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Author Topic:   MT170 Cruise Speeds
Maximus posted 09-08-2003 06:15 PM ET (US)   Profile for Maximus   Send Email to Maximus  
Curious to what the crowd cruises at.

I've found a few happy throttle settings: (motor all the way down)

3100 RPM 17-18 MPH
3600 RPM 20-21 MPH
4100 RPM 25 MPH

I probably spend most of the time at 3600 RPM.

Moe posted 09-08-2003 08:03 PM ET (US)     Profile for Moe  Send Email to Moe     
What prop did you wind up with?

--
Moe

Barney posted 09-08-2003 09:01 PM ET (US)     Profile for Barney  Send Email to Barney     
Max, Sounds right with motor down all the way. I have the 18 pitch Vengeance SS. Pulling skiers ranged from 4100 to 4300 RPM, trimmed up about 5 bumps. I broke the prop free yesterday in a hard turn to retrieve a downed skier with that setting. I normally run 7 to 9 bumps on the up trim. Jim
Maximus posted 09-09-2003 11:48 AM ET (US)     Profile for Maximus  Send Email to Maximus     
SS Vengence 18P Prop

Maybe the real reason for the thread is my concern over motor life. I would never run my car's engine under load at 4100 RPM for a half hour straight...

Sunday, I ran 20 miles or so at 25 MPH (4100 RPM). This got me thinking and got me a little worried.

I find the best compromise over speed and minmal engine RPM to be at the 3500 RPM range.

Is there a prop setup that gives you the best cruising speed with the lowest engine RPM? From what I have seen, prop selection is based upon WOT RPM vs. Redline.

Moe posted 09-09-2003 12:46 PM ET (US)     Profile for Moe  Send Email to Moe     
Is your car a DOHC 4-valve with 6,000 rpm redline?

4100 is just under 70% of redline and 70-75% is a good rpm limit for max continuous usage.

Selecting a prop is like choosing a gear on a 5-speed manual transmission. In most cases, 4th is as high a gear as you can use and still be able to reach the engine's maximum rpm range, which is where maximum horsepower is developed. Given that an overdrive 5th gear may be .70:1, 4th gear provides 40% more torque to the ground (like torque to the water), for better acceleration.

If you choose 5th gear (or a prop with more pitch), the maximum speed of the car (and hence the engine) will be limited by aerodynamic and rolling drag at a road AND engine speed where the engine is developing less horsepower. In some cases, a car is actually slower in 5th than it is in 4th, because the power at the lower rpm is insufficient to raise the speed higher.

5th gear (or a prop with more pitch) will lower the rpm at cruise speeds, decrease fuel consumption and engine wear, but it will also significantly reduce acceleration, and may actually lower the maximum vehicle speed, contrary to what it might seem. With a boat, the loss of rpm and torque with more pitch can also RAISE the speed at which the boat falls off plane.

That's why the rule of thumb is to select a prop that will put the engine in the max rpm range (without overspeeding the engine) at WOT and lightly loaded. It's a good compromise between acceleration and rpm/mph.
--
Moe

Rodger posted 09-11-2003 12:45 PM ET (US)     Profile for Rodger  Send Email to Rodger     
I have recorded the following performance numbers using GPS and optimal trim in calm water. 2003 MT170 with 90HP 4-stroke running 18P SS Vengence prop.

MPH RPM
4.0 1000
5.5 1500
7.0 2000
9.0 2500
16.0 3000
22.5 3500
26.5 4000
30.0 4500
34.5 5000
39.0 5500
42.0 6000

3500 to 4000 RPM seems to be best for long distance cruising.

Moe posted 09-11-2003 01:45 PM ET (US)     Profile for Moe  Send Email to Moe     
Roger,
What was the fuel, people, and cargo weight load? Bimini up? Any wind?

Thanks,
--
Moe

Rodger posted 09-12-2003 01:29 AM ET (US)     Profile for Rodger  Send Email to Rodger     
Hi Moe,
The above figures were taken in pretty ideal conditions:
10 gallons fuel, one person (200Lbs), no bimini, no wind or waves, and minimal equipment (approx 50lbs).
Rodger
Moe posted 09-12-2003 05:42 AM ET (US)     Profile for Moe  Send Email to Moe     
Thanks, Roger!

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