crazy 30meter beam stability question

Discussion in 'Stability' started by yacht picasso, Sep 15, 2007.

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yacht picassoJunior Member

hi all,

imagine this:

• A mono hull boat is 62meters long;

• it's devided into 2 sections from point 0: stern is at 43m forward, and bow is at 19m aft.

• at 0 it has a maximum beam of 30m (from strbrd to port), to then reach 0 at stern and bow.

let's exclude the superstructure.... what stability issues would it encounter (many i am sure), but what exactly? how do you counter them? would you need a deeper draft?

thanks!

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sottorfmember

you are talking about a hull with length to beam ratio L/B=2.0 if this is a monohull it sounds more like a barge than anything else.

You dont mention the speed but unless it is designed to plane :?: i dont think it will have stability problems.

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yacht picassoJunior Member

Remmber that the hull is basically 2 triangles put together, joining opposite to each other.
So one triangle facing west with hight 19 and base 30. The other triangle facing east with hight 43 and base 30. Both starting at the same point.

How would you advise the hull below LWL be formed? The impact with the on coming hydrodynamics would generate much friction, right!?

So, what do ya think?

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marshmatSenior Member

Let me get this straight. The thing is more or less diamond shaped when viewed from a helicopter, and is twice as long as it is wide? Sounds more like a hydrostatics professor's idea of a bad joke than an actual boat. Could you try posting a sketch of what you're thinking of, to confirm that it's really as strange as it sounds?

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yacht picassoJunior Member

No sketch needed, it's exactly what it sounds like. A diamond shape (exactly). But it's not a joke, see it more as a way to understand designers thinking outside of the box and normal conceptual designs.

How would you reach maximum stability on such a design? And maximum fuel efficiency?

Thanks!

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marshmatSenior Member

Hmm. For maximum initial stability in something like that I think I'd want as much buoyancy out at the perimeter (especially the tips of the short axis) as possible; ie. a fairly squared-off bottom shape. Since it's pretty close to the least efficient planform I can think of in terms of drag and fuel use, optimizing fuel efficiency would be best achieved by having it stay still whenever possible. If it has to move, that's another story. Perhaps a run in Michlet will reveal a point on the speed/drag curve where the bow , mid and stern wakes (I'm pretty sure it will create a secondary wake at the sharp corners, as well as the normal bow/stern wakes) manage to partially cancel out; this would be its ideal cruise speed.

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yacht picassoJunior Member

I agree on the secondary wakes. What if the beam points where rounded instead of being pointy?...would the wake be less turbulant?

What is Michlet? Is it along the lines of maxsurf seakeeper or autohydro?

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GuillermoIngeniero Naval

picasso,
Somebody recommended you the reading of Gerr's "The Nature of Boats" at the other thread on this 62 m yacht designing of yours. I friendly and strongly support that reccommendation. Most educative and entertaining reading.
I think you will enjoy it a lot as well as get from there many claryfying concepts (Well, there are some things I do not totally agree with, but my overall opinion is frankly positive for a divulgative book like this).
Cheers!

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yacht picassoJunior Member

Thanks mate.

I guess I'll give it a read before posting more.

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alan whiteSenior Member

I recall, there was a recent post and commentary involving a diamond-section keel with the sharper end forward. A skinny version of Picasso's idea.

A.

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Hotel LimaJunior Member

Does it have to be a mono hull? You could make this about as efficiant as it gets and square (more deck area) AND stable if it were a catamaran.

Just my 2cents

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marshmatSenior Member

Going back a few posts-
Michlet http://www.cyberiad.net/michlet.htm is a useful little program courtesy of Leo Lazauskas which can give pretty good estimates of hull resistance and wake properties, within certain limits of geometry and speed. Whether it would work on this diamond block of yours- it would certainly try, but the results won't be the most reliable, I think because the water flow is going to be completely turbulent and utterly unpredictable aft of the pointed wings. The format and interface are a bit tricky to get used to but are actually quite efficient.

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yacht picassoJunior Member

Thanks guys! (I much prefered the sheldric's morphogenetic field version).

The idea is to keep it as a monohull. But it is not necessary to keep the wetted surface "diamond" shaped. The top view needs to remain original though.

I'll give Mitchell a try, and see how it compares to the other 2. Thanks for the tip!

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alan whiteSenior Member

Yeah, except I was serious, but I thought it might seem as if I was making fun.
A.

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sottorfmember

I assume you are referring to the diamond shape for the shape of the deck or at the waterline. For te underwater sections you should try to get rid of the sharp corners so that there is little or no flow separation. Given the length to beam ratio being so small the hull will have a huge hump resistance and will only be capable of very slow speeds.

There will have to be some very strong overiding reason to go with a hull this shape, ease of fabrication and cost definitely don't justify the fuel cost of operating a vessel with a hull like that. What is the application supposed to be?

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