fuel milage

[atrometer]

Completed the 80 mile round trip up (to the south) the St. John's River from Astor to Sanford on Lake Monroe.

The current varied with the width of the river, but we averaged 4.7 MPH going and 6.4 MPH coming back.

I have estimated that we burned 1 gallon per 5 hours of running time upriver.  At almost 5 MPH that would be 25 MPG.

 

Looks like between 25 - 30 to me (LOL)

[reedtg3]

Thanks for the report, Ron.  I hope you will have time to favor us with a full trip report, including photos, in the Journeys, Anchorages and Destinations section of the forum.
Cattail Tom

[alsantini]

I worked at a boat place years ago and the owner would tell people to expect to use .4 gal per hour on a 4 hp 4 stroke at hull speed.  If someone bought a 9.9 he would say it will use just under 10.  Put a decimal point ahead of the hp and you are good to go.  Last week I spent 3 hours under power at around hull speed or slightly lower and used 1.5 gal of fuel.  That is using my 4 hp 4 stroke at about 1/2 - 2/3 throttle.

[hoddinr]

Oh yeah, Don!  Excellent fuel milage! 
That's probably about 80 mile range on 3 gallons! 
With an additional 1 gallon tag along tank it's over 100 miles for 4 gallons.

Very good economy!

Ron

[FireDrill]

Just some observations on this very interesting topic:  If we use OntarioSuncat's figures (4 stroke 5HP Nissan)  earlier in this thread and convert from his units to Miles per gallon  he gets 23.2 mpg.. If we use Hoddinr's (4stroke, 4 HP Yamaha)  exact number of 4.7 and convert to MPG we get 23.5.  VERY SIMILAR !!   Both were going approx. the same speed approx. 5mph (5.1 and 4.7 respectively).  Those two numbers are better documented than the rest but it seems many think 25 mpg.  My estimate earlier (4hp Yamaha) of 30 was based on less carefully measured numbers averaging 2 or 3 mph(not good data). Capt_Nemo literature reference of 60mpg (he cites it's unclear data) may be true but I'll bet it was for something like 1 mph?  The old days boat yard estimate works out to 15 mpg but I wonder if that is for 2 cycle engines? or full throttle?   So I guess I'd go with 23.4 as this thread's best number for (4-5 hp 4 stroke Tohatsu, Nissan, Yamaha etc) so far,   but remembering its for 5 mph.   Any thoughts?       

[NateD]

So I guess I'd go with 23.4 as this thread's best number for (4-5 hp 4 stroke Tohatsu, Nissan, Yamaha etc) so far,   but remembering its for 5 mph.   Any thoughts?       

I got a little over 18 mpg with a 9.9HP 4 stroke on a Com-Pac 23 at 5 mph, 20-25 seems pretty reasonable for a smaller motor/boat at the same speed.

[hoddinr]

If I said I was going 4.7 knots, I mis-spoke.  I was measuring MPH.  I used to do everything in Nautical Miles, but now that I'm mostly inland and Intracoastal I'm using MPH.

The only thing that's not clear to me is how the river current affected the speed and fuel consumption.
  Best would be to average the upstream and downstream mileage to get the overall mileage for the whole trip.  Since I haven't topped off the tank since returning, I don't know for sure how much I used on the way back.  My bet would be LESS.  I tried to keep the same RPM, and throttle position on the handle both upstream and downstream.  Upstream I was going 4.7 MPH - Same RPM downstream yielded 6.4 - 6.5 MPG.  A lot less time to cover the same distance at the same engine RPM.

Ron

[atrometer]

If I said I was going 4.7 knots, I mis-spoke.  I was measuring MPH.  I used to do everything in Nautical Miles, but now that I'm mostly inland and Intracoastal I'm using MPH.

The only thing that's not clear to me is how the river current affected the speed and fuel consumption.
  Best would be to average the upstream and downstream mileage to get the overall mileage for the whole trip.  Since I haven't topped off the tank since returning, I don't know for sure how much I used on the way back.  My bet would be LESS.  I tried to keep the same RPM, and throttle position on the handle both upstream and downstream.  Upstream I was going 4.7 MPH - Same RPM downstream yielded 6.4 - 6.5 MPG.  A lot less time to cover the same distance at the same engine RPM.

Ron

Top Off, Top Off, Top Off!

PS  I think you meant MPH, not MPG on your downstream trip (LOL)

[hoddinr]

Sharp eyes, Gus.  Yes... MPH not MPG.

[atrometer]

Sharp eyes, Gus.  Yes... MPH not MPG.

My Mother was a "Proof Reader" for the Buffalo Evening News back in the day, so I grew up with that and was "Properly" trained.

[FireDrill]

Hoddinr - good comment about the effect of the current! 

I believe the difference between your two speeds divided by 2 is the effect of the current. i.e. 1.7/2 = .85 mph (+ or - depending on direction ).  Since you kept RPMs the same that means we can apply results to MPG  calculation at that rpm.  So the speed upriver without current would have been 5.55 mph (getting near hull speed of 6mph in still water)  going 27.75 miles on 1 gallon of gas at your fixed rpm.  So its more like 28 MPG !.  The actual current may have been greater than .85 mph,  that is simply the calculated effect on your suncat's hull... (I am an optical engineer, not a mechanical engineer so this may be too simple of an explanation)  Looking at some manufacturer's data on various outboards, there is generally a sweet spot on fuel mileage in the lower rpm ranges, and I think you said you were mid throttle - i.e. mid rpm range probably.

Any  thoughts.? Other insights on the effect of current? 

Atrometer - I grew up in Buffalo - back in the day -  and loved reading the Buffalo Evening News - no wonder it was so good ! 

   

[atrometer]

1/2 way through 4th grade in Kenmore, moved to Michigan - small world & my Dad was a U of M mechanical engineer.

[hoddinr]

OK.. so I topped off the 3 gallon tank today in preparation for this weekend's Cayo Costa Cruise with the WCTSS.

It took .95 gallons.  So let's say 1 gallon.  I used a 1 gallon spare tank on the trip, and had an attendant at a pump put in .8 gallons.  So total used for the 80 mile St. John's River Trip was 2.8 gallons.

That figures out to be 28.57 miles per gallon. Even better than I thought!

So I can be pretty safe in saying I have a 75 mile cruising range with the 3 galloon tank, with room for safety.  That is if the wind and waves are not right in my face.  That changes everything, and would be a good reason to start sailing!

Ron

[Mike Howell]

Good information Thanks.

Mike