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-   -   Dinghy Swinging - Heavy Load (http://www.cruisersforum.com/forums/f117/dinghy-swinging-heavy-load-8701.html)

FY44AC 11-06-2007 11:10

Dinghy Swinging - Heavy Load
 
1 Attachment(s)
Please help! I recently took delivery of a Hunter 44AC with custom made dinghy davits. I have a couple of issues with the davit system and I am looking for your help. The first this is that the dinghy when in the hoisted position it swings all over the place. Is there a better way to secure it? The second issue is when I am hoisting the dinghy up… It weighs about 200 lbs and I find it very difficult to hoist. I do not want to remove the motor (the davits are rated at 600 lbs) and I am looking for a better way to hoist. The picture attached is the way they installed it. It swings all over and the only thing holding it up is the cam locks. I would appreciate your suggestions and pictures if you have a similar davit system.

henryv 11-06-2007 12:38

controlling the dingy
 
You will need extra lines to control the dingy. You can use the painter snugged to a low point on a rail at one end. A couple lines crossed diagonally from the lower blocks tightened to low points on the davits will help a lot. Each boat & davit combianation is different so you will need to experiment to see what works best for you.

Lifting can be made easier by going to a set of blocks with an extra sheave so you get more mechanical advantage. Having larger diameter sheaves will also help. You should only have to do this at the end with the engine.

Pblais 11-06-2007 12:43

Davits usually are rated to 300 lbs but you don't want any more on them than required. I never carry the engine on the davits. It makes it too easy to steal and promotes more sway. I had another boat with a hard dighy and I made a set of X straps from the tip of the davits back to the boat and dropped the dighy into the set of straps to support the bottom. It didn't sway very much then. Mostly it was because the dighy never was made to be hung so I never trusted the lifting eyes, but it also stopped most of the sawy.

A stiff breeze pinning you against a bulkhead can be a trick to get away from without crunching the davits. You always have to be watching out for them. Also note in a following sea the dighy will be snapped off clean with one wave. Anything that can come down on it will weigh far more than the davits can hold. Maybe the davits will rip out clean or maybe they will take out a chunk of transom too. Also be sure you allow for drainage and do not forget to plug the hole later on before deployment. As a rule covers are nice but they all leak. The cover is more for UV protection than water penetration.

The current boat uses cam cleats for the lines and I just secure the bitter end with a few turns to make sure it can't jump the cam through a freak accident. For a passage I would stow it deflated below or lashed to the deck. You'll want a real liferaft not a dinghy.

Terra Nova 11-06-2007 13:43

Quote:

Originally Posted by FY44AC
Please help!
The first this is that the dinghy when in the hoisted position it swings all over the place. Is there a better way to secure it? The second issue is when I am hoisting the dinghy up… It weighs about 200 lbs and I find it very difficult to hoist.

I do not want to remove the motor

Yo FY,

take...the...motor...off.

best, andy

Vasco 11-06-2007 14:08

Get 4 nylon ratchet straps, Walmart has them. Two to pull the dink in against the stern and two for lateral movement. I regularly carry my dink with the engine on - 10' 06"AB RIB with a 15 hp yamaha 2 stroke on it. The secret is to have absolutely no movement whatsoever.

Pblais 11-06-2007 19:19

Quote:

Get 4 nylon ratchet straps, Walmart has them.
I find the Walmart straps die from UV in about 2 years. Tubular nylon webbing lasts a lot longer. I found some 316 SS rachets that seemed to work nice. You do need something easy to operate otherwise you can be looking at 1/2 hour to secure up the dinghy and that gets to be a problem. It needs to be quick and solid to work well so you do it right every time. A sloppy job could cost a lot.

GordMay 12-06-2007 02:24

1 Attachment(s)
Working loads for these nylon ratchet straps are:
700 lbs. for the 1” wide (plenty, if used for restraint & not hoisting)
900 lbs. for the 1-1/2”
1,500 lbs. for the 2”
Goto: Ratchet Buckle w/Strap, 304 Stainless Steel, Precision Formed & Machined

Pblais 12-06-2007 04:04

Gord,

I have the exact same buckles from Bosun and they hold up quite well and are well made. Not a spec of rust on any part after 4 years of always hanging off the boat even when the dinghy is not. I found a 3/4 inch tubular webbing like used for rock climbing lasts a very long time in the UV. The color fades long before the material will show wear. 6 or so years might be expected.

