17' boat 120hp what prop size do you run?

S

sun runner

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What prop do you folks run on a 17' boat?
trying to get an idea of what size mine should be as the previous owner I beleive changed it to a bigger prop (15.5 19p 3 blade) and its not getting up on plane very well. tried a 14.4 17p dual geometry 4 blade but didnt see much of a change.
post up you prop size please.
 
What RPM's do you get at WOT? What is the recommended WOT RPM for your motor?
 
Try running a 14.5 X 17 - 3 blade. If you know someone that has one for you to try, do it that way. I believe you will get good performance out of that size.
 
quote:
Originally posted by cwms

What RPM's do you get at WOT? What is the recommended WOT RPM for your motor?

Click to expand...

WOT for motor is 4200 rpm.
my highest 3800 rpm at WOT (5000 ft altitude)
was hard to get on plane. then bought the new prop and put it on, went to a lake higher up (6500 ft altitude)
and it ran worse 2800rpm max, no plane at all. tried original prop did worse 2400 rpm max, no plane at all.
motor runs great and everything looks good,
the plugs looked good too pulled them today.
I'm stumped on this one.
 
There are "altitude HP loss" calculators and articles on the net. At 6500 feet the calculator I found puts your HP below 100.

That is IF your O/B is putting out its rated 120HP.

First thing to do before selecting a prop is to be sure your engine is up to snuff. With outboards, the shops have "test wheels" which are installed in place of the propeller. Boat then moored at a dock, or just outboard put in a test tank. The wheel consumes power without moving the boat. In this test, the WOT RPM must reach the spec for the wheel.

Many variables here, beside altitude. Hull design, hull condition, load, trim angle, transom height.

4800 sounds low for an outboard's rated HP at WOT. What's the Make/Model of this OB?
 
quote:


4800 sounds low for an outboard's rated HP at WOT. What's the Make/Model of this OB?

Click to expand...


Actually, the OP said 4200... Which is really low for an outboard.
 
Motor is a mercruiser I/O 153ci 4 cylinder,
120 merc foot.
I'm going to bump timing 2 degrees and see if that helps.
 
Chevy 4 cylinders (depending on gear ratio) don't usually pull 4 blade props very well on runabouts. Better off with a 3 blade. If you get a chance take it down to sea level and get it right. Then use that experience to down pitch for a higher altitude.

4 blade props (with higher gear ratios) are used on house boats for thrust at lower speeds.

Give the length/weight of your boat , HP, Altitude, and gear ratio to your prop supplier and he can recommend a diameter and pitch.
 
I think your problem is related to the carburetor settings and high altitude. Assuming the boat was running right previously - before you took it to the higher elevation, I could see a tweek in the timing and maybe a 1" change in the propeller pitch (less pitch), but that's not going to fix a 1200rpm difference at WOT. Generally 1" of pitch = 200-400rpm. Altitute and air density has a big impact on an engine. For example - my Mercruiser 4.3 in an 18' bowrider, I have two proellers a 21p and 23p. We are in central NC, altitited is around 700ft. In the summer, high humidity low air density it will turn 4800rpm with 21p and 4200rpm with the 23p, in the winter with cooler air temps and denser air, it will hit the 5000rpm revlimit with the 21p and turn 4600rpm with the 23p. I generally swap the props around between summer / winter. Call Mercruiser directly, I have and they will answer technical questions for you, I am sure there is a standard setup for high altitude.

Google "high altitude settings for mercruiser engine" and you will find links to the Mercruiser guide for a 3.0 that talks about high altitude operation.
 
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