Davits

ChrisP

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Got a nice shiny pair of davits all ready to go on the boat this winter. The problem is how far apart do you fit them. The back of the boat is not prepaired for them so all the interior support will be made up to suit. Is there a standard dimension that the manufacturers work to or is it guess and god and hope they line up with the dingy(which has yet to be bought).
 
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There is certainly an ideal spacing for simple support of a beam or dinghy with uniform distributed weight. Problem 1 is that the dinghy does not uniformly distributed length (engine at one end) and Problem 2 is you don't know ho long it will be and problem 3 is erm I've forgotten how to work out the flippin formula, and resolving at one end of beam with 2 supports shows that L=L.

The Law of Lazy Obviousnes might indicate that it must be about a quarter of the way in from each side spose, so each half is sort of balancing eh? Prob bit less for each support taking a bit of the others, bit more to allow for heavy engine. So each davit in a quarter of the whole length of the way along seems fine to me.

Jfm is also mech eng but unlike me is paid for knowing things, whereas er well anyway I bet the davit manufacturers will have a good idea. Except of course it entirely depends on which end the engine is on, and how heavy, and it's bound to be wrong if you decode on one side but change to t'other for access or mooring etc. Lining them up with dinghy should not be prob cos davits have to fit most dinghys with straps, bits of wire etc.

I am certain that fixing the davits really well, and making sure that the dinghy doesn't bounce around at speed is miles more important (shock loads, fatigue blah) than exact spacing, exact calculation of which wd require sawing dinghy and engine into slices and weighing each bit for total accuracy, plus variance to allow for heavy petrol can in or out and mebbe at one end or t'other.
 

miket

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Chris.
Before you start ripping the boat apart I should try and contact FW Booker, the manufacturer of your Humber.

I would be surprised if the strengthening is not already in place. You may just need to find out where.
 

paulineb

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Two methods of achieving this:

1. Buy your dinghy. Line up empty lager bottles inside your dinghy between the handle thingies. Count the bottles and then line up the same number on your transom and make the holes either end of them; or

2. Get Mrs P to lie stretched out on the transom, making sure her centre of gravity is in the centre of the transom, then make the holes either end of her so she can hang a hammock for her to relax in after she's cleaned the boat, done the shopping, cooked dinner, changed the beds and fixed the drinks.

Pauline B
 

BarryD

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I like the hammock plan, regretfully SWMBO is a foot shorter (not one legged Matts) so the calculation will be skewed. Also assuming an average bottle of lager is 7cm wide, to fill the transom will take about 28 bottles, a little in-set for balance and that's 20 bottles to be drunk and then power tools to play with near water...

Serious question someones drilled holes and put metal clips with eyes in them into the swim platform of MV2 sorry Firefly. How could I find what davits should fit on them? I figure a picture post to MBM with "Missing pointy uppy sticky bits to go on these any clues(?)" might work.

Barry D.
 

jfm

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A very scientific analysis of weight and stuff

The scientifically optimum distance apart that matts refers to is meant to reduce bending stress in the thing that will hang on the davits, in this case a dinghy. But that makes sod all difference, a dinghy is so light that the bending stresses that it will endure by being dangled from 2 points are irrelevant, it doesn't matter how far apart the two supports are. So 1/4 in each side would be fine, except for one thing...the outboard.

The outboard changes everyting. Lab coats on please. It's obvious that there is more weight on the davit near the outboard normally. But if the davits were an infinite distance apart the weight on each davit would be the same, I mean 50% of the total dinghy weight would be on each davit. However, the closer you move them together the more the weight transfers onto the davit near the outboard. There comes a point where the line crosses the axis, and 100% of the weight of dinghy plus outboard is on the davit nearest the outboard, and zero weight is on the other davit - this point is reached I gess when the davits are about 2 feet apart.

So, the davits should be as widely spaced as possible, to even up the weight on both davits as far as possible.

