Sail twist - what you were afraid to ask

[Rick Macdonald]

Don and friends -I've gleaned the following detail about sail twist from your book and your forum posts over the last couple of years, but I don't believe you have ever spelled it out quite as I have done below.Don, I'm inviting you to confirm or deny my thoughts here. If anything is found to be incorrect I should edit this post so as not to mislead anybody who reads it later. If I've over-simplified by ignoring factors that are too great to ignore, please chime in with that too.I'm perhaps only slightly anal: There are many scientific facts that I can do without knowing when sailing: boundary layers, induced drag versus parasite drag (would you believe Dp = CDp x S x 1/2*r * V*V?), drag coefficients, etc, but I've gotten a lot of insight from using simple vector diagrams for true and apparent wind.You state that the wind is 60% stronger at the mast head than at deck level. I drew some vector diagrams for close-hauled and beam-reach assuming a boat moving at 6 knots with a deck-level true wind of 10 knots. This means 16 knots of true wind at the top of the mast.I won't try to draw the diagrams here, but the result of the wind vector additions is such that due to the higher wind aloft, the apparent wind actually has a different direction at the top of the mast compared to deck level. This is the key to it all and why we must talk about twist in a sail. (Airplane wings have twist as well, but for a completely different reason: you want the wing to stall at the root first so the nose drops instead of a wing tip dropping, which can lead to a spin).When close-hauled using the numbers above, there is about a 5 degree difference between the apparent wind at deck level and the apparent wind at the mast head. On a beam reach, this difference is about 15 degrees! As a side note, the apparent wind at deck level is 14 knots close-hauled and about 10 knots on the beam reach for this example.So, the idea of twist in a sail is to get the angle of attack consistant up and down the entire sail. This literally means 5 degrees of twist close-hauled and 15 degrees on a beam reach for our example. This also means that twist set for close-hauled will need to be adjusted if you fall off to a beam reach.The right twist combined with the right angle of attack gets all the telltails streaming.By now everyone knows that over-twisting a sail "spills air out the top" and depowers the sail (reducing heel). The top portion of the sail is actually as if head-to-wind and generating no lift. Just now I can't recall if the top of the sail actually starts to luff under this condition. Perhaps it's not actually twisted _that_ far.Questions for Don -1) If you _under_ twist the sail above, the top portion of the sail will actually have too great an angle of attack and stall. The top telltail should show this. I'm assuming that there is never a reason to do this intentionally, but I thought that I'd ask. Would you ever do that? Saying that the top of the sail is stalled might sound like you'd get less lift and therefore less heel, but there would be way more drag, and the heel could perhaps be the same or worse.2) Is there ever a case to try to adjust for zero twist? Not that you could achieve it anyway due to sail cut and the forces on the sail?....RickM...

 

[Don Guillette]

Rick: Good thoughts but I'm just a common seaman. If I go to deep into things my mind gets confused.The wind speed differential between deck level and the top of the mast came from Dennis Conner. I heard him tell about it in a talk he gave a number of years ago. Made sense to me but at the time I was not sure what he was talking about because I didn't even know what twist was.Zero twist is easy. You would do that when you are trying to squeeze every bit of power you can out of the sail. I'm not sure what under twist is. I assume it is beyond zero twist. I don't know why anyone would go there as there would be no point to it.

 

[Joe]

My take on rick's comments....

...is that he is suggesting there is always a need for twist in a sail because there is always a difference between top and bottom of the sail conditions. Yet there are scenarios in your sail guide (and others,too, I might add) that call for NO TWIST. I think he is asking for an explanation of this discrepancy. PS Rick if this is incorrect I apologize for butting in.

 

[Don Guillette]

Joe; No or none is a relative term. There is a need for twist because of the wind difference from deck to the masthead and that is why sail makers build it into the sail. As I've stated a million times, you can never completely eliminate it even though it may appear to the eye that it is gone. The boom vang and mainsheet are used to control or dial it for the wind conditions and point of sail and experience level of the sailor sailing the boat. Some of these discussions are interesting but if we get to technical, we turn off the group that I am attempting to help and that is the beginner to intermediate. The same thing happens with the racers forum and that is why so few beginners to intermedites frequent those site. I don't even bother with them. In my early days of educating myself regarding sail trim, some of these discusions confused me. Actually, they still do. In reality I know what we are discussing but in my seminars I tell the folks to forget the technical material and just let me explain how the stuff like lift/drag, twist etc can be made to work for you.

 

[Rick Macdonald]

Getting closer!

It probably sounds like I'm spliting hairs but there is a simple question lurking in here. Don's and Joe's comments are helping me to rephrase the question.So, we know that the optimum or proper twist is the point where the top and bottom telltails break at the same time. From this point, adding twist (over-twisting) spills air to reduce power.Don, you say you can reduce twist to "squeeze every bit of power you can out of the sail". Given that reducing twist increases the angle of attack of just the upper part of the sail, why wouldn't I instead raise the traveller and slightly increase the angle of attack of the entire sail? Or do you do a little of both?Since this is less twist then optimum twist, I called it under-twisting. In other words, too little twist and the bottom telltails break first. Zero twist is even less: no twist at all.Page 16 of my favorite Sail Trim book says "too little - or too much - twist reduces power". Trying to sort out this subtle point is _way_ more fun than my day job!...RickM...

 

[Jeff Bacon]

Wind velocity gradient.........

.....is what the phenomonon is called when wind is stronger at the masthead versus the deck.Not to challenge Dennis Conner, but a 60 % increase sounds like a lot. (20 mph at the deck...32 mph at the masthead ?)However, there is definitly an increase.As the wind speed increases, it moves the apparant wind aft, which means that the top of the sail has to be trimmed farther out than the bottom of the sail, (also called twist). In essence, the entire sail will then have different angles of attack as you move up the sail in order to take advantage of the Wind Velocity Gradient moving the apparant wind aft as you get closer to the masthead.....Jeff