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Old 20-08-2014, 11:19   #1
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Davit System - Strain On Backstay

Hi, I have a davit system where two arms are hinged to the pulpit (and connected to create a raise/lower frame assembly) and a line is led to the backstay {about 3metres} and down to a winch to raise or lower the dinghy. I have been concerned about the pressures of the dinghy on the backstay {although I understand this davit system is widely used??}. The system is stron enough (barely) to lift a light dinghy.

My Question, Does anyone think the system puts too much extra pressure on the backstay (35kg 85.lb dinghy] or otherwise is inadequate for blue water? I plan on only having dinghy on davit for coastal and keep on deck for blue water. Very tempted to build new SS solid one which might double as a swinging derik crane for motorbikes etc. But dont want to spend the money if unnecessary. thanks for any input. not sure if this thread should be located under rigging elsewhere?
thanks for any input.
Gordon
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Old 20-08-2014, 11:44   #2
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Re: davit system - strain on backstay

Hiya Gordon,
Easiest way to tell is leave your tender in the water and with no load on the davits, go to the foot of the mast and put your face hard up against it and look up it, it should be dead vertical. (If its not, you need to tune your rigging until it is). Now hoist your tender up on the davits with the outboard on it and lash it off. Then go to the foot of your mast and look up it to see if it is bowed aft.

cheers,
JJT
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Old 20-08-2014, 11:49   #3
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Re: davit system - strain on backstay

Would think that the extra weight and the variance in weight would have some impact on the tension of the backstay and subsequently the tension on the forestay and the overall angle of the mast. That may effect your sailing in some manner if not compensated for when you have the dinghy up or down.

As far as weight is concerned, the forces on a sailing boat are in the thousands of pounds.. For general use while anchored and putting about I doubt the rig would notice the weight of even a 200lbs dinghy.
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Old 20-08-2014, 18:33   #4
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Re: davit system - strain on backstay

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Originally Posted by jolly jack tar View Post
Hiya Gordon,
Easiest way to tell is leave your tender in the water and with no load on the davits, go to the foot of the mast and put your face hard up against it and look up it, it should be dead vertical. (If its not, you need to tune your rigging until it is). Now hoist your tender up on the davits with the outboard on it and lash it off. Then go to the foot of your mast and look up it to see if it is bowed aft.

cheers,
JJT
great suggestion should settle the matter!
thanks
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Old 20-08-2014, 18:40   #5
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Originally Posted by natew View Post
Would think that the extra weight and the variance in weight would have some impact on the tension of the backstay and subsequently the tension on the forestay and the overall angle of the mast. That may effect your sailing in some manner if not compensated for when you have the dinghy up or down.

As far as weight is concerned, the forces on a sailing boat are in the thousands of pounds.. For general use while anchored and putting about I doubt the rig would notice the weight of even a 200lbs dinghy.
thanks, my clubs boson, thought I was being paranoid given the strength of rigging. Im still nervous (or perhaps really want another project) and will apply Jolly Jacks tests. thanks for input
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Old 20-08-2014, 18:56   #6
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Re: davit system - strain on backstay

If you want to know if your davit system is offshore worthy, hoist your dinghy and then fill it with water to simulate a pooping. If you're not comfortable with that, it's not offshore ready.

This is why davits are for coastal sailing only, unless you put a ton of thought and effort into it. If you're going offshore, only bring a dinghy you can keep on deck lashed down, or in the bilge deflated.
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Old 20-08-2014, 19:02   #7
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Re: davit system - strain on backstay

Quote:
Originally Posted by jolly jack tar View Post
Hiya Gordon,
Easiest way to tell is leave your tender in the water and with no load on the davits, go to the foot of the mast and put your face hard up against it and look up it, it should be dead vertical. (If its not, you need to tune your rigging until it is). Now hoist your tender up on the davits with the outboard on it and lash it off. Then go to the foot of your mast and look up it to see if it is bowed aft.

cheers,
JJT
Actually, many masts are set up with significant pre-bend, so unless you (JJT) are familiar with this specific design, this advice is flawed.

Jim
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Old 20-08-2014, 19:39   #8
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Re: davit system - strain on backstay

It seems you would have a lot of movement with a system like that in any waves....even coastal. If you cant fully stop the movement, shock loads from from the dinghy will be severe.
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Old 21-08-2014, 02:36   #9
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Re: davit system - strain on backstay

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Originally Posted by Jim Cate View Post
Actually, many masts are set up with significant pre-bend, so unless you (JJT) are familiar with this specific design, this advice is flawed.

Jim
The OP is concerned with the potential effect that the weight of his tender has on his stays and mast. I suggested a simple method to identify if this is the case when the boat is in still-water conditions.

I don't see how it is flawed unless you can give me "many" examples of cruiser boat masts which are pre-bent when load free.

cheers,
JJT
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Old 21-08-2014, 02:54   #10
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Re: davit system - strain on backstay

Quote:
Originally Posted by jolly jack tar View Post
The OP is concerned with the potential effect that the weight of his tender has on his stays and mast. I suggested a simple method to identify if this is the case when the boat is in still-water conditions.

I don't see how it is flawed unless you can give me "many" examples of cruiser boat masts which are pre-bent when load free.

cheers,
JJT


Ask any rigger. Pre bend is standard on most rigs. Even my in mast furling system has some.
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Old 21-08-2014, 04:03   #11
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Re: davit system - strain on backstay

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Ask any rigger. Pre bend is standard on most rigs. Even my in mast furling system has some.
So, Minaret has already said it and I won't repeat. Will add that all three of the boats that I have cruised over long distances had some pre-bend. Most of the modern French type boats with 7/8 +/- fractional rigs and swept back spreaders have some... and on and on.

Jim
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Old 22-08-2014, 03:00   #12
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Re: davit system - strain on backstay

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Originally Posted by Jim Cate View Post
Actually, many masts are set up with significant pre-bend, so unless you (JJT) are familiar with this specific design, this advice is flawed.

Jim
Thanks, good point, will check with rigger on next job.

regards
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Old 22-08-2014, 03:13   #13
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Re: davit system - strain on backstay

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Originally Posted by funjohnson View Post
It seems you would have a lot of movement with a system like that in any waves....even coastal. If you cant fully stop the movement, shock loads from from the dinghy will be severe.
Thanks Hi, I will need to check next time conditions come up but I am not too worried about the dinghy or the system moving in coastal conditions (it is reasonably braced) further the system holds the dinghy and braces it so that it cant move in cradle (unlike normal davits where you can get a swinging motion). But having said all this I would not trust it with a dinghy full of water at sea and would prefer fixed davits to provide this sort of load strength.
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