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Old 08-04-2009, 14:03   #1
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Question Adjustable Backstay

A question for all you cruising racing types. I have a mast head rigged 26 foot monohull with an adjustable split back stay. Is this meant to control mast rake or mast bend? If I understand what I have been reading here and elsewhere the back stay on a fractional rigged sloop controls mast bend but will it also bend a mast head rig or just remove the sag from the head stay and will it also add some rake to the mast? The boat was set up to race when I bought it so i have no idea how to use the back stay while under way. I await your expert replies.
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Old 09-04-2009, 04:40   #2
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Increasing the backstay tension straightens the forestay and enables the boat to point higher. Take care not to over tension the backstay.

Generally:

1. Ease the backstay to add draft to the genoa, for more power in light-to-medium conditions.

2. Tension the backstay, which tightens the forestay and bends the mast, to remove draft, for better pointing in heavier air.

3. The backstay adjuster should be eased, to release the load on the rig when not sailing.

The Grampian Owners website may have some general and/or specific information:
index

G26 Manual:
http://grampianowners.com/G26/g26manul.pdf
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Old 09-04-2009, 05:13   #3
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My thirty footer has a similar set up - split backstay - the system is easy to over crank - my mast has minimal bend but the forestay can certainly be tightened - adding height to windward. Over tensioning seems to stop the boat and make it feel dead. Under tensioning bags the headsail and reduces angle of attack. Sometimes I play with this for advantage - gaining height if around others towards marks for example. Always ease down wind and reaching. Does your boat have an inner forestay? That can change mainsail shape significantly via backstay tension which in turn will influence the "slot" between headsail and main. Every boat is different. Experiment with when you are around other boats of similar speed. You might be surprised how a little backstay tweak will alter your relative speed.
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Old 09-04-2009, 06:39   #4
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Most masthead rigs tightening the backstay pretty much only tightens the forestay as others have said, but some are set up different like the Ranger 26. It has the aft lowers in line with the mast and then conventional forward lowers, the spreader attachments have slotted holes to allow for the mast bend. The forward lowers are set tight enough to keep the middle of the mast from moving aft as the mast tip moves aft with increasing backstay tension, so you achieve mast bend as well as tightening the forestay. Of course this is not nearly as dramatic a bend as you can get on a fractional rig or with an adjustable baby stay on a mast head rig.

Mast bend flattens the mainsail decreasing power and making a better shape for pointing.

John
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Old 10-06-2009, 22:53   #5
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Mast bend flattens the mainsail decreasing power and making a better shape for pointing.
Seems counter-intuitive. Doesn't bending the mast allow the main to be fuller, and more powerful?
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Old 11-06-2009, 00:47   #6
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Seems counter-intuitive. Doesn't bending the mast allow the main to be fuller, and more powerful?

One of the ways sailmakers achieve sail shape is with luff curve. Put the fingers of your left hand together and have them straight. The tips of your fingers is the curve of the luff of your mainsail. Take the index finger of your right hand, hold it straight. Now put the luff of the mainsail, tips of your left fingers on the mast (right hand index finger) such that you're touching the inside of the finger (mast finger can curl toward the luff fingers). Keep the mast straight, let the middle fingers of your luff hand bend such that all the tips of your fingers are touching the straight mast. Your fingers on your left hand are now bent demonstrating the draft developed by putting a curved surface on a straight stick. Now bend your mast, as your right index finger bends, the luff (the tips of the left fingers) follow and straighten out. The sail is now flat or close to flat. A flatter sail develops less lift and the wind can come from a direction closer to parallel to the sail without causing it to luff. The sailmaker has to know something about the bend characteristics of the mast in order to put the curve in the right place on the luff.

On the other hand if you bend the middle of the mast aft you will get a fuller sail (Or break your index finger).

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Old 11-06-2009, 00:49   #7
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Depending on the set up - bending the mast can de-power the top part of the main which can make boat handling easier in heavy conditions.
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Old 11-06-2009, 01:03   #8
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Found some videos. The top two show the sail flattening with mast bend. Dinghies without backstays use the leech of the sail to bend the mast.

Finn Sailing; Video Sail Controls

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Old 11-06-2009, 04:45   #9
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Depending on how the rig is set up you can bow the mast dramatically. The mast head rig will often have a crane which is an eccentric. You can pre-load the rig with lowers or you can also pre-load the rig with the partners. If the rig is fairly high tech it is quite easy to induce 2~3 sections at maxd tension.

Play around with your rig tune sighting up the spar as you go. If you develop a hard crease in the main from the clew to mid mast section you have over bowed the rig.
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Old 11-06-2009, 07:13   #10
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Great visualization cal40john! I guess I was visualizing the leech getting rounder; your explanation makes complete sense.
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