Physics/leverage/backing plate question

[ERTF]

I attached a couple photos to clearly demonstrate what I'm considering.

I just installed a davit (red in drawing) to lift my 1000lb skiff. I need to install a new engine and paint the bottom. There isnt really any spare room on the hardtop to work on it up there. I was thinking I could install 2 heavy duty diamond pad eyes on the underside of the hardtop the same distance apart as the bow & aft lift points on the skiff. Then i would lift the skiff with the davit. Attach the (yellow) lines. Then let out the cable on the davit until the lines were holding the skiffs weight. Then the davit would be free to hoist the new engine into position. Also when painting the skiff (from a dinghy in the water), instead of hanging out on the arm of davit for 2 days, the skiff would just be pulling almost directly vertically on the lines from the pad eyes in hardtop. That seems to me like it would be less strain(?), but I'm not an engineer...

The hardtop looks to be 1 inch balsa core with not alot of fiberglass on each side. I would need some serious backing plates on the topside of the hard top for the pad eyes.

My friend says I should just install the engine with the skiff in the water, and paint the bottom while it is dangling from the davit.

Looking for input from the peanut gallery. Particularly, if the cored hardtop is strong enough for the compression forces and how thick/large of backing plate it would require?

 

[O C Diver]

There's a big difference between static load and shock load. Any wave action or rocking on the boat will dramatically change the force exerted.

You were unclear as to whether the attachment points were temporary or permanent. While it doesn't seem worth the effort for a temporary use, if that was your intent, I would lay a 2×8 the length of the hard top and through bolt to it.

Personally I wouldn't risk a major repair for this temporary convenience.

Ted

 

[ERTF]

It would be permanent so i could repaint the bottom annually or touch up as needed. I would go anchor back in a protected cove to do it.

 

[dhays]

Hard to tell from the photo how much support is really provided to the hardtop. A sufficiently large backing plate (ie BIG) would solve the issue of bolting the pad eye, but I would worry about the total strength of the hardtop structure.

 

[koliver]

I paint the dinghy bottom with anti fouling annually. I can access the port side of the hull from the swimgrid, but to get the Starboard side I need either a convenient dock, a kayak or another dinghy. Seems easier than what you are contemplating.
Lifting the engine from or to a suspended dinghy seems awkward, at least, dangerous at worst.
Suspending from a Rube Goldberg like you suggest seems unsafe. Though I am sure you can ultimately do it, it could be expensive to get to the point of being safe enough and it would still be awkward.

 

[ERTF]

Hard to tell from the photo how much support is really provided to the hardtop. A sufficiently large backing plate (ie BIG) would solve the issue of bolting the pad eye, but I would worry about the total strength of the hardtop structure.

Just seems to me that if the hardtop is secure enough to not get torn off the boat horizontally when lifting the skiff with the davit, then the structure should be enough to hold it vertically.

 

[High Wire]

My concern would be the whole hardtop being pulled over the side like folding up a cardboard box. Are there substantial supports where those vertical supports meet the hardtop? How are the vertical supports anchored? Then what supports the support anchors?
Lifts that I've seen have a support pole that goes all the way down to the main deck or stringers.

 

[dhays]

My concern would be the whole hardtop being pulled over the side like folding up a cardboard box. Are there substantial supports where those vertical supports meet the hardtop? How are the vertical supports anchored? Then what supports the support anchors?
Lifts that I've seen have a support pole that goes all the way down to the main deck or stringers.

This was my thought as well. I wouldn't trust the crane to simply be mounted to the hardtop unless that structure is whole lot stronger than I imagine it to be. I had assumed that your crane would have a support pole going through the hardtop to a strong mounting position below as High Wire described.

 

[ERTF]

This was my thought as well. I wouldn't trust the crane to simply be mounted to the hardtop unless that structure is whole lot stronger than I imagine it to be. I had assumed that your crane would have a support pole going through the hardtop to a strong mounting position below as High Wire described.

