Let's design a dingy crane?

[MVCalypso]

We installed a St Croix Junior removable manual crane on an IG36 Europa now sold.
<snip>

Deflection under load persuaded me to fix a line when in use to support the crane vertical shaft back to the mast.

Was that deflection of the St Croix Junior crane, or deflection of the deck where the crane was mounted, or some combination thereof?

 

[mvweebles]

You’re close with this idea Peter, but the vertical section should be a hat shape. Or a rectangle tube or channel welded to the upright plate. Move the gusset out enough to cover 3/4 of the horizontal plate. Try to maintain a 3:1 ratio of height to length on the gusset(s) if there’s headroom. Or give them a nice radius shape so they don’t look so industrial.

For those that think this is overkill, I do remember some discussion of upgrading the tender at some point. Better to plan for it now than do it again when a good looking rib comes along.

OP - you may save yourself time and money with a hybrid solution. Pipe davits are two pieces: the boom, and the stand pipe. Buy the boom from Nick Jackson, fabricate the stand pipe. Unfortunately I don't remember exactly how the boom section fit down into the stand pipe section so I don't know how far down the boom sleeves into the stand pipe, but it has to be a better solution than having the whole assembly fabricated from scratch.

Peter

BTW - guidance from Bmarler is excellent. From other posts he's made on stainless steel, he seems to really know his stuff. Welding a channel down vertical section is a good idea.

 

[MVCalypso]

The one that resulted in leaks into the saloon (Post #1)?

Just to paint a bit more detailed picture - I believe the leaks started from rust in the crane base around the bolts. That let water into the wooden interior of the upper deck block, which then rotted over the years. Interestingly, there wasn't any sign of flex (no stress cracks etc) in the frp laminate that encapsulated the wood block that got wet and rotted. The 12v wires for the crane winch went up thru the center of the old crane and the water followed that path down thru the FB deck and in thru the Saloon wall (the wires then run above the headliner), where it found a low spot and started dripping.

 

[MVCalypso]

Trying to upload an image of the original 3" O.D. x schedule 40 pipe that cracked under a 1,300-1,400 lbs lift load.
https://photos.app.goo.gl/iT21j3s2H7rW4f6G8

This may sound odd, but I find this somewhat comforting...

The NA calcs said that the sched 40 pipe would start to bend around 1400 lbs static load. Your tender was 1300-1400 as a static load, add in some factor for dynamic loading I bet your crane had more than 1400 lbs dynamic load on it.

So the fact it cracked is consistent with what I was told I could expect - your experience seems to tell me I can trust the quality of the NA calcs.

Your welding repair mod also gives some idea of how much strength was needed to withstand the 1400 static load range - so now I can roughly bound the pipe material sizing that would be required.

Sorry you had a problem in the past, but thanks for doing the experiment - I think it will help me out.

 

[MVCalypso]

Leaks, which could happen with a 5000 lb crane and MVCalypso reports no structural issue?

If I spent more than $1-1.5k on the entire project that would be too much,
but I can design, fabricate, weld, wire and engineer, etc.

Cool estimate - A prelim estimate for the cost of materials and some welding labor came out to be between $1-1.5k. I wish I had the welding skills, but I learned years ago that welding is a skill you have to keep up * I don;t do enough to not need lots of practie each time I try it. I've only done some amateur torch & MIG work. I tried some Al experiments with my small mig setup, but it appears I've only mastered "no penetration" or "melted puddle". :lol:

 

[MVCalypso]

Folks, I'm really appreciating the discussions. My thanks for the inputs!

I am finding this a bit like drinking from a fire hose (there are a bunch of very knowledgeable folks on this forum) - so I think I'll try to respond to the key topics instead of each post.

Mounting strength:
I'm still absorbing the suggestions for strengthening the deck mount arrangement. Part of me says to trust the NA that I asked for advise; part of me says stronger is better. Aesthetics come into play here - the mount area is overhead - where one walks under it each time you go to the side boarding gate. I have the feeling that gussets tend to look like headbangers... the admiral would not like that.

Dink requirements:
When considering this originally, I made some estimates:

The current Dink is an Achilies SPD4FL w a 8HP 2 stroke Evenrude. All in with gas, some misc stuff (small ice chest, radio, seat etc) I estimate it to be 200-250 lbs total.

The Achillies is removable floor board dink. I'd like to have a RIB. But in any case the dink will never be longer than about 10ft - that's all the space there is on the FB for the dink.

I looked up several RIBs and 15hp 4 stroke OBs and estimate that the package would be <less than 350lbs.

Thus I arbitrarily picked 500lbs static load as the "future proof" design goal and figured if I can get more margin for not much cost increase, I'd do that. Hence the 1000lb value as a stretch target.

Pipe Cranes: Due to the fixed radial reach nature of the pipe cranes, I'm thinking I have to stick to my idea of a lifting boom. I need to be able to pick up stuff from a couple of locations that are not equidistant from the crane mount location. There are some motivating reasons for this that would take a bit to explain - if anyone is interested, I can try to add that info tonight or maybe tomorrow.

 

[KnotYet]

Cool estimate - A prelim estimate for the cost of materials and some welding labor came out to be between $1-1.5k. I wish I had the welding skills, but I learned years ago that welding is a skill you have to keep up * I don;t do enough to not need lots of practie each time I try it. I've only done some amateur torch & MIG work. I tried some Al experiments with my small mig setup, but it appears I've only mastered "no penetration" or "melted puddle". :lol:

I can imagine a dinghy crane that would work and require almost no welding.
Your ability to draw and CNC machine parts should not be minimized! I wish I
wasn't 400-ish miles away because we could tear into this and have fun doing it.

I use a TIG for AL but the right MIG with a spool gun can work great and is faster.

 

[MVCalypso]

I've always liked the guideline that one should buy good tools, if you buy a cheap throw away version, you'll end owning two of them.. the cheap one and the one you really use.

Now the thought that this is a reason (excuse? justification?) to get a tig set up and learn to use it has popped into my head... but that clobbers my project budget and the time line... but then again...

 

[Capt Ray]

I have been very happy with our design.
If doing it again, we'd make it heavier duty to handle twice as much weight or more. But it works for what we are doing.

These posts are at different stages of our evolution:

https://shellerina.com/2021/04/18/new-dinghy-davit/

https://shellerina.com/2021/05/02/davit-with-dinghy-tested-underway/

https://shellerina.com/2023/07/20/new-highfield-dinghy/

https://shellerina.com/2021/05/15/endurance-marine-capstan-winch-installed-today/

Changes being made this weekend to accommodate a 6ft distance between pull points on the new Highfield RIB. So stay tuned for updates.

 

[BruceK]

Was that deflection of the St Croix Junior crane, or deflection of the deck where the crane was mounted, or some combination thereof?

5 years since we fitted it so testing memory. The deflection surprised me, considering the modest weight, but we were at full extension. I think we found some cracking in the paint under the crane, so we strengthened and extended the plate under the deck. Probably deck deflection but possibly combination.Supporting the crane mast tube back to the mast helped a lot.
My current boat is a refinement of the IG36. I would not have gone the same course again but it was out anyway, the dinghy wouldn`t fit on the aft FB deck. We fitted snap davits to the hull integrated swimstep and carry the dinghy there, I thinks it`s an abomination but it fits, sits well, transom gate still opens, convenient and easy.