Replace my lift bunks…using joists bolted to my dock while lake is dry. 23RSB.

Vpracer

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So our lake is way down and I currently have dry ground underneath my boat which is in a lift. Want to take advantage of that by replacing my lift bunks. I have a fixed wooden dock with 6x6 posts. My thought is to bolt up some joists underneath the boat (19’ across) and lower the lift so that the boat rests on the joist while I work on the lift runners. Thinking that a 2x8 single joist for the front and then a double joist in the back would be enough to support the boat. Please tell me if you think this could work or will my wife find me under the boat buried in the mud…..ha. Thanks.
 
I think that many in this forum would like to help but you have failed to supply sufficient detail. Last year for a different purpose I successfully lowered bunks while supporting the boat in a different manner. I used vertical posts and horizontal joists but the devil is in the details. You have a thought provoking project that many would enjoy sharing. Please send more detail.
 
Thanks for the reply and your thoughts. Not sure what other info I can provide. Basically the boat is hanging in the lift now. No water underneath. I want to lag bolt some joists to the boathouse supports (6x6) posts at the front and rear of the boat. The joist would be outside the cradle itself so that the Cradle can move freely up and down. The front and rear of the toon logs would rest on the joists. Then the lift cradle will lower down in between the joists and the boat pontoon logs will rest on the bolted up joists. The cradle itself will lower to the ground where I will put on new aluminum bunks, from ground level I.e. the lake bed. The distance between the post is around 18’ so the joist would span that distance.

I thought of trying to hang the boat from the rafters using the lifting eyes but I am afraid of bending or hurting the floor of the boat as the weight comes down. The rear eyes specifically are way underneath the boats decking so a rope or cable can’t go straight up. This Is why I shifted to the idea of joists holding the boat about 7’ in the air while I work underneath on the cradles. I may use a 2x10x20 board as my joists. I would double them in the rear given the weight. Rear joist would be 2 boards glued and screwed together for extra strength. I don’t think a vertical solution works because of the sandy lake bottom.

Just wondering thoughts on whether the joist idea would support the boat or whether anyone else has done something similar?
 
I think this is the worst idea I've ever seen in any boating discussion. Anywhere. And I've been boating for 33 years.

I'm not an engineer, but I built large decks as a side hustle for 20 years. You are proposing to span 18 feet and support a load of unknown weight overhead while you (and maybe others) work under it? And you plan to use one 2x10x20 in the front and two in the back? And you plan to lag screw them into the uprights instead of through bolting?

I just took a quick look at a floor joist span table. I'm going to take a WILD GUESS your boat weighs 4000 pounds. (It would have behooved you to weigh it before it became stranded on the lift). I'm also going to GUESS 1/3 of the weight is forward of the midpoint of the boat and 2/3 is on the back due to the motor and fuel tank. So the front joist would need to carry about 1320 pounds and the rear double joist about 2680 pounds.

Divide those numbers by the 18 foot span and you get 73 pounds per LINEAL foot on the front and 149 pounds per LINEAL foot on the rear.

I found a span table for 75 pounds PER SQUARE FOOT live load. A #2 grade SYP joist ON 16" CENTERS with a 75 pound PER SQUARE FOOT load can span 10' 9" --- close to half of what you are proposing.

I can't find any span table over 100 pounds per square foot.

Even if the span table rated the joist to carry the load you are estimating, NONE OF IT APPLIES! Joist span tables assume the joists are used in a system. The span calculations don't apply for a single member acting on its own. Joists in a system share the load and rigid attachment to an end structure (and mid span blocking for long spans) prevent the joists from laying over. Tables also assume a minimum bearing surface on each end, not screwed to a vertical member.

Just skip this project until the water comes back. Take your boat off the lift and replace the bunks without the boat overhead. Do it when the water is 3 feet deep or less if you want to or do it with the aid of a boat. Or pay to have someone do it. I'm guessing there are tens of thousands of boat lifts in this country and people replace the bunks periodically without suspending their boats over their heads.
 
Thanks for the reply and your thoughts. Not sure what other info I can provide…
Just wondering thoughts on whether the joist idea would support the boat or whether anyone else has done something similar?
You have provided a significant amount of new information. Overall a great idea.Now that you have explained what you are doing I can see that what I did to support my boat is not appropriate for your application. Your proposal to support the tubes on edge of the joists might deform the tubes do the very low contact area. On the other hand it might not because Bennington has a solid keel. I had a neighbor that dragged his boat up on the beach on round posts. This dimpled the bottom. His widow gave me the boat and I recently replaced the tubes do to this and other reasons. If needed you can reduce the high contact stress by increasing the contact area. With regard to the bending stress of the joists, In the middle of a 18’ span you have 2 high loads 6 foot apart. I am mildly concerned about that. If needed you can install a mid span support screwed into the side of the joists and going down to a pad on the ground. To find out if any action is needed slowly lower the boat on the joists while observing deflection of the tube and joist. If the deflection seems excessive take appropriate action. Lastly be careful and take things slow.
 
Well thanks for the additional commentary DVW.

This is the internet and frankly, I asked for it......so I read and try not to take comments personally.

Maybe I should win an award for having the dumbest boating idea in the last 30+ years?

Helpful suggestions of course are always appreciated.

I shifted gears a bit because I'm not an engineer, as such I could not discern a whole lot of real world practicality from span tables in this particular situation.

I did however stay at a holiday inn express last night so I came up with another idea and completed the project yesterday except for the center toon bunks that I'll finish today.

I suppose the moral of the story is never tell a country boy that he can't do something.

Thank you all for the comments.

Boat Lift.jpg
 
^ now THAT was a good idea! Glad you got the project finished.
 
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