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post #21 of 36 Old 05-20-2016
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Re: Mast dilemma

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Originally Posted by bobperry View Post
The choice of mast is not a good place for guessing. I have never seen a grp mast.
If I read this from Bob Perry, my choice would be made!
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Quote:
Originally Posted by MastUndSchotbruch View Post
If I read this from Bob Perry, my choice would be made!
Agreed
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post #23 of 36 Old 05-20-2016
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Re: Mast dilemma

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there was a day when they said you can't build a stiff boat hull out of fiberglass it just will not work. I built a fly a fiber glass airplane and it is all about fiber orientation. A fiberglass mast is very doable . they were expensive to build and the resin technology was being developed at the same time as carbon fiber technology so now carbon makes more since. if they had not developed carbon fiber and had todays resins masts would be fiberglass.
I didn't say you couldn't, I said you shouldn't. A fiberglass mast on most boats very quickly runs into a material properties limitation.

................................fiberglass......al uminum......Douglas fir
Stiffness (psi)............1.2 x 10^6 ...10x10^6 .......1.8x10^6
Density (lb/in3).......... .055 ........... .1 ............... .02

So a fiberglass mast of the same dimensions would have about 60% the stiffness and weigh almost three times more. And when compared to an aluminium mast it would have about 20% the stiffness but weigh half as much. But masts aren't designed based on weight, that's just a necessary evil, masts are designed based on stiffness. So a fiberglass mast is going to have to be much heavier than either a wood or aluminium mast at the same stiffness.

Of course the RM(30) of the boat is going to go down with the heavier mast, so you may not need quite as stiff a mast as you otherwise would, simply because the boat will heel over further sooner.


The only boats I could see reasonably using fiberglass in a mast section are those boats that need radical flexibility of the mast. Something like a DN ice boat. But even there I am pretty sure they are using carbon designed to flex, not fiberglass. I know on fast catamarans flexibility is designed into the carbon laminate schedule.


If the fiberglass section is actually lighter than the wood, then one or both of them is seriously under built.
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Last edited by Stumble; 05-22-2016 at 02:57 PM.
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post #24 of 36 Old 05-20-2016
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Re: Mast dilemma

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Quote:
Originally Posted by Stumble View Post
I didn't say you couldn't, I said you shouldn't. A fiberglass mast on most boats very quickly runs into a material properties limitation.

................................fiberglass......al uminum......Douglas fir
Stiffness (psi)............1.2 x 10^6 ...10x10^6 .......1.8x10^6
Density (lb/in3).......... .055 ........... .1 ............... .02

So a fiberglass mast of the same dimensions would have about 60% the stiffness and weigh almost three times more. And when compared to an aluminium mast it would have about 20% the stiffness but weigh half as much. But masts aren't designed based on weight, that's just a necessary evil, masts are designed based on stiffness. So a fiberglass mast is going to have to be much heavier than either a wood or aluminium mast at the same stiffness.

Of course the RM(30) of the boat is going to go down with the heavier mast, so you may not need quite as stiff a mast as you otherwise would, simply because the boat will heel over further sooner.


The only boats I could see reasonably using fiberglass in a mast section are those boats that need radical flexibility of the mast. Something like a DN ice boat. But even there I am pretty sure they are using carbon designed to flex, not fiberglass. I know on fast catamarans flexibility is designed into the carbon laminate schedule.


If the fiberglass section is actually lighter than the wood, then one or both of them is seriously under built.
Thanks for the info. The wood one is heavier and slightly thicker. 2 cm in diameter. Both are 13m. So if you had your choice, what would it be?

I've talked to a reputable woodenboat builder here and he suggests to use the fg mast, but keep the wood one off to the side for a year to feel it out. He likes the wooden one because its traditional and he knows wood. If I'm not satisfied with its performance, I can always change it out to the wood mast. The rings and stays are compatible. I feel it is slightly too much weight for the boat, but that's what electric plainers are for, no?
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Re: Mast dilemma

I honestly don't know what to tell you. A solid wood mast is a ridiculous use of materials, almost all of the stiffness is coming from the outer 1/2" or so of timber, the rest of the weight is just there because of poor construction processes. The fiberglass mast, as discussed, is just as silly because it's fiberglass.

If it were me I would put whatever was on the boat before back on and use it as is. But then I would either be in the market for an aluminium mast in the near future. I don't trust wood masts because I live in a swamp and wood rots down here very quickly, in a cold climate they make more sense.

But a wooden mast should be hollow not solid, at which point it actually becomes weight comparable to an aluminum one. Maybe a little heavier, but not outrageously so.

The other option is to measure the moments of intertia of the two masts and then decide, but I have never done this and wouldn't know where to tell you to start.

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post #27 of 36 Old 05-21-2016
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Re: Mast dilemma



enough said!

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post #28 of 36 Old 05-22-2016
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Re: Mast dilemma

This is a very interesting post and I'm looking forward to the ultimate outcome!

At the risk of looking stupid after Bob seems to have accepted the argument below, I wonder if it really offers a legitimate comparison. To compare those numbers as absolute, wouldn't you be comparing a mast of solid wood to one of solid Al to one of solid FRP? Or a hollow mast of the same dimensions of the three materials. Of which neither situation has been suggested.

IF the FRP mast has been properly engineered and built, and has been used successfully in sisterships, I'd certainly consider it.

That said, I am not familiar with the loads and consequences of the loads of the gaff jaws on the mast when reefed. Too much flexibility could come into play with that.

Quote:
Originally Posted by Stumble View Post
I didn't say you couldn't, I said you shouldn't. A fiberglass mast on most boats very quickly runs into a material properties limitation.

................................fiberglass......al uminum......Douglas fir
Stiffness (psi)............1.2 x 10^6 ...10x10^6 .......1.8x10^6
Density (lb/in3).......... .055 ........... .1 ............... .02

So a fiberglass mast of the same dimensions would have about 60% the stiffness and weigh almost three times more. And when compared to an aluminium mast it would have about 20% the stiffness but weigh half as much. But masts aren't designed based on weight, that's just a necessary evil, masts are designed based on stiffness. So a fiberglass mast is going to have to be much heavier than either a wood or aluminium mast at the same stiffness.

Of course the RM(30) of the boat is going to go down with the heavier mast, so you may not need quite as stiff a mast as you otherwise would, simply because the boat will heel over further sooner.


The only boats I could see reasonably using fiberglass in a mast section are those boats that need radical flexibility of the mast. Something like a DN ice boat. But even there I am pretty sure they are using carbon designed to flex, not fiberglass. I know on fast catamarans flexibility is designed into the carbon laminate schedule.


If the fiberglass section is actually lighter than the wood, then one or both of them is seriously under built.

Last edited by ggray; 05-22-2016 at 12:04 PM.
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post #29 of 36 Old 05-22-2016
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Re: Mast dilemma

Grey: In a standard mast calculation the gaff jaw loads would not enter the equation.

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Re: Mast dilemma

Thanks; good to know.
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