Prop questions

[utfyrfytr]

Last winter I put a 1989 Yamaha 70hp 2 stroke on my 1997 16' aluminum Crestliner. I don't know the make of the prop but it is a 13.5"X17 SS 3 blade. The boat weight is probably around 2000 lbs loaded. Currently I am running at 4600 RPM at WOT. It doesn't jump onto plane but it is okay. I get conflicting info about WOT RPM's as 4500-5500 and 5000-6000. I think through research that the lower RPM is correct. My question is should I change to something like a 13.5X15 to increase the RPM's to something closer to top end or just leave it as is? I would also like to know if anyone knows for sure what the WOT range truly is. If it is the higher 5000-6000 then I should really look at changing props. Any info is appreciated.

 

[jlh3rd]

I'd prop it for hitting the higher rpm range, if not the max. Then when I add weight, the motor rpm won't drop below the min.

 

[boscoe99]

Red line RPM for a Yamaha 70 HP two stroke is 5500. Yamaha suggests propping the motor so that when at WOT, boat loaded, motor trimmed up to the point of best efficiency, the motor turns at or near the red line RPM. Best performance, best engine life. Even though you may never run at WOT again.

If the boat is operated at a lighter than normal weight, mind the tachometer.

 

[jimmbo]

Going from 4600 to 5500 is going to require quite a drop in Pitch, about 4 inches. I would make sure the Motor is performing at its Best too.
Compression, Is the Timing Advancing fully, are the Carbs opening fully too
Also is the Tach set correctly, and is it accurate?
Is the Boat loaded evenly, and is the Engine trimmed for best performance? If the Motor is trimmed in too close, the Bow will plow and Engine Rpm will be lower than what it could be

 

[Chris1956]

Lowering the prop pitch 2" gives about 400 RPM gain. Your boat may vary, but that is the rule of thumb.

 

[utfyrfytr]

I checked the compression before buying, and cold it was 110,106,110. I'm not sure how to check timing advance. I'll have to look that up in my manual. I'll check to make sure the carbs are opening all of the way. The tach is an older Yamaha digital that is probably about the same era as the motor. It's going to be a couple of weeks before I can get it back on the water so I can check some of these things. Thanks for all of the input and maybe I'll check back in a couple of weeks with more details and see what you think about prop size.

 

[Texasmark]

Really some helpful answers here.....from the guys who know. Couple of things in adding my "2 cents": Slow planing is too much pitch or wrong trim or both. If the engine is tuned correctly, biggest speed reducer is too much hull in the water. My last boat was a 2002 Crestliner 1750 Fish Hawk and it had reverse chines which help the boat ride "ON", not necessarily "IN" the water. When boating alone, and best trim at speed, the bow wake exited at the intersection of the transom with the side of the hull. The more you tuck the engine in, the farther this bow wake would move forward easily telling you that you didn't have trim out far enough and you were limiting your top end (RPMs) with too much hull in the water.