Repowering Your Engine

Undoubtedly some of you will have old, tired engines that need some attention and often it comes down to a choice between rebuilding the existing unit or replacing it altogether.
In these situations it can be very easy to be tempted in to re-building an engine as this appears to be a cheaper/easier option than re-powering.  However, this can often prove to be a false economy, particularly with old raw-water cooled engines as more harm than good can be done through disassembly, due to a high level of internal corrosion.  This corrosion is a result of years of direct contact with salt water.  Engines built with wet cylinder liners can suffer similar issues even after a costly rebuild.  They can still be likely to suffer from oil emulsification.

If your current engine is always a problem to start, is low on power, constantly admits excessive smoke from the exhaust, leaks or uses a large quantity of oil, then its maybe time to think about replacing your engine altogether.  Particularly if your engine presents more than one of these symptoms.

The biggest advantage repowering has over rebuilding is you can be sure any or all of the above issues will be cured in one hit.  Additionally most modern diesels will be smoother running, quieter and more fuel efficient than engines of a previous generation.  And another bonus with a new engine is the availability of spare parts.  After all, sourcing parts for an older model can prove to be timely, difficult and expensive.

So….you’ve decided on the repowering option.  How do you go about choosing the correct engine for you?
An important consideration is your required horsepower (hp).  This is a personal preference, particularly in sailing vessels depending on how often you use the engine in your boat.  For example, if you mostly sail and only use the engine for manoeuvring in and out of port, you may feel inclined to have less hp than someone who wants to cruise at hull speed for sustained periods.  A rough guide to hp requirements in boats is 3-4hp per ton for racing yachts and 5hp per ton for cruising.  It is important to have a decent idea of the designed hull speed of your boat, as once you have reached hull speed it is impossible for displacement hulls, such as with sailing boats to go any faster, regardless of hp.  Another consideration is the size of prop that your vessel is able to swing.  There is a limit to the amount of hp any given size propeller can transfer to the water.  If prop and engine are imbalanced, i.e. the hp is too great for the diameter of prop, you’ll gain no benefit from the extra hp you have invested in.  The ideal scenario is to have an engine and propeller balanced to provide you with enough hp in the water to drive you comfortably at hull speed in ideal conditions with some power in reserve to deal with unfavourable winds and tides.

There are many other factors to consider when re-engining such as physical size of the engine and space available.  Typically a new engine will have a different footprint to the engine its replacing so it is important to have a plan as to what modifications may need to be made to your engine beds and surrounding space.  Whether you intend to install your engine yourself or have an engineer do it.  Installation issues, such as fuel & exhaust systems and whether they need to be replaced or not will be specific to the engine that you ultimately choose.  Ensure you discuss these issues with the relevant experts.