April 16, 2019

Electric Winch for Dinghy Davits

  ➛ ➛ From our list of Stuff we have and use [and do...] in the right sidebar ➛ ➛   


This is part of a series describing some of our boat system refits and their operation.

We refer to these often not only for our own use, but also when asked specific questions about systems on Denali Rose, and when participating in discussions on various forums. 

We should also mention that we wait a while [1+ years in this case...] before publishing new articles— allowing enough time for the concept [and/or product] to demonstrate itself worthwhile to us [or not]...
We aren't implying our choices are the best, unique, or only way to go; they just happen to be the decisions we made at that moment in time...

–––– Latest revision 18-Apr-2019 [by Bill] ––––
[Minor revisions to Winch Rope Drum Divider Disk drawing]

Overview:

To make it easier to raise and lower our 11 foot long fiberglass RIB [with 15hp outboard motor and 6 gallon fuel tank...] on our fixed davits— and to make it a one person job— we replaced the two manual 5-part tackles [one per davit] with a single 12V DC ATV winch with a 3000 pound capacity.

I'm sure we are not the first to take this approach, but will share what we did in case any aspects are interesting or useful to others...


Details:
There is a lot going on in this photo:
  • The 12V DC ATV winch with [white] spool dividing disk is dead center on the spreader seperating the two davit arms
  • The aged dinghy chaps are loose on the partially deflated dinghy [winter cold reduces its air pressure... that is snow to the right on the chaps...]
  • The orange lifting line is 1/4 inch HMPE line [e.g., Dyneema] 
    • Each end has a thimbled Brummel splice variation with a locking carabiner that clips to its respective bridle lifting point in the dinghy
    • Note the orange lifting line is slack in this photo because the white w/ red hash doublebraid line is the safety [tensioned] line when the dinghy is stowed— optionally removing the load from the winch
    • The orange lifting line requires two blocks on our davits to fair lead it to the desired vertical drop location [on both sides]
    • The dark smude [looks like grease] on the orange line and lines behind it [just left of winch in photo] must be an artifact of increasing the exposure [brightening] after the photo was taken 
  • A portion of the inverted kayak cartop J rack can just be seen lower center in the photo [more about this below...]
  • [Note to self:] This photo also reveals we are still using the 4 snatch blocks originally installed during the proof-of-concept stage... That reminds me we need to reclaim them at some point- replacing them with dedicated single blocks.
  • Unrelated to the winch:
    • The light directly above the winch is our 'back-up' light [900 lumens]
    • The large light to the right is the stern navigation light
    • The smaller all around light under the stern nav light is a dusk-to-dawn anchor light that serves two purposes:
      • It shows the stern of the boat when at anchor [There is also one on the bow and one amidship to clearly show our vessel at anchor— in addition to the masthead anchor light]
      • It acts as a 'garage' light when using the dinghy in the dark
    • All this is sheltered under two 165 watt solar panels structurally joined and pivoting on their athwartships centerline

Here is a wider angle shot taken from the same spot— showing both davits:


Since the [one piece] orange lifting line runs in opposing directions on the winch drum, the rope drum is divided in half by a home made disk:



The spool dividing disk was fabricated on the dock using drill bits and hole saws on a small sheet of 1/2" Starboard [what we had on hand...] 

The first hole drilled in the disk was a small diameter [which became a slot] in the ID of the disk [where it contacts the winch drum- parallel to the drum] to allow a single piece of Dynema line to run through the hole/slot, and then to blocks on each of the davits, then down to the dinghy. [No multi-purchase tackle necessary because the winch handles the load easily.]

I next drilled the OD then the ID using hole saws.

Then I drew a line through the center [that would be the cut line] and drilled through the circumference for bolts to tie the two halves together once it was cut in half.

This disk was then sliced in half and bolted back together in the center of the rope spool on the winch. 
Note: After using a while, I learned a couple of wraps of silicone rigging tape around the denter of the rope drum where the divider disk is installed helps keep the disk from 'walking' to one side or the other over time...
A machine shop would take less time and produce a better looking result, but my home made version works just fine for this very low RPM, low load use case...


Operation: 

Running the winch winds up the lines from opposite sides simultaneously. [One line on top of the spool; the opposing line over the bottom...]

