Fuel use for 75 honda

UncleRichie · Posted: Wed Jun 11, 2014 10:52 pm Post subject: Fuel use for 75 honda

There has been some questions about how much fuel a C-Dory uses found this from a early broshure from Mark.

800 Lb. people, Fuel & Gear

Single Honda 75 Four Stroke carb model

13 3/4" x 15" prop, 3 blade, aluminum

R.P.M. G.P.H. M.P.H. M.P.G.
2500 - 1.50 - 8.5 - 5.67
3000 - 2.40 - 12.5- 5.21
3500 - 3.10 - 16.2- 5.23
4000 - 3.85 - 20.3- 5.27
4500 - 4.70 - 24.7- 5.26
5000 - 5.60 - 27.6- 4.93
5500 - 6.80 - 30.4- 4.47

Would think the new 90s with fuel injection would be pretty close to the same.
Richard

rjmcnabb · Posted: Thu Jun 12, 2014 11:14 am Post subject:

Hi Richard,
I've been thinking about this stuff a lot, lately. On NoddyBleu I was delighted by the fuel management system, and all that could tell me. Our present boat, Luna, is much more bare bones-no speedo, knot meter, GPS, etc. She is powered by a 50HP carbureted Honda. So, I started keeping track by weighing fuel consumption and calculating mileage from charts. Over a range of conditions I got burn rates of 2.5 to 4.4 lb/hour. Those gave mileages in excess of 20 nm/gal, which is totally unreasonable.
This summer, I have a simple hand-held GPS which I will try to use for speed and distance. That might get me closer to reality. Rolling Eyes

An interesting (at least to me) calculation I made from the data in your table was the RPMile. Here, I have appended them to your table. This suggests to me that piston wear might decline as one covers a set distance at increasing speeds, up until prop slippage becomes severe, say above 20 kn. I suppose the discontinuity between 2500 and 3000 rpm is due to transition from displacement speed to planing mode.
Rod

800 Lb. people, Fuel & Gear
Single Honda 75 Four Stroke carb model
13 3/4" x 15" prop, 3 blade, aluminum

R.P.M. G.P.H. M.P.H. M.P.G. R/mile
2500 - 1.50 - 8.5 - 5.67 17647
3000 - 2.40 - 12.5- 5.21 14400
3500 - 3.10 - 16.2- 5.23 12962
4000 - 3.85 - 20.3- 5.27 11823
4500 - 4.70 - 24.7- 5.26 10931
5000 - 5.60 - 27.6- 4.93 10870
5500 - 6.80 - 30.4- 4.47 10855

UncleRichie · Posted: Thu Jun 12, 2014 11:58 am Post subject:

That sounds right at 2500 it plows more than planes.
Richard

thataway · Posted: Thu Jun 12, 2014 12:07 pm Post subject:

One test result which has been around for a long time, is the Boat Test.com
Since the cost of the boat was about 33.8K, I suspect this is the carb engine. Time frame, maybe about 2000?
Prop 13 7/8" x 15" Solas

2 people 5.8 fuel

RPM - SMPH - MPG
1000 - 3,9 - 7.7
1500 - 5.2 - 6.9
2000 - 6.6 - 6.0
2500 - 7.5- 4.1
3000 - 9.4 - 4.4
3500 - 14.2 - 4.5
4000 - 19.1 - 3.8
4500 - 24.7 - 3.6
5000 - 27.4 - 3.4
5500 - 30.9 - 3.4
6000 - 32.9 - 3.5

DuckDogTitus · Posted: Thu Jun 12, 2014 10:29 pm Post subject: Re: Fuel use for 75 honda

thataway · Posted: Fri Jun 13, 2014 1:09 pm Post subject:

For the 90 HP Honda, the WOT RPM should be at least 5500, and preferably 6000 RPM. If you cannot get those numbers you are over propped.

DuckDogTitus · Posted: Fri Jun 13, 2014 1:44 pm Post subject:

Mine is a 2005 Merc, but I just haven't bothered pushing it wide open. I like the way it sounds and feels at 4500-4700. That's definitely something I could explore further though.

ssobol · Posted: Fri Jun 13, 2014 3:19 pm Post subject:

My CD22 with a Honda BF90D cruises most happily at just over 4000rpm and about 19mph. At WOT it will do 5600rpm and just about 30mph. The one time I checked the mileage I got 92 miles on one tank (23 gal). That works out to 4mpg. Almost all of that was above 4000rpm.

Sunbeam · Posted: Fri Jun 13, 2014 3:21 pm Post subject:

It is good to give it a couple of WOT runs just to have the figures in your baseline data files (usually there is some wind or current, so I do an up-and-back and then average them, of course trying for a calm time). Once you see what the maximum RPM you can attain at WOT is, you can use that to make sure you are "geared correctly," which on an outboard you tweak by changing the prop. Sounds like you are already aware of it (but just of course don't typically run at WOT), but if you don't get into the proper RPM range at WOT, then you are actually lugging the engine at any speed. Once you can attain an RPM in the suggested range at WOT (usually better to be in upper half of range, as Thatway says), then you are good at all speeds.

It changes with altitude. Powell is around 4000', and at that altitude I was below even the lowest suggested WOT RPM with the prop I had (that worked at sea level), so I got a different one just for Powell and other places of similar altitude. Of course it's good enough to put on as a spare at sea level, too.