Mast Pre-Bend

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Morgan Dewees
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Mast Pre-Bend

Post by Morgan Dewees » 14 Nov 2007, 19:22

I am trying to understand the purpose of mast pre-bend. I believe it is to allow one to maintain a straighter mast when the rig has significant head and back stay tension. I have found using a 7075 grade mast that i can just barely bend the mast with rig tension as it is so i am wondering if i am missing something.
Morgan Dewees

Barry Fox CAN262
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Design: V8
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Post by Barry Fox CAN262 » 15 Nov 2007, 01:44

On my "good" boat I found that similar condition when I first built its rigs. I was trying to be trick so I carefully bent (pre-bent?) my masts, built them up and put them on the boat and found what I thought was that same condition. Way too much tension just to get the mast back to straight.

Then as I sailed it I found that I had some little speed ripples in the luff of the main that I could really only seem to limit by reducing back stay tension. That worked but eating away at the back of my mind was too little back stay tension. But it worked so I was happy.

But one day I had to go out and rescue someone else's boat so I handed my radio to someone to sail my boat. While I was out rescuing I decided to watch the boats from the water and was amazed at how much the jib (on many boats including mine) were sagging in what was really not much more than just very nice wind.

On shore I comiserated with one of my fellow sailors wherein he allowed that he had the same kind of thing happen to him so he discovered that way more mast ram tension than he thought to be needed coupled with the back stay tension he wanted to use helped him eliminate those speed bumps on the sail. So I tried that and presto I had the back/fore stay tension I wanted and could tune out the bumps.

So then the other day I got to watching the mast bend discussion on the Victoria forum and the lightbulb came on that I should run some prebend so that I can bring the stay tension up and not have the mast ram as extended as I have been doing. So one day I am going to take my rig apart and give it a try. It should work. . . . . .I think.
Barry Fox
CAN 46
Vancouver Island, BC, Canada

Morgan Dewees
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Post by Morgan Dewees » 15 Nov 2007, 01:55

What grade of Aluminum is/was your mast, i think mast stiffness is also a factor?
Some full size boats, particularly Lightnings, point higher and faster if the jib is sagging so i'm not certain that's all so bad as long as it is not oscillating
The ripples, where exactly were they on the luff, i have some in the bottom 20% or so that i cannot get rid of with downhaul?
Morgan Dewees

Ralph Knowles
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Post by Ralph Knowles » 15 Nov 2007, 03:18

Barry

If you have a keel stepped mast and free movement fore and aft at deck level, then with the rig set up for windward sailing, gently press the mast aft at the deck level. This should flatten out the wrinkles. Take note of how much you have moved the mast back, then screw your ram back to there. If you don't have a ram, then fit blocks or wedges into the deck slot to make up this movement. A deck stepped mast is not so easy to adjust, you would need an angled screw adjuster or lower adjustable stays.

Regards

Ralph.
Bring on the Breeze!

Ralph

Barry Fox CAN262
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Post by Barry Fox CAN262 » 15 Nov 2007, 05:47

Thanks Ralph that is my discovery. I am keel stepped so the mast ram is in place. I used to just run it back until it just touched the mast before loading the back stay. Now I am so much smarter that I can finally use some more of the tools!!!

Morgan - Now for more ignorance on my part. Seems I used to know teh answer to your tubing question but now I can't remember except that they came from GBMY.

If the winds were steady and you knew how much it would sag then maybe you could let it move but what I observed from water level was that the sag was moving almost all the time. It appeared the sail would load up, the sail would sag and release some pressure and then the cycle would start all over again. My racing car experience tells me that you don't want the vehicle adjusting itself as you go and that you want settings that you know are consistent.

The ripples were just slightly back from the mast and generally in the lower half. Not very long but visible.
Barry Fox
CAN 46
Vancouver Island, BC, Canada

Barry Fox CAN262
Posts: 354
Joined: 21 Apr 2007, 17:54
Sail number: CAN 46
Club: VMSS
Design: V8
Location: Vancouver Island, BC, Canada

Post by Barry Fox CAN262 » 15 Nov 2007, 06:04

Morgan - Incomplete answer last time.

