.mercruiser 3.0 on Suntracker party deck 21

[cmartin5693]

I have a tracker party deck 21 that i bought new in 1999. It has a mercury 3.0 with alpha 1 outdrive. I have changed the prop 3 different times in the years i have owned it. I usually use this boat on a small lake with a 5mph limit. Recently i have taken it out to a larger lake and noticed a significant drop in top speed, used to be 38-40 now it is 28mph. I have changed plugs on it. I have a 15.25"/15 pitch quicksilver aluminum prop. I am certain i have the wrong pitch but have no idea what the original pitch on this boat should be. I am getting 4800rpm. Anyone have a y suggestions on correct prop? My elevation where i use the boat is 1400ft. Boat weight is about 2400lbs with fuel, no passenger or gear.

Thank you

 

[alldodge]

You Were getting 38-40 mph and now only 28
Motor reaching 4800 rpm so I would do a compression test.

Could just need a tune up, but your reaching the recommended WOT, but may not be producing the same HP. Pull the plugs and see what they look like

 

[cmartin5693]

You Were getting 38-40 mph and now only 28
Motor reaching 4800 rpm so I would do a compression test.

Could just need a tune up, but your reaching the recommended WOT, but may not be producing the same HP. Pull the plugs and see what they look like

Thank you for the reply. I replaced the plugs with out much benefit. I put a new 15 pitch prop and hub on it, again not much gain there. I keep thinking that the oem prop was a 17 or even a 19 pitch but that should theoretically reduce my WOT rpm. Of course i am not 100% that the tach is accurate after all these years either. I have ordered a new cap and rotor since its 21 years old. Trying to find out what the oem prop would have been. I bought the boat in sacramento but the dealer is no longer around from what i heard.

 

[alldodge]

The issue is, the prop is what the motor uses to apply its power. If the prop has the correct pitch for the power the motor produces, then it will turn the prop at the motors max rpm. If your tachometer is accurate, your motor is turning the 15P prop at max WOT rpm (4800).

So if you change the prop to more pitch 17, 19 then the motor WOT rpm drops. Rule of thumb is 200 rpm for each inch of pitch. So going from 15 to 17 you will loose 400 rpm.

Your already at max rpm, so you have a weak motor or something else is going on

 

[cmartin5693]

The issue is, the prop is what the motor uses to apply its power. If the prop has the correct pitch for the power the motor produces, then it will turn the prop at the motors max rpm. If your tachometer is accurate, your motor is turning the 15P prop at max WOT rpm (4800).

So if you change the prop to more pitch 17, 19 then the motor WOT rpm drops. Rule of thumb is 200 rpm for each inch of pitch. So going from 15 to 17 you will loose 400 rpm.

Your already at max rpm, so you have a weak motor or something else is going on

It seems like at about 80% throttle it tops out on speed but continues to raise rpm another 200-300 rpm as i open to wot which is why i question the prop pitch. I understand if i go higher it drops rpm so i am not clear if i should go up in pitch or down?

 

[alldodge]

The IF, so if the motor does reach WOT rpm with a given prop then your good. If it does not reach the rpm the pitch needs to decrease, or increased if it exceeds the rpm

 

[cmartin5693]

The IF, so if the motor does reach WOT rpm with a given prop then your good. If it does not reach the rpm the pitch needs to decrease, or increased if it exceeds the rpm

I have been doing some reading and went out and inspected the hull and I have a film from being stored in the Lake all summer so my son and I just scrubbed the hull. I will take it out and test it in the next day or 2. Do you know of a good product I can put on that will prevent algae build-up when it's stored in a Lake? It's a painted aluminum hull. Thank you for all of your input and I'll follow up with the results of my next test.

 

[alldodge]

Only thing I know that appears to work for short periods is Ceramic coating. It still attaches but takes next to nothing to remove and the water itself can do it. Ceramic coating is expensive. Otherwise there is nothing out there that will keep it off that I'm aware of.

 

[Scott Danforth]

Do you know of a good product I can put on that will prevent algae build-up when it's stored in a Lake? It's a painted aluminum hull.

elbow grease and wax

 

[Faztbullet]

This.....My fiberglass guy uses it..lil pricey but worth it.
https://www.auroramarine.com/store/v...ttom-coat.html