# Multihull stability

Discussion in 'Multihulls' started by Mefistofele, May 14, 2009.

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### MefistofeleNew Member

Hi I'm new on this forum. I introduce myself. I'm an aerospace engineer student at Pisa university and for my final project I'm designing a 90x90 multihull.

I have to develope a VPP program and find a optimal configuration..

Does anybody know where I can find something about stability and dynamic of a multihull?

Thanks

2. ### Guest625101138Previous Member

I expect you are talking about a sailing boat.

In the limit it will be a monohull with a counterbalance weight keeping it close to upright. The forces to achieve this condition are strongly related to the beam and total displacement. There will be slight dependance on the size of the sail and the wind condition - big sail less wind, small sail more wind. So the point of force application will change slightly, causing different overturning moment, and this will alter the hull trim, which will make a slight difference to the hull drag as well.

For the above to be true for a trimaran the leeward hull needs enough buoyancy to support the total displacement. If this is not the case then the righting moment is limited to the buoyancy of the ama. One possibility of exceeding this would be if you have foils that can apply additional down thrust or up thrust under dynamic conditions.

Rick W

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Your best bet for understanding and working out a VPP for yourself is
the classic books by C.A.Marchaj. Has all you need to know, and more!
There are 2 very good ones:
1) Sailing Theory and Practice
2) Aero-Hydrodynamics of Sailing

PS...i could scan in some of the pages for you, but there is a lot and refers to several other chapters at the same time...so may confuse too.

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### MefistofeleNew Member

Thanks

Thanks a lot for yours replies.

Yes, it's a sailing one.. My university team up with italian +39 team for America's Cup. They asked to join in the race with Alinghi and BMWO...

You confirm what I already think.. I'm relieved

I noticed that the foil on the ama is curved. So this foil can generate vertical thrust, besides the thrust needed to balance the lateral force of sail. Am I wrong?

Ah Doc, thanks for your offer, after your suggestion I looked for these books and I find Aero-Hydrodynamics of Sailing.

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### daiquiriEngineering and Design

If you need to start back from Marchaj then it will be a very long way to the victory...

6. ### Guest625101138Previous Member

Yes it will provide some lift thereby reducing drag on the leeward ama.

This shows full foiling set up:
http://yachtpals.com/hydroptere-4036
Notice the "T" rudder. I expect this can provide down thrust so under dynamic conditions you have more righting moment than the displacement of the boat is able to provide. Also on such relatively light boats the crew are going to make a difference as well.

Rick W

Last edited: May 15, 2009
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### MefistofeleNew Member

Sorry but I can't see the T-rudder..

mmm what's a reasonable displacement for this kind of ship?

I can have the necessary knowledge of aerodynamic but i'm a total beginner in ship design.. Especially multi hulls..

From the studies I have already done I found out that is necessary an hydrodynamic down thrust on the leeward ama because the thrust connected with displacement is insufficient..

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### Ilan VoyagerSenior Member

You won't find any useful book or paper on a such specialized design. All the good data is kept jealously for good reasons (money) by the handful of NA able to design these race boats. I can say that the "generalist" books won't be of any help on fast multihulls, except understanding some general concepts of sail boats.

These boats are as different from a common sail boat than a SR71 Blackbird is from a Piper Cub, or a Ford Ranger from a Ferrari Formula 1. I had a very little experience in the structural engineering of this kind of boats by working in this field a few years ago. There has been a such evolution that now I'm totally obsolete...

The design, engineering and construction of such multis is the reserved domain of a few N.A., structural engineers, shipyards and sailmakers mainly French (and one English) who accumulated a more than 20 years experience. The others (english, aussies and american) are one or more steps below.

Definitely out of reach for someone without a solid experience in the domain, even if he is an aerospace engineer. We have just to remember the costly mistakes of the Dassault engineers (and they are very good in fighter planes) when they tried to play in the field of fast multihulls.

Even for skipping these multis, you need a very solid experience. The attempts of pushing a bit such multis, even by experienced monohull skippers, end in a flip-flop. It's similar to aerobatics at high level, just a few can do it after years of training.

About a VPP and optimal configuration try to contact a NA (Irens or Cabanel, or another french NA) and try to have some clues on the subject. Nobody in this forum will be able to give good and reliable information for making a good set of (working) equations. I think you have to produce something mathematically solid and not a few vague assertions for your final paper. This work have been already done and I guess you'll can have access at least to the general principles by contacting a specialised NA or hydrodynamicist, and maybe making a stage.

After you'll have to work hard to produce your own set of equations, everything changes after the 30 knots step and become complicated at the around 40 knots step with the cavitation phenomena. There are very few reliable data (and a lot of "toxic" ones) as we enter in commercial ferries domain (the designers won't give the results of a very costly work, you imagine INCA giving away their data?) and worst in a military domain.

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Ilan Voyager summed it up pretty well. Which are the reasons of the reference to Marchaj. As IV noted, you are not going to understand and design one of these "over night", and certainly not by knowing high speeds are attained simply by minimum weight maximum sail area. There is more to it than that.

