Safe cruising rpm and WOT rpm the same or different

[delmarjr]

Are safe cruising rpm and WOT rpm the same or different? I have a 1999 Mercury 2 stroke outboard on a 18 ft Bass Tracker Pro Team 185 boat. What is a safe?efficient cruising RPM and what should the WOT rpm be? How long can you run an outboard or a sterndrive motor at WOT safely?

Thank you

 

[BaileysBoat]

Re: Safe cruising rpm and WOT rpm the same or different

WOT is the engine running with the throttle wide open. The rpm should be at
or close to upper limit of the operating range with the usual boat load.

This is determined by using the correct pitch of prop etc.

Safe cruising speed is the boat on plane, getting the best fuel economy.

 

[delmarjr]

Re: Safe cruising rpm and WOT rpm the same or different

At what rpm should the boat be cruising at or is WOT acceptable for a 2 stroke outboard?

 

[crazyclam]

Re: Safe cruising rpm and WOT rpm the same or different

I do not know about numbers...but you can tell for the most part by sound if you revs are too high......dont wanna lean it out......under a load it WOT should be ok for a while while cruising around from point A to B

 

[chris.olson]

Re: Safe cruising rpm and WOT rpm the same or different

How long can you run an outboard or a sterndrive motor at WOT safely?

If the engine is in proper tune, propped so it spins in the rated speed range for the engine, and the cooling system is working properly you should be able to run a stock marine engine at WOT all day without hurting it.​

Maximum engine speed is a function of the boat, engine and prop combination, and load in the boat. You should select a prop that allows the engine to run near the top of its rated speed range at WOT with a light load in the boat. As the load increases in the boat, the maximum WOT speed will be lower. Two different boats might take two different props with the same engine, so unless you get more specific about which '99 two-stroke you have, telling you what the maximum WOT speed is supposed to be would be just a guess.​

When you talk about "cruise speed", I assume you mean max economy cruise. Finding that is trial and error and you almost need a fuel flow meter to know for sure. Otherwise, if the boat is on plane and you don't have the throttle pinned, you're "cruising". So really, cruise rpm can be anything with the boat on plane and not running flat out.
--
Chris​

 

[Chris1956]

Re: Safe cruising rpm and WOT rpm the same or different

Chris is correct in theory. If the motor is correctly timed and the carbs are clean and adjusted, waterpump and thermostat work perfectly and the fuel is good, an outboard motor should run all day at WOT, especially a smaller motor. However, you pay a large fuel mileage penalty at WOT operation, when compared to 75% or less throttle, on any motor.


I like to think that cruising is the speed and RPM that is comfortable for the motor, boat and passengers. On smooth water, with an outboard, I think cruising speed is about 75-80% throttle. I like to run the I/Os a bit slower. Rough water or little kids and pets cause me to slow down a bit.

 

[chris.olson]

Re: Safe cruising rpm and WOT rpm the same or different

an outboard motor should run all day at WOT, especially a smaller motor.

I think stressing the "especially a smaller motor" part is important. The higher the power density, i.e. power output per unit of mass and displacement, the quicker temperatures can build above safe limits in critical components.​

100% power duty cycle in any marine engine is a function of metal mass and cooling system performance to dissipate heat. The more metal mass you have in the engine components, the more heat it can take without distortion. For instance, in a gas MerCruiser stern drive operating at continuous 100% power, cylinder head, oil and exhaust temps will eventually rise to a point that overwhelms the cooling system and you'll get shortended valve seat and valve face life, and shortened bearing life, as compared to operating the engine at a lower continous power setting. However, a Cummins MerCruiser will run all day, around the clock, at 100% power without hurting it. This is due the iron mass in the Cummins diesel, as compared to the gas power unit that may have the same hp rating - the Cummins has enough iron mass to take extremely high operating temperatures without dimensional tolerances changing in the engine.​

But like you say, the fuel penalty for continuous full throttle operation is huge. I really can think of few situations outside of racing where you'd want to run your engine at much above 85% power, continuous.
--
Chris​

 

[chris.olson]

Re: Safe cruising rpm and WOT rpm the same or different

The higher the power density, i.e. power output per unit of mass and displacement, the quicker temperatures can build above safe limits in critical components.

