Prop Blade/Pitch/RPM

[Gwoo]

I recently bought a 1998 Larson Flyer 166 Outboard with a 125 Johnson. I figured it would be great for skiing. It came with a 21 Pitch 3 blade with a horrible hole shot. I did research and went with a 17 pitch 4 blade aluminum. Hole shot is great, but the RPMs are over the recommended limit. These are also Solas Aluminum props. I bought 2 to have 1 as a spare and am going to return one for a different prop.

At 61 and with a surgically "repaired" back, I am way more focused on Hole shot to get my arse out of the water than I am with top speed. With the 17 I can get the boat up to 35-36 and keep the RPMs within recommended allowances, but the boat porpoises. Driving full out and trimming to speed is better for waterskiing, but it does put the RPMs in the 6000 range when 5250-5400 is the recommended top end.

So ... how much Hole shot will I lose going back up to a 19? I am definitely staying 4 blade. How much of a difference does it make going with the pricier stainless steel? The prop finder suggests pitches all the way down to 13, which I can't understand relative to RPMs, as I thought dropping pitch kicked up the RPMs.

Any ideas?

 

[alldodge]

With the 17 I can get the boat up to 35-36 and keep the RPMs within recommended allowances, but the boat porpoises

I am definitely staying 4 blade

With these 2 different issues, why are you so definite about staying with 4 blade?

 

[Gwoo]

Hole shot. I find a 4 blade to be much better off the mark.

 

[alldodge]

Hole shot. I find a 4 blade to be much better off the mark.

You went from a 21P 3-blade to an equivalent 19P 3-blade (19P 3blade = 17P 4-blade). Going to the 19P 4-blade puts you back to the same 21P 3-blade. The main difference is the 4-blade provides more lift. The more lift in the stern to more chances to porpoise.

I would suggest a 19P 3-blade, less lift and about the same hole shot

Either that or put some cup on your current

 

[at101]

I keep reading about prop finder but there does not seem to be any such on this site. Does one exist or is this just a general statement about finding a prop?

 

[WesNewell]

I keep reading about prop finder but there does not seem to be any such on this site. Does one exist or is this just a general statement about finding a prop?

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[Texasmark]

I recently bought a 1998 Larson Flyer 166 Outboard with a 125 Johnson. I figured it would be great for skiing. It came with a 21 Pitch 3 blade with a horrible hole shot. I did research and went with a 17 pitch 4 blade aluminum. Hole shot is great, but the RPMs are over the recommended limit. These are also Solas Aluminum props. I bought 2 to have 1 as a spare and am going to return one for a different prop.

At 61 and with a surgically "repaired" back, I am way more focused on Hole shot to get my arse out of the water than I am with top speed. With the 17 I can get the boat up to 35-36 and keep the RPMs within recommended allowances, but the boat porpoises. Driving full out and trimming to speed is better for waterskiing, but it does put the RPMs in the 6000 range when 5250-5400 is the recommended top end.

So ... how much Hole shot will I lose going back up to a 19? I am definitely staying 4 blade. How much of a difference does it make going with the pricier stainless steel? The prop finder suggests pitches all the way down to 13, which I can't understand relative to RPMs, as I thought dropping pitch kicked up the RPMs.

Any ideas?

Back in 1972 I bought a new Johnson 125, power tilt only...trim came out in 1973, on an 18' Caravelle FG bow rider, tri-hull.....heavy boat with probably 15* of dead rise at the transom. I had a 17P OMC SST prop...their first SS prop, Teflon coated, the second year (1973) as they came out that year and my OEM alum. prop was on the boat Sat. and Sun. and at lunch hour on Mon. they were headed for the repair shop with bent blades.

I didn't have a tach but alone it would never go above 39 and at a company party one day, with me in the boat alone, I was able to get 5 persons up on skis.....for about 200 yards before one fell off. I used to sit in the jump seat adjacent to the engine (like used in I/O boats but being an OB they just made a place for an icebox between the seats)...moving on, I used to sit back there, wife driving, drinking my beer and listening that that engine purr...not scream, purr.

So, with that background and owing to the fact that 150-200 rpm change is the standard on this site for a 1" pitch change, a 19, of the same make and model of your current prop ought to get you the best of both worlds. Change models or brands and you are throwing curve balls into the results.

 

[Scott06]

What was the WOT RPM with the 21"/3 blade?

I would think a 19" 3 blade or 18" 4 blade like a michigan vortex would put you where you want to be

 

[racerone]

I tested my boat one time.-----115 HP.------19" pitch prop ---54 miles / hr and 5800 RPM.-----21" pitch ---54 miles / hr and 5400 RPM.----23" pitch ----54 miles / hr and 4800 RPM.-----Boat speed was the same.----All testing done one after the other.

 

[Texasmark]

According to go fast prop slip calculator if your 115 is like most witha 2:1 gearbox...prop turns half the speed of the crankshaft, your slip...the difference turning in water vs a screw turning in wood is: 19 = -4; 21 = 0 (the screw "is" turning in wood; 23 = -4 also.
Common denominator here is the gear ratio.

So, backing the truck up. a popular Mercury gear ratio for V6 engines is 1.76:1 Running that gear box, for the numbers posted in percent gives 8-11-8....a lot more believeable.....but the gear ratio for that engine isn't. Guess I'll have to dig up an old sales brochure and see what OMC was running back then.

Well here it is: https://issuu.com/liquidnirvana/docs/1998---johnson-outboards--x---wm--67.2mb
About page 23 are the specs. Seems they have 3 different V4 loopers (90, 115, and 130) and 4 configurations. 1 has a 0.44 gearbox and the rest have a 0.5 So, figuring the .44 probably for the 90 hp like Mercs 90 hp at 2.33:1, and like Merc, the 115s have the .5....2.0:1

Soooo something is wrong with the inputs Mr. Gwoo is receiving.