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Boat lift design advice requested


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Greetings,

Last fall I purchased my first Malibu, a '98 Response, and am looking for advice on the best modification needed for my Shorestation. What I have is a 3600# steel constructed cantilever design that is no longer manufactured by Shorestation. Boat support is designed by way of an adjustable 'V' cradle that conforms to the hull shape towards the rear of the boat, which is mounted on the rear steel/wood crossmember of the Shorestation. The front of the boat is supported by the hull center ridge simply setting on the front steel/wood crossmember. Rear contact point is approximately 12" in front of where the prop shaft passes through the hull. Front contact point is approximately 114" further forward. My question is; is the dynamics of this type of weight distribution acceptable (more importantly, detrimental),  or would longitudinal bunks, similar to what the Malibu trailer has, be a better solution. The issue concerning longitudinal bunks is that the hull contour changes from approximately 8 degrees from horizontal (per side) at rear contact point of boat, to 24 degrees at front contact point. I doubt that a 2X6 would twist that much in 114" to allow for a flat contact surface, full length. For what it's worth, the Malibu trailer is designed with a 16 degree contact surface. I would really like to use the existing Shorestation design, simply to avoid the modification effort, but I definitely do not want to damage the structural integrity of the boat. I'm sure that I'm over-thinking the issue, but that's me. Any advice is greatly appreciated.

JK    

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You'll definitely need new bunks that go parallel to the keel.  I just went through conversion from I/O to Malibu with a ShoreMaster a few years ago.  It probably doesn't matter that they don't make your specific model anymore.  The bunks are typically aluminum and have adjustable risers that fit on the cross members and they don't change those dimensions very often if ever.  Are you sure your lift is steel and not aluminum?  I would suggest going to a boat show where you can look at and talk about boat lifts.  Before you go take pictures of your lift and measure the cross member length and cross sectional dimensions.  If Shorestation doesn't have the right parts I'm sure another manufacturer does.  If your lift really is steel think about tossing it out.

Welcome to The Crew!

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+1 on the bunks parallel to the hull, those are a must.

Here's a pic of my 3600lb cantilever lift, an old Harbormaster.  I installed the hull bunks and the guide on bunks also (the ones that are higher than the hull bunks).

Windsurf_board_lift_install.thumb.JPG.c1f7ca69c9723bea75eb2c960de90654.JPG

Edited by Michigan boarder
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minnmarker

My lift is definitely steel construction.  2.5-3.0" dia steel tubing, depending upon the component. The lift is a Shorestation model 3698 (3600#, 98" beam) that ceased production sometime during the 90's. The good thing is, it's so heavy that a high wind will not capsize it if the boat isn't setting on it. I would prefer a newer aluminum, but my canopy is in such good shape, etc,etc, and, you can't hardly give away a steel lift let alone sell one.

So, it appears that parallel bunks are preferred. Is that because of a possible better hull weight distribution rather than front and rear loaded points with no support in the middle?

What about hull contour change from front to rear? If the rear part of hull sets flat on the bunks, the front will set on the inside edges of the bunks. No big deal?

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47 minutes ago, S2000 said:

minnmarker

My lift is definitely steel construction.  2.5-3.0" dia steel tubing, depending upon the component. The lift is a Shorestation model 3698 (3600#, 98" beam) that ceased production sometime during the 90's. The good thing is, it's so heavy that a high wind will not capsize it if the boat isn't setting on it. I would prefer a newer aluminum, but my canopy is in such good shape, etc,etc, and, you can't hardly give away a steel lift let alone sell one.

So, it appears that parallel bunks are preferred. Is that because of a possible better hull weight distribution rather than front and rear loaded points with no support in the middle?

What about hull contour change from front to rear? If the rear part of hull sets flat on the bunks, the front will set on the inside edges of the bunks. No big deal?

Remember your engine is in the middle of the boat, instead of near the rear like an I/O would be.  That's why I would definitely do the parallel bunks and not just the two V's.

My bunks rock back and forth, so the front can rock/twist to shape the hull of the bow and flex while the back stays flat.  I set mine up a little closer together in the front to help cradle the boat.  It is a 2x6 bolted to aluminum U channel, and all of it does flex to match up the hull.  If you used steel or something else more rigid that might be a problem, but I'm guessing you can get aluminum and U-bolt it as easy as you could attach a steel component.

