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Old 03-05-2015, 12:21   #1
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Mounting double backstays to the davits instead of hull transom?

I have had this idea for some time now of having custom built davits fabricated for my boat. I presume davits would be the easiest way to store the dinghy while on the way as I don't care about the windage and I'm not a racer. Also suffering from a slipped disc it just seems like the most convenient way to launch the dink (and to haul out), as opposed to from the deck, or just towing it behind all the time. Furthermore, this would enable me to enlarge my old hard solar bimini, giving me more shelter in the cockpit from the tropical sun and the tropical rains too. And of course more solar watts. I tried to draw the bimini to scale, and I feel it really isn't big enough at the moment.
I would like to use the space on top of the davits for solar panels also.

But for now I wanted to ask if anyone has built a system like the one I sketched below? Hang on there... Specifically, on the pic you can see where the old double back stays attach to the hull's transom (black line), and my idea is to attach those backstays to the bends of the davits instead; the pic I hope illustrates this better than my attempt in writing. I'm a bit concerned about how the new mounting point for the backstays would affect the support for the mast, in a situation where you have say 70-80 kg dinghy hanging from the davits, and get some swells. I guess one should build the Davits almost rock solid so the weight of the dink would not cause oscillating forces to the backstays.. Or not? Anyone ever done or seen a setup like this and what are your thoughts? Is it a bad idea to mount the backstays to the davits? Any fotos or links?
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Old 03-05-2015, 12:38   #2
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Re: Mounting double backstays to the davits instead of hull transom?

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Is it a bad idea to mount the backstays to the davits?
Sorry, but if you'd like the mast to remain standing, I'd suggest that Yes, that is probably a very bad idea...

:-)
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Old 03-05-2015, 12:40   #3
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Re: Mounting double backstays to the davits instead of hull transom?

I'm not a naval architect, only a cruiser who loves to sail, but this seems like a very, very bad idea. You may not be interested in racing, or windage, but your backstays are a rather integral part of the safety of your vessel, as in holding the stick up straight. I would not want the extra sideways forces of a dinghy in a blow on it. Plus, the transom is a very solid mounting point, what with the corners there. I can't imagine mounting davits that would be nearly as strong and integral to the hull that would be safe.

Personally, without the advice of a reputable NA, I absolutely would not do this.

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Old 03-05-2015, 12:53   #4
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Re: Mounting double backstays to the davits instead of hull transom?

From an engineering standpoint, the biggest problem is attaching the stays to the outboard end of the davits.

Look at your diagram. The stays on the davits will not be pulling in a straight line to the hull but instead pulling the top of the davits inward. The davits will work like a long lever and the force from the back stays will pull on top of that lever and apply a much greater force to the other end of the lever. In this case that would be where the davits attach to the deck.

Most likely outcome the force would rip a big hole in the deck at the base of the davits. Only solution would be to build really strong, rigid davits, a very, very large, heavily reinforced area on the deck where they attach and a very, very large metal backing place to spread the load around.
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Old 03-05-2015, 17:00   #5
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Re: Mounting double backstays to the davits instead of hull transom?

I am not willing to say it can't be done, but I wouldn't even consider it without having an engineer run the numbers. My guess is that you would need bob-stays, massive amounts of metal or carbon fiber, and to completely rebuild the transom.

My absolute random guess is $50-60k to do something like this by the time it's done. And I am not sure what it gets you. Just sew in a hole for the backstay like everyone else does for the biminie.
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Old 03-05-2015, 17:15   #6
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Re: Mounting double backstays to the davits instead of hull transom?

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From an engineering standpoint, the biggest problem is attaching the stays to the outboard end of the davits.

Look at your diagram. The stays on the davits will not be pulling in a straight line to the hull but instead pulling the top of the davits inward. The davits will work like a long lever and the force from the back stays will pull on top of that lever and apply a much greater force to the other end of the lever. In this case that would be where the davits attach to the deck.

Most likely outcome the force would rip a big hole in the deck at the base of the davits. Only solution would be to build really strong, rigid davits, a very, very large, heavily reinforced area on the deck where they attach and a very, very large metal backing place to spread the load around.
Bingo... it could be done but the way you have it shown would require massive reinforcement. I wouldn't suggest trying it without professional (not a random fiberglass guy) assistance.
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Old 03-05-2015, 19:51   #7
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Re: Mounting double backstays to the davits instead of hull transom?