I know you dont have this much freedom on a real boat (if the width were infinite, think what the LOA would be) so, as others have said, the main thing is to get massively strong transom supports which you're already doing, and fasten dinghy tight to minimise shock loads.

Gotta go, some trains to spot...... :)
 

jfm

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Re: Davits snap davits

Barry they may be for snap davits. As it happens I have a pair of pukka snap davit bits that you attach to the dinghy, which might well fit the bits in your transom. I got them by mistake, bort a tender by mail order and the supplier accidentally put them into the packeage and then told us to keep them. So cost me £0. You're most welcome to have them for a small lager. They're on the boat but I'll pick em up soon and deliver to Shenley manor or wherever. Check what's on MV2 when you're there this w/e and lemme know.
 

BarryD

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Well, JFMs a gent.

Thanks John - I will see if there is anything inscribed on them, and take a digital picture. You're welcome round Shenley anytime, always got copious booze tucked away around the manor and it only goes off if it doesn't get drunk...

Barry D.
 
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Deleted User YDKXO

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Our learned friends on this forum may have pushed back the boundaries of physical science but they have'nt answered your questions. The point is that, ideally, the davits should be in line with the lifting points on the dinghy or you'll get lateral loading on the dinghy/wire/davit - I am sure jfm will explain in mathmatical terms why this bad.
The most likely situation is that your boat already has reinforced positions on the transom for mounting davits in which case you wont have a choice as to where to mount them so you'll have to buy a dinghy which can be fitted with lifting eyes the same distance apart - consult the dinghy manufacturer
If your boat does not have reinforced davit points, then more than likely you will have to glass in some wooden pads behind the transom. Obviously, then you have a choice where to place the davits but they should be placed as far away as possible from openings in the transom such as transom doors, hatches and windows. Decide roughly where the davits could go and buy a dinghy with lifting points to suit. You could offset the davits from the centre line of the boat but it could create trim problems with the boat or difficulties with mooring lines
Remember that the total weight of the dinghy/outboard and fuel should not exceed the safe working load of ONE davit
 

jfm

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Re: Cosine of sod all

Oooer! Actually I disagree. Yes, if the davits are not directly above the dinghy lifting points then there will be lateral load on the dinghy (not, as you said, on the wire. The wire is always in straight tension it ca never have lateral stress). But so what? What's wrong with a bit of lateral load? The extra tension in the wire needed to hold up the dinghy if the wires aren't quite vertical is the cosine of sod all, so it makes no difference to anything. IMHO :)
 
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Deleted User YDKXO

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Re: Cosine of sod all

Obviously, a slight angle of the wires makes sod all difference but a few inches difference between the davit centres and the dinghy eyes can make an appreciable angle, the cosine of which may be a bit more than sod all. I suspect that davits and particularly the arms are not designed for lateral loading and I guess thats why you see some boats with spreader bars between the dinghy and the davits to ensure a vertical loading.
I'm no expert but I think the less load you put on them the better, speaking as someone who's had to tow his dinghy quite a few miles thanks to the bloody things breaking!
 
G

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The Humber 38 will be already reinforced as they are built lika brick s t house anyway, you could ring Roger Whitall at whitall davits or Ross Hale at Ron Hale marine at Southsea, he can reccomed the correct distance spacing, I think most of the big manufacturers set them at 60 inches, only the stupid Taiwanese set them at about 7 ft, as they did on our HISTAR 42 some years ago.

Paul js
 
G

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Re: Big spacing is good

Not really jfm, bought avon 310 rib, steering console, nice 15 hp mariner, tried to hang the thing on the davits, wires ended up at all angles, looked a right mess, phoned ross hale up he said, poxy taiwanese boats , we have loads of trouble with them, what the histar was set up for must have been something lik Alan Priddys boat, the build quality would have carried it I can tell you, as it only did 21 knots flat out with 2 375 cats in it and needed a tanker behind it, would use 90 gallons from Hythe to Weymouth, sold it 3 years ago. Sorry Fred drifting a bit there.

Paul js
 
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