lugging a two stroke merc

[fishdog4449]

My dad insists that switching between my SS 19P and aluminum 17P isn't necessary just to pull skiers/tubes. He said that the motor is just as happy running WOT at 4000 rather than 5800, it's just "working harder". I know that the motor has to be propped right otherwise it will lug and perhaps cause damage, however I don't know the mechanics behind it so he doesn't believe me. Could anyone explain why a two-stroke motor such as mine (76 merc 1150) will be damaged by not running at the proper WOT rpm?
Thanks,
MJK

 

[Chinewalker]

Re: lugging a two stroke merc

Hi MJK,
Think of it this way. If you are towing a heavy trailer, up hill, you would downshift to bring the motor RPM up and maintain your speed. If you don't do that, your car's motor would likely start pinging and your engine temperature would likely rise. The motor is working harder, for sure. Continue without downshifting and you could conceiveably burn a piston due to excessive pinging and heat buildup.

Same thing happens with your outboard. In a boat, you are ALWAYS going uphill, so you prop the motor so that it will maintain it's max RPM without pinging and building up that excess heat. You don't have to run it an max, but it should certainly be capable of reaching it when needed. If you prop it to only reach 4000 RPM at wide open, then you are essentially leaving your transmission in overdrive while driving uphill with that trailer.

Hope that helps...
- Scott

 

[fishdog4449]

Re: lugging a two stroke merc

Thanks Chine,
Just what i needed. He's really good with cars, but has zero experience with outboards.

 

[Silvertip]

Re: lugging a two stroke merc

Another way to look at this is that if the engine is propped wrong for wide open throttle, it is propped wrong throughout the rpm band. If overpropped for example, one would need more throttle opening to maintain a given speed than you would if the engine was propped correctly.

 

[DJ]

Re: lugging a two stroke merc

I have a 2001 Merc. 75 HP, on a toon, with over 1200 hours.

It runs-fine.

It's propped to run at its high end of the RPM range.

I attribute its life to good fuel, good oil and propped right.

DON'T LUG IT.

 

[dingbat]

Re: lugging a two stroke merc

This brings up a little different question. Why prop a motor at WOT? Most of the motors I've dealt make max HP below WOT and then drop off pretty quickly

For instance, the advertised WOT on my motor is 5800 rpm. The motor is power rated at 5000-5500 RPM so when the ?experts? set up my boat they propped with a WOT of 5500 rpm.

Is this right, wrong or indifferent?

 

[dajohnson53]

Re: lugging a two stroke merc

Thanks Chine,
Just what i needed. He's really good with cars, but has zero experience with outboards.

I addition to Chinewalker's excellent downshifting vs. overdrive explanation, just point out to your dad that the engine has an actual specification for the proper wide open throttle operating range (in rpm). Even if you or he didn't understand the gearing explanation, why would you prop your engine at any thing other than what's recommended? I'll bet your dad wouldn't even think of questioning or not following other specs like spark plug, oil or gasoline type, torque specs. etc.