Making new engine stringers

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Back to Home ... s/v Stella Blue home ... Repower Page ... Projects

May 2006 --

This engine has been sitting on this pallet since November 2005, and I have hauled out for the express purpose of rebuilding my stringers to fit the new engine mounts.
My old Perkins had 24" feet, but
this engine only has 16" feet.
That's a big difference, but after talking with a bunch of folks, I began to believe that I could fit this engine in my boat, so I bought it.

The final big step is to modify the stringers to support the new engine. Before hauling, I checked with the yard boss to make sure that it was a good time and that the right people would be available to do this job.

That turned out to be a problem.

The new stringers will be about 1-3/4" higher
at the rear of the engine (forward in the boat.)
At the front of the engine, they'll be about 3/4" higher. (As we found out later, that was a mistake. It should have been only about 1/4" higher.)
Unlike my old Perkins, the new Universal M40B
sits much more level.

To change the angle, I knocked a bit of glass off the top of the old stringers, where they have that odd "hump". It was interesting to learn that the hump was lined with plywood for reinforcement.

The whole area was ground down
to glass laminate. One can't
fiberglass the new stringers onto paint.

This is a key picture.

Exactly where the new stringers should be built had always been a big question. The yard mechanic put this jig together, but then the project stalled.
So I took another day away from work and drove down there, cornered him, and made him show me exactly how it was supposed to fit
so I could build the stringers.
The trick is to set the engine on top of two planks and mark where the mounts sit, then fit another piece of wood over the bushing where
the shaft mates with the transmission.
It's important to get the angle correct,
so the bushing fits flush against the board.

Once the template is all glued together, the whole thing is moved onto the boat. By putting the bushing on the shaft and making the template flush with the bushing, the stringer locations and
angle are determined.

With a V Drive, it was a bit tricky.

The stringers are made from G10 fiberglass, from McMaster-Carr. G10 is incredibly hard and strong, and is really tough to work with.

You can see from the picture below that the glass to resin ratio is very high. The resin is
some kind of epoxy.

To get the angle just right, I cut some scrap fiberglass into little blocks, and put a big glop of epoxy on top. The epoxy is thickened up with
fiberglass chop, colloidal silica,
and high density filler.

Then I covered the template with waxed paper and set it down, making sure the shaft bushing was positioned correctly.

The bushing actually didn't fit perfectly flush, but the mechanic at the yard said it was close enough that the engine mount adjustments would handle
the difference.

Note the *small* holes in the old stringer base. It's hollow there, but since I'm going to brace the new stringers down to them, I'm going to
fill that gap with epoxy.

Here's a view of the stringers trial fit in place.

The gaps between the old stringers and the new surface will be filled with scraps of G10, packed with epoxy.

For this application, I used
West System epoxy, thickened up with high density filler and colloidal silica. The high density filler has the best compression characteristics, and the colloidal silica helped keep it from
sagging out of the gaps.

I cut the stringers to match the template using a metal wheel
on a Skil saw, and added braces to hold them in position.

Then the bottom faces were roughed up with a grinder.

Not all of the little blocks are G10. I used whatever scraps were handy, since grinding the G10 down was a real job.

That stuff is really hard.

Then the whole thing was epoxied into place. It took about seven hours.

Here's a view the next morning.



It took about three hours to grind the surfaces down and smooth everything out to prepare for a good epoxy bond.

Then I glassed the living heck out of it. That was another six hours.

There are four layers of glass over the back edges, and two on the sides and on the braces.

The idea is to leverage all the strength in the existing stringer construction for the new stringers. They really aren't tabbed down
to the hull.

Six weeks after haul, they still hadn't started. I got some other things done while waiting, but I ran out of patience and humor. I didn't want to take another afternoon from work to drive down and discuss it and have a wonderful conversation about what was going to happen. I wanted something to actually happen.
To make a long story short, I started using the "F" word effusively and did it myself.

This fiberglass tape has two layers of unidirectional strands, that run perpendicular to each other in an X pattern.
At Tap Plastics, it's called Knytex, but other fiberglass shops just call it "X cloth with mat."

The mat is not impregnated with styrene soluble binders, so it works with epoxy. The whole thing is sewn together and is pretty thick, so it's easy to work with and is very strong. Using this, four layers of cloth is really twelve layers, but goes down
much faster and easier.

Here's a good look at one of the center braces.

I laid really thick epoxy fillets along the joints, and then when I painted the surfaces with thin epoxy to wet the surfaces out, it also smoothed down the fillet.

Here's another huge epoxy fillet, that just smooths down the transition from the new stringer surface onto the old stringer.

The tape runs down to the end of the stringer, and wraps over.

Day three of this project.

It took about three hours to grind it smooth and form the final shape. I used an electric grinder for the surfaces, but a small air powered die grinder with a 1-1/2" 24 grit sanding pad for most of it. The little slot between this stringer and the liner required my Handy Dremel Tool with a sanding wheel.

Then the whole boat was hosed down (again) to get all the dust out. During that part, I wet sanded the stringers with 60 grit so they're really nice and smooth.

I wanted to make sure there were no little sharp edges where fiberglass fabric sticks out. That's important. In the years to come,
I want to be able to work back here
without cutting my hands up.

Here's another look at that brace, after it's been ground and sanded into shape.


The whole objective is to make sure that these things are strong enough to support the engine if the boat's knocked down.
I think I can be heeled over without worrying about anything moving.