shows how to balance the longitudinal vs lateral demands. And one see by the trim angles that sheer in the outrigger is a valuable way to maintain waterplane area and reserve buoyancy forward to prevent diagonal capsize, which is the biggest hazard for a trimaran.

Shuttleworth's stability indices can also be depicted the same way:


The footprint plot shows that it's not just the buoyancy of the outrigger that counts, it's where it's distributed and when. Not all of the volume of the outrigger is available at any given time, because different parts of the outrigger are out of the water. For example, to oppose the heeling moment going upwind,most of the outrigger's length will be immersed. But to provide diagonal stability, the stern of the outrigger is likely to be out of the water. And it's vital that there be waterplane area forward in order to stabilize the boat against increases in sail loading, even when the bows are depressed.

 

rmoolman, I think you specifically said power tri...a'la AUSTRAL high speed tri ferrys or RV Triton? As I understand it, demi-hull (not ama as there are no akas?) location and waterplane are more important than displacement. We are using thin ship theroy here. The location and length of the demi-hull are used to manipulate the wave train to advantage, while the waterplane is selected to stablize the ship at a designed roll period based upon top level operational sea states. This is a design sprial item. I'd start with a total up weight, set my main hull then the location of the demi-hull bows then the needed Iwp then the length then the demihull bredth then the demi-hull displacement.

 

Just being curious: is this a model case or are you talking about a solid designstudy.
I see quite a few of such theoretical discussions, but nobody issues any internal weight. What I know about a powertri is that at a certain speed the hull will find it's own stability, generated by the specific speed. Up to a critical point, the ama's become less important. They are only required for a certain speed trajec being 0 to ...... speed, afterwards the hull balances on it's own.

Is that correct?

 

It's for a model case. So far I only need to determine the static stability. I can wait 'till the models are tested, but I want to prove it theoretically.

But it's a interesting point you made that the centre hull stabilizes itself when reaching a certain speed. To tell the truth I also don't know if that is the case. This will probably be the true if the vessel is travelling in still waters.

 

just got this e-mail about the sale of the good ship triton. you can e-mail qinetiq marine with a question.

 

power-tri

I have just started building a power tri thanks for all the discussion

 

We've done some investigations of long, very slender power tris. Models were designed to allow fore/aft and athwartship placement variation of the amas. The best overall performance configurations seem to have had quite small side hulls, located very far aft, and not very far away from the center hull. In fact the amas were so small and so close to the main hull, that they didn't even call them trimarans, but "buoyancy stabilized" ships. Length to Beam ratio of the center hull was on the order of 12-15:1, and the length of the side hulls ended up being about 1:6 of the overall length, with similar L to B ratio as the main hull.
Of course everything depends on the specifics of your application, and explicit computation of the RA-RM is always a good idea.

 

Can you please specify what the displacement of the stabilizers are compared to the main hull? We found that the position of outriggers with a displacement of 6% to that of the center hull did not influence the total resistance too much. The wave resistance differ for different positions, but doesn't have an influence on the total resistance. The length was 60% of the center hull and this was due to the configuration of the hydrofoil system. No resistance results on that yet.