Fuel consumption on 7.5 & 8.0 ribs

smokie · 08 February 2005, 16:44

I would like to someday buy a 7.5 to 8.0 rib to make a long trip where there are few fuel facilities. I favor outboards simply because I do not want space taken up inside the boat.

Can we hear from people that have ribs in this range.

What brand rib?
Single or twin outboards?
Engine brand?
Two stroke or four stroke?
Size of fuel tank?
Speed and burn rate at optimum cruise.

Brambles · 08 February 2005, 20:20

Quote:

Originally Posted by smokie

I would like to someday buy a 7.5 to 8.0 rib to make a long trip where there are few fuel facilities. I favor outboards simply because I do not want space taken up inside the boat.

Can we hear from people that have ribs in this range.

What brand rib?
Single or twin outboards?
Engine brand?
Two stroke or four stroke?
Size of fuel tank?
Speed and burn rate at optimum cruise.

Hi Smokie,

Here are some thoughts:

Go for single (big!) outboard, plus small aux engine to get you home.

Engine brand - new 'rude Etec is probably the most efficient 2 stoke available now. The Suzi 4 strokes look good on paper. In my view the Merc Verado is too heavy to offer any real advantages & I have heard of a few mechanical failures on early models.

2 stroke or 4? Well, the latest generation of 2 strokes seem to be very efficient and are much lighter than the 4 stroke alternatives. It may all come down to whether you want the hassle of topping up the oil on a regular basis.

The size of fuel tank will probably be set in the boat spec by the builder, but at least 230 litres I would say.

Fuel rates on the Scorp with the Etec 250 (with a 26" raker prop) at 38Knts we were burning 15gph, at 44knts 18gph & 48knts 21 gph. All US gallons. 35 knts is probably the optimum cruise speed.

On the question of which brand, my preferance is well documented on this site. However, what I would say is which ever brand you chose, go for the 8m rather than 7.5, even if it means spending less on the engine of choice. We have done some tests in the same brand of boat and the increase in sea keeping & comfort is tangible in the larger boat.

All IMHO of course!

Regards

Ian.

gtflash · 09 February 2005, 12:51

hows the new engine brambles

Squirrel · 09 February 2005, 13:08

What brand rib? Prosport 7.5
Single or twin outboards? Single 200hp 2 stroke V6 with 4hp aux.
Engine brand? Yamaha
Two stroke or four stroke? 2
Size of fuel tank? ~60 gals*
Speed and burn rate at optimum cruise 30-35kts 2 litres per nautical mile.

*Fuel tank was 100l (20 gal ish?) but sprung a leak, so has just been replaced and enlarged at the same time. 100l was restrictive in that it was fine for going on a jolly, but if you want to open it up or go wakeboarding, its gone in no time. 50 miles range isn't really that much if you want to keep some in reserve.

The new tank is custom built by pro-sport to fill the bilge under the console and quite a long way forward (ie between the front and back bulkheads) and no-one is quite sure how big it is, but the estimate is at least double. We'll find out at the weekend when its back in the water! They now fit 300l tanks as standard.

My fuel consumption is pretty accurate because I spent most of the summer on 20l auxiliary tanks and had to watch the fuel levels closely!

The auxiliary engine is a new addition for the season too. 4hp is pretty weak for a big RIB, but I'm told it should be enough to potter back and keep out of trouble. Went for a 4 stroke with its own tank on the basis that it could run off the main fuel tank provided fuel contamination isn't the problem!! Went for Yamaha purely to keep the servicing all in one place.

JIY · 09 February 2005, 17:48

Sorry to sidetrack this thread, but I'm a little stunned at the fuel consumption figures quoted here. My 8.5m Tornado with 250hp Yanmar burns just under 6gph marine diesel at 33 knots (that's a little under one litre/nautical mile), with 4 people aboard, a full 200 litre tank and slight/moderate seas. I reckon the boat weighs a little over 2 tonnes with this load.

Even if the marine diesel VAT exemption is removed in the UK, this still makes diesel appear to be the most economical way to RIB (and the most practical up here in Scotland, where very few marinas have petrol pumps).

Admittedly initial purchase costs for a new diesel boat are high, but the price differential between diesel and petrol second-hand boats is significantly less.

What sort of fuel burn are others experiencing with this size of boat and a petrol OB?

Brambles · 09 February 2005, 18:06

Quote:

Originally Posted by JIY

Even if the marine diesel VAT exemption is removed in the UK, this still makes diesel appear to be the most economical way to RIB (and the most practical up here in Scotland, where very few marinas have petrol pumps).

Admittedly initial purchase costs for a new diesel boat are high, but the price differential between diesel and petrol second-hand boats is significantly less.

Good point JIY but,

how much more did the diesel version of your boat cost, compared to the petrol O/B equivavlent? I can't remember the extra cost for a diesel version of the 8.1m Scorp, but it was many thousands of £, which equates to one hell of a lot of hours before "you get your money back". It is a similar scenario to diesel engined cars v petrol equivalent. Last year a magazine published figues proving that a diesel Ford Focus type of car is more expensive to run that the petrol version over the average time an owner keeps the car.

Then factor in the uncertainty of diesel tax concessions and it starts to look like an expensive option.

The availability of petol is a very valid point though!

Regards.

Ian.