Looping one end through a D ring around the davit and just burning the other end was enough. Quality snap hooks can be a bit spendy. I attached the close end of the ratchet to the boat and eliminated the fixed strap. It was easier to reach.

Nylon isn't that UV resistant but makes a strong strap with very low stretch. Ideally the straps should break before the davits give out in any type of disaster. You might then save the dinghy with trashing the stern of the boat.

Sunspot Baby 12-06-2007 10:13

Island to island, short hops, it is nice to leave the motor on. Makes it easy to launch and take the dog for shore patrol. Big weather or long passage, off comes the motor.

It used to take us 30 minutes to remove the motor, fuel, etc. Stow them, and then hoist the dinghy onto the deck. Same amount of time going back in the water. That's why we got davits. Ours are custom made and much beefier than the normal ready made. Don't know what the rating would be, but welder that fabricated them works for major big yacht builder and used large diameter stock and good bracing.

We are still experimenting with reducing swing. Getting close to a good solution. Some pictures of our davits in my photo gallery.

George

Pblais 12-06-2007 13:04

Quote:

Ours are custom made and much beefier than the normal ready made. Don't know what the rating would be, but welder that fabricated them works for major big yacht builder and used large diameter stock and good bracing.
Might have been better if it were lighter. A wave large enough to fall down on top of it will exert tons of force and probably not break the davits but rip a hole where they used to be attached. Same thing if you hit something. The mass of the boat exceeds the strength of the material. If you bent one it would be cheaper.

The ones on our CSY are St Croix davits and were rated to 300 lbs a pretty standard rating. The Gozzard are custom and braced stronger. Maybe something in between is called for, but you really want them to break or bend before they rip loose from the boat backing plates and all. Perhaps the dinghy would break up first but it won't be a fun time finding out.

GordMay 13-06-2007 02:50

Ocean Marine Dinghy Davit Installation:
”... The ideal position for the dinghy at sea on davits is to have the dinghy as high as possible, sitting level and with the tube of the dinghy tied firmly into the "armpit" of the davit. There are several very effective ways in which to minimize dinghy motion. All of them should be employed at once. First and foremost, the lifting points in the dinghy should be low - as close to the floor as possible. If the slings are not down low then you cannot lift the dinghy up high. Secondly, it is important to adjust the davits so that the outer ends are positioned closer together than the distance between the bow and stern lifts in the dinghy ...”
Goto:
http://www.oceanmarinesystems.com/INSTRUCT.pdf

Sunspot Baby 13-06-2007 05:18

When we roughed out the design, I really considered the transfer of load to the hull. Area where mounted must have been intended for davits because it is about 1/2" thick. Then I put aluminum backing plates behind to spread load/stress. Without doing finite element analysis, but with many years of experience in industry, I believe failure mode will be for arms to bend aft of the support. That will be a real OH S**T if it happens, but should be survivable.

Weather predictions are not perfect so I can't say I will never have the dinghy in the davits during heavy weather, but for a passage of more than a couple of days, we would deflate and stow.

I looked at the St. Croix but because I have a swim platform right where the dinghy should hang, there was no good way to mount them. Being cantilevered out to clear the platform was better. My mounting system spread the load far better than the St. Croix mounts.

Vasco 13-06-2007 06:04

Quote:

Originally Posted by GordMay
Ocean Marine Dinghy Davit Installation:
”... The ideal position for the dinghy at sea on davits is to have the dinghy as high as possible, sitting level and with the tube of the dinghy tied firmly into the "armpit" of the davit. There are several very effective ways in which to minimize dinghy motion.
Goto:
http://www.oceanmarinesystems.com/INSTRUCT.pdf

I got the davits for La Belle Aurore from Ocean Marine and was very satisfied with the much needed support as I installed them myself. If anyone's thinking of getting davits I would recommend a visit to their site. It's very informative.

FreeSail 13-06-2007 06:28

Isn't the main stress point for a davit on the stern rail, where the top of the davit mounts to ? I just received the davit I ordered from Garhauer Marine. They added an extra bar to the top so I can mount solar panels. I have not installed them yet. BTW, no directions as to installing the davits in any of the boxes have been found.

Sunspot Baby 13-06-2007 07:16

Dinghy mounts vary by brand and boat. See ours on this forum at http://www.cruisersforum.com/gallery...&imageuser=223

George


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