Boom for jet ski lift

admin

I'm in the process of designing and soon building a boom for a jet ski lift to bolt to. A friend recently had a dock built at his lake property and had the builder set the dock 1 foot out past the pilings. The problem this created is the jet ski lift is meant to bolt to the piling, allowing you to lift the jet ski from the water and onto the dock. Sad part is the builder is also the one who installs the lifts and should have known better than to build the dock that far out. The builder's remedy was to add two 6x6 posts with threaded rod run through them, the piling and lift bracket. The jet ski can be lifted and brought about 2/3 onto the dock but the whole setup is shaky and my friend is not comfortable to try and pull the jet ski off the lift and onto the dock... I don't blame him. I sketched up a boom that is built off a 8" schd 40 pipe that is 48" long. My plan is to route the piling (a job in itself) to allow the pipe to fit over it. The hinge of the boom will go on the side of the piling where the electrical is currently mounted which will allow the entire jet ski to be placed down on the dock. The sketch you see attached is currently with an engineer and I'm waiting to hear back on his recommendations. The jet ski weighs 800lbs and the lift weighs 300lbs.In my drawing, the plates and gussets that hold the boom arm to the pipe/piling will likely be 3/8" or half inch and everything else will be 1/4". I'd like to hear any recommendations for the hinge pin and pipe combo. I looked at the ID of 2.5" schd 80 and it seem a pin between 2 to 2.25 inches would work well. Is there a better tubing than schd 80 to use for this application? One last thing, The bottom arm of the boom is two square tubes, one slipped inside the other, to allow adjustment of the boom to correct for sag. There is alot of sag in this current setup, but it's not real visible in the attached photo.Thoughts please Attached Images
Reply:I have a friend that has one of those lifts and something just looks wrong.  After seeing a picture of one installed properly I see the problem that you guys are having.  The mount for the lift is supposed to be on the side of the piling, not the face.  Even if it was on the side, there would be a problem with the ladder (aside from the obvious inability to swing all the way).  Sleeving the piling with the schedule 40 pipe appears to be a lot more work than it is worth.  Every piling mount lift that I have seen has some kind of a bolted clamp arrangement.  Off the cuff, I have a strong feeling that your design would be more than adequate but a bit overkill.  It seems easier to remove the 6x6's off of the face, mount the lift to the side of the piling, notch the deck for the cradle/jetski and move the ladder.  Wood is cheap and easy to return to a normal configuration if he ever gets rid of the lift.  A little sag on a boat lift is normal and not really worth the trouble to get rid of beyond shimming.  The only reason to really want the ski over the dock is to make space to get into a slip and for maintenance and fueling.  If you could just make sure that the ski goes over the dock nose first, fueling is resolved.  Maintenance is better done on the trailer anyway.  Hopefully, if there is a slip access issue, you can get it out of the way far enough.  The lift is designed to support the entire load 100% of the time.  So, while it seems like it was a bit of a waste to get a swinging lift when the design ended up a bit screwed up, it may be just fine as is.  Your new design shouldn't have any problems except for the amount of time and money that end up in it.  I would highly recommend sending the whole thing out to get galvanized if you build it.  Rust stains on a new dock are ugly and inevitable without it.Good luck and post pics if you build it,ReneLast edited by zerepener; 07-27-2009 at 11:28 PM.
Reply:Zerepener, Thanks for the feedback, I was actually in the same boat  as you on this one but he wants to be able to fully land the jet ski on the dock. At this time the project is back to the drawing board after I got him a quote for the steel today. Apparently building this swing boom would cost as much as the lift did  I'm trying to work out a simpler solution for him so we can stay under budget, though he hasn't told me what that is yet  Whatever the solution is, it has to be built off the piling. He's telling me that the solution he has now is causing the end of the dock to press way down when he lifts, so there isn't good support in the framing to build off of.
