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Old 06-02-2020, 08:02 PM   #1
Lt. JG
 
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Default 3 blade prop to 4 blade

Hey Everyone!

I’ve been doing a lot of research on holeshot/water skiing and would like to change from a 3 blade to a 4. I also would like greater control at lower speeds. We don’t have a system where I live that you can borrow props and test them out, so trying to get this right first time (if I can!)
Boat is a 2001 Maxum 1800SR. I have an Alpha1 gen2 1.8gear outdrive connected to a Mercruiser 4.3 LH 1996 engine. Engine Manual Says MAX RPM at WOT should be 4400 to 4800.

Out on the water I get exactly 3000 RPM (not a typo!) at WOT according to the dash RPM meter with speed roughly 40MPH. They are newish gauges and do not know if they were calibrated correctly?

Two adults on the boat, a full fuel tank and nothing really extra in weight. The current prop is a Solas Aluminium 14.3” with 21” pitch SOL1511-143-21 which was on the boat when I bought it. I genuinely feel this is over propped in some way.

Couple of questions after a few weeks of research on here and youtube:
  • I feel that a 21” pitch at 3000 RPM should be performing a little better than 3000RPM? Shouldn’t it be up in the 3800-4000 region?
  • Should I buy a manual tachometer and check the belt?
  • What size pitch should I be aiming for if I do get a 4 blade? I read every inch in pitch roughly equals 150-200RPM give or take…. and to also drop 2 pitches going from a 3 to 4 blade
  • My gut says 15” pitch on a Solas 4 blade? Don’t seem to see any lower than 15 at the local shop.

Would love to get a stainless steel but feel I should “try” an aluminium first in case I break it (again!)
Many thanks as always!
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Old 06-02-2020, 10:53 PM   #2
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Fester, I'd first recommend that you make sure your tach is working correctly. There is a selector knob on the back of the tach, make sure it is set on "6". You can also get a loaner tach from an auto parts store.
Low speed control is always a challenge in an 18 ft boat with planing hull. The hull creates turbulence for your prop. You can try trimming the drive out a little more.
I'd recommend against a 4 blade prop, but that is just my opinion. A 3 blade prop gives plenty of bite for your size boat and motor. Blade pitch is much bigger factor than number of blades. Is your boat a dog during hole shot now?
With you motor and 1.8 ratio, you should have not trouble getting to at least 4k rpm with the 21p blade. I suspect there is a wrong number here somewhere and my guess would be the tach.
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Old 06-03-2020, 12:08 AM   #3
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I considered going to a 4 blade also. My 1995 SCR 2400 with Bravo 2 is running 18 1/4 X19 3 blade prop and performs very well. I repowered 11 years ago with a 350 MPI Mercruiser Horizon and added the Merc Monitor that reads off the ECM. It shows my analog Tach running high by 120rpm. I can get to 46 mph at WOT at 5K rpm so found no reason to go to 4 blade.
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Old 06-03-2020, 12:57 AM   #4
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I have a 21’ with a 350 and I went from a 3 blade 21 pitch to a 4 blade 21 pitch as my WOT was a few hundred RPM low and I would never go back. Boat performed well with the 3 blade but the hole shot is incredible now and I stay on plane at significantly lower speed which is great as I pull my kids on the tube. I also noticed a vast improvement on “prop walk”, the boat is way more controllable at low speeds, it really improved the slow speed control. I lost about 6 miles per hour of top speed but I don’t care as I never run wide open anyway but it still pulls 48-50MPH if I needed it to.

Just my 2 cents,


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Old 06-03-2020, 03:42 AM   #5
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Hey Overboard ....you now have me interested in trying out a 4 blade prop. Just have to get the right size....
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Old 06-07-2020, 02:58 PM   #6
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Thanks @jrsick. I had a look at the back of the gauge and you are right.. It does look off. The engine is a V6 and its currently set to 8 cycles? I'm slightly confused between cycles vs poles however I'm guessing this should be set to 6 CYL at position 2 on the gauge setting?
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Old 06-07-2020, 04:28 PM   #7
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Sorry just occurred to me 6CYL is probably cylinder? #Noob!
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Old 06-08-2020, 12:34 AM   #8
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You got it!
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Old 06-08-2020, 10:13 PM   #9
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Good find Fester. That still puts your WOT at 4000 rpm though. A 17p will make your boat leap on your hole shot. 3 or 4 blades at this pitch will give you a significant improvement.
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Old 06-15-2020, 06:10 PM   #10
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OMG! the boat is like a bullet!.. I went for a 4 blade 17 pitch stainless and it just jumps on plane. I never knew the engine could be that powerful. It even sounds much better and its only at 4200RPM on the gauge. Maybe I should have gone for 15" .. Control at slower speed is much much better! I also tried 4 burly adults on the boat and a ski'er towed on the back with no bother to it at all.. It still jumped on plane asking for more..............

Thanks so much everyone.. I am beyond ecstatic!
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Outdrive: Alpha1 Gen2 with 1.8gear
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