Am I hurting things by not running WOT?

[62Scout]

80-something Bayliner Trophy 17ft bass boat, Force 85HP motor. Been browsing prop threads lately, where I see a lot of people attempting to chase another 2-3mph out of their boats, along with the common advice of "make sure you're getting X RPM at WOT".

I don't have a performance problem per se that I'm attempting to resolve, though I'm not sure I'd even be able to recognize I have a problem if I did. I'm just looking at this as attempting to fine tune my operating skills.

I have absolutely no idea whatsoever what my boat's top speed is, have never looked, and never cared. I run small inland lakes exclusively, with the lake I'm at most often being roughly 5 miles end to end. The difference between 25mph and 35mph is 5 minutes end to end. The biggest one is 15 miles, and I haven't been there in several years at this point. I have never been in a tournament, and likely never will. I fish for fun, with the mindset of "a lousy day fishing still beats a good day at work", lol. The day that 5 minutes becomes an issue is the day I need to find a new hobby...

That said, I've always run this boat with just enough throttle to maintain plane. Jam it full to get out of the hole, and as soon as it pops up on plane, pull throttle to just enough to keep it there with minor adjustments to trim & throttle as conditions change. For all I know, I'm only running 15mph at this point, and my primary concern is keeping it safe, and minimizing my fuel usage. The boat never had a tach before, I did add one recently, though it hasn't yet been on the water since.

But now I'm curious - am I actually using more fuel by running it slower? Am I causing accelerated wear or some other kind of damage by not running it WOT when the water is clear? Is "chasing that last 2-3 mph" on prop changes actually making it more efficient at slower speeds too?

 

[JimS123]

My feelings are quite similar to yours. My approach has always been to optimize with data and then go for best economy. I rarely run at WOT, except maybe once a year to blow the stink out.

Fortunately, performance data has been readily available for my current boats. The data shows speed and fuel usage for each rpm. From that I pick the optimum mpg and that's where I typically run.

Early on I did do a few WOT runs to verify that the prop was correct, and also to determine my prop slip.

Since published reports for an old boat like yours may not be readily available, find data for a comparable engine on a comparable size boat and you will probably be in the right ballpark.

 

[Chris1956]

You want to prop your boat so it will run at max recommended RPM at WOT. That gives keeps you in the motors powerband properly. Lugging the motor is bad on several levels.

Once you have that, pick a speed that you like and run it.

As for best economy, solidly on plane and trimmed near max, is probably the best. A fuel flow meter would be necessary to get closer than that.

 

[JohnSz]

You are running as close to being the most efficient without additional gauges. Myself, I like to run WOT for a few minutes on every outing. Especially with an older 2 stroke like you have. Helps clean some carbon out. With the tach, you now can check if you are hitting the recommended WOT with your current prop. If you are currently popping on plane quickly, unlikely you are lugging.

 

[cyclops222]

What a simple sounding sincere statement and question.
Bless you for being honest about a complicated topic.
A couple of basic facts / conditions needed.
Accurate tachometer at all rpms / speeds.
Motor trim gauge is accurate.
Boat is loaded with maximum # of people you take on a trip.
Gas tank is full of normal gasoline.
Prop is set at correct depth to bottom of boat.
New correct gapped props.
Spotter boat running with you. To help you set correct hull angle at WOT.
Prop has been checked and set perfectly.
Accurate hand held GPS speedo checked in a car for accuracy.
Pencil and paper to keep recording values to reach maximum engine rpms.
Possible a 1/2 hour for all the best runs to reach a maximum.
All important runs to be into waves less than 12" high.

Mostly done.

( I have never done this. )

 

[racerone]

I believe the most efficient RPM for a big 2 stroke is around 3600 / 3800 RPM.----It was that way for me with a 1966 Johnson 100 HP.-----Your results may vary.

 

[cyclops222]

internal combustion engines are variable air speed pumping machines we add in gasoline so they can do useful work / things for us.

Many normal gasoline engines reach a peak intake vacuum and efficiency at around 2800 to 3600 rpms. I like any engine to do work in that range.
The real danger of NOT reaching the peak WOT RPMS when called to ? Is causing the engine to run hotter than designed. Heat equals a earlier death.

 

[dwco5051]

I have found over the years running state boats where engine hours, fuel used records were kept a on a daily basis that by running almost exactly as you describe gave the best economy. Get on plane quick and then back off and run 100 or so rpm more than it takes to hold it on plane.

 

[tpenfield]

Generally speaking, vehicles of any type do not run efficiently at WOT.

You will probably find a 'sweet spot' at around 60% of WOT RPM.

Also, consider the life span of a NASCAR engine (which is run at/near max RPM much of the time) vs. a typical automobile engine.

 

[KD4UPL]

WOT is a standard baseline for measurement. If you hit rated RPM at WOT then you know you're propped right. After that run whatever speed you like. The point isn't to operate the boat at that speed. It's to verify it's set up corectly or to document how things may have changed with a modification.
I'm not a speed junkie. My boat will do about 55 mph but I rarely go over 30 mph. Any more than that and I get an earfull frim my wife. But, I have verified that I hit rated RPM at WOT. The wife was on shore for that.

 

[dwco5051]

WOT is a standard baseline for measurement. If you hit rated RPM at WOT then you know you're propped right. After that run whatever speed you like. The point isn't to operate the boat at that speed. It's to verify it's set up corectly or to document how things may have changed with a modification.
I'm not a speed junkie. My boat will do about 55 mph but I rarely go over 30 mph. Any more than that and I get an earfull frim my wife. But, I have verified that I hit rated RPM at WOT. The wife was on shore for that.

I think I might have married her sister.