Sea Sled madness. It’s in my brain.

I'm pretty tired, but for a jib crane in a boat, don't you need to decide the failure mode/location? You wouldn't want the failure to be through the hull bottom. Nor would you want it to fail out the ship side. So, don't you need to put some sort of something into the design to limit the load?

A friend of mine built a homemade crane with a semi truck wheel bearing somehow. For a moment, picture an axle that spins. He welded stop limits on its rotation to avoid it going past a certain point. The i beam is like 5-8" steel and maybe 12' long and welded to the wheel. The thing is in a solid steel tube like a foot around filled with concrete and bedded
a couple feet into the floor. Obviously more than you need, but food for thought.

Anyhow, all I got.

 

Fallguy, you follow my thinking exactly. I was pretty good at vector mechanics in university, and that was before I spent years desugning and building bridges. I'm following the load paths, seeing where the structure may fail. I can arbitrarily strengthen the post so it doesn't snap off at the deck, but building the deck/side deck/bulkhead 1 intersection strong enough is a real challenge. Most likely failure mode at the moment is the post tearing through bulkhead 1

 

...and BH 1 is not where you want your failure to occur (duh).
But where: the cable, the post, the I-beam?
So, if you "overbuild" the BH-post joint then perhaps a
progressive failure can be designed somewhere less catastrophic.

Another thing to consider is load control.
Swinging loads combined with rolling boats can be ugly.
A few options come to mind:
Control lines attached to the load, but require a person on each rope,
a brake to control crane rotation, both?

 

Further to my random thoughts yesterday, here is a sketch showing a section through the forward cockpit looking forward, based on various dimensions given to me by John. Including outline dimensions for the all terrain vehicle (ATV) - which is a heavy beast, weighing around 400 kg.
I was optimistically thinking that I could try to arrange a luffing davit arm rather than a fixed horizontal cantilevered jib, but I don't think this is possible - there simply would not be room.
I have shown the outlines of the ATV resting on the cockpit sole and when lifted such that the bottom of the wheels are 100 mm / 4" above the deck when it is being swung ashore or recovered.
I think that the whole assembly is going to have to be bigger than what John initially envisaged.
I have not done any calculations - this sketch is just to see how everything fits.
Re the 200 mm deep 'I' beam, I am thinking it could have a little trolley running along the lower flange to which a chain block hoist is attached. The position of the hoist will have to be movable along the length of the beam as the davit is rotated, especially so for cargo that is loaded / discharged over the port side rather than over the bow.
To all of John's loyal followers, please feel free to offer lots of constructive criticism! And any other alternative ideas that you have for getting a 400 kg ATV on and off the boat.

I also had a crazy idea involving a gantry crane...... the loads on this would be easier to resolve in some ways, but the whole structure would be a lot more complicated.

Edit - I started off drawing this on an A4 sheet of paper, and quickly ran out of room, so I glued another sheet on, to make it a bit smaller than A3. No scale is mentioned, as it is now not to scale (as I took a photo of it), so I have put a few dimensions on for reference.

 

This is great Martin! I see now that I'm crazy!

Lol. Possibly, but mostly I'm persistent.

 

Make the gusset a failure point? Oh ****, I bent the gusset is a winner.

why eff with a four wheeler up n over..just gonna break ****....

make an equipment barge...probably easier if you can store it somewhere cheaply

 

The first big job you get; you'll think, darn, gonna need that barge that damned fg talked about all the time..

is damned okay?

 

Excellent Martin, very nice.

I'm wondering if 1500 would be sufficient I-beam length
as there are installation points in all four corners of the boat.
However, dockside reach may be more important than on board reach.

I like the idea of a travelling chain block hoist, my only concern would be control.
Perhaps they have locking mechanisms available, I don't know.

 

Post like this.

 

Nice post!

 

John did mention this to me as a possibility - a 'mini' sea sled type of barge.
Maybe a boat like the TX 18?
https://www.boatbuildercentral.com/StudyPlans/TX18_STUDY.pdf
And it could have a simple RO/RO ramp for loading equipment like ATVs.

There was a thread on here last year by @Darren1234 who wanted to build a flat top barge for carrying an excavator on a lake.
Flat top barge https://www.boatdesign.net/threads/flat-top-barge.63920/

 

Yes, I think that dockside reach would probably be more important - it looks like the bow is about 1 m. / 3' forward of the bulkhead (?), and the width of the ATV is 1.2 m., so that is already about 1.6 m. of reach required just to place the ATV on the edge of the dock. when discharging over the bow.
And it would be safer to land it a bit further away from the edge, so maybe 1.8 m. minimum arm length?

 

I can't quite see if it would hurt the overall structure, but if that top step in the bow were stretched all the way across instead of being in a notch, it would be easier to bring stuff aboard over the bow.

 

I think if we can get our test case up over the bow, continuing the rotation another 90° to set it down beside the boat would be sufficient... no, no it wouldn't. Might very well be bow on to a low cliff, or sea wall. Well, 1.6m was the jib length I was thinking. Although I was wrong about sufficient jib clearance above the gunwale. Martin's drawing is extremely illuminating.

 

And I'm thread hopping about. Let's focus this one in the jib crane thread, in case members like cranes but hate sea sleds.