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Parmenter · 01-07-2014, 23:16

On the back of the cat I am building I could have davits or I could have a walkway between the hulls for cleaning fish and storing a dinghy but for this I will need a dinghy crane to lift it up and onto.

What I was thinking of was a deep fiberglass tube say 4 inch dia x 3 ft long glassed into the back step with a smaller size tube inside on some sort of plastic bearing material and where the inner fiberglass tube exits the deck put 1.2 inch ply cheeks/sides (red) with internal gussets and tops and bottoms making a box beam and glassing the whole thing together with epoxy and double bias cloth.
Bolt a boat trailer winch on one end and fix a sheave at the other and hey presto, a cheap composite dinghy crane is born.

Can anyone see any reason why this shouldn't work?

sparrowhawk1 · 02-07-2014, 04:45

It needs to be much taller. And I don't think I would have it extend more than half the beam of the dinghy

Parmenter · 02-07-2014, 05:26

Quote:

Originally Posted by sparrowhawk1

It needs to be much taller. And I don't think I would have it extend more than half the beam of the dinghy

Its not drawn to any scale and it needs to be at least half the LENGTH of the dinghy

sparrowhawk1 · 02-07-2014, 05:37

Why? Just get it on deck and rotate. The longer the arm the more stress in the system. And depending on the weight of the dinghy you'll probably want a block tackle long with the winch

Parmenter · 02-07-2014, 05:40

Quote:

Originally Posted by sparrowhawk1

Why? Just get it on deck and rotate. The longer the arm the more stress in the system

Its a catamaran, there is a large back beam in the way stopping it from coming on fore and aft.
It gets lifted fore and aft and spun mid air to face athwart ships therefore need to be hanging from the middle of the dinghy length.

Quote:

The longer the arm the more stress in the system. And depending on the weight of the dinghy you'll probably want a block tackle long with a winch

Block and tackle?
Most trailer winches are already 3:1 or 5:1, even 10:1

It'll be foam composite so lighter than an inflatable and lighter than production glass

valhalla360 · 02-07-2014, 05:58

I would expect the trailer winch to rust out in a short time if you are in a salt water environment. It's likely a good deal worse than on a trailer parked a couple miles from salt water. At $40-50, it might be worth a try.

Agree with keeping the arm as short as possible. It only needs to extend 1/2 the beam of the dingy past the edge of the main boat hull. If you can bring the dingy to the stern, it would avoid the need to reach half the beam of the main boat plus some.

Otherwise, as long as you figure out the stresses, it should work fine. Might want to look at some of the trawlers with cranes to see how they go about it rather than starting the design from scratch.

Parmenter · 02-07-2014, 06:19

Quote:

Originally Posted by valhalla360

I would expect the trailer winch to rust out in a short time if you are in a salt water environment. It's likely a good deal worse than on a trailer parked a couple miles from salt water. At $40-50, it might be worth a try.

I guess I'll just have to use one of those S/S ones then

https://www.whitworths.com.au/main_i...138&item=93783

Quote:

Agree with keeping the arm as short as possible. It only needs to extend 1/2 the beam of the dingy

And how will that work looking at the pic below
There is a 11 ft gap between hulls, crane needs at least 5 1/2 ft of reach to place dinghy

Quote:

Might want to look at some of the trawlers with cranes to see how they go about it rather than starting the design from scratch.

Or not.
Trawlers are different to catamarans, they do only need half a dinghy width of crane, usually lift considerably heavier dinghys and mostly run expensive hydraulics setups.

valhalla360 · 02-07-2014, 06:38

Quote:

Originally Posted by Parmenter

I guess I'll just have to use one of those S/S ones then

https://www.whitworths.com.au/main_i...138&item=93783

And how will that work looking at the pic below
There is a 11 ft gap between hulls, crane needs at least 5 1/2 ft of reach to place dinghy

Or not.
Trawlers are different to catamarans, they do only need half a dinghy width of crane, usually lift considerably heavier dinghys and mostly run expensive hydraulics setups.

Just verify that it's actually decent quality stain "less" steel. At $99, it's likely the low grade stuff that won't hold up much better than the $50 dollar ones.

