What makes a boat point high?
Thanks
Don
i think there is some talk going on about this here:
http://www.rcgroups.com/forums/showthread.php?t=776144#post8609190
Hi Again Don,
You ask a great question. The answers could fill a book!
There are at least 2 answers, as your question could be interpreted 2 ways.
The first and simple question is how to point high. The simple answer is flatten your sails, pull the backstay tight to tighten the forestay, trim the sails in close, and you will point high. BUT that may be very slow!
The other deeper question is “how do I get to the windward mark in the shortest time” or " “how do I make the best VMG (velocity made good) to windward”?
The problem with ‘just’ pointing high is that the boat slows down (due to drag). As the boat slows, the flow over the fin is reduced. The fin looses its ‘bite’ or lift in the water, stalls, and the boat goes sideways.
To get the best VMG to weather requres good speed through the water for the fin to work. So driving off a couple of degrees and easing the sails a ‘tad’ can produce a much better overall effect.
To get the best sail trim for this is what we all strive for. It includes sufficient shape in the sails, proper ‘twist’, enough halyard tension to place the draft of the sail in the correct place, correct trimming, sufficient backstay/forestay tension, sails shaped to fit the spar and rig, and the correct amount of ‘helm’. I’m sure others can add to this list.:zbeer:
This is what I was after to start. The simple answer.
enough halyard tension to place the draft of the sail in the correct place
Where would that be? Most of the big boat stuff I’ve read seems to say that the draft should be at 50%. While searching for stuff on pointing today I found a site that says for pointing the draft should be somewhat forward of 50%. If my sails are cut for 50% draft will the Cunningham pull it forward the correct amount?
Don
I like a draft position of abut 40 to 45% for the jib with a rounded entry and a flat exit. For the main, I like about 45 to 50% with a flat exit off the leach.
In full size sailing, there is a lot of pressure on the sailcloth that causes stretch. In model boats we don’t have the same degree of problem. In fullsized boats hard on the wind, the sailcloth stretches and the draft moves aft. Applying cunnningham brings the draft forward again. Also on fullsized boats, if you have the draft at (say) 50% and apply backstay and bend the mast, the front of the sail pulls forward and effectively the draft moves aft, so again you pull on the cunningham to correct. Running off the wind, the pressures drop and so you need to ease the luff tension or get a vertical wrinkle in the sail just behind the mast.
You can get this effect on a model boat by using the downhaul (no need for an additional line called a cunningham). There is a diagram on Lester Gilbert’s site for a ‘self easing’ cunningham. ie you set the tension for the beat, and as you ease out the main for the run, the geometry of the downhaul eases the pressure on the luff. Be careful not to overdo the luff tension or the boom may get too tight to cross the centerline on a tack or gybe.