Outboard Conversion On A Gemini 105M

I recently repowered my Westerbeke and Sillette equipped Gemini with a Honda 15 HP high thrust outboard. Overall the outboard conversion wasn’t that complicated but it did take a few days of work to complete. The work was all done with the boat in the water.

Here is a summary of the entire process. I’m not going to go into great detail on every little step, because I’m assuming that as a Gemini owner and just as a boat owner you’re already familiar with some of the tasks involved.

Removing The Sillette

The first thing I had to do was drain some of the oil out of the top of the Sillette drive and remove the drive from the boat. As usual, one of the pivot pins was stuck, and would only come out part way, but it was enough to get the drive off without much trouble. The next step is to remove the transom plate.

There are 6 screws holding it on that are relatively easy to remove with the exception of one that’s under the muffler, which still isn’t much of a problem to get out. After the screws are removed you need to pry the plate off of the transom, which can be a long process if you’ve never done it before, thanks to PCI’s excessive use of 5200.

Removing The Westerbeke

This step isn’t necessary, but if you’re doing an outboard conversion you’re going to remove the diesel at some point anyway. Since you’re already removing the exhaust thru hull and patching the transom, you may as well remove some of the parts now, because it will give you more room to work. The isolator coupling is attached to the transmission by 2 bolts that are easily removed as long as they’re not seized.

The muffler is held in by 4 screws. The one in the back corner requires an extra long screwdriver to get to…or you can just break the corner off like I did. Once the muffler is out you can remove the shelf either with a chisel, or if you smarter than me a Dremel or Rotozip will cut it out much faster.


I was originally going to grind a full bevel on the holes and do a proper fiberglass patch. After thinking about it, though, I realized it’s not going to be load bearing so I wanted to just fill them in as quickly as possible. First I epoxied a couple of pieces of wood in the holes as you can see in the photos.

I know it doesn’t look too professional, but the sole purpose of the wood was to be a filler so I didn’t have to lay up 100 layers of cloth. Next I filled the gaps around the wood with thickened epoxy, with pieces of fiberglass cloth and woven roving mixed in. Finally I laid up just 2 layers of cloth on both the inside and outside of the patch, and faired the outside.


I painted over the repair with Beyer Ultra exterior paint from Home Depot. The purpose was simply to protect the epoxy from UV exposure. It costs a fraction of what gel coat or any marine paint does. My gel coat isn’t in the greatest shape as it is, and my decks and sides are due to be repainted in the not too distant future anyway.

So it just doesn’t make any sense to spend the extra money and drive myself crazy painting a small patch now. Also the hole from the drive is almost entirely covered by the engine bracket anyway.

Engine Mount

I traced the holes from the engine bracket onto a piece of parchment paper that I taped to the transom as a guide for drilling. Why parchment paper? Because that’s all I had. I used all 12 holes on the bracket, because according to Garelick the more the better.

After consulting the oracle and a few boating boards it was determined that skipping the middle holes and going with just 8 screws would have been more than enough. The holes were all countersunk, and the screws were put in with 5200, just enough to form a gasket, not an entire tube like the old transom plate.

Trim Switch And Dashboard

I cut a hole for the trim switch with a Dremel just below the engine control. In the middle of the project someone bought the admiral panel from the Westerebeke. This left a huge hole which forced me to cut a new panel. If I had known ahead of time I may have installed the trim switch right on the dashboard.

I cut the new dashboard out of Starboard. It may not be as nice and shiny as Acrylic, but nothing on my boat is. More importantly it’s much easier to work with, and there was much less chance of cracking it when cutting the holes for the instruments.

Cables And Harness

I reused the control and the cables from the diesel, both to save some cost and to make the install a little easier. The gear shift arm had to be reversed from the Westerebeke, and the cables do need a little adjustment. The wiring harness was run to the helm using the kill switch cable from the Westerbeke to pull it through.

Both the control cables and the wiring harness run out the back of the boat through the hole from the old hydraulic line. The hole did need to be enlarged to 1-1/2″ to allow enough room for the multi pin plugs on the harness. When running everything out the back of the boat it’s easiest to start with large connector first, the smaller connector, and then the control cables.

Steering Lines

An eye bolt was put in on each side about half way between the engine and the sugar scoops, with a small block attached. This allowed me to use the steering arm on the engine rather than drill holes in the back of the engine to attach the steering lines. The lines lead from the blocks to a large eye bolt on the engine and up to the cleat.

Each line is run through bronze ring that is attached to the eyebolt on the engine with a carabiner. The reason for this is because when the engine is tilted up, the steering arm tilts down. This causes the steering lines to tighten, so one or both of the lines to be unclipped when sailing, so the engine doesn’t turn when it’s tilted out of the water. This is still a work in progress and I may make one last small adjustment.

One Last Thing

Since the engine is on a catamaran some people recommended a wave diverter. Others didn’t think one was needed. The bridge deck on the Gemini is less than 1′ from the water at the stern, and the engine bracket provides some protection, so I skipped this for now. Going down the ICW the few times the engine did get hit with a decent splash, it was the wake from a boat, and from the side. A diverter wouldn’t protect it in this instance.

It did take a little more splash coming down the bay, because of the high winds. But even then it seemed like most of the splash was from the engine planting as the stern pitched down, not from it getting hit head on with a wave. I have a piece of 1/2 starboard that I may fasten to the bracket to act as a wave diverter.

I still haven’t decided if its really needed, though. What do you think?

2 Responses to Outboard Conversion On A Gemini 105M

  1. Nice work on your conversion. We have a similar conversion with a 40hp (and custom long-shaft) – here’s a clip: https://www.youtube.com/watch?v=U0O_BcArQHE

    My biggest annoyance with the setup is all of the rigging required between the outboard and the tiller arms at the rudder posts. My goal over the winter is to get rid of all of those cables, pulleys, and springs – replaced with a hydraulic steering ram attached to the lifting transom. (Specifically so I can also easily decouple the motor from the rudders while sailing to get rid of the “flip-flopping” motor every time we tack).

    I’m not sure a wave diverter would really do anything other than slow you down. On ours, I’ve noticed that as the bow waves start to build and “swamp” the motor, if we raise the hydraulic life (vertical, not tilt), it keeps everything happier (as long as you are sure to put it back down when you slow in port (to keep the impeller below the water).

    • Thanks Steve. Wow, that’s a really nice setup! You even went with an electric jack plate. I agree, the steering setup on the Gemini is annoying. I’ve had some in depth conversations with another own, while delivering my boat, about hydraulic steering an other possible improvements to replace those lines.

      Thanks for the tip on the diverter. I still haven’t added one, and don’t really plan on it at this point.

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