mast sizing?

[PlPhillips]

hello
thanks to all who lookd at and helped with the triandle 20. now it is time to think about puting this thing togher. can any one tell me weather it is best to size the mast by boat lenght or displacement? and how ? she is 20' and displaces 2,300 lbs.
thanks
paul

[SlowButSteady]

The fancy answer is to calculate sail areas, lever arms, righting moments, etc., etc. But, as a general rule, boats of this size have masts a little longer (10-15%) than one boat length.

[okawbow]

Originally Posted by PlPhillips

hello
thanks to all who lookd at and helped with the triandle 20. now it is time to think about puting this thing togher. can any one tell me weather it is best to size the mast by boat lenght or displacement? and how ? she is 20' and displaces 2,300 lbs.
thanks
paul

I have a bristol 19 that is 19'6" long, and displaces about 2700# with a 2'11" draft. The mast is 26' tall. It used a Kenyon "D" mast section. (same as a Cape Dory Typhoon.) I would think you could use that as a guide, depending on your keel depth and weight. It would also depend on where you sail. For inland lakes, a tall mast is good. For San fransisco bay, a shorter mast would be wise.

[SlowButSteady]

Originally Posted by okawbow

I have a bristol 19 that is 19'6" long, and displaces about 2700# with a 2'11" draft. The mast is 26' tall. It used a Kenyon "D" mast section. (same as a Cape Dory Typhoon.) I would think you could use that as a guide, depending on your keel depth and weight. It would also depend on where you sail. For inland lakes, a tall mast is good. For San fransisco bay, a shorter mast would be wise.

At first 26' sounded really tall, until I realized that a Bristol 19 is a fractional rig. The rule of thumb I used in my earlier post was for masthead rigs. A fractional rig will probably be another 10-15% or so; in other words, the mast would be a boat length plus 20-30%. With that shallow a keel, I wouldn't use a mast too much more that a boat length, however. Also, keep in mind that a shorter mast will be easier to raise/lower and transport.

[okawbow]

I looked up the triangle 20, and using their picture, it looks like they used about a 24 1/2' mast. Their rig is a fractional, with a small headsail. You can print out a picture and compare the known length of the hull with the mast, to get close to the right length.
TRIANGLE 20 Sailboat details on sailboatdata.com

[Jeff_H]

Assuming that the drawing that you linked to is drawn correctly to scale (which it appears to be), then the mast appears to be 27 feet 6 inches from the top of cabin to the top of mast. In terms of spar section and detailing, you really should not wing it, even on small boat (too heavy a spar section and the boat will be tender and will not spill off the wind in a gust, and too light a spar section and it will end up over the side).

Also if you have not built a spar before, especially a fractionally rigged spar with jumper struts, you should talk to someone who has. There are critical details involved in terms of how the shrouds attach to keep the mast from twisting or crushing.

In a general sense, fractional rigs are typically 1.3 to 1.4 times the length of the boat above the waterline. Mast head rigs are more like 1.25 to 1.3 above the waterline. But of course both vary widely with each specific design.

Jeff

[PlPhillips]

it all sounds good.
would deck height off the water line play into the height and length of the mast? I think the same size mast on 2 boats will act diferntly given a difrence in deck/cabin hight.

[Jeff_H]

When you are designing a boat, you use rules of thumb and your objectives for the boat to set sail area, mast height and sail plan proportions. When you are dealing with an existing design, you typically want to get close to the original unless you really know what you are doing.

It occurs to me that you must still have the mast step attached to the boat. In those days, most production boats either had Dwyer or Kenyon spar sections (a few had Proctor). You should be able to carefully measure the mast step, and then go to the Dwyer or Kenyon webpages and figure out what the spar section was. The rest is pretty easy.

Jeff