Genoa's & the Slot Effect

[Loren Beach]

Greetings,
While considering the addition of an easy-to-tack headsail for our boat, I had a nice talk with the local North loft mgr yesterday. I admit to being influenced by my short sail on an Alerion 28 last April while at Strictly Sail in SF.
Self-tacking is... Ahhh.... Wonderful.

Having one headsail, a 135%, for our masthead rig boat for the last 9 years, we do tire of winching. Whenever the wind gets over, say, about 12 or 14 knots, we feel that we could get by with a smaller jib.

What I was told is that while the lure of self tacking is seductive, what I would lose is the slot effect that makes the pointing significantly better, even with a small overlap such as with a 105% or a 110%. Proportionate to the whole rig, the larger that the main is, the better one can get by with a self-tacker, was the gist of the advice.

The rig on the O-34 is "ballanced", i.e. the J= 13'3" and the E= 13'6". Our main is a significant part of the driving force, but still much less than on the fractional Ericsons's like the "+" series or the E-33RH.

I know that we have some thoughtful sailors checking in here and a sail builder or two, as well.

Any comment on the 'Save the Endangered Slot' argument?

Class? Bueller? :nerd:

Thanks again,
Loren in PDX
Olson 34 Fresh Air

 

[Seth]

Semantics?

I would hate to take issue with a representative from the Blue Menace aka Evil Empire(, but would not say that a non-overlapping headsail eliminates the slot effect, if by slot effect you/he refers to the phenomena of the air "leaving" the leech of the headsail feeding or flowing onto the backside of the main, accelerating the air on the back of the main relative to the air on the windward side of the main-making the mainsail more efficient (more lift) than it was before the exhaust air of the headsail was added (for example by trimming in the headsail).

Thus, I have always thought that the total lift produced by sailplan was more than simply the sum of the individual lift produced by both sails. It was the lift of the headsail, the lift of the mainsail, plus the "boost" added to the mainsail by the presence of the headsail.

It is true than a 135% will produce more lift (assuming the right wind range) than a 105%, but I think the effect is still there-just that main + 135% makes more than main + 105%.

Also, on modern boats, we go to a blade headsail specifically in order to point higher. A high aspect headsail (small overlap) is generally more efficient than a low aspect headsail (larger overlap)-again, assuming the proper wind speeds.

The best example is right at the crossover point for going to a non-overlapping sail from a genoa, somewhere near 14-16 knots upwind.

If the water is flat, and you don't need the extra punch of a big genoa to get through any waves (which slow you down), the good boats will go to the blade right away, since they will point higher. Once you are past the crossover windspeeds, you need the little sail all the time so as not to be overpowered, but if you are in that grey area and it is choppy, the good guys hang onto the genny just a bit longer because the waves are slowing them down to the point they are better off giving up a bit of height to maintain boatspeed...

Sorry to ramble-I have about 10 projects going on right now-did this help at all?
Happy T-day all!
S

 

[Nigel Barron]

Big main, small jib. Debate is over. I agree with Seth on everything he said. To add to it though, even with a blade or jib the simple addition of inhaulers will negate any lost point. Also, even with a jib you can still get drive through waves by easing the sheet and twisting a little. Certainly not as powerful as a genoa. Look around at the performance boats out there these days and look at their sails to see what they are doing. There are not a lot of boats with genoas anymore. Just my 2 cents.

 

[Loren Beach]

Maybe 100% is enough, after all

Nigel,
What with the E-27 rig ballance having some IOR influence, and the smaller main that goes with it, I take it you have made the blade jib idea work well with your boat.

And Seth seems to be encouraging my heretical thought of putting a "Euro style" curved traveler in front of my mast for a self tacking jib...
I mention the track idea because the Hoyt jib boom would seem to clash with our foredeck anchor locker and it costs a bundle of B.U.C.'s, as well.
Maybe there is hope for a full-hoist 100% blade for our boat...
This will be an interesting project for the winter of '07-'08. The "track" could be SS, or even composite, as I envision it.

And please do keep the comments coming!

Thanks,
Loren in PDX

 

[Seth]

pointing with a blade

Nig makes a good point about the inhauler, and it DID make me think of ONE possible reason the idea was floated that one might not point as high with a blade as with the bigger genny: on the more modern e-boats and similar types, the sheeting angles for the #1 and the #2 genoas are closer than the standard position for the blade (front end of the inboard track)-so, without some effort to improve this, I could buy that argument..But, the more serious race boats will almost always either add inhaulers, or install a blade track closer to centerline to improve pointing.

So, I would have to say, that yes, a 135 on the inboard track MIGHT have slightly better height than a blade on the rail, or on the front end of the inboard track, or otherwise not coaxed inboard.

BUT, unless you are racing against a boat who DID go to this effort, I doubt you would notice the diff-Hoyt track or otherwise.

Enjoy little jibs if it fits your profile!

S