After a multitude of conflicting advice - this post outlines the things I have learnt should be considered if you are thinking of changing your engine
There are more myths around on the matching of horse power to boat than you can imagine. So if you hear them – stories like “one hp per foot” or “always better to have some power in reserve” – disregard them!
Boat speed (for displacement
boats) is limited by waterline length regardless of horsepower. So you need to identify the maximum hull speed
and then use one of the formulas on the web to determine the smallest engine you can use to reach that speed. Unless you frequently pull excess weight – either acting as a tug, or due to load of another sort.
People will say that if you have a diesel
engine that is over powered you can easily cause glazing (see below). Engine load is relative to throttle position, so provided the correct propeller
is fitted for the engine it very unlikely that the engine will glaze when it is working. The risk is more of an issue when the engine is run purely to charge the batteries.
Using an engine with a shaft that is too thin will cause the shaft to distort and causes serious damage to its mountings, one of which is the exit tube and that will allow water
into your hull!
The weight of the engine has to be within the design range for the boat. If overweight, the engine may cause the boat to sink a few centimetres resulting in unfavourable sailing performance, exhaust
being more easily flooded with water
(which can be pushed back through the exhaust
valves into the cylinders). HOWEVER for my boat (34’) 750kg of additional weight will drop the boat about 3.5 cm (1.4”) in the water (350kg makes this 1.7cm) if the motor
is centred – maybe 1.5 times that if not! I can’t see that this can be an issue! After all the diesel
and water tanks
would contribute over 400kg. The heavy engine I have been considering weighs 750kg, but the one that is in place must weigh half that.
Remember that the engine must be moved into the engine space (and the old one removed) if it is too large (particularly if it has to be man handled down the companionway
and into place) it can result in damage to you or the yacht finishes or even not fit!
The fitting of the new engine needs to be appropriate for the engine bed
. There are two major aspects. First, the engine must be aligned with the shaft, and there are two major possibilities - the shaft may extend horizontally out of the crank case, or it may go through a gearbox
that effectively runs parallel to the crankcase at a lower level. Second, the engine mounts on the two rails in the engine bed, if these are not sufficiently far apart, or if there is insufficient room below the level of the mounts there will be a problem.
If your new engine differs from the old on either of these it may be necessary to rebuild
the engine bed. Such a rebuild
can be done, but adds to the expense and pain.
If the exhaust volume is significantly different (exhaust volume is dictated by a combination of RPM
and engine capacity/displacement) to the old there is the possibility that the old exhaust system may not allow sufficient gaseous exit. The ideal exhaust is determined by a mixture of volume of gas exited and velocity of gas exited. Thinner pipes increase velocity - so thick pipes is not the easy answer! Building back pressure will be noticeably detrimental on performance. You may have to restructure your exhaust system - remember to protect the engine from a water influx via the exhaust!
If the new engine and old engine are cooled differently - e.g. old is raw water
cooled and new is clean water cooled then it may be necessary to upgrade the water inlet piping, stop cock and strainer. This is not a biggie - but again adds cost.
It is quite likely if you change your engine for a new one, and the hp of the engines are mismatched that a new propeller may be required. Again - not prohibitive on its own, but can add to an already bulging price
So after all that – what conclusion have I reached – well you won’t believe it but for every opinion I have others to contradict. Is it really a case of you can have any engine size you want, so long as you can deal with the consequences – as set out above?
Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil
film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.
This poor combustion leads to soot formation and unburnt fuel
residues which clogs and gums piston rings, which causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly-sealing piston rings, causing the lubricating oil
on the cylinder walls to 'flash burn', creating anenamel-like glaze which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface which are there to hold oil and return it to the crankcase via the scraper ring.
Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption
(blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures.
past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.
The problem is increased further with the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.
This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.
Under-loaded running inevitably causes not only white smoke from unburnt fuel but over time will be joined by blue smoke of burnt lubricating oil leaking past the damaged piston rings, and black smoke caused by damaged injectors. This pollution is unacceptable to the authorities and neighbours.
Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning
and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures allows the piston rings to scrape glaze off the bores and allows carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves, this will not have any effect.