Question about strengthening fiberglass transom to handle larger outboard.

[mallardtone-man]

Ok guys, I have a 1970?s 16 Malibu fiberglass tri hull that I am restoring. I have gutted and restored 90+% of the boat already and re fiberglassed the hull and transom. I have drilled some exploratory holes in the transom and know that it is not 100% because there was some moisture in it and the wood was dark but solid, far from totally rotted. I think that gutting it and totally rebuilding is beyond my capability so instead I have opted to strengthen it. It is already a pretty beefy transom, around 4? thick and runs forward a but. I stripped it and laid a sheet of 1708 biaxial fiberglass matting over top of the whole thing as well as much as the interior/underside as I could access. I replaced the outer board and mounted my 1999 120 hp mercury on it. this motor weighs around 670lbs and I know that is pretty big for this boat but I think it can handle the weight. I put my full weight on the motor and rocked up and down aggressively and there is absolutely no flex in the transom. I have had the boat on the water (not running) and it floats fine, I have at least 1 1/2 feet of freeboard at the lowest point on the back of the transom. It is a wide boat (7' beam) so I think it will be stable with the big motor on it. I have installed a transom saver to help while towing.

My biggest concern at this point is that the motor is going to generate a lot of torque and could possibly even rip the transom off (I have heard of this happening). I would like to do something more to strengthen it just to be on the safe side. I am thinking I will redo the corners with two more sheets of the same 1708 matting wrapping them around extending around 12? in each direction for the first sheet and 8 or 10? for the second. The other thing I had considered doing was trying to strengthen it with some metal brackets. I was hoping to get or make some that are about 12? long on each side and mounting them at the very top outer corners of the transom, either on top of the two sheets of biaxial or possibly between them. Has anyone ever done this or does it sound like a bad idea? On one hand I could see it adding a lot of strength, on the other hand I could see those holes I would have to drill through the corners serving as weak points where cracks could begin to form or more water could get in allowing the rot to spread. Does anyone have any advice for me on this? I have included the only 2 pics I have right now. I will take some better ones later but won?t be able to do so for a couple of days. Thanks in advance for any help! James

 

[jigngrub]

Damp dark wood isn't that far from rotten, it's adolescent rotten wood.

Overpowering a boat isn't recommended on this site, and putting an overpowered motor on a half rotten transom is a really really bad idea.

You should read and study the other resto threads in this forum, they will give you the confidence to replace your bad transom... you'll also learn that you need to tab your decking to the hull around the whole perimeter of the boat. Deck to hull tabbing adds a lot of strength to the boats structure.

 

[Watermann]

Looks like you've got a ton of work done on the trihull so far.

I'm not a glass boat guy but I have repowered upward a bit, although the boats an IO config. Looking at the pics I can see that the transom looks to be around 1.5" thick rather than 4" even with that outer wood pad which appears to be raw ply since I can see the layers. Not sure anybody here would offer suggestions on how to over power an OB boat especially with a "not 100%" wood transom that you're not replacing. When restoring boats the main concern is safety for you and your occupants.

So what's the rated HP for the boat?

 

[Woodonglass]

While I DON'T condone what you've done, and I Highly recommend you and all of your passengers always wear PFD's when on the water and have Cell phones on the ready to call for help, If indeed the transom is not showing any flex with the 120hp motor hanging on her now, then you'll prolly be ok for a while. However the Dark Damp wood will eventually turn into dry rotted wood and the transom will be a danger. You will need to keep a close eye on it for your safety and that of your passengers. There is no way of knowing how long that will take. I really see no need for the steel. If you have done the proper tabbing of the transom using 1708 all the way around to the hull sides and bottom it should support the motor for now. You should be aware that anytime a boat is overpowered, if you are in an accident on the water you will be at fault and your insurance will be null and void.

 

[gddavid]

That motor should weight a bit over 300#, nowhere near 670# but still not a good idea for that boat. I would sell that motor and buy something smaller and lighter. The boat probably won't handle that well with that engine at most speeds, the only time it would really be fun is running it hard and then you will have a big engine jumping around on a flexible transom when you hit the smallest wave which is unnerving to say the least and takes the fun away.

I really think you would be better suited to take a step back and replace the transom completely while you are still working on it. It is certainly no fun to do so but you will thank yourself later one when you no longer have to worry about it. Like others said the wood is only going to get worse and as it turns to mush the whole think will act as solid as a overstuffed peanut butter and jelly sandwich.

 

[mallardtone-man]

you'll also learn that you need to tab your decking to the hull around the whole perimeter of the boat. Deck to hull tabbing adds a lot of strength to the boats structure.

what do you mean by this can someone explain I am NOT a experienced boat builder just a novice trying to get this project done as safely as possible

 

[Woodonglass]

The Picture of the Floor in your boat does not show it attached to the sides of the boat. Is this how you intend to leave it??

 

[Bondo]

Ayuh,.... Welcome Aboard,...... It's ashame to put that much work into a hull, 'n not start at the Transom,.....

The transom is the most important aspect of a hull,.....

 

[mallardtone-man]

The Picture of the Floor in your boat does not show it attached to the sides of the boat. Is this how you intend to leave it??

More or less yes. I was planning on adding flotation foam under the floor but want to wait untill all of the wiring is done. I am going to screw the floor down and was going to silicoln all the seams but probably wont fiberglass it it. How do I tab the decking to the hull? I have never done this before. Do you mean fiberglass it in or are there tabs I can buy or make to attach it?

 

[Woodonglass]

You will learn a lot from reading and studying this...http://forums.iboats.com/forum/boat.../8127378-83-lund-sirius-total-gut-and-rebuild The deck portion starts around page 13 I think. but you should read the entire thread. This guy does quality work. This link will also help you understand how to install a deck..Fabricating Decks, Stringers, and Transoms