I'm in the commercial field of designing multihulls, and it is a piece of cake compared to sailing multihulls.

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### Ilan VoyagerSenior Member

I agree with you Ad Hoc. Design of motor multis is a piece of cake compared to design of fast sail multis.

I would say that design of motor boats is often simpler than sail boats unless you design a slow and heavy one. In this case you are going to design a barge that moves at a few knots with some pieces of cloth on one or several masts. Nothing difficult.

But to make a fast and reliable sail boat: that becomes an exercise of High Design, combining solid engineering and strong instinct.

For example the design of a trimaran with the following requirements: longuest possible for one man, easy to sail, able to cross the south Pacific not surviving but racing, reliable but light, fast but safe enough, and that this design will be able to make a circumnavigation skipped by a lone guy at almost a mean speed of 20 knots, I say "chapeau" (hats off) and it's the case of Irens and Cabanel design IDEC.
How many NA, engineers, shipyards, and sailmakers are able to produce a such boat? (without asking how many solitary skippers are able to stay 55 days at max speed?)

The design of a fast patrol boat is a piece of cake compared to that.

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IlanV.
Nice to know someone with the same thinking and understanding as me..

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### daiquiriEngineering and Design

Ilan, you have elaborated perfectly my single line of comment.

Now, it is not to say that Mefistofele shouldn't give it a try and perform the task he has been given. It is simply to say that the results will not be good and he (and whoever has thrown him into the AC design inferno) should be (and probably is) well aware of that. You can't start from the basics (quite obsolete basics like Marchaj, I would say), have no experience in boat design and pretend to make a good multihull design. Let alone creating a winning one.
We are talking about a design field which is advancing daily, where the experienced design teams are racing shoulder-to-shoulder and where the details make the difference between the victory and the oblivion. The improvisation leads to the later outcome.

Ask mr. Ceccarelli, one of the excellent italian designers of commercial sailyachts, what happened when he accepted a bid to design his first AC yacht (Mascalzone Latino), back in 2002/2003... They won one race maybe, or maybe even zero - I should perform a search to refresh my memory.

As about the aerospace experience - I am an aerospace engineer too, and I have a strong background in both aerodynamics and in CFD. It was a complete shock to me the first time I dicovered (years ago) the effects of the free surface and the complexity of its' numerical analysis - just to say one example. It is an order of magnitude more complex than the CFD of an aircraft.
But your knowledge of aerospace engineering could be of big help for the design of sails and hull appendages, that's for sure.

The most logical way of doing this task, Mefistofele (well, we do have some big and famed names in the forum ) would be to embed you in a design team which has the relevant experience, where you will be able to learn the secrets (because it's all secrets, as Ilan pointed out) of multihull racers design. What you can learn in this forum is just a bit more than what you can learn by reading commercially available books and googling through the internet.

Imho.

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### MefistofeleNew Member

daiquiri, IlanV, thanks for yours directness. Like you said I was thrown in the AC inferno (quite suitable for Mefistofele ). And the point is that my supervisor has experience in AC yachts, but the classic monohull. Also the team that join up with my university has no special skills in designing multihulls, i think that they are quite insane...

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### Ilan VoyagerSenior Member

Yes the problem with boats that they are on the surface separating two very different fluids. Worst they move everywhere, pitching, rolling, sliding. Nothing common with a plane immersed in air or a submarine. I remember a "miraculous" keel designed by a team of aeronautic engineers. Truly on the paper a marvel of "hyper" laminar profile. A lone problem, boats do nor move in straight line and do pitch and roll, seawater is dirty with algae, plastics pieces. Nothing worked.

Once I was consulted because a fast 30 feet boat had a very poor handling (euphemism; it was highly dangerous). I see the rudders. Magnificent high gloss job with a very beautiful laminar profile. All looking at me like if I was the Messiah, waiting for my words of wisdom. I asked a sander with 180 grit sandpaper and I "dulled" all the last 60 % of the profiles and added 3 degrees of ackermann steering under the horrified eyes of the owner.
Ok in theory it's not good at all. But the boat became controllable. After we modified a little more with simpler all around profiles with better results.

Nice to get a very low drag, but if the boat is do not stay in straight line, and control in curves is nihil what's the use?
That's field experience...

I would like to end with a positive note: it's a very private club but not closed. They are always looking for good brains as the matter is becoming more and more complicated, and there is a lot of job to be done.
So if you're really interested in the field and truly able you'll be cooptated in the club.
Do not expect to earn some money. Surely any job at Piaggo or MV will pay better, more sure any commercial post selling equipment will pay far more.
It's a fact that working for rich private clients in yachting do not pay well: corporations pay better on long time.
But you can have some fun, pleasure, and excitement.

Try to contact some NA like Van Peteghem and other. They are always searching some free slaves for the hard job, and a formation of aerospace engineering is highly appreciated. Maybe you'll earn just enough to buy a sandwich and a coffee a day. Do not do that; keep the money and find a middle aged girl friend (they cook better, and have no illusions about the future) to get a good bed and free meals in exchange of some physical activity. And you'll learn a lot.

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