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Old 06-15-2010
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Inducing Mast Bend

Hello,

I have a Islander 37ms, a keel stepped masthead sloop with single spreaders that run straight out from the mast, fore and aft lower stays, roller fuller on the fore stay, and a single back stay. The aluminum mast extends 40' above the deck.

It is very hard to get any mast pre-bend on this rig without cranking down the forward lowers and being a little light on the aft lowers, and then it ain't much bend.

I am going to paint the spreaders and was thinking of having a shop put a 3 or 4 degree angle on the spreader brackets. The thought here is that I would then add some pre-bend to mast by the always snug upper stays pushing the spreaders at the forward 3 to 4 degree angle instead of straight into the mast. Then I could just set the lowers to hold it where I want it to stay.

I am looking for opinions. Do you think this work without causing other issues, or problems with the spreaders/mounts ?

Thank you in advance for your assistance
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Quote:
Originally Posted by Grumpymx View Post
Hello,

I have a Islander 37ms, a keel stepped masthead sloop with single spreaders that run straight out from the mast, fore and aft lower stays, roller fuller on the fore stay, and a single back stay. The aluminum mast extends 40' above the deck.

It is very hard to get any mast pre-bend on this rig without cranking down the forward lowers and being a little light on the aft lowers, and then it ain't much bend.

I am going to paint the spreaders and was thinking of having a shop put a 3 or 4 degree angle on the spreader brackets. The thought here is that I would then add some pre-bend to mast by the always snug upper stays pushing the spreaders at the forward 3 to 4 degree angle instead of straight into the mast. Then I could just set the lowers to hold it where I want it to stay.

I am looking for opinions. Do you think this work without causing other issues, or problems with the spreaders/mounts ?

Thank you in advance for your assistance
I have to ask why you feel you need to redesign your rig.

Maybe you are as qualified as the guys who designed your boat. I don't know. But I generally figure that the Naval Architects who designed the boat are likely to have done a better job than I can do.

Having said that, I would probably start with an adjustable backstay.
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The rig does not have much adjustment to it and is set a little too far back on the boat so it develops quite a bit of weather helm as the wind picks up.

I normally sail in 12-20kts in the afternoons, and have been looking for a way to get rid of the weather helm other then reefing the main at 14kts.

I moved a bunch of weight from forward to just aft of mid ship which helped a lot.
I got a new main sail, which helped a little, but with both the old and new main I can not move the center of effort of the main far enough forward to get rid of that much weather helm and heel when pointing.

Thank you for your suggestion but I have a masthead, not a fractional rig, tightening the back stay does not induce mast bend. It just pulls on the fore stay.

I was asking for opinions here because I have seen some very knowledgeable folks on this board.
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Last edited by Grumpymx; 06-15-2010 at 11:55 PM. Reason: add
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Old 06-15-2010
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For such a single spreader rig, the max. of 'pre-bend' should only be about 3/4". I have a monsterously stiff (Seldén) mast and such requires to maintain a 15% cap shroud tension and a 3/4" pre-bend, I need to apply 20% tension to the lower forwards and loosen to about 10% on the lower afts. I also have intermediates and runners, so Im not too concerned about the slack aft lowers.

Changing the spreader angle by sweeping them aft will also require that you also relocate the chainplates ... I wouldnt do that.
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Old 06-16-2010
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Great response, thanks.

The "move the chain plates" was the over sight on my part that I was looking for. Glad you saved me from that one.

I have been loading the forward lowers to 15-16% and 8-10% on the aft lowers and it moves just off center, but the mast is pretty straight.
3/4 of an inch should do it. I just need to pull the middle of the sail forward a little.

I'll try loading the forward lowers more, and see what happens.

Thanks again
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After thinking about this for a minute I have a further question.

Would not the masthead, spreader tips, and chain plates to the uppers still align as they do now because the slight rearward rake in the spreaders would push the mast to move forward to create the alignment?

I can see that 3 to 4 degrees is the wrong way to think about it.

If the spreader tips were 3/4 of an inch behind the mast at the spreader junction with the direct line between the masthead and chain plate would that not make the mast pre bend to hold the line?

This is also a stiff Selden mast also, so we are talking apples and apples
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I pre-bend my trimaran mast (with a spreaderless masthead rig) with a tensionable babystay.

Works pretty well, was easy and cheap to fit and best off all it is a fully and easily reversible modification which I can loosen (or even remove) at all times...
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Two thoughts:

1. Call Selden. I have a Selden mast and was on the phone with them a couple of days ago with questions (including asking about inducing pre-bend in my masthead rig (although I have a baby stay)) and they were quite helpful.

2. How much rake do you have in the mast? If there is a lot of rake, that might account for the weather helm.
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Originally Posted by Grumpymx View Post
The rig does not have much adjustment to it and is set a little too far back on the boat so it develops quite a bit of weather helm as the wind picks up.
Then it isn't pre-bend you want. What you want is more less rake. Different thing altogether and used to accomplish a completely different end.

Please read Giulietta's most excellent rig tuning guide at Adjusting Your Rig before going any further. (Sorry, you'll have to copy and paste the URL. Sailnet's brain-dead text-munger would destroy it if I tried to do a handy link.) I'd also highly recommend Ivar Dedekam's Sail and Rig Tuning. You might wish to consider obtaining a Loos gauge, tho one can get by w/o it.

Btw: Normally you want aft lowers on a rig like yours to be little more than hand tight.

Jim

Last edited by SEMIJim; 06-16-2010 at 01:44 PM. Reason: Corrected recommendation
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In the Selden page you can find manuals for rigging your mast and other issues.

By the way, the giuliettas explanations is good and easy to understand.

the forum dont allows me to sending links but the page is in
3w.seldenmast.com
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