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Old 27-02-2017, 14:04   #1
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Davit Reinforcement?

Recently I saw a beautiful Oyster sailboat that had done something that looked interesting to me.
They had run a line, probably either wire cable or dyneema from the top of the mast to the outmost end of the davit arms. It would seem that this would greatly increase the strength and reinforce the davits, since it is carrying part of the load of the cantilevered arms.
Is there any reason why this should not be done?
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Old 27-02-2017, 14:09   #2
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re: Davit Reinforcement?

Should have said DAVIT REINFORCEMENT!
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Old 27-02-2017, 14:52   #3
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re: Davit Reinforcement?

That "line" must have served some purpose other than reinforcing a davit or davits. Perhaps an SSB aerial. If the davits couldn't handle the load placed upon them it's time to find a solution to that issue, not put the entire rig in jeopardy.
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Old 28-02-2017, 01:13   #4
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Re: Davit Reinforcement?

I've got a single davit on my BUCAN 37 and have added lines from 3 points ...the back stay and both the port and starboard bimini frames. (Much stronger support) fyi
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Old 28-02-2017, 01:23   #5
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Re: Davit Reinforcement?

We have static topping lifts from each davit to the mizzen. They've been in place for over a decade and the boat has been all over the Carribean and Central America in that time. No issues.

An added bonus is the original running backstays are unnecessary unless the wind really picks up.
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Old 28-02-2017, 02:44   #6
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Re: Davit Reinforcement?

Quote:
Originally Posted by Jimholden View Post
Recently I saw a beautiful Oyster sailboat that had done something that looked interesting to me.
They had run a line, probably either wire cable or dyneema from the top of the mast to the outmost end of the davit arms. It would seem that this would greatly increase the strength and reinforce the davits, since it is carrying part of the load of the cantilevered arms.
Is there any reason why this should not be done?
I recently considered a similar idea except that mine was to run the double backstays to the davits, it was pretty much shot down; here:

http://www.cruisersforum.com/forums/f116/mounting-double-backstays-to-the-davits-instead-of-hull-transom-145603.html
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Old 28-02-2017, 02:53   #7
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Re: Davit Reinforcement?

I've used a pair of adjustable support shrouds from the top of my mizzen to the distal ends of my dinghy davits for the past twenty years.


Here you can see the port support with a turnbuckle between the solar panel and the davit.


At the top of the mizzen these two supports lead from the stainless tangs within the halo antennae and pass well clear of the aft end of the mizzen boom.

I should add that my stainless steel pipe davits are well designed to function without these supports, but I have habits to employ redundancy. I've considered the possibility as an earlier post suggested that this would "compromise the rig"; however, it can also be interpreted as an added link that forms a more secure support completing a uniform frame in a full circle, - a "unibody" as termed for some automotive frames.

I do recognize a potential for faulting this plan, but I've been pleased with it for a little more than two decades.
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Old 28-02-2017, 06:16   #8
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Re: Davit Reinforcement?

Never sailed in a ketch, but with a single mast the backstay (or stays depending on your setup) is used to adjust the tension of the headstay, and to give the main a proper shape by bending the mast. It's a common trim point for anyone that has ever raced, but guess that with the setup you propose you can't do it.
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Old 28-02-2017, 16:53   #9
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Re: Davit Reinforcement?

Quote:
Originally Posted by Jimholden View Post
Recently I saw a beautiful Oyster sailboat that had done something that looked interesting to me.
They had run a line, probably either wire cable or dyneema from the top of the mast to the outmost end of the davit arms. It would seem that this would greatly increase the strength and reinforce the davits, since it is carrying part of the load of the cantilevered arms.
Is there any reason why this should not be done?
If your thesis is correct then you are adding a variable dynamic load to the mast. Peak loads will easily reach 3 to 4 x the static davit load with even a minute flex in the davits.

Lets assume that is a peak load of 1000kg. Are you concerned yet? I would be.

You've now introduced additional compressive load to your mast and its variable. You've also introduced a significantly greater variable load to the rigging which you have to preload.

You could be significantly increasing the buckling sensitivity of the mast. Without crunching the numbers I'd quantify actual davit deflection as a good proxy for whether this is a concern.

Could that line be an aerial? That would be a better alternative.
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Old 28-02-2017, 17:57   #10
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Re: Davit Reinforcement?

I'm with Leftbrain. There are too many ways where motion could make things worse. It is almost certain to increase fatigue, and just because someone else has gotten away with hanging wires there does NOT make it a good idea. There is no proof it helped, only a case where it did not fail. Instead, have an engineer evaluate your davits.

Finally, in the case of a bad wave strike to the dinghy, do you really want that load transferred to the masthead? Loosing the dinghy is probably better.
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Old 28-02-2017, 20:16   #11
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Re: Davit Reinforcement?

I would be concerned, but my lines down from the mizzen masthead are on a quarter inch nylon line tensioner and the davits are 2&1/2" stainless steel pipe on 1/4 inch stainless plates bolted through 1&1/2" solid fiberglass pads sandwiched in the middle.



That's a 100 lb Caribe 9L,- the "L' is for light,- 'no double floor and I never keep the outboard on the dinghy when offshore. I'm not denying the validity of your concerns, but then my boat is not one of those "feather-dusters" either. My modest redundant supports do suspend a solar panel too, but they are not at risk of bringing down my rig, deforming my hull or tearing off my transom.
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Old 01-03-2017, 00:15   #12
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Re: Davit Reinforcement?

Also I'll mention my davits are Bremer 350 cast aluminum I-beam lifeboat style davits with backing plates joining them to both the deck and transom. These are way heavier duty than any of the modern off the shelf davits I've seen.

But the PO also used to hang 650+lb of Boston whaler and 25hp console steering outboard from this setup 24/7 and when I bought the boat I (200lb) could stand in the whaler and nothing would move a millimeter. We've since converted to a somewhat lighter Rib but that was due to tender capacity, not weight concerns. The whaler setup did a ton of miles with zero issues.

So you've got a couple of people saying "I've never done it but it's a terrible idea". And you've got a couple other people saying "I've done it, with beefy davits it works great."

I'm curious if we get any response where someone has actually installed topping lifts to davits and had a bad experience.
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