RPM/Speed, GPH for MKII 35MB

[Kent & Jane Overbeck]

Tom,  I'm starting a new thread on this.  I'm curious what other experience.
My RPM and speed, in flat water (3 blade prop), clean bottom and loaded for cruising, follows.

1500 / 4.9 MPH

1800 / 6.0 MPH

2000 / 6.4 MPH

2200 / 7.0 MPH

2500 / 7.6 MPH

I generally cruise at 2200 and use .6 GPH of fuel.  Fuel usage drops to less than .4 GPH at 1800 and 3.5 at 1500.

What do the rest of you guys get?

[tommyt]

Kent, thanks for starting the new thread. I should have.

As I said, at 2100-2200 the boat went about 6.1-6.3 kts. and used .6 GPH. At 2500 the boat was closer to 6.7 and I never tracked GPH at that speed. Of course, the big question is: Was the tach accurate? The boat has had three tachs. The tach would always work, but the hour meter would not. I always tracked the hours so caught it quickly, but never checked the tach for accuracy. Two blade prop.

Scott, are you inquisitive enough to find out?

[Stu Jackson]

I wonder why one uses MPH instead of knots for a boat.  Even for inland waters.

You will also find lots of discussions about this if you do a search on "fuel consumption" with the " marks.

IIRC, the M35 skippers here are seeing from .8 to 1.0 gph, we get 0.493 gph on our M25 at 2650 rpm, 6 to 6.5 knots depending on bottom condition, 3 blade fixed 15/10 prop.

[Kent & Jane Overbeck]

Stu, I used MPH because all the inter-coastal charts and rivers use statute miles for the mileage markers.

[Stu Jackson]

Thanks, Kent, now I understand. 

[Ron Hill]

Kent : I assume that the RPMs stated are laser "tach ed" and are actual engine RPM!?!  NOT readings from your boat engine tach instrument!?!

I also assume that your speed is thru the water? in a no wind no current condition? with a calibrated MPH meter!?!

You need to state the above parameters if you want any kind of a valid comparison.

A few thoughts

[Kent & Jane Overbeck]

Ron, the speed was calculated using my GPS on Lake Chickamauga, Chattanooga.  There was no wind and very little current but I averaged both upstream and downstream values. The RPMs have been "laser tached". On the "Loop" I used these benchmarks to establish just what kind of current I was encountering and it was nice to have them available.  
I have no confidence in a "calibrated MPH meter".  I am assuming you mean a through hull paddle wheel instrument. Grass, weeds etc. can change the accuracy in a heartbeat.  Besides, just how do you check the accuracy?  With a GPS!!  I guess that I should have stated the parameters initially.

Thanks for your reply,

Kent

[Mick Laver]

For the past couple of years we've done a round trip to Two Harbors on Catalina Is. with some friends who own a 2000 C34. The trip involves a long uphill slog to Dana Point from San Diego, then over to Isthmus Cove, back to Oceanside, and on to San Diego. We fuel before the trip and then again in Dana Point before heading over to Catalina.

SOG of course varies with the current since we kept the RPM constant when motoring (which was too much.) We have a 9x15 3-blade Flexofold, and at 2300 rpm we'd do 5-6 kts against the current and 6.5 -7.5 with the current coming back. Our friends have a 10x15 3-blade Flexofold, and with the higher pitch they only had to run at about 2100 rpm to stay even with us.

So here's the interesting question. Flexofold recommends the 9x15 for the M35bc, but a 10x15 should work fine. Does the 10x15's additional load result in higher fuel consumption? Here's the result of our admittedly very unscientific sample. SOG is the same.

Prop                   RPM                       Course                            GPH
9x15                  2300                      SD-DP                             0.68
10x15                2100                      SD-DP                             1.01
9x15                  2300                      DP-TH-O-SD                    0.76
10x15                2100                      DP-TH-O-SD                    0.83

We did another similar trip on CINNAMON by ourselves a couple of weeks later and the results were about the same: 0.76 GPH uphill to DP, 0.80 GPH back.

Our engines have about the same number of hours (1200 for our '99, 900 for their '00) and are well maintained, so the answer to the fuel usage seems to be "it depends." Higher pitch seems to make the engine work harder against the current (with n=1 I'm sure the statisticians out there are howling) but other than that the answer seems to be " At 2300 rpm we use about 0.8 GPH at about 6.5 kts. Depending."