Snatch box use with Crane Davit?

[Streff]

Can a snatch box be useful in decreasing the weight/load in crane operation when pulling a dinghy up.

My 1000lb crane davits had a Warn 3700 winch when we purchased the boat. It seemed to have been on the boat for years. In the process of exchanging the wire rope with Dynema, the winch negative post was damaged.

I called Warn to seek guidance and they were adamant that a hoist would be the way to go. I ordered the 1000lbs 12VDC hoist but it won’t go in the Crane opening without a good amount of modification. I am working on that starting with intermediate mounting plate. Moreover, Warn indicated that the drum of their small winches are not designed for synthetic ropes. That’s another story.

Yesterday I ordered a Warn 4000 winch as a backup player in case I don’t get the hoist installed before heading out and wondered about using a snatch box to reduce the load on the winch. Just thinking about it, I am not sure if the pulling load will decrease with a snatch box. I realize for winching operations on the ground.. this thing cuts the weight up to 50% but slows things down.

Has anyone used a snatch shackle with their crane davits and it decreased the load?

Thank you in advance

Streff

 

[DNT99611]

You can get to the fine print and it makes a difference. There are lots of things to be factored in. But the real life answer is what it does is makes life easier for the winch but it takes longer to do the job.

 

[Alaskaflyer]

Assuming you are placing the snatch device at the dinghy end of things, and that you are hard tying the standing end of the line/cable up top...it will decrease the load on the winch. You've constructed a 2:1 mechanical advantage. It's not a perfect 50% reduction due to the increased friction of the system but it will still deliver an advantage with a corresponding reduction in speed.

The load on the lifting point (davit) will remain the same regardless, which is the weight of the dinghy plus rigging.

Will you have enough line?

 

[Alaskaflyer]

BTW if you do a search there is a thread here from the last recent few months discussing replacing original winch cable with Dynema or Spectra and some of the associated issues/difficulties/advantages they encountered.

 

[Streff]

Thank you both. I believe that do have enough line by the numbers but I will watch carefully that a few wraps remain on the drum.

I did read the Dynema thread, but will reread it again. I have to reverse mount the winch due to the crane arm orientation. So the reason for the switch is to allow the winch to pull from the bottom of the drum instead of how the PO had it pulling from the top of the drum.

Will see how that goes.

Streff

 

[redog]

Yes I have a snatch set up as you describe and lift a very solid 4.1m tinny with 30 HP.
One winch on a boom and another one lifting the boom. Both have snatch pulleys on the lifting hook.

As stated correctly in replies above, load on winch is halved but time doubles. Electric load reduces but of course applied for a longer period of time. Loading or weight on lifting davit or boom remains the same.

Yes a snatch reduces load on winch whether lifting or winching horizontal. It gives you more mechanical advantage.

 

[DNT99611]

I will add that if you move the winch from out on the davit back to the boat you will be loading the davit more. Just something to think about if you were to relocate the winch.

 

[Streff]

I finally installed the new winch with a Dyneema rope instead of wire rope. the dinghy weight came in at 632 lbs. I then installed a snatch tackle.. reweighed the dinghy. It came in at 574 lbs with the tackle in place. A 10% drop in static weight. Is this the correct way to analyze the effect of the tackle in my setup?

I installed a Warn 4000 utility winch so that we get started on our trip. Ultimately, I plan to install the 1000# hoist.

Streff

 

[HiDHo]

I don’t think so. If your running rope from the winch drum out the end roller on the boom to a snatch block and back to a shackle on the end of the boom you will lessen the strain on the winch motor. But you will still be lifting the same dingy weight with regards to the crane boom capacity.
Basically what Alaskaflyer said.

 

[SteveK]

Any pictures to share on this discussion?

 

[Alaskaflyer]

I finally installed the new winch with a Dyneema rope instead of wire rope. the dinghy weight came in at 632 lbs. I then installed a snatch tackle.. reweighed the dinghy. It came in at 574 lbs with the tackle in place. A 10% drop in static weight. Is this the correct way to analyze the effect of the tackle in my setup?

I installed a Warn 4000 utility winch so that we get started on our trip. Ultimately, I plan to install the 1000# hoist.

Streff

Something doesn't compute here, at least inside my own brain as I try to visualize what you are saying.

Mechanical advantage doesn't reduce the load (weight), it changes (reduces) the force necessary to move that load. Which is why setting up a mechanical advantage system doesn't relieve one of the requirements for certain load specifications in your line, crane, or hardware. The change in weight from your old system of dinghy plus cable and existing hardware should now subtract the difference in weight from old cable to new nylon line, but then add the weight of the newly introduced snatch block and take into account any weight changes in the downstream hardware such as snaplinks, rings, carabiners etc. Did replacing the cable with nylon but then adding the snatch block result in a 58 lb weight reduction?

 

[Streff]

Thank you all for the great comments. I had the same thoughts about the drop in measured weight. It did not make sense to me. The weight measurements were taken back to back. Same dyneema & warn winch setup. Except I added the snatch tackle and of course looped the dyneema back to a loop attached to the boom. Possibly the angle/vector of force where the lifting harness attaches changed a bit, leading to a 10% drop in measured weight.

I setup the harness with longer legs to avoid pushing on the center console and still keep the dinghy balanced…which does not leave much room for a snatch tackle even at max boom elevation.

I will try to measure the time that it takes to lift the dinghy with and without the snatch tackle.. if the tackle slows down the process then I think a mechanical advantage does exit.

I started questioning this advantage after speaking with a Warn winch CS rep. Who was also adamant about not using a winch in this setup.. the proper unit is a hoist. I did purchase the hoist but it and possibly the boom will need modifications to get it installed.

Thanks again for the input.

Strega

 

[angus99]

I’m hoping Streff’s questions are all answered and he doesn’t mind me piggybacking a related engineering query for some of the intelligentsia.

If I’m lifting a 550-lb dinghy using two 4-ft tall levers—basically a typical transom-mounted davit (pictured)—is the lifting force on the arms throughout the 90-degree pull the same as a deadlift from directly above?

I’m trying to find the strain I’m putting on the attachment point on the transom (arrow in photo).

Many thanks.