Last week I fitted a fuel flow sensor to the boat. A Lowrance EP-60. This will give me accurate fuel useage info on my trip Round Britain. For those that don't know... it is a Merry Fisher 625 with a Suzuki 115hp outboard. Conditions were lively in Plymouth Sound but planeable but outside the Sound 1-2m sea and not planeable. Wind was easterly F4-5.
Based on about 100 hrs of use last year I estimated that I used about 1 ltr per mile. After the first sea runs today I can confirm that I am using about 1 ltr per mile! [img]/forums/images/graemlins/smile.gif[/img]
That did not come as much of a surprise but something else did... I was expecting that at displacement speeds I use less fuel per mile than when planing. Not so.
The surprise was that I am using about the same consumption per mile either on the plane or off! 5kts showed about 5 ltr/hr, 15kts showed about 15 ltr/hr and 20kts showed about 20 ltr/hr.
I do know that once on the plane then consumption per mile normally remains fairly stable until it nears WOT but I didn't expect that it would remain so at displacement speeds.
I should say that in the range above 6kts until properly on the plane at about 15kts the useage dropped to anything up to 2 ltr per mile. This was not surprising.
What does this tell me? I suppose it says that for my engine/boat configuration and in the conditions I had today, there is no benefit to either planing or not as far as fuel usage per mile is concerned as long as I don't operate between 5kts and 15kts.
One thought... the propeller is quite a coarse pitch at 21" and I could imagine that it is not running efficiently at low speed. I will be trying the same tests with a 19" pitch prop next week so it will be interesting to see any differences.
I have a lot more testing to do and if I find anything else surprising I will post.
An interesting point though is that I felt a strange feeling seeing in figures how much fuel I have used instead of what is more normal at seeing how much is left... kind of emphasises how much it has just cost!! [img]/forums/images/graemlins/blush.gif[/img]