My statics may be a little rusty. But I believe you have 2 forces to think about
- the base of the davits
will support the weight of the dinghy (and davits). So the base must support a little over 300#. There will be very little lateral force on the base
- the attachment to the stern rail at 1.5 ft above the base must counteract the moment arm of the dinghy hanging on the davits
. The dinghy will generate 300 lb-ft of torque (100 lb x 3 ft moment arm). Therefore the attachment to the rail must support 200 lbs (300 lb-ft / 1.5 ft).
As previously stated, all those forces are static only. It's reasonable to assume the forces could increase by 2x or 3x in waves.
A couple thoughts.
- Don't forget that the weight and forces will be distributed between 2 davit arms, so no single
point will bear all the weight.
- stuff in the dinghy (outboard, gas, anchor
, ...) should be added to the calculations
- The dynamic loading is much worse than any static analysis.
- Add water
in the dinghy and things get much worse very fast - catch a wave and the davits themselves are prone to failure, probably before the attachment points.
However you slice the numbers, it's further justification to never make a crossing with a dinghy hanging on the davits. Put it on the foredeck. Davits are for calm water