Time for new bunks

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tom7219
Just Enlisted
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Joined: Wed Mar 01, 2017 4:20 pm
Sailboat: MacGregor 25

Time for new bunks

Post by tom7219 » Fri Jan 26, 2018 3:03 pm

My bunks are getting old. The starboard bunk is in especially bad shape. The sections forward and aft of the rear bunk support have warped downward and the bottom of the hull has a depression in it from too much weight resting directly over the steel support. My question is two part. First, how do I bend the new bunk board to fit the natural curvature of the hull? Second, is there anything special I should do about the dimple in the hull, or should that settle out to conform to the new bunk?

Have already added a second axle to make the trailer more stable. With two axles, it rides far more smoothly. Before the second axle, the trailer bounced alarmingly. The second axle also has an electric brake, which should decrease stopping distance. Other than the bunks, the trailer is in good shape.

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Tomfoolery
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Re: Time for new bunks

Post by Tomfoolery » Fri Jan 26, 2018 3:42 pm

tom7219 wrote:First, how do I bend the new bunk board to fit the natural curvature of the hull? Second, is there anything special I should do about the dimple in the hull, or should that settle out to conform to the new bunk?
To the first question, I'm guessing 'you don't'. The boat will put the bend in it, thereby spreading the load over the length of the boat. If you pre-bend it to conform to the hull, the load will be concentrated at the steel supports underneath. Might as well just use short blocks at the supports.

Just a guess without seeing it, though. In woodwork clamping, the concept manifests itself in the form of a caul, which is a slightly curved piece of heavy stock that, when clamped at the ends, distributes the clamp force over the length of the caul as it flattens. Similar to flattening a leaf spring if you clamped the ends to a flat surface.

To the second, I suspect the dimple was caused by the permanently bent bunk which no longer distributed the support along the hull, and I suspect it will mostly come out on its own over time.

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topcat0399
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Sailboat: Venture 2-24
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Re: Time for new bunks

Post by topcat0399 » Fri Jan 26, 2018 4:17 pm

I changed mine a couple of years ago.
Just put the new boards on and they will
assume the new shape from the boat in no time.
No pre-shaping required.

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The Mutt
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Re: Time for new bunks

Post by The Mutt » Wed Jan 31, 2018 3:07 pm

The timber under the carpet on Imagine's trailer bunks started collapsing about 7 years ago, I removed the old beds repainted the steal then replaced the beds with future wood (wood/plastic composite left over from a deck I made) and topped off with a layer of Ultra-high-molecular-weight polyethylene, that the really hard slippery plastic used on ski boat trailers.

Can't tow without strapping Imagine down to the trailer now, the trailer beds are too slippery.

Before changing the beds over Imagine use to stick to the carpet beds every time we went to launch, the 40hp outboard had problems breaking the bond.

Fair winds
Glenn

tom7219
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Sailboat: MacGregor 25

Re: Time for new bunks

Post by tom7219 » Tue Feb 13, 2018 7:18 pm

Thanks for the guidance. Still have one concern. As the new bunk bends, if bolted at both ends, it won't be free to conform - the length when curved is greater than with a straight line. Or is that difference not significant? Any insight will be appreciated - I don't want to damage the hull. Also, how long does it take for the wood to bend to the right curvature? Thanks :P

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Tomfoolery
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Re: Time for new bunks

Post by Tomfoolery » Tue Feb 13, 2018 7:41 pm

Wood will stretch, connections will elongate, and the steel will also flex elastically (or plastic-ly, if loaded high enough). And the curve is pretty shallow, so not much chord length change. And besides, you want the bunk to resist at least somewhat, so the load is distributed along its length and not just at the hard points where it's mounted.

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