Strength of M10 threaded rod

Forty_Two

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I want to use an A4 SS threaded rod 50cm long bolted horizontaly to lift points on the transom of my dinghy. This will be lifted on a davit from the centre of the rod.

Total dinghy/outboard weignt about is 75kg. Most weight obviously is at the transom so best to consider this the static load. The rod bends a small amount, is this well within the M10 capabilities.

When raised the dinghy is tight against the davit so no movement.

Couldn't find an answer on MrG for this type of bending load

Thanks Paul
 

superheat6k

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You don't state the length between the load beairng positions presumably at each end of the rod. Likely OK but bear in mind any threaded rod loaded sideways has a myriad of stress raisers at the root of each thread. I would use a piece of plain rod and thread the ends with a decent die.
 

dunedin

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I just drilled the transom and use a couple of 10mm eye bolts and a rope bridle to lifting tackle - A4 316 MARINE STAINLESS STEEL LIFTING EYE BOLTS M6 M8 M10 M12 FULL NUTS WASHERS | eBay

Me too, except for the second dinghy I used U bolts, for no particular reason other than easier availability. And dyneema bridle, just because it was a nice bit of suitable rope.

The threaded rod right the way across with load in the middle looks the wrong solution, and would make an engineer weep :)
 

Forty_Two

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Me too, except for the second dinghy I used U bolts, for no particular reason other than easier availability. And dyneema bridle, just because it was a nice bit of suitable rope.

The threaded rod right the way across with load in the middle looks the wrong solution, and would make an engineer weep :)
I'm not exactly happy with it myself ?

I will be replacing the lift wires with dyneema next year. Problem is the lift points are too high for a bridle to work, the dinghy needs to go right up to the davits. As it's a shiny new dinghy with an aluminium Hull I didn't want to start drilling holes just yet & the nice welded lift points are a bit too high. It's also tricky with the outboard mount.

The rod threaded at the ends sounds a nicer solution till I change the dinghy lift points which is the right thing to do.
 

Forty_Two

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Massive stress on the GRP brackets at each end of the rod as they will be pulled inwards. Far better to have a heavy duty stainless bracket bolted thrpugh the transom at the lifting point.
The single fixing could possible work instead. Brackets currently are aluminium welded to aluminium transom. I used a rod to make the load vertical on the brackets whereas with say a short wire strop there would be a bending pull on them.
 
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Replace the rod with a length of 1" or 1.25" SS box section, 3mm wall, bolted at both ends to the brackets and with a hoop welded to the top for the davit hook. It won't bend and it'll remove the inward pull on the brackets.
 

dunedin

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Replace the rod with a length of 1" or 1.25" SS box section, 3mm wall, bolted at both ends to the brackets and with a hoop welded to the top for the davit hook. It won't bend and it'll remove the inward pull on the brackets.

Also a good solution. But unless there is some issue with the transom construction, drilling a couple of holes and fitting brackets in a better location might be a lot simpler and cheaper :)
 

William_H

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Simple answer might be to for a thicker threaded rod. Or as said use solid rod thread tapped on the ends. Use clamps to locate the centre attachment point.
What might work or at least alleviate the bending stress would be to add another piece of threaded rod under the existing rod with plates connecting the 2 at the cetre and near the ends. Obviously the bending forces will be near the centre. Clamp these plates in place with nuts. (actually this latter a horrible suggestion) ol'will
 
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Also a good solution. But unless there is some issue with the transom construction, drilling a couple of holes and fitting brackets in a better location might be a lot simpler and cheaper :)
I don't disagree but the OP was averse to drilling holes in his new boat!
 

rogerthebodger

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Massive stress on the GRP brackets at each end of the rod as they will be pulled inwards. Far better to have a heavy duty stainless bracket bolted thrpugh the transom at the lifting point.


This is how I would do it.

Using a M10 threaded bar over a span of 500 mm is such an awful design that I shudder to think what would happen to your dingy in a seaway, even pulling the dingy up against the davits a you suggest would bend the M10 bar such that it would not stay tight for very long.

I would make up a bracket something like this with 2 loop and bolted through the transom.
 

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Forty_Two

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Thanks for all the suggestions. The transom is hollow all 3mm aluminium.

I will use U bolts lower down being careful not to overtigbten & bend the aluminium. This will let me use a strop, either wire or dyneema. At least being ali I don't have to worry about the transom strength.

The davits are good old Electric SL 175's installed in 2000. Rather substantial as are the mountings to my GB's transom. No problems with that installation ?
 

andsarkit

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I would be suprised if the transom doesn't have some reinforcing behind the outboard clamps. If you can find the extent of it by tapping for a hollow sound and then fit the U bolts near the edge of it.
 
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