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1 hour ago, S2000 said:

So, it appears that parallel bunks are preferred. Is that because of a possible better hull weight distribution rather than front and rear loaded points with no support in the middle?

What about hull contour change from front to rear? If the rear part of hull sets flat on the bunks, the front will set on the inside edges of the bunks. No big deal?

Yep. Better weight distribution over a wider area of the hull and room for the prop shaft and rudder to get through.  Remember, they are ahead of the transom.   You would have to be perfectly centered, or better,  to get the prop or shaft through the middle of a V bunk.

The aluminum bunks will twist to accommodate the different hull angles although they should be about as far apart as the chines at the bow to be resting at a good relatively flat spot.  Recommend you look at a few examples before you commit to parts and design.  Th bunks need not be parallel or exactly the same height off the cross member.

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@S2000 I picked a older used lift, shorestation also to which also had the "V" bunks. I converted it to bunks via a vertical 2 x 10, top edge cut at a 30 Deg angle and capped off with a 2 x 6. then I built brackets, alum. tubing to go over crossmembers (slide) and welded angle to said tubing and bolted new bunks to them.

once these were built and on the lift, i measured the gap on the trailer and matched on the lift.

not sure if any of that makes sense........

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@Michigan boarderThat's kinda what I figured too - with the engine weight pretty much centered between the lift crossmembers it would put less of a strain on the hull using parallel support. In my opinion, your lift is of a much better cantilever design than my Shorestation. I picked up a couple of carpeted 2X6's at the local marina for free - items that they removed and were going to trash. The 2X6's must be of some dense wood because they are much heavier than your local lumber yard stuff. They also have a flanged 'V' galvanized support piece running almost full length on the bottom side for added rigidity, with pivot brackets attached at each end. All in excellent condition. I intend to use them in hopes they will twist to conform to the shape of the hull. I still have to fabricate front and rear mounts but I will be able to modify the components already removed from the lift, fairly easily.

@minnmarkerWhen my Response is positioned on the lift cradle, the point at where the propshaft exits the hull is 6-12" behind the rear lift crossmember. The only concern remains with tracking fin clearance, which is solved by launching the boat with the cradle lower in the water. I can always raise the bunk supports to allow for fin clearance if that becomes necessary.

@mackie12When you speak of "gap on the trailer" I presume you mean the distance between the bunks. I intend to space them at 34" outside to outside measure, which is what my Malibu trailer is.

Thank-you all for your input and willingness to share. It's a very big help.

Jim K

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@S2000:  I do agree that longitudinal bunks are a sound and the much more common solution.  Just to be the devils advocate here and give you more food for thought, many vessels are either hung from the lifting eyes at the front / aft (and noted in several manuals as a viable support method), slung via U straps roughly in the location you are noting or supported via V bunks.  Given it is boat show season, simply take a look at how the boats are supported at various shows, they are put on what are basically 3 jack stands, 2 at transom & 1 under the V.  Just for reference, the engine is attached to the stringer which acts as a long 'bunk' or like a vertical 2x6 to provide the vertical stiffness needed.  Just remember a flat bunk is actually very flexible unless it has a stiff frame under it so the weight will be concentrated at the vertical support bars either way.

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@Woodski"more food for thought"; that's for sure - good point. I suppose I could entertain the entire 'moments of inertia' dynamics that occurs in a cross member support system where you have 3-4' of boat behind the rear crossmember, 9-10' of boat between the cross members, and the remaining 6-7' of boat in front of the front cross member. Surely I would think that would cause less stress than a boat suspended at the extreme front and rear. I'm not knowledgeable enough in the physics of the whole thing to know which is worse. I do know this; a guy I know owned a mid 90's Supra Sunsport (wood stringers) and when he put the boat on the trailer (Supra as well) the walk-through windshield would no longer close, and the stringers were in good shape. There had to be some undesirable forces occurring to cause that to happen. Oh, and then there's the issue concerning higher psi loads against the fiberglass, due to smaller contact area using a crossmember 'V' cradle opposed to longitudinal bunks.  At any rate,  I'm sure I'm over-evaluating this whole thing.

In regard to longitudinal bunks being the common solution; with todays design of aluminum lifts using rectangular tubing and lowered center cross members it would appear that longitudinal bunks are a simple accommodation and easily adjustable. I'm not sure I've seen a modern design lift using anything different. So in todays world, are longitudinal bunks a necessity or more of a manufacturing/marketing issue?

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