Calculating load capacity

[HeadedToTexas]

The previous owner of our 2007 Mariner 37 sailed from South Carolina to the Bahamas, then up the AICW to New York, across the Erie Canal, through the Great Lakes to Grand Haven, MI before the Covid hit. We bought the boat in Grand Haven.

Through those few thousand miles, the (now our) Highfield CL310 and manual start, tiller steer Yamaha 15 hung on St. Croix davits secured without backing plates to grab handles on the transom and swim platform. I have to think that they encountered at least some rough water.

I left Grand Haven September 29, 2020 for Sandusky, OH. It took four tough days to get out of Lake Michigan. While it was easy sailing after that, the early October conditions on Lake Michigan were memorable. Day 1 was cut short by 4-6 on the nose, day 2 was off shore, and days 3 and 4 were tailwinds. A buoy north of Grays Reef recorded 13 foot waves for the time we sailed past it.

It was somewhere in Lake Michigan that the stern of our dinghy bent a gate carabiner leaving it hanging on by the Dyneema lines. I did not discover it until tied up at a marina in Cheboygan. We shored up the stern and continued on in far calmer seas for the rest of the trip.

Among my 2021 projects was replacing the St. Croix davits with a Dinghy Butler system. Haven't gotten around to assembling that story, but it turned out great and is everything I expected. The dinghy now hangs at bridge deck height leaving the swim platform completely usable without interfering with the view from the cockpit nor fly bridge. The lift mechanism is swift, secure and easy.

Here's my question. The hinge plates are backed by 6x10 inch plates of ½" FRP bonded to the inside of the transom. The winch deadman and tie down eyes are all mounted with backing plates bolted through solid columns through the foam core deck. The whole thing is solid. The boat hangs from chain. The winch "cable" is Dyneema. That said, how would I go about calculating the load capacity of all that?

 

[O C Diver]

Don't know how to calculate the load capacity. Would expect Dinghy Butler to give you some parameters. Regardless, momentum will likely be the cause of that type of systems failure.

I'm not being critical of that system, but...

In a beam sea, you have elevated the weight and mass much further up from the boats pivot axis. Visualize the boat in a beam sea, pivoting in about the middle (port to starboard) of the swim platform. It is absolutely critical to prevent port to starboard movement of the dinghy, especially with the outboard attached.

Now Visualize a thousand pound dinghy, shifting a couple of feet port to starboard. The potential for shock loading is significant.

Ted

 

[HeadedToTexas]

I know what you mean, Ted. The dinghy is just up in the lift for that picture, but usually (and always while underway) it is strapped to the deck and stabilized against the keepers. It doesn’t move at all even in significant beam seas.

 

[Comodave]

The sideways movement would be my concern too. Do you strap it to the flybridge deck when underway?

 

[Caballero II]

Texas Bound,
I just happened to notice and believe in the picture of your 3-part pulley system that you may have threaded the line incorrectly for 3 part blocks. I have the same 3-part pulley system for my davit system, and was shown by a professional rigger how to rig the line. It is recommended to do it that way to eliminate drag and kinks. You might google it and see if you've got it right.

 

[mvweebles]

Most shackles have the load ratings stamped on them. That looks like a Suncor (angled/bent bail). 3/8" had SWL (safe working load) of around 1000 lbs. 1/4" is around 400 lbs (both guesses). Breaking is 5x so to deform like that, you had considerable load.

You're lucky the line was caught in the hook of the bail. Would not have been pretty had the dinghy gotten wild.

Peter

 

[HeadedToTexas]

The sideways movement would be my concern too. Do you strap it to the flybridge deck when underway?

There are two eye plates mounted to the deck of the flybridge through solid thickened resin with stout backing plates. The dinghy is strapped to them when not in use. It does not move in that state. Many here contributed to my learning about mounting hardware on cored decks in a 2021 thread. Those eye plates were among the subjects.

 

[HeadedToTexas]

Texas Bound,
I just happened to notice and believe in the picture of your 3-part pulley system that you may have threaded the line incorrectly for 3 part blocks. I have the same 3-part pulley system for my davit system, and was shown by a professional rigger how to rig the line. It is recommended to do it that way to eliminate drag and kinks. You might google it and see if you've got it right.

