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Author Topic:   Prop diameter vs pitch
kshoaps posted 09-15-2002 10:16 AM ET (US)   Profile for kshoaps   Send Email to kshoaps  
I've seen discussions of prop pitch and of pitch selection to allow the engine to max out at the proper rpm but very little regarding how the prop diameter figures into prop selection and engine performance. It seems that for a given pitch, a larger prop would require more power and therefore would not reach the same rpm as a smaller prop. I s this correct or does the engine just accelerate slower but reach the same rpm? How do you adjust pitch when changing diameters? Are the larger props better suited to 2 or 4 stroke engines in relation to hole shot and top speed? I will be getting a 150 Sport in the spring probably with the 60 hp 4 stroke "Bigfoot". Any advice as to prop size and pitch? Thanks
raygun posted 09-15-2002 11:42 AM ET (US)     Profile for raygun  Send Email to raygun     
I have been wondering about this subject as well. My new Johnson 90 is set up with a 14x17 (which is doing great) and my old motor had a 13 1/4x17, and they had very different shapes. What does a diameter change affect?
Dick posted 09-15-2002 12:51 PM ET (US)     Profile for Dick  Send Email to Dick     
As your 150 Sport will be coming as a package I would guess that it will be shipped with the prop Whaler has tested and deemed best.
Bigshot posted 09-16-2002 11:19 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Raygun....try a 13x19 and see what you like. Diameter keeps a boat on plane better but has more drag so less top end. Basically more like a fan blade. My Montauk with a 13x19 tops out at 5400. With a 13 3/8x17 SS it will hit 6200, so the 400rpm for 2" of pitch does not apply.....props are basically trial and error. On light boats, go with biggest pitch you can get max rpm's with. Diameter is great when you have a 18' outrage with a 90hp on it.
raygun posted 09-16-2002 03:41 PM ET (US)     Profile for raygun  Send Email to raygun     
with the 14x17 I'm at 5450 rpm. would the 13x19 maybe still spin that high with more pitch but less diameter? maybe then I would do better than 42mph?
lhg posted 09-16-2002 03:50 PM ET (US)     Profile for lhg    
Props are only meant to be selected with respect to pitch. Let the diameter fall where it may, as this a complicated relationship to pitch and style of prop, best left to the propeller engineers.

I have noticed, however, that as pitch increases, diameter tends to decrease.

Bigshot posted 09-16-2002 04:38 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Correct Larry!

Raygun....I doubt so in your case being you max at 6000 rpms I believe. You may want to try your old 13.25x17 and see how she does.

Sal DiMercurio posted 09-16-2002 11:19 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Engine size determines the prop diameter size, for instance.
"V" 6 engines diameter size goes from 14" to 15"
You cant put a 15" diameter on a "V' 4, as it won't clear the antivent plate.
Theres really not much difference in performance between a 14.25x19 & a 15x19.
At most, you might gain 75 rpms [ MAYBE ]by going from a 15" to a 14" but don't depend on it.
Pitch , cup, & rake is what gives you speed, as pitch equals thrust, dia equals power out of the hole or holding a boat on plane at lower speeds.
One of the best buys is a Stiletto prop, cost less then half of Merc or Omc Bombardier costs.
For instance, a 14.25x19 s/s Stiletto costs $249 compared to $600 for Merc or OMC, plus the Stiletto is a much better prop then omc, but about equal to Mercs.
OMC props look like they belong on a bedroom fan, plus no lifting qualities at all.
I went to a Stiletto on my 20' outrage & 2000 - 200 hp FICHT & the whole boat rides much higher on the water then it did with an OMC, plus I was able to raise the engine to the 3rd hole without blowing out as it did with the OMC prop.
Alot less boat in the water with the Stiletto, thus more speed with the same dia & pitch.
Sal
jimh posted 09-16-2002 11:27 PM ET (US)     Profile for jimh  Send Email to jimh     
As Sal points out, the aperture of the outboard motor lower unit limits the diameter of the propeller.

With highly raked props the zinc trim tab is often removed and replaced with just a flush zinc to allow the raked propeller to have more room and to operate more efficiently.

As LHG comments, let the wizards of propeller design figure out the diameter. Getting the prop working right is still an art form.

Bigshot posted 09-17-2002 10:05 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Sal.....I hate to disagree with you so I will leave it at this....Stilettos are $269 for a reason.
ewalsh posted 09-17-2002 02:39 PM ET (US)     Profile for ewalsh  Send Email to ewalsh     
Sal,

What is the top end on your 20' Outrage with 200 FICHT?

Thanks, Ed

lhg posted 09-17-2002 04:45 PM ET (US)     Profile for lhg    
Mercury's performance line of props, superior products in my estimation, for any brand engine, are not as expensive as Sal indicates if you shop around a little. A year ago, I bought a pair of mid-range HP Laser IIs, complete with adjustable venting plugs and new interchangeble plastic hub design, for $365 each. Had them shipped to me from a dealer in Ft Myers FL, and with no sales tax to boot! Maybe they're $380 now, but I think worth the extra cost over the Stiletto, which does not have the interchangeable hub and adjustable vents.

Performance wise, however, it's entirely possible the Stiletto performs well, with characteristics as Sal has accurately described, as it seems to be a copy of the Mercury Laser II series, as is the newer Michigan Wheel "Rapture" series. Also, not all of OMC's props are bad at all. Kingfish has an SS Raker on his OMC 225, also similar to the Mercury Laser II, and it runs really well, complete with the necessary bow lifting properties. I think Bombardier is re-introducing these propeller products.

Sal DiMercurio posted 09-17-2002 06:03 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Keep in mind my boat is called a 1980 - 20' Outrage, but is really 19'10", with hull only weighing 1,600 lbs.
With barely any load, like 10 gallons of fuel & by myself, I was shot on police radar 3 times, on a 1' following sea, trimmed out to max, turning 6,100 rpms on a 14.25 X 21 s/s 3 blade Stiletto, engine up on the 3rd hole at 60.3 mph, 60.1 mph & 60.3 mph.
My boat is rated at 175 hp maximum, & the engine is putting out 222 hp at the prop, darn near 50 hp over max rating.
This 200 hp FICHT handles this boat like putting a 25 hp on a canoe.
I over trimmed her one day, while racing a bass boat at 60 mph, & the bow went straight up, with i believe only the engine in the water, kinda hairy getting her back down to normal, as you don't just shut it down, you must let her back down straight, or your gonna turn over, or get thrown out.
A good friend of mind couldn't believe 60 mph, so he clocked me in his new pickup from the levee, right at 60 mph, blew his mind.
GPS won't give an acurate reading here on the Sacramento river, as we have strong currents that will cause you to read faster or slower then your true speed, as GPS measures speed over the bottom, not the surface of a current driven river with tidal flows.
When I was clocked on radar, we did it at slack tide, no current.
Sal

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