Dinghy Davit Reinforcement (Stern Rail)

[rslifkin]

I finally got the dinghy hoisted in my new davits over the weekend. But I definitely have a little more flex in the stern rail assembly than I'd like, even with added bracing to the swim platform (at an unfortunately shallow angle).

I've been thinking of a few ideas. Either tie the supports down to the platform to the deck to tension them inward slightly. Or removable stays to the back of the trunk cabin (or the deck) that get removed when docked for easier passage on the decks.

I've included pictures of the davits both with and without the dinghy so you can see how the setup is laid out. Unfortunately the rail sits too far in from the transom, so I had to put the davit bases on the deck for everything to fit together nicely rather than putting them on the transom.

I figure some of you guys have dealt with a similar concerns, so what are everyone's thoughts on how to best add some bracing to prevent the rail from bending outward?

 

[angus99]

I think your best option is the second one. Tying to the deck would be an extreme angle for the support guys and probably not provide the stabilization you’re after. Guying to the trunk, as close to level as possible, would give maximum resistance to forces pulling against the rail. Just a possible trip hazard when docking or when nature calls at night.

When we installed our Dinghy Butler, the manufacturer was adamant about keeping the angle of the hoist cable below 30 degrees. But the attachment point on our boat is a king post on the transom, not the rail.

 

[rslifkin]

I think your best option is the second one. Tying to the deck would be an extreme angle for the support guys and probably not provide the stabilization you’re after. Guying to the trunk, as close to level as possible, would give maximum resistance to forces pulling against the rail. Just a possible trip hazard when docking or when nature calls at night.

When we installed our Dinghy Butler, the manufacturer was adamant about keeping the angle of the hoist cable below 30 degrees. But the attachment point on our boat is a king post on the transom, not the rail.

That's what I was thinking. If I did a support to the deck, it would have to be from the swim platform strut so it could be almost straight back to the deck. If I sit on the deck and pull hard inward on those it does pull the rail in, but I don't know how much pull they'll take before bending.

 

[mvweebles]

A friend used something similar to bimini support brace hardware such as THIS from Sailrite, though his was smaller diameter and the end pieces were welded to the tubing. Took 15-seconds to release the support for easy passage.

Peter

 

[rslifkin]

A friend used something similar to bimini support brace hardware such as THIS from Sailrite, though his was smaller diameter and the end pieces were welded to the tubing. Took 15-seconds to release the support for easy passage.

Peter

Something like that would definitely work. I'll have to do a little measuring to see where I can place them. If they can be opened like a gate, that'll make releasing them as we come into the dock easy enough. The biggest pain will be that the port side one likely has to be outboard of the step down from the upper deck, so that one (fortunately at the lighter end of the dinghy) will have to be released before approaching a dock (we almost always dock port side to).

 

[mvweebles]

Something like that would definitely work. I'll have to do a little measuring to see where I can place them. If they can be opened like a gate, that'll make releasing them as we come into the dock easy enough. The biggest pain will be that the port side one likely has to be outboard of the step down from the upper deck, so that one (fortunately at the lighter end of the dinghy) will have to be released before approaching a dock (we almost always dock port side to).

My friend leaves the supports unattached except when underway. Maybe setting/releasing the supports can be part of your docking routine along with lowering fenders and setting lines etc.

Good luck -

Peter

 

[rslifkin]

My friend leaves the supports unattached except when underway. Maybe setting/releasing the supports can be part of your docking routine along with lowering fenders and setting lines etc.

Good luck -

Peter

That's exactly what I'm thinking we'll do. Just have to figure out the attachment points and how they'll fold out of the way and stow.

