# How big a sail and keel do i need?

Discussion in 'Boat Design' started by stonedpirate, Nov 30, 2009.

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### stonedpirateSenior Member

Is there a way to calculate how much force a sail will generate?

I mean, i my hull has a hull speed of 3 knots, is there a way to have a sail that will give me 3 knots but not over power me?

Basically i am trying to find the smallest sail that will give max speed.

Same with keel and ballast, what is the smallest keel and ballast i need?

I heard someone say that paranoid builders have keels and ballasts 4 times more than they need so i am assuming there is a way to calculate those needs.

Sorry for such a noob question. Just after some basic physics calculations if they exist

Thanks

2. ### dskiraPrevious Member

I am sorry but is like everything else. If you are sick you go to the doctor.
So you have to hired and pay a naval architect.
Good luck
Cheers
Daniel

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### stonedpirateSenior Member

lol

I do my own motorbike repairs but never did a mechanics trade.

A few links to a textbook or a webste that explains it will suffice. I will learn it the same way naval architects learned it. Reading....

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### stonedpirateSenior Member

Are you suggesting that amateur designers quit?

Or go do an engineering degree to design an 8 foot sailboat.

hahaha

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### Crag CaySenior Member

'How to design a boat' by J Teal is full of useful ratios to answer all those questions as well as put these things into some sort of context.

The 'normal' limits of ballast / displacement / sail area / lateral surface area are well known. However, stray outside these and this empirical approach will have to be replaced by something more detailed and rigourous.

http://boatbooks.co.nz/design.html

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### stonedpirateSenior Member

Thanks a lot Cray.

I'll check it out.

Cheers

7. ### dskiraPrevious Member

It is you life, not mine, so who cares
Good luck
Cheers
Daniel

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### stonedpirateSenior Member

Naval architects built the titanic.
Just sayin

Sorry, no sale.

9. ### dskiraPrevious Member

Stonepirate, you didn't get it. I was not trying to sell you a design or having a customer, I was just trying to put you in the right direction.
But that seams not rational for you, so be it.
As for the Titanic, (so old joke, please choose an other vessel) do not forget that naval architect help to winn WWI and WWII. Plus they allow you to import everytyhing you need, in time aand at good price.
Just saying.
Good luck
Cheers
Daniel

10. ### dskiraPrevious Member

And just for you information you realy have to know that naval architect do not BUILT ship, they DESIGN ship.
It is quite a difference.
Good luck
Cheers
Daniel

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### stonedpirateSenior Member

Dont take that the wrong way.

Of course naval architects are worth their salt.

Just think you can over engineer an 8 foot boat

I'm not building an ocean liner.

12. ### dskiraPrevious Member

StonePirat if the boat is 8' long do not put ballast. Your own weight will suffice. Of course it is suggesting you are building a dinghy for a short sail.
As for the sail area I will say 40 Ft2, assuming your boat will weight 100Lbs
8' by 4' wide and a draft of 6" and a total draft with centerboard down of 1'8" will be aproximatly the norm. Of course I am talking some kind of dory type.
as for the speed, as we say: depend of the wind
Start along these lines and read the J. Teal book Crag Cay recomend. full of good stuff. you will enjoy.
A cardboard or balsa simple model is always useful.
Good Luck
Cheers
Daniel

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### stonedpirateSenior Member

Thanks a lot Dskira.

Will definitely get that book.

Cheers

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### PARYacht Designer/Builder

Basically, what Daniel is trying to say is if you take a person into an unnatural environment, there has to be a reasonable level of compliance or at least assurance, the design will meet minimum safety targets (like floating right side up on launch day).

Considering the very basic nature of your questions, you are not a novice designer, your are just an interested party, with literally not a clue what's involved. This isn't meant as a personal dig, but a real warning, that you're in way over your head at the moment.

This doesn't mean you couldn't get the information you need, but it does mean you'll have to boost your education on the various subjects you'll be encountering on this project. An on line course is a good idea.

The bottom line is the unnatural environment element. Flying along a 30,000 feet in an airplane, places a person in an unnatural environment. Were there something to go wrong, particularly as a design error, the results will be less then desirable and likely messy. It should be reasonable to assume the designer had a clue about engineering and aircraft design. The same is true of a boat (any kind) that takes you farther from shore then you can swim back to. The result of a design failure or omission can and will likely kill you.

With this in mind and given the fact you're attempting to design, then build a highly specialized boat, makes most of us wonder if you're nuts. It's likely you're not nuts, you're probably young and inexperienced about boats or boat design, clearly engineering principles and general physics and haven't truly grasped the whole picture.

There's no free rides, you need an education. This isn't the best format to get it, though if you have specific questions, here is an excellent place to come. Of course considering your gross lack of understanding of the multiple subjects involved, you'll quickly wear out the tolerance of most here, if you attempt to get all or most of the answers for free.

Look in the library section of this site or try your favorite book seller. There are dozens of texts that can help bring you up to speed. I'd strongly recommend the latest revision of "Elements of Yacht Design . . ." as a start. If you can get through this, then you're ready for the next step.

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### alan whiteSenior Member

Just because you asked, there is a way to size a sail to a hull but it's roo complicated to cover here.
You then asked about what would be needed to reach 3 knots. The question is somewhat meaningless, since it is the boat's length that usually determines its speed potential and not the sail area. These are all basic questions.
I suggest Dave Gerr's "The Nature of Boats" as a primer. Read it and learn the basics. It's a very well written, readable text.

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