tacking angle

[rexy]

some theoretical stuff about sail theory's and how they work i came across

http://www.arvelgentry.com/techs/Origins_of_Lift.pdf
http://www.amasci.com/wing/airfoil.html
http://www.arvelgentry.com/techs/The...nteraction.pdf

[paulk]

Arvel Gentry is the guy. Like Giulietta says, theoretically it's possible, but other factors like heeling, waves, boatspeed (target numbers) interfere. On our Soling, we routinely tacked through 60-65º on a beat, but like Giulietta also mentioned, you can adjust jib and main travelers on a Soling. Other boats may not have that capability, and (because of reduced boatspeed) it may not be desirable to change the angle of attack much. On the other hand, we've found in light air it often makes a lot of sense to foot off and keep moving, rather than head as high as the sails might allow , but go slower. It's a sailboat - a series of changing compromises all the time.

[oldironnut]

This is another question rather than a reply. My boat is a 22' Oday shoal draft (23"). My problem is leeway when pointing as high as possible. It seems that I have to bear off a little more than my competitors to maintain speed therefore a better VMG. To check between my compass and my gps I'll motor downwind where leeway won't be a factor. My compass on the bulkhead and heading on the gps is very close. When I'm sailing upwind sometime I get as much as 8 degrees difference. I'm not sure if it's a matter of just design or my sailing skill and sail trim. Ready to learn.

[Sailormon6]

Originally Posted by mr31

does wind velosity, light air to medium air change the tacking angle when on a beat ?

I know this is an old thread, but I just ran across it, and I think the conclusion reached by Giu and others is not entirely correct. He is correct when you only consider the limiting factors of the boat's basic geometry, i.e., the sail shape, location of genoa track, length of spreaders, etc. But, generally, boat speed increases with wind velocity. An increase in boat speed increases the efficiency of the keel, and that reduces drift to leeward. Thus, in a lower wind velocity, a boat can't point to the full extent which can theoretically be achieved within the limits of it's basic geometry. As the velocity of the wind increases, and the boat speed increases, the efficiency of the keel increases, and, at some point, the boat is able to point to the full extent which can theoretically be achieved within the limits of it's basic geometry. The boat's optimum tacking angle is determined by it's optimum pointing angle. I think Giu's error was in not factoring in the reduction in the keel's efficiency as boat speed declines.

[RichH]

With reduced wind flow velocity - the higher you can 'point' ... several factors combine to make this happen.

1. the relative speed of the boat vs. the relative speed of the oncoming air ... creates a slightly larger 'relative' amplitude 'upwash' of air flow in front of the boat - for higher angle of attack relative to the 'true' wind.

2. the 'slot distance' between the jib and main is (should be) more closed or reduced in light winds vs. higher strength winds ... giving a boat in light winds a higher 'angle of attack' relative to the 'true' wind because the clew can (should) be closer to the boats centerline than when in 'heavier' wind as well as better 'aerodynamic dumping velocity' (bootstrapping) between the jib and main. You precisely adjust the 'slot distance' in accordance to the output of the speedo (and forget or ignore any so-called 'backwinding' you see in the luff of the main) .... max speed is the goal when adjusting how far open or closed that 'slot' is. Of course your sails are perfectly trimmed and shaped (all tell tales flying 'perfect') and with VERY slight 'weather helm', before you adjust the slot-open distance to get MAX. SPEED/VMG out of the boat when beating!!!

For the case of the shoaldrafter and pointing ability, you have several things against you:
1. a shoaldrafted boat will more easily skid to leeward (helmsman erroneously blaming 'weather helm' because of the side-impact of water against the rudder due to the normal skid) .... so, the position of maximum draft of both sails has to be 'more forward' AND the wake coming off the stern is at an angle of no more than 2-3°. The usual 'set up' is to get MAX. speed out of a shoaldrafted boat when attempting to 'beat' (to maximize VMG) by having a near 'dead fish' / neutral helm with *VERY* slight helm pressure and by checking the stern wake (turbulence caused by the keel and rudder ... is coming STRAIGHT off the stern, not any appreciable angle .... get that boat MOVING instead of skidding to the lee (for better VMG!!). With a near dead-fish helm you will consciously have to work the wheel/tiller for the boat to go 'upwind' and not constantly 'bear off'.
Also you will have to learn how to 'turbo sail' : Footing Off - SailboatOwners.com .... post#12 and this also applies to deep fin keelers who want to 'fly' their keels.

