1972 Keel Up Restoration

The 1/2" transom is just the 1st of 3 laminations. That is going in 1st to keep things light because I will be flipping the hull for sanding.

 

Hey tj309:

I know that you don't want to use deficient materials on your rebuild to have to deal with again but marine ply (or microlam material for motor mounts) is still an industry sandard for transom material for rigidity and strength. I can see where you would be a little paranoid about the materials below the water-line but the transom has got to have a lot more structural loads, both vertical and torsional that may stress the foam core materials more than the "tried and true" traditional materials. I know that matercraft went into foam core materials back in the late 1980s for the same reasons that you have but they also were focused on the inboard market at that point.

As to the core materials, I've owned three (currently 2) post 1970 hydrodynes (1973, 1975, and 1985) and none their balsa core structure(s) have failed. In addition, I sailed for 10 years (1994 to 2004) on a 1989 Tarten Ten which is a balsa core lay up and the owner never had issues in the mid-west. If your going to do the core in foam for moisture intrusion, then power to you, but I would consult with local rebuild shops about transom materials

Kevin-

 

Kevin,

I am with you, but I read up on this blue water 26 and it is pretty tough stuff. I think it is a bit heavy for core though.

jim

 

Kevin thanks. I have consulted with a local boat builder and currently have an active thread on S&F and the consenus is that Coosa 26 is fine for both my transom and floor and stringers. I hope so because I spent $1400+ for 7 1/2" 4x8' Coosa boards. Jim explained why I should put the core in the hull 1st and then the stringers on top of that and even though it goes against my reasoning it does make sense because the designer of my boat did it that way. I may hedge my bets on the core and have 1/4" holes every so often in the core where the stringers go to tie the stringers in directly to the shell with resin. There will be 5 knees to tie the transom to the core. 2 to form the sides of the widened motorwell and 3 for stringers. That should be about as bullet proof as is possible.

 

Kevin we crossed posts. I plan on using Divinycell H80 for the core.

 

Sounds like a plan!

jim

 

I have restored 2 18's and used divinicell for both floors and stringers. My cores were good or I would have used the foam there too. IMO it is stronger and much lighter than plywood. The cost is higher but minimal compared to the overall cost of a project this big. You will need scrim backed block foam for the core to conform to the compound curves, and sheets for the floor. You will not need the cross stringers. For the pylon mount, I made a dam out of foam and filled it with a mixture of resin, chopped glass and micro balloons. Keep a fan on it during cure as it gets pretty hot. Your cutting board is likely UHMW which will not bond to anything.

 

Nice work Ski! You are probably right about the kitchen cutting board not bonding to anything so I will probably enclose the fin area with coosa as you did with your ski pole pylon area and drop a piece of plywood in and glass it in and thru-bolt the skeg to it. That way if the integrity of the fin mount ever leaks it will be contained. I may even build an inspection hole in the floor to check but that is much later...

From what I have learned so far Divinycell is for cores and is a foam. How can you use that for stringers? Glass the heck on it? I plan on using 1/2" coosa laminated to 1 and 1/2" for stringers.

I also have another idea that I want to bounce off you guys. I plan on putting the new transom in with 3 1/2" laminated coosa boards and then put in the sides of the expanded motor well in with coosa doubleing as knees and tie them directly to the hull shell. Probably 1" thick except on the motor well side where they will be 1 and 1/2" and provide a support for the floor of the motorwell. Then install the core. All this is still too wide for where the normal stringers would go so I plan on using the 3 center stringers installed over the core as the original designers did (thanks Jim). I can make the 3 center stringers also support the motorwell and tie all this together to make a really bullet proof transom.

Jim mentioned that the strength of a Hydrodyne comes from the deck being tied into the rest of the hull especially at the stern. Makes sence to me and my idea will certainly do this.

One more question - I removed a steel I-beam in the hull shell from the front of the motorwell. The I-beam appeared to be 41 years old and tied the top of the hull together and fitted under the front top of the motorwell riser. Is this original and should I replace it with coosa?

 

That was probably added by a club to reinforce the pylon or other tow point. The transom and pylon of a multi-engine club boats see a lot of stress. Your call.

One thing that I did to my boat (a converted I/O) was to make the motor well as small as possible with just enough room to tilt the motor up. Large motor wells are a waste of space in my opinion. This leaves room under the motor well for batteries, oil tanks, and flotation if you like. It also gives a better load path for transom loads into the deck. It also leaves the option to run all the hoses and wiring through flex hose through the center of the motor well like the modern hotboats do it. This gives a little cleaner look provided you use hydraulic steering. Some modern outboards have no motor well, granted they usually use a jack plate which would not be appropriate on an 18 imo.

jim

 

You read my mind. I do plan on making the motor well much less deep than it is presently, and I do plan on using flex hose and hydraulic steering in the rigging. I have given all aspects of this project many hours of thought but have only presented the immediate stuff on this forum so far. Thanks to all for the input!