JIY · 09 February 2005, 18:16

Hi Ian,

I paid £40K for the boat new 2 years ago - I notice that Barnet have just sold their boat show demo 7.5 Tornado with 250 OB (Mercury Saltwater, I think), advertised at £38K. Perhaps it was just a great deal at that time though, because a second hand (local dealers demo boat) Revenger 29 with 300hp Yanmar was £54K!

Of course, I guess servicing costs are a little higher - every 120 hours for the Yanmar (about £300) and every 100 hours for the Bravo 3X leg (about £175).

Regards,

Jim

jwalker · 09 February 2005, 18:57

I too think the outboard consumption figures are high, especially when most are quoted for the new generation 'economical' 2-strokes. I used to get about 1 ltr per mile on a 6.5 rib which was quite heavy and not at all a performanve boat. It had 3 engines during it's life, Merc 200, OMC 150 and Suzuki 200. The Suzi was a little more economical than the others.

Re the the diesel rib, there is another factor. For some reason it seems easier to pay upfront and have lower running costs than to continually spend a small fortune on petrol.
The notion of paying £100 to go 100 miles becomes frightening after the early enthusiasm has worn off.

Brambles · 09 February 2005, 19:06

Quote:

Originally Posted by jwalker

Re the the diesel rib, there is another factor. For some reason it seems easier to pay upfront and have lower running costs than to continually spend a small fortune on petrol.
The notion of paying £100 to go 100 miles becomes frightening after the early enthusiasm has worn off.

Very true JW,

it seems to cost us a £100 every time we have a day out, but boy do we have fun with a 250HP 2 stroke on the back!

Getting back to my earlier point, I was just wondering if anyone had actually done the maths for the "whole of life" cost of a diesel v petrol installation in the same model boat.

Ian.

codprawn · 10 February 2005, 11:04

Quote:

Originally Posted by Squirrel

What brand rib? Prosport 7.5
Single or twin outboards? Single 200hp 2 stroke V6 with 4hp aux.
Engine brand? Yamaha
Two stroke or four stroke? 2
Size of fuel tank? ~60 gals*
Speed and burn rate at optimum cruise 30-35kts 2 litres per nautical mile.

That is scary - 60 - 70L per hour or 14 - 15gph - £60 per hour at uk prices!!!

Alan · 10 February 2005, 11:17

It also seems a bit(!) high. I generally figured on 1-1.5 lph tops at cruising speed on both my DS21 with 150 Opti and Blue Ice with her 200hp HPDI. Cruising speed being generally around 75% of WOT and of course very dependent on sea conditions. (e.g. driving in a bigger sea usually uses lot of throttle movement and burns more fuel.)

I did do some comparison a while back on petrol vs diesel (I think there are a number of threads on this subject if you search) as I know did Mike C who runs a humber/1.7 merc. The variables are how you are funding the boat e.g. cash, finance etc and your perspective on resale value. If a diesel boat has a higher initial purchase price and a higher resale value then the economics can work in you favour, but if you have to borrow lots to fund it then it all falls apart again. Its absolutely true that the monthly or upfront payment for the boat is a lot easier to stomach than buying vast quantities of Mr Esso's finest unleaded every time you go boating. Ask Nasher, he runs (IIRC) a Johnson 150 VRO, the thirstiest outboard known to man (been there done that). After that experience an Opti is pure joy!

The compelling case for diesel for UK/European cruising is not so much the red diesel concession but the availability of diesel waterside vs the very poor availability of petrol. Smokie needs to make that part of his thought process when spec'ing out a new boat for his cruising area.

For me, planning ahead to the time hopefully when I have a high speed powerboat for cruising the wilderness of Newfoundland, I have to make the same considerations. Waterside petrol is virtually unobtainable, waterside diesel is widely available (albeit in fishing boat sized quantities usually!). Fuel cost is virtually the same for either (about a third of UK domestic petrol). I suspect I will end up with outboard power and probably seriously consider twins given the many miles I might have to motor on an aux to get back to civilisation. Its an interesting conundrum to think about whilst I should be working!!

Squirrel · 11 February 2005, 08:31

My figures are based on journeys in open sea (eg Guernsey to Sark), where even on the calmest days, there can be a decent swell and a strong tide running through the passages. This must affect fuel economy as the engine is working harder in that sort of sea.

Don't even try to work it out for wakeboarding, all that stop start action. At least you go slow once you're on the plane!

Brambles · 13 February 2005, 20:28

Quote:

Originally Posted by JIY

Sorry to sidetrack this thread, but I'm a little stunned at the fuel consumption figures quoted here. My 8.5m Tornado with 250hp Yanmar burns just under 6gph marine diesel at 33 knots (that's a little under one litre/nautical mile), with 4 people aboard, a full 200 litre tank and slight/moderate seas. I reckon the boat weighs a little over 2 tonnes with this load.

Hi Jim,

I am equally surprised by some of the fuel consumption figures claimed by the diesel users. My, admittedly limited, experience indicates that a performance diesel boat uses more fuel that its petrol equivalent.

I was reminded today about a trip we did to the Class 1 powerboats at Plymouth last Summer. The Scorpion (with O/B Honda 225) and a Revenger 29 (with inboard Yanmar 300 diesel) met up close to Poole and cruised together to Dartmouth with similar fuel loads of approx 300 ltrs. When we left Dartmouth the Revenger was nearly empty and needed refuelling. The Scorp still had about 120 lites left.

Hardly scientific I know, but that's the way it was.

Regards.

Ian.