Trimaran question

Is there a special name for the power tri's that have short ama's to the rear instead of the full length ama's on sailboats?

Does a tri with the short ama's to the rear prevent the vomit comet effect that some wave piercing cat ferries have exhibited?

Example, and link to more-http://www.lomocean.com/projects/cutting+edge

 

Stabilised monohull

I always though if I went crazy and sold my house to build a boat,may as well be over the top and it'd be something like this.
But I'm happy with things the way they are

 

You wonder about the ultimate stability of these things, but I guess the answer there is, "it depends".

 

Catamarans do have a much greater tendency to hobbyhorse than either trimarans or monohulls. Shorter amas, forward on sailboats and aft on power tri's, do help dampen hobbyhorsing by unbalancing the fore and aft buoyancy. Most monohulls dampen hobbyhorsing with their fine bow sections and fuller aft buoyancy. The reason amas are forward on sailboats is to prevent tripping or pitch-poling, which will ruin the whole day. Powerboats don't have that problem and can place the amas where they wish.

The "vomit comet" has a slow twisting kind of roll that can be uncomfortable in response to long period ocean waves. Looking at the action of these cats close up leads me to think that roll damping is achieved by steering of the thrust jets to induce counter roll to oppose wave induced rolls. May be in error in some regard there, but that is what it looked like to me on a long trip across the channel in the remains of a storm.

 

No. Tri..meaning 3. Thus 3 hulls. Where they are located and how long is a detail and not related to the definition of TRImaran.

That is a seakeeping question alone.

If you focus on one aspect of a design to create an absolute, in terms of an answer, you won’t achieve a well balanced design. Since a good design is a comprise of all these “absolutes” which are often in conflict with each other. Being able to balance them and achieve the desired goal, the SOR, is the key to a successful design. Focusing upon an absolute inevitably means the other aspects, which are equally as important, never get their fair share of attention in the holistic sense.

 

You could just call it a outrigger, sort of !!

 

A few years ago I attended a lecture given under the IMARest banner by a representative of Austal and that was certainly the claim made with reference to the trimaran ferry Benchijigua Express. The lecture made the point that the ama's on this style of vessel serve mainly to provide some stability in harbour and at low speed. When the vessel is travelling at cruising speed it is effectively a monohull with active control systems controlling both pitch and roll. There are three continously adjustable hydrofoils and a continuously adjustable interceptor plate aft. According to that lecture, a catamaran is less suited to active roll control since its waterplane area distribution tends to cause it to follow the profile of the waves, a stabilised monohull or small float trimaran can ignore the profile of the waves, at least up to a point.

 

In aviation we refer to the vomit comet type motion as a "dutch roll" motion and it makes most people eventually puke. http://en.wikipedia.org/wiki/Dutch_roll

It makes sense to me that the trimarans like the Austal 102 would reduce this motion as there are no widely spaced hulls out front. I think the problem occurs with catamarans as one hull is lifted and the opposite rear corner drops, then the wave hits the second bow, it is lifted while the other drops so you get the corkscrew dutch roll motion.

Here is a link where Austal claims "Studies show that motion sickness on the trimaran will be approximately 56 per cent lower than on a 100 metre catamaran operating in head seas. Even larger benefits are realised in other headings."

http://www.austal.com/en/media/media-releases/10-05-18/trimaran-named-industrial-product-of-the-year.aspx

 

I don't think you'll find a company selling something which shows their product to be inferior!! It is after all a sales pitch, excuse the pun

Saying better in other headings assumes the reader will link the wow factor of the sales pitch to the claim of better in other headings.

You need to see this demonstrated and also compared against like for like.

Anyone can make a claim...especially a sales dept

 

Whilst that might be true in purely structural sense, it is actually important to size and locate the amas to optimize a tri design for wave/wake interference and drag.

 

Of course. However, I hear that placing amas to benefit from bow wave interaction has often failed to deliver promised results at other than at specific speed and water conditions. Does this mean that there may be increased drag at other times over drag when amas are outside the interference zone?

 

Yes.
And small amas cannot reduce wave resistance much. They do not make waves of suffiicient size to cancel large waves made by the central hull.

 

That makes sense.