The davit does have a support pole that is seated on the deck below the hardtop. I just installed it yesterday so thats why I had to draw it in the mock up I posted.

Even though the standpipe is mounted to the deck below, there's still gotta be side loading on the hardtop.

 

[KnotYet]

ERTF, you can fabricate (or have made) a hoist dedicated to lifting the motor that
attaches to the column supporting the davit. This could be clamped to the pipe in
such a way that it could be removed when not being used.

Visualize a jib arm that projects horizontally from the pipe and extends as far as you
need it to raise your outboard. I would trust the support column more than the hardtop.

If you need the full swing provided only by the davit then maybe the jib arm method
could support the dinghy but the loads will be greater.

 

[Benthic2]

I would be afraid even lifting the outboard only for fear it would swing into the side of the hull. I think I would install the motor on the dinghy at a local boat ramp with the help of a few strapping young lads. For painting I'd hang the boat on the davit over a dock with the mother ship side-tied to the dock.

 

[Comodave]

Personally I would not be worried about painting the bottom or installing the outboard. Those are not everyday events. You can always pay a yard to haul it and let you paint the bottom. I would just be concerned with being able to haul and launch dinghy for every day usage. Much simpler issue.

 

[koliver]

ERTF
Please tell us that we have talked you down from this rooftop, where you would hurt both your boat, your dinghy and likely yourself.

 

[ERTF]

I would be afraid even lifting the outboard only for fear it would swing into the side of the hull.

If that's the case then you would be even more fearful of ever lifting the entire dinghy to store in on the hardtop.

 

[ERTF]

ERTF
Please tell us that we have talked you down from this rooftop, where you would hurt both your boat, your dinghy and likely yourself.

This is a sensational comment. I'm listening to all inputs, but nobody has stated a reason why the forces would be GREATER hanging directly vertical from a pair of ropes vs 65" out on a davit arm. That still doesnt make sense to me logically. From a physics point of view, it actually seems it would be SAFER hanging from the ropes vs davit.

I am considering the horizontal bracing of the hardtop as the primary concern.

 

[Benthic2]

I am not a structural engineer, and I didn't stay at a Holiday Inn last night, but......The people who installed the davit didn't trust the roof to hold the weight and mounted the davit to the deck below.

And lifting the dinghy would not be as scary because its presumably got a rub rail, would make contact over a larger area and wouldn't have 4 feet of swinging room to gain momentum.

 

[Bacchus]

This is a sensational comment. I'm listening to all inputs, but nobody has stated a reason why the forces would be GREATER hanging directly vertical from a pair of ropes vs 65" out on a davit arm. That still doesnt make sense to me logically. From a physics point of view, it actually seems it would be SAFER hanging from the ropes vs davit.



I am considering the horizontal bracing of the hardtop as the primary concern.

My $0.02
The forces may not be any greater but they will be very different.
The crane has vertical and side loads. The vertical has been handled by the support pole. The side loads will be spread out and represent torque on the support pole/crane. That will exert a load towards boat center line at bottom of the support pole and opposite/outward at the hardtop. I will bet those points were checked or reinforced to take those loads.
What you are suggesting are loads at different points. They will be mostly vertical and spread out away from the crane support pole. The support pole will help only if the top is strong / or reinforced enough to carry the vertical load from the lift points to the pole.
I would at least have someone draw an accurate loading / force diagram to see what & where the loads will be and is that area capable of supporting it.
One addition that could help significantly is if the crane extends sufficiently above the hardtop to add an attachment point such that the dingy lift points could be through the hardtop and cables added from lift point back to the top of crane/support pole and get maybe a 45° angle. That would transfer a portion of the vertical load back to the support and the remaining forces at the dingy attachment points would be horizontal towards the support. I will guess the hardtop is stronger in the horizontal than vertical or more easily reinforced if necessary. The result is then a pair of triangles and very strong... think bridge support framing.
I'm not too great w computer/tablet drawing but hoping the description is enough to get the idea and you can get a techie to draw & estimate the loads & directions.

