Prop Size Question

[AndyD]

I have a 20' Hydra Sports Seahorse with a 1999 150 Johnson. Presently have a 14.5 x 19 stock OEM aluminum prop Max RPM is 5,000.

Manual says min RPM is 4500. I've heard that I should be targeting mid 5,000's. I want to by a new stainless prop. My options for some good prices are;

• 15.00 RH 17 Michigan Match Stainless for $229
• 14.25 RH 19 OMC Rebel Stainless for $263
• 13.50 RH 18 OMC Raker Stainless for 269

I think that all these should allow my motor to turn a little bit faster but I'm not sure by how much. 1st in list increases dia by .5 inch while reducing pitch by 2. The second keeps the same pitch but reduces dia by .25 inch, and the 3rd reduces pitch 1 inch and dia 1 inch. Prices seem very good any reccomendations?

 

[AndyD]

Re: Prop Size Question

Well, I just found a brand new OMC SST 15 x 16 for $200.

Using 150-200 RPM Increase per 1" of pitch that would get me up to 5450-5600 RPM's in theory. I'm not sure how the extra 1/2" in dia would affect that either. My guess is that this would put me on the lower rather than the higher side of the increase.

What do you think? My max RPM is supposed to be 5500 should I go with the 15" x 16" pitch SST or with the 15" x 17" Michigan Wheel. The Michigan would get me theoretically somewhere between 5300-5400.

 

[Bifflefan]

Re: Prop Size Question

How does the boat perform with the prop you have?
What is the problem you are trying to fix?
How often are you at WOT?

Dia of 1/2 will not make a difference.

I have a 14x17p Alum i use for skiing and such, and a 13x19p SS for speed.
The 19 goes on a couple times a year if that.
I also very rarely run at WOT for more than a few minutes. Most of the time is spent getting up and slowing down.

 

[AndyD]

Re: Prop Size Question

It's not so much how much time I spend at WOT it's rather having the motor correctly propped for long engine life. My motor though 10 years old only has about 30 hours. I want to get a long life out of it. They say that operating near the upper end of recommended RPM range is best for a two stroke. 5500 is supposed to be max for my motor although many recommend going as high as 5800. As I understand it two strokes are not supposed to work too hard and that operating in the higher recommended RPM range results in substantially longer engine life.

I was hoping someone would weigh in with their experience. Before I plunk down $200-300 I want to be as sure as possible that I've made the right decision.

 

[Bifflefan]

Re: Prop Size Question

The max RPM on mine 5500, and the range is 4500 to 5500.
My motor has not seen 5000 in probably 30 years (not that i have had it that long). But i know the boat.
As long as you keep it tuned and dont abuse it, it will run fine for a long time.
Run some seafoam in it at least once a year, some people run it in every tank.
This will keep the carbon build up to a minimum.
Once you on plane the motor is not working hard.
you can drop a pitch or two if you run the boat heavy alot, But from the info you gave you have a good match prop i think.

 

[Silvertip]

Re: Prop Size Question

You like many others are confused about engine longevity and engine rpm at wide open throttle. You need to understand that the prop is the transmission for your engine and it represents only one gear. The proper prop is one that allows the engine to operate at or very near the manufacturers maximum recommended wide open throttle rpm with an average load. Note that I said maximum WOT rpm, not minimum rpm. The prop must bet the boat out of the hole, on-plane, and run at WOT without over or underreving. If you install the wrong prop the engine will lug trying to get on plane or when towing water toys or skiers (too much pitch) and may over rev on the high end (too little pitch). Therefore, if you install the wrong prop it is wrong across the entire rpm band, not just one end or the other or the middle. You need to read the "Read This First" sticky at the very top of this forum. Without any existing performance data it is impossible to tell which of the props you found would be best for your application.

 

[AndyD]

Re: Prop Size Question

You need to read the "Read This First" sticky at the very top of this forum. Without any existing performance data it is impossible to tell which of the props you found would be best for your application.

Here's the info in order;

1) Performance issue you are trying to correct.

Buying a new prop. Presently have aluminum and need SS. There a lot of shallow water around here.

2) Current prop manufacturer, model, aluminum or stainless and as a minimum.

OMC Aluminum stock propeller

3) Current prop diameter and pitch (required).

14.5" x 19"

4) Wide open throttle RPM and speed with an average load (very helpful)

5000 RPM with full fuel tank, 2 persons, and fishing gear

5) Engine/drive make, model, year, and HP

1999 Johnson 150 HP, Model: J150PXEES

6) Boat make model, year, length and weight

Hydra - Sports Seahorse 200 CC (19' 06" LOA, 7' 09" beam), 1704 lbs (less Engine), 65 Gals Fuel Tank.

