Fuel consumption:Leanburn O/B Vs. Diesel.

[rbcoomer]

Brilliant and rubbish in that order.....

Actually truth be told it wasn't as simple as that....

I had a Mercruiser 5.7V8

It burned around 18gal / hr at WOT

When the LPG was first installed, it was buring 25 gal / hr at WOT so would never have saved much money - however I refused to let that one go, and demanded tests and so forth or I wanted my money back due to false advertising.

After much research, we got the gap down to about 18 gal / hr on petrol and 22 gal / hr LPG which would have been better.

I also got a big refund which helped!!

But then I found out that the data for the timing on the Mercrusier was American and based on using 92 octane petrol - which meant that the ignition settings were for that.

LPG has an octane of over 100 and so it wasn't running anywhere near as well as it could have.

By advancing the timing well over the max in the manual we got the LPG engine to run at about 20GPH at WOT which compared well to the 18 of the petrol.

Back then, Petrol was 75p per litre on the Marina, and LPG was 38p - so the boat went from costing £62 per hr to around £34 per hr at WOT - and a fair bit less at cruise - so at the original conversion cost of £3500 it wouldn't have been worth it - but after I got £1500 back - yes it was worth it.

I note that several years later the same boat came up for sale on eBay and the owners had removed the LPG purely as they didn't have it locally to them so wanted the space back in the mid-cabin.

If you can get a car system fitted and can actively expel the vapours in the bilge, then I think you'd do fine - I would suspect a simple vapour LPG system to cost around £300 + fitting and you could fill up at any Autogas forecourt which would save a lot of money.

You will need to be trailing the boat though - anything marina based wouldn't work

That's really interesting feedback and echos some of my own experience in car LPG conversions. I had a Series 1 Land Rover Discovery with an early OMVL LPG conversion first and that was similar to your initial experience. It needed the LPG to be half the price of petrol for the numbers to work as it used almost twice as much gas as petrol - 15.5mpg average on petrol (3.9 V8 injection) & 8.5mpg on gas! There was a vacuum timing adjustment when on LPG, but it worked by spraying vapourised gas into the inlet manifold. The current tug is a Range Rover P38 (4.6L V8) has a much later BRC multipoint LPG injection and this works out between 15mpg and just shy of 20mpg on LPG. This is a cost equivalent of close to 40mpg! The Fletcher Arrowbolt however has a 5L carb V8 and I fear it would be closer to the discovery than the P38. I might replace with a repower base 5.7 and 4-barrel carb, but adding perhaps another 1000 to cost for LPG kit doesn't seem a viable unless the base engine and kit were MPI? I do trail the boat and we have quayside LPG in Torquay, but I don't think it would be a saving when you factor in the kit cost and probable fuel burn with carb... The big plus for me is that it's so clean - my emissions test at MoT are astonishing when compared with petrol and that alone suggests the duty will remain lower. Will look at the tinleytech kit in more detail however before I make a call on the engine. :encouragement: