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Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/23/2008 12:08 PM

hello all, our commercial diving business just purchased a Zodiac M-7 inflatable rib boat that has a 5000 lb load capacity. The boat was used for searcha dn rescue as well as a tow boat service. The rigid hull is constructed of kevlar reinforced fiberglass. on the front is a tow bit that is detachable and slips out of a 3 1/2 or 4" alluminum pipe. The pipe is structurally connected to the hull and looks solid. We are looking to build a crane to slip into that pipe that would support lifting ABOUT 1000 lbs max. any ideas on how thik of material (steel) we would need, max length on crane arm, and how tall it could be to support 1000 lbs? Oh, and we will be using a 1000 lb winch attached to the base of the crane.

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#1

Re: Trying to put a lift crane on a boat, ability of approximately 1000 lbs.

10/23/2008 1:16 PM

Is this what your boat looks like? It isn't very heavy. It may be able to carry 5000 pounds centered in the boat but 1000 pounds eccentrically applied will likely swamp it.

The length of crane arm and height of mast will be limited by the eccentricity required to upset the boat. I assume you want the arm to move through a complete 360ocircle. The mast will not remain vertical when the load is raised. It will tip towards the load and affect the eccentricity.

The outboard motor tends to balance the load applied at the front, but when the crane arm extends to port or starboard, the boat will tip sideways.

To do a proper analysis, the weight of all parts and dimensions of the boat would be required. Don't build the crane until you have the design checked out by someone who knows what he is doing.

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#2
In reply to #1

Re: Trying to put a lift crane on a boat, ability of approximately 1000 lbs.

10/23/2008 1:56 PM

Absolutely correct. Even if a you could hoist an item out of the water you would never get it into the boat.

For safety, you should be thinking of a fixed overhead rail with a winch. It should be attached to 4 strong-points, The winch travel must be positive controlled

This way the load is always centered over the keel

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#3

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/23/2008 10:42 PM

Yes all of this makes sense, and yes that is the exact boat, but isnt that the 733?? the guy in our business that purchased the M-7 actually has talked to you a few times about that boat. He ended up purchasing one in Millwaukee. But anyways, back to the crane, i did think about it tipping, but again this would be a crane made for 1000 lbs MAX, it would never lift anything of that weight. We are commercial divers, and it would be nice to just clip on our gear and jump over the floats to get back into the boat. Then just lift the gear out with the crane, which may weigh up to just over 70-80lbs. In this case, it would be no problem for tipping. As for using it lifting stuff overboard to us, we dont know what the future holds, but we have never worked with something over a couple hundred pounds, so again a few hundred pounds at the maximum. Haha, so i guess i should have worded my original post better.

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#4

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/23/2008 10:51 PM

You need a barge with the lift mounted in the center of the barge and a opening on the deck for the crane to drop and lift from. A small barge should do but you need steel to support the weight of the crane and the wight of the life. The size of what your lifing might cause a problem with the working hole of the deck.

Have you considered AIR BAGS ? To get things to the surface and then pull onto shore or ramps.

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#5

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/24/2008 3:41 AM

If you are commercial divers then you are not likely to make fundamental mistakes with simple engineering/rigging and wetted area of the hull so I'll just point you to this site for a suitable crane. I think this will be perfect for you for lifting tanks in/out the RIB. Suggest you mount it in the bow not off the side - looks like you have a suitable mounting point for the base socket there anyway. The small model is what you need and it can be easily removed from its socket and kept undercover when not in use - runs off 12V. You will find good used ones on e-bay also. www.pennyhydraulics.com

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#6

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/24/2008 11:41 AM

I'd think one of these cranes might work well. (See: http://www.davit.com/remcranes.html)

The 40" reach version would probably extend far enough overboard for small loads and would not be too difficult to control as the boat rocks. If your existing post is really secure, perhaps the crane's post could be slid right in, with suitable bushings from nylon, etc to take up any slop (you'd want these to be permanently attached to the crane tube, no doubt.)

You may need to provide 2 support rods that would meet the crane post at about the location where the rail bracket would otherwise be. These would terminate at hard points on the hull, port and starboard, about 4' back from the post, and would operate in compression and tension. Three cables could be used, but it might be hard to find a secure enough point far enough forward of the crane post.

Best not to make the crane too high. With a load swinging from a long lever arm, and with a few crew positioned in the wrong places, a capsize is a real possibility. maybe after putting the thing together, you could put a test load on the hook, and do some experimentation to get a feel for working stability: even if the post ends 4' above the gunnel, there could be some sea state in which you would not want to use the crane. Better to experiment and know that in advance.

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#10
In reply to #6

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/25/2008 12:10 PM

Another good job Ken!

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#7

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/24/2008 1:04 PM

dadw5boys- yes we do use bags all of the time, but again, the purpose of this crane is for EASE of getting things out of the water, you dont always have a ramp or any place at all to remove something from the water, with the crane we could lift it with bags to the boat, then crane it into the boat to remove.. A barge, yes spectacular idea and they work well, we work with them often, but we dont want to tow a barge around with us.. this is going to be made for EASE of getting things into the boat and be as versatile as possible. This boat is being trailered to the jobsite every time we do something, as for job efficiency. thank you very much for the crane websites, i will surely look at them, they seem like good ideas that WILL work, as for all who doubt this idea, we'll see..

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#9
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Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/24/2008 3:22 PM

I'm not sure that anyone doubts the idea will work, but you have to be careful to prevent overturning of the boat.

Let's say the weight of the boat is "W" and the load on the crane is "P". Say that the dimension from center to center of pontoons is "b" and the crane extension is "a". The crane mast is centered between pontoons. Neglecting the height and tilt of the mast,

Overturning Moment = P*(a - b/2)

Stabilizing Moment = W*b/2

If the overturning moment exceeds the balancing moment, the boat will overturn. Otherwise, it will not. The situation may be improved by adding weight to the middle of the boat, i.e. increasing W. It can be improved even further by adding weight to the pontoon remote from the load (the crew could sit on it).

The crane suggested by Blink is rated to carry 200#. If that is adequate for your purposes, it would seem to be a good solution.

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#8

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/24/2008 1:19 PM

Please clarify what you want.

You state a 1000# crane and 1000# load, but later you state you won't lift 1000#. Which is it?

I would suggest derating whatever is installed for safety purposes. You could use a 500 or 1000# winch and crane, but clearly and permanently mark it to state a MAXIMUM load of 100 or 200#. (if that is the true intent of the design)

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#11
In reply to #8

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/25/2008 12:28 PM

Thast why I suggested a barge as a working platform. From there 1000# of weight and 1000# of load would be possible.

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#12

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/27/2008 7:38 PM

You're gonna need an outrigger for stability or load your gear in the outrigger

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#13

Re: Trying to Put a Lift Crane on a Boat, Ability of Approximately 1000 lbs.

10/27/2008 8:04 PM

Hey all, no need for an outrigger, i think we need to take a step back and see what exactly this boat is, its a Zodiac M7 , NOT a 733, they are two totally DIFFERENT hulls, the M is for the military issue that does have a 5000 lb load capacity. Gear: each persons gear weighs maybe in all a little over 100#s. 2 sets of gear. We will have a welder on board also. A barge, yes it would be awesome, but again, very inefficient, bulky, and most of the time non transportable. We did purchase a crane made of alluminum that has over a 500lb capacity. obviously if we will be lifting something this heavy it will be off the bow, not the side, common sense. Thank you all for the replies and ideas it helped us in our project.

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