Loose Footed Main - Which Side of the Boom?

[Sam Salter]

What side of the boom should a loose footed mainsail hang?

One theory was that on the leeward side it set better and the shape could be controlled easier.
Another theory was that on the windward side the boom/sail acted as a sort of boundary layer fence and helped keep the pressure differential from equalizing.

As we sail on a lake with no stable wind patterns (It's often possible to do a 360 whilst beating to windward all the time) it's almost impossible to answer the question using the speed puck.

Joe and RichH can jump in anytime here, or simply put it down to heat stroke and move on to the next thread!

sam

 

[RichH]

The apron is designed to be on the 'downwind' or leeward side ... and the strength of the wind is supposed to 'push' it there.

 

[Joe]

Well, it wouldn't be a loose foot main if the foot was captive on the boom.. even by the lack of wind. I can't really see air flow being enhanced with the sail's foot being hung up on the windward side.

 

[Jackdaw]

I want to know how many rum drinks were harmed in the creation of this post! ;^)

 

[Don12364]

Hey!

Is this 'apron' free and unmeasured extra sail area?
If soo why not have a 'roach' supported by 3 or 4 soft battens and just push these over the boom on each tack?

 

[Cayennita]

If you were attempting to go upwind in almost non-existing wind, then the main should be flat and the outhaul tight. hence the skirt would be on top of the boom.

 

[Don Crowther]

which ever side doesn't block the view IMHO

 

[stanz4]

If you were attempting to go upwind in almost non-existing wind, then the main should be flat and the outhaul tight. hence the skirt would be on top of the boom.

I was taught the opposite: in light winds you want more draft in the sail. A deeper draft gives more lift (more power) and is suited for light wind conditions. As the wind increases, you want to depower the sail (by increasing the outhaul tention to flatten the sail, i.e. less lift) to reduce healing (to optimal heal angle or less) while maintaining the right angle of attack (windward and leeward tell tails streaming aft).

I would suggest testing this sometime in steady light air. Measure your speed and adjust only the outhaul. You should see better speed with a deeper draft (outhaul slack) than you would with the outhaul tension tight and a flat sail.

 

[Don Guillette]

Stanz4: I don't think so mate and here's why. In very light to light winds, the wind does not have the power to get around a sail with any amount of draft depth and the sail stalls. You see it all the time in very light to light conditions -- skippers bag out the sail because they think it makes good sense as they're trying to capture what little air there is.

As the wind starts to pipe up you should then start to gradually induce draft depth up to a max point of about 25%. Over 25% becomes counterproductive.

 

[bffatcat]

Loose footed main

seems obvious to me a loose footed main is going to lay to port on a starboard tack, an starboard on a port tack. you could always stretch out the outhaul in light air and get the sail to sit on top of the boom

 

[RichH]

I was taught the opposite: in light winds you want more draft in the sail. A deeper draft gives more lift (more power) and is suited for light wind conditions. As the wind increases, you want to depower the sail (by increasing the outhaul tention to flatten the sail, i.e. less lift) to reduce healing (to optimal heal angle or less) while maintaining the right angle of attack (windward and leeward tell tails streaming aft).

I would suggest testing this sometime in steady light air. Measure your speed and adjust only the outhaul. You should see better speed with a deeper draft (outhaul slack) than you would with the outhaul tension tight and a flat sail.

The reason for keeping sails FLAT rather than with more draft is that light wind velocities dont have much energy to transfer to the sail, nor to 'stay attached' to the leeside especially where the leeside flow is curved --- such as at the point of max. draft.

Simple test next time youre in super light winds:
while watching your speedo and tell tales (to keep them perfectly flying), gradually change the amount of draft and see whats happening on the speedo. What you will invariably see when deepening the draft is that there will be a point of maximum draft at which the tell tales at midcord of the main and jib will simply 'drop' and hang almost straight down signifying that the flow on the leeside has a 'separation stall' and the airflow has become unattached. This happens because there isnt enough energy to keep the flow 'attached' especially at the 'roundness' or max. draft of the sail ... lessening the draft or make the sail flatter will keep the flow attached in 'light winds'.
Flat sails or sails with minimal amount of draft are better in light winds because the flow across the leeside can remain 'attached' and attached waaaay down to almost 'zero' air flow.

 

[Ken Prichard]

I agree with keeping the sails flat in light air and I do it all the time. But I have only heard of the outhaul being used and that's all I have ever used. Which means the draft is being reduced in only the bottom 3rd. That brings a question to mind.....
How about the top of the sail ? Should the backstay be applied to flatten the top ? It seems to me if it's beneficial to flatten the bottom it should also be beneficial to reduce the draft in the top of the sail.

 

[Don Guillette]

Kiprichard: Yes, the sail must be flat from top to bottom and here's why. If the top 1/3 of the main or jib is twisted off or open then you're spilling what little air you have to work with. You're defeating the purpose. The mainsail sail trim controls that are used to adjust TWIST are the boom vang (if you have one) and the mainsheet. The TWIST sail trim control for the jib is the fairlead.

Unless you have a "bendy mast" like a "J boat, (and maybe you do), you can't flatten
the mainsail using the backstay. If your mast is like a telephone pole, when you crank on the backstay all you're doing is raking the mast back and that causes other problems. With a bendy mast, when you crank on the backstay, you're taking the fullness out of the middle portion of the sail which is exactly what you want to do and then use your boom vang to deal with the top portion of the sail and also your outhaul.

You need to use every mainsail and jib sail trim control that deals with flattening the sails.

 

[stanz4]

Stanz4: I don't think so mate and here's why. In very light to light winds, the wind does not have the power to get around a sail with any amount of draft depth and the sail stalls. You see it all the time in very light to light conditions -- skippers bag out the sail because they think it makes good sense as they're trying to capture what little air there is.

As the wind starts to pipe up you should then start to gradually induce draft depth up to a max point of about 25%. Over 25% becomes counterproductive.

I stand corrected. Thanks for the explanation. I will most definitely try this the next time I find myself in extremely light air.

 

[Sam Salter]

Don - kiprichard has got a bendy mast and we can flatten the top of the sail with the backstay - although we haven't been - that will change in very light air on our next outing!

As we sail on a lake with no current to speak of, we can use the speed puck (GPS driven) to test the effectiveness - it's real accurate!

The outhaul was tight to flatten the draft as there was very little wind. In fact so little that on a starboard tack the foot was laying against the starboard side of the boom - hence the original musing/question.

sam