Sail wing tips?

I was wondering in my ignorance why they dont have wing tips on sails to stop the air spilling over or does the air not spill over as much in a sail due to its varying camber.
If you had another small sail at the top of the mast parrarlel with the deck wouldnt this help stop air spilling over the head of the sail?


Alex

 

You are definitely thinking along the right lines. The mast head is a very inefficient area and people are always looking for improvements. Some method of generating an 'end plate' effect is worth considering and was thought decades ago to be the reason that topsails on gaff rig seem so advantageous even when the gaff topping lift prevented the topsail developing any camber. The 'square head' sails first seen on windsurfers and now sailing boats are a way some people are addressing similar issues.

When experimenting, its worth considering that, as in so much of yacht design, it will not be all gains and you will also have to evaluate the balancing costs, such as increased drag, increased weight aloft at its most disadvantage position, increased heeling moment, etc.

Get experimenting!

 

This is my take on the issue of generating an end plate effect on a sail while also creating a bunch of other advantages. There are, as Crag has observed, also disadvantages to running objects up at the top of the sail and I have not had these designs analyzed for that purpose yet.

The benefit has to outweigh the gains you may see for the power of the sail, so I need to look at that reality more closely before producing this product for a test.

I see the end plate system as made from a soft, semi-flexible foam with a cover made from sailcloth. Because it is foam, it has the potential to resist a full turtling capsize on mulithulls, doing away with the quirky, Hobie Bob style, of mast head float. A multihull that does not turtle, is in much better shape for self-righting than one that has gone all the way over and needs assistance.

The foam is an integrated functional part of the sail at the mast head and could be attached quite easily with some of the newer, more tenacious Velcro so it cold be transferable from sail to sail, or removed altogether, returning the sail to its original form.

I see a companion piece on the mast head so that optimal streamlining could be enjoyed, further reducing the drag signature created by the device and creating additional end plate effect.

Just like the tip foils on modern aircraft, there is a size where the drag cost/performance benefit crosses over to the positive side of things.

I'm not an aeronautical engineer, so there are probably a few holes in the concept from a technical point of view, but I think it could work and because it has secondary benefits for rigting, there is a large incentive to see it as an overall improvement to the extra power being generated by the sail.

The illustration of the boom section is intended to show that the whole sail could be given the same end plate treatment, while also providing a much stronger boom section. There's nothing new about the boom itself. It has been done on many boats before, though they did not carry the idea to a full end plate design as far as I know.

Chris Ostlind

 

Well i suppose i could experiment on my rc boat.
On an rc boat i think i can make the boom very close to the deck anyway so the deck might act like an end piece. I really cant see that many disadvantages it would also help like you said if the boat capsized. Obviously it would have to be tight against the top of the gaff.

I am not sure if it would actually have any benefits on a model my scale low reynolds number or something.

Alex

 

In my messing around with this problem, I became convinced that a fenced square top messed up the flow when heeled- that is not adding forward thrust, so I started adding peak to the top of the sail, but then the flow was strange when the rig was upright. One idea I didn't try was a fence that rotated from the luff, automatically aligning itself to the flow direction. Although if you look at model airplane and glider practice with wing end dihedral, they tend (almost insist) to a tapered semi round form. They are at the bottom end of the Re scale though.

The foam fence Idea is what I was fiddling with, although I was using ep. I'd use epp now, if nothing else for impact resistance.

It's fun to mess with. Check out some of the RC and model soaring sites. If your messing with small rigs, even the Re # will be close.

Paul

 

Peaked Up Square Top

I did testing of RC versions of squaretops(America One) to "Peaked up square tops"(S 50 and F3) like in the photo below on both monos and multies. The "peaked up" version seems to work best(leach longer than luff) and in two boat testing in monohulls was always faster upwind. I think that for a variety of reasons its better than using an endplate. The type shown in the picture has an upper outhaul that works just like the normal outhaul and allows the top half of the sail to be changed in camber easily. The "gaff" is simply a carbon rod molded in a silicone tube as you see it and fitted in a low friction bushing in the top of the mast. The gaff rotates(independently of the boom) and twist and leach tension are controlled by a vang or fixed boom/outhaul system. This is an advantageous system as it allows more nearly a rectangular planform without requiring full battens on a model.

 

That is cool!

 

The wide boom shown is a "Park Ave Boom' used in the 1800's and on some of the J boats.

The wide top is similar to many Dutch work boats of the early 1900's.

A curved gaff that adds lots of area with little work at the peak.

FF