View Full Version : Sprit Boom for a Trad Spritsail
05-27-2003, 09:09 AM
I want to experiment with a sprit boom on my 86 sq ft spritsail. Question is, where should I place the attachment point for the boom on the mast? Thanks, Dennis
05-27-2003, 11:35 AM
I've seen snotters placed at various heights on the mast. Two things that enter the equation are the angle of the sail at the clew and the stiffness of the mast. The more acute the clew angle, the higher the snotter must be to give adequate vanging of the sail. The higher the snotter, the more the mast will bend in a gust.
The usual answer is to tie the snotter to the mast with a rolling hitch and experiment with the height. In any case, it is best to have the snotter set up so it can be adjusted underway for different wind strengths. This means that the loose end should be cleated low on the mast or led aft instead of secured to the sprit where you could be swept overboard trying to adjust it.
05-27-2003, 12:07 PM
The heel (forward) end of the sprit-boom should be higher than the aft end, but I'm not aware of any specific set of parameters, so a little experimentation may be in order. I might tend to start up about as close to the heel of the regular sprit as I could get without the two fouling each other in use and see what happens. That would be the longest possible configuration for the sprit boom. If you then decide that it works better moved down on the mast, it can always be shortened as needed.
I would also do my best to make sure that the boom's clew end can be securely attached to the sail so that it can perform the function of holding the clew corner outboard, off the wind, without having to run a lot of tension on it's snotter to keep it from coming loose at the clew. It's quite possible for sprit-boom tension to pull all the draft out of the bottom half of the sail, which isn't generally desired. A lot of tension on the sprit-boom's snotter may also form a crease in the sail from the tack corner to the clew corner and may leave whatever foot round the sail has hanging loose below the crease as sort of a useless flap. In most conditions, you probably want to maintain a draft across the lower portion of the sail equal to 9"-12" of draft for every 10' of horizontal width (chord width) and adding too much tension to the snotter will eat that up fast.
The other variable may be mainsheet angle. Whether the sheet is attached to the tail of the boom or just to the sail's clew corner, you essentially have end-boom sheeting and it's on a boom that is free to float forward and aft a bit at the mast, rather than being deadened with jaws or a gooseneck (especially if the sprit-boom's snotter is left a bit slack to maintain some draft in the sail). This may not be an ideal formula for creating good sailshape and may be one reason that a boathook or whisker pole clipped to a fitting on the mast end may actually work better on a regular spritsail (though a jaw or jaws on the forward end of a sprit-boom and a snotter just to keep it from sliding down the mast might be an interesting idea).
Anyhow, there are a lot of possibilities to explore and as far as I know, not many rules of thumb.
05-27-2003, 12:32 PM
The higher the attachment point, the tighter the lech relative to the foot of the sail.
My Shearwater Yawl has a pair of log-o-mutton sprit sails. The foot of the main is horizontal, and the snotter block for its sprit is fastened roughly 155 to 18% of the total luff length up from the tack.
Mt mizzen has perhaps a 10-degree angle at the foot (clew higher than tack) with the snotter block attached 25% to 28% up from the tack.
The mizzen is mostly a riding and trimming sail, so its leech is tighter in relation to the foot than the mainsail.
Your sail should not require a block. My Mizzen could get along very well without one. My previous Dovekie has a single-part snotter, though some Dovekies have two-part snotters. The main on my Shearwater has a two-part snotter.
Good Luck, Moby Nick
05-27-2003, 12:58 PM
Tom, Todd, Moby -- Thanks for the replies. They are greatly appreciated.
05-27-2003, 01:46 PM
Sort of like this
You can imagin how the mizzen would set.
05-27-2003, 09:24 PM
Don't let this get too complicated because simplicity is the real plus about the sprit boom anyway.
There is one other thing that was alluded to but not really mentioned. Fix the mainsheet to the clew and not to the sprit. That way, when you take off the sprit, the sail is still captured and under control.
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