RPM Too High?

DWhitefield

Cadet
Joined
Jun 15, 2021
Messages
11
1) Performance issue you are trying to correct.
WOT running about 5K RPM at 44 MPH, I believe it should be max 4800 RPM.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Unknown manufacturer/model, aluminum prop.
3) Current prop diameter and pitch (required).
14.5 x 19
4) Wide open throttle RPM and speed with an average load (very helpful)
Running with two people, got to 5,000 RPM at 44 MPH.
5) Engine/drive make, model, year, and HP
1994 Mercruiser 4.3L, I'm not 100% on HP, 210? Maybe?
6) Boat make model, year, length and weight
1993 Maxum 1800SR, 18ft. Unknown on weight.

Really just trying to figure out if I need to change props due to RPM running too high. I don't run at WOT much, just do it for a speed test and saw that the RPM was high. I do have a stainless prop, 14.5 x 19 that I was thinking about putting on there. From my little research, stainless will reduce RPM by 100-200?

Thanks!
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Make sure you know what the Recommended Rpm Range is. If it is 4400-4800, then you are a couple of hundred over, and you will need to add an extra inch of prop. However most Prop Lines use a 2" difference, so you would be looking at about a 400 rpm drop, which puts you in the center of the range.
I never experienced with an Rpm Drop when switching to Stainless, unless the Aluminium was blowing out.
There is more to Props than just Material, Diameter, and Pitch. Rake and Cup can, and will have an enormous effect on how the Boat rides and handles
 

Scott06

Vice Admiral
Joined
Apr 20, 2014
Messages
5,546
You are propped perfectly as long as your tach is accurate. You are better off as you are being slightly over propped then under propped and lugging the engine.

I wouldn't say you have to change the prop based on being 200 rpm over , but since you own the SS I would go ahead and try it for sure. You will likely see better overall performance
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
You are propped perfectly as long as your tach is accurate. You are better off as you are being slightly over propped then under propped and lugging the engine.
You have that backwards
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,310
I would not worry about turning 5000 RPM
 

DWhitefield

Cadet
Joined
Jun 15, 2021
Messages
11
You are propped perfectly as long as your tach is accurate. You are better off as you are being slightly over propped then under propped and lugging the engine.

I wouldn't say you have to change the prop based on being 200 rpm over , but since you own the SS I would go ahead and try it for sure. You will likely see better overall performance
Hey all,

Ended up switching to the SS prop. At WOT it did tend to stay at about 4800RPM, but when I started playing with the trim it would increase back up to about 5000. I did pick up a few MPH with the SS, running at 47MPH into a head-wind with a lot more fuel than I had in there when I did the original speed test.

Thanks for the feedback, I'll probably just leave it how it is.
 

Scott06

Vice Admiral
Joined
Apr 20, 2014
Messages
5,546
Hey all,

Ended up switching to the SS prop. At WOT it did tend to stay at about 4800RPM, but when I started playing with the trim it would increase back up to about 5000. I did pick up a few MPH with the SS, running at 47MPH into a head-wind with a lot more fuel than I had in there when I did the original speed test.

Thanks for the feedback, I'll probably just leave it how it is.
If you got 3 mph more at the same rpm with the SS prop then that is a lot less slip vs the original prop.

Trimming out will get more of the hull out of the water and allow the engine to spin up more. Also you can reach a point where with too much trim the prop can cavitate especially in turns.

Likely you will be at a lower RPM with a few people in the boat . With that set up 47 is doing pretty good.
 

airshot

Rear Admiral
Joined
Jul 22, 2008
Messages
4,222
On my small boat, I typically prop it to run 100-200 rpm over when alone with a light load....I do watch my tach !!
This works better when I have passengers or extra weight. Have done this to all my boats and get better performance overall. Over propping and lugging under load can cause other issues.
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
What age is the 4.3 ? If you are using the dash tachometer… it’s likely not too accurate anyway. The most recent of the carb 4.3 (into the 2000’s certainly) had a limiter at or before 5000 anyway.
 
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