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New Lower Unit Lost 10 mph

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ringato77

New member
We have a 2004 Kenner 21' with 2004 Mercury 150HP Saltwater EFI. We had a shop put a new lower unit on it when the original went to crap but we noticed some time after that we lost about 10mph off the top end. Seems to jump on plane the same. Looks like that motor could've come with 1.87:1 or 2:1 ratios. Any chance that we got the lower ratio and it could cause that much of a reduction in WOT speed? My calculation is that at WOT the prop would be spinning 400 RPM less with 2:1 vs 1.87:1 which seems like a stretch to be the culprit.

As always thanks for any insight you guys have.
 
You note the difference in speed, but what about rpms?

Did you swap over the prop from the old lower or did it come with a new (or another) prop - a change in pitch OR diameter could likewise explain your top end speed difference (but again, the rpms would also be different).
 
Unfortunately we trashed the old one and I doubt the dealer can tell us what they put in the new one since it was just a rebuilt for $900. Theoretically though, with a WOT of 5750 RPM and going from 1.87:1 to 2.0:1 you think that could account for 10 mph? We also installed a T-top around that time I wouldn't be surprised if that caused a few mph loss too just from a reduction in aerodynamics.
 
Let's see how my math is -

At 5750 rpms a 2:1 gear ratio would give you 2875 "prop rotations" per minute.

With a 1.87:1 gears you would get (about) 3075 - so 200 more "prop rotations" per minute.

If you kept the same prop, and lets say just for the sake of this exercise, it was a 17 pitch.

So all else being equal, keeping the same rpms, the lower gears would push the boat (200 x 17" = 3400"/12) 283 feet "less" every minute or 16,980 less feet per hour OR 16980/5280 or 3.2 mph "less", so doesn't account for the entire 10 mph, but that is assuming your rpms are constant and NOTHING else changed.

If it's another prop with even a slightly different diameter or different pitch, then you could experience, easily a doubling or tripling of the effect.

(or if you were say running a 21 pitch prop, that would compound the difference compared to my 17" example - but still not a full 10 mph)
 
Thanks Graham that's good info and what I was thinking just didn't know how to calculate it. Not sure of the prop size or pitch but I don't think that changed. We didn't ask for it to nor where we made aware if it did is all I can really say for sure as we didn't take notes of what was there before.

Long story short, we bought the boat in a somewhat neglected state, got her tuned up and all the warning buzzers to go away and took off with our newly installed GPS and she would hit 48 MPH with three big dudes and 30 gallons of gas. Lower went out, got it replaced, and a few months later and the next opportunity we had to open it up and we noticed it would only get to about 38-39 mph. The gauges are old too so we can't say for certainty that we are getting to WOT. Sounds like it and it pulls hard but we just ordered a smartcraft system monitor to see what's going on if anything that's not obvious with the eye test.

All in all, I should say, this isn't really a problem for us we are happy with how the engine runs and it's not very often we ever need to go faster than about 30mph but we are just mostly curious where the heck the top speed went all of a sudden.

Thanks again for the feedback.
 
Ahhh, more to the story.

So, the lower unit could account for some of the speed change.

BUT, just different seasonal conditions can have a HUGE effect on a motors performance.

If you hit 48 mph in the spring or the fall, when the temperature is lower and the humidity is down then on a hot/humid day in the middle of summer you can "expect" to lose 5 to 7 mph if all else is equal, simply because the motor is having a harder time breathing.

Although almost nobody does it, it is advisable to have two different props, one with a little less pitch (which is easier for the motor to turn) to use in hot/humid weather IF you really stretch your boating season.

Since most boats are pretty much "summer toys", selecting a prop in the summer is usually good enough.
 
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