The friction drag from that pulley system did not help. St. Croix makes a decent davit, but it just was not for us. The main problem was that they eliminate the bulk of the swim platform and completely eliminate access to the port side of the platform when the dinghy is up. That is our primary means of accessing the boat from our side tie slip.

Lifting the boat manually got old quickly, though incorrect threading of the line through the blocks would have amplified that problem.

 

[HeadedToTexas]

Most shackles have the load ratings stamped on them. That looks like a Suncor (angled/bent bail). 3/8" had SWL (safe working load) of around 1000 lbs. 1/4" is around 400 lbs (both guesses). Breaking is 5x so to deform like that, you had considerable load.

You're lucky the line was caught in the hook of the bail. Would not have been pretty had the dinghy gotten wild.

Peter

Whew, that is for sure. I was very lucky. And I believe the previous owner sailed that boat with that dinghy hanging from those carabiners for several thousand miles. I replaced them with pelican hooks before replacing the whole davit system with the Dinghy Butler.

 

[Steve1.0]

Assuming the dingy is secure when stowed, it appears to me the weakest part of the lift will be the single pins in the hinges. The shear strength of them might be an ultimate breaking load.(and a point of regular inspection) Don't know how they figure a safe working load

 

[HeadedToTexas]

I will have to get the specs on them. If I recall correctly, they are 316 M8 bolts. They form the axle about which the milled aluminum arm ends rotate in the milled aluminum hinge plates.

 

[O C Diver]

Here's my question. The hinge plates are backed by 6x10 inch plates of ½" FRP bonded to the inside of the transom. The winch deadman and tie down eyes are all mounted with backing plates bolted through solid columns through the foam core deck. The whole thing is solid. The boat hangs from chain. The winch "cable" is Dyneema. That said, how would I go about calculating the load capacity of all that?

So I went to the Dinghy Butler website. The aluminum version is rated for 350 pounds and the stainless steel version is rated at 700 pounds. Now I'm assuming that the rating is for the dinghy and outboard where most of the load (because of the outboard) is on one arm.

Further, there is no reinforcement (plate welded to the arms to increase strength and reduce flexing) commonly found on pipe style dinghy cranes. While items mounted in the stern see less shock loading than in the bow, there is still the potential which could bend and kink the arms.

How much above 350 pounds do you want to go?

Ted

 

[HeadedToTexas]

My calculation indicates that our Highfield 310CL and Yamaha 15 hp outboard plus the tank, fuel, and small amount of stuff we carry in it adds up to 284 pounds. I want to add Highfield's FCT helm option and that would increase the weight to 364 pounds.

I will keep the pull start and manual tilt. Removing the tiller arm will offset the addition of the Yamaha steering linkage and the rest of the component weights are included in the FCT package.

 

[O C Diver]

I'm not a naval architect / engineer, nor have I played one on TV or TF.

If I were doing the calculations, I would start by determining the weight load on the Dinghy Butler arm next to the outboard. Then I would determine whether the helm weight would increase the arm with the outboard or the forward arm. I would also see if the fuel tank could be relocated further forward, atleast while out of the water. In a perfect world, I would get a weight scale that could be attached between the arm and the dinghy to see how different the weights are, and see how much I could improve the balance.

Ted

 

[HeadedToTexas]

The small 3 gallon fuel tank is in the dinghy bow locker. The FCT option mounts forward of the transom, but well back on the 310 model. The holes are already drilled and tapped in every 310 and while moving the seat and helm forward might help distribute the weight between the davit arms, it would also negatively impact the boat's attitude on plane.

 

[O C Diver]

The small 3 gallon fuel tank is in the dinghy bow locker. The FCT option mounts forward of the transom, but well back on the 310 model. The holes are already drilled and tapped in every 310 and while moving the seat and helm forward might help distribute the weight between the davit arms, it would also negatively impact the boat's

I think you're at the point where you need to determine how the weight distributes between the 2 arms.

