Roached Mainsails!

Zagato

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At a recent Drascombe Rally a friend of mine had a 'roached mainsail' (wider white area at the top of the sail supported by a batten as pictured). The sail can be used to good effect in light airs, although the usual way of 'harbour furling' the main, e.g. Rolling it up against the mast is hindered and reefing looks a bit nuts because you still have the large white area on top.

image.jpg1_zpsonwruwe1.jpg


Question from a novice, I am totally wrong obviously but I would of thought the last thing you want is sail area on top of a sail as it will make the boat heel more. Why not have a larger area below, maybe as there is no boom!? To all those comics yes I know Drascombes will never be quick, they are not about being quick ;)
 
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Daydream believer

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Seems like a sensible improvement. Better than a separate topsail as it gives a better shape to the sail
if you took the area of the white bit & stuck it on the bottom you would have to raise the brown bit up -or shroud the crew in sail so no point in it going on the bottom as the gaff would be a problem
Reefing is not a problem , one just furls the sail at the bottom & moves the main sheet up the leech
If you want to reduce mainsail area more then put in another reef point a little higher up
 

bbg

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The first thing it does is add sail area. If you are looking for more power, that is a good thing. If you are overpowered you can always reef.

I think it may also improve the efficiency of the sail. A pointed top will have all kinds of vortices that don't help efficiency.

Certainly racers think it is a good thing to add sail area to the top of the sail.

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bitbaltic

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Question from a novice, I am totally wrong obviously but I would of thought the last thing you want is sail area on top of a sail as it will make the boat heel more. Why not have a larger area below, maybe as there is no boom!? To all those comics yes I know Drascombes will never be quick, they are not about being quick ;)

Mainsails have a roach to increase the sail area high up because the wind gets faster the higher you go, and move away from surface wind speeds to gradient wind speeds. It is essentially a characteristic inherited from racing designs. Fractional rigs are designed in part to exploit this effect and give the top part of the main fast, clean air when sailing to windward.

Building a sail for gradient winds seems kind of pointless on a boat of this size and intended application.

As bbg says the roach also increases total sail area so helps speed when sailing downwind. It won't do much for speed to windward as the lift comes from the luff.

John Vigor argues that a roach is not required for cruising boats (which would certainly include the drascombe) here: http://johnvigor.blogspot.co.uk/2011/11/dump-that-roach.html
 

JumbleDuck

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That looks just like the mainsail on Kelpie, an Innellan class boat from the Clye which friends of my parents owned forty years ago. A previous owner had combined her tan mainsail and white topsail into one, replacing the gaff with a wishbone. I understand that a later owner restored the original sailplan.

I've asked before without success - if anyone knows where Kelpie is now, please get in touch. The last thing I heard she was at Ardmaleish boat yard, awaiting repair. Her surviving former owner, is now old and ill and would like to know what happened to the boat she owned and loved for many years ... though if the answer is "broken up" I'll keep it quiet.
 

prv

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Normally, roach just refers to the leech of the sail being made with a convex curve, adding a bit of area but not substantially changing the shape. Your photo looks more like a sort of pseudo gaff topsail, hence the different colour to help with the illusion. Looks probably have as much to do with adding area that way as do more prosaic considerations like the height of the CoE :)

Certainly racers think it is a good thing to add sail area to the top of the sail.

Aren't so-called fat-heads just a way to get more sail area onto a mast whose height is restricted by rating rules?

Pete
 

lw395

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Normally, roach just refers to the leech of the sail being made with a convex curve, adding a bit of area but not substantially changing the shape. Your photo looks more like a sort of pseudo gaff topsail, hence the different colour to help with the illusion. Looks probably have as much to do with adding area that way as do more prosaic considerations like the height of the CoE :)



Aren't so-called fat-heads just a way to get more sail area onto a mast whose height is restricted by rating rules?

Pete
Some truth in that.
But a triangle is not the most efficient shape, the ideal is allegedly an ellipse like a spitfire wing.
But a trapezoid can be quite efficient.
Those top battens are also important in gust response and can allow the sail to auto-depower.
There are a lot of subtleties!

I suspect the influence of an ex Merlin Rocket sailor?
 

Woodlouse

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The reason for fat head mainsails on high performance boats is due to the extra drive you get out of the top of the sail due to the extended chord of the aerofoil. It doesn't take much to realise that in terms of cross section a wing with some width to it is going to produce more lift than a tapered apex.

As for the drascombe sail plan, it's purely aesthetic though the extra sail area should improve performance off the wind.
 

JumbleDuck

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The reason for fat head mainsails on high performance boats is due to the extra drive you get out of the top of the sail due to the extended chord of the aerofoil. It doesn't take much to realise that in terms of cross section a wing with some width to it is going to produce more lift than a tapered apex.

It also produces more drag. The optimum shape for highest lift/drag ratio is, as someone pointed out, an ellipse. This is basically because the vorticity which creates the lift is shed evenly over the span in a vortex sheet wake and isn't all dumped at the tips, where it produces lots of induced drag.
 

Buck Turgidson

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Some truth in that.
But a triangle is not the most efficient shape, the ideal is allegedly an ellipse like a spitfire wing.
But a trapezoid can be quite efficient.
Those top battens are also important in gust response and can allow the sail to auto-depower.
There are a lot of subtleties!

I suspect the influence of an ex Merlin Rocket sailor?

Small correction. It's not elliptical plan that's efficient. It's elliptical lift distribution which can be achieved in many ways but very difficult to do it with an elliptical sail as the sail has twist.
 

Daydream believer

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It also produces more drag. The optimum shape for highest lift/drag ratio is, as someone pointed out, an ellipse. This is basically because the vorticity which creates the lift is shed evenly over the span in a vortex sheet wake and isn't all dumped at the tips, where it produces lots of induced drag.

Have to tell those glider designers that they have the wing shape wrong then
 

Zagato

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Thanks for the answers, better understanding now, I won,t bother getting one, my Drascombe, for me is to keep it simple and easy. You can buy a bigger jib, main and mizzen which is an alternative but it makes little difference apparently so will stay with the standard set up, I am in no rush to get anywhere ;). Off now to make a whisker pole for the main out of an extendable boat hook, rig a chord up to keep the tiller in place for hands free er... private moments and think about making a board with shelves, hooks etc to go on the inside of the bulkhead!
 

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Question from a novice, I am totally wrong obviously but I would of thought the last thing you want is sail area on top of a sail as it will make the boat heel more. Why not have a larger area below, maybe as there is no boom!?

Sail area higher up is more effective than low down as there is a wind gradient with velocity at the surface up to 1/3 less than a few metres up. Also the air is cleaner up there as it's not obstructed by bodies and other protrusions.

I sail a Longboat and find that setting the main any lower means the poor people on the lee side can't see with a face full of sail and it tends to upset them, especially when gybing.

We have to sail very conservatively as we carry disabled people so our reefing technique tends to involve leaving the mainsail & gaff on the pontoon!
 
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