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drive gear help needed 1:36 vs 1:50

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Old 06-13-2002, 08:21 PM
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Dan..Going throught the same thing with a 35 Fountain and 500 H.P Efi's with Whipples. Something that has not been mentioned yet that I noticed changing from 1.5 to 1.35 is what your boat needs as far as lift. I was running a 36 Bravo 1 at 5400 rpm with 4 people and full gas tanks, if you do the math its about 22% slip because the boat never caught up to the prop before the port drive let loose. I was running 98 on GPS. The boat seemed like it was running down hill. Because of the angle of the blade to make that much pitch it really made alot of stern lift. It the tanks were 1/2 full and only 2 people who knows how much it would have lifted the stern. After talking to several people Including Mark at BAM Marine, the 1.5 gears seemed to be the fastest. Went to 1.34 IMCO's so I could run smaller pitch props and a little less stern lift so the boat was more balanced. To date with IMCO Xtremes, and 1.34 gears turning a 30 Bravo 1 The boat has run 95 at 5100 rpm. Better balanced not so much stern lift. With the 1.5 gears they were stock Bravo drives no nosecone. I think that their is more to be had working with props need to get the rpms up around 5400. Tried 31 5 Blades from Hydromotive. Could only turn them 4700 at 93 MPH. Will try 29's to get the rpms up.
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Old 06-13-2002, 10:33 PM
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It is my understanding that the 1.5's are faster in the midrange. I do know that if you go to the 1.5's that you'll be looking at a bigger prop. I have also heard that when you go over 32" that the efficency starts to drop off.
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Old 06-13-2002, 11:34 PM
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Dan,

Congradulations on the boat. It sounds like it's working real well for you. Mathmaticaly you are right on the money !

Drive gears or reduction gears are simple to decifer if you understand the relationships. We know from your test:

1:36 reduction driving a 32" P (lab) @ 5100 rpm

5100 crankshaft rpm = 3700 prop rpm

3700 prop rpm x 32"P = 95 mph

@ 15% slip (very good for a step bottom boat)

So in order to make a 1:50 reduction do the same we need,

1:50 reduction driving a 33" P (lab) @ 5400 rpm

5400 crankshaft rpm = 3600 prop rpm

3600 prop x 33"P = 95 mph @ 15% slip

By letting the prop spin more freely the load on the drive will be slightly less with the 1:36 but no more than a good throttle arm with some finesse. You can see why prop selection would run out with higher reving motors.
I would be most concerned with where the motor builder wants the rpm to be and prop accordingly !
Also, boxes may help by placing the props in cleaner water further reducing the % of slip and drag. Either way the boat is running very efficiently, and should provide lots of enjoyment.
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Old 06-14-2002, 10:01 AM
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Thanks for the replys, and Bosco, thanks for the compliements., and the clear explination. Pat is trying to explain almost the same thing, but I got a thick head.
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Old 06-14-2002, 03:29 PM
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Hey Bosco, I'll send you my calculator.
5100rpm with 1.36 drive is 3750 prop rpm. (5100 divided by 1.36)


Let's look at this:

1.36 32"lab 5100rpm 113.63mph(theo.) = 95mph = 16.4%slip

1.5 32"lab 5625 113.63 95 16.4

1.5 36" 5100 115.90 96.9 16.4

1.5 36" 5100 115.90 95 18.1%

What does all this mean?
1- if you change the drive ratio by ~10% (1.36 to 1.5) you can increase the rpm by ~10% and get the same mph.
2- If you change the drive ratio by ~10% (1.36 to 1.5) you can increase the prop by ~10 % and get nearly the same mph.
3- but larger pitch props have more slip so a 1.5 with 36" will run the same 95 mph as 1.36 with 32".

The labbed prop will affect the numbers slightly.
The 36" will give a bit more stern lift because the blades are flatter and if the boat needs it, it will run a little faster.

Your testing will be very informative. Please keep us all informed.
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Old 06-14-2002, 07:47 PM
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Another point that needs looked at is the Fact that gear reduction is torque multiplication enabling the engine to have more mechanical advantage across the propellers pressure face, (look at the extreme gear ratios of the outboards "Tim") so you can be a lot more “creative” in the prop selection.

Plus’s The Active Thunders Step bottom is the best in the business and Patented (you could not pay me to say that)
They have run mid 90s with much less power before in fresh water.
The Engines KEs what else needs said.

Like any other new setup test work is needed good Luck Pat and Tim.
Best Regards


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Old 06-14-2002, 08:16 PM
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Steve,

I do not understand, can you explain the gear reduction is tourqe part for me. Do you think we are going to have better results with the 1:5 or the 1:36?
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Old 06-14-2002, 08:17 PM
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I am confident that Pat and Tim will find the ideal setup.
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Old 06-14-2002, 08:50 PM
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Gears Ok a quick glance would reveal devices where you can trade force for speed or vise versa governed by the ratios in and out
We have an engine RPM of 5000 and at that a Torque of 600’# (just using numbers) with a 1.35 gear that’s 1.35 x 600 or 810 foot pounds of Torque with the 1.5 that’s 1.5 x 600 or 900 foot pounds of Torque.

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Old 06-14-2002, 09:27 PM
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Thanks Steve
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