Deisel RIB -v -small four stroke?

[C2 RIBS]

I have been talking to a colleague who is considering buying a small up to 6 metre RIB for running regularly between Portsmouth and Cowes.
The question I cant answer relates to Deisel use. We are unsure of fuel use on a deisel boat against say a 50-70hp four stroke. We are not talking about a blast over and back but a sensible run back and forth.
Clearly there are different purchase costs even on second hand but running costs? If they settle for a small four stoke the boat size may be reduced. The boat will stay in the water in marinas so this may affect costs/servicing.

I appreciate this is a wide question which will throw up some varied responses. Anyone with thoughts and knowledge including strengths/weaknesses/price of purchase etc all welcome.

[HUMBER P4VWL]

or modern injected two stroke? if youre buying juice from forecourts.

If you can get your hands on red diesel!! ooh arrgh farmer giles

Then there's no comparison if you do any mileage.

[Polwart]

C2 - one major difference will depend on whether he can get away with claiming the partial lower rate duty (for non propulsion use). In theory it doesn't matter because technically he should be claiming only for the proportion of fuel actually used for heating etc. However HMRC have indicated that they are unlikely to look for evidence if you are claiming 60% is heating etc - but to be credible that will need a heater and a cabin. If he is looking at using it all year round for commuting those may be desirable anyway.

[C2 RIBS]

I am not sure the fuel duty reclaim is the issue here. I cannot answer fuel usage on a deisel as I never used one in anger, the fuel use on small four strokes we can work through but I/we must be missing some finer points, service costs, initial costs,
Costs for leaving the water as I lift mine almost daily and with an outboard we can lift most clear of the water but I guess this will not be rinsed through

[DGR]

I suspect that a diesel will be more fuel efficient/economic, but you need to balance that against additional costs for storing a longer boat (with an inboard) etc.

I bought diesel for economy, because red diesel was so much cheaper (34p at the time) - and also there was no petrol easily available - but there is diesel alongside. The only advantage (to me) of choosing an outboard would be that I could lift the leg higher/go shallower than I can with a B2X leg - but that's all.

For fuel numbers, I ran my 7.8 Ribcraft commercially for 3 years - and the highest fuel consuption I saw (averaged over a tank) was 20 l/hr, and that was at 35+ kts for a lot of the time. How would that compare to a 300 petrol?

Hope this helps!!

D...

[Searider]

5gph per 100hp used is a rule of thumb.

So, 120hp Mercruiser in a 6.5m will do about 33 knots for 6gph.

Slow down to cruising speed, say 22-25 knots for about half that consumption 3gph.

The problem he may have is finding a 6m RIB with a diesel engine. There's very little choice of engine to put in a boat of that size.

120hp Mercruiser is the only one that comes to mind. Will need to keep on top of maintainance and keep the wastegate moving.

Have only seen one or two small RIBs with Volvo AD31s of 130hp or 150hp which I think is a more robust engine.

D3s being fitted now but in boats of 7m plus generally.

Is he looking for new or used?

[C2 RIBS]

The discussion was for a second hand

[willk]

Parker 6.3s with Mercruisers come up fairly often. Not much below that size...

[Polwart]

Originally Posted by willk

Parker 6.3s with Mercruisers come up fairly often. Not much below that size...

I've noticed that too - not sure if there are quite a few around or its the same one being recycled a lot!

[willk]

There's a few around. At 120hp with a mercruiser lump in the back, they must be a bit gutless. Nice rib though, if you don't want to go speeding.