Reply:Has he considered driving a new piling specifically for the lift? It should definitely be cheaper that a steel solution.  It won't be able to go in the same spot unless he saws the existing piling down to deck level (which might not be a bad Idea when I think about it).Rene
Reply:*** Advice wanted ***I talked to my friend and he really wants to go with the boom setup so I got started on it this weekend. In an effort to keep the cost to near zero as possible I scrounged up as much available stuff as I could. All the plate is off the scrap pile, the 1.25" pins are used from a small dump truck (mine) and the motor couplings were donated to me by my local Kubota dealer. I visited them on Saturday morning hoping to find a matched pair of bent cylinder rod ends to use as the hinge for the boom arm but all they could come up with was this pair of couplers. They had no idea what they were for and had no inventory on them, so they donated them to the cause.I changed the design a little to match up to the parts I now have. The point I got to is what you see in the pictures. Everything (except the 3/8" long arms) was cut by hand with a reciprocating saw then all edges were filed and sanded for a smooth finish. All edges to be welded are beveled, so I'm almost ready to get it all set up and tacked together. I need to find a larger plate to make the center gusset for the boom arm before I can proceed. I think the only piece we will need to buy will be a 48" section of 8"x8"x.188" box tubing to go over the piling.This project is my first real use of a MM180 I bought a few months back so I'm trying to get the machine "tuned" (which means I'm trying to figure out what the heck I'm doing). The welds you see were done with the machine set to 8, wire speed 70 and .035 wire. I tried the "autoset" feature and immediately turned it off. I am probably doing something wrong cause the wire was feeding way to fast on "autoset" giving me weld mountains instead of beads.*** Advice wanted *** Attached Images
Reply:A little more progress today. It's almost finished, just waiting on the measurements of the holes for the "factory" bracket mount before drilling and welding the front plate on. The swing arm came out great but the long back plate cupped on me a little when I welded 2 full length pieces of 2x2 angle down the back. I think I need to learn better patience when stitching to flat plate or use a specific pattern to help eliminate the cupping. Comments welcomed Attached Images
Reply:I've been holding off updating this thread because the first revision did not pass the "use" test. To state it humbly: it failed miserably Somewhere on this site, in at least one thread maybe two, I'm sure someone states "Understand how the unit will operate and all the forces that will be applied to it". Wish I had read that thread To make a short story long, here's what happened. We installed the boom back in mid August and attached the jet ski lift to it and proceeded to lift the jet ski out of the water. Everything seemed fine, the jet ski lifted straight up to well above the dock floor without a creek from the boom assembly or the wooden piling. The boom arm was level, the lifting forks were level. Basically everything was perfectly balanced. The moment we applied pull to the boom arm to swing the jet ski onto the dock that perfect balance was interrupted and the boom arm flexed. The top of the arm went east and the bottom of the arm went west. I would estimated the total flex from top to bottom was about one full inch but when you multiply that over the entire lifting mechanism and out to the jet ski you get.... well - PANIC!As you would expect the flex happened in an instance so there was no time to react. Oh, for the "weak reader" don't worry, no one gets hurt and more importantly no welds were injured or broken in the making of this story The panic came from watching the nose of the jet ski raise about 1.5 feet and the back of it lower the same.. all in an instant. It never slid off the forks so all was well. After we unclenched our cheeks (lower pair) we simply lowered the jet ski back into the water.So, what was the problem?? Turned out to be quite simple and apparently the joke played on first year mechanical engineers. It's called the 3 sided box. When I was building this unit I only had in my head the upward lifting force that would be applied and completely left out any diagonal rotational force applied when swinging the arm.The fix was simply to add a "backbone" which closes in the box. I also added an "X" brace on both sides of the center gusset just to be sure. The unit was reinstalled this past weekend and it works perfectly... err "as designed" Below are some pics, first one explains my error and the fix. The others are the unit installed and operational. Attached Images
Reply:That looks really good.  I like how you came up with the solution.  Good job
Reply:I really like the idea of getting the PWC on the dock, seems more safe and the fact that you got to build something that can handle that wight and stresses is also a bonus.Miller Dynasty 300dxSpeedglas 9002XMiller Spectrum 375 XtremeLincoln PowerMig 255xtLincoln PowerMig 140cMilwuakee Portaband
Reply:Very good job on both the build and the thread, very informative.  I like the way the lift is designed to swing all the way onto the dock.
Reply:Excellent job. Very robust. I'm not a welder, but been around boats, docks, and lifts all my life. I got to this site looking for ideas on a davit style mount or hinge to mount to a piling. My only comment is the mounting of the whole assembly looks to be just 4 large U-bolts around the piling. I hope this isn't the final assembly support - it just doesn't do justice to the stoutness of the rest of the design. Great work, though - I wish I had one of those, now.