Prop question?

[Guest]

Would someone please explain what the outcome would be in swapping an 18 pitch prop for a 20 pitch would be? More topend, better holeshot etc.
Thanks

 

[soggysod]

assuming same prop (diameter, and cup) you will change 400 rpm for a 2" pitch change.(in your case go down 400 rpm w.o.t.) sooooo if you are running near the top of your rpm on your boat with your current prop you can expect more top speed, but less holeshot. kinda like going on a car from 4.10 rear end to 3.45. but if you are pulling near the lower end of your redline unloaded, this can hurt top speed AND holeshot, because the motor cant pull that tall of a gearing. a good example is my boat as it is unloaded with one person can only pull 4800rpm with a 19 pitch, it should be near 5500 unloaded, so i went down 2" in pitch, and gained a ton of holeshot, but it actually helped my topend a little. this is because now i am nearer the 5300 trimmed out, and it gets in to the meat of the powerband quicker. so quicker revs means better holeshot, but higher rpms, even though a lower gear makes for same or better top end. does that make sense?
the real kicker is that 2 same pitch props of different brands or materials can operate totally different on a boat, so the best bet is to get with someone who has the same boat and motor and has experimented or get with a good prop shop and experiment on your own.

i am by far not at pro at this, just played with it a little back in "the day" before the kid and lack of expendable income!
good luck
soggysod

oh by the way where in lafayette do you live, and you fished our lake any lately?

 

[Guest]

Here is the deal with my boat. It has a rev limiter that kicks in at 6000 rpm. Unloaded I can hit the limiter real quick. Loaded it takes a bit longer but I can still hit it. My boat runs 37 to 38 mph on the GPS. I have a 14 X 18 ss prop. I have been talking to a guy in NC that has the exact same boat and motor. He claims that he is seeing 45 to 47 mph but he has a 14 X 20 prop. I normally would not spend much money trying to squeak out 2 or 3 mph but 10 mph gets me to thinking I might like to try the different prop. Is it possible that this change in prop can make that much difference?

I live about 7 miles south of LaFayette in the Waterville community. Have not fished Queen City lake in several years. Fishing is pretty good there but the ramp stinks and they won't let you even start the big motor. I just go elsewhere.

 

[soggysod]

wouldnt surprise me to see that much change if you are hitting the limiter that quickly, i had a 150 merc on a little fish and ski, with a 23 pitch it would run 69-70 without trimming the motor too much or fiddling with it, but would only turn 5200 rpm. but it wouldnt get out of the hole well. i put a 19 pitch on for skiing, with 4 people in the boat and a skier it would give you whiplash. it would only top off at 53mph loaded unloaded whatever that was bouncin the limiter. so i agree if you hit the limiter that quickly, you probly have alot left. what motor and boat are you runnin? reason i ask, is if available for your motor, ive had luck with the compprops they are cheap. they will flex some, and not give the best performance but either way it turns out you have a good spare. and then can buy a ss prop based on your results.

soggysod

 

[Guest]

I have a 21 foot Carolina Skiff with a 115 hp Suzuki four stroke. I can get the prop that the guy in NC has for $239 to my door. It is a Stiletto Advantage. I am very limited in prop selection for this motor.

 

[wrechin2]

Soggysod,

Just readuing old threads and found this one. Your speedo has to be way off! The numbers don't match up. Not being a smart a__ Just want to explain. With a 1:87 gear, 5200 rpm, 0% slip (impossible amount of slip) 23 pitch prop would be 62.6, most boats will have at least (if perfect set up) 8-10% Best I have ever seen is 5%. At 8% slip it would have ran 58.1. if it was a 2.0L merc with at 2:1 case it would only run 54.4 mph. If tach is off with a 69mph, 1:87 gear , 8% rpm would have been 6171 RPM's, with a 2:1 case same would be 6600 RPM's. Most merc V-6 will turn rpm's in this range. 6200 is common, 6600 is hard with a stock ported 2.0L engine and without the addition finger ports. Most 150's were 2.0L. Very few were 2.4's or 2.5. They were XR4 and XR6 and some not special purpose. Would be harder for a stock 2.0L to pull a 1:87 lower with a 23 through the hub prop to 6200. The 2.0L's came with 2:1 lower to help get most boats started easily due to the lack of torque from the 3.125 bore. The 1:87 would be like pulling out in 2nd gear and the 2:1 like pulling out in 1st gear on you car. You will get more speed (generally) out of the 1:87 than the 2:1 with the same rpms. You would have to turn the 2:1 approx 400 more RPM's to match the 1:87 case. The 2.0L are a good work horse engine that LIKE rpms they have to be made to turn them. I am building a race 2.0L TUBED engine that will turn 9500 RPMs (many have been built, not BS), with a 2:1 lower (hole shot and accel) 24 et drag copy, 8% slip, I should see 103.5 mph, 5% 106.8 MPH. Just my 2 cents worth. Hope does not offend. Did not mean to.