mast location

I'm building a small trimaran using an Inter 20 hull for the center hull. I have shortened the hull and would like to attach the crossbeams and mast base.
Is there a formula for locating the mast( how far from the bow and/or stern)? The mast will sit on the front crossbeam.
Thanks!

 

I'm not a designer so my terminology may be off and since no one else is responding . . .

There isn't so much a formula as how you want the boat to perform. If you do or do not want the boat to have lee or weather helm, then you put the mast so the center of effort of the sail is aligned over your centerboard/daggerboard. If the center of effort is in front of the board, you will have lee helm and the boat likes to head down wind and steering can be difficult. If it has weather helm, the center of effort is behind the board and the boat likes to turn into the wind and stall and steering can be difficult.

Sorry you get a schlep such as I to answer your post.

 

Thanks! I have only an empty hull, as of yet. The daggerboard has not been installed yet. The board will be installed after the crossbeams and the mast. Thanks for the tip on lee and weather helm.
I guess I wanted to know how factory boats are assembled, in order to be in line with their placement of masts.

 

You would be best advised to make a scale drawing of the rig and place it over the available lateral area center in an optimized way. In short, you should have a plan as, taking a wild swing will just cause you to cuss a lot on launch day. The location of the CE in relation to the CLP can vary by as much as 20% of the LWL, so it's fairly important to get it right.

Contrary to advice given, you'll want some weather helm, for two main reasons: the first is if you expect to go up wind reasonably well and the second is a safety feature, so the boat will round and luff if the helm is released (like if the skipper falls over board while sailing solo).

There can be several variables to CE placement over the CLP (and yes there are formulas) so you can do some study, take a guess or have someone pen up a reasonable placement for you. If you have accurate drawings of your appendages and hull, plus the sail plan, this is a simple and often requested task of designers. It doesn't cost much and you can launch with the assurance, it will good manners during sea trials.

 

For a quickie intro into what I alluded to and what PAR is answering, try:
http://www.jimsboats.com/15oct10.htm

For another quickie intro for lots of other info, John Teale's book How to Design a Boat is a handy reference.

I would still go with PAR's recommendation of seeking a professional for some extra guidance. Otherwise, prep your moaning chair before getting your boat wet.

 

Correct placement of the mast
http://www.faze.co.za/Little Tri/Little Tri 4.html
http://www.youtube.com/user/faniefaze?feature=mhum#p/a/u/0/pEYyCq92mnU
The daggerboard was just ahead of the centre of force.

 

Michalak's technique of placing the CE over the centerline of the daggerboard works in small boats fairly well, though honestly I've found it'll be a more refined placement if it's over the leading edge of the daggerboard.

On a 20' tri, I suspect skipper and crew placement will have much less affect on trim then the 10' to 15' boats Michalak usually plays with, therefore you'll need to work in some lead. Other wise your comfort level may suffer, because the required to trim crew and skipper placement may not be in the best interest of the available seating.

In short, a dinghy can live with a fairly out of trim design and the skipper can slide his butt forward or aft to compensate. A 20' boat is big enough where you're going to want the hull form, it's appendages and sail plan to balance out nicely or it'll just be "cranky" to sail and probably just suck to windward, to use a technical term.