 

[psneeld]

I am with your friend...just lift the motor on and off while the dingy is in the water, I do it all the time with my boom that doesn't even extend vertically over the mounting spot.

Sure you need protected water, but this isn't a daily occurrence.

As far as painting the bottom....anywhere near the boat might be pretty messy if you use a roller, no matter how careful I try to be, small dots appear all over when I finally finish and look around.

I do things with my lifting setups that many on here consider dangerous. Well that's because I completely disagree with their understanding of physics and usually apply the common sense check that pretty much may not prove my version, but shows me I can still safely do it.

 

[ERTF]

The people who installed the davit didn't trust the roof to hold the weight and mounted the davit to the deck below.

I have bad news for you (or maybe me?) -- I'm the person who installed the davit.

You guys have made me a bit apprehensive about the side loads this davit will be putting on my hardtop (because all the supports are vertical. However when resting on the hardtop, the load of the skiff will be split between the port and starboard supports. But with the outboard hanging over one side, it will actually be more like 2/3 of the 1000lbs on one side. So thats my rationale for it not really being that much greater of a step to hang the entire boat from one side.

So first things first, I will see if the whole thing collapses just from lifting the skiff ?. Will start out with a smaller dinghy to see if it's stressing the supports, and work my way up.

 

[ERTF]

I am with your friend...just lift the motor on and off while the dingy is in the water, I do it all the time with my boom that doesn't even extend vertically over the mounting spot.

Sure you need protected water, but this isn't a daily occurrence.

As far as painting the bottom....anywhere near the boat might be pretty messy if you use a roller, no matter how careful I try to be, small dots appear all over when I finally finish and look around.

I do things with my lifting setups that many on here consider dangerous. Well that's because I completely disagree with their understanding of physics and usually apply the common sense check that pretty much may not prove my version, but shows me I can still safely do it.

Well my main concern with installing the engine in the water is trying to line up the bolts while skiff is shifting and water will probably be splashing in the two bottom bolts holes. But I may go this route just to get the engine on now, and stew on the bottom painting for a few months to see if I can improve my attack.

You and me are of similar good enough / git'r'done mindstates. I was just laughing at your post about letting your teak go bare yesterday -- cuz mine is on the long flaky road to bare as we speak ?.

 

[FF]

"The hardtop looks to be 1 inch balsa core with not alot of fiberglass on each side. I would need some serious backing plates on the topside of the hard top for the pad eyes."


This construction describes a light surface built to walk on , not bolted to

 

[hmason]

Why would you paint the dinghy if you can store it on the hardtop? I can leave my Boston Whaler in the water for about a week without seeing fouling. Then up she goes, bottom gets hosed off and engine gets flushed. Since the outboard should be routinely flushed after use it would be difficult to do in the water. Perhaps lifting her out weekly would be an all around better and safer scenario.

 

[dhays]

Why would you paint the dinghy if you can store it on the hardtop? I can leave my Boston Whaler in the water for about a week without seeing fouling. Then up she goes, bottom gets hosed off and engine gets flushed. Since the outboard should be routinely flushed after use it would be difficult to do in the water. Perhaps lifting her out weekly would be an all around better and safer scenario.

Thank you for asking this question! I’ve never painted the bottom of a dinghy. However, my dinghies normally never sit in the water for long. They have either been stored on the boat, or on the home dock at my slip. I have spent time towing a dinghy for a week or two, but usually the dinghy was moving through the water every couple days. I am also in colder water. I’ve only had an issue with bottom growth once, and that was when the dinghy had sat in the water for a month in the summer.

I can imagine though that for folks who boat in warm water and often leave their dinghy in the water for extended times, bottom paint would be important.

 

[ERTF]

Why would you paint the dinghy if you can store it on the hardtop? I can leave my Boston Whaler in the water for about a week without seeing fouling. Then up she goes, bottom gets hosed off and engine gets flushed. Since the outboard should be routinely flushed after use it would be difficult to do in the water. Perhaps lifting her out weekly would be an all around better and safer scenario.