Being an engineer by profession, I feel I should further clarify this.

Marine engineers typically design their stock, non-racing applications with the capability to operate at 100% power, continuous. I'm not aware of one that doesn't. That doesn't mean there isn't one - just that I'm not aware of it.​

However, here's the kicker; while you may not get a rod hanging thru the crankcase because you operate at continuous 100% power, you WILL ultimately shorten engine life because you're operating it under higher stresses. You may end up with an overhaul at 2,500 hours instead of 4,000, or whatever.​

Marine diesels are the exception - they are MADE, and their drives geared, to run flat out and that's where they get their best fuel economy. But the diesel (heat) cycle is inherently more efficient than the Otto cycle, so that's a whole 'nother can of worms to open.
--
Chris​

 

[delmarjr]

Re: Safe cruising rpm and WOT rpm the same or different

The Seloc manual says WOT should be 5000-5500 rpms and thats what I do read at WOT. Usually about 5500 is what I see. What RPM do you recommend I should cruise this engine at for good efficiency and durability? Or is it designed to run at WOT all day long for both?

 

[chris.olson]

Re: Safe cruising rpm and WOT rpm the same or different

I should cruise this engine at for good efficiency and durability? Or is it designed to run at WOT all day long for both?

Economy cruise is usually the sweet spot where the boat is comfortably on plane and you're using minimum throttle opening to keep it there. It varies depending on the boat, how the boat is loaded and rigged and how you got it trimmed.​

"Cruising" at WOT leverages a huge penalty in fuel economy. So unless you're a bass racer trying to make weigh-in, or just like to go fast and don't care how much fuel you burn, "cruising" at WOT is usually not recommended.​

When we go fishing on the Lake Of The Woods with our Alumacraft Navigator with a 150 OptiMax I find that a 40-50 mile run up the big part of the lake to get to the islands, running at about half throttle, is a good compromise between the time it takes to get there and how much fuel we got left to get home on. We could get to our fishing spot a lot faster if we ran at full throttle all the way, but we wouldn't have enough fuel left to get back home.
--
Chris​

 

[Silvertip]

Re: Safe cruising rpm and WOT rpm the same or different

Your question was answered several times. Yes -- run it wide open safely. If it wasn't safe to operate at that RPM the manufacturers would set the rev limiter much lower than they do. But understand it won't be as economical as running it at cruising speed nor will it last as long. Do you think your car would be as economical and last as long if run at wide open throttle every time you go in it. Cruising speed is whatever you are comfortable with and other than that "Cruising Speed" has no specific definition. Unless you have a fuel flow meter, you can only guess what the most economical rpm is. But generally, 3500 - 3800 rpm will produce the best economy and that should be somewhere around 2/3 throttle. But because that RPM provides the best "economy", that does not mean that is someone else would consider the best cruise "speed".

 

[saumon]

Re: Safe cruising rpm and WOT rpm the same or different

I've always done the "gun out of the hole then throttle back" thing, and i adjust it by the sound. On my particular setup (17 ft aluminum side console fishing boat + Merc. 60hp 2-strokes) the engine is happy and made a sweet sound at around 4000 rpm, wich translate to 26 mph. If I push it to 5500rpm, i only get 33 mph (and a winding high pitched sound), wich is not a great gain for 1500 more rpm.

It seems to be confirmed by theses tests on 50, 90 and 150 2-strokes OB on various setup where the peak efficiency (MPG) is around 4000 rpm, ranging from 3500 to 4500.

http://www.mercurymarine.com/look_d...=Title&Section=outboardChecks&hp5090150efi=20

Naturally, for all this to work your rig has to be set right i.e. being able to reach the engine upper range of the recommended rpm at WOT.