Maths/physics genius required

The steel cable is a tension only member . It cannot transmit a bending moment . It can only transmit a direct tension force.
Therefore the lever arm can only be from the base to the cable where it runs over the pulley.
 
MartynG said:
Yes it does
Because the steel rope is always running over the pulley to the davit at the same location.
The dinghy may move away from the base of the davit but the lever arm to the pulley remains at the same distance.
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No, it doesn't Draw a picture with different vert and horz dimentions. You need to know the height and length to understand the direction and extent of the deflection from the neutral axis. For all he told us, the arm could be horizontal with no vertical leg (they are on the PDQ), or the vertical leg could be 6'. Big difference.

Without a drawing it's a trick question.
 
I agree with thinwater.

You can split the tension in the cable into a vertical (V) and a horizontal (H) component (H = V tan(10deg)).

The vertical component applies a moment to the davit base of V * horizontal distance from base to pulley (Dh)
The horizontal component applies a moment of H * vertical distance from base to pulley (Dv)

The total moment should not exceed 1500 N * Dh (= the moment at rated load) - you can thus work back to find a maximum acceptable load from the cable.

V = Load in Newtons = 1500 * Dh / (Dh + Dv * tan(10deg))

10 degrees is considered a small angle, so weight and load from the cable can be considered equal, but don't forget to make some allowance for drag up the transom.
 
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john_morris_uk said:
I’m not going to enter the maths debate. It’s a long time since I did ‘Applied Mathematics’!

However, I was advised that each davit should have a load max of a least double the weight of the dinghy (and out board if you are leaving it on - and what’s the point of davits if you aren’t leaving the outboard on the dinghy!)
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I agree with that. And don’t forget to allow for extra weight of water gathering in the dinghy, the fenders that end up going in for ease of storage, and a bit of safety margin for when you lean on the dinghy when doing up the ropes.
And therefore the 10 degree angle on the rope is largely immaterial as with 150kg rated davits you probably want to have a dinghy and outboard with dry weight of 80kg or so max to start with.

The key thing is generally really good reinforcing of the stern deck that the davits are bolted onto. We had to add some 20mm ply bonded in before fitting the Plastimo 150kg davits. Been a great investment though.
And a 10degree inward angle for the rope may make more stable when stowed, by pulling against the davits, but could mean needs a bit of help clearing the stern on the way up and down.

PS I have a degree in maths, but I assume we are talking bits of bent s/s tubing here, not aerospace engineering!
 
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Blimey there are some clever folk on here. I think I’d haul it up and then lean on the elevated dinghy to see how the davit arms moved. Not much would equate to good, lots would be bad.

Remember the USA spent $squillions to develop a biro that would write in space.....the USSR on the other hand gave the cosmonauts a pack of pencils!
 
GHA said:
You have an anti gravity machine? How else can you reduce the force imposed on the system by gravity acting on the dinghy?
Or is that just plain completely wrong... ;)
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Perhaps I should rephrase it
The safe working load capacity is reduced due to the angle of the cable being at 10 degrees to vertical
 
MartynG said:
Perhaps I should rephrase it
The safe working load capacity is reduced due to the angle of the cable being at 10 degrees to vertical
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Thanks all. Much appreciated input. I suppose I could furnish the forum with all the dimensions but I think the general view is that dynamic and extraneous loading is more significant than an odd "angle of dangle" in the ballpark of 10 degrees.

The reduction in SWL as calculated by Martyn (thanks Martyn) is not overly concerning. I hadn't planned to keep the outboard on it anyway so this reduces the stress quite a lot.

Cheers all.
 
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