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Author Topic: Formula for Mast height and sail area  (Read 10728 times)


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Formula for Mast height and sail area
« on: August 10, 2011, 11:29:04 am »

I have an old B Boat sailing boat that I am restoring. It is 26 inches long and the beam is 5 inches and deck to the bottom of the keel is 8 inches. Can any advise how tall the mast should be and the maximum sail area please.


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Re: Formula for Mast height and sail area
« Reply #1 on: August 21, 2011, 10:29:40 am »

Hi Beeky

There is no formula, as such. You can have much larger than, or smaller than, standard sail areas.

However, there are other things to consider.

Do a search on this board (Dry dock, yachts and sail) and on the R&D board for yachts and sail

 for 'centre of'. There are several consideration to do with 'centre of lateral resistance', 'centre of area' (a.k.a. 'centre of effort)'.
Also search for 'lee helm' and 'weather helm'.

Why no formula?
Because there are too many variables.
Hull shape, keel length, keel area, keel bulb mass, rudder size.
Masts, mast height, mast position (fore/aft), number of masts.
Sails : The ratio of canvas in front of the mast compared to to behind the mast.

You can buy books on this. But I recommend you go for trial and error.
So what can go wrong:
-Mast too high, the wind will knock the boat over or she will 'broach'.
-Mast too low, she won't reach her maximum speed (but you won't be racing).
Everything else is down to tuning.
NB every real boat changes its sail plan to match the weather, and so there is no best sail plan, they are changeable.

So, where do you start?

Find a picture or silhouette of and similar boat and compare length of hull to hight of mast. Work out the ratio.
This gives you a mast height that won't be far off.

Buy 1.99 shower curtain and cut out some rough sails based on the silhouette you used before.

3/ Put it on the water and tell us what happens.

4/ We can then suggest how to tune your rig.

Can you see where the mast should go?
If so, is it one hole or many?
Can you see if there used to be a bowsprit?
A photo of the deck would help.

Sorry for the long delay in replying.
The only stupid question is the one I didn't ask


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Re: Formula for Mast height and sail area
« Reply #2 on: August 21, 2011, 01:21:56 pm »

Hi tigertiger

Thanks for your input. I have now actualy found (to be fair through a subscriber on VintagE Model Boats FORUM) a picture of the orignal advertisement for the boat in Model Sail Yaghts and Speedboats - 1932. The sail confirguration is basicall a Bermuda format. For the mast I have done exactly what you suggested and scaled from the picture. many thanks for your help.



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Re: Formula for Mast height and sail area
« Reply #3 on: August 22, 2011, 01:17:32 pm »

For my scratch yachts I use a rule of thumb of mast to be 25% longer than the hull, works out about right.  Where to put it needs a little extra work.
Put the hull in the water and find the point where if you push it sideways it does not turn in either direction.  This is centre of lateral resistance.

Work out the sail areas and their combined centre of area (very easy) and this point should be 4 % ahead of the CLR at the waterline.
The mast then has a position, if the mast has a fixed point to fit into then work backwards on the sail areas, adjust jib and main areas.  This is possibly the only use that I have ever found for basic geometry.
Hope this helps.

regards Roy


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Re: Formula for Mast height and sail area
« Reply #4 on: August 22, 2011, 07:47:59 pm »

Thanks Roy

I think this is about where I am at at the moment. 25% seems about right.
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