Advice for my transom modification appreciated!

That'll work . . .

 

Hey PAR!
Do you think it'll be OK when I cut through the new hole? If the plug was to stay whole then I can imagine it retaining its strength however I'm a little concerned about how strong the 'leftover' parts of the joint will be once I've cut the new hole. I'm thinking about cutting the hole 5mm larger and then glassing from the inside to the outside, through the transom, what do you think?

 

Connecting the skins is a good idea, though not wholly necessary on a technical level. On the practical level, it's always a good idea to encase the core and any openings in it. For example holes would be drilled over size, filled with thickened epoxy, then drill again for the appropriate fastener shank diameter. What this does is prevent moisture from getting at the core. All penetrations should be treated this way, but manufactures often don't simply because they only offer a 1 year guarantee and they figure their crappy work will last at least this long.

 

Thanks PAR. That's what I think I'll do then. What do you think about the strength of the joint after cutting through it for the new transom shield?

 

About strength (and stiffness), it's all about how things are "tied in" from new to old. In general, assuming the core is good, you just have to worry about the transition from the old laminate to the new. This is where the tapers really come to play. If they're too shallow, it'll crack, but you can never be to long, in terms of the slope of the transition from old to new laminate. If you have to error, do so on the side of too much laminate and/or tabbing. I use a 7" or 10" grinder disk, with ridiculously coarse grit (16, 24, etc.) on it. I plow out as wide an area I think practical, grinding down until I create a feather edge around the opening, tapering back as much as I can. I do admit to keeping in mind the widths of the tape I might be using, so for example if the laminate skin is 1/4" thick, I'll grind 4" back from the opening in a gradual slope. This is a 10:1 taper and generous enough for your project, though is areas you don't have room, well you just have to do what you have to do. At the bottom of the opening in your case, I would grind right down to the hull shell and use the tabbing area for part of this transition. It'll get filled back in eventually as you bulk up the new stuff and under putty and paint, no one going to know what butchery has taken place under it. Again, the load on an outdrive setup are like that of an outboard, so you're save, just use as much area as practical and if some bulging laminate shows when you're done, know one will see it once the bell housing is back in place. Simply put, "when in doubt, make it stout" . . .

 

I've only read the first two posts and perhaps came rather late to the discussion but I think a solution like in the attached figure is highly desirable.

 

This is an outdrive equipped boat and this type of reinforcement, isn't necessary on the current transom, as it already has two vertical abutments and two engine beds, that carry nearly all the loads, for this type of drive assembly.

 

Beg to differ, PAR, but there are two points of stress concentration because the bottom longitudinals not have adequate conection to the transom. That, always, should be avoided in a structure. With more reason in this area where the relative movements, longitudinal / transom, are obvious.
Even the solution in the picture may be more appropriate.

 

What you're showing is the typical arrangement, the engine beds usually don't land on the transom, but stop short, to prevent what you've described. Again, the beds carry the bulk of the load transmission to the bottom, not the transom in an outdrive setup. The transom is simply a part of the hull shell envelop, to keep the crew's socks dry.

 

There are two fully independent effects:

• engine weight and rotation of the crankshaft, which are loads transmitted through the engine mounts on bottom longitudinals, and
• the component of thrust of the propeller perpendicular to transom, which in this boat acts directly on its transom.

The second is what I'm talking about.

 

The drive is directly hooked up to the engine and it's weight (torque, loading, etc.) borne by the engine beds. The transom has little influence of load transmission. The transom is sandwiched by the gimble housing and bell housing, but is little more than a hull gasket, preventing leaks though the shell. There's no debate on this issue here, if it wasn't for the obvious leak that would occur, there's no need for a transom on an outdrive setup.

 

PAR Okay, I will not get into that discussion. So I see the transom only serves to keep out water. I do not care. Only if I were Gazzainnorway would be more careful with the forces that mirror "is not supporting".

 

Thanks for the advice TANSL. At the moment I'm concentrated on patching the transom hole.I'll be using the payson technique as suggested by SamSam, discussed and agreed apon with PAR, as far as structural support goes, I'll look into that after I've bedded the engine (VP AQ140) which I'm rebuilding at the moment. I'm planning on a VP 290 drive with douprop.
The engine only has two engine mountings and they are not 'dividing' (which would give you little or no weight transferred onto the transom) the full weight of the engine, so there will be weight on the transom from the engine however I'll have to weigh this once the engine is off the rebuild stand.
My first aims are:
1) to patch the huge hole in the transom.(need better weather)
2) strip down and rebuild the motor. (busy with now)
After that I'll look at space in the engine bay and most probably look at building a boxing aluminum frame with diagonally supports , for extra support and to supress any possible torsion moments. If need be. However as PAR pointed out there are already abutments supporting the GRP and tabbed onto the transom. This will be the easy part...
PAR thank your for you help and support. I'll be posting pictures of the plug and payson work once I start on it. I was wondering about what to do with the hull and you answered that question. And BTW I believe my GRP is approx 1/4" thick. Total transom thickness in 2 1/2". That's a 1/4" thick glass on either side

 

I disagree with this description of load distribution, this was correct with the OMC drives of yesterday but not so with the drives of today. Some today are even fitted with jack shafts and the only loads transmitted between engine and drive are rotational through universal joints.
Gerald

 

Yes, there are some differences with drives, but most still have a solid mechanical connection between gimble and bell housing, which is what I was referring to. Jack shaft drives aren't that common yet, though admittedly there are exceptions to this rule, he's got a Stinger and putting a VP on, both are hard mounted.