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My guess is that you would need bob-stays, massive amounts of metal or carbon fiber, and to completely rebuild the transom.
Certainly, I can't imagine how it could be done without resorting to some sort of bobstay configuration...

Which would likely render the davits themselves highly unsuited for dinghy stowage...

:-

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My absolute random guess is $50-60k to do something like this by the time it's done.
In which case, it might be cheaper to simply convert the thing to a B & R rig... :-)
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Old 03-05-2015, 20:12   #8
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Re: Mounting double backstays to the davits instead of hull transom?

don't do it. all the above comments are generally correct, it will impart huge bending/prying forces on the mounting pads of your davits. It can be done, but it is not trivial or even advisable, unless the transom, davits and general stern deck area are design with this in mind from the start.

i am an engineer, and i do design structural fitments for commercial vessels and yachts. but i have not done calcs on this, nor would I do them.

best regards,

bg
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Old 03-05-2015, 20:23   #9
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Re: Mounting double backstays to the davits instead of hull transom?

I have an arch with two additional backstays that are a thinner diameter running to the edges of the arch as extra support for the davits.



here is a bad photo but you get the idea.
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Old 04-05-2015, 08:36   #10
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Re: Mounting double backstays to the davits instead of hull transom?

I for one would not do it, as you will be changing the stress points on the hull and trim of the boat....
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Old 04-05-2015, 09:04   #11
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Re: Mounting double backstays to the davits instead of hull transom?

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I have an arch with two additional backstays that are a thinner diameter running to the edges of the arch as extra support for the davits.



here is a bad photo but you get the idea.
Huge difference with the mast supporting the davits versus the davits supporting the mast.

It appears on you example, the original stays are still in place and carrying pretty much all the normal rigging loads.
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Old 04-05-2015, 09:12   #12
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Re: Mounting double backstays to the davits instead of hull transom?

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I have an arch with two additional backstays that are a thinner diameter running to the edges of the arch as extra support for the davits.



here is a bad photo but you get the idea.
In this case what you have built is 2 hangers to support the aft end of your solar panels. If you hang a dinghy from there as well, no problem. It is probably a slightly bouncy way to suspend the dinghy and panels. But practically, there is nothing here to support the mast whatsoever, not when considering the huge forces the wind on sails would provide. Those wires can't be called backstays in any meaningful way.

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Old 04-05-2015, 09:14   #13
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Re: Mounting double backstays to the davits instead of hull transom?

You will probably break the davits at the point where they are attached to the boat (and the mast will collapse).
You can attach bobstays from davits ends to stern close to the waterline to counter the pull of the backstays - but then you complicate things at the stern and probably would not be able to hang the dinghy on the davits.
Forget the idea...
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Old 04-05-2015, 09:41   #14
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Mounting double backstays to the davits instead of hull transom?

I think your idea is a good start but incomplete.

As several posters have pointed out the davits would rip out if loaded up as drawn. Somebody mentioned bumpkins. The davits themselves can act as part of the bumpkin. To make them work you would need to run bobstays down from the davit to the hull. Also you would want to flatten the angle of the vertical leg of the davit to decrease the load it carries. This pushes the hoisting end of the davit further aft.

The biggest problem I see is securing the bobstays to the hull and the geometry of the situation.

You should provide a plan view (top down) of the boat with davit drawn in.

You could provide chain plates through bolted to the hull but this puts them underwater when underway which is not such a good situation given that they will work and move as loads cycle. Alternatively you can build up the structure under the tip to the stern and pop the chain plates out thru the top of the transom. More work but less likely to lead to water intrusion and related damage.
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Old 04-05-2015, 09:57   #15
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Re: Mounting double backstays to the davits instead of hull transom?

If you did the bobstay arrangement, you would have to assess the compression loads on the davits themselves along with where they attach to the hull. Also the tension on the bobstay where it attaches to the hull could be tricky. When you see a bobstay on a bow, the pull is usually in line with the centerline of the hull. The hull has a lot of strength to resist loads at this point. Assuming you kept the attachments out of the water, you don't have a similar high strength point to attach to.
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