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Old 20 August 2010, 10:29   #1
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Deisel RIB -v -small four stroke?

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.
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Old 20 August 2010, 10:34   #2
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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.
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Old 20 August 2010, 10:36   #3
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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.
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Old 20 August 2010, 11:17   #4
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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
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Old 21 August 2010, 03:09   #5
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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!!

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Old 21 August 2010, 08:31   #6
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Diesel useage

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?
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Old 21 August 2010, 12:22   #7
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The discussion was for a second hand
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Old 21 August 2010, 12:40   #8
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Parker 6.3s with Mercruisers come up fairly often. Not much below that size...
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Old 21 August 2010, 12:51   #9
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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!
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Old 21 August 2010, 12:58   #10
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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.
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Old 22 August 2010, 02:48   #11
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I have a feeling the size of boat is going to be inefficient for short crossings with an inboard diesel, also the difference in purchase price will leave a fair amount over for a few liters of petrol.

We do 40-60 mile rounds trips in our 6m boat with a 90HP four stroke and have found it a very economical way to travel.
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Old 22 August 2010, 14:44   #12
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6.0m and 6.5m Coastlines with 120hp mercruisers come up from time to time.

Borrowed one for a day before I got the Scorpion. Was getting into its stride at about 30 knots just as it was running out of steam.

I decided that for my use i'd pay the extra fuel cost of an opti.

Have seen a 6.5m Solent with a diesel as well.
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Old 23 August 2010, 09:49   #13
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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

Rinsing through probably not an issue if it's being used daily........
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Old 24 August 2010, 16:28   #14
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Hi I have Humber 6.3 ocean pro with volvo D3 160hp 170lt tank 4 blade 21 pitch

I have just done 160nm on 160 lts of diesel
approx 90 nm at 20/25knt 2500/3000rpm economy cruising
70nm anything from dead slow to wot

Humber 6.3 ocean pro with verado 150. same mileage same speeds same fuel consumption 3 blade 23 pitch
(Portsmouth to Cows and back approx 40lts)
petrol £1.12 lt
Diesel £0.80 lt

both boats are used in the Solent

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Old 25 August 2010, 02:20   #15
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... same mileage same speeds same fuel consumption .... both boats are used in the Solent
Interesting. So the boats are the same apart from top speed, fuel cost and purchase cost?
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Old 25 August 2010, 06:07   #16
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1L per nm sounds a bit thirsty for a diesel? or is that normal? I thought they were a lot more frugal
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Old 25 August 2010, 06:53   #17
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1L per nm sounds a bit thirsty for a diesel? or is that normal? I thought they were a lot more frugal
The yammie 285HPs cruise (c.2900rpm) at 1.1L Nm. Dunno about small units.
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Old 25 August 2010, 08:58   #18
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250 F/Stroke Yamaha about 4500 to 5000 rpm approx 35 kts = 1.25 lts a N/mile
on a run say Hayling Island to Cowies or Yarmouth
the comsumsaion dose very acording to how many people on board and how much fuel in the tank which is 400 lts
Full service at Lansdale Marine Chichester £325 with a new impeller
i hate to think how much a diesel service would be then there is the cost of maintaining the Leg
Still bloody exspensive
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