Novice question regarding I/O efficiency

[jumpjets]

Please help me perfect my technique for maximizing fuel efficiency in my 5.7 Alpha one Sterndrive boat. See my sig for complete details.

My current technique is to use a high power setting to get on a plane, then pull the throttle back to ~2100rpm and trim the nose down so that I am moving as slow as possible while still on plane. I usually get 18-22 knots.

2100rpm is about the minimum that my boat will maintain a plane, depending on load-out of course. If I motor around at 2000rpm, I'll be going ~10knots with my nose pointed 30 degrees up like an aircraft taking off.

Is this a good way to travel? Is there a more efficient profile? Should I be cruising faster?

 

[Silvertip]

Re: Novice question regarding I/O efficiency

The only way to know what technique/cruising speed provides the best "fuel economy" is to install a fuel flow monitor or to make some test runs over a known distance. Every boat is different so one setting does not work for all boats. Many boat manufacturers and boating magazines have test reports that would provide the information you are seeking.

 

[bnicov]

Re: Novice question regarding I/O efficiency

Most efficient is with the boat comfortably on plane, not struggling to stay on plane. Trim it up just right and she'll cruise as efficient as is possible for a boat. With mine, it'll barely stay on plane at 2500rpm, at 2800rpm, it cruises just fine.

 

[paultjohnson]

Re: Novice question regarding I/O efficiency

After I get on plane, which only takes about 2100 rpm to maintain 19 mph. I bump her up a little to get the max MPH for the least RPM. Fine tuning the trim at the same time My sweet spot is about 2300rpm which runs me about 24-25 mph. To me that feels like the most efficient way to run. Without a fuel flow meter to know for sure Above or below that speed the RPMs creep up and the MPH creeps down.

 

[Kaplooi]

Re: Novice question regarding I/O efficiency

If you truly want to maximize fuel economy, the best speed is around 1000rpm moving at hull speed around 5 knots or so. Basically going as fast as you can before the bow starts to try and climb up out of the water even a tiny bit. Travelling at 10 knots with the nose up in the air is about the most inefficient way to travel. With that said, your best planing speed efficiency is not likely to be the slowest speed at which your boat stays on plane. If you look at the fuel economy curve of most planing hull boats you'll see that there's typically a bump in the curve around the middle of the rev range where speed and fuel consumption is optimal. If you're having to trim in to give more stern lift to run at 2100RPM you're also pushing the bow down and putting more of the boat in contact with the water which increases resistance. If you ran at a higher rpm you could trim up more and get more of the boat out of the water. Given that you're also going to get more speed at a higher RPM in addition to lowering your drag your fuel economy should improve somewhat. As Silvertip said, the only way to know for sure is with a flowmeter.