Boom position when on the wind

[alec]

It is often advocated that the boom be positioned on the centerline when going to windward in light to moderate winds.

I notice that this is often seen on more modern boats but also on many other types also. I have always trimmed the main to the jib but would be interested to know what others think.


Regards,




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[Phoenix of Hamble]

Alec,

I think this is very boat/rig specific.... IMHO the most important thing is that the boom position supports sail trim properly... a well trimmed sail, with effective airflow, good attachment, and clean release will be the fastest... a well sorted set up, non centred will be much faster than a centred, poorly trimmed main.... centering the main is often the cause of a hooked leach, and hence poor release...... which is devastating to boat speed in light winds..... so my personal approach in light winds is to trim the main through a combination of kicker, outhaul and cunningham to get a nice full(ish) shape, and a leach that is fairly tight, more open in the top 3rd, and not hooked, and watch any telltales that you have fitted on the leach to make sure that they are flying staight out... you get the feel eventually... I then trim the boom towards the centreline with the traveller loosening the kicker at the same time to get it as close as I can but without 'stalling' the sail....

I often find that in really light winds, its nearly impossible to get the main centred without destroying sail trim....

and as you quite rightly identify, balancing the main and jib is also critical... leach profiles should be quite similar on both sails in the light stuff upwind...

Thats my opinion for what its worth...

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[alec]

Thanks Nas for the response. I enjoyed it.

Do you think that booms have got closer to the centreline because of better jib sheeting angles ?

I notice also that my largest jib ( in the foot) requires a closer to midships angle than say my number three.

Regards,




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[Sunnyseeker]

In light winds its important to look at the top of both sails, you need them twisted off to allow the wind out cleanly from the top, and a flat entry at the front (loose tack/halyard)
You should still be able to centre the boom, but release the kicker to allow the boom to rise, this also means you need a traveller, so that the traveller is to windward but eased so that the boom is centred but higher...jib tack higher as well.
Sheeting position (entry angle) should be similar with flat sail and more twist....in really really really light stuff <5knots ish then you have to ease sheeting angle to stop the boat stalling and stopping when wind shifts slightly, and sail 5 degrees off your normal wind angle, keeping the boat moving is then whats important as you generate your own wind to help keep sails filled.
This could be a total load of horse pooh, but its worked for me....if you come last it wasn't my idea

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[ex-Gladys]

The critical aid for this situation is leech tell tales. You need three, attached between batten pockets. They will tell you if you have smooth flow off the leech. If they don't fly your leech is effectively hooked to windward (either traveller too far up or main in too hard). The way to use them is to sheet the main in until you lose flow and then ease enough to make them fly. They don't tell you (as luff telltales do) when you are sheeted out too far.



<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[john_morris_uk]

Sound advice - with the addition that in really light ghosting conditions some boats sail better with slightly flatter sails - because even the fulness of the powered up sail is enough to stop the flow and release of the air. As soon as the wind increases to more than a knot or two then you can power back up as the air will flow correctly.

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[Phoenix of Hamble]

Thanks Alec!

I think that sail cut generally has a lot to do with modern sail trimming activity..

My opinions on some of the comments.....

A full sail (ie one with the deepest point close to the mast) will provide a lot of accelleration through generating a lot of lift..... but it will be very poor in three other respects, the release, and what is technically known as the attachment bubble (this is the area of sail behind the mast where the air is disturbed and not attached to the sail surface, and the drag.... this means that while it generates lots of lift, it will ultimately be slower due to inefficiency.... so a flatter sail will both enable higher pointing and better maintenance of high end speed.....

Therefore it depends upon your boat type and sailing mode as to which is best..... lots of tacking (eg round the cans) then a fuller sail will help, although the fastest crew will also flatten off after terminal speed is reached, and subsequently fill out again before or during the tack.... and if you sail a heavy boat, you may either find that the power for acceleration helps, or in heavier seas, the weight of the boat will maintain momentum so a flatter sail will help.... light boats in heavy seas will get speed knocked off far more easily, and might therefore set a fuller sail.... trial and error i'm 'fraid!

As for sail cut affecting trim... absolutely.... sail makers besides overall plan shape have two main weapons in their arsenal.... broadseam and luff curve... sail panels may be cut with straight or curved edges before they are joined, and this curve is known as broadseam.... generally more broadseam = fuller sail (but not always - depends which edges, and which panels!).... luff curve is the amount of pre curve built into the sail, this is also clearly affected by mast characteristics especially softness, and also rigging set up, especially pre-bend and running backstays....

Finally, the other factor is overall sail pattern layout..... increasingly you will see modern sails cut with a radial pattern (eg all panels leading to the corners) rather than the traditional horizontal patterns, and with the advent of modern computer modelling this allows both much more intended management of sail shape prior to trimming up too much, and also much more refinement of characteristics in different parts of the sail..... so yes.... with computers, sail design is more technical, and there has been substantial improvement in shape and the ability to drive power out of it....

This alas does also provide plenty of opportunity to get really rotten performance out of a good set of sails through poor trimming!

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