This approach keeps the tensile loads balanced on the winch mounts. [The winch isn't pulled excessively one direction or the other...] With the tensioned lines running athwartships, it has the added benefit of reducing operator risk if lines ever disconnected or parted while under tension... [The winch operator stands on the aft deck using either a hard wired up-down momentary toggle switch, or a cabled remote— all positions out of harms way...]

For safety lines [double-securing the dinghy in the 'parked' position] individual double braid lines [~5ft long] are tied to the fore and aft lift points in the RIB [white with red hash lines in the above photo]. These safety lines are used when the dinghy is stored in the davits, and allows us to release the tension on the winch. [Once the dinghy is raised to stow position, these safety ties are manually fed through the same blocks the orange lifting line runs through, then forward [relative to mothership...] to a cleat on each davit. After cleating, the winch is jogged to release tension on the winch and lifting lines...]

For rapid dinghy deployments [e.g., Emergency egress] the disengage knob on the end of the winch is facing the aft deck. This means if we need to deploy the dinghy in a hurry [or in the event of a 12V DC power outage to the dinghy winch] one person can uncleat the two safety lines [one on each side] and flip the disengage dial to let the winch free-spool, lowering the dinghy by gravity and making it ready for passenger loading in 10-15 seconds total.

This has all been working great for 2+ years now, and the relatively cheap winch is holding up well. 
To help mitigate the affects of our salt water environment on the ATV winch I replaced any fasteners I could with SS and reinstalled with anti-seize. Any unreplacable yet accessible fasteners were removed, coated with Corrosion-X, and reinstalled with anti-seize. Everything susceptible to corrosion is kept coated with Corrosion-X [including the two 12V DC electrical connection points on the winch...] 
All wiring to the winch is 6 AWG. 
For safety and security, an off switch was installed inside the boat for disabling the winch. [Switch off the ground wire that controls the winch relay— an ~18AWG wire— to disable the winch controls...]

This didn't take long to fabricate and install, and was fairly cheap and very rewarding as boat projects go.

Click image for larger version

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How things look from the aft deck... 
A few more details to note:
  • Each of the two davit arms has a topping lift to the mizzen masthead
  • Each davit arm has a cartop kayak J rack mounted upside down for the dinghy inboard pontoon to nest into when hoisted into travel position  [easier to see in the short video, below]
  • Our feline first mate and engineering officer, Gus, signed-off on the installation


Following is a candid video showing the very first time we tried this new fangled electric dinghy lift. Among other things, it revealed some minor changes were needed to balance the lifting eye locations and bridle lengths. It also demonstrates the winch is very capable of the task...





What else needs to be done?
  • Replace the snatchblocks used for the proof-of-concept with permanent single blocks.
    • This will require more steps than appear at first blush since I made permanent eyes with thimbles on each end of the lifting line [orange Dyneema] where it attaches to the dinghy... [At least I planned for this (and other eventualities) by spooling as much extra Dynema on the winch as would fit before cutting to length... Hopefully that will be enough...]
  • Safety stop: When the lifting line is re-run, install a microswitch [low amperage disconnect for winch control relay... wiring already run...] near each primary block. This will serve to stop the winch automatically when the dinghy reaches maximum 'height'. 
    • Right now stopping the winch is incumbent on the operator... [A distraction could produce interesting results... Not unlike those which have occured with other electric winches or anchor windlasses...]
  • Install a polyurethane rubber plug [snubber] directly behind the newly spliced lifting line thimbles to mitigate damage to the blocks should the line be reeled tightly against them...
  • Implement an easy way to make small adjustments to the length of the two lifting lines so the dinghy comes up parallel to the davits
    • This may become a part of improving the lifting eye/ bridle components

Would we change anything if we did it again?
  • To slow the final stage of 'docking' the dinghy in the davits, I might experiment with a one part tackle using a single block that would have a locking carabiner attached for connecting to each of the two dinghy bridle lifting points
    • The bitter end of each of the two lifting lines would be rove through a single lifting block and led back to the end of each respective davit and secured
    • This would halve the up/down speed [and the load on the winch- which isn't necessary for this winch, but will likely make it last even longer...] 
    • To fit enough additional line [~14 feet] on the winch drum to accomplish this we may need to use a smaller diameter HMPE line [e.g., 3/16 inch Dyneema— which would still be plenty stong for the purpose intended @ 4,900 lbs breaking strength...]

If you mechanized the raising and lowering your dinghy, do you have any as-built details you are willing to share? 

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