Lagging the Motor

[branman1971]

I was told with my 70hp Yamaha 2 stroke that my RPMs should be closer to 5500rpm at WOT than the 4500rpm that I am getting with this prop. I have a new prop on the way. My question is, if I am lagging the motor at WOT at 4500rpm, am I not lagging the motor at any and all speeds? I mean, if I am cruising 8mph at 2000rpm, is it still "lagging" because of the less than ideal prop?

 

[Dhadley]

Re: Lagging the Motor

Yes, if you are lugging the motor at wot then you are lugging it throughout the rpm range. Putting on a smaller pitch prop is always a last resort when trying to gain rpm. Always work the set up first.

This is assuming the motor is healthy and capable of reaching full rpm.

 

[branman1971]

Re: Lagging the Motor

So no matter my speed or rpm, it is still bad for the motor?

 

[steelespike]

Re: Lagging the Motor

That is lugging not lagging.
Anything higher than displacement speeds will agravate lugging.
I certainly wouldn't run over 2,000 for any extended time.
Motor isn't going to fail instantly but gradual detereoration will begin.

 

[mthieme]

Re: Lagging the Motor

I have a 50 HP Mercury in the garage...

 

[branman1971]

Re: Lagging the Motor

I have a 50 HP Mercury in the garage...

Hey mthieme, you misspelled "Garbage"

 

[towtruck1]

Re: Lagging the Motor

If you are below the recommended RPM range at WOT because of an improperly sized propeller, then that propeller is also improperly sized for running at less than WOT. Therefore, you would always be lugging the engine.

If you are below the recommended RPM range at WOT because of an improperly tuned/operating engine, that is a different issue. Fix that first, then check your rpm.

BTW, it would be helpful to tell us what the old prop was (dia. x pitch etc.), and what the new one is.

 

[branman1971]

Re: Lagging the Motor

Sorry, I put all that info in the posts under "Pontoons", probably should have gone here.

It's a 2003 Yamaha 70hp, 2stroke, with a 13.75 x 15, and i should be getting my new 13pitch in the mail today. I am sure the motor is in great shape, I just took it off of a 16ft runabout where it ran like a gorilla.

 

[towtruck1]

Re: Lagging the Motor

..I just took it off of a 16ft runabout where it ran like a gorilla.

... and put it on a ______? If its a heavier boat, that could cause you to not get the rpm you need. Sounds like the 13 pitch should help quite a bit. Of course you won't get the same top speed as the gorilla boat.

 

[branman1971]

Re: Lagging the Motor

I put it on a 20ft pontoon boat.

 

[towtruck1]

Re: Lagging the Motor

Ooops! Now ya tell us! It all makes sense now.

That barge is a whole different animal. You are moving a lot of weight rather slowly (you cannot plane even w/ a bzillion hp) as opposed to moving less weight on plane with your runabout.

You probably need to go down to a 10 pitch to get your rpm where you want it (5500-4500 = 1000 rpm, divided by about 200 rpm/1" of pitch). That of course will mean a bigger diameter prop. Before you buy, make sure it fits (make sure you've got clearance with the anti-caviataion plate).