Stainless steel plate: Overkill?

caguy · May 6, 2012

That is why I uised 3/16" stainless.

rgranger · May 7, 2012

For me, the biggest barrier to fixing stuff is just getting started... so I take the attitude of... well since I have it all torn apart I might as well upgrade as much as I can now since I don't know when I'll have the time or inclination to tear it up again.

If you are going to have the motor mount off... I'd replace that backing plate while I was at it. Stainless is clearly the gold standard here and if money is not an issue go for it. Don't paint over the stainless.

I have found that trex (fake deck wood) makes good backing plates and I use large SS washers on the bolts. I think of it as the bronze standard

kenn · May 7, 2012

In most boats the transom has been designed for the load of an outboard, but prudent augmentation never hurts.

A full 1/4" backing plate, in steel or ss? Massive overkill and not worth the effort, in my opinion.

My approach is to try to analyze the load, and it's pretty clear that the greatest load on a motor mount occurs on the top bolts, where the weight of the engine, acting on the lever of the motor mount (fulcrum is the lower bolts) will want to pull the top bolts straight out. Ditto for the engine's forward thrust, this time with the leverage of the whole motor leg. So the goal of bracing would be to spread that force horizontally as widely as possible. To acheive that, I'd use a piece of 1 & 1/2" aluminum right-angle maybe 12 or 14" long, with holes drilled near the middle to catch the top two mount bolts. Assuming your transom is relatively flat at this point, the piece would go right against the transom, and any pull on the top bolts would be spread across the bar's width.

The lower bolts could be backed just with big washers, or maybe a 8" strip of flat or angle aluminum to add even more horizontal stability.

(Ok, my solution is overkill too, but it's cheaper and less effort. Marine ply or TREX, and big washers would probably serve admirably)

Marty A · May 7, 2012

Yep, I'd agree that a full plate is overkill. I like Kenn's suggestion best with a hat tip to the TREX solution.

Now for the boarding ladder, I'd be more supportive of a larger plate. The mounting points for the ladder will easily have far more stress placed upon them than the outboard. Just think of a supersized individual (ok, so I resemble that remark) stepping up the ladder, particularly if if your transom has a significant counter (thereby providing a significant lever arm).

And good on you for doing the repair right instead of (not) 'just good enough'.

drumbeater · May 7, 2012

I think overkill is a good thing when the alternative is the motor bracket ripping out you transom and the motor going to the bottom of the sea. Best case, worse case you boat sinks.

Really what's that piece of stainless cost? $40 maybe?

caguy · May 7, 2012

I used ss because I can pick a piece out of reminant scrap from Industrial Metals 2 miles from my school for $1.65/lbs. I usually have to buy extra because there is a $10 minimum.
I don't have to seal it like wood so I save time in using it, and it last forever. Here is a pict where I used it for my transom ladder.
Try a local scrap yard.

FastOlson · May 7, 2012

"Pastics!...."

Short answer -
log in to Mcmaster carr.
order up a piece of G10 and epoxy it to the inside when you tighten up the bolts.
http://www.mcmaster.com/#standard-plastic-sheets/=hfjwgd
"Structural Plastic"

Half inch might be overkill, but 3/8 should be fine.

It'll be the strongest part of your boat....

Delivery is about two days or so. They're quick. One of the fastest and most efficient db setups I have ever encountered on the 'net.

If you have a local plastics warehouse, they can sell you the same stuff. Ask for "G10/FR4".

LB

Merlin Clark · May 7, 2012

If your transom is curved and you use a big hunk of metal that doesn't curve, you are likely going to end up with stress cracks and maybe some laminate damage (at the edges of the metal plate) when you crank the nuts down hard enough to force the hull to meet the center of the plate since the fiberglass will give before the plate does. Better to have something that curves with the hull and still spreads the load.
The manmade lumber would be great for this if it is stiff enough to spread the load. All the decking I've seen requires additional support over wood decking otherwise it will sag. Careful of the composite as the wood used to form the "composite" in those is pulp wood and literally garbage.
The right plywood is fine for this. Marine ply is great stuff for any boat project. if you don't want to toss out the $$ for marine or if it's not available in your area, exterior fir plywood will work fine. It's the same glue and wood, just more plies, no voids and 2 finish sides on the marine ply.
The reason stock plywood backing plate rot is because boat manufacturers use cheap plywood that can't stand up to the environment.
I wouldn't go over 3/4" so that it can conform to the shape of the hull.
Just my 2c.

