Questions about Wishbone rigs

Seeing the work some designers have done with unstayed masts and wishbone/spirt rigs has stirred my interest. However, I am finding it difficult to find information about what is the proper relationship (angle) between the mast and the wishbone boom. Equally elusive is how the running rigging is laid out.

Is there a book or technical paper that would provide answers? One person suggested going around to marinas and looking at boats, however, I not very good at climbing over gates or running from rent-a-cops.
Thanks for any guidance.
Bruce

 

The angle between mast and boom is dependent on the way the sail is designed. A sail that has a steep rise at the clew end will use a boom that angles downward from mast toward clew. On small boats there are some clever advantages to this arrangement. By moving the boom up or down the mast, you can control leech tension and if using a snotter you can control draft position and depth. Vangs are not necessary, so that bit if simplification is welcome.

Unstayed masts may or may not use running rigging. It seems that runners defeat the purpose of unstayed masts. If you step and or unstep the mast with each use of the boat, as in trailer borne boats, then the wishbone can be a slight pain. The reason is that the mast must be inserted thru the inside of the wishbone and the WB is in the way while you are finnagleing the mast into its step.. You can get around that by using a half wishbone but if you do it that way you will need a lazy jack to hold the half bone in the desired position. Lazy jack is usually not a problem. You can also use a straight boom on one side of the sail, which is a very simple and cheap option. The sail will pull better on one tack than the other but not as much difference as to eliminate the possibility of a striaght boom.

Free standing masts, that are allowed to rotate, are good for several reasons. Not least is that the sail can be freed to swing forward of the mast or even weather cocked such that the boom is over the bow. A huge convenience in some cases. All this depends on the size of the boat and its sails along with the intended use. Small boats that are cat rigged have a lot going with this method. If there are to be foresails then I see little advantage in wishbones. And if you plan to have fore sails then running backstays will surely be needed

 

And one more thing.......If the sail is to have a high cut clew, there is a potential problem as follows. The high clew is tempting in that the foot of the sail is less likely to interfere with the crews head. The wishbone will also, out of practicality, be sheeted from the outboard or clew end. That means that the sheet will exert a larger than normal downward force on the end of the boom. That can cause leech tension problems that can only be cured by a topping lift of some kind. Excessive leech tension can cause unanticipated mast bend too. Keep in mind that the wishbone will be located up the mast somewhere. The bone exerts a forward pressure on the mast at the snotter location. Since the location is higher than on a conventional rig, there are increasing beam load considerations that must be addressed when designing the mast. Make some sketches of this layout and you will see what I am trying to explain.

 

Again Bruce, it would be helpful if you where a little more specific. Have you picked up a Freedom missing it's wishbone and are looking to make a new one?

 

Presuming that you also do away with the boom vang, the angle that the wishbone boom makes will be the determinant of the relative amounts of foot and leech tension. I would then expect that a high aspect rig would require a more vertical boom and a lower aspect rig with a high clew could use a lower angled boom.

 

Wishbone Rig

The idea I have in mind, for a wishbone rig, is similar to what we see wind-surfers using and the very successful rig Par seems to have in mind used by Freedom yachts or the beautiful Hinterhoeller Nonsuch cat boats, but that can be misleading...

http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999.html

The Hinterhoeller/Freedom yacht rigs seem to be a good choice for general apps, but I am a sailor, not a NA, and look to others for guidance. Personally, I like them.