Log XXVIII - Artemis 1 (Last Days of My Engines) - Part 3

What to consider when choosing a suitable diesel engine to replace your current petrol engines?

1. Engine Power

Of course when you change your engines, you will want your boat to go as fast if not faster than your current cruising speed. Therefore it is important for you to know what is the required horsepower to power your boat. Take note that petrol engine produces less torque than diesel engine. Therefore you need not find the same horsepower diesel engine to replace the current petrol one. However modifications to the propellers have to be done to utilise the higher torque produced in diesel engines.

2. Engine Room Size

Diesel engine generally takes up more room than petrol ones. So if your current engine compartment is a just nice fit, it is very likely that you have to choose a lower horsepower diesel engine to fit in. But as mentioned above, lower horsepower does not mean less torque which does not mean slower cruising speed. Also take note that you cannot use up every inch of space in the engine compartment as you will need access to do the usual servicing of the engine, such as changing of engine oil/ fuel filter, impeller, belts, etc.

3. Gear Box

Photo: Gear box for cars but similar concept as boats except that for boats, there are only forward and reverse gears.

You can use back the same gear box when you change the engine. But you may require an adapter so that the new engine can be bolted onto the existing gear box. The position of the gear box is actually the benchmark of where your new engine should be seated. Measurements of the new engine positions are all taken reference from the connecting point between the gearbox and the engine body.

4. Propellers

As mentioned earlier in item 1, propellers have to be modified to utilise the higher torque from diesel engines. There are two ways of increasing the torque of a propeller. One is to increase the diameter, and the other is to increase the pitch. Increasing the diameter is limited to the clearance of the prop to the soffit of the hull. Too little clearance will result in excessive turbulence which reduces the efficiency. I will suggest modifying the pitch of the props instead. With a deeper pitch, more water is being pushed out in one rotation hence more power in one rpm. However do not rush into this modification until everything is completed and the speed of the boat is clocked with various rpm. Only with sufficient data can you or your mechanic make a decision of how many pitches to be adjusted.

5. Condition of the Diesel Engine

Needless to say, this is one of the most important thing to consider. You will not want to install a chunk of problems into your boat, for which I did. So there will be a lot more posts on engines in the future than I wish for. Anyway, a few key things to check on the re-conditioned engine - condition of the pistons and cylinders, heat exchanger (is there any leakage?) and gaskets/ seals (make sure every gasket and seal is replaced). There are of course many more stuff to check and you will find out along the way.