2. You're simply not going to get that shoal draft keel to develop very much LIFT to weather, in comparision to a deep fin.

[Sailormon6]

Originally Posted by oldironnut

This is another question rather than a reply. My boat is a 22' Oday shoal draft (23"). My problem is leeway when pointing as high as possible. It seems that I have to bear off a little more than my competitors to maintain speed therefore a better VMG. To check between my compass and my gps I'll motor downwind where leeway won't be a factor. My compass on the bulkhead and heading on the gps is very close. When I'm sailing upwind sometime I get as much as 8 degrees difference. I'm not sure if it's a matter of just design or my sailing skill and sail trim. Ready to learn.

Oldironnut, I have raced against Odays in that size range, and they point about as well as most other cruiser/racers in that size range. A boat with shoal draft will usually lose a very little pointing ability. But, what really reduces your boat's pointing ability is poor sail trim, poor helmsmanship, and anything that reduces your boat's speed. If the underwater surfaces are fouled by dirt, or slime, or many layers of old, peeling paint, that roughness creates drag, and disrupts the smooth, laminar flow of water over the hull and keel, and it creates turbulence. Serious racers at least scrub the bottom before a race, and are meticulous about preparing the bottom each spring.

No matter how much skill the skipper has, a sailboat can't be competitive if it has a foul bottom and can't generate speeds that are comparable to it's competitors, so your first priority should probably be to make the bottom smooth and fast. Then, work on sail trim, boat handling and helmsmanship, and practice, practice, practice. Also, volunteer to crew for other, experienced racers, and watch how they trim their sails and handle the boat, and you'll learn fast.

[chris_gee]

Agree the slower you are going the more leeway, which can offset the theoretically better higher vmg from pointing higher if in fact there were any advantage at a particular point of sale. The gps is showing actual course made so 8 degrees difference is significant.
I think the original thread confused angle to the apparent wind, with tacking angle. One might sail at 30 deg AWA and that is related to the angle of the wind on the sails. There can be a difference in curvature on the luff for different wind speeds so that angle may vary somewhat with windspeed. That is the luff might be tighter in higher wind so you could point higher as the curvature is less. But that depends on sea state you might not tighten it if waves are slowing you and you want more powerinstaed of pointing a bit higher. But then you might have excess power so can point higher. It depends.

But if you are pointing at say 30 AWA your angle to the TWA will vary depending on windstrength as the wind increases the wind effect due to boat speed increases less relative to the true wind. But in a lighter wind the apparent wind may move more ahead so that if you maintain a fixed angle to the AW you are actually sailing at an increased TWA.
Since the windward mark is in the direction from which the true wind is coming in lighter winds you are in fact sailing at a larger angle to the true wind even if sailing by say the windex you are sailing by the AW. You will therefore tack through a greater angle than if the wind were higher.
If the AWA is 30 you are not tacking through 2 x30 but maybe 80 relative to the true wind or even 110 in lighter winds.
In lighter winds therefore although you may keep the same apparent wind angle in fact since the awa moves ahead you are sailng at a greater angle to the true wind as you pick up speed.
I don't know how clear that is as it can be difficult to get one's head around it.

[RichH]

Let me put this another way. The faster you go the more you will 'bear off' when beating to hold a perfect/precise set/trim in accordance to maximized aerodynamics as indicated by perfect flowing tell tales. This effect is very apparent on planing hull sailboats and iceboats when they are 'accelerating' to faster speeds.
Simple trigonometry when applied to dynamic acceleration or increasing speed of the *boat*. The same is true for in increasing wind strength/velocity
;-)

[chris_gee]

Agree with the first part - disagree with the second, that you bear off more with increasing wind speed. You only bear off in relation to the true wind not the apparent wind. The apparent wind changes more at low ws and relatively little at higher speeds.

[puddinlegs]

Originally Posted by RichH

2. the 'slot distance' between the jib and main is (should be) more closed or reduced in light winds vs. higher strength winds ...

In light air (0-5kts) it's important to keep the slot open and flow moving over the sails. Strapped sails in light air are very very very slow.. higher winds (5-12/14), the slot closes and angles tighten. Main sheet on hard, traveller up or down for a good helm or changes in wind pressure. 14+ and you begin to change head sails, or sail shape (open the top of the main and jib and flattening the body (outhaul, backstay, moving jib leads back a bit) depending on sea state, etc..., 20'ish + and most non race boats will start reefing the main, yadda yadda yadda....