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Speed / Prop / RPM questions

Eaglenva

Eaglenva

Member
OK, I know, the infamous question with a million variables needed and I know about 2 of them... lol but with the calc's available they say I should not be going as fast as I seem to be going so I must be entering something wrong.

I have a 24' Enforcer, was told it has close to 300 hp (not sure I can trust the previous owner) and 260 came stock. The engine is a replacement but this is what seems to be the numbers.

350 v8, Alpha one 1.47:1 drive, 21" pitch Stanless, approx 3300 lbs - was having trouble getting engine up to speed (maxing out at 3300 rpm). Handheld GPS shows around 51-53 mph and of course the built in brag o meter shows closer to 57-59.

The boat handles like a dream at this speed, can do a full hard turn with no problems at these speeds. I replaced plugs, fuel filters, etc. Same speeds.

Then one day one time I really got on it hard from a stand still, the boat rpm got up to the correct operating range of 4600-4800 and the boat acted like i was on ice :0) Since the label in the boat advises that above 60mpg to not make any aggressive moves I am guessing I was getting into that area but was just trying to watch what I was doing instead of looking at the speedo.

Any thoughts?
 
The power to weight calculation comes out to be 260hp x 3300 lbs = 52mph so it seems that your engine is putting out it's stock power or maybe you just weigh more than that, did you account for fuel, battery, people, misc. etc. a few hundred lbs. can make a big difference.

-P.S. I need to go back to the built in brag-o-meter, these darn GPS's are depressing! hehe
 
Sounds like you may have some other issues, rpm's way off? My quick calculations show 1.47 drive/21P/3300rpm theoretical speed should be 45mph, 52 mph would be a negative slip of -16%, impossible. Running that drive with that pitch, @ 4800rpm your theoretical speed should be about 65mph, factor a slip percentage of 11-13% should bring you around 58-57mph.
 
So its likely the tach is incorrect, the confusing part is when it seemed to go fast enough to be on its bottom pad and I had little steering control the rpms read correct levels.

I cannot reproduce the effect as I found water in the oil that night (cracked head and block). Inspection shows replacement Jasper engine in the boat has already had 3 sleeves put in it. Trashing block and heads, re-using crank, pistons, rods and intake (which is steel instead of cast iron- intake, not the crank) - everything else will be new. Also a local shop said the prop had been reworked, possibly twice which will be changing the DAR % making the slip higher I would think.
 
what shape is the gimble ring in? your icy feel could be a loose/worn ring - just double check - huge safety issue.
 
:brickwall: Probably beyond my experience level to determine. I do know that the starboard side of it that sticks outward has some wear as the paint is gone. I have not measured to see how much. I have a Merc shop manual but if it is a safety issue I should probably have a pro look at it as there are other parts to it that need to be checked.
 
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