Alumium vs wood mast weight? - SailNet Community
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post #1 of 21 Old 04-05-2008 Thread Starter
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Alumium vs wood mast weight?

I know this question is going to sound like "how long is a piece of line" but here goes...

I have a 1975 Formosa 41 single headsail ketch. She was originally built (and designed to have) wooden masts. The boat I have has recently built and installed aluminum masts.

People have commented that the aluminum masts will "save considerable weight aloft". But how much? Is there any way to guesstimate how much masts of each type would weigh? Or how much of a percentage decrease in weight aluminum spars give?

Main is 52' high
Mizzen is about 38' high.

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post #2 of 21 Old 04-06-2008
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Aluminum spar ... easy ... lbs/ft depending on extrusion ... pretty close.

Wood ... solid or hollow core, fir or spruce, calculate cu/ft of material required and multiply by lbs/cuft ... good ballpark.

Moitessier used telephone poles and seemed to do just fine.

Cough up a few bucks and consult an expert for the exact figures.

Weight aloft? Sometimes we get sooooo hung up on numbers that really don't mean squat ... like how much does my ballast weigh.

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post #3 of 21 Old 04-06-2008 Thread Starter
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Originally Posted by cockeyedbob View Post
Weight aloft? Sometimes we get sooooo hung up on numbers that really don't mean squat ... like how much does my ballast weigh.
Well I suppose it does sound a little like a personal question. "You seem nice but how much does YOUR ballast weigh?"

It's actually a question I ask to better facilitate sleep. Having numbers all over the map for my boat and hearing rumors of Taiwan shipyards tossing beer cans and scrap into the keel for ballast I am looking for reassurance that when the storm tries to capsize us our boat will smartly stand to attention. If the ballast is lacking it wouldn't be too big a job to add more, but we're heavy already and wouldn't want to do this if I don't have to.

Cheers!

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post #4 of 21 Old 04-06-2008
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I doubt you will have an issue. Your original post states its outfitted with Aluminum masts already. IF you can find the weight for the original wood masts - its should be about a 30-40% (conservative) weight savings over solid wood. Your not going to have much issues with righting with that boat - Formosas are well regarded for being performance oriented and excellent cruisers (over 200 of them built)...Doug Peterson the designer .

You should be able to sleep well - your boat will take of you...

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post #5 of 21 Old 04-06-2008
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Heheheh gota a sense of humor ya has.

"Well I suppose it does sound a little like a personal question. "You seem nice but how much does YOUR ballast weigh?""

To answer your question, not certain yet ... 8,424 lbs of wheel weights yielded 7,291 lbs of 50 lb ingots ... with propane, cost came to about .247 cents/lb ... add keel plate weight of 10.2 lbs/sqft and bob comes up with a number that really doesn't matter much 'cause ol' bob's not really into numbers ... I'll load 'bout 80% and once rigged and stores are aboard, I'll add 'til she's on her lines and later adjust accordingly ... how much? well, I 'spose 'bout just enough, whatever that comes out to be.

Don't know how much ballast in the Capri either ... seems to be enough 'cause I haven't tossed the mate ... yet.

"... I am looking for reassurance that when the storm tries to capsize us our boat will smartly stand to attention."

Sometimes the sea wins ...

oh yeah ... project boat ... aluminum spar 361 lbs, douglas fir (hollow) 619 lbs, steel tube 414 lbs ... haven't decided yet ...

bob
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Last edited by cockeyedbob; 04-06-2008 at 05:08 PM.
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post #6 of 21 Old 04-06-2008
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This is a post that is wandering all over the place. Starting with the basics, the answer is heavily dependent on the particular species of wood used. It is further complicated by the fact that aluminum spars are designed around their stiffness (E x I) while wooden spars are designed around they strength in bending (S x F) and further complicated by the fact that wood is designed with a larger safety factor than typically used for aluminum.

Strictly speaking a Sitka Spruce spar will weigh within 5- 10% of an aluminum spar when designed for Deflection only. When designed for bending with proper safety factors the Aluminum will be 10% to 15% lighter than sitka spruce, especially when you consider the lighter hardware and few lighter fastenings typucally used with aluminum.

When you talk about Asian built boats like the Formosa in question, heavier weight species of wood were used and so the aluminum spars can even lighter than that. (Steel spars are generally 20-55% heavier than properly designed aluminum for the required strength and stiffness partially because steel does not come in a shape that is efficient for building a mast.)

Jody: This is not the mostly well built and nicely designed Peterson designed, Kelly-Peterson 44 and46, but the alledgely Garden designed Formosa 41 which used all kinds of different materials and amounts of ballasting and which are the poster child for the dubious quality character boats that gave Taiwan is questionable reputation.

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post #7 of 21 Old 04-06-2008
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Jody: This is not the mostly well built and nicely designed Peterson designed, Kelly-Peterson 44 and46, but the alledgely Garden designed Formosa 41 which used all kinds of different materials and amounts of ballasting and which are the poster child for the dubious quality character boats that gave Taiwan is questionable reputation.
I wasn't aware of that - thanks for the clarification and valuable correction...

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post #8 of 21 Old 04-06-2008
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I read wood was about 8 lbs/ft, Alu was just under 6 lbs/ft and CF was just over 3 lbs/ft

I think my deck stepped wood mast weight around 416 lbs

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post #9 of 21 Old 04-06-2008
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Sitka spruce weights 25 to 27 lbs per cu ft, and Aluminum weighs165-168 lbs per cu ft but that provides little info about their relative weights as a finished spar since the wall sections are so different.

PDP your numbers (8 vs 6 lbs) would be pretty close to the correct weight proportion if your mast were douglas fir rather than sitka spruce. (Roughly 6.03 to roughly 7.6 lbs per foot comparing a Kenyon 6092 cruising mast section to a Douglas fir spar with equal sectional properties adjusted for modulus of elasticity) The weight for wood is a little too high for Sitka Spruce.

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post #10 of 21 Old 04-10-2008 Thread Starter
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...Formosa 41 which used all kinds of different materials and amounts of ballasting and which are the poster child for the dubious quality character boats that gave Taiwan is questionable reputation.

Jeff
Whoa there Jeff! That's my girl you're talking about. Since we're mud slinging now I'll tell you something that I was going to leave unsaid. I saw YOUR boat the other day rafted up to a sleek racing sloop of ill-repute in a dark, secluded harbor. Now I won't make any assumptions, but I will say that those two boats were pitching and yawing quite a bit for a harbor with calm water.....

How about that for a boat with a dubious reputation??

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