Prop specs.

[Kng15012]

4.3l merc on a 19 foot open bow. So last night my numbers are 4600-4800 rpm wot, approx 40 mph. My prop is a 19 pitch. So my question is are these numbers correct?

 

[JoLin]

Well, your WOT rpm's are right where they oughta be, so I'd say yes.

 

[Kng15012]

Thanks! I thought everything seems good. What would be a good pitch to move to for a little more top speed? I’m content with my current setup just curious.

 

[thumpar]

List what engine you have and year. 4.3l engines have been around for a long time in different configurations and there are some RPM differences.

 

[Kng15012]

The boat is a 1991 celeb 19 foot 4.3l 0D601730 serial

 

[stonyloam]

4700 rpm, 19” prop, 1.81 gear ratio, 10% prop slip = 42 mph (in theory), so yeah you are in the right ballpark.

 

[Bondo]

Thanks! I thought everything seems good. What would be a good pitch to move to for a little more top speed? I’m content with my current setup just curious.

Ayuh,..... Yer outa horsepower, no more speed to get,....

Goin' up in pitch will lower the rpm, movin' it to lower torque, 'n horsepower,....

 

[Kng15012]

It might be time to move on to a v8. Lol

 

[QBhoy]

I’m not so sure.
If you are not at altitude, that boat should run quicker than that trimmed well and likely with a 21”

 

[achris]

I’m not so sure.
If you are not at altitude, that boat should run quicker than that trimmed well and likely with a 21”

Agreed. Something is 'off'. I had a 4.3L with the 1.84:1 drive in my boat before the MPI, and with a 21" prop I was at about 40 KNOTS (not sure about these 'mph' things you're talking about are, except for landlubbers ) and pulling about 4950rpm. A 19' bow rider is significantly lighter than my boat. You should be running a 23" prop, and pulling about 45 knots at about 4700rpm. So yeah, only 35 knots at 4800 with a 19" is off the money somewhere. My prop slip/Angle of attack calculator says those numbers look ok, so I would start with an engine 'health check' (compressions, timing, carb opening properly/fully, etc). Also check the engine revs with a proper 'shop tacho'. The dash unit can be off by a few hundred revs... And the speed is with a GPS...



Chris.........

 

[Rick Stephens]

(not sure about these 'mph' things you're talking about are, except for landlubbers )
Chris.........

Knots... isn't that where you throw a hunk of wood tied off to a bunch of rope that has a knot tied in it every 4 feet or so. If you got 40 knots through your hand in 30 seconds, that's how fast you were going. We can use other methods to measure speed now.

 

[dingbat]

Knots... isn't that where you throw a hunk of wood tied off to a bunch of rope that has a knot tied in it every 4 feet or so. If you got 40 knots through your hand in 30 seconds, that's how fast you were going. We can use other methods to measure speed now.

Boats, airplanes and rockets travel in knots (minute of arc at the meridian).

The automobile industry choose to base their units of speed off a Roman unit of measure (the distance of the left foot hitting the ground 1,000 times) and that is somehow a more modern unit of measure?....lol

 

[Rick Stephens]

Boats, airplanes and rockets travel in knots (minute of arc at the meridian).

The automobile industry choose to base their units of speed off a Roman unit of measure (the distance of the left foot hitting the ground 1,000 times) and that is somehow a more modern unit of measure?....lol

That's great.