Prop diameter/pitch issue

[inthedirtagain]

I posted last year on wanting to replace props on my grandfathers old boat, and have done so recently with the help of my local shop, but didn't get the results I was looking for. This is on a 1969 Silverline DeVille with the original Merc 1000 ToP. This 16ft boat/motor combo comes in around 1300 lbs, and with an old 3-blade 17p prop, I can reliably hit 5200rpm @ 37mph with me and my 13yo son. When loaded to its rated capacity of 6 people, plus a cooler and our skis and boards, my rpm drops to about 4900-ish. I'm not concerned about speed, but would like to maintain the upper rpm limit of 5300 when at capacity.

My local shop sold me a 13.25 x 16p Black Max and with just me and my boy, we're only able to hit 4400 rpm. I've verified the timing and max throttle advancement, as well as the compression (all within spec, 130psi x6), and I've tried different trim positions. The older 17p prop is smaller than the new prop, so I'm wondering if that is where the difference in rpm is coming from. Its a beat up 12.5x17p that I've cleaned up as best I can in my garage. The shop thinks the newer prop is more efficient at grabbing the water, and I would agree as I can go into a much harder turn without any prop blowout.

Since we ski and board with this boat, pulling power and keeping on a low-speed plane are more important to me than top speed. I'm interested in a 4-blade prop, but want to maintain a 12.5in diameter. The shop won't take their prop back, and I don't want to waste anymore $$$. Any help here would be appreciated.

 

[steelespike]

Is your 37 by a gps? We need the rpm and gps speed. Also for the new prop.
your old prop is definitely undersized. With its age I'll bet the old prop diameter is less than original.
It also is likely the new prop has included better technology than the old prop.
Your problem isn't too much diameter but too much pitch.
If you look up 17" props for your 1000 you will find that they are all over 13"You will also find that as pitch
goes up diameter goes down. But even the higher pitches are still 13" diameter.
You also may want to confirm the prop is actually marked as a 16"
If your tach is original it is suspect.You may want to check it against a shop tach.
You can't use the 16; Put it on Craigs list get what you can.Wide open throttle for the 1000 is 4800 to 5300.

 

[inthedirtagain]

The in-dash tach has been verified against 2 other tachs, and the speed indicated was by GPS. The old prop isn't necessarily undersized if it allows the motor to reach its optimal rpm, but I want it to spin a smidge faster, especially when loaded with 6 people and gear. Prop slip calculator puts the old prop at 12%. Never turned on the GPS with the new prop because I couldn't come near my target rpm range.

 

[inthedirtagain]

Just for giggles, I did a search on the Merc 1000 of this era and found others with a 12.5in prop. I'm guessing that manufacturers aren't making them anymore due to costs and lack of market.

 

[inthedirtagain]

Stopped by a different boat shop this morning and spoke about my prop issues. They let me borrow a Turning Point Hustler 13.75x15p. Took my two boys out this afternoon and in some pretty good chop, we managed 4900rpm and 34mph @ WOT. No doubt those numbers would improve slightly with smoother water. Put the original 12.5x17p back on and knocked out 36mph @ 5200rpm. Im positive the diameter is limiting my revs. I did notice that holeshot was better with the 15p.

Im going to search for a 4-blade 13p so that I can maintain revs with a full load. Dont mind shaving a few mph to get the pulling power. Thoughts?

 

[Sea Rider]

You should always test props on flat calm no wind water cond with a tach and GPS at wot. Is it possible to get a new 12.5 x 14 or 15 prop ? Will achieve better hole shot with slight reduced top end speed and probably be inside max wanted 5300 revs with full load. Bear in mind that when boating alone or with less load rpm will shoot, so watch tach close if liking running wot.

Happy Boating

 

[inthedirtagain]

I'll keep the old 12.5x17p for fishing, but the new 4-blade 13p will be strictly for watersports. We don't typically do anything @ WOT when pulling. Just bought the Solas Rubex from iBoats for $117 shipped, and that included all hub hardware. Can't beat that! Now if only I can sell the Black Max 16p........

 

[inthedirtagain]

So my new Solas Rubex 4-blade came and I had a chance to test it out today. Its exactly what I needed! I can now get a good holeshot and maintain plane at a low speed for when my youngest is on the board. My boat produces a nice, fat wake around 15-16mph, yet still remains on plane. Top speed was a reduced at tad, but I'm finally able to rev out. On some mild chop and the trim set on the 2nd pinhole from all the way out, I was able to knock down 35mph at 5600 rpm with just me and my boy. When fully loaded, it will still hit it max of 5300 rpm and have that great holeshot. Its like a night and day difference in getting up on plane very, very quickly. I'm happy!

 

[SkiDad]

Nice !