The proper prop is one that allows the engine to operate at or very near the manufacturers maximum recommended wide open throttle rpm with an average load.

Agreed! So, I would deduce from my reading that I need to reduce pitch by 2"-3" to get there. Do I go with 2" and be 'safe' or 3" and risk being a tad bit high?

 

[junior1113]

Re: Prop Size Question

have a 93 hydrosport 2000 cc vector (3600loaded down) with a 93 200 evin. ocean pro great boats. i run at 5800rpm@47mph with a 4 blade 17"pitch turbo ultima. had a 3 blade ss 19 5500rpms and the 17"4 blade put it to shame. i save gas $. i would'nt run that boat with that prop. spare only. 5500 would be the least. hard to hit 6000 with todays ethanal. any where in between will ensure maximum engine life. if i were you the first thing i would do is see if engine height is good before just buying a prop. where is cavitation plate in reation to bottom of hull? should be above 1-2" mine with a 5-6" off set jack is 4". rpms can be achieved if motor needs to go up. the very same prop i have would make that boat run like it should. the only thing is with todays prop technology the only way you can take advantage is by adjusting motor height. i had to raise motor with the 4 blade cause it is a surface prop hope this helps..

 

[AndyD]

Re: Prop Size Question

Junior,

I think 5000-6000 is the operating range for your engine, right? by going from a 19" to 17" you got much closer to the top of that range.

I'm kind of thinking that for my rig if I likewise go from a 19" to a 17" then that should get me 5300-5400 which is closer to the upper bounds of the recommended RPM range (4500-5500) without exceeding it. I don't know that a 16" pitch prop would not exceed the recommended RPM's

 

[junior1113]

Re: Prop Size Question

i've had 150's in the past and never ran them under 5500. never had any problems with them. with todays gas i would run ANY omc to 5800-6000 and out run the one that runs 5000-5500. just my experience with omc. still think youd be happy with 4 blade on that setup.

 

[Silvertip]

Re: Prop Size Question

Regardless what the manufacturer says, that motor likes to be in 5800 RPM range. And as was suggested, work with setup (engine mounting height) before you start spending money on props. Buying stainless props gets expensive (but a moot point I guess if you have deep pockets).

 

[AndyD]

Re: Prop Size Question

The motor height is good... about 2" cavitation plate to hull bottom.

Also, I'm not buying a stainless prop just for the heck of it. First I need one since I bay fish in the winter when I can't get offshore. We have a lot of grass flats around here and I frequently run in shallow water. Nobody drags bottom on purpose around here but some of the 'channels' are only 3' or so deep. Aluminum props just get chewed up in pretty short order, while stainless props just get shiny. Been there done that with my other boats. Second as stated earlier I want to run at optimum RPM's for my motor.

So, with all of the information about the boat provided, engine height set properly what do you guys think would get my engine turning the right speed the 15" x 17" prop or the 15" x 16" prop?

 

[DocShock]

Re: Prop Size Question

You said shallow waters around there as well...if you hit something hard withthe aluminum, the prop will bend or break.
If you hit something with the ss, the lower unit will break (again, a moot point if you have deep pockets).
SS does give you better bite in the water and performance, though.
I would go with the 15x17... it is one of the most commonly used setups, and drops your pitch 2" to increase potentially 400 rpm's or so (after you verify the height/placement of the motor). The increased diameter will not affect you much.

 

[junior1113]

Re: Prop Size Question

15x17 will put you 5500-5600 which is fine. id still see if engine can go up with out cavitating and hit 5700 maybee

 

[AndyD]

Re: Prop Size Question

Yeah, I think I'll go with the 15 x 17. Currently bidding on a very nice used one on Ebay. If I win the bid good, if not I can go new. Thanks to everyone for the help and advice.

 

[Silvertip]

Re: Prop Size Question

I think you can forget the 13.5 diameter prop as that is likely for the intermediate size gear case. Diameter is a function of prop design so as pitch goes up, diameter decreases slightly and therefore is not a major consideration in prop selection. Since you need to pick up 600 rpm or so (assuming the engine is in a proper state of tune and is not tired), A 16P should get revs up in the desired range. But also understand that all 16P props are not created equal. Things like rake and blade shape also affect peerformance. This is why prop selection is such a crap shoot. Check right here on iBoats as they may still have the program where you can exchange a prop if you buy one and need to make a slight adjustment.