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Old 15-03-2015, 04:30   #16
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

I had to go through something similar. My manual Simpson davit gear box on the engine side was on the point of failure with bending teeth and seizing plain bearings. It was inadequate for the design load, so I couldn't just repair it.

I considered doing away with the gear box like you too and had a number of ideas that probably would work fine. In the end I designed a re-engineering of the gear box, replacing the gears with high end stainless gears and supporting them in new roller bearing blocks. I am really pleased how well it has worked. The effort to lift is less than half of what it was before and the design is so bullet proof and well made I am sure it will outlast me. So consider re-building your motor/gearboxes to address whatever failing they have. I/we may be able to help if you give more details about what is wrong.

If you are dead set on ditching the existing equipment you could replace it all with new from Simpson Cooney. They did quote me for a replacement electric motor/gearbox/pulley set, but later changed their mind about supplying as they didn't want to deal with the complexities of the install they said. It didn't ring true as I was going to deal with all that anyway. They said I should just buy a whole new set of davits. I gave up with them. You may have better luck and persuade them to sell you new parts and install yourself. £2200 from memory for the parts.

I don't like your beam idea as is I think it will be too unstable when operating. It depends on your dinghy, but if the lift point is low like that the dinghy can even flip especially if there is not much weight low down and with a motor on. I think it will be at best be a handful to control. Worth testing first if you can.

The solution I was going to go with, had I not re-designed the gear box was to keep the existing bridle, remove the motor/gearbox, run a new 8mm double braid line through a new sheave at the end of the davit, then through the inside of the davit beam. Fit a Karver KJ10 jammer (the only one that will fit) at the inboard end inside the davit. Fit hinges onto the rear plate to access the jammer and line. Fit if necessary a guide sheave at the outlet side of the jammer and run the line to a winch.

Another solution, which I also came close to doing is to do as above, but without the jammers. Run the line inside the middle of davits into the lazarette and fit a 4x4 vehicle winch under the base of the davits. They will be oversized as only the bigger ones have brakes and will be a robust solution if you can make them fit.
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Old 15-03-2015, 06:22   #17
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

Two reasons to over build. Shock loads and corrosion. I think you need that 60 footer with a dingy garage. Get rid of pesky Davis and carp all over the place.
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Old 15-03-2015, 06:23   #18
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

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Originally Posted by conachair View Post
Another thought, if there is a heavy outboard attached and the centre of gravity ends up above the pickup point, which it could well do with a spreader beam low down, then only thing the dinghy wants in this world is to turn upside down...
I didn't think of that. Drat . . ..

That seems likely to be fatal to the whole idea
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Old 15-03-2015, 06:25   #19
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

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Two reasons to over build. Shock loads and corrosion. I think you need that 60 footer with a dingy garage. Get rid of pesky Davis and carp all over the place.
64 footer ...

I agree, but it's not going to happen before I depart for the Baltic on 1 May, so meanwhile I have to make do somehow.
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Old 15-03-2015, 06:34   #20
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

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I didn't think of that. Drat . . ..

That seems likely to be fatal to the whole idea
Maybe not...
If it's just a temporary measure if the other mechanism breaks, could you do a single fall on a bridle on the dinghy and have a double reeved block closer to the winch? Is it double reeved just to get the load lower at the winch? There's always a way....
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Old 15-03-2015, 06:51   #21
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

Quote:
Originally Posted by poiu View Post
I had to go through something similar. My manual Simpson davit gear box on the engine side was on the point of failure with bending teeth and seizing plain bearings. It was inadequate for the design load, so I couldn't just repair it.

I considered doing away with the gear box like you too and had a number of ideas that probably would work fine. In the end I designed a re-engineering of the gear box, replacing the gears with high end stainless gears and supporting them in new roller bearing blocks. I am really pleased how well it has worked. The effort to lift is less than half of what it was before and the design is so bullet proof and well made I am sure it will outlast me. So consider re-building your motor/gearboxes to address whatever failing they have. I/we may be able to help if you give more details about what is wrong.

If you are dead set on ditching the existing equipment you could replace it all with new from Simpson Cooney. They did quote me for a replacement electric motor/gearbox/pulley set, but later changed their mind about supplying as they didn't want to deal with the complexities of the install they said. It didn't ring true as I was going to deal with all that anyway. They said I should just buy a whole new set of davits. I gave up with them. You may have better luck and persuade them to sell you new parts and install yourself. £2200 from memory for the parts.

