Lost 200 rpms!?!

[sutor623]

Hey guys trying to figure out how I lost 200 rpms. Speed of boat seems to be within .5-1mph of where it always has been. It is a 20' finerglass boat, no jack plate. Motor is evinrude 130hp (1995 v4 looper).

Anyways, motor runs great, and speed/rpms doesn't seem to be affected by how much gas is in the tank or if there is an extra person on board. Have gone out three times in the recent past and all 3 times I was running at 5500 rpms instead of my usual 5700 rpms.

There was one incident that happened but I don't know if it triggered this or not. I ran into some shallow water and chopped up some sand. There are a few very tiny chips (I mean very tiny) on the leading edge of the prop. The rpms dropped sometime after that incident, although I can't pinpoint it to that incident exactly.

Would that be enough to shave off a whole 200 rpms? I feel like the old girl is lugging a bit. The wot rpm range is 5500-6000 and I don't wanna be on the bottom side of that. Thanks fellas!

 

[SkiDad]

You probably bent the prop slightly when you hit the sand. A prop shop could true it up.

 

[Texasmark]

....and blade leading edge thickness matters......one of the key differences between SS high performance props and alum and it matters. Other thing is that your little minute dings could generate little vacuum pockets as the water flows across them known as Cavitation along the edge of the blade which can reduce the density of the water and allow the prop to "slip" more than it was before. However, reducing the density of the water would allow your rpms to increase, not decrease. Maybe as Ski Dad said you dinged the blade and now it possibly has more pitch at least on one blade and that accounts for the drop. Course if one blade is affected, you should notice some engine vibration that you didn't have before.

Let's face it. You were good to go. You have this accident. Now you're not as good to go. Smoking gun. Problem solved. What do you think?

 

[sutor623]

Yea I Agree with you guys. Tex I have no problem replacing this prop. I have this boat loaded down pretty good so it wouldn't kill me to step down in pitch. How much weight do you think it would take to bring the rpms down that far?

 

[Texasmark]

Depends....as the saying goes. No real way of telling unless you play with different things till you get your answer. Not trying to be vague, but something as simple as the temperature when you run your test vs a previous test could make the difference. Case in point, next time you go boating, if you go on a cool, early morning, get everything setup and take note of how things are arranged and all and your rpms at WOT, optimum running conditions. Run the same test at 2 PM. You could easily have lost your 200 or more.

 

[Sea Rider]

When running at 5700 revs loaded as stated, was it at wot, or it's a personal cruising speed with slight less throttle than full ?

Happy Boating

 

[sutor623]

Thanks for the replies fellers. Sea rider, 5700 was WOT.

Tex, I think you are right about temp and other things changing the WOT settings. I had been running the boat all winter and through early spring. I put the new powerhead on in the winter so it hadnt even been run in summer since the rebuild. I believe the temp was much higher (20-30 degrees) in the air and maybe the same with water temp. So it could have been the denser colder air kicking the RPMs up where I was "used" to seeing them.

Next could be bad tach, although I doubt that is the issue. I found my prop slip to be right at 10%, so at those numbers, if I truly lost 200rpms it would have equated to a 1.5mph speed drop. I will have to check my GPS speed and see if that lines up.

Last but not least, I have smart tab SX trim tabs. I wonder if I had changed the setting on the tabs.

Anyways, Ill be on the lake this friday so I will change the tabs setting and check my speed vs. rpms and let ya know what I find. Id like to get the rpms up a bit, but I do not want to sacrifice the top speed drop that I will inevitably get by going down in pitch.

 

[sutor623]

So, I changed the smart tabs down to the lowest setting, didnt have the 400lb. bait tank in the boat, and trimmed the engine out to max and I hit 5600rpms. This was with three people aboard. I think when I ran it with nobody else on board I was topping out at 5800 rpms, so 5600 with two others on board seems about right. I am getting a little bit of vibration if I trim out a little too fast when getting on plane, but other than that it seems smooth. This prop used to be perfect and now it is not. I think I may want to experiment with some different props. Sure wish there was a shop around here that would let me do that.

Anyways I think it was the added weight that was the main cause for the lower rpms and possible a bit of prop damage.