cautionary tale - broken davit cable

Wanted to share a cautionary tale about a series of events that ultimately led to our davit cable failing while under load while lowering the dingy into the water. Thankfully, no one was hurt and the dinghy only fell about a foot. But very scary situation that could have been very damaging or even deadly in a different scenario.

Background: the davit is a 600 lb rated UMT davit/bow crane with a dutton lainson winch. The winch is spooled with 25' of 3/16" stainless cable. The cable breaking strength is listed as 3700 lbs. I replaced the winch last season so the winch and cable were nearly brand new. The dinghy is a RIB weighing approximately 400 lbs with fuel and gear - also brand new, replacing my previous RIB that was about 50 lbs lighter.

Part 1: We were anchored outside the reef off of Guana, and had run the dinghy in to have lunch at Nippers. When it was time to leave, I was diving on the anchor to make sure it wasn't caught on a rock and my wife and teenage daughters were raising up the dinghy to the bow, which they have done many times before. However, this time, my daughter did not fully pull out from the boom all of the wire connecting the remote to the winch. With the dinghy suspended over the water but not quite above deck level yet, she learned in a flash of sparks that the control wire got tangled up in the winch cable and spooled onto the drum. The wire insulation chafed off in several places and the energized conductors shorted to the winch cable. The short popped the breaker. With the storm bearing down, we managed to heave the dinghy on to the bow of the boat to take the load off the winch cable. It took us a while but we were able to get things untangled. I cut out the bad sections of the control cable and reconnected everything. The winch cable appeared undamaged from what i could see, other than what looked like superficial charring from the arc/shorting incident, but I didn't spool it all the way out (more on this later). With the davit working again, we were able to get the dinghy back on the chocks and get on our way just as the storm hit.

lesson 1: always pull out all of the cable from the remote control when operating a davit. The guard on the front of the winch is insufficient to keep the electrical cable from getting pulled into the winch.

lesson 2: if your winch is so equipped, make sure you know how to operate it manually and have the correct tools to do so. UMT failed to provide me the emergency manual crank handle when they supplied the replacement winch despite me asking for it and them promising it was included. Then they made several other promises to send it and never did. I eventually stopped following up - shame on them but shame on me too.

Part 2: A few days later, once again my wife and girls were lowering the dinghy into the water, this time at the marina. When the dinghy was within about a foot of being all the way down, the cable snapped and the dinghy dropped into the water. One was operating the crane and the other two had taglines controlling the load. I'm still unsure what caused the cable to snap but my strong suspicion is that the control cable binding up in the winch cable on the drum deformed the winch cable and created a weak point. I don't know if the heat from the arcing could have also contributed. The failure in the cable occurred much closer to the drum than to the boat, so it failed in a place I had not fully inspected. The only other explanation (which is even scarier) is that the cable had a latent manufacturing defect and was far below its rated strength. Still, this was a very, very surprising turn of events given we were working with a load that was an order of magnitude less than the breaking strength of the cable. There were no dynamic loads - we were in a marina so the boat wasn't rocking and winds were not particularly strong. The dinghy was probably a bit on the heavy side due to water trapped in a compartment I was at the time unaware of (we'd drained the main bilge but there was a second chamber with a different plug). My guess is the load was still under 500 lbs though so I doubt this was a major factor.

lesson 3: Never allow anyone under a suspended load. Everyone involved should be aware of the hazards and should understand there is always a chance of failure.

lesson 4: Understand how to properly tend a tag line. Never allow the line to be wrapped around your hand, a finger, ankle, or any other part of your body. Be ready to release the line in an instant (for example, to keep from being yanked into the water by a falling load).

lesson 5: When you have a failure of a safety critical system, fully inspect the entire system after repairs. In this case, i should have spooled all of the cable out and inspected it before lifting with it again.

lesson 6: Don't let a large factor of safety make you overconfident. There are many ways your actual factor of safety can be reduced in a system under load. Always assume the system can fail.

lesson 7: Water is heavy and sloshes around creating dynamic loads in addition to the added static load from the waters additional weight - make sure you get as much out as you can including from any secondary chambers or places where it can get trapped before lifting a dinghy.

lesson 8: Fully inspect lifting gear regularly and replace any safety critical component with any sign of damage. Specifically, I will never again trust any steel cable that shows any sign of deformation whatsoever.

After doing some research, I decided to replace the stainless cable with 3/16" dyneema. I think the benefits outweigh the downsides in this application, but YMMV. That's probably a different thread...

Is the concern the dynema will slip over the rim of the pulley and bind up?

I looked up what I could find in terms of pulley/sheave specs for dyneema and I’m well beyond the 8:1 D to d ratio recommended. The only other thing I see is on channel shape but the dyneema seems to fit in nicely without being squeezed so I think I’m ok but if there’s something else to look at let me know.