I don't like your beam idea as is I think it will be too unstable when operating. It depends on your dinghy, but if the lift point is low like that the dinghy can even flip especially if there is not much weight low down and with a motor on. I think it will be at best be a handful to control. Worth testing first if you can.

The solution I was going to go with, had I not re-designed the gear box was to keep the existing bridle, remove the motor/gearbox, run a new 8mm double braid line through a new sheave at the end of the davit, then through the inside of the davit beam. Fit a Karver KJ10 jammer (the only one that will fit) at the inboard end inside the davit. Fit hinges onto the rear plate to access the jammer and line. Fit if necessary a guide sheave at the outlet side of the jammer and run the line to a winch.

Another solution, which I also came close to doing is to do as above, but without the jammers. Run the line inside the middle of davits into the lazarette and fit a 4x4 vehicle winch under the base of the davits. They will be oversized as only the bigger ones have brakes and will be a robust solution if you can make them fit.
The center of gravity problem seems fatal to my original idea

If I keep the slings, then I have zero space for lifting tackle under the davits. That means I have no choice but to lift using a line inside the davits, either using the original mechanism, or something like what you're talking about. Drat.

Or lift from a point alongside the davit, maybe using a cheek block. That last will probably be simpler as I can use a normal Spinlock clutch (50 quid vs 300) mounted conveniently externally, which allows me to leave the original mechanism in place.


Another thing to do might be to take the two existing mechanisms, take them both apart, and try to find enough good parts between them to make one good one, and use that for the outboard motor end of the dinghy.

Hmmm.
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Old 15-03-2015, 06:51   #22
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

Agreed on the issues with lifting point so low.

Only other suggestion I can add is do see if you can dispense with the blocks as you have drawn, and have your purchase prior to the davit sheave. That would give you a gain at least half a foot-- whoops--150 mm.
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Old 15-03-2015, 07:02   #23
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

Quote:
Originally Posted by conachair View Post
Maybe not...
If it's just a temporary measure if the other mechanism breaks, could you do a single fall on a bridle on the dinghy and have a double reeved block closer to the winch? Is it double reeved just to get the load lower at the winch? There's always a way....
I meant fatal to the idea of using a lifting beam.

I think I will go with a cheek block on the side of the davit. I think that ought to work. I'll have only single purchase, but I wanted multiple purchase only to reduce the load on the clutch -- I'll be using a big electric winch to do the actual lifting.

I'll use the existing slings, then, and there won't be any problem with center of gravity. I'll back up the clutch with a cleat, for safety's sake.
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Old 15-03-2015, 08:17   #24
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

The solution does depend on how permanent the fix is to be. I think I would think along the lines of having a nice, tidy, reliable internal motorised system first and that most likely will mean finding a way to get the original or similarly functioning alternative system up and running.

For a quick back up, how about a snatch block snap shackled to the end of the davits. The snatch blocks would sit along the inside face of the davits and the lines through both would be attached to the passarelle halyard to hoist the dinghy from a mast winch (or thru another block to an electric winch). When up and tight, just secure with rope or straps. This way no modifications are needed and nothing extra needs to be bought, engineered, drilled or tapped to deal with a davit breakdown. You probably have a couple of snatch blocks and plenty of rope spare.
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Old 15-03-2015, 08:36   #25
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

Quote:
Originally Posted by poiu View Post
The solution does depend on how permanent the fix is to be. I think I would think along the lines of having a nice, tidy, reliable internal motorised system first and that most likely will mean finding a way to get the original or similarly functioning alternative system up and running.

For a quick back up, how about a snatch block snap shackled to the end of the davits. The snatch blocks would sit along the inside face of the davits and the lines through both would be attached to the passarelle halyard to hoist the dinghy from a mast winch (or thru another block to an electric winch). When up and tight, just secure with rope or straps. This way no modifications are needed and nothing extra needs to be bought, engineered, drilled or tapped to deal with a davit breakdown. You probably have a couple of snatch blocks and plenty of rope spare.
Yes, that was one of my temporary solutions last summer. It worked ok, except: (a) you do the winching at the mast, using a manual winch (no electrics on my mast); (b) you can't even see the dingy from up at the mast, so no way to do it single handed; (c) I didn't like the dinghy hanging off the masthead, pulling the mast back.

I do however continue to rig the spare mainsail halyard to the dingy as a safety. I actually broke (!) one of my davits in a storm in the Baltic last year, and the mainsail halyard saved the day -- keeping the whole mess out of the drink until I could get the davit welded (at extortionate cost on Gotland; I posted about it).

