2.27mpg.....

[realslimshady]

Took the kids to Whitecliff Bay on Tuesday-put 60 quids worth of unleaded in at Gosport, by the time we returned it had all gone (ish-judging from the fuel gauge, not hyper-accurate calculation)-at about 2.27mpg....from a Fairline Weekender (21.5 feet for those unfamiliar) with an AQ200D and 280 leg...does that sound right?
Previous 20' Bayliner with 3.0litre Merc used to do 4.5mpg-should the Fairline use twice as much?

 

[jhr]

Seems possible, particularly if the Merc was an EFi - the AQ's an older, less efficient engine (?I think?), boat's slightly bigger but (I'd guess) a fair bit heavier, etc. etc.

However - never rely on a boat's fuel gauge for accurate measurements.....

(Edit: On the assumption that you filled her right up last time, why not see if you can get another sixty quids' worth in now? If not, adjust your calcs accordingly)!

 

[Whitelighter]

What is the condition of the hull (fouled, rough etc)?
Where you punching tide there and back
What was the sea state
How many were on board

MPG is a very difficult measure on a boat as there are so many things that affect it. I had an accurate fuel measuring system on one boat which gave a MPG reading from GPS speed data and it varied +/- 25% on any given day dependant on different factors.

Its why you will find all manufacturers only give gallons or litres per hour, and even then with a buch of caviats.


The fairline will be heavier than the Bayliner, and the Bayliner being newer will probably have a better hull shape so I would expect the Fairline to be worse on fuel. However, one run is no-where near enough time to get any kind of idea about average fuel useage because of the reasons above.

Keep really accurate records for 20 or so hours of running (and not just a feeling based on an analogue fuel guage which, incidently, is way less than reliable in the first place) and that will give you amuch better idea of what fuel the boat really uses.

 

[Moonshiners]

Can imagine that weekender being quite thirsty in rough chop if that's an explanation for the higher than expected, The weekender being much heavier etc... you wouldn't be able to run at similar revs per MPH if that makes sense. I think you may find you'l get better MPG's if you adjust the revs and cruising speed expectation to the heavier boat.

This is all on the presumption your trying to use the weekender in the same way you did the bayliner. Sorry if this isn't the case, but a common cause.

EDIT - Took me as long to write that as it did for the other responses to get in first!

 

[realslimshady]

I'm obviously expecting the Fairline to use more than the Bayliner, just found that a bit eye-watering!
It was rough chop on the way across, so we only pootled at 8 knots, then planed it back at 20.
As I said, only a rough estimation-didn't brim the tank and then refill (as I used to on the Bayliner)-just wondered if that was 'normal' consumption for a boat of this size/weight.

 

[Whitelighter]

If you were doing 8knts in a 21footer that is just about the worst speed you can pick. Well over displacement speed and not on the plan, pushing a whole bank of water in front of you. With a planing boat, for economy, you either do displacement speed (for you about 5knts) or you get yourself on the plane fully.

Anything else and you might as well be setting light to £20 notes

 

[realslimshady]

If you were doing 8knts in a 21footer that is just about the worst speed you can pick. Well over displacement speed and not on the plan, pushing a whole bank of water in front of you. With a planing boat, for economy, you either do displacement speed (for you about 5knts) or you get yourself on the plane fully.

Anything else and you might as well be setting light to £20 notes

5 knots it is in future!

 

[barfly4657]

[QUOTE it had all gone ish-judging from the fuel gauge, not hyper-accurate calculation

The only accurate way to monitor fuel consumption is to fill right up at the start and again on your return. My gauge shows empty for about three hours before the warning alarm shrieks - and I can still get back home!

 

[jimg]

5 knots it is in future!

maybe 5.5!!

1.34 x sqrt of 18 feet?