FastOlson · May 7, 2012

merlinuxo said:
If your transom is curved and you use a big hunk of metal that doesn't curve, you are likely going to end up with stress cracks and maybe some laminate damage (at the edges of the metal plate) when you crank the nuts down hard enough to force the hull to meet the center of the plate since the fiberglass will give before the plate does. Better to have something that curves with the hull and still spreads the load.
The manmade lumber would be great for this if it is stiff enough to spread the load. All the decking I've seen requires additional support over wood decking otherwise it will sag. Careful of the composite as the wood used to form the "composite" in those is pulp wood and literally garbage.
The right plywood is fine for this. Marine ply is great stuff for any boat project. if you don't want to toss out the $$ for marine or if it's not available in your area, exterior fir plywood will work fine. It's the same glue and wood, just more plies, no voids and 2 finish sides on the marine ply.
The reason stock plywood backing plate rot is because boat manufacturers use cheap plywood that can't stand up to the environment.
I wouldn't go over 3/4" so that it can conform to the shape of the hull.
Just my 2c.

Plywood will certainly work, but remember to coat it all with epoxy - especially the edges. Moisture is the enemy. If the hull has some curvature, use a couple of sheets of ply, like quarter inch each. (Same advice if using frp plates, too.)

Best,

L

caguy · May 7, 2012

merlinuxo said:
If your transom is curved and you use a big hunk of metal that doesn't curve, you are likely going to end up with stress cracks and maybe some laminate damage (at the edges of the metal plate) when you crank the nuts down hard enough to force the hull to meet the center of the plate since the fiberglass will give before the plate does. Better to have something that curves with the hull and still spreads the load.
The manmade lumber would be great for this if it is stiff enough to spread the load. All the decking I've seen requires additional support over wood decking otherwise it will sag. Careful of the composite as the wood used to form the "composite" in those is pulp wood and literally garbage.
The right plywood is fine for this. Marine ply is great stuff for any boat project. if you don't want to toss out the $$ for marine or if it's not available in your area, exterior fir plywood will work fine. It's the same glue and wood, just more plies, no voids and 2 finish sides on the marine ply.
The reason stock plywood backing plate rot is because boat manufacturers use cheap plywood that can't stand up to the environment.
I wouldn't go over 3/4" so that it can conform to the shape of the hull.
Just my 2c.

A 3/16 or 1/4" plate of stainless can be bent or curved to fit before installing it.

Merlin Clark · May 7, 2012

caguy said:
A 3/16 or 1/4" plate of stainless can be bent or curved to fit before installing it.

That make sense. I was thinking on a wide backing piece and a thin hull.
Your install looks great.

GorillaToast · May 10, 2012

I didn't think so for my '82 C22. 3/16th s/s plate inside and out. Cost $35.
Cheap insurance.

Merlin Clark · May 10, 2012

I stand corrected. I didn't think 3/16" s/s plate would bend that well.
I don't know what else to use to seal up ply, maybe thinned oil based paint?

thinwater · May 10, 2012

Actually, I always think of sufficient FRP, laminated to the existing structure, to be the gold standard for many applications. More stiffness per wieght because it works as a unit and no corrosion. There can be no movement over time and so sealing is easier. In my case, the builder used a foam core where they should not have:
http://sail-delmarva.blogspot.com/search?q=transom
FRP lamination fixed it nicely.

You never permanatly seal ply; eventually it cracks and leaks. Also, epoxy MUST be painted or it will fail in a few years.

All marine environments are humid, at least down low. At night the water is warmer than the air and humidity rises. Dew is common at lake's edge even in the desert.

Bad Obsession · May 10, 2012

When it comes to boats....I don't think that there is such a thing as overkill. Having more strength than you think you need is peace of mind. When you go into heavy weather and you are depending on your outboard it is one less thing to worry about.

kenn · May 10, 2012

Bad Obsession said:
When it comes to boats....I don't think that there is such a thing as overkill.

There's overbuilding... and there's going too far. For the motor mount of a fiberglass sailboat, a 1/4" stainless steel backing plate is not going to perform measurably better than a same-sized plate of 3/16" aluminum. The transom will be destroyed before either plate gets anywhere near failure.

GorillaToast · May 10, 2012

kenn said:
There's overbuilding... and there's going too far. For the motor mount of a fiberglass sailboat, a 1/4" stainless steel backing plate is not going to perform measurably better than a same-sized plate of 3/16" aluminum. The transom will be destroyed before either plate gets anywhere near failure.

In my case, the transome was already dented in at the bottom of the motor mount, so I felt I needed at least 1/8th inch plate. I chose stainless over aluminum mostly for looks. The $35 I paid included machining both plates.

kenn · May 11, 2012

A nice polished SS plate looks sexy, no question

You did inspect and repair the transom dents before covering them up, right? (I've been gelcoating all week, so I'm all about fg repair right now

)

Stainless steel plate: Overkill?

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