By the way, thread drift -- but for everyone using Simpson davits -- make sure and torque the attachment bolts regularly. That was my mistake last summer -- I wasn't used to the short, sharp seas of the Baltic and the resultant sharp boat motion, which worked the davit bolts loose and created snatch loads which broke one of them


I think adding cheek blocks to the inside sides of the davits should make it comfortable to use the existing slings, which solves the center of gravity problem. This is still simpler than the lifting beam -- just attach the block and add a clutch, and hopefully Bob becomes my close relative. I can then lift effortlessly using the electric sheet winches. This will not interfere with the existing mechanism at all, should I manage to get it working again, and indeed will be a useful additional safety line even when I'm using the internal mechanism. Seems sound to me. Only downside is the blocks which fit will only take 8mm rope -- plenty strong enough, but unpleasant to handle.

Biggest problem will be drilling the heavy stainless of the davits -- really don't look forward to that.

I'll need to do this in any case, not just for backup, but because I am unlikely to repair the internal mechanism in the only six weeks I have before my cruise starts.
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Old 15-03-2015, 09:44   #26
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

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I'll need to do this in any case, not just for backup, but because I am unlikely to repair the internal mechanism in the only six weeks I have before my cruise starts.
I had the parts for my gearbox in just 2 weeks. It should be doable. What's wrong with them?
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Old 15-03-2015, 10:51   #27
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

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I had the parts for my gearbox in just 2 weeks. It should be doable. What's wrong with them?
Three things:

1. The plastic gear (what crap design!) in the electric motor of one is stripped.

2. The little nut and hub holding one sheave on is missing.

3. They are in general a carp Rube Goldberg design with a particularly poor method of friction braking, which is incredibly hard to adjust. Very expensive, complicated, brass construction, poorly conceived and executed.


The plastic gear is really tough. I haven't tried to have one machined for it -- I guess it's possible, though likely to cost a fortune. The little hub and sheave would probably be less of a problem.



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Old 15-03-2015, 11:40   #28
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

I like the jeep/land Rover winch idea. Mounted in the lazarett. You could keep the existing geometry and add line clutches to belay the lifting lines. Winch her up set the clutches, make fast the tails of the lifting lines and close and dog the hatch. You done.
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Old 15-03-2015, 12:00   #29
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Re: Please Critique the Engineering of My Dinghy Lifting Beam

Quote:
Originally Posted by Dockhead View Post
Three things:

1. The plastic gear (what crap design!) in the electric motor of one is stripped.

2. The little nut and hub holding one sheave on is missing.

3. They are in general a carp Rube Goldberg design with a particularly poor method of friction braking, which is incredibly hard to adjust. Very expensive, complicated, brass construction, poorly conceived and executed.


The plastic gear is really tough. I haven't tried to have one machined for it -- I guess it's possible, though likely to cost a fortune. The little hub and sheave would probably be less of a problem.



Attachment 98893

Attachment 98894
I can't see from the pictures what is broken. It is a completely different design from mine and I can draw no parallels. I guess you probably have a worm gear box and the plastic broken gear is the worm gear? Possibly made from peek or similar engineering plastic. Maybe acetal. If it failed it is probably under-engineered and there we have a common problem in my gears weren't up to the job.

You could have it made in something stronger. Bronze or steel maybe. A specialist gear cutting shop will be able to reverse engineer a copy for you. There are likely to be capable people in Southampton. If no joy there I can give you the name of the people I used in Manchester to look at the job. Probably photos with dimensions will be all that is needed to quote. Wild guess -less than £250.

The sheave problem I also can't really appreciate without pictures, but again, it sounds like a small job for a machine shop.
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Old 15-03-2015, 12:58   #30
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Quote:
Originally Posted by poiu View Post

I can't see from the pictures what is broken. It is a completely different design from mine and I can draw no parallels. I guess you probably have a worm gear box and the plastic broken gear is the worm gear? Possibly made from peek or similar engineering plastic. Maybe acetal. If it failed it is probably under-engineered and there we have a common problem in my gears weren't up to the job.

You could have it made in something stronger. Bronze or steel maybe. A specialist gear cutting shop will be able to reverse engineer a copy for you. There are likely to be capable people in Southampton. If no joy there I can give you the name of the people I used in Manchester to look at the job. Probably photos with dimensions will be all that is needed to quote. Wild guess -less than £250.

The sheave problem I also can't really appreciate without pictures, but again, it sounds like a small job for a machine shop.
OK, thanks. You've inspired me. I'll work on it. Cheers
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