gap between mainsail luff and mast

[dionysis]

I was reading in Pierre Gutelle's "The Design of Sailing Yachts" that there is virtue in having a gap between the mainsail luff and the mast. He thinks that this will increase the lift some 15%.

Profurl also recommend this approach. Here is the link:

the slot effect

They consider a 15% increase in power as well.

Has anybody got any thoughts on this idea?

Interestingly, this idea is going around the very competitive IOM (Internatinal One Meter ) model raceboat class. The theory applied to models says about a 5mm gap for the luff which is about 60". The rumor has it that this came as a result of some computer based analysis done during the last America's Cup since several IOM model sailors also worked for AC syndicates.
I'm still looking for more detail...

[dionysis]

Yes it seems like a promising avenue.

The models proportions works out to be about a 3 inch gap behind a ac class yacht.

This seems to be very small, so I don't think the gap scales linearly with size of sail. I wonder what the actual theory is?

You could model it in say a multi-element aero code like MSES. This would give a good indication of the aerodynamics involved.

Not much on the net about it though.

cheers, dionysis.

[Tim B]

It looks like they're just moving the sail into clean air. The leading edge on the sail now gets critical. I suspect, that it will improve performance off-wind, but it will pay penalties over a narrow mast upwind. That is to say, that the advantage gained is a function of the angle of attack of the mast and sail. I would consider also, the effect of main sail chord, since the flow may attach better at wide chords, but worse than it did before at the mast-head (often it is the top part of the sail where most power is generated). You should be careful, though not to confuse this with the slat effect, which is the interaction used between jib/main and on jetliner wings.

Cheers,

Tim B.

[Stephen Ditmore]

It was tried by the French America's Cup challenge in 1995 (I think). It was only beneficial if the measurers did not measure the open space as sail area, but I think they ruled that the open space was to be measured as sail area.

[dionysis]

If the gap was large enough to allow the luff clean air upwing it would have to be very advantageous. The lift to drag ratio of a jib compared to a main is all the proof you need.

The question is though, how to engineer the mast to act as a bow, that is, to take the large bending moment so as to keep the luff straight.

The French AC effort sounds interesting. It would seem that the effect is not so dramatic as I would think. I will look into it. Thanks for that Stephen.