Downhaul tension change didn't help any of it.
Barry Fox
CAN 46
Vancouver Island, BC, Canada

Lester
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Re: Mast Pre-Bend

Post by Lester » 15 Nov 2007, 10:23

Morgan Dewees wrote:I am trying to understand the purpose of mast pre-bend. I believe it is to allow one to maintain a straighter mast when the rig has significant head and back stay tension.
Hi Morgan
The end result that is desired is greater jibstay tension, so the jib luff doesn't sag (and then oscillate or pant) excessively at higher wind speeds. Of course, if you could guarantee higher wind speeds, you could always cut a higher jib luff allowance that accomodates the forestay sag, and you wouldn't need the mast prebend. But most IOM jibs are cut with very small luff allowance to allow effective performance ih lighter winds.
I have found using a 7075 grade mast that i can just barely bend the mast with rig tension as it is so i am wondering if i am missing something.
The ability of an aluminium mast to bend is much more a function of its wall thickness than of its alloy. Perhaps you have a heavy (thicker-walled) mast.

The other factor is whether your mast is deck- or keel-stepped. You will struggle to induce mast bend due to rigging with a deck-stepped mast, since it needs something to react against. A keel stepped mast reacts against the mast ram, some 120 - 150 mm above the mast foot, while a deck stepped mast foot is perhaps 30 - 50 mm below the ram. As Ralph notes, you need lowers (checkstays) here if you want to take this seriously on a deck-stepped design.

Good luck!
Lester Gilbert
http://www.onemetre.net/

Morgan Dewees
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Post by Morgan Dewees » 21 Nov 2007, 19:13

I'm back to thinking about bend again. It sounds like one wants their headstay as tight as possible, ie piano string? What does the pre-bend look like, how much, through what extent of the mast is typical and how do you bend the mast in a controlled manner? The mast i am working with came from Great Basin, 36" lengths of 7075 aluminum made by Easton.
Morgan Dewees

Ken Dobbie
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Mast Bend

Post by Ken Dobbie » 22 Nov 2007, 04:28

If you refer to the TS3 manual at http://www.tsdesigns.com.au/ts3Instruct ... al_1_0.pdf Page 7 will describe how to go about applying pre-bend. My high tensile aluminium jig is 800mm not the 650mm shown in the manual, I find this works better for me.


Regards


Ken

Dick Carver
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Post by Dick Carver » 22 Nov 2007, 18:04

My experience has been : 6061 T-6 will bend pretty easily. You can get a nice curve in a 6061 mast.

Don't try to bend a 7075 T-9 Easton mast. Chances are pretty good it will snap well before you can get a significant amount of pre bend into it.

The 7075 Easton sections should be plenty stiff enough to get good jib stay tension without mast pre bend.
Dick Carver

Steve Landeau
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Post by Steve Landeau » 22 Nov 2007, 21:37

Dick Carver wrote:
Don't try to bend a 7075 T-9 Easton mast. Chances are pretty good it will snap well before you can get a significant amount of pre bend into it.
The 7075 Easton sections should be plenty stiff enough to get good jib stay tension without mast pre bend.
Agreed. The Easton spar is tempered to an extent that it will not take a bend easily. The only way we were able to efficiently bend it was with a compression wheel assembly, but it was ineffective because getting it bent in the right places was too difficult this way.
The 12.4mm section is plenty stiff enough to get the job done without prebend. I've been using it for years with good results. I think it's a great spar especially for the new IOM skipper because prebend is not necessary. Prebend obviously works, as most skippers are using it, but it's a learning curve that you're best not to have to worry about in the early going.
The 11.1mm section will be good in all but the highest #1 rig conditions, but I prefer the 12.4.
Steve Landeau
AMYA 10859
IOM USA 112
Finn USA 112
Cal 25 #548

Morgan Dewees
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Post by Morgan Dewees » 22 Nov 2007, 21:37

Thanks for the good info, TS Designs jig looks easy to build but not relevant unless i want to break my mast.
Morgan Dewees

Nigel
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Post by Nigel » 14 Mar 2008, 15:51

This thread is a little old, but I don't come here all that often anymore.

Can anybody name suppliers for 7075 T-9 Easton mast tubes that are IOM legal?

Many thanks!
Nigel Winkley
GER 87

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Olivier Cohen
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Post by Olivier Cohen » 14 Mar 2008, 17:08

PG Modelisme in Pierrelatte sells 7075 tubes, in 2 meters long

http://pgmodelisme.free.fr/

Nigel
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Post by Nigel » 24 Apr 2008, 17:44

Thanks for the info. I managed to get some. 8) They are about 25g lighter than the regular masts with about the same stiffness.
Nigel Winkley
GER 87

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