Most Efficient Cruise Speed

[Mark Laurnen]

Hello. I have a 2004 31 TE with twin 310 Yanmars. Does anyone have a recommendation for most efficient cruising speed? Running 19 knots burns approximately 20 gal/ hour when fully loaded and properly trimmed. I’m now finding that I have more time to enjoy the journey, and will be happy to slow down to do so. Without a flow meter, and short of a precisely measured refueling log, I am uncertain what the speed vs fuel consumption curve would look like. What recommendations would any of you have for this hull? Thank you for your advice.

[RobS]

Keep her on a "clean plane" cruise or drop down to ~ hull speed.

[Norseman]

Well, not sure about the most efficient cruise speed. (I have a single diesel 28TE)

Running less than hull speed will get you a lot of miles per gallon.
(On my boat 6-7 knots will go forever)

Yanmar have a chart on the net showing the fuel/load curves for the 6LP, but the hull shape or displacement is not shown of course.
If you have a Flowscan or similar fuel computer you can figure it out yourself if hooked up to the GPS.

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[Mark Laurnen]

Thank you for your quick answers. I think you have confirmed what I suspected, on plane or hull speed makes perfect sense. Thanks.

[RobS]

Mark, if you really want to make sure all is setup 100% and dialed in as best as possible start with making sure you are correctly propped. If you are over-propped all bets are off. Get a non-contact photo-tach (laser) and check the accuracy of your helm tachs and note any delta's in the higher rpm ranges.. Then be sure your 6LP's are turning as close to 3,950 rpms as possible (see attached from Yanmar) and check this with the boat fairly heavy with fuel, water and gear. Only then can you rely on the accuracy of that fuel burn chart and be in a position to do your own MPG calcs.

[Mark Laurnen]

Thank you Rob. I know the analog tach’s are notoriously inaccurate, so I hear you. At the sea trial, with both the general surveyor and engine surveyor present, WOT produced 3950 at each engine, but that was with three persons aboard and presumably light fuel and water, and also using the factory tach’s I suspect. I did get a tach, but to be honest I’m a little hesitant to get down there to use it at full throttle. I believe I’m spinning 20X20’s and I have wondered if I’m over propped. I guess I will perform the study myself with a heavier payload. Thanks again!