Hydraulic effect on swim platform

[RedRascal]

Making a new swim platform for a CHB 34 and noodling on the hydraulic forces on the platform in a following sea. A 10' rib rides on the platform on weaver davits. I measured a few CHB 34 platforms and they seem to be about 17-18" in depth. New platform averages about 18.75" in depth. The factory platform was a grid of 1.25" wide teak slats followed by an 1.25" relief gap. The original wood backing for the platform brackets have been replaced with stronger fiberglass in the transom. So the question is how much relief is needed to keep the platform, platform brackets, dinghy and transom happy. I am not sure if the bigger issue is a wave on top of the platform or a wave pushing up from the bottom side. Seems like a wave on top of the platform is a bit limited by the dinghy. I am thinking about creating about 1/3 of the relief that the factory allowed for. Anybody running a solid platform and making out okay in a following sea?

 

[Comodave]

We has a solid glass platform on our last boat and no problems. On our current boat I put a 5’3” platform extension on that is solid glass with only minor cutouts. It overlays the factory platform about 2’ so it sticks out about 3’ and have had no problems. It is about 6 to 8” above the waterline.

 

[psneeld]

I replaced my broken, teak, slotted platform with a similar sized solid plywood one.

I wish I had made it at least 6-10 inches wider.

I did cut about a 1.5 inch slot with breaks in it, following the outer edge of the platform. It was about 1.5 inches in from the edge and was great for holding onto when in the water and as a place to tie lines to if necessary. I figured this may alleviate some stress because it being out on the end of the cantilevered platform.

If I had added any length to the platform, I may have added another row of slots and maybe small grates inboard as you see on many of the solid glass ones.

It survived 20,000+ miles on the ACIW and both the Delaware and Chesapeake Bays plus the sounds behind Hatteras gave it a pretty good workout.

 

[O C Diver]

My caution would be where you plan to cruise. Dinghies are very slow to drain. If you get hit by a wave from behind, that's a lot of weight in your dinghy. Even several small waves could cumulatively have a devastating effect. I don't know if a cover with an elevated ridgeline would solve the problem. Maybe determine the height above the water of the side tube. Having a wave catch you from behind in a following sea, could be horrific!

Ted

 

[FWT]

https://i0.wp.com/helmsmantrawlers.com/wp-content/uploads/2021/07/1500x1000-HTL-38E-Gallery-49238.jpg?fit=1500,1000&ssl=1

Best image I could find

Deeper, guessing more than 24 inches

Supports are the plates, the thinner tube being a swim ladder.

Both supports and platform bolted to the transom. It handles a load pushing the platform both up and down. I've never heard of one getting loose.

That's a Helmsman 38

 

[Comodave]

My caution would be where you plan to cruise. Dinghies are very slow to drain. If you get hit by a wave from behind, that's a lot of weight in your dinghy. Even several small waves could cumulatively have a devastating effect. I don't know if a cover with an elevated ridgeline would solve the problem. Maybe determine the height above the water of the side tube. Having a wave catch you from behind in a following sea, could be horrific!

Ted

When we carried a dinghy on the swim platform it was tipped up on its side. Not sure I would want to carry one upright for this very reason. Maybe it would be fine but I would not want to take the chance.

 

[RedRascal]

Didn't read any stories about a wave ripping a platform loose so I just added sets of three slots spaced out between the mounting brackets. If you guys would have comeback with a bunch of bad stories I would have increased the slot count and made them bigger. The boat had a slightly bent swim platform bracket that I straighten so it made me gun shy about making the platform deeper than the original. We'll see how this one does this season.

OC Diver, Dave is correct that the dinghy rides on it's side or perpendicular to the water. Many boats carry dinghies this way however it makes me nervous in a following sea.

 

[johnma]

the formula for hydraulic force is : Force = Pressure x Area. So it would be the area of the platform and the force of the wave under it. That is difficult to determine. liquid is not compresible (or almost not compresible) so the pressure of the wave would be in pounds per square inch (psi). the pounds would be the weight of the portion of the boat that the wave is trying to lift. Reducing the working area is important. (by either length, width, or slotting the platform itself to allow the water to move through. I would expect that the naval architect and engineers worked out the mounting hardware when they designed the boat for the size of the platform they used. So, if your new platform is close in size and number of slots as the original and you used the original sized hardware and mounting location you should be good to go.