I live on the hook. My skiff lives in the water.

Engines can be flushed with boat in the water. My friend with a wet slip did it. But flushing an engine doesn't make any sense to me. Water gets "flushed" thru it every time you use the engine. My outboard is getting used constantly, so what's the point of flushing freshwater thru it in the darkness after a long day of fishing just to run saltwater back thru it again tomorrow or the next day. Do you think most charter fishing captains flush their outboards everyday after fishing? What about the engines on your trawler? Do you flush the heat exchangers with fresh water after every use? No, you never do.

 

[psneeld]

Actually, yes the charter captains I know with outboards do flush evey day. Critical? Probably not, like cheating on eating healthy food...occasionally yes, all the time bad idea.

The times I have left my dink in the water, no I haven't flushed every day, but generally after a few. And yes with my transom shower with a short hose it is fairly easy with the new motors that don't have to be run to flush.

Heat exchangers on diesels aren't aluminum....that's the real point of flushing outboards and mostly are not made from different metals like outboards. If they are, they often have much shorter life spans and require greater care.

 

[ERTF]

Actually, yes the charter captains I know with outboards do flush evey day. Critical? Probably not, like cheating on eating healthy food...occasionally yes, all the time bad idea.

Well I'm currently replacing a 24 year old outboard. Just about everything else has failed on it, but the cooling system is fine. Who knows what the last guys did, but I've had it in the water for 5 years and never flushed it. I was inside it a year ago and there was no notable corrosion. So for sake of science, my new $6700 outboard can be the guinea pig, I will report back in 10 years if it rotted out from the inside.

 

[psneeld]

Well I'm currently replacing a 24 year old outboard. Just about everything else has failed on it, but the cooling system is fine. Who knows what the last guys did, but I've had it in the water for 5 years and never flushed it. I was inside it a year ago and there was no notable corrosion. So for sake of science, my new $6700 outboard can be the guinea pig, I will report back in 10 years if it rotted out from the inside.

It's not only corrosion, Salt does build up.

Not sure which outboards have all what cooling components in their systems, but that can be an issue on some. Once Salt does cake up, can be a bit * to get rid of.

The other issues are some components are defined and others Rust. The tilt plate on my 2011 Yamaha 8 has major rust....a real PIA to replace so I hesitate. While rinsing and flushing are 2 different things, they do go hand in hand.

But go ahead, one person's experience is all that's needed in life.....just passing along the pros do it, it's recommended and a lit of these guys too know what's best or overkill.

 

[Xlantic]

I have bad news for you (or maybe me?) -- I'm the person who installed the davit.

You guys have made me a bit apprehensive about the side loads this davit will be putting on my hardtop (because all the supports are vertical. However when resting on the hardtop, the load of the skiff will be split between the port and starboard supports. But with the outboard hanging over one side, it will actually be more like 2/3 of the 1000lbs on one side. So thats my rationale for it not really being that much greater of a step to hang the entire boat from one side.

So first things first, I will see if the whole thing collapses just from lifting the skiff ��. Will start out with a smaller dinghy to see if it's stressing the supports, and work my way up.

The side force would be equal to the vertical force (weight of the skiff) multiplied by the ratio of the horizontal distance to the vertical distance. The vertical and horizontal distances are those between the hoisting point on the fly and the lifting point on the skiff. So, it would not be very large.

The above refers to the static forces. As mentioned before, the dynamic forces can be much greater. If the mother boat starts rolling from the wake of a passing Bayliner the forces can be much greater.

 

[JESSEDIVER49]

Your question boils down to the ability of the hardtop to handle the stress of the skiff and outboard loads. The backing plates where the attachments are placed are certainly a valid consideration but I'd also be concerned about the hardtop vertical supports buckling from a load they were not designed to handle. I'd either get a structural engineer involved or take your friends advice. I would not gamble on the hardtop being stout enough.