Custom Extended Swim Platform

I would suggest that you contact a mechanical engineer. Even with a two foot extension placing a 1,000 lb weight on the aft edge of that platform is going to generate a lot of torque where the platform is attached to the built in platform and your boat. I'm not saying it's impossible to do but for that amount of force I'd want an engineers opinion instead of seat of the pants design. I built a platform of my own a couple of years ago and can tell you from experience that the loads are tremendous as you extend from the back of the boat. Do it once and do it right. Good luck, MIA

 

Yes, I have the good fortune of having been advised by a mechanical engineer in the nautical structures industry as I have proceeded so far. I'm an engineer as well, but not specifically mechanical and not in the marine industry.

I have seen your thread on the swim platform. Do you have any specifics to share regarding the platform that you built in terms of what you needed to do to adequately secure and support it?

 

If you've looked at that post from 2016 you'll notice a picture from below showing 4 brackets that bolt through the transom and the 4 stainless tubes supporting everything. After I used the boat for a season I did some careful measuring. Even though I moved the outboard mount closer to the transom (in order to minimize the torque forces on the supports from the outboard) there wasn't quite enough strength in the area of the outboard mount. I ended up adding some 1 inch thick 6061 aluminum bar stock to the tops of the brackets that you see bolted through the transom. I set the bar stock on the top of the brackets, ran it to the rear of the platform and through bolted it. This modification stopped the thrust of the kicker from twisting the back edge of the platform down. It also proved that the tubing supports weren't doing much to support that back edge. I was surprised that I had to do this as the platform is 1-1/2" thick, dense Philippine Mahogany where the supports attach. After strengthening the transom I added 1/4" thick backing plates (6061) where the brackets bolt through. These were made out of 4" wide plate, 12" long to spread the load out. I'll post that photo I'm talking about below.
Just a side note for anyone else who's reading this. I added a Garmin TR-1 autopilot to the kicker in 2017. One of the best things I ever did.
MIA

 

I had wondered about all of the thru-bolts when I looked at the picture in your thread.

I can probably do some force calc's, assuming 1,000 lbs. at the edge of the platform, then see what looks reasonable.

 

Putting my engineering hat on, I have calculated some of the forces on the platform, using a 1,000 lb. load in the center 0f the platform as a worst case. I also considered a 1,000 lb. force upward for any wave slap that may occur.

So here are the force diagrams for worst case 'people' load; 1,000 lb in the center. . . which normally would be distributed across the beam of the platform in some manner. Placing the load in the center makes for the maximum bending moment on the platform, as shown below.

Aft View


I assume that the center strut is not going to do much since it is way inboard of the perimeter of the platform - I'm actually considering doing away with the center strut. So, in the middle of the platform, I'll have to make sure the lower edge/skin can tolerate a 6,000 lb load. I have done some searching on the tensile strength of fiberglass . . . 15,000 psi seems like a worst case. So, I would need to make sure there was at least 1-2 sq. inch of glass making up the cross section of the lower skin.

Side View - assuming all of the load supported by the struts . . .


Alternate Side View - assuming none of the load supported by the struts.


Nothing really overly stressing the fiberglass, which has about 15,000 psi tensile strength at a minimum.

For Wave Slap


This assumes 1,000 lbs. at one side of the platform, or 2,000 lbs more evenly distributed. . . Since the struts are the furthest out from the center of rotation (top of the platform ) they will take most/all of the load. Probably need to thru-bolt the struts into the transom. If the strut took none of the load, the next thing to take the load would be the lower corner 'L' brackets, which would be about 1,800 pounds.

Nothing earth shattering here; key take-aways are:

1) Need to make sure there is sufficient skin or structure on the lower side of the platform supporting the center area.
2) Thru-bolting on the transom struts (not lag bolts)
3) Reinforcing pads inside the fiberglass area where the corner 'L' brackets attach.

 

Quick Update . . .

I heard back from my tech support contact at Formula Boats. They secure the swim platforms to the cap portion of the boat with 4 through bolts that are molded into the platform underside during layup. They also use adhesive sealant between the platform slab and the built-in platform. No struts, just a cantilever.

So, it looks like I'm good to go with my approach to fastening the platform to the boat.

 

Based on the information (and pictures) I got from Formula Boats' Tech Assistance Group (TAG), I may change the way I secure the lower edge of the platform in order to simplify the design. Instead of having the 4 thru-bolts for the 'L' brackets going through the lower edge of the platform, I may mold in 4 bolts into the underside of the step-down portion of the platform during the layup and resin infusion. The step-down is about the same thickness of the built-in platform (5-6") so the 'L' brackets could just be a flat plate bracket, eliminating the need to make the 'L' bend in bar stock, and also eliminating the need to fabricate the 3" ridge on the underside of the platform. Overall this would probably be easier and lighter with no appreciable loss in strength.

