Mainship 30 Pilot II fuel use by the numbers

[rgano]

We just got back from a 244-mile round trip over the last few days, our first trip beyond 20 miles from home in this boat. Our former boat was a Grand Banks 42 with twin naturally aspirated 120 HP Ford-Lehmans which I ran at 1600 RPM for 9.5 MPH over my 29 years of ownership averaging 2.5 MPG (3.7-3.8 MPG); so I have been interested how this faster, lighter Pilot II would perform with its vastly different turbo-charged Yanmar 315 HP engine.


The first thing I did with this Pilot after I bought it last June was to add a calibrated fuel level sight gauge like the ones I had in the trawler. Level measure tick marks are in 10-gallon increments.


Thanks to the kind soul who posted the Yanmar-provided fuel usage data for this boat and engine, I was able to make up an Excel spreadsheet with as graph including MPG, GPH, MPH, and knots all versus RPM which gave me a starting guesstimate and proved surprisingly accurate.


I this bi-directional trip ran in one direction at 3100 RPM getting 19-20 MPH and in the other direction at 2700 RPM getting 14-15 MPH (almost 16 for a short period with a favorable tide). At 3100 RPM I used 9.2 GPH, and at 2700 RPM, I used 6.5 GPH. These numbers compare well with my graph; so I will feel comfortable in the future using it at the helm when I want to know what to expect for fuel usage.


What really pleased me was to find that fuel economy at 15 MPH and 20 MPH were respectively 88 and 72 percent of the "economical" trawler while running respectively 58% and 110% faster.

 

[DavidM]

Rich:


Those are good numbers. But are you a little over propped? I recall reading a post from you on Yahoo where you said that your engine would only do 3,600 rpm at wot. Also your fuel burn seems a little high at 9.2 gph at 3,100 rpm which could be due to over propping.


Your engine will be much happier cruising at 3,300 rpm to make the same speed rather than 3,100 if it can get up to 3,800 at wot. If you are over propped by 200 rpm a good prop shop can take out 2" of pitch and get the numbers closer to where they should be.


But as always, make sure that your dash tach is correct by calibrating it against a photo tach.


David

 

[rgano]

First, let me correct the GB info. I typo'd the 3.7-3.8 GPH as MPG.

The numbers I found for GPH are right on the curve I plotted from the Yanmar data for this boat and engine, and I am no longer getting ANY soot around the exhaust as I did when running a series of short trips around our local bays. I wonder now if I am over-propped, even though my digital handheld tach says at WOT I only get 3600 RPM.

I will get the prop balanced in the coming months and may let the prop shop take off the one inch of pitch they previously recommended.

 

[DavidM]

[ I wonder now if I am over-propped, even though my digital handheld tach says at WOT I only get 3600 RPM.]

Rich:

Since the definition of being over propped is if your engine will not hit the manufacturer's rated rpm at wot and for that engine it is 3,800 rpm, then you are over propped.

Most props can be repitched by up to 2" but no more. A 2" pitch change usually increases wot rpm by 200 and it would not hurt a thing if it is 300 rpm.

So I would take 2" off the pitch of your current prop so you at least hit 3,800.

David