16' searay 115 mercury prop pitch

[breeze4820]

I just boat a 1990 16' searay with a 1990 115 mercury. It came with a 21 pitch prop which pushes the boat 38-40 mph at 4500 rpms. If I drop down to a 19 pitch will I be able to pull tubes,skiers, and wake boarders? Looking for all around good pitch. Any suggestions would be greatly appreciated.

 

[steelespike]

Assuming the tach is accurate I think a 19" is a better choice.Should improve hole shot.
Is the present prop an aluminum 3 blade?
You might also consider the Merc Spitfire 4 blade in a 19" It should improve hole shot, Have better grip,
stay on plane better at reduced throttle and respond better around the dock.
it is said to maintain rpm like a 3 blade.

 

[Barramundi NQ]

Going to a 19 pitch prop will generally increase your revs by 500 rpm. So assuming that your engine is in good shape it will now run closer to 5000 rpm. Do you know the WOT maximum rpm for the mercury 115? You should be ok but just a bit of due diligence to make sure that you don't exceed the maximum rpm and kill your motor

 

[Texasmark]

5250 for WOT rpms upper recommended limit from the book. Even without propping for water skiing, you are at the bottom of the range and starting to lug the engine which results in poor performance, carbon build up, fouled plugs and hard starting. Due to the physics of the 2 stroke where the HP per charge has to be realized in the first half of the power stroke you need to run rpms since you don't have cubic inches nor the lugging ability like you would have in a 4 stroke, like a diesel: low rpms, long stroke, power all the way down sort of thing. Your engine is 113 cu in putting out 115 hp usually measured at the center of the operating range. I'd say you need the 19 for current use and drop down to a 17 if you want to ski. 21 to 19 is 2" drop at 200 per inch is +400 rpm added to your current 4500 puts you up at 4900 and much closer to where you need to be in the first place.

I have been running Mercs in this hp range at 5500 to 6000 WPT since 1989; nothing special just stock engines. They love it, have no problems with them, great performance, and haven't lost an engine yet.

I ran a 19 ported Laser SS on my 17' Ranger with a 115 tower which is a pad boat and ran 6000 at WOT all day long. Had it for 7 years and sold it to my BIL. Only thing I did was to replace the water jacket cover over the cylinders due to a water leak. Don't even remember replacing the impeller. Due to the port, my son (alone in the boat) could easily pull me up (250#) on a slalom. It had a 2:1 gear ratio like your engine.

HTH,
Mark

 

[jestor68]

Your rpm range is 4750 5250.

Base your pitch selection on the rule of thumb that each inch of pitch change results in 150-200 rpm change IF your dealing with props of the same material and design.

Pay attention to the manufacturers max rpm rating if you want your motor to last for more than a season or two.

The engine manufacturers rpm rating is not open to debate.

 

[Texasmark]

Y
The engine manufacturers rpm rating is not open to debate.

How do you figure that? Seems to me that you have the rated rpm to produce the advertised HP with a +/- 500 rpm for equipment errors. For the record sir, I have run the engine that produced the Avatar wake at more than 7000 rpm....tach limit is 7000 and that was several years ago due to a problem I had fooling around with carb/spark advance linkage.

What is the difference in a stock engine and a race engine that may run up to 10,000? If racers couldn't run their mils up there without blowing the engine just why would they do it, SIR?

Besides I don't need your assumptions. I published what I DID, not what is in a sales brochure, and the engine is still running like a top today, SIR!

Mark