Jib question

I agree with what you say. GP boats do have narrower sheeting angles than in the old days. There are a couple of reasons for that. First, the boats are much more easily driven (lighter, straighter buttocks, narrower entries), so they don't need the more powerful, fuller sails. Second, they are sailing faster relative to the true wind, so the apparent wind is much more forward.

To clarify about the I14 comment, the sails are still the same fullness and AOA as the sails on a conventional I14. If the board can gybe at 4 degrees this allows the hull to rotate the 4 degrees to match the true direction of travel (hull now zero AOA with less drag, board still 4 degrees AOA and still creating the same amount of lift). The tack is now some inches (say 3) to leeward of the position of a normal rig. So to keep the same AOA of the jib the clew would have to also be that same 3 inches to leeward.

The sail size and shape is the same, as is the AOA. The "more powerful" thinking seems to come from the more open slot. As I say, there was a good amount of disagreement about this.

 

Thanks Paul, I'm no expert in this subject so I rather follow the discussion (thou I can draw my vectors). Moving jib tack to lee would IMHO mean as Mikko and Eric noted, reduction in aero drag component. There's also some more total force from the jib, anyway for a ballasted mono (moving tack closer to vertical there's less vertical aero force) For me, I'm not a racing so I keep my current heading, this means a bit more speed and a bit less leeway ie better vmg.
BR Teddy

 

I would love to see the vector diagram you have drawn that supports your claims.

You should be careful about putting words in other people's mouths. here is the actual quote, and it seems to be quite different from what you are representing:

 

Would be interesting to read Aerorig users interpretation of Mr. Sponberg's claims, since what they do is exactly the opposite.....

I would, however, like to mention - for history - the early "Moth Europe" designed (and raced) by my colleague and friend Jacques Fauroux as an example of "hull does not follow sails", although sort of the other way around what Mr. Sponberg describes.

His boat had been built using one single sheet of plywood, giving a shape similar to what you would get by making a fold on the center of the short side of a sheet of paper, thus creating a "bow", and leaving the rest of the sheet making a "hull". The shape obtained was somehow comparable to the hull of an Open-60. This boat was sailing, just like what these big racers do, along a line at an angle to its center-line the moment it would heel, but Jacques' trick was to have his centerboard articulated (vertically) around it's aft edge, thus having the leading edge moving to windward, with a controllable angle of course. I am not sure if my description enables anyone to see the picture....
The fun thing is that, when pointing to windward, the hull was actually looking like falling off the wind while he was simultaneously letting his main-sheet traveler go all the way down, but actually pointing very high, this with his boat heeled more than others.....a bit confusing for those sailing besides him!
Well: he won the World championship three times in this series, the first time with with that "one sheet of ply" boat he had designed and built at the age of 19...World championship won by his sister on the same boat the year after!

 

Most of the discussion has focussed on the jib but surely it is the main that is critical.
If the hull is pointing 10 degrees higher the mainsail will need trimming in by a similar angle. Possibly the end of the boom will now be to windward of the centre line of the boat.
In any event the forward component of the forces on the mainsail will be significantly reduced.

 

As I understood Mikko's opinion it was merely a doubt of the amount of the reduction in aero drag, not the reduction itself. But maybe he can comment that himself..
Nobody has commented about the reduction of vertical aero force which IMO is bigger gain than in the aero drag component.
BR Teddy

 

Why is there a reduction in "vertical aero force"?

 

Becouse (in a ballasted mono hull) the tack is more vertical (though tack moved to lee) and so the forces affect more in the horizontal plane.

 

if you change a overlapping foresail to a tight sheeted jib the boat will go higher without re trimming the main.
The boats that sail the closest/fastest to the wind can only do this without a foresail.
I think Eric's work is/was focused on cruising boats not race boats

 

I think you are correct. If you simply draw the sailplan on some acetate (or on a different layer on the computer) then overlay on a sketch of a hull it becomes clear. When the hull is rotated to windward both the main and the jib are now trimmed "leech up" relative to the hull.

I was out sailing last Thursday on my boat. It is a big main/small jib so it sails well under main alone. So while sailing under main alone I pulled the traveller all the way up (right around 10 degrees). I then pointed higher by 10 degrees, which realigned the mainsail to the same AOA and trim as before. This simulates the trim in the original claim by Eric.

I can say there was a distinct reduction of performance. Anyone can try this and see the result.

 

You are correct about this, but what is the extent of the gain?

Consider a 50 foot "I". For each foot you drop the tack to leeward you gain one degree of vertical angle.

So if you are sailing your 40 footer at the typical 15 to 20 degrees of heel your headstay will be seeing 14 to 19 dgreees.

 

Gain depends on the boat. In my case 35'er ketc, masts 'bout 30' from the keel, lugger typical heel 20 to 25 deg and tack dropping would be 3'. So I'm dropping both lugs, not the jib.
BR Teddy

 

OK. I guess I was confused by your mention of jib in your earlier post.

I did notice your sketch shows the clew is also set to leeward, as some of us have mentioned. So this is obviously not in line with Eric's theory.

This may help your lug rig. The only thing I know about lug rigs is I have no interest in them.

You have still not explained how this will improve leeway and VMG.

 

If I get a bit more drive from my sails (without bigger increase in the heeling force) => it means more speed => more produced lift from the keel => less leeway
And less leeway + more speed = better VMG

 

I see that there is an ongoing tendency here to consider sails isolated from the rest of the boat, probably because it is simpler to visualize mentally just the forces from the sail.
But there is also the underwater part of the boat which has to be considered, and the equilibrium of forces and moments which have to be satisfied, which in turn depend on both the sail/keel AoA's and on their speeds relative to the fluid.
Any modification of the trim of the sails will also modify the hydrodynamic condition of the keel,which will modify the hydro drag, which will modify the speed, which will modify the aerodynamic speed vector, which will modify the force and moment form the sails, which will require a new trim, and so on... until a new equilibrium of the whole boat is found.
Clearly, it is not a trivial problem and hence no conclusions can be drawn without considering the sailboat as a synergy of its aerodynamic and hydrodynamic parts. Sails are just one half of this story.