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Old 06-28-2016, 05:24 AM
Admirals Club Admiral's Club Member
Join Date: Apr 2016
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Default Building Catamaran Lift Bunks

Hey THT,

I just bought a new (to me) catamaran and needed to convert my existing 10k lift into one that could support a catamaran. I searched the forum for a How-To of sorts to lead me in the right direction, but couldn't find anything super useful. I figured that since I took the time to do it myself, I might as well post it in case it can help someone else out in the future.

I looked at a few different ways to do it including purchasing aluminum bunks of different varieties, using 2x12s to make bunks and even using no bunks at all! (The next three photos are not mine and I found via google searching)

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I cruised around the canals around me and found a few Cats sitting directly on the I-Beams, but this felt really uncomfortable for me (whether or not it hurts the hull is something I didn't look into nor something I wanted to risk). Purchasing aluminum bunks was going to be expensive and I'd have to wait for shipping (something like $1,400+ from what I could find), so I was left with making my own wooden bunks.

Since water level is not an issue for where I am located, there was no limit to how thick I could make the bunks. My I-beams are spaced such that they are close to 9' 6" center to center, and I opted for using 4x6x16' pressure treated lumber. Wood will compress under loading and provide support to all areas of the hull in contact with the surface.

I went with sets of 4x6 as opposed to 2x12 because I wanted to bond the boards together perpendicular to their loading direction. Bonding boards horizontally also came with the added benefit of not having to counterbore all of the top face bolts.

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I used 1/2 x 12" Stainless Bolts with washers and lock nuts. Bolts were spaced every 2' 8" with an extra bolt 6 inches from the aft end of the bunks. The bolts allow the load to be transferred between each piece of lumber in the event that the catamaran is parked slightly askew and rests on only one of the 4x6's. This large of a bolt is definitely overkill, but with everything related to boating, better safe than sorry! As a final measure, I bought outdoor carpet and used a staple gun to attach it to the lumber, while making sure it was taught to reduce the chances of it bunching and tearing. The bunches ends on the sides are where it overlaps an exposed bolt head. It has since been cut so the bolts are exposed for easy inspection.

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Next, I removed the old aluminum brackets holding the aluminum V-Hull bunks to the I beams.

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Left alone, they would be too tall to reuse on my new cat bunks, so I decided to cut them. Using a 10" metal cutting wheel installed in a miter saw, I cut the top ~2.5 inches off the aluminum brackets.

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After that, I did a quick test fit to make sure I cut them low enough and reinstalled.

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I attached the old aluminum brackets to the wood bunks via galvanized lag screws. Each bunk is attached to the brackets in four places, twice in the front and twice in the back.

I may go back and make another set of brackets to support the opposite side of the bunks if I deem it necessary, but I'll wait and see how these do for now.

All in all the project took me a little over 4 hours and cost about 300 dollars in parts!