Maximum RPM

[deppong]

Is running an outboard close to maximum RPM for any length of time a good idea. If I have a 5500 maximum RPM limit - what is that telling me. Is that the absolute maximum I can run the engine or as long as I dont run it over that threshold I'm OK. I can take a guess and say that the lower the RPM the better it is for the engine for any extended time or am I being too cautious? Anyone have a feel for what makes sense?

 

[JB]

Re: Maximum RPM

Howdy, Deppong. If you exceed max rpm engine damage is likely. Running at max rpm (w.o.t., wide open throttle) for more than a few minutes at a time is a bad idea. Your engine is probably happiest and most efficient at "cruise" rpm, typically 3000 to 4000 rpm, depending on your boat and prop. Cruise should be achieved at half to 3/4 throttle.<br /><br />If you have the ideal prop you have to get it out of the water to exceed max rpm.<br /><br />The time to use w.o.t. is getting a planing hull up on plane. Once she's up, reduce throttle to 1/2 to 3/4 for most efficient operation and longest engine life.<br /><br />Happy Boating.<br />JB

 

[MERCMAN]

Re: Maximum RPM

If your max rpm's is 5500 you won't to prop it to run that rpm even though you might not run it at that rpm. It does not hurt to run at max rpm, It's better to run at the recommend range than at because it keep the carbon blown out.Outboards are made to run not to babed.As long as you run good oil and 87 minimum octane and perform perventive mantinance you should be OK.

 

[deppong]

Re: Maximum RPM

Sounds like good advise - Thanks!

 

[krosemond]

Re: Maximum RPM

I have just been through this process of changing my prop pitch down to 15 to achieve max rpm in the 5500 range (79 Johnson 150), but now she starts to fall off plane under 3500. The boat performs best in the 4000-4500 range, but is that too much to cruise for the health of the engine? Between 3500-4000 I can't find a place with the throttle where she isn't picking up or losing speed. That is a pain, because I am constantly tapping up or down on the throttle. I know the solution is more horsepower, but that is not an option right now. Whataya think?

 

[deppong]

Re: Maximum RPM

krose - I originally had a 21 pitch and went to a 19 pitch and it seems to perform better. My RPMs went up (expected) but I plane a lot quicker (Force 120) - I would think the lower pitch will provide more torque and get you out of the whole faster but after that you get diminishing returns. Torque gets you out of the whole but then speed gets you to plane. The higher pitch will give you more speed. What do you think - make sense???

 

[krosemond]

Re: Maximum RPM

I am aware of how the prop effects performance of the boat, but I was only getting about max 4500 out of the 15 and although she was a little slow out of the hole, she did perform better at 3500-3700 rpms. That is 82% of max and I was told that bogging the engine would kill it. With the new prop, if I cruise at 3700, it is 67% of max. I am willing to sacrifice top end speed for the sake of engine health and reliability. My question is: Now that I have the boat propped correctly for it's<br />design, weight and average load, is it ok to cruise at the higher rpms (4000-4300)for better boat performance and not sacrifice doing what is best for the engine. I think what has happened over the years is that what was dry foam in the hull is now wet foam and I have pick up some extra lbs. If I repower at some point, I will go to 200hp.

 

[deppong]

Re: Maximum RPM

Does not sound like you are getting max RPM. But according to the others it does not sound like running at 4300 will cause a problem. I guess the general rule of thumb is - running at lower RPM is better for the engine (will last longer) if you can get the performance you want. As far as weighing to much try going thru the truck weighing stations - they have no problem with doing that. Later!

 

[JB]

Re: Maximum RPM

Ahoy, Krosemond. If you are getting 4000 to 4300 at 3/4 throttle or less, that is a good cruise for your boat/prop combination. It is best to have a prop that allows you to reach, but not exceed, max rpm. If you can't hit that target bull's eye, it is easier on your engine to under prop (less than perfect pitch) than to over prop. Sounds like you and Deppong have made good choices.<br /><br />Keep your lines wet!<br />JB

 

[krosemond]

Re: Maximum RPM

I made an error in that earlier post, I was only getting 4500 out of the 17,(that was over loaded and on the lake), but at the coast last weekend, it turned really close to 5500 with the 15 and fully loaded with gear, but only 2 adults. I think the gain of 1000 was a combination of factors: less pitch, lighter load and better engine performance due to some ignition work. Here is a interesting question: Does the increased buoyancy of saltwater allow for better planing and faster speeds when compared to running the same rig at the lake? As a kid, I once asked my dad why it was harder to cut on a slalom ski at the beach, and he explained it away by saying that saltwater was more buoyant. Never thought about that again until just now.

 

[JB]

Re: Maximum RPM

Yeah, Krose, saltwater is heavier so theoretically your boat will ride higher, plane easier and go faster. You would need very sensitive instruments to measure the difference.<br /><br />Good luck.<br />JB

 

[krosemond]

Re: Maximum RPM

JB, it was just a thought, i don't think it is worth measuring.

<br />I've got to ban myself from this board, ever since I found it I have been analyzing everything about the boat. I used to just get in it and go.<br /><br />If I started doing that with my golf swing, I would be in the woods all day. I think they call it "paralysis through analysis".<br /><br />Its been fun though, I have enjoy everyone's virtual company.

<br /><br />Ken