I have a mast length question

I am planning to build a small sailboat that is only 11.5 feet in length. At the widest point, the boat is 59.5 inches (4' 11.5"). My problem is that I don't know how long to make the mast. I was thinking the thickness of the mast would be maybe 3", but still I don't know the length. I think the keel is going to be a fin style keel maybe about 17 or 18". Any help on this would be appreciated!

 

Since you are doing all the design work yourself, you wont have the benefit of expert professional calculations, so you need to 'play it by ear'

So, the best bet is to buy a cheap, secondhand mast from a dinghy class of a similar size, and start experimenting.

 

Ah, thats what I was afraid it was probably going to come down to. Thanks for the fast reply.

 

reckon around 2'' might do much better.. The height depends how much you weight and how far on the weather side you feel comfortable..

 

At least attempt to preform the appropriate calculations for your boat and it's rig. Small boats need to be well thought out or they end up being overly heavy and out of balance. The extra weight reduces carrying capacity and kills performance ability.

A good guide for sail area is a percentage of the wetted surface. Assuming generally normal hull proportions for your 11' 6" boat, you'll look at 2.2 times the wetted surface as a trainer, with 2.4 and 2.5 offering better preformance when you gain experience. With the sail area amount in hand and a few other "figures" you can easily calculate how big and thick a mast you'll need.

So, assuming about 32 to 35 square feet of wetted surface on a 11' 6" boat, you'll be looking at 75 to 85 square feet of sail area. Also assuming a low aspect Bermudian sloop rig, with a masthead jib, a 10' tall stick, 2" mast heel, with a continuous taper to 1.5" at it's head will do, with a reasonably safety margin. If it's hollow birdsmouth built with a 20% stave thickness, then it'll weigh about 3 pounds if Sitka and about 4 pounds if Douglas fir.

If it's a gaff sloop, then increase the heel and head dimensions by at least 1/8". If it's a cat then another 1/8" more.

 

off topic here...just joined but cant see how to start a thread. can someone direct me to the new threas button?
thanks

 

kenbker, welcome to the forums. When you are viewing an individual forum such as http://www.boatdesign.net/forums/boat-building/, at the top and bottom of the thread list you will see a blue button "new thread" to start a new topic.

 

thanks muchly

 

Standard for small punt or stubby sailing skiff with sprit rig is mast will just fit in the boat, so 11'+, sprit the same length or a bit shorter. Mast dia rule in old days is 1/4" per foot of length so 11 quarters or 2 3/4" at the thwart or partner, 20 percent less at heel and head or say 1 3/4". This is for a round, easily rotated, mast of solid fir, picked out of the lumberyard very carefully for best piece, fewest knots, lightest weight.
Mast as far forward as you can get it, say 10% back from bow.
Leeboard of simple Peruvian type with no hardware, or a center board or dagger board. Stay away from leeboards that use hardware as it is not necessary.
I have rigged many skiffs and punts with the above formula and it's usually about right.
No boom, almost rectangular sail, close to vertical leech for Scandanavian style, or as the picture for US workboat type.

 

I just finished a dinghy a little smaller than yours. It's 7 ' 10" by 48". The mast is 11 feet with a sail area of 23 feet. Admittedly that is a postage stamp sail. it could have been twice or even three times the sail area. To see the building of Sunflower see http://newboatbuilders.com/pages/Dinghy.html



The plans called for pine or fir, 2 X 2's. I used clear cedar. (clear means no knots) I bought two 12 foot 1 X 2's and glued them together. Then I cut off the corners until I had eight sides and then planed it down to approximately round (eyeball round). It is about 1 5/8 in at the bottom and tapers to about 1 inch at the peak.

For the sail I wanted to keep it simple too. No high tech here. I bought 4 yards (way too much) of heavy nylon. I made a pattern using a cheap plastic tarp and used that to cut out the sail. This sail has no halyards. I sewed a tube into the leading side, closed at the top, and just slide the tube over the mast like putting on a sock. I sewed a length of line to the tack which gets tied to a cleat on the mast, and a length of line to the clew which goes through a hole in the end of the boom and gets tied off. The bitter end is the main sheet for the sail.

This was pretty simple. If I had wanted to I could have done a hollow square or a birds mouth, but for this little boat that is overkill.

The boom was made of the same clear cedar.

 

Seems to me you need to decide what kind of sail you are going to use before attempting to decide how long the mast will be. A lateen sail will have a different mast length compared to a Lug or a Sprit sail and a Bermudan will take an even taller mast... all with the same sail area. I go along with PAR in that you need to determine what your sailing experience is, determine from there how many square feet you feel you can handle and figure out what style of rig you want...then you can determine how long the mast will be. Once you determine the size of the sail and what type then you should look at what kind and how large a keel you should use and where to locate it.