Re: Help with 5.0 220 horse on Lake Tahoe... engine RPM's and Speed?
I don't understand why elevation should matter sea level vs 6200 feet *given* that RPMs are constant. For example if I am running 4000 RPM at sea level and 4000 rpm at 6000 feet elevation, shouldn't that mean the prop would be turning at exactly the same speed, which would mean it's moving exactly the same amount of water, which would mean I'd be going the same speed at same RPM? Sure, it may take longer to get to that speed due to being at higher elevation, but it seems to me that 4000RPM is a constant, for example, whether at sea level or Tahoe.
Absolutely right. And if you can get it to run 4000 @ 6000 MSL, you would see the same speed.
The problem here is that you said you're turning 5500 RPM. That's basically impossible
if the boat is "propped" correctly AND it has the correct drive ratio.
Secondly, I will definitely research the details you asked me, but in the meantime can someone give me some basic bench marks that I should be looking for with my boat? For example what's recommended cruising RPMS and how fast should that be in MPH? Also, WOT, is this just for going fast for a few minutes so as not to burn up the engine? My RPM gauge goes all the way to 6000 RPM, and it is an inboard/ouboard. Do I ever want to run at 6000 RPM or will that burn up my engine?
In a word, NO. and again if you're propped right, ratio is correct, the engine will never make it to 6000 RPM.
In general, your engine should turn in the manufacturers recommended RPM range when at WOT. This is generally true for any marine engine I/O, OB, or inboard.
AND specifically just about all the engine makers suggest that you slect a prop that will give you an RPM right at the top of the range when lightly loaded and running at the elevation where you'll run the most.
When you get it set up like that you'll get the best all around performance out of the boat.
The recommended range will depend on what you have. Most newer engines have rev limiters so it's impossible to get the engine to exceed a preset RPM.
In general, most I/O's have a recommended upper limit of about 5000 RPM. The range varies around 4200-5000 I think the 5.0L engines have a range somewhere around 4400-5000RPM.....if it has electronic ignition, the rev limiter may be set at 5100RPM. The normal range should be in your owners manual. Since this is a Mercruiser forum, you must have a Merc. I think the drive for a 5.0L V-8 is 1.5:1
You can use a prop slip calculator to get in the ball-park of what your boat *should* do.
Prop Slip Calculator
Mercury has a prop calculator that will suggest an approx pitch to start at if yours is way wrong.
So if you had a 1.5:1 drive, 19p prop and you ran your boat at 5000 RPM, you would get a theoretical speed of 60 mph (no slip)
Since ALL props "slip" (about 15% for a single prop) your boat should do about 50 mph at WOT (5000rpm @ sealevel)
This is all at Sea Level though. The same engine and prop at Tahoe might only do about 4000 RPM or less and that would be less than recommended so you would maybe go to a 17p prop (or even a 15) because your engine would only make about 75% of max power at that alt.
(But you would still want it to run in the recommended range of RPM)
I took my 21ft 1987 Four Winns Liberator (454/Bravo III) to Tahoe a few years ago.
I have 26p props on it and at approx sea level, I can get it to run 4600 at WOT and 57mph.
At Tahoe, I installed 24p props and was only able to get it to run 4200 RPM. I don't remember what speed it ran...somewhere around 40 mph. I was surprised it even planed...I had 8 people and 3 dogs in it .
By the way, one of the things Mercury does is recommend lower drive ratios for "high" altitude operation. It's all about getting the engine to operate in the recommended range.......... You can't always just change the props (but it usually works.)
etc) issue!!