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mth180 06-28-2016 05:24 AM

Building Catamaran Lift Bunks
20 Attachment(s)
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!

Track Junkie 10-16-2016 05:30 AM

Just came across this...
Very informative and looks great. Did the 4 x 6s not add too much flotation to the lift when empty, to sink when lowering?

I'll need to fab something very similar in the next month or two.


mth180 10-16-2016 05:46 AM

Originally Posted by Track Junkie (Post 9574722)
Just came across this...
Very informative and looks great. Did the 4 x 6s not add too much flotation to the lift when empty, to sink when lowering?

I'll need to fab something very similar in the next month or two.


I ended up adding a 50 pound bag of sand to each ibeam to handle the floatation.

1NO REGRETS 10-22-2016 05:45 AM

the boat on the lift was mine, it was on that lift for 7 years with no problems. On top of the I-beams was 2x6 and conveyer belt RUBBER, it held no water. The bad thing about using carpet under the hulls is that it could promote blisters.the carpet holds water in the wood and itself. I was advised by a Pro. Since my lift is an elevator, it made it simple to slide on and off. It was the perfect solution with double I beams. That kept the hull away from the angled welded aluminum.

sugarloaf 10-22-2016 07:03 AM

I just ordered a second pair of bunks from the trailer manufacturer and bolted them on my lift.
Owen and Sons Trailers. Easy Peezy!

Thelimey1 02-11-2019 02:01 PM

Bunk types for catamarans
this is my first time posting and appreciate any insight to my questions. I have a 36 Aquila Catamaran on a lift which has two vertical bunks for each keel. We cannot get the hulls to evenly sit on the two bunks it always favora the outboard bunks. The hulls are assymetrical and i believe the outside of the hullhas a more pronounced curve than the inside. Several people have told me to just replace them with flat bunks and several people have said dont use flat bunks. Of course each has many different and often conflict8ng reasons for their opinions. After a few months of use it clearly looks like the outer bunks have taken on a curved shape that conforms to the hull side. The inner bunks however are still pretty straight with contact just in the middle. My next step is to move the inner bunks in a few inches in the hope they two take on the shape of the hull and hopefully the boat weight will better distributed. This could all be the wrong way of going about it so I am reaching out to all for comments and advice. Attached are two pictures that show the bunks and the shape the outboard bunks have taken on after 2-3 months of use.
Thanks in advance.

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