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ContinuousWave: Whaler Performance
Props for Montauks
|Author||Topic: Props for Montauks|
posted 10-25-2001 03:29 PM ET (US)
I have a 90hp Johnson on a Montauk. I have a poling platform and a front deck. This accounts for an extra 100lbs at all times. I am not convinced that my current performance is the best. My friends all have Outrages or Standards so they can't help. What is the best prop for this setup?
posted 10-25-2001 04:31 PM ET (US)
Howdy 88. The only sure way to "prop out" a rig is on the water, doing what you want to do with the boat and loaded the way you load it.
The ideal prop will allow the engine to the top of its operating range at WOT, but not over that.
My Son's Montauk/Merc90 uses a 19" SS prop. My Montauk/SuzukiDF70 uses a 18" SS prop.
Red sky at night. . .
posted 10-25-2001 10:10 PM ET (US)
When I repowered my '79 Montauk this summer I was nervous about getting the right prop. Maybe it was just luck, but by using the "propeller" information in the reference section I got the prop just right. Here's how I did it.
First I used Clark Robert's formula (in Part 2) to estimate the maximum top speed of my boat/motor setup. My new engine is a 70 HP Evinrude 4-stroke and I estimated my boat weight (hull, engine, 2 people, gas etc.) to be 2000 lbs. Using Clark's formula...[Square Root (70/2000)]x 200 = 37.4 MPH. This is the maximum achievable speed for my boat and motor.
Next I used the formulas in Part 1 to determine the propeller pitch. My motor manual gave the full throttle operating range as 5200 to 5800 RPM. I picked the mid-point of 5500 RPM. The manual also gave the lower unit gear ratio as 2.42 to 1. Using those two pieces of information I calculated the propeller shaft turing speed at 5500 RPM...(5500/2.42) = 2272 RPM.
Now I know that...When my propeller is turning at full throttle (2272 RPM) my boat should be operating at maximum speed (37.5 MPH).
Convert MPH to inches per minute by multiplying by 1056...37.5 x 1056 = 39,600 inches per minute maximum speed.
Divide 39,600 inches per minute by 2272 revolutions per minute and you get the inches moved by 1 revolution of the propeller...39,600/2272=17.4 inches per revolution. Inches per revolution is the prop "pitch".
Since all props slip, I was prepared to discount the pitch downward.
Examining the standard (3 blade)props offered for my motor I noted the diameters were generally 13" to 14". My dealer had the 14 inch diameter, 17 inch pitch prop (14x17 aluminum) in stock and I bought it.
After break-in at WOT my RPM is 5500. I don't know my top speed but suspect its right around 37 MPH. FYI my hole shot is great and I'm very pleased with the boat/motor combo. Luck? Maybe. But as with all the things I've tried using the combined knowledge of this forum it worked for me...Bob M.
posted 10-26-2001 02:09 AM ET (US)
My old '79 Johnson 85 hp is turning a 13 1/2 x17 aluminum, and moves my loaded '79 Montauk to a max of 38 MPH in windless conditions on flat water. Since I never see that where I do most of my boating, I am more impressed with the solid holeshot and low planing speed. In gentle swells, I can keep her on plane at just 15 mph.
posted 10-26-2001 08:37 AM ET (US)
RWM--nice analysis of the prop situation! I like it, and the results sound good.
It is hard to predict the "best prop for Montauk" without taking into account the gear ratio in the engine's lower unit. In mid-range outboards the gear ratio varies quite a bit among various brands and models. Just looking at Mercury outboards in the 50-90 HP range, the lower unit gear ratios vary from 1.83 to 2.33. You would have to take that into account when choosing the propeller pitch.
posted 10-26-2001 09:18 AM ET (US)
Thanks for the input. Now I know my prop is killing me. Top speed with my 90 is only about 39mph. I will say my hole shot is somewhat break neck though and I can maintain plane at 17mph.
posted 10-26-2001 10:45 AM ET (US)
88 it is real simple. What are you running now(rpm & prop pitch)? Your motor redlines at 5500 I believe so if you are 5300-5500 WOT you're good. If you are only doing 5100 you are overpropped. 5700 you are under. Every inch of pitch reduces/increases you by about 200rpm's. Most Montauks with 90 OMC's run 17" props so if you are hitting 5600(tach being correct also) you can add a double cup(roughly 1") and it should drop down to about 5450.
|John from Madison CT||
posted 10-26-2001 12:33 PM ET (US)
FWIW, ,my '89 Montauk has a 90hp Yammie. The prop is 17" and it tachs out at 5400RPM's (5500 is Max Spec.)
posted 10-27-2001 03:16 AM ET (US)
Something I have found out from much trial and error is that rake and blade shape have much to do with performance. In most applications I have found that round, "elaphant ear" props offer less performance than a "pointy" blade with a straight back. Sorry for not knowing the correct terminology. A 90 Evinrude I had on a Montauk ran much better with an OMC "Wide Blade" than a Michigan of the same pitch. I run a Yamaha 19 black SS prop and get great holeshot and top end, while a Yamaha Performance polished 19" SS prop with extreme rake (swept back blades) and heavy cupping and was horrible at everything. A friends 13 Sport/35 Honda runs well with the stock aluminum (pointy). When he hit a rock he got a Michigan aluminum, same pitch and diameter. Hole shot and top end suffered. Went to dealer and got another stock prop and runs great.
posted 10-27-2001 02:24 PM ET (US)
90 horse Johnson, 99 model on a 62 Nauset. I've got a stainless 17 pitch Viper prop (OMC high performance, heavily cupped). Initially, I could run the engine up to about 5800 RPM, so I was obviously underpropped. However, I've added weight as I got the boat rigged, and now with the baitwell full it turns about 5400 at about 41 to 42 mph. I've got the engine set all the way down too, and will have it raised one hole when in for next service. Holeshot is best characterized as "instant".
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