Sail Interaction

[PI Design]

Does anyone have any feel (or knowledge) for how useful the part of a jib below the boom height is?

On the plus side by bringing the jib down to the foredeck, the endplate effectively doubles the aspect ratio and thereby reduces induced drag. But does this really work out in practise? There are not many designs where there is absolutely no gap between the jib and the deck along the whole length of the foot.

On the downside, the part of the jib below the boom is working in isolation, and is not helping the flow over the main.

In classes where individual sail shapes and sizes are free (within the confines of an overall max area), could there be an advantage to removing the part of the jib below the boom and adding that area elsewhere?

I know it's one of those "it depends" questions, but is the trade off between lower jib aspect ratio and having the sail area in a more useful place worth it? The Dart catamarans have a high jib foot, but I can't think of any dinghies that do.

[Tim B]

With a Catamaran you're trying to get the sail area into clean air. With a monohull the jib can seal against the deck as there is less turbulence at the deck (than a multi), and yes, it definitely does work in practise.

It works particularly well of dinghies where they can be sailed flat.

Cheers,

Tim B.

[gggGuest]

The question got answered in the 60s when *every* dinghy class moved their jib as close to the foredeck as they possibly could within their rules. Before that it was routine to fly the jib on a short strop so that it overlapped the main more.

[Mychael]

With my 26foot keelboat. Using Genoas, I fitted strops to raise the foot of the headsails between 1 foot and 2 foot. Depending on the sail I have up at the time.
It leaves the sail free from fouling on the safety rails and lets it sit better. So in this case would my setup be a better trade-off then a "seal against the deck"? Or is this thread more in the context of dinghy's.?

Mychael

[PI Design]

Thanks ggggggg, I didn't know that. That's exactly the sort of thing I was thinking of, but shan't bother now!

Mychael, I was thinking in terms of dinghies, but the factors are the same for keelboats, just with the additional factor of fouling the guard rail.

[Crag Cay]

No there are other considerations with keel boats, fouling the guard rail being the least of them as they usually set inside the rails:

But one is visability - especially for cruisers or short handed sailors who don't have the option of having someone constantly keeping an eye under 'under the genoa'. The next is shedding green water. There is a picture of Velsheda around at the moment with her (very expensive) deck hugging genoa completely blown out when she took green water over her bow. And finally, there are trim and set issues; deck huggers 'twist off' badly when you crack the sheet and don't set well if reefed with a roller without adjusting the cars. So to get the best from them you ideally need fully adjustable fore and aft and athwartships tracks and cars to control a deck hugger. Lift the clew and you trade off a little efficiency against better safety, durablity and versatility.

So even on big offshore boats, only the light #1 was truely deck hugging.

[PI Design]

Good points Crag. That'll teach me to think before typing!

[Mychael]

That' true Crag. I've had my 120% Genny dip under the water and act like a scoop. Also I have to run it outside the rails as it would otherwise foul on my shouds. Tacking the larger sails requires some care and you are totally blind when they are up.
Even with the Strop it still interfears a bit with the safety rail, I'm thinking of getting the foot modified to give me some more clearence.
I have adjustable cars.

Mychael

[jim lee]

deck huggers 'twist off' badly when you crack the sheet

I can attest to that. My machine uses "Deck huggers". They set wonderfully beating to weather. As soon as you crack off the're just a mess. But.. when cracking off, it doesn't matter so much does it?

-jim lee

[Crag Cay]

Here's the photo of the moment Velsheda's owner started to reconsider the advantages of the deck hugging foresail.