Boat lift advice

[flyboycj84]

Trying to get some advice about this lift mod. The boathouse came with a center mount lift that isnt strong enough for my new boat. I plan on hanging a 24" drive pipe support from the block setup, with through bolts, in the picture below. There will be one of these at each corner of the lift, with 2 drive pipes and motors running parallel to the boat. The total lift needs to lift about 10k. the motors are all good, im just looking for advice as to whether or not this 2foot section of 6x8 lumber will support 2-3000lbs as there is one at each corner. i gotta think this short of a piece of lumber is pretty stout. im chatting with a dock guy tmrw, but was just lookin for some thoughts here if anyone knows how strong 6x8 lumber is. Thanks! pics a few posts down

 

[KC8QVO]

I don't think your upload of the picture worked right, I can't see it.

That having been said, I would imagine that piece of lumber would be OK. However, make sure the load on it is well distributed (don't hang all the weight off one bolt through the middle - use a few and a steel plate on the opposite end as the load to distribute). Same goes for how the lumber is connected to the rest of the structure.

Also keep in mind that lumber will weather and deteriorate over time, even the treated lumber. Out boat house used to have an old ~20ft Limen hanging in it. That's been gone for years now, but the general structure I'd never trust hanging 1/4 of the boat these days.Most of our docks and cribbing structure is 4x6 and 6x6, but where the hoist used to be is about the 6x8 size, maybe a bit bigger.

 

[HalfFish5087]

Go to photobucket, create an account, upload the photo and copy the html coding in your post. A short piece of 6x8", 2 feet in length, should support around 20,000 lbs. But, connections make all the difference. I would suggest throwing a cable over or fashioning a support to hang on the lumber vs. drilling any supports. Side note, treated lumber actually gets a reduced factor because of the injection process, but will of course last longer.

 

[flyboycj84]

 

[flyboycj84]

Go to photobucket, create an account, upload the photo and copy the html coding in your post. A short piece of 6x8", 2 feet in length, should support around 20,000 lbs. But, connections make all the difference. I would suggest throwing a cable over or fashioning a support to hang on the lumber vs. drilling any supports. Side note, treated lumber actually gets a reduced factor because of the injection process, but will of course last longer.

thanks for the input. Do you think drilling significantly reduces strength of the wood? The cradle cable will be compounded, and will terminate on a 5/8 stainless eye bolt that goes throught the actual piling (leftover from old lift) so I imaging the piling will be supporting some of the load too.

 

[HalfFish5087]

thanks for the input. Do you think drilling significantly reduces strength of the wood? The cradle cable will be compounded, and will terminate on a 5/8 stainless eye bolt that goes throught the actual piling (leftover from old lift) so I imaging the piling will be supporting some of the load too.

Alright, good pics and now I have a better idea of what were looking at. I can't take credit for the attached as a friend worked it up. Based on what you stated the boat weighs 10K lbs and assuming there are 4 supports, the 5/8" bolts are A307 or better, and the lumber is southern pine or better:

The setup looks good and checks, but you will need use at least 1/4" thick, 2-1/2" square bearing plates. The calcs are good considering that there is no impact loading, e.g. when your boat is lifted no wakes are going to try and lift and then drop it...

View attachment Boat Lift.pdf

 

[flyboycj84]

Thats great info, thanks. So u mean the 2.5 bearing plate will go on top instead of washers correct? Also, the local lift guys sold me 16" long 1/2" bolts, there will Be 3 bolts per corner/hanger. They said they use 1/2" bolts on all their pipe hangers. The eye bolts that hold the cable, that go thru the pilings, are 5/8 stainless.

 

[flyboycj84]

I actually just responed before opeing that pdf, i missed it. Wow! Thanks for the great info! Its nice to see that kind of detail, gives me piece of mind. Thanks a ton! Also, there will be no wakes or waves or anything so i think we're good.

 

[jdlough]

I must say that I didn't understand all that on the pdf. However, I'd recommend you move everything up one notch.

My pier was built to decent spec, to withstand the 100-year storm and all. I thought it was stupidly overbuilt and way too tall over the high tide waterline.

Hurricane Sandy changed my mind. Ripped the end of my pier off and snapped several 5/8 inch bolts. Tore the Home Depot washers right through the stringers.

I rebuilt with 3/4" bolts (2 for each stringer instead of one) and 1/4"x3" square plates, plus 1/4" thick corner brackets everywhere. I rebuilt with 3"x12" lumber instead of 2x12. This new pier ain't going anywhere.

The old pier looked like an incredibly strongly built deck. It washed up on my lawn after the storm. The new pier looks like some Civil War fort.

You will never regret spending the money for the next size larger bolts and washers.

 

[flyboycj84]

A Civil War fort eh? sounds stout!! I didn't quite understand all in that pdf either....I did, however, like seeing the computations, and then the "OK" next to every one of em. We are pretty well insulated from the ocean and intracoastal here in Merritt Island, there is no real tide to speak of or storm surge thankfully. We're technically not even on a flood plane, which I find odd.

 

[HalfFish5087]

Thats great info, thanks. So u mean the 2.5 bearing plate will go on top instead of washers correct? Also, the local lift guys sold me 16" long 1/2" bolts, there will Be 3 bolts per corner/hanger. They said they use 1/2" bolts on all their pipe hangers. The eye bolts that hold the cable, that go thru the pilings, are 5/8 stainless.

Yes, plates in lieu of washers. I would go to the steel shop and get a 4' long stock of 3" wide, 1/4 thick stick and cut and drill your bolt hole. Though I'm sure the 1/2" dia bolts will exceed the requirement your looking for, I would personally go with the 5/8" dia for comfort. I would also carefully inspect the hangers and place washers on underside to prevent them from pulling through.

 

[flyboycj84]

Sounds good. I think the dock store has those plates pre-made, hopefully just a few bucks. At first I was going to reuse my hangars, and decided that for $30 a pop....I'd feel better with new, so those are good. Bought new drive pipes too, as the old one had a weld in it from being extended (the lift was a center mount before) And yes, I have the biggest washers in those hangers that will fit. Thanks