DarkHorse

Smart/Not Smart - booms above centerline

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So we have seen boats pulling the boom WAY above centerline, and doing just fine... so, can our sail maker friends give us the skinny:

- is it driven by the large twists normally found in squaretops

- does it require a different design than your standard phrf main

- is there some upper limit

- etc

Lets get some real 'trickle down' from the pros!

 

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We sailed the Ensign with the traveller up 9” above centerline and the boom 1 1/2”-2” above centerline and dump the traveller a few inches coming into large puffs. We sailed higher and faster than all the others who were below centerline and lost very few races over the next 8 years

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There is no magic about the centerline, its just one stop on a range of settings that have only to do with one thing, AWA. Which has the square root of fuck nothing to do with where the middle of the platform is relative to the boom at any given time.

Used to go North of the centerline all the time with the wing, worked fine. Tell tales flowing? then its working. Do it on other boats all the time, tell tales flowing? Then its working.

Now maybe on some boats its a bad idea due to foil configuration etc etc but are the tell tales flying?

Is your rudder stalling? Maybe its too far up then.

 

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I was going to write a reply based on my experience with doing this as a jib trimmer but it is pretty much covered here...

For the most part in flat water and moderate wind it is all about the extremely tight jib sheeting angles (well under 6 degrees) and getting the main out of the slot...

 

 

 

  

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Has been feature for a couple of decades as yacht design refines. In summary, below max heel and short of max efficient rudder angle, the boom above cl with plenty of twist has better lift/drag for the sails and the hull.

Really summarised, in say 8 tws, with the 1/4 height  main leech on the cl, the positive effects are:

1. Improved flow over the jib

2. Greater sideforce, because the total apparent wind deflection over the rig is increased.

3. The additional twist improves the l/d ratio, probably  (mostly) because the tip vortex is reduced

4. Helm is up, with rudder angle towards optimum, (say 3 degrees) so leeway is reduced, reducing hull drag from crossflow.

The slot is fairly irrelevant.

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It's very simple.  Think flaps down in an aircraft.   High lift, high drag, high aoa.   There is a point drag exceeds lift and you go slower.   Either because wind is stronger or aoa is too high. 

Work your tell tales appropriately. Keep your head out of the boat with one eye on the knot meter. 

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Yup, Jumbo jet at takeoff....

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Nothing new here; we've been sailing Etchells for decades with the boom well above centerline, until it starts to blow over 15 or so.

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2 hours ago, billy backstay said:

Nothing new here; we've been sailing Etchells for decades with the boom well above centerline, until it starts to blow over 15 or so.

Interesting that this applies in fairly old school Etchells as well. I observe from my position on foredeck/trimmer on Atlantics that the consistently faster boats seem to do the same, while those of us who sail with more old school skippers have booms right on the center line. But it must not be just as simple as having the traveler way up and boom above center line because that seems to cause many complaints of weather helm. Does that weather helm not occur on Etchells? Or do you do something else to combat it?

Apologies for the newbie question from an observer of mainsail trim! 

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2 minutes ago, fprintf said:

Interesting that this applies in fairly old school Etchells as well. I observe from my position on foredeck/trimmer on Atlantics that the consistently faster boats seem to do the same, while those of us who sail with more old school skippers have booms right on the center line. But it must not be just as simple as having the traveler way up and boom above center line because that seems to cause many complaints of weather helm. Does that weather helm not occur on Etchells? Or do you do something else to combat it?

Apologies for the newbie question from an observer of mainsail trim! 

 

Weather helm usually doesn't become an issue until breeze is up around or above the 15 kts', I mentioned, as the time when we ease the main trim and boom goes to, or below centerline.

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.... and because the onset flow gets deflected  ore, the jib can be deeper as well, i.e. higher lift, so the longitudinal ce goes forward again.

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Oops....deflected earlier 

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