I'll post some diagrams of this new thought process in the next day or so.

 

Here are a couple of diagrams showing replacement of the 4 main 'L' brackets on the underside with straight brackets and the 2 embedded studs in the extended platform for each bracket location. The underside of the existing platform is still thru-bolted.





The underside structure of the ESP will be much simpler with this approach, similar to the way Formula does their platforms. I'm still going to keep the outside struts, even though Formula does not use them.

 

I would be hesitant in just embedding the stainless bolt heads into the fibreglass as the bearing edges, ie the 8 sharp edges of the bolt head are small. With even a reasonable amount of torque
you run the risk of the bolt heads spinning in the glass and then you have a major issue.
If you take say a 4 inch by 3 inch flat plate, drill it, push the bolts through, weld the bolt head perimeter then embed the entire plate, then you should be ok. Dry run the nuts to be sure that
the threads are clean of weld spatter. Whenever we welded up blind bolts, we would just run a bunch of nuts finger tight over all the threads to ensure that absolutely no weld spatter gets on the threads. And lube the threads when you are making up the final joint

Stainless 316 bolts and plate.

Stainless, certainly the poorer grades have a tendency to gall, pack up the threads and then the nut will not come off.

 

Yes, I plan to push the bolts through a plate so that the bearing surface is much greater that the bolt head itself, just did not show that much detail in the diagram (sorry ). This will also hold the studs in place during the fiberglass layup.

Here are a couple pictures from my 3D model with the underside cleaned up, more under-skin shown, the new brackets shown, and the center strut removed. The studs and reinforcing plates will be buried in raised areas of the platform on the underside of the step-down portion. The underside of the step-down will be approximately the same height as the underside of the built-in platform +/- . . . so the straight brackets should do the trick, maybe with a shim, if needed.





At this point, I should press on with the finishing touches of the plug.

My plan is/has been to:

Make the Plug by end of January

Make the Mold by end of February

Make the Platform by end of March

Fitting and hardware by end of April

All in prep for the 2109 boating season which typically starts (for me) late-May or early-June.

 

My main point of the plates was to weld the bolts to the plate to keep the heads from turning in the fibreglass.
I see that in one of your drawings you had tabs on the ends of the tubing, this is the right way to do this instead of just putting a bolt through the tube itself

 

Yes, absolutely . . . my 3D design skills are still a bit fledgling

 

Here is an updated 3D with realistic looking struts. My plan is to have 1-1/2" inch (1.5 in) diameter SS tubing with flattened ends as the struts. They will be approximately 42" long. I have done some buckling calculations (Euler's critical load formula. . . ) and it seems to be fine with 3/32" (+/-) tube wall thickness.

Some folks think 1.25" would even be adequate, but I am not so sure . . .

Pictured below are 1.5" diameter 316 Stainless steel struts.. . . 42" long

 

Idea of the day . . .

It looks like I can get some stainless steel 'squared' U-bolts of a reasonable size from trailer part suppliers and other resources. So, for the embedded studs, I could merely use an SS U-bolt as the 2 studs and put it through a backing plate to help hold it in place during layup. No risk in the studs breaking loose and a lot easier to fabricate.

Here is a diagram of the idea, showing the embedded U-Bolt in place of the 2 embedded studs.

 

The plugs are done, so now onto waxing and prepping to make the molds.

I plan on using a bunch of CSM, tooling gelcoat & resin, along with some wood bracing for the molds.

Looking ahead to the platform itself, I am thinking about using some PVC foam to bulk the laminate up in the flat sections. One concern about the platform's laminate is weight vs strength. . . i.e. less weight, more strength. I have come up with a layup plan (schedule) for the platform as shown below. Fastener and stress areas will have a few more layers of 1708 and no foam, but this will be typical of the flat sections:



There will be the gelcoat, CSM layer, then 2 layers of 1708, then 1/2 of the PVC foam strips oriented as shown, then another 1708 layer, then the remaining PVC strips in the opposite orientation, then a final layer of 1708. All of this will be pressed under vacuum and then resin infused. I'm thinking the total laminate will be around 5/8" (0.625), thick, which should be fairly light and fairly strong.

Here are some resin/glass calculations. . . The surface area figure of 42 sq. ft. includes the sides/shapes and the 2 ladder hatches.


I think with the CSM and gelcoat layers the weight of the platform will be around 75-80 lbs, not including the hardware and other fittings. All said and done about 140 lbs.

Thoughts on the layup schedule ???