Downwind sail trim

[Rick Macdonald]

Hi Don!Here's a question I haven't been able to answer for myself. It has to do with the transition from the main generating lift (as on a beam reach) to the main generating only drag (deep broad reach and run) as you need to pick points of sail farther off the wind. Light and shifty winds on my long narrow lake make real testing difficult.When bearing off from a beam reach, at some point my mainsail and boom are up against my swept-back shrouds and spreaders. Let's say this happens when the wind is 110 degrees off the bow. Somewhere beyond this point, say 115 degrees, the wind no longer gets behind the main and it's generating drag only. At this point the vang and leech cord are tight to minimize spillage of air and maximize drag, but the main and boom are not perpendicular to the wind. Do I leave the boom near the shrouds, or do I pull the boom back 40 degrees to put it perpendicular to the wind? Pulling the boom back to place the main perpendicular to the wind would increase the sail area it presents to the wind, and minimize air spillage, but the larger force generated by the main is now a vector 40 degrees farther off the bow compared to when the boom was against the shrouds. Therefore, not all of the increased force would actually contribute to the forward direction, and perhaps it's even less?If pulling the boom back is the right thing to do, I could do so for all headings from that point until it hit the spreaders again, which would occur when the wind is 155 degrees off the bow. ...RickM...

 

[Bill Roosa]

The vector thing again

Use your knot meter to trim for max speed. There is no easy way to calculate the size (strenght or length depending on the school you went to) of the vector at any given position and how that will vector sum to increase the net force powering the boat forward. You can, however, look at the knot meter and it tells the tale.As a general rule this is the way I trim all my sails. Let it out a little and see if that increased the speed. If not bring it back in beyond where is was initally and see if that increases the speed. Repeat untill you have the boat going at the max speed for that wind and boat direction. ALWAYS trim the foresail first, then the main and then retrim the foresail and then the main again. The foresail(s) modify the slipstream for the main when going upwind. Going down wind the order is not as important as the foresail is either in the "shadow" of the main or is polled out to the opposite side.Happy sailing

 

[s sauer]

Teltales?

Assuminy you have teltales on luff of the Genoa and leech of the main they should tell you what the airflow is doing and if portions of the sails are stalled. Even with the main up against the shrouds, if the air is flowing smoothly thru the slot you should be getting better performance than sheeting in the main and having a back flow off the leech with teltales flowing backwards around the leech.

 

[Rick Macdonald]

Yes to Teltails

I do have all those teltails, plus two sets on the middle of the main, one below and one above my one (deep) reef point. This weekend I'll try to see how deep I can point and still keep them all streaming.I rarely have much luck using the knotmeter as a sail trim judge on my lake. Invariably, I can change some aspect of trim and see the boat speed up. Then I put it back to where it was, expecting it to slow down again, but it speeds up even more. So, I repeat the test and it may slow down in both cases. The winds are that variable in direction and speed here, not to mention being affected by the nature of the upwind shoreline (cliffs, trees)....RickM...

 

[Alan]

I have to agree with Bill

The goal of the trim is to produce the best speed possible. That is exactly what your knot meters job is. Do exactly as he suggests. Ease and watch the boat speed, adjust in the direction of the greatest boat speed. The knot meter takes into account all the variables and generates the final result.

 

[Don Guillette]

Downwind

Rick: I'm not sure I'm going to answer your question correctly because I'm not exactly sure what your trying to accomplish.Coming from closehauled down through beam reach the wind is pulling you and reading the flow across the sails is everything. As you go past beam reach on your way to a run and end up with the boom against the shrouds you have gone from being pulled through the water to being pushed along by the wind. The deeper you go towards a run the less effective the telltales are because there is no flow over the sails no more than there is a flow of wind over a kite. The telltales become useless. I don't even bother looking at them on those points of sail. Instead I focus on the masthead fly.If your bow is pointing DDW with the boom on the shrouds and you pull the boom back you'll stop. Before I left Lake Havasu, AZ this summer I was watching a mate give sail board lessons. He was using a "Lazy Susan" type thing with a mast and sail on it. As I watched I wondered how I could apply that to talking about sail trim. In other words, if you could set your boat on a lazy Susan and set your sail for closehauled and then spun the boat what would happen to the set of the sail relative to the wind. Nothing would happen as the boat would spin and the boom would remain where it was.Do I have that right and can anyone see an application?

 

[Bob7291]

off the wind

Many casual sailors make the mistake of sheeting the main too much when off the wind - they think it just doesn't look right to have the boom that far out. When I began racing my boat I invited a guy to crew who had raced on his college team. Wow, I learned more in that one day than I had in the 20 years previous about sail trim. And one of the most valuable lessons was to keep the tell-tales streaming until you just have to go ddw, that is, when the wind is up and the boat is making at or near max speed to the mark. Whenever you can sail the boat up a little and get flow over the lee side of the main, it will be faster. The question becomes, will it be faster in the vector toward the mark? The answer is the lighter the boat or the lighter the wind, the higher it can sail and still make better vmg to the mark. Even with the genoa poled out on the other side and exhibiting drag, the main is more efficient with flow over its lee surface. In fact, some of the good guys talk about getting reverse flow over the headsail, with the leech acting as its "leading" edge, so to speak.

 

[Rick Macdonald]

Rephrased question

Somewhere, I know I read the statement "put the boom perpendicular to the wind" as sail trim advice for sailing downwind; perhaps directly down wind. I can't find it now; it doesn't seem to be in Don's book or Sail Trim Chart. It's been so long since I read anything else, I don't know where I got this tidbit from! It's always stuck in my mind and I've been trying to figure out when, if ever, this is called for. Perhaps it isn't!I'm happy to let this rest and just see what gives me the best speed if ever I can be in stable enough conditions long enough to see what works best.But, here's a rephrased question in case it brings anything out...Many sailors don't like and avoid sailing DDW, including me. Well, if I do sail DDW it is only wing-and-wing with a whisker pole on the jib and a preventer on the boom. Then, it's quite enjoyable.Anyway, say your boat does not have swept-back spreaders and your boom will actually go out 90 degrees if you care to put it there. So, instead of sailing DDW, you are sailing up some, what 15 degrees? 20 degrees?My question is, sailing 20 degrees up from DDW, is the boom nearly against the shrouds? Does anybody ever put the boom perpendicular to the wind?...RickM...

 

[Don Guillette]

Swept back spreaders

Rick: Now I get what you were saying. What a novel approach - sort of like sailing closehauled backwards!! If it works you could be a trend setter. You would be the fastest downind sailor on the planet.

 

[Rick Macdonald]

Simulator

I imagine the simulator in the link below is over-simplified. It has a main only. But, it shows the boat goes faster with the boom all the way out instead of perpendicular when sailing up a bit from DDW.I Googled "wind perpendicular to the boom" and came across a couple of sites that seemed to be talking specifically about a dead run....RickM...