Magical 7 degrees of mast rake

How come fast upwind boats I have raced ie Cal 20, J22, lightning, Viper 640 all have 7 degrees of rake? I read the artical from the North sails designer "span squared" but why 7 degrees?
Looking back the schooner America and the Baltimore clipper also were 7 deg.
Thanks

 

Holy Figure

7 is an holy figure in many religions and in free-masson esoterism as well !!

Not sure it can help

Cheers

 

Another magic number. Magic numbers seem to be "in".

How accurately did you measure the mast rakes of the boats referenced? Is the rake 7 degrees +- 0.1 deg or 7 degrees +- 6 degrees. The plans I've seen of Baltimore clippers show considerably more mast rake than most boats.

The North Sails Span Squared article has a rather simplistic explaination of why aft rake might reduce induced drag. The factors it claims reduce induced drag are probably smaller than other factors which are not mentioned which would likely increase drag.

 

7 degrees

I used a truss framers pivot square, on the aft side of the mast at deck level. http://chhanson.com/layoutFraming.html#pivotsquare. Boats floating level on the waterline or checked with the designers line drawings.

 

It will be better 4 degree forward. Like the gliders. Boats doesn't goes fast enough to rake the sail plan aft, but forward is helping.
Lister

 

Lister 4 degrees is also an esoteric number - four seasons--four directions brainwaves in meidtative states come in fours and sevens..and the list goes on..7 and 4 are reoccurring numbers in nature.. 7 x 4 is the natural lunar month- might be that 7 and 4 just happen to be the best natural angles for a mast...

 

Why would that be?

 

I am talking about windward performance.
It seams wind tunnel show that around 4 degree will be right angle forward (added for clarity).
Gliders, Piazzo airplane, Manfred Curry and other seams agreeable to this number. Some Star Class even tried with success.

Now your thinking about esoteric number belongs to you and only you. I was expecting more from you. I find your posts generally very interesting.

Lister

 

I did not expect so much esoterics on this rather technical subject.
So, there is my 1.5 cent:
Raked mast is so aesthetically pleasing, that your competitors admire you, loose concentration, speed and pointing.



(this post has nothing to do with my signature about stresses ... )

 

Whoa, I called Dan Brown he said it's all in my book, 7 is the angle of the dangle in Rosslyn Chapel. He said 4 would be right if the boat was level at the dock but the Templars caravels sailed 3 degrees bow down 4 + 3 = 7
Yes, yes 7 is truly the divine number for the procurement of pickle dishes....
Your humble servant McFast

 

The angle of the mast is only one of many other factors, and likely any angle can be accommodated with cleaver design to give the same performance. Small changes make small difference in performance.

So if there is a current trend to use 7 deg, it is just fashion, superstition, or a variation on "monkey see, monkey do" which is the most likely. A boat wins in a class with a 7 deg rake, so everyone else copies it for the next race.

There is no specific advantage to 7 degrees, unless it is part of an overall design trend that include the aspect ratio, sail plan, profile shape, camber, etc.

 

Mandatory aft rake

Aft Mask rake is sometimes mandatory in order to be able to open the mainsail downwind, otherwise the shrouds will push on the sail.

If the sail is a wing, it will destroy it, but if the rake brings the hound just above the shroud attachements on the hull(s), it should make it.(C-Cat are a good exemple of mandatory aft rake)

Schooner America has even more aft rake than the magic numbers above- mentionned. > 10°. It seems to be mandatory aft rake as well.

A bit less esoteric, I guess....

Regards to all

EK