replaced drive and lost performance with existing prop

NicksterFL

Seaman
Joined
Oct 29, 2014
Messages
61
Boat: 19' T-craft Runabout style
Engine: Mercruiser 3.0 (140hp)
Drive: Pre-Alpha (1.98 ratio)
Prop: 13-3/4 x 21

Here's the story. last month my drive was toast (long story) but water intrusion resulted in the lower unit failing. It was a pre-alpha upper and an alpha 1 lower unit. I bought an entire new(er) pre-alpha 1.98 ratio drive. installed and put the boat back in the water this past weekend.

By myself, the boat handled normally, maybe a little less throttle to get to cruise speed then I'm used to, etc. but when I had a load (family, etc) I noticed a significant decrease in performance. Slow to plane (30 to 45 seconds). Top end was very slow. 2500 RPM's max at about 25mph (gps) at WOT. The only thing that has changed is the drive. same prop as before.

with my old drive, i cruised at about 3200 RPM's going about 25-30 at 3/4 throttle. I could easily push over 30 mph and it still had some spunk at the top end of my throttle if I needed it.

Am I over propped now? should I go with a wider diameter and lower pitch? I'd prefer to be on plane at lower speed and have some power. we don't ski or pull tubes, but mostly fish and occasionally get into some choppy waters. so I'd want the power when i have a load.
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Had you run the boat recently, as in this year, with the old drive? Just wanting to eliminate any engine driveability problems introduced as a result of a winter layup. If the engine ran good this year just before the drive gave it up, then the drive was replaced, then it almost has to be that the drive is not the same final ratio. Just by observation. If the engine had not been run this year then problems could have occurred with the engine over the winter.

If it is just a drive ratio issue then I think you can make up for it with (gulp) propping, but the numbers you give now do present quite a gap. If you feel the engine is fine then a cheap diagnosis tool would be trying a 17p prop.
 

NicksterFL

Seaman
Joined
Oct 29, 2014
Messages
61
yeah, i'm in FL, we boat all year. I was out several times in april until my old drive (lower unit) died. Then i got a whole new(er) drive. Turns out my old drive had a 1.98 ratio and this newer drive has a 1.84 ratio, and with the 21p prop, it was just too much with a loaded boat (3 adults 2 kids, cooler, etc).

I have a 15p prop on order and that should help.
 

NicksterFL

Seaman
Joined
Oct 29, 2014
Messages
61
So I got my prop (13-3/4 x 15p) and it didn't fit. ordered the wrong one. the hub outside diameter was too small where it buts up against the drive.
Fortunately the Marina had a 14-3/4 x 17p prop and we threw that on there. With 1/2 tank gas, and just me on the boat (light load) i could only get about 3k RPM's
 

NicksterFL

Seaman
Joined
Oct 29, 2014
Messages
61
turns out my tach gauge in my console is wrong at higher RPMs. took a shop tach out and was getting 4500 rpm's with the 14-3/4x17p prop.
 
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