Reproping my zuki 225 efi on my 85 222CCP

[smitty 1]

I'm running a 14 1/4" X 18 S/S prop on it now, it came on the motor when I bought the motor and I don't know what size boat it was on prior. Suzuki offers up to a 15 1/2" prop on that motor. Any thoughts, is bigger better? not withstanding pitch. I'd like to get the most out of the motor for my fuel buck. Any recomendations?

Smitty

[fitz73222]

Hey Smitty,

Its all up to what your tachometer say's at WOT. The rule of thumb is to prop the engine to be in the upper range of the recommended max RPM. If the factory says the range is 5500-6000 rpm; you want to be in the 5700-5900 with your normal load. If you're propped in that range the engine will be loaded correctly throughout the entire RPM range. In other words, if you like to run her @3800 for your cruise rpm the engine will be correctly loaded for the timing advance, fuel injection delivery curve and torque. If your main interest is fuel economy and mid range efficiency, you cant beat a 4 blade propeller for bite and fuel consumption. You will lose about 2 mph at wide open throttle over a 3 blade because of the efficency loss or interference of the addtional blades proximity to the next one, but at lower speeds the extra blade really helps with bite. You have to decide what is most important. I run 4 blade props on my 115's and could not be more pleased with the mid range. I`ve actually ran her down to 3200 rpm @25 mph with 5 people and a tower, those props really harness the mid range torque and with gas predicted to rise again; I`m OK with being a 40 mph boat instead of 43 and 24 gallons per hour at WOT!

[fitz73222]

One other thought Smitty, I just finished a pot of coffee since I've been up since 3:30 and I'm on the jazz right now!
I think these new big diameter, big ear 3 blade props are being designed to bridge the gap between 4 blade bite and 3 blade speed. I think Flounderpounder just went through this on his CCP and used a Mercury Enertia propeller to really wake up his 225 Yami and he gave some great feedback on the results. My point is, I may be a little behind the times with what is going on with this propeller technology. You and Flounderpounder are running big 4 3/4" diameter gearcases that can handle these big diameter props. My 4 1/4" gearcases on my 115's cannot, so the four blades work fine in that application. Once Marc reads this post he will steer you with his conclusions.

[flounderpounder225]

Hey Smitty,
(Fitz) haven't had a cup yet, just got through putting 200 pounds of BBQ on the smoker, that's my day today.

On the prop, You want to run the biggest wheel you can, and still achieve the WOT RPM like Fitz was referring to.  As you can see below my data with the Mercury Enertia was amazingingly different from any other prop I had tried.  The Enertia was designed for bigger heavier boats with bigger outboards, I have a 250 HPDI, but not that much difference between yours and mine.  The big blades of the enertia, keep it biting in turns, no blow outs, zero cavitation on take off, and great mid range cruising and economy.  Honestly, you need to try a few different ones, I would definitely go to the 15" diameter, I would call Ken at "propgods" down in Miami, I did, he sent me this 500.00 prop for the price of shipping and let me try it, it is worth every penny.
Marc

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[GoneFission]

One of these days we should probably do a "Propeller Facts & Fiction" section in the Resources forum, but that's a different story...   But here's some info for ya to chew on - large diameter props have a larger swept area than small diameter, so they move more water.  Lage diameter props are good for torque and getting the boat up on a plane, but they have more drag due to the larger swept area.  The problem with large diameter props is cavitation (not ventilation) at high spin speeds or RPMs.  The tips of a prop can only go so fast through the water before they start vaporizing the water, causing cavitation.  The tip speed increases with the square of the diameter, so a little increase in OD makes a big difference in tip speed.  Spinning a large diameter propeller at high speeds results in cavitation and a dramatic increase in slip.  So if you have the engine torque to turn it, you will find a large diameter prop gets you on a plane faster, but you may find top speed is impacted (regardless of the pitch seleted) due to the tips of the blades starting to cavitate.  Sometimes folks complain that the boat doesn't go much faster at 6000 RPM than it does at 5200 RPM and they think it's wind resistance.  Wind resistance also is sqare factor - of speed - but often the problem is the prop is beginning to cavitate just over 5200, and is really cavitating and slipping at 6000.  The fix is one of two things - reduce the diameter of the prop or reduce the gear ratio in the lower unit to reduce prop spin speed.  It's usually easier to change the prop than the gear ratio...  

This is why 4 blade and even 5 blade props are popular with heavier boats with higher HP motors.  You can have the same swept area of a big-diameter 3 blade with a smaller diameter 4 blade.  A 14" prop with 4 blades has a swept area like a 15" big-ear prop, but may keep the tip speed below cavitation level to acheive good efficiency at high RPMs.  A basic rule of thumb is the faster you want to go, the smaller diameter prop you need, and if you want to go really fast, you have to deal with drag, so you start reducing swept area and blades.  The fastest boats you will see (think unlimited hydroplanes with 12,000 HP turbine engines) are running little 2-blade props at really high speeds.

[smitty 1]

Thanks guys for the information!
Marc, or should I say Master Chief, sounds like your getting ready for some serious BBQ eating. I'm ol Navy as well,
( Seabee ) not lifer, just one tour, 64 -68, in a little place called viet nam in 67 -68.

Smitty

[flounderpounder225]

Thanks guys for the information!
Marc, or should I say Master Chief, sounds like your getting ready for some serious BBQ eating. I'm ol Navy as well,
( Seabee ) not lifer, just one tour, 64 -68, in a little place called viet nam in 67 -68.

Smitty

Just a lot coming up, superbowl next weekend, I am providing a bunch of BBQ for that, and a catered party this coming Friday.  Try to get all the long cooking knocked out this weekend, and it was a long day....  Thank you for your service, Shipmate...
Marc