 

[koliver]

I have "custom" SS plate davits. They were originally designed for a PO and when I bought the boat a Whaler 9' was on them. They worked well for that and for a soft 11' Achilles inflatable. When I upped the load to a Caribe 10 + 20hp Yamaha, they started moving and cracking the cap rail teak, so I added a brace under each. The attachment of the original davits is a piece of SS plate made the height of the bulwarks with a ~6"x6" plate bolted to the bulwarks, with a 6"x6" horizontal welded on top and supported by a pair of plate triangles to connect it all together, then the davit sits on top of that. My brace modification is more 1/4" plate extending down the transom 18" and bolted on down there, providing both stiffness and support by pressing on the transom. I also added a substantial backing plate on the inside of the bulwarks. That worked so well to eliminate the bending of the bulwarks that I was able to upgrade to a Caribe 12' with a Honda 40, for a total weight of ~750#

Could yours be stiffer with a brace to the lower reaches of the transom?

 

[rslifkin]

I have "custom" SS plate davits. They were originally designed for a PO and when I bought the boat a Whaler 9' was on them. They worked well for that and for a soft 11' Achilles inflatable. When I upped the load to a Caribe 10 + 20hp Yamaha, they started moving and cracking the cap rail teak, so I added a brace under each. The attachment of the original davits is a piece of SS plate made the height of the bulwarks with a ~6"x6" plate bolted to the bulwarks, with a 6"x6" horizontal welded on top and supported by a pair of plate triangles to connect it all together, then the davit sits on top of that. My brace modification is more 1/4" plate extending down the transom 18" and bolted on down there, providing both stiffness and support by pressing on the transom. I also added a substantial backing plate on the inside of the bulwarks. That worked so well to eliminate the bending of the bulwarks that I was able to upgrade to a Caribe 12' with a Honda 40, for a total weight of ~750#

Could yours be stiffer with a brace to the lower reaches of the transom?

Hmm... So almost like running a big vertical brace downward from the davits onto the transom? That way it would have to compress the brace downward to be able to tip the davits and rail outward?

 

[koliver]

Hmm... So almost like running a big vertical brace downward from the davits onto the transom? That way it would have to compress the brace downward to be able to tip the davits and rail outward?

I think the object of the exercise is to transfer the forces from the rail to the transom, ideally, remove the rail attaching point completely, or at least limit forces at the rail to lateral rather than fore and aft.
Note that mine have no attachments at the rail.

 

[rslifkin]

I had an idea while working this morning, so I did an experiment during lunch. Added stays (temporary using 3/8" line for now) from the bimini frame to the davits. See the pictures below.

The stays attach where the 3 pieces of canvas meet in the first picture (there's a gap to feed them through) and pull down and back on the aft bow of the bimini just above the angled supports. Those angled supports get loaded in compression and hit the rail right next to a stanchion, so the load gets pretty much transferred right to the deck as compression load. That part seems to work well enough that after tensioning the lines a good bit the canvas wasn't any tighter than when I started, so nothing in the bimini frame and upper deck rail is flexing.

The lines have a bit of stretch which means there's still a little movement, but less than 1/4 of what I had before. And the stretch means load should get shared between the stays and the stern rail (and swim platform supports).

This probably isn't the ultimate solution, but for now it's a good start and maybe part of the final solution. And I've got 6'3" of headroom to walk under the stays, which is plenty for me (and a little more than many other places on this boat). So they can be left in place all the time.

 

[angus99]

You’ve probably already thought of this, but if the drain in your dinghy happens to clog during a rainstorm, the weight on your davits and supports could increase dramatically. Ask me how I know . . .

 

[rslifkin]

You’ve probably already thought of this, but if the drain in your dinghy happens to clog during a rainstorm, the weight on your davits and supports could increase dramatically. Ask me how I know . . .

That's a good thought. Might be a reason to invest in a cover so the drain is just a backup for any rain that happens to get in.

 

[Jeff F]

Aren't those davits typically based on the swim platform or transom and supported at the deck? What was the problem with that approach?

The top support is critical in my experience. Don't discount the athwartship loads when in a cross sea. Bracing should be diagonal when viewed from above.

 

[rslifkin]

Aren't those davits typically based on the swim platform or transom and supported at the deck? What was the problem with that approach?

