fuel efficient cruising

Larryrsf · #1 06-05-2014, 12:17 PM

My boat is a 74 V20 with an Evinrude 150. An outboard mechanic evaluated my engine and said it is in "new" condition internally, obviously from the compression numbers having had a recently overhauled power head. He said the carbs should be cleaned at the end of the season for optimum performance. I add the right amount of the recommended carb cleaner to the fuel tanks.

I have it in a marina on San Diego Harbor. It will be there until October at least. The harbor is usually choppy, making a very rough ride at higher speed than 20 MPH or so, when the boat is just planing.

How should I set the trim for best mileage? Obviously I start with it trimmed all the way down, which keeps the bow low enough to see over. But as it starts planing should I adjust the trim for optimum speed with the lowest RPM? It does 22MPH (accuracy of speedometer unknown) at 3000 rpm.

Obviously the smoothest ride is with the trim down and minimum speed to plane. But that may not be the most fuel efficient.

Larry

Destroyer · #2 06-05-2014, 01:25 PM

Larry, you don't say if you have trim tabs or are you just trimming the boat with your engine? Tabs make a hell of a difference.

Larryrsf · #3 06-05-2014, 02:04 PM

At the risk of appearing stupid about boats, what are "trim tabs?" There is nothing like that on the transom of my boat. I have an optional swim step and the speedometer pitot tube extending down back there.

Larry

Blue_Runner · #4 06-05-2014, 03:21 PM

They are electric or hydraulic stainless steel flaps on each side of the transom that help control porpoising and makes a better ride. And yes, they do make a big difference.

But to answer your question, you are right in that all the way down isn't the best economy and highest speed / lowest RPM work in your favor. As you trim up more hull is coming out of the water, steering gets easier, speed increases and so on. My trim indicator died, but I can tell by feel exactly how my engine is trimmed.

I don't have a fuel flow meter but I believe the best RPM range for fuel economy on a V20 w/ 150 hp o/b is somewhere between 3700 - 4000 RPMs. In your scenario of going around 20 mph or so I wouldn't worry so much about economy as adjusting the trim for the best ride, including pounding and spray from the hull. I've found there are times when speeding up and adjusting trim you can get on top of the chop and find a better, dryer ride than if you tried to go slower. Not always the case, but sometimes.

Larryrsf · #5 06-05-2014, 04:02 PM

Thanks! Saturday was busy out there, every kind of boat including the heavy Harbor Cruise vessels. Lots of sailboats, even skidoos and a few runabouts really flying. So crossing those wakes was scary for my newbie-to-boating wife. I was praising the Wellcraft deep bow for efficiently slicing through most of that stuff. But we did get slammed by big waves a few times-- and since she wasn't standing with knees flexed like I was, she got slammed too, ha. I bought cushions for our "Bluewater" boat seats today. Hope that helps, but standing up is really the best answer to rough water.

Larry

smokeonthewater · #6 06-05-2014, 05:01 PM

trim for the highest speed at a given throttle and then trim down if you need to to stop porpoising

Blue_Runner · #7 06-05-2014, 09:42 PM

Bean bags!!! Cannot stress it enough. You can get one called the "Big Joe" at Walmart for about $30. Money well spent. You won't be sorry. The lady will hug your neck, I promise! My crew catches quite a few snoozes while I'm at the helm banging the fillings out of my mouth.

Destroyer · #8 06-05-2014, 10:09 PM

Quote:

Originally Posted by Larryrsf

Thanks! Saturday was busy out there, every kind of boat including the heavy Harbor Cruise vessels. Lots of sailboats, even skidoos and a few runabouts really flying. So crossing those wakes was scary for my newbie-to-boating wife. I was praising the Wellcraft deep bow for efficiently slicing through most of that stuff. But we did get slammed by big waves a few times-- and since she wasn't standing with knees flexed like I was, she got slammed too, ha. I bought cushions for our "Bluewater" boat seats today. Hope that helps, but standing up is really the best answer to rough water.

Larry

Just remember that you have a steering wheel to hang onto... she doesn't. If you're standing up in the boat MAKE SURE you both have on life preservers.... Accidents happen, and if you're standing, they can happen faster. Getting tossed overboard by a wave you didn't see coming and weren't ready for is NOT high on the fun things to have happen while boating.
I'm 70, a good swimmer, a certified Red Cross Lifesaving Instructor and I always wear my pfd while underway. NEVER be embarrassed to wear one. Get the canoe/kayak kind that you wear like a vest and has a zipper and wide arm holes and in minutes you'll forget you even have it on.

jamesbalog · #9 06-06-2014, 01:33 PM

[QUOTE=Blue_Runner;214237]They are electric or hydraulic stainless steel flaps on each side of the transom that help control porpoising and makes a better ride. And yes, they do make a big difference.

But to answer your question, you are right in that all the way down isn't the best economy and highest speed / lowest RPM work in your favor. As you trim up more hull is coming out of the water, steering gets easier, speed increases and so on. My trim indicator died, but I can tell by feel exactly how my engine is trimmed.

I don't have a fuel flow meter but I believe the best RPM range for fuel economy on a V20 w/ 150 hp o/b is somewhere between 3700 - 4000 RPMs. In your scenario of going around 20 mph or so I wouldn't worry so much about economy as adjusting the trim for the best ride, including pounding and spray from the hull. I've found there are times when speeding up and adjusting trim you can get on top of the chop and find a better, dryer ride than if you tried to go slower. Not always the case, but sometimes.[/QUOTE]

here on lake erie where we usually have a chop speeding up and trimming up keeps things much more smooth and dry

Larryrsf · #10 06-11-2014, 11:23 AM

My boat is in a Marina on San Diego Harbor. So I am constrained to do my boating on that harbor. It is big enough that even light winds can kick up chop and even swells. And BIG boats, even ships pass through and create big swells from their wake. So the harbor is seldom smooth enough to plane without scaring the crap out of my passengers, ha.

My boat speedometer shows low 20s with 3000 RPM. It is on top and planning at that speed. I read that this boat with a good 150HP engine can hit 50MPH with just one or two people aboard. Maybe I will find lake-smooth conditions some morning and let er' rip!

Larry