Transom support questions

[quincyscuba]

Hello Everyone,

I purchased a 1965 50MY last year. We are in the process of having a new custom swim platform fabricated at a local machine shop. My question is about the supports that are on the vessel. I believe the swim platform on it may be the original (about 28” deep). The original supports look to be 3” flat stainless with an angled down support. They each have what looks to be 2- ¾” machine bolts through the transom. There are 5 of these supports I can only see 4 of the ends of these as the water tank blocks access to the center one. They do not have bolts on them and appear to have just been fiber glassed in. The new platform will be deeper (46”deep) and will have a davit to lift our RIB. I am concerned about if I should remove the old supports and replace with longer stronger supports or should I just leave these in and just beef them up. I also thought I could add two more supports and place backing plates on the ones I have access to. I have been told there may be a metal plate inside the transom that the original supports were threaded through.
Thank you for any advice or experience you can share on this topic.

 

[SKYCHENEY]

I don't know about what they did in 1965, but in later years there were backing plates glassed into the transom.

 

[quincyscuba]

Is it possible to unscrew the bolts and then reinsert bolts with out damaging the transom or the backing plates?

 

[Andy B]

Depends if the bolts are stuck in the plate or not.

 

[SKYCHENEY]

Mine came out and went back in just fine. YMMV

 

[Keith]

I have a 1966 50' yacht fisherman. I am not sure how mine are held inside the space that can't be seen because of the water tank but, I do know one thing. However they are held in they are extremely stout and have taken numerous hits without bending, leaking, cracking, etc. I have taken out a couple of the bolts to look for crevice corrosion. I have found no problems with the bolts and they threaded back in and were torqued down with no issue.

Keith
Lady J

 

[quincyscuba]

Thanks for the info. I will take a pictures next weekend and post them so you can see what the backside looks like on mine.

 

[Vincentc]

If I understand correctly, you are going to extend out your 28" swim platform to 46" and add on the end of the swim platform davits for a rib. That sound like a significant added load on a lever arm that is about 70% longer than original.

The platform creates an up load from wave action under the transom as well as the down load from what ever weight is on top of the platform.

What you will find when you remove your original bracket is not certain. All but the bottom bolts for the brackets on our 43DC were accessible from inside the boat. The bottom bolts were not and were 3/8" lag screws into 5200(?). One night at a rough anchorage generated enough force to pull them out.

I would give serious consideration to the load you are creating on a transom that was probably not designed for that kind of load. I do not know about your boat, but the 1983 43DC has a foam cored transom and the fiberglass skins are not that thick. An under engineered platform bracket could do more than damage the platform.

I extended our platform about as much as you plan, but it is hinged and only extend out beyond the original width when we are at anchor and using it. Even then I have support wires running from the transom sheer to the corners of the platform extension to carry the occasional load of may be 4 people. The details of my efforts are posted on the forum. see http://www.samsmarine.com/forums/sh...im-Platform-Completed&highlight=swim+platform

I am interested to see how you do this project and how it works out.

Regards,

 

[sgharford]

I'll second Vincent's concern about upward force on the swim platform waves produce. I've had 3 swim platform bolts shear off at transom from the slapping motion of waves on my 43DC with stock swim platform which is about 28" wide. I've seen water splash 20' high from the swim platform slapping the waves at my dock. Make sure the design you use allows for water to pass thru swim platform to lesson force. On my 1976 43DC, the transom is solid glass with an aluminum plate embedded in it that is threaded for the silicon bronze bolts that hold swim platform brackets. There were no nuts on the inside of transom (I added as safety after replacing the 3 sheared bolts). I added a St. Croix dinghy davit to platform 2 years ago and added an additional swim platform support at the outboard motor end. I simply drilled thru transom and tapped (threaded) the embedded aluminum plate for mounting the additional bracket. By the way, You can buy these brackets stock at http://www.yachtsupplydepot.com/exterior-boat-accessories/swim-platform-brackets/cat_621.html. 46" deep is some serious torque at the transom, I would probably remove all the existing brackets, glass in the holes, and put in 5 or more properly sized brackets that extend the full 46". Adding weight of RIB, Davit, and Person(s), custom brackets are probably way to go. I think the key aspect to proper design would be using large enough safety factor to accommodate for the force all this weight produces as it changes direction from swaying with wave action. The means you use to fasten brackets to transom is probably priority one. Anyway, gotta get back to work but hope you get more feedback and will post what you do.

 

[bobk]

And I'll 'third' Vincent's recommendation. I had my stern to the waves while in Dowery Creek Marina several years ago and the wave action during a blow pounded on and under the platform. After a night of this, one bolt hole in the 5/16" thick brackets was worn through and two others almost failed. My platform was extended by a PO and is virtually solid glass with only one slot to allow water to pass through.

Bobk

 

[quincyscuba]

I have done many searches trying to find info on a project like this. Thanks for your experiences. I will definitely add more slots for up-swell to pass. It is an issue I had not given due respect to (we are located on a lake that is usually calm but do plan to move to the ocean in about 4 years). I had planned on adding a little height to the platform and making some slots for water to pass through, but will add more. I also am adding spiral stairs that will somewhat do the same thing as your wires do on yours, Vincent. It will tie the swim platform to the transom at about 50” up. I figure this will really strengthen things up quite a bit. The davit I am building is a hybrid to ones such as Nick Jackson and Jatco Marine designs. The pivot/anchor point will be about 6” to 12” from the transom. My plan is to build and install platform, take measurements for stairs, build and then attach stairs. Final step will be to build and add davit. I hope to complete this all while the boat is on the hard this spring getting bottom job done.

One thing that did surprise me is that you said yours was foam cored. I was not aware that there was a possibility of it not being solid fiberglass. My PO did place a new hole in hull and it was solid. I will try and post a photo. Of the plug that was cut out.

Thanks again for all of the responses.