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Author Topic:   How to determine "cruising speed" for your engine
Buckda posted 01-12-2003 08:13 PM ET (US)   Profile for Buckda   Send Email to Buckda  
Is there a rule of thumb for how to determine the "cruising speed" (that speed at which your engine runs most efficiently?

I have a '95 Evinrude 2-stroke (70 HP) on my '89 15-GLS. The only instrumentation is a tachometer.

I know WOT RPM should max out at 5,800 - so I don't exceed that (although current prop will allow me to over-rev) - but how do you determine the most efficient zone for engine operation?

Is it simply the slowest speed that the boat will go but be "on plane?" Like JimH has noticed with his 15, mine "hunts" for a faster speed once it's up on plane - tough to keep the RPMs from creeping up on me.

Second part of the question: Approximately what gas mileage should I expect under "normal operation?" (Trying to determine range.) I have 2 tanks - a 12 gallon main tank and 6 gallon reserve.

Thanks in advance for sharing your experience.


Clark Roberts posted 01-13-2003 06:52 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Buck, if you cruise with engine at about 100rpm more than max torque rpm (see engine specs) that should be best fuel mileage. Of course there are so many variables like prop pitch/type, hull type/condition, load, weather/waves etc that the average will vary all over the place! Also you will get many opinions re: your question. Some data supports the following: Big V6 best mileage at around 3500, 3 and 4 cyl engines around 4000 to maybe 4500 in some cases. Generalities aside, for your specific engine, a loop charged 3 cyl develops some definite harmonics due to the tuned exhaust and you can hear it "get on the pipe" with a melodious tone when the exhaust tuning is efficiently "scavenging" (pulling exhaust out(this is what loop charging is all about!). You should hear the first harmonic at about 3000rpm (up on plane) and it will come on like a bugle if you are listening! Second harmonic will come on at maybe 3800rpm (just guessing here) and third harmonic at about 4400 (you will be flying by now). Cruise at any of these harmonics and that should get you the best mileage for that speed range. The middle harmonic may provide best in terms of MPG... Not trying to be technical (not smart enough) but the above I have experienced (emperically) from many, many hrs. with 3 cyl loopers (OMC, Suzuki,Yamaha and Merc) from the small 30hp Yams and Suzis to the big bore 85 cubic inch Mercs and all display the above discribed characteristics... whew, don't know when to stop.. happy Whalin'.. Clark.. Spruce Creek Navy
PS> all engines produce harmonics at some point but the 3cyl loopers really shout at you!
Bigshot posted 01-13-2003 09:36 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I ran my 15 with 13x19 made into a 20". It would hit about 5900(redline 6k) and would cruise nicely at 3500.
Buckda posted 01-13-2003 01:18 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Clark and Bigshot - thanks for the feedback. I'll listen to the engine more closely next time I'm out on the boat.

I've made it from South Haven and back (from St. Joseph) on about 8 gallons, but that was a very calm day and I ran at varying speeds. I think that is about 40 miles which puts me at roughly 5 miles per gallon with two average sized adults aboard.

That's pretty good!

I'm currently running a 13x15 prop and planning to trade up to a 13x19 - to give me greater top end speed, and keep me from over-revving the engine.

How should this affect my fuel consumption?

Bigshot posted 01-13-2003 01:39 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Yikes.....a 15". WAAAYYY too small. Amazed you can get 35mph. The 19 will increase your MPG.
Buckda posted 01-13-2003 02:01 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Bigshot -

Yeah..well, it's what came with the boat. Cabelas never seems to have the Michigan Wheel Props in stock, so I'm waiting (want the Cabelas points).

One thing about the 13x15 - the hole shot is incredible. The boat will literally throw your passenger over the transom if they don't hang on to the railing and almost launches the boat straight out of the water if you start with the motor trimmed up. Time to plane: a hair past a freckle - it's fast. Makes pulling up a skier nice 'n' easy, but I'm looking to upgrade this spring and keep the 15 as a spare on board "in case."

lhg posted 01-13-2003 02:24 PM ET (US)     Profile for lhg    
I have always thought that optimum cruising speed/economy is a combination of boat speed and RPM's. I think most Whalers would have an optimium planing speed not related to engine size. This would be where the boat planes easily, high on the water, but not to the point where wind resistance starts to take over and decrease economy. Wave condition can also be a factor. My guess, is that 25-30MPH would be this range for many Whalers. AS an example, I have noticed that my 25 Whaler has a faster HULL max economy cruising speed (about 30 mph) than the 18 Outrage (about 25 mph). Then, when your engine is sized to give this speed within it's most efficient range, usually 3000-3500 as Clark mentioned, you have a perfect combination.

This usually results in another situation where it is unwise to underpower. Using a hull's full rated HP will usually give the situation described above. If your boat's most efficient planing speed is around 30, you don't want an engine turning 5000 to achieve it. I am convinced that a 50 or a 90 pushing a Montauk at 30mph will result in the same fuel mileage, but wear and tear on the 50 will be twice as great, as will be engine noise.

Bigshot posted 01-13-2003 02:49 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
You are dead on Larry. That is why I was skeptical about a 70 on my Montauk but that worked out better than anticipated and obviously I recommend that engine. If I had the choice I would always chose the bigger engine unless it was a situation where you lived on a lake with a 30mph speed limit or something. My Montauk loves to cruise at 25-28mph and I feel if you can do that under 4000 rpm's than you have a suitable package. On bigger hulls you want it to be about 28-32 cruise speed, especially on deeper v hulls. Putting a 50 on a 17 may be worse than coughing up for a 60-70 which would be more suitable over the long run....maybe not for some though. Just like with a 100hp on a montauk, a 15' is great with a 50-60hp and the 70 is a tad overkill but FUN.

Buckda, the holeshot with a 19" will throw them overboard as well. I could slam the throttle back and forth and literally jump the boat out of the water. My 15' with a 48 was fast and had some scare factor. The 70 could get downright dangerous:)

BQUICK posted 01-13-2003 05:12 PM ET (US)     Profile for BQUICK  Send Email to BQUICK     
My 70 50 Merc has a nice feature where when the control is level the timing is maxed out after that it's all throttle. (verified with cover off.) The best mileage (cruising) is there for me.


Doug Weaver posted 01-13-2003 07:55 PM ET (US)     Profile for Doug Weaver  Send Email to Doug Weaver     
I just read a really interesting article about this subject. I am considering purchasing a floscan setup to replace the tachometer, fuel gauge and hour meter that died when my water pressure gauge took a wizz all over the inside of the console and corroded everything. Using the floscan is a great, but expensive (about $2-300) way to find the sweet spot or optimum cruising speed. I would make sure you have the right prop and that everything is tuned properly before installing this system. The floscan will tell you your fuel usage in real time, changing with the way you drive, weather conditions, weight etc. Also tracks fuel level and running hours. I don't think there is a more accurate way of determining your boat's sweet spot, although there are other more general ways to determine what speed or rpms you should run to optimize your fuel economy. But, these cannot always account for changes in wind, weight and so on. Good luck. Will let you know if and when I get around to talking my wife into letting me buy yet another boat toy.
Bill C posted 01-17-2003 09:20 PM ET (US)     Profile for Bill C  Send Email to Bill C     
Dave. If you decide to try a fuel flow meter check Boaters World. A Navman for $150. Identical to and made by Standard Horizon. Only difference is brand labeling. The SH name costs more. Getting ready to install on my 15SSL. My reason for this, aside form determining best cruise RPMs, is remaining fuel. Gauge can be set to show starting gallons and counts down as fuel is used. My tanks are covered by the rear deck during use so this gauge will let me know when to head home.

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