Optimum boom angle downwind?

B

bcardarella

Member
70
5
Hingham, MA
Is there a general rule like: "If you are sailing downwind and the AWA is X then your boom angle should be Y" ?

My boat is an asym so sailing off the wind is what I need to do. I usually just let the boom go until it seems OK but I figure if I have an AWA value then it would stand the reason that the optimum boom angle could be a discreet value.
 
neuronz

neuronz

Anarchist
929
102
europe
Depends a lot on your actual AWA and conditions. At small AWAs you can use the telltales which will show you how far you can sheet in without stalling and also the twist. However, sheeting in too far might induce too much heel and/or load up the rudder which would be slow which is when you would start to ease/increase twist. seastate also plays a role here as waves might push you over etc.
 
K

Kenny Dumas

Non Binary About Anything
1,397
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PDX
Hold the sheet in your hand. When it pulls the hardest, let it out a little bit more. The sheet vector points further forward the more you let it out so the optimal trim is more eased than maximum sheet tension. And stalling is slow so you want to error on the eased side anyway.
 
Ex Machina

Ex Machina

Super Anarchist
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New Zealand
YouTube is your friend . Watch onboard vids of Shaw 650s etc and anything sportboat in Aussie like Hamilton island or Airlie beach onboard vids
 
Steam Flyer

Steam Flyer

Sophisticated Yet Humble
47,940
11,643
Eastern NC
bcardarella said:
Is there a general rule like: "If you are sailing downwind and the AWA is X then your boom angle should be Y" ?

My boat is an asym so sailing off the wind is what I need to do. I usually just let the boom go until it seems OK but I figure if I have an AWA value then it would stand the reason that the optimum boom angle could be a discreet value.
Click to expand...

It's all about apparent wind. What is the angle of the wind approaching the luff of the main? Be aware the it will be different along the boom and up at the top batten, that's why use of the vang is so important, as mentioned above.

Another important factor is the helm. The spinny is exerting a lot of leverage out at the end of the bowsprit, the mainsail leech is the counter-balance to that force. You play the mainsheet to keep from having to drag the rudder one way or another.
 
M

munt

Super Anarchist
1,438
466
The belt
As a multi guy (rotating masts, no vang) I would really love a fairly simple explanation of how the vang is being used in that very cool video. Thanks!
 
hendrixharlow

hendrixharlow

New member
46
5
Boston
bcardarella said:
Is there a general rule like: "If you are sailing downwind and the AWA is X then your boom angle should be Y" ?

My boat is an asym so sailing off the wind is what I need to do. I usually just let the boom go until it seems OK but I figure if I have an AWA value then it would stand the reason that the optimum boom angle could be a discreet value.
Click to expand...

We just had a nice Saturday up in Marblehead with some (new owners) of the Viper fleet practicing tuning upwind, tuning downwind, tack practice upwind, and gybe practice downwind. On the tuning legs, we spent a lot of time working through settings, and I can definitely tell you the biggest speed improvements on the downwind was based on mainsail settings... In particular, at least on the Viper, and I suspect other roachy mainsail Asso boats, it's all about preserving the slot between the kite and the main (up high). If you ease main too much, you basically can close the slot... I've seen a few boats in our class plaster a few more telltales up high on the main (ie: on either side of the main) to give an indication of slot flow downwind.. It pays attention to watch this on your boat..
 
JulianB

JulianB

Super Anarchist
1,388
2,091
Sydney mostly
Bcardarella, all depends on the weight of your boat (all up, so inc crew), length of your bowsprit/prodder/pole, your RM [Righting moment] and sail area.

Week ago we were sailing in Arlie Beach Race week [ABRW], I was on a FE28R which is 1350kgs, plus 5 crew so another 450kgs, 28-29ft LWL. Against us, among others, were Skeeter and Vivace, bit shorter, 600kgs (boat), 500kgs of crew, same sail area (all 3 sails) lots of additional RM, longer bow-sprits.

We turn the corner, often in front, throw everything up, trim for max VMG, all crew weight up to windward (real error putting anyone to leeward), jib down, (pole is only 2m) so you know how to float and trim a spinnaker, so I wont go there. Main, ease Down-haul and then, because you main is operating in the down wash of the spinnaker, a) as much camber as you can get in the foot, 10-15% is enough, and bring the sheet on until it's out at about 10-15° and then look at the leach, and using Vang and Mainsheet you work until you get all the leach ribbons flowing all the way up and down the sail and that will require a pretty massive ease on the vang. Once they are all flowing, bring the main-sheet on about 2° so they are 50% of the time, flicking to leeward. Re test every 2-3 mins.

So the down wash from the spinnaker (spinnaker is about 18% camber) low down is massive for 3 reasons, a) it's low down so you can stand it,
b) it's end plated with the water surface so it's efficient and c) because we sheet at deck level so it's squeezed in regardless. Up hi, it's far more open for almost exactly the same 3 reasons so the main should mirror that.

In 2d, it a big like flaps on a aircraft, then all need to work as one "element" to maximize effect.

In 20knts at ABRW we came around the top mark set up and did 13-14knts down hill, with a TWA of 145° ish.

Vivace and Skeeter, came around the same mark, similar sail area, everyone hiking hard, sail less twisted (we had a A1, they had A3's), sheeted in a lot harder took off at hi teens, occasionally low 20's at TWA of 165°, and they left the jib up, (poles 2.5-3m off the bow) but main would have been sheeted over the qrt, with far less twist. And they left us for dead in just about all races.

But they were looking at the same leach ribbons and doing the same exercise. Plus they had the added complication of jib trim.

So it all depends on your boat and it's numbers, and it's all about getting the 2 (or 3) sails working in unison!

jB
 
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