From a mental exercise point, there will be a difference in weight between the two arms. The manufacturer has to recognize there will be a weight disparity and rate the system accordingly. Without knowing the weight disparity, it's hard to guess how close to the limit you already are.

As a side note, stuff expands to fill available space. I'm looking at the storage box under the seat. Also, assuming the dinghy has a double bottom, water has a lot of weight, so both the space between the floors and the dinghy itself need to be kept drained.

Ted

 

[HeadedToTexas]

The yard removed the dinghy when the boat went into storage. That might be a good opportunity to get actual weights on each end.

 

[Jeff F]

Most shackles have the load ratings stamped on them. That looks like a Suncor (angled/bent bail). 3/8" had SWL (safe working load) of around 1000 lbs. 1/4" is around 400 lbs (both guesses). Breaking is 5x so to deform like that, you had considerable load.

You're lucky the line was caught in the hook of the bail. Would not have been pretty had the dinghy gotten wild.

Peter

I like davits. But in heavy conditions they scare me. Shock loading can be huge. I've seen my dinghy become weightless in heavy head seas. And as you say, the consequences of a failure can be ugly.

My Highfield 340 tows just fine. It's well under the rated capacity of my davits, but if I encounter heavy weather I put the dinghy on a towline, and worry far less.

 

[rslifkin]

I like davits. But in heavy conditions they scare me. Shock loading can be huge. I've seen my dinghy become weightless in heavy head seas. And as you say, the consequences of a failure can be ugly.

My Highfield 340 tows just fine. It's well under the rated capacity of my davits, but if I encounter heavy weather I put the dinghy on a towline, and worry far less.

That's the exact reason why my dinghy is strapped up, down, and sideways (and snugged tightly against the davits for fore/aft movement) when lifted. Without something holding it down, I wouldn't trust it not to bounce up and cause a massive shock load.

 

[Shrew]

What is the rating on the carabiner? That does not look like it has a high load capacity for a couple of reasons.

1) wire gate. This is meant to prevent the carabiner from slipping. A true 'gate' acts to complete the loop and strengthen the carabiner.

2) the eye is bent, not cast or welded.

That looks like something to be used to strap down a load, not to be lifting with. I absolutely wouldn't climb with that style carabiner.

 

[rslifkin]

What is the rating on the carabiner? That does not look like it has a high load capacity for a couple of reasons.

1) wire gate. This is meant to prevent the carabiner from slipping. A true 'gate' acts to complete the loop and strengthen the carabiner.

2) the eye is bent, not cast or welded.

That looks like something to be used to strap down a load, not to be lifting with. I absolutely wouldn't climb with that style carabiner.

Interestingly, there are wire gate carabiners on my Garhauer davits. They're stamped with a working load rating though (990 lbs IIRC).

 

[mvweebles]

What is the rating on the carabiner? That does not look like it has a high load capacity for a couple of reasons.

1) wire gate. This is meant to prevent the carabiner from slipping. A true 'gate' acts to complete the loop and strengthen the carabiner.

2) the eye is bent, not cast or welded.

That looks like something to be used to strap down a load, not to be lifting with. I absolutely wouldn't climb with that style carabiner.

Funny you should mention this. I happen to be rigging lines for my flopper stoppers and have one on my coffee table as I write. Pic attached.

Because it has a bent gate, I believe the OPs snap clip is a Suncor. Working load is 1200 lbs - more than enough for the setup on the OPs pic. Breaking load is either 4x or 5x so in the 5000 lb range. Assumes it's a 3/8" shackle. Also assumes it's a Suncor. BTW - the bent gate allows more throat clearance - nice design feature.

I am really surprised the OPs clip deformed the way it did. Only explanation I can think of is that the dink was not secured from swinging and developed point loads from mementum. He dodged a bullet.

Peter

 

[HeadedToTexas]

I do not recall what the rating was on the wire gate biners originally on our St. Croix davits, but they clearly were inadequate for the heavy end of our dinghy.

I replaced the with pelican hooks (don't recall the rating on those either), but those never saw the forces like the originals saw in Lake Michigan.