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Old 10-01-2013, 06:50   #1
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If a 100lb dinghy is suspended @ the end of a 36" davit, is the actual weight load or force on the back of the transom increased by the leverage and if so anyone have an idea how much? Or is it simply increase the force on the mounting plates etc and not the downward force on the stern of the vessel by more than the dinghys weight?
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Old 10-01-2013, 06:52   #2
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Re: Dinghy davit question

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Old 10-01-2013, 06:56   #3
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Ok ill need a pen and paper lol
Thanks!
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Old 10-01-2013, 08:15   #4
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Re: Dinghy davit question

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Originally Posted by Dulcesuenos View Post
If a 100lb dinghy is suspended @ the end of a 36" davit, is the actual weight load or force on the back of the transom increased by the leverage and if so anyone have an idea how much? Or is it simply increase the force on the mounting plates etc and not the downward force on the stern of the vessel by more than the dinghys weight?
If you assume a simple installation of a straight davit bar connected to a single end point then to calculate foot-pounds of force exerted at that end point is simple. It is just effort (weight in lbs in this case) x distance. So 3' x 100lb = 300 ft-lbs.

This of course assumes a very simple installation. Other configurations would make this calculation more complex.

Same principle as a torque wrench or a pry bar.

Also important to keep in mind is that the load of the dinghy is not static on a vessel underway. In heavy seas the inertia of the boat moving up and down will in effect make the dinghy heavier (or lighter) momentarily.
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Old 10-01-2013, 10:46   #5
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It would be connected at the base and 1.5' up it would be connected to the safety rail.
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Old 10-01-2013, 10:55   #6
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Re: Dinghy davit question

I havent found it to make a huge difference in boat attitude on larger boats. For the type that mounts on a base plate and then is supported by the pulpit, I wouldnt leave the motor on. Can be very handy just moving anchorages for the day (90%). For a blue water trip (10%), you could still put the dink on deck.
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Old 10-01-2013, 11:24   #7
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Re: Dinghy davit question

My statics may be a little rusty. But I believe you have 2 forces to think about:

- the base of the davits will support the weight of the dinghy (and davits). So the base must support a little over 300#. There will be very little lateral force on the base

- the attachment to the stern rail at 1.5 ft above the base must counteract the moment arm of the dinghy hanging on the davits. The dinghy will generate 300 lb-ft of torque (100 lb x 3 ft moment arm). Therefore the attachment to the rail must support 200 lbs (300 lb-ft / 1.5 ft).

As previously stated, all those forces are static only. It's reasonable to assume the forces could increase by 2x or 3x in waves.

A couple thoughts.

- Don't forget that the weight and forces will be distributed between 2 davit arms, so no single point will bear all the weight.

- stuff in the dinghy (outboard, gas, anchor, ...) should be added to the calculations

- The dynamic loading is much worse than any static analysis.

- Add water in the dinghy and things get much worse very fast - catch a wave and the davits themselves are prone to failure, probably before the attachment points.

However you slice the numbers, it's further justification to never make a crossing with a dinghy hanging on the davits. Put it on the foredeck. Davits are for calm water and anchorages.
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Old 10-01-2013, 14:31   #8
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Re: Dinghy davit question

Thanks all, I spoke to a family member who is an Aerospace engineer, he explained it similar to Paperbird, Though Said based on the length of the vessell, etc etc (lang I didnt understand at all) the actual downward total weight on the transom would be only about 20% +/- more than the actual dinghy weight, so roughly 120lbs not incl other forces. Which though they would be increased from waves etc the metal flexes, and boats movement etc etc ( more lang I didnt understand) wouldnt quite be the 2-3x the dinghy weight many claim to be exerted as If the boat were actually dropping off waves onto a hard surface than that would be more the case.
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Old 10-01-2013, 14:50   #9
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Re: Dinghy davit question

Beware those aero guys (yeah - me, too).

There are 2 separate questions at work here. Beware getting them mixed up.

One is how much force will be generated by the dinghy/davits/etc on the davit mounting system. That's what I was calculating. And the dynamic loading could indeed get pretty high. If you've ever dropped off a 10-15' wave, you'll appreciate the forces involved.

The second, and it's the one your family member and a previous poster was addressing (I think), is how much impact will the added weight located at the aft end of the boat have on the overall balance of the boat. I agree that it's not that much. Roughly equivalent to a large person walking around the back of the boat. Is it better to avoid weight in the ends so as not to induce hobby-horsing? Sure. But then you're getting into racing, not cruising. So not an issue here.

Bottom line - the mounts will, in all likelihood, be stronger than the davits themselves. Most davits seems to be rated for 350-500 pounds. Any bracket system you use should handle that and more.

Is your family member still practicing in the aero field? I bailed from engineering a long time ago - switched over to the consulting side.

Cheers!
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Old 10-01-2013, 15:26   #10
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Re: Dinghy davit question

He retired From Navair after 25 years or so. I dont want to over load the stern of our cat, so wanted to do some pre davit purchase / build digging. On our prior two we always dropped the dink and towed it or stored it on deck in the rough stuff. But Davits sure are nice for calm weather cruising / island hopping. Hoping to buy a AB 10 UL in the future when we can afford!
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Old 10-01-2013, 16:53   #11
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Re: Dinghy davit question

As previously stated, all those forces are static only. It's reasonable to assume the forces could increase by 2x or 3x in waves.

A couple thoughts.

- Don't forget that the weight and forces will be distributed between 2 davit arms, so no
single point will bear all the weight.

- stuff in the dinghy (outboard, gas,
anchor, ...) should be added to the calculations

- The dynamic loading is much worse than any static analysis.

Yep. These type are sold often though so I guess they must work ok. Good to know what they have to withstand though.
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