Why Most Plymouth Pilots Use Inboard Diesel Engines

I was going through an old photo album recently and came across a picture of a Pilot 16' with an outboard mounted in a well at the stern. It is not the only one I have seen over the years, but they are rare enough that it always stops me for a moment. The boat in the photo looked perfectly tidy. It had been built deliberately, the well was properly finished and the owner had clearly thought it through.

It is a useful reminder, because the standard answer at Plymouth Pilots is an inboard diesel. That is what we build and that is what nearly every Pilot on the water has. But the photo is a fair prompt for the question that comes up often: why is that the default, when most small boats at the marina seem to have an outboard?

The short answer is that the engine has to suit the hull. The longer answer is worth a few minutes of your time, particularly if you are weighing up a Plymouth Pilot against other small boats.

The hull decides the engine, not the other way round

A Plymouth Pilot is a displacement boat. It moves through the water rather than climbing on top of it. The hull has a long, solid keel running the length of the boat, a pair of bilge keels either side, and a skeg at the stern that protects the propeller. The cockpit sits low, the centre of gravity is low and the boat is heavy for its length.

None of that is accidental. It is what gives the boat its directional stability, its ability to take the ground level, its steady motion in a chop and its willingness to keep going when the weather turns.

The engine has to match that brief. On a displacement hull, more power does not give you more speed. The Pilot 18' cruises at around 4.2 knots and tops out somewhere near 7.5. Fitting a larger engine does not change that. It just burns more fuel, makes more wash and adds weight in places you would rather not have it.

What the hull actually needs is steady, predictable thrust at modest revs, hour after hour, year after year. A small marine diesel does that job about as well as anything ever invented.

What an inboard diesel does for the boat

Three things in particular.

It sits low and central. The engine is heavy and where you put heavy things on a small boat matters. An inboard diesel sits in the bilge, close to the centre of the boat and low down. It pulls the centre of gravity in the right direction. You feel that under way, especially when the boat is loaded with people and gear or when a swell catches the stern.

The propeller is in the right place. A shaft-driven propeller sits under the hull, in clean water, just ahead of the rudder. It pushes water across the rudder, which means the rudder actually steers the boat at any speed, including dead slow against a tide. That matters when you are picking up a mooring single-handed, coming alongside in a crosswind or holding station while someone steps off onto a pontoon.

Everything is protected. The keel and the skeg sit lower than the propeller. If you touch the bottom, which on a tidal mooring you will eventually do, the boat takes the ground on its keel and bilge keels. The propeller stays protected. On a boat that is going to spend twenty or thirty years in tidal water, that is not a small thing.

Why diesel rather than petrol

The other half of the choice is fuel.

Diesel is less volatile than petrol. On a boat with an engine compartment inside the hull, that matters. Petrol vapour is heavier than air, so any leak settles in the bilge, which is exactly where you do not want it. Diesel does not behave that way. A small marine diesel, properly installed, is a calm thing to live with.

Diesel engines also tend to last. A well-looked-after Beta or Yanmar in a Plymouth Pilot will run for a very long time without much drama, provided the basics are done: oil and filters once a year, raw water impeller when it needs it, coolant every few years and a careful eye kept on the stern gland. None of that is difficult and both Beta and Yanmar offer a five-year self-servicing warranty for leisure use, which a competent owner can take advantage of without much trouble.

Annual service costs sit in the £250 to £350 range if you pay someone to do it, less if you do it yourself. For an engine that might give you twenty years of useful life, that is not a heavy bill.

What outboards do well and why they are not the standard here

Outboards are popular for good reasons. They are simple to fit, simple to lift off and replace, light, easy to service and they get out of the way when you tilt them up. On the right hull, they are an excellent answer.

The right hull is usually a lighter, planing or semi-planing boat, often designed from the start with the outboard in mind. The transom is built to take the weight. The hull is shaped to climb out of the water. The boat is meant to be quick and the engine is meant to make it quick.

A Plymouth Pilot is not that boat. The hull is built to plough steadily through water, not to climb on top of it. Hanging a large outboard on the back puts weight aft and high, which on a heavy displacement hull affects the trim. The propeller ends up behind the transom rather than under the hull, which changes how the rudder works at low speed. And the working parts are no longer protected by the keel.

Those are real differences. None of them make outboards bad. They make outboards a poor fit for this particular boat.

The Pilot in the old photograph

The boats like the one in my photo do exist. They are uncommon and most of them are older builds where an owner wanted something specific, often for ease of maintenance, lower upfront cost or because their use case was unusual. They can be perfectly usable boats.

If you come across one on the second-hand market, the questions to ask are about the conversion itself. How was the well built. Has the transom been properly strengthened. Has the trim changed noticeably. Does the boat still sit on her lines when loaded.

None of those are dealbreakers, but they are different questions from the ones you would ask about a standard inboard Pilot. A surveyor will take a careful look. So should you.

What a first-time buyer should take from this

If you are looking at a Plymouth Pilot and you have been wondering whether the inboard diesel is just tradition, it is not. It is the engine the boat is built around.

The hull, the keel, the skeg, the cockpit layout, the weight distribution and the way the rudder works under power are all parts of one design. The diesel inboard is another part of the same design. Change one and you change the rest.

That is why we build them the way we do, and that is why most Pilots you will see at moorings and slipways look the way they look. The boat and the engine are doing the same job.

If you want a boat for steady cruising on the south coast, for fishing the same marks year after year, for taking your family out for the afternoon and bringing them home dry, the standard Plymouth Pilot is a well-considered answer to that question. The engine choice is a part of why.