# 28' sail cat

Discussion in 'Multihulls' started by mihari, Sep 4, 2009.

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### boat fanSenior Member

Shuttleworth :

"the center of buoyancy ( is ) being moved forward in the immersed hull"

Hard to explain ( at least for me).... I will try :

As the windward hull attempts to lift , the leeward hull`s center of buoyancy moves forward because the leeward hull is simultaneously forced forward and down ward.The stern attempts to lift out of the water.The waterline gets shorter , and c of b moves forward.
The boat " pivots " at a diagonal axis to abeam of the boat.

Does that make any sense ?

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### mihariJunior Member

Without actually doing any calculations, I think that the c of b is not moved any more than a few cm at normal conditions, and less than half a meter at extreme conditions. We are talking about a cruiser here, not a racing cat. The lighter/faster the boat the more extreme this momentary occurance is.

Thank you boat fan you have been of great help so far.

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### mihariJunior Member

Hull width calculation.

OK, I made my wife stand in the kitchen as if she was working on the counter to measure how much space we need. As I thought, the absolute minimum (for our average weight) for 2 people to cross without touching is 60 cm at bum level. A 5cm narrower space on either side is needed at the feet. At 55 cm effort is needed not to touch. At 50 you can be sued for sexual harrassment.

I think you cannot go any more minimal than this... for a cruising (maybe liveaboard) cat.

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### boat fanSenior Member

Yes c of b does not appear to be moving much.

However , a 28 ft cat like you are proposing will essentially be relatively lightweight , even if cruiser.You will still have to watch your weight or the boat WILL be a "pig" , that is to say it will be sluggish and wet and plow through the water , instead of gliding
over it.A 28ft cat can be nice .If you weigh it down you kill some of the very attributes that make a cat so appealing in the first place..The first commandment of building a good cat should always be " KEEP IT LIGHT " cruiser or not.

A c of b shift of .5 m (or 19 inches ) on a ( more or less ) 28 ft waterline could well make the difference between a boat that " bucks " or "hobbyhorses " and one that behaves in a more sedate manner.

I think you really should have at least 600 mm width to move around. A little wider in the hulls on a 28 footer will not necessarily be bad . In fact , I believe that the gains far outweigh the extra wetted area of a slightly wider hull, at least at this size.
Just the gain in displacement will float her higher .That has got to be a good thing. Some people may be surprised to find that your boat may well be FASTER with slightly wider hulls , given equal loading / weight / displacement.

I had a conversation with Ann Snell ( Australian Easy Cats ) about this. Peter`s smaller 29 ft should not carry more than 1ton. If you want more they strongly advise to build the 32. Like Ann said , there is no difference in cost.

As you get bigger ( longer ) things get a little more forgiving as far as weight goes.

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### mihariJunior Member

Of course, as I said, that is what I thought to be an absolute minimum. I never said I was going to use that. I will probably go to something more rummy. This is a rough sketch of what I am thinking of...
...how the heck can I attach an image?

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### boat fanSenior Member

If the image is already on the web somewhere , right click with your mouse select properties and copy the url into your post here.

If the image is on your computer its a little more messing around :

You need to join an image hosting website ( free ).Here is one:

http://photobucket.com/

BTW there is nothing wrong with the 28 ft size cat.
It`s just that you really need to trim what you carry on board. The other thing is getting headroom in saloon is near impossible under 40 ft.
Just saying ........

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### jamezSenior Member

Or you can just select 'go advanced', then 'manage attachments' and attach them to your post.

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### boat fanSenior Member

Just learned something ....thanks Jamez.

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### sailsocalJunior Member

No it doesn't. How does the C of B "move aft in the raised hull" as Shuttleworth states?

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### boat fanSenior Member

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### catsketcherSenior Member

Movement of Cb in flared hulls

Gday all

I use Shuttleworth type hulls in my little folders. They are tricky to design right. If you go slightly over weight the boat will be a pig. As to the CB question.

If you look carefully at Shuttleworth's hulls the flare is different fore and aft. This makes the CB move as the hulls are raised or immersed. The tricky thing to remember in cat design is that the hulls are always moving up and down - leeward always down and windward always up. You can use this to make the CB move where you want if you are clever.

You should not use low flare hulls on a boat with a high immersion rate. Go slightly wider than you think because every boat comes out heavier than you think and you will be dragging the flare around if you stuff up.

cheers

Phil

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### mihariJunior Member

(I hadn't seen the little button "manage attachments" at the bottom)

So, here is a rough section of what I had in mind. You are right, it is tough to get standing headroom in the salon without making it look like a bus. I will make a small compromise and give it a shot.

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13. Joined: Sep 2009
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### mihariJunior Member

Originally Posted by sailsocal View Post
No it doesn't. How does the C of B "move aft in the raised hull" as Shuttleworth states?

As the C of B moves forward when the bow dives in the water, when it comes back up it moves stern-wards. I guess this would only be a momentary effect.

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### boat fanSenior Member

Do you have any hull sections I could look at catsketcher ?

That is real good advice catsketcher . I agree 100 %

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### boat fanSenior Member

That " Art meets science " thing is coming into play now ....

You could devise a saloon " pop top " to reduce height.

Richard Woods novel approach :[/QUOTE]http://www.sailingcatamarans.com/studyplans website/Sango/popbottom.htm[/QUOTE]

One question , how did you arrive at the .34 m2 cross section for your displacement ?

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