High Altitude Prop Selection

[mikeinwestla]

Hello, I have a 1996, Four Winns 190 Horizon with a 5.0 L, Fuel Injected, 190 HP engine and a Volvo Penta, Cobra SX outdrive. The Full Throttle Operating range is 4000 - 4400 RPM's. The outdrive has a standard gear ratio of 1.66:1 and 1.85:1 at altitude according to engine specs. I currently have a 14.25" by 21 pitch, 3 blade prop by Michigan Wheel (Standard Black). I live in Los Angeles and mostly use the boat on lakes at @1500' elevation. I now travel to Lake Tahoe at least once a year and my performance is drastically reduced. Lake Tahoe has an elevation @6200'.

The boat is 19'6" and weighs @ 3850 lbs. if my memory serves me correct.

Will a different prop alone increase my boats performance or do I have to modify the engine in some way? I would like to tow a wakeboarder/skier and not lose too much overall speed.

Any prop/engine adjustment suggestions would be greatly appreciated.

Thank You.

 

[Titanium48]

Re: High Altitude Prop Selection

Loss of power with altitude is unavoidable unless you want to invest in forced induction (turbo- or supercharging). The atmospheric pressure at 1900 m (6200 ft) is only about 80% of the pressure at sea level and 85% of the pressure at 450 m (1500 ft). Your engine is fuel injected so the computer will automatically adjust the amount of fuel accordingly, but power and torque are still reduced proportionally across the full RPM range. Less power means a lower top speed, as the drag on a boat is primarily from the water and does not decrease with altitude (unlike your car where the reduced power is compensated by reduced aerodynamic drag in the thinner air).

I'm guessing you will want a 19 pitch prop for high altitude. I'll leave it to the prop experts to say for sure, but I suspect that restoring sea level holeshot would require 20% less pitch (17 instead of 21) to compensate for the 20% less torque. However, your reduction in top speed will be less than 20% (drag is not linearly proportional to speed), so a 17 would probably let your engine overrev at WOT. A 19 would still improve holeshot (just not as much as the 17), but would likely allow operation at WOT without redlining the engine.