# how to determine sail area

Discussion in 'Boat Design' started by stonedpirate, Feb 13, 2012.

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### stonedpirateSenior Member

hello,

sorry for all the questions.

Just doing the final sums on my design and need to get a rough idea of my mast height.

Is there a formula for sail area?

Specs

10' loa and lwl
4' draft

aiming for full sails at 10knots reef til 25knots no sail above 25knots.

Thanks

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### sharpii2Senior Member

The general practice is take the cube root of the Displacement volume, which in the case of a 1,000 lb displacement would be: 1000/64=15.6 cubic feet. Cube Root of 15.6=2.5. 2.5 Squared=6.25.

This number is then multiplied by, usually, 15 or more. In your case, I would stick with the lower number, because your boat will likely be using a considerable amount of its stores as ballast and will probably have low initial stability for it's weight, as most heavy boats do. The size of the mast effects stability greatly. Especially if its to be sturdy enough to brave extreme off shore conditions. So, lets go with 15. 15*6.25 = 93.75 square feet.

This may not seem like a lot of sail area, but, if you were to use a masthead rig (which Gerry Spies did) and keep the entire rig within the length of the deck, the mast would have to be nearly 20ft tall.

You can cheat, of course. you can add a bowsprit, for the fore stay, and a boomkin for the back stay. If each extended 2ft past its end of the boat, you could end up with a mast 4 to 5 ft shorter and have the same Sail Area.

You could also start with less sail area for your Working Rig, say 12 * 6.25 instead of 15 * 6.25 and have a really big light air sail, such as a Genoa jib, which extends almost to the transom. Being that this boat is intended to spend much of its time on the open ocean, the ability to short tack will probably not be on top of your priority list.

The advantage of this is the mast can be shorter, sturdier, and more securely stayed mast. Keep in mind that the capsize moment of a mast varies with the square of its height. If you want a new mast that is twice as tall as the old mast, for instance, it will have to weigh half as much as the old one to have the same capsize moment.

To put this in perspective, let's say you decide to go with a 16 ft mast instead of a 20 ft one, using the dodges and cheats mentioned above. Now, lets say you want to keep the sixteen ft mast the same sectional area (lbs per ft) the same. Now, say the taller mast would weigh 25 lbs.

16ft/20ft = 0.8
O.8*25 lbs = 20 lbs.

Your new mast is only 5 lbs lighter, which seems hardly worth the effort.

Now lets figure the capsize moment of both masts from the deck up.

To keep things simple, lets say the mast has a uniform section, all the way to its top.

The capsize moment of the old mast is:

20 ft/2*25, or 250 ft lbs.

The moment of the new, shorter mast is:

16 ft/2*20, or 160 ft lbs.

As you can see, this 5 lbs of actual weight savings has quickly turned into 90 ft lbs of capsize moment reduction. Definitely worth doing, I hope you would agree.

Sorry for the long, divergent lecture, but I think it was necessary to show you that desirable Sail Area is more than a simple number.

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### stonedpirateSenior Member

you have given me a lot to think about and research.

Thanks a lot

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### stonedpirateSenior Member

Well, a far out boat needs a far out rig

I will try out a mast aft, genoa only rig. I know its not as good as marconi but with such a small boom and thin mainsail, it would seem kind of useless.

Also, cant use bow sprit as it adds to LOA and will ruin record attempt.

Its a freestanding, 18ft mast. Since i am not hoisting a main up it, it can just be a simple pole with no flex to keep forestay tight. No idea if an 18ft no flex pole strong enough to withstand the max force of 25knots on reefed genoa exists. Will need to do some physics

With no backstays, i need to find out the force generated at base of mast to build transom mount strong enough.

Its a crazy rig but with a small boat with no sail up beyond 25knots, maybe the forces are light enough to allow for freestanding mast.

Its a simple rig that can be controlled by 3 lines from inside the boat. Furler makes reefing a sinch and 2 jib sheets can be tensions by 2 winchs that can be turn from inside and outside boat.

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### YobarnacleSenior Member

Might consider chinese lug sail. They were set on low unstayed masts. Balanced and fully battened, they were hundreds of years ahead of modern western design. I believe the Hobie Cat was first fully battened western sail.
Clipper ship logs regularly reported being passed by raggedly looking chinese junks. Out pointed and out sailed!

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### viking northVINLAND

StonePirate--you migh want to read over the thread--"Ok Can someone explain this to me" to gain some insight into the characteristics your proposed rig style.

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### StumbleSenior Member

Stoned,

What record are you attempting to beat? From what I discovered you would need to shave about 7 foot off of this vessel to set a trans-Atlantic record.

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### stonedpirateSenior Member

smallest boat circumnavigation record is 12'

considered chinese junk and lug sail, just dont like the looks of them

thanks for that link, good to see other aft mast rigs

i am aiming for 2 to 3 knots downwind and reaching and would be happy not to lose much ground upwind.

Doesnt have to be perfect, just basic sailing

boats like these are more about drifting on currents

if it sux upwind i will just drouge it

cheers

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### stonedpirateSenior Member

would a simple 6 meter steel pipe do the job?

Like antenna masts?

They are cheap and sturdy, not sure about weight

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### YobarnacleSenior Member

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### YobarnacleSenior Member

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### StumbleSenior Member

According to what record keeping authority? I have been unable to find such a record being kept by anyone.

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### stonedpirateSenior Member

guiness book of records and general public concensus that serge testa set the record

its open to debate whether his outboard and later added bow sprit is included in loa

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### daiquiriEngineering and Design

Stonedpirate,
If you want to keep the weight down, then it is much better to use multiple tubes (or extrusions) for your mast, in order to diminish mechanical stress on each aluminum tube.
For example, you could arrange it in a 3-tubes mast (triangular layout: two with deck mount fwd, one aft, tubes joining at the top) and thus cut in half the mechanical stress in each tube. In this way the tubes can be made with smaller diameter and with thinner walls, hence lighter (up to 60% less weight) and less draggy for the same ultimate (fatigue included) stress.
Cheers

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### PARYacht Designer/Builder

Slavi, this is a kid with a dream and no engineering background, nor anything that reasonably resembles hydrodynamic understanding. I think we are just "enabling" this kid, in spite of the negative replies to his "quest", he's empowerment grows with each post. I question the wisdom of this among fellow posters.

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