transom outboard backing board - SailNet Community

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  #1  
Old 05-09-2010
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transom outboard backing board

Looking for advice on dimensions for a transom outboard backing board. I've been searching and asking around for a while now and can't find any answers!!

My boat is a '78 Columbia 8.3. The outboard going on the transom is a Mercury 9.9HP (long shaft) two stroke, which will be mounted on a spring loaded bracket.

I'm planning on using marine ply for the backing board, which will be epoxied to the inside of the transom ... all pretty standard.

My questions are:
  • What's a god thickness for the backing board?
  • What is a reasonable height and width for the backing board?

Thanks.

--Tim.
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Old 05-09-2010
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It depends on what the backing board is made of. I would recommend going with G10 garolite fiberglass board, rather than marine plywood. Garolite G10 fiberglass board doesn't rot, unlike plywood. Make the board as large as possible, and preferably epoxy it to the hull. The forces that the outboard will transfer are fairly large, especially if you use it in heavier seas.
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Any suggestions on dimensions? For the plywood or the G10?

"As large as possible" is a bit vague and not really helpful .... I could glass in a 1" thick piece over the entire transom, which would be "as large as possible," give an enormous safety margin, but is complete overkill.

I'm looking for what's reasonable and safe.


--Tim.
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Old 05-10-2010
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I would go with a piece that is the width of the transom if you have access to do so, but say 8-12" high based on the height of the outboard mounting bracket. Glass it into the hull on the outboard edges to give it extra strength and support.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 05-11-2010
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Thanks sailingdog, appreciate the specifics.
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Old 05-11-2010
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Glad to help.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 06-15-2010
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Well, I sorted out this transom a couple of weekends ago. Being me, I went for overkill and ended up using Nida-Core pourable transom compound. Grandma sent me a few boat bucks

The result is a power-boat style transom, 2.5" thick, that will never rot. Total weight added was about 70lbs.

This was a big project. It entailed about 20 hours of work down in the lockers under the cockpit, grinding off all the bilgecoat and primer, laminating a skin the same size as the existing transom, then glassing it into the boat, 2" away from the inside of the transom. This created a void for the Nida-Core PTC to be poured into.

Once everything was glassed, cured and any wax/dirt/etc removed, the Nida-Core PTC was simply poured into the void from above decks, using a tube and a big funnel.

Here's one of my favorite DIY boat pictures: 10 gallons of Nida-Core PTC poured into a milk jug funnel taped to a bit of PVC tube. My daughter was probably thinking "Dad, you're crazy!"




I know, I know .... ridiculous overkill and expense, but now I have peace of mind and will never have to deal with rotted core or wet backing blocks!!


--Tim.
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Old 06-15-2010
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It'll also stop small arms fire.

70lbs in the transom, wow, only thing worse would be to put it in the mast head.
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I agree with your daughter— YOU'RE CRAZY....

Quote:
Originally Posted by tmalcolm View Post
Well, I sorted out this transom a couple of weekends ago. Being me, I went for overkill and ended up using Nida-Core pourable transom compound. Grandma sent me a few boat bucks

The result is a power-boat style transom, 2.5" thick, that will never rot. Total weight added was about 70lbs.

This was a big project. It entailed about 20 hours of work down in the lockers under the cockpit, grinding off all the bilgecoat and primer, laminating a skin the same size as the existing transom, then glassing it into the boat, 2" away from the inside of the transom. This created a void for the Nida-Core PTC to be poured into.

Once everything was glassed, cured and any wax/dirt/etc removed, the Nida-Core PTC was simply poured into the void from above decks, using a tube and a big funnel.

Here's one of my favorite DIY boat pictures: 10 gallons of Nida-Core PTC poured into a milk jug funnel taped to a bit of PVC tube. My daughter was probably thinking "Dad, you're crazy!"




I know, I know .... ridiculous overkill and expense, but now I have peace of mind and will never have to deal with rotted core or wet backing blocks!!


--Tim.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 06-16-2010
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Quote:
Originally Posted by zz4gta View Post
70lbs in the transom, wow, only thing worse would be to put it in the mast head.
Wind Gypsy is an old Columbia 8.3. At 27' 4" OAL, with 9' 8" beam, the weight changes are going to put her trim forward, not aft. Even fully loaded she already sits forward and to port.

From a weigh perspective, I'm pulling out a 300lb engine, a 20 gallon fuel tank, and the OB and 'new' transom adds the total weigh of a new 4 stroke 10HP outboard, about 150 lbs.

Long term, this transom/OB project is part of an electric repower, which may be inboard or OB, haven't decided yet. The old inboard engine will be pulled next month, which will give me the opportunity re-trim by moving stuff around and by moving a lot of batteries to the engine compartment.

Can't sail until I've finished the s'brd deck re-core anyway, so worse case, she sits in the slip for 2 the next 3 weeks, slightly bow down and to port.

--Tim.
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