Trimaran Rigging Question

[Ryerson]

I've got a Cross 34 R/C that was stretched to 38 ft. during original construction. The boat was built to be in the trade winds and has only a 40 ft. mast. I'd like to build a mainsail with a lot of roach for the boat and increase performance in light air. The boat has the Cross cruising fin keel and draws only 3.5 ft. Here's what I'd like to do: I'd like to move the twin backstays to the positions used on a three point rig. I would be able to get rid of the regular shrouds that come from the masthead and lose a bit of weight aloft and would still have forestay, staysail stay, 4 lower inner shrouds & running backstays, in addition to the shrouds in the new location. My question is, how far aft of the mast do I need to put the chainplates for proper support of the mast and will the fact that the boat has a masthead rig interfere too much with how far the sail can go out when running off to make it practical. I realize that there will be some added weather helm because of the added area but because of the longer fin keel I think it will be minimal.

[RHough]

Originally Posted by Ryerson

I've got a Cross 34 R/C that was stretched to 38 ft. during original construction. The boat was built to be in the trade winds and has only a 40 ft. mast. I'd like to build a mainsail with a lot of roach for the boat and increase performance in light air. The boat has the Cross cruising fin keel and draws only 3.5 ft. Here's what I'd like to do: I'd like to move the twin backstays to the positions used on a three point rig. I would be able to get rid of the regular shrouds that come from the masthead and lose a bit of weight aloft and would still have forestay, staysail stay, 4 lower inner shrouds & running backstays, in addition to the shrouds in the new location. My question is, how far aft of the mast do I need to put the chainplates for proper support of the mast and will the fact that the boat has a masthead rig interfere too much with how far the sail can go out when running off to make it practical. I realize that there will be some added weather helm because of the added area but because of the longer fin keel I think it will be minimal.

Get out the geometry text.

You can calculate the loads on the rig as it is. Then move the stays to the new positions on paper and see if you maintain the loading on the mast. If the mast is still supported properly, all that remains is to analyze the structure of the boat to see if the hull will take the loads at the new attachment points.

If this is beyond your ability, take some sketches to a N.A. and have them do the math.

My guess is that the boat's structure was designed to take the loads where they are and will not be happy with those same loads transferred to areas that were not designed to take them.

[Ryerson]

Yes, thanks for your suggestion, a Naval Archetect would be better at the math approach than I would for sure. Attaching chain plates to the hull in an area without bulkheads would definately concern me. If the right geometry for using the new main required having the chainplate in an unsupported area I'd back it up between the stringers (wood boat, double diagonal construction) with 3/4 ply with a cap of 3/8 above the stringer. (stringers are 3/4) All WEST System. An alternate way to do it is seen in some of the Kurt Hughes boats where he has a shroud coming to a tri plate and a wire going to both forward crossarm and aft crossarm, thus putting all the loads on the crossarms and avoiding compression at the shearline. Basically I tend to overengineer things as a cruiser who likes performance rather than a racer who occaisionally cruises. I'm kind of hoping someone else has done this change in a similar boat.