Can't figure out which prop

Blackbart5

Seaman
Joined
Jun 8, 2015
Messages
68
I have a 1988 harris 28ft flotebote with the 3.0 i/o alpha one. The engine runs tip top( new cap rotor, plugs, wires, points), tuned it up, dwell and timing are correct(6btdc and 32 dwell). purrs like a kitten. Rpm at wot is supposed to be 44/4800rpms. I just switched props AGAIN to a 3 blade aluminum 15 1/2x11. Rpm's were reading 36/3800rpms....thought maybe my gauge was off, so I hooked up my meter to the engine and yup, it's right. All five of us on board at wot was 18/20 using gps. Choppy water......I'm out of ideas to get to my rpm's, this is my 3rd prop.............17pitch, 15, and now this 11............diameter has only gone up 1/2 through all 3. Any ideas please????
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
I've always used 1" of pitch = 500rpm, but you can double check that. Using that, it looks like you're looking for a 9" prop.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Perhaps the tubes have water in them?Can you do a light load test just you for wot rpm and gps speed.
Have you checked compression?What were the rpm results with each prop.
Its hard to believe a healthy 130 or 140 hp can't make rpm.
Have you done a cylinder drop test.Plugs all look healthy?
 

Blackbart5

Seaman
Joined
Jun 8, 2015
Messages
68
Perhaps the tubes have water in them?Can you do a light load test just you for wot rpm and gps speed.
Have you checked compression?What were the rpm results with each prop.
Its hard to believe a healthy 130 or 140 hp can't make rpm.
Have you done a cylinder drop test.Plugs all look healthy?
Tubes....everytime I get home, I pull plugs to check for water-nothing........just me, gps 21/22@40/4100......compression, 148/149/145/146........17pitch was 2800, 15 was 3100.....brand new plugs, points, cap, rotor. Although I'm wondering if my coil might be bad, when my gauge was hooked up to the engine, as soon as I hit 4000rpms, gauge was bouncing all over(3600,4200,4100,3600,ect).....back to props, I was told originally to go with a 9pitch but I'm only seeing them in a 14dia. I was told that performance starts getting effected around 1in loss/gain of diameter.
 

Blackbart5

Seaman
Joined
Jun 8, 2015
Messages
68
I've always used 1" of pitch = 500rpm, but you can double check that. Using that, it looks like you're looking for a 9" prop.
That was originally suggested but...I'm only seeing them in 14in diameter. I have a 15.5 now. How much will an 1.5" decrease will affect me. I was told I want as much water moving as I can with a big boat????
 
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Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Prop diameters are designed for the gear cases they will be installed on. Because of this a decrease in pitch results in a slight increase in diameter and vice versa. Do not buy a prop based solely on diameter. Buy a prop specified for your engine and then pick the pitch and design characteristics you need. As for your specific issue, props are not the issue in your case. Something else is wrong if 2-inch pitch changes result in only a couple hundred rpm change. Are you certain the outdrive gear ratio is correct that engine? Someone may have installed a V6/V8 outdrive (taller gears) that the 4 banger cannot twist properly.
 

smoedog

Petty Officer 1st Class
Joined
Mar 25, 2012
Messages
238
What silvertip said. I had kind of the same situation in a used ski boat I bought. 17ft that had a 4.3 (originally 3.0) but it still had the 3.0 gear on it. after trying every prop I throw at it. I had a boat that would only do about 33mph at r WOT for that motors RPMs lol
 

MaPaHa

Petty Officer 1st Class
Joined
Jul 6, 2012
Messages
239
I'd identify the gear box ratio somehow before making another change. Maybe someone can put a socket on the front of the crankshaft while in gear and turn it 10 times while someone else counts the turns on the prop. Example: 10 engine turns to 4 prop turns = 2.5:1 ratio. Then you can do a prop calculation and see where you're at.
With that said, that's a lot of boat for a 135 hp inboard/outboard and my gut tells me that there's no way it would pull a 15 pitch prop and I doubt it would pull a 13 either, but the gear box ratio is the key factor.
Pontoons are known for "big ear" props to keep the ventilation down, hold their bite and keep prop slippage to a minimum. I wouldn't confuse a big ear for a larger diameter. You may have to alter the diameter slightly if selection is a problem, but I'd agree with Silvertip that the drive unit should call for the diameter.
If this is a twin-toon with no lifting strakes then you're not going to see much more speed i'm afraid. Under skinning helps in mild to rough rough water and will keep some of the drag down. If you're looking for as much speed as you can get, a stainless 3 blade may be your best bet. If the boat is known to ventilate at the prop in turns then a SS 4-blade may work best.
 
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