Pretty much without question, you're going to need to strengthen AND stiffen the transom. And were it me, use bigger backing plates
as well. Although with a properly beefed up transom, they needn't necessarily be stainless steel
(inside the boat at least). Especially if they're properly sized (for what they're made out of, the load on them, & what they're mounted onto).
With a good sized stiffener added to the transom, 3/16" or 1/4" aluminum
would be fine, assuming that you use good sized washers. Although, with the internal transom stiffener, it's likely that the stainless plate might suffice.
A flex check will tell you easily enough.
And, of course, one of the perks of aluminum
is that it doesn't require any special tools for cutting or shaping. But when properly sized, it's plenty strong enough for use as backing plates
for the winches on Maxi's.
BTW: All of this information can be found on the WEST System site. Both in their smaller, user guides, & in their excellent book On Boat Construction
I'm 99% sure that it's also covered in their "Projects" (menu) section, plus in the archives
of their free magazine www.Epoxyworks.com
Ditto on the info being on most other major epoxy
manufacturer, & distributors websites, along with video links too.
In terms of upgrading your transom, don't overly fret over how much access space you have inside, with regards to squeezing in one big piece of plywood back there. You can use multiple layers of thinner pieces of plywood, in smaller pieces, in order to get a proper sized reinforcing Core
Pad. - Over which you'll be laminating some fiberglass
When you're installing the pieces, you butt the seams together in order to get a large enough pad. Then when you put the next layer on top of that one, just put the seam in a different spot, several inches away from the one in the layer underneath of it. And you can still put a layer of tape on each seam if you want to, though strictly speaking, it's not necessary when building via this method.
Also, if you like, you can; pre-scarf the individual pieces of plywood, prior to mounting them, & glue them together along the scarf line at the same time you're gluing them to the transom.
That, or you can do a rabbett, lap, or finger (etc.) joint to connect them if you like. Ditto on using a biscut joiner or similar.
For the pad, you can use say, 3 layers of 1/4" Exterior Grade, or Marine
plywood for this. And you'll want to bevel the perimeter edges of each layer of plywood, to avoid creating any hard spots via this reinforcing.
Exterior is fine, as it has waterproof glue, like Marine
ply, & few voids. Plus, it's cheaper.
- With the first layer of plywood (the one going up against the transom), paint
it with unthinned epoxy until it's saturated.
- Similarly, paint
the taped off, & cleaned & abraded section of transom to be reinforced, with unthinned epoxy. Then with a layer of thickened epoxy as well.
My favorite for structural applications is to use milled, or chopped fiberglass as a thickener.
- Bolt the plywood to transom, & allow to cure. Use the backing plates as a clamp so that the plywood bedded in the thickened epoxy is well seated, with no gaps.
- Then, similarly, glue on the other layers of plywood. Making sure that the seams where the pieces meet, aren't in the same place on any of the layers.
In fact, you can turn each layer 90 degrees to each other, so that there's no chance of this. But make sure that you orient more of the face layers of the wood vertically than horizontally, so that you get more strength from the wood.
Also, remember that the edges of the wood need to be beveled. Plus, each successive layer of plywood in the pad should be slightly smaller, dimensionally, than the one below it.
- Once the epoxy in your plywood pad has cured, you can start laminating on layers of fiberglass reinforcement. Stitched axials (such as triaxial) are best from a strength perspective, as well as being pretty easy to handle as compared to perhaps anything but woven cloth (even wetted out).
NOTE: Epoxy will not properly dissolve the binders in fiberglass mat, as it contains no styrene (unlike Polyester Resin). So you're better off skipping anything with mat if you can. If not it isn't a crisis. And I only mention this, as for some reason the styrene/mat thing, seemingly isn't common knowledge.
But when using layers of mat in between layers of say, woven roving, if the binders don't get dissolved, then the mat can't flow between & connect the layers of roving as it was designed to. Resulting in a weaker laminate, overall.
~ The easiest way to handle a piece of wetted out glass in this application, is to wet it out while it's laying on a piece of visquene, & wait until just before it begins to gel, to put it in place. Then roll it on firmly.
- When adding these reinforcements, their sizes will be the reverse of the plywood. You'll start with the smallest layer first, with each successive one being slightly larger.
Were it me, I'd add layers until I had at least 3/16"+. That way, between the cores, & the glass, there won't be any worries about strength. Plus, the reinforcements & thus, load, will be spread out over a good sized area of the transom.
*** - About your question as to how to tighten nuts by yourself. Skip using Vise-Grips on the threaded portion of the bolts inside of the boat method. As you'll wind
up damaging the threads on the bolts that way. And then either have to cut the bolts shorter (to remove or add nuts), or use a jeweler's file to fix the threads on the bolts.
What you can do, is put your primary nut onto the bolt, & spin it into position (or close there to) by hand. And then put 2+ more nuts onto the bolt near it's free end, using two wrenches to heavily lock the two nuts against one another.
When you get them locked tight enough, you should be able to grab the pair with (dreaded) Vise-Grips, & hold them, & thus the bolt in place, while you tighten your primary nut (the one @ the transom/backing plate) into place.
That, or sometimes just a wrench on the inner nut, of the locked together pair will provide sufficient resistance to allow you to tighten the primary nut sufficiently.
FYI. Since this is an engine
mount, & thus subject to a lot of vibration, it'd likely be wise to put a few drops of Blue Loctite on the nuts, for their final mounting. After, of course, making sure that both they, & the bolts are clean.
As I said, most/all of this info is on the WEST System site. Especially if my technical descriptions are unclear. But either way, once you see it in pictures, on their site, 99% of it will become crystal clear, & very simple.
- Just print out Two copies of it to have on hand as guides when you're working. One to use as a reference, & the other to use & get epory on when you're actually gluing & fiberglassing things.