You only need to get the center of gravity over the back of the hull and then let out line to shift over. You can make it work with a longer arm just make sure to beef up the supports by a significantl degree.

I think you are confusing the trawlers with the big sports fishers that have the dingy on the bow. A lot of trawlers use a simple manual crane on the upper deck. But if you want to reinvent the wheel have at it.

Parmenter · 02-07-2014, 07:06

Quote:

Originally Posted by valhalla360

Just verify that it's actually decent quality stain "less" steel. At $99, it's likely the low grade stuff that won't hold up much better than the $50 dollar ones.

So I go upmarket and get the $250 one
https://www.whitworths.com.au/main_i...tAbsolutePage=

I am yet to see any 304 rust out overnight

Quote:

You can make it work with a longer arm just make sure to beef up the supports by a significantl degree.

Of course

Quote:

A lot of trawlers use a simple manual crane on the upper deck.

Usually steel from what I have seen , weren't you just worried about rust?
And the others usualy have some hideous large base bolted to the deck like an afterthought

Quote:

But if you want to reinvent the wheel have at it.

Nothing to do with reinventing the wheel - just because a bunch of people have something agricultural, heavy and unsightly does not mean its right nor does it mean that another cant make a better version from better materials.

Roy M · 02-07-2014, 07:16

This is very cool. I have been wrestling with this issue for the last week. Here is my solution: use what you already have, i.e., your main halyard and your massive sheet winches. I have a trimaran and a 10'6" RIB with a 15 hp outboard, a total of about 350 pounds with fuel tank (horizontal liquid propane, composite, 7 gallons). I am building a roller on my port quarter deck to support the weight of the RIB when I roll it off aft, over my wing deck. Retrieving it was the bigger issue, mainly with the dink getting cocked around as I lifted it around various obstructions (the port ama extends further aft than the wing deck). The roller I am building extends perpendicularly across the beam from the ama stern to just under the wing deck of my sterncastle (center cockpit, therefore, I have an aft cabin). I will bring the dinghy up to the stern, stand on the bow locker, then sit on the deck of the mothership. I stand up, and attach the dink's painter to a line on the deck which leads to a snatchblock, attached to a folding pad eye in the deck (and bolted into the forward crossarm), which then leads aft to my cockpit-located genoa sheet winch (Harken 50). I pull the bow of the dink up to rest on the roller and take a turn on the winch to hold the bow in place. I then attach the main halyard to a bridle which attaches to two eyes on the transom of the RIB. I then raise the transom of the dink with the halyard, which then pulls the bow of the dink forward using gravity. I will probably have to jiggle the stern of the dink because the angle of pull to the masthead causes the boat to twist a bit. I will also be adding some additional rollers as I tweak it. Pictures will follow as I actually start building stuff. The roller beam is of structural foam with unidirectional 20 oz. carbon fiber, 2 layers, on the top and bottom, followed by a couple layers of 10 oz. fiberglass cloth, and the necessary forming of the roller pads and supports. The beam will be epoxied and attached into the hulls. I did this before when I had a pure inflatable hull and a 10 hp outboard. It was so cool that I wanted to emulate the experience with a rigid hull and larger outboard (to carry dive gear and 2 big divers on a plane).

Panope · 02-07-2014, 07:36

I made something similar to what you are considering. The purpose of my unit is to act as a davit that can rotate 180 degrees to shorten L.O.A. (cheaper dock fees). Also good for Radar mount, VHF antenna mount, Deck light mount, All around light mount.

Primary pole is 2.5" schedule 80 aluminum pipe.

3 to 1` purchase is adequate for lifting this 72 pound dinghy by hand.

Here it is in the rotated position.

Both the upper and lower sockets have a UHMW insert to decrease friction and eliminate galling of aluminum structure.

Close up of the custom (for my dingy gunwale) channel that keeps the dingy secure in the horizontal plane.

Roy M · 02-07-2014, 08:26

I just loaded some pics to show the starting point. Here are some shots of my original sketch, the actual wing deck and a couple shots of the actual dink sitting on deck. I also included one to show the necessity for the perpendicular roller bar that I will start building this morning. Note that the lifting sling is made of 1/4" Amsteel, with individual "Whoopie slings" to allow quick and precise length adjustments. Details for this trick are on YouTube.