Don't get me wrong, I am not against diesels for propulsion
, I have 2 on my current
cat. However, given the right circumstances, petrol outboards can be a good option.
First, the boat must sail well to windward and in light winds. However, if the owner chooses to motor over sailing, then to me that tips the balance towards diesel. However, if sailing is the thing, lifting those outboards clear of the water is a big advantage.
Petrol is widely available, basically where there is powered road transport cars, scooters, motorbikes etc you will find petrol. Agreed it may not be at a refuelling berth.
Petrol may go stale when stored for a long while, but in 30 years of bring my spare 5 gallon containers ashore for a UK winter storing in my shed and then re-using them the following season, I never had a problem. Diesel can have the bug problem which does cause problems. The only problems I experienced with petrol was contamination with 'bits' which were easily filtered while filling the service
While most outboards are designed for lightweight relatively high speed boats, there are a number of outboards designed as sail auxiliaries. These have a larger, slower revving prop so are designed to give a high thrust at slower speed. They are usually 4 stroke
and hence very fuel efficient compared to a 2 stroke
although they will be slightly higher consumption
compared to a diesel. I am only referring to these high thrust types.
Outboards are prone to cavitating due to ventilation when the prop is too close to the surface due to pitching. Extra long shafts and siting the o/b as far forward as possible or in a well will help.
Cost and weight are big factors in favour of an outboard. You get the whole propulsion system and even a tank for ~£2000 and 45Kg (Yamaha 9.9 High Thrust in UK used to power my previous 10m cat). Plus you can pick it up and put it in the car to take home to service. That's about the cost of a decent folding prop. (I know, as I am looking at buying
2 folders to mitigate towing 2 windmilling sea brakes through the water when sailing)