The top support is critical in my experience. Don't discount the athwartship loads when in a cross sea. Bracing should be diagonal when viewed from above.

They're meant to mount to either the transom or deck at the bottom and the rail at the top. Given an install mounted low enough on the transom they could presumably be supported by the deck, but that would leave a lot of unsupported height, as I wanted to be able to lift the dinghy above deck level. The braces between the rail and swim platform were included with the davits (but with my tall deck and short platform they're not at an ideal angle). The braces to guide the dinghy outward away from the transom when lowering though (and avoid worrying about it swinging and hitting while lifting).

One issue is that davits of this style are often mounted on sailboats, which tend to have much stiffer / stronger stern rail setups than my boat.

I would have mounted them a few inches down on the transom rather than to the deck, but with the shape of the hull/deck joint and the rail leaning inward by design, I would have needed the rail clamps modified to account for the davits being 4 - 6 inches further aft of the rail. It would have avoided some of the excess height I've got (and avoided the very slight dinghy to swim platform interference I've got when lowering), but the dinghy would have also added even a few more inches of length that way.

I'm thinking that if my stays work well this weekend, I may replace them with a rigid version for more stability in the long term, possibly with some further reinforcement to the bimini frame to allow it to take more load. At some point I'll hard top the boat and extend the hard top back further, so then it'll be easier to come up with a well triangulated upper support like this.

Lateral support is much less of an issue for this install. It's already very stiff in that direction (the rail has much more stiffness laterally). The 3 piece cross brace between the davits does a good job of keeping them tied together as well. Plus, I have the dinghy tied down such that the tie down lines should absorb most of the lateral loads down lower, so they won't be applying leverage at the top of the davits like the vertical loads can. I can't get anything to flex side to side if I start pushing, wiggling, and bouncing with the dinghy tied down. The davits and rail pretty much flex as a unit and only a bit in the forward/back direction (which is much reduced with the stays taking some of the load).

 

[bill.kimbell]

St Croix makes "sportfish mounts," which are vertical struts from underside of transom to deck. Adjustable, with three stainless struts each side. I used them for transom mounted davits on Mainship pilot. Worked perfectly.

 

[jflongwell]

I used Kato davits and thought the rails were insufficient for an inflatable with 15 hp.
I recommend mount yours to the swim platform and make the upper mount attach to the transom with through fasteners.
You may want to upgrade in the future.

 

[Comodave]

Maybe put some hefty through bolted padeyes on the top rear cabin. Then some S/S rachet straps from the padeyes to the davits. Also 2 more straps in an X from port padeye to starboard davit and vice versa to the other side. Easy to install and remove and they will pull tight pretty well. With the X it will stop some of the racking side to side.

 

[Raybo]

If it were my boat, I would not hang it from the rails. They aren’t designed to hold equipment. I would mount it to the transom. Plenty of strength, even if it filled with water.

 

[rslifkin]

For those saying to mount the davits to the transom, that's not an option. I'd have to sell these davits and have a new set made 2 or 3 feet taller. If the dinghy can't be lifted above the top of the transom, I've gained nothing from just using flip up davits.

I did finally run the boat with the dinghy in the davits this weekend. The stays to the bimini frame work wonders. Biggest seas we saw were about 2 feet, but it didn't matter if they were on the nose or the beam, there was no detectable movement or flex. The dinghy just sat rock solid. The stays also got rid of the flex while lifting the dinghy and pulling on the lift lines.

From trying to flex stuff at the dock, it seems like I've got plenty of stiffness in side to side with the big cross brace and the rails being pretty stiff in that direction. It's the vertical and fore/aft forces trying to bend the rail that cause concern. And the swim platform struts only help a little.

At this point, I think my next steps are to investigate whether legs from the davits down to the transom would help and then further reinforce the bimini frame where the stays attach and replace the stays with something rigid. That should further stiffen things beyond what my temporary stays do as the stays are slightly stretchy. Plus, the stern rail assembly can handle a little bit of forcing, just not a large amount.