Custom-building new davits...any tips?

[Greenheart]

With ambitious plans for a bigger and heavier tender (eleven foot, 120kg) than I've used previously, I'm thinking of building a swimming-platform and stern ladder, and substantial davits to deal with the boat.

Is it a rotten idea to wire the davits to the masthead for extra support, rather than just drill through the deck and reinforce their backing-plates?

I'd like to hear any accounts of setting up davits, especially of cases where they've failed in use - both how and why, and what extra technique the fitter learned and would recommend.

 

[oldsaltoz]

First off I must say I'm not a fan davits, most davits I have seen ore on craft that were never designed to have them.

The extra weight hanging on a cantilever over the stern will effect the performance and in some cases the safety and handling of the boat.

Having said that, the design should be based on the rear support being as close to transom as possible and very close the outer edge of the hull, this will provide the strongest support on a hull that had no davit in the original design.

The forward leg should be as close as posible to the outer edge of the hull and the area between the hull the hull and deck reinforced before any backing plates are added, the area of reinforced hull/ deck shout be a minimum of 0.5 metres on a small craft and around 0.3 metres wide covering hull and deck.

The rear support should be near upright, the forward support should well ahead of this and extended over the forward support with a brace under the buisness end back to the base of the stern fitting.

A single bracing bar between davits with braces to resist and sideways loading also helps and should be included at the base of the davits as well.

Good luck.

 

[prv]

Is it a rotten idea to wire the davits to the masthead for extra support

I've never seen it done, which must say something.

It ought to be pretty clear that the distance between masthead and davit won't stay the same to the millimetre forevermore. So unless you want to be stressing the base attachments, I think you'd have to design for that assumption by making the bases flexible/pivoting and placing all responsibility for vertical support on your mast wires and not the frame.

There are tables for calculating the force (for each apparent wind strength) that each metre of a given size of rope or wire will apply as windage. Go work it out for your two(?) wires and see whether you want that much force holding you back when trying to get to windward. I don't honestly know how big it would be for the sort of wire that would be appropriate here, but certainly for reasonable-size rope the forces aren't immaterial.

Instinctively, it feels like it could be done but probably shouldn't be.

Pete

 

[eagleswing]

davit design...

i cannot imagine that a masthead to davit wire would be feasible. any pumping of the mast would transmit a force to the davit which would operate to put the retaining bolts at the hull into tension. cyclic loading of those fastenings would not be. wise. a better design is what is used by davit manufacturers..imagine an upside down letter l where the long leg attaches both to the rear stanchion and to the deck. the leg of the upside down l which fastens to rear stanchion is bolted and is in. tension. the lower leg of that piece fits onto the. deck and is. in compression. with a strong backing plate , this style of davit can support up to 400 pounds per pair of davits. forespar is a prominent davit manufacturer who utilizes this design, you can find their install directions on the web. i have their davits on my 33 and have been pleased with them. my dinghy is only 100 pounds. and. has a cover to prevent rain or waves from filling the. dinghy with. water, which. would cause undesirable added weight and loads on the davits.

fair winds .

 

[john_morris_uk]

I agree that a wire from the mast head to the davits is wrong for all the reasons already given reference movement of the mast etc.

What many people fail to appreciate is that each davit ought to be designed to take the entire weight of the dinghy and outboard. As the mother craft lurches and rolls around in heavy seas, then the load comes onto each davit in turn and you need a safety factor built into the calculations.

Adding davits to Serendipity was one of the best things we have done, and has made life at anchorage very much easier. In fact cruising generally is easier: the dinghy is always ready with its outboard already in place ready to go.

If you are designing your own davits you also ought to think about the winch arrangements for hoisting. Our davits are from Simpson and have winches built in that have a ratchet brake inside, but the action of winding anti-clockwise automatically disengages the ratchet. Wind up or wind down using the same handle. Very easy and very simple but clever design all in bronze and stainless inside the box section of the davit.

 

[Ex-SolentBoy]

Attached is a picture of a new Rustler 44. This is the first one where the owner requested a swim platform, davits and stern boarding ladder. It actually uses off-the-shelf davits themselves, but the rest of the fittings were custom made.

Looks like quite a neat solution to me.

 

[Greenheart]

Thanks, my eyes are open now.

Dear me, I don't think I knew there were so many worms in the can!

Presumably, the longer each davit's base, and the more serious its reinforcement beneath the deck, the better able it will be to support a loading. The problem always struck me as being the strength of the decking to which a davit is attached, rather than the metalwork itself.

Any remarks on davits for a canoe-stern?

 

[GrahamM376]

Dear me, I don't think I knew there were so many worms in the can!

Presumably, the longer each davit's base, and the more serious its reinforcement beneath the deck, the better able it will be to support a loading. The problem always struck me as being the strength of the decking to which a davit is attached, rather than the metalwork itself.

Any remarks on davits for a canoe-stern?

Thinking a bit further ahead, do you intend to fit solar panels? If so, a gantry which doubles up as davits can be built which spreads the loads over a greater area.