Join Date: Jul 2000
Thanked 235 Times in 222 Posts
Rep Power: 18
Re: Can someone explain roach to me?
The "more sail area" argument just wasn't holding water with me. Many new boats that have no rule limitations on design are building roachy heads into their mains. If it was truely just sail area, then they'd make the mast taller right? ...
******* yes, probably thats whats going on in the high class venues. A taller mast would increase the heeling moment -- its like supply and demand ... needs 'balance'. I strongly feel the roachy heads are for sail efficiency increase .... and the more SA thats aloft the better the upwash is forming waaay out in front of the boat, more SA up top the more 'upwash' aloft. With strictly triangular tops, the upwash directly in front of the top is less, simply because there's less sail area 'coming forward' to create that upwash. all this for 'upwind' sailing.
Your explanation may also explain something I read in an article by Carol Hasse on small jibs whereby she was saying that if you have an extremely long luff with very little LP you end up with a lot of narrow sail at the top of the luff that does little to create lift but does a lot to induce heel.
******* the preponderance of the 'lift' comes from the luff section, the 'trailing sections' through to the leech make the luff section more efficient (simplistically but NOT in direct proportion to 'length' ... and you can see the scarcity of large overlapping jibs on high end boats these days, too); making the luff longer increases the heeling moment. see sketch below.
On my boat I have a VERY low aspect mainsail (P=42, E=17), which would mean that I have a larger angle at the head of my sail than I high aspect rig. Would I benefit (in theory) from battens at the top, or is my rig low enough aspect that I already have enough sail area at the head? Is there a painful formula that I can use to figure out how much sail area should be at the head or how much roach would be idea for my sail? Sail what you have, or at least 'fill in' all the space from the backstay towards forward with roach.
****** But your boat has a staysail and if flown 'correctly' when upwind (and usually above 7kts) the staysail 'can' become a forward 'extension' of the mainsail system. Here's how: http://www.arvelgentry.com/magaz/The...e_Head_Rig.pdf. If your boat isn't tender after it initially 'hardens up' on a beat --- easily heels over to a quite constant angle and no more; then a proper 'matching' of a new mainsail with a full 'shoulder' (top panels with extra draft) would probably be the best way to go, will reduce your light wind performance a bit but could add a lot more 'power' (not speed) to the top portion of the mainsail. This is an individual sail design that you're not going to get from a 'stock sail' loft AND you're going to need a sailmaker who actually 'deals' with such design changes. A sail with lots of 'shoulder' will develop lots of extra power and the boat needs to be somewhat stiff or in racing a crew that can hike waaaaay out. ... or a 'blend' between plain vanilla and full shouldered.
An alternative to full-shoulder, is to 'hook up' the leech by over tensioning the mainsheet until the second batten 'just' points above the boats center line, then 'very' slightly ease the traveller until that #2 batten is parallel to the CL., such can create a lot of increased 'draft' up top (and the mid section and foot, so dont overdo it) To do this the sail has to be correctly/perfectly raised (if dacron) and the luff boltrope cant be 'shrunken' (an age problem).
Last edited by RichH; 01-28-2013 at 11:02 PM.