In the 25th anniversary issue of the Multihull magazine in the year 2000, there was a real good article about Goran Marstrom and the Sail Centre of Sweden. In that article, Goran was quoted as saying the optimum wingmast shape for aerodynamic purposes is a 4 length to 1 width ratio. Wouter kind of brought this legnth to width issue up in the posts on the SC20TR, so I figured I would see what he says about this.
In looking at what to do for a wingmast on an A-Cat, I have been toying with the idea of making a mast so have drafted out a 1 meter NACA 64020 shape (Half of the 20% foil width would be the optimum 10% sail camber) on paper. An interesting side note is that the mast sail combo measured 1.04 meters around the outside of the sail which I considered to be a very small gain in legnth for a 10% width change and back. Anyway, one thing I read somewhere indicated that to get the proper mast shape, you draft out the foil shape and then take the desired percentage of the front of the foil shape and mirror the shape on the other side to get the optimum shape. I found that there are a couple of considerations that need to be addressed to fix problems with it, but if you do what this guy says for 10 percent of the foil legnth, you get a foil shape that is about 3.6 inches long to nearly 1 inch wide or the 4 to 1 ratio. A mast of these dimensions would be real flexible sideways if we actually used this method, but it would be the correct aerodynamic shape for the front of a foil shape. Almost all of the sailing books I read specifically talk about the front of the foil as being the single most important part of the foil as a whole to avoiding early flow seperation. Thus the front of the mast sail combo should be the most important element too.
Currently in the A-Class, we are using a mast that is untapered and is 5.75 inches in chord by 2.375 inches wide sideways for a 2.43 ratio that is the same as the mast on Wouters boat. When looking at the percentage of the chord of the whole sail mast combination, my current mast is about 7% at the bottom to about 21% at the top depending on the sail. It is my opinion that this introduces twist into the sail as a matter of course in the top of the sail, or may just introduce an increase in sail camber, and poorer aerodynamics as the sail has to make too sharp a turn near the top of the mast. At any rate, it has been interesting to consider, and would sure be interesting to look at research to see how a mast of continuous 10% legnth of mast sail combo would rate against an untapered mast sail combo.
I have actually started a mold to make one of the 10% chord tapered masts, but am reconsidering this as the top of the mast would have very little buoyancy for those times that I push just a bit too hard.
I have actually heard that John Lindahl had built a tapered mast that was 25% of the sail chord. Now that is interesting. One book I read stated that the sails get more efficient as the chord of the mast increases as a percentage of the total sail area. I bought a Marchaj book expecting to read that in there, but it was not there that I read. Maybe I purchased the wrong book.
Anyway, what opinions do you guys have on this?
