Diesel Inboard or Petrol Outboard?
Deciding which type of power unit for your RIB is an important decision. This guide provides an overview of the age old debate of the two main types: diesel inboard compared with a petrol outboard. It also compares 2 stroke outboards with 4 stroke outboards.
Cost of fuel: Diesel compared with Petrol
The Treasury Minister, John Healey, confirmed on 7 December 2006 that the European Commission has rejected the UK Government's application to renew the UK's derogation on red diesel for private recreational boaters.
The difference in fuel price between duty paid petrol and low duty diesel on marina fuel berths was about 55p per litre. The current difference between diesel and petrol available on marina fuel berths is a few pence. Generally an diesel inboard would consume less fuel per nautical mile, than a petrol outboard. This combined with the significant cost differential meant that it was a fairly simple equation to calculate how many hours at sea you needed to justify the additional expense of a diesel inboard, together with the higher installation costs. The calculation today is not so straight forward.
Longevity of a diesel inboard compared with a petrol outboard
Many years ago the were significant differences between the longevity of petrol outboards compared with diesel inboards. However, today with the advances of modern technology there is little, if anything to choose between the two types. There are many outboards still going strong after a couple of decades of leisure use. The key with all things mechanical is to ensure the engine is serviced in accordance with the manufacturer's instructions. In any event, it is a relatively simple option to carry an auxiliary outboard for a RIB. It might not get you home very fast, but it should have sufficient power to get you out of danger.
Installation and Layout
There are important installation and layout considerations. Essentially a diesel inboard installation is heavy than a petrol outboard of equivalent power. However, the diesel inboard option locates this weight lower in the RIB. Whereas the petrol outboard raises the RIBs centre of gravity and moves it aft.
Another key consideration is that the installation of a diesel inboard will take up space inside of the RIB. For some, this is considered an advantage as it provides a useful sun bathing platform. For others, it is simply a waste of valuable space.
Finally, large capacity outboard engines require transoms to be more heavily built to withstand the enormous stresses.
Safety aspects of diesel compared with petrol
If you ask any sailor whether they would prefer a petrol or a diesel inboard ... probably 95% would say diesel. The few that would prefer petrol usually have a single deciding factor that is often unique to their situation. This is because diesel is inherently a much safer form of fuel than petrol.
Availability of fuel
On the south coast heading west, a popular cruising stretch, there is petrol available in Poole Harbour, Weymouth, again in Dartmouth, Plymouth and then nothing until Falmouth.
Trailer towing and weight
The weight of the different engine options is something else to consider. Modern petrol outboards are much lighter than equal power diesel inboards. For towing your RIB around to different launch sites this has an implication over and above the RIB itself, an outboard powered RIB is much easier to tow than one with a 500Kg inboard engine. This may mean the difference between towing with a normal family car, or having to resort to a 4x4.
Residual Value
The residual value of your RIB is also a worthy consideration to influence decision making when it comes to choosing the engine(s) for your RIB. Diesel engines have always enjoyed higher residual prices.
Conclusion
There are many factors to take into account when deciding whether to install a diesel inboard or a petrol outboard, including cost of fuel, cost of the engine, cost of installation, together with practical implications of installation, operating performance, towing weight, etc.