Effect of altitude on pitch requirement?

[Mel Taylor]

I have a 1995, 30 horse, Evinrude on a 1984, 14 ft., Mirrocraft Utility Series, Aluminum boat.

The boat weighs around 300 pounds empty. I usually carry myself and one passenger (370 - 400 pounds total) plus trolling motor, one size 24 battery, six gallons of gas, tackle boxes, etc.

The boat has only been used but a few days a year until now and, for the most part, at an altitude of about a thousand feet above sea level. Now it will be used more often and primarily at Elephant Butte and Caballo Lakes in New Mexico. where the altitude is between 4000 and 4500 feet.

The motor came with a 10X15 prop on it and that's what I've been using since the motor was new. I've been told by a local boat mechanic that with the change in altitude I probably need to go to a 10 X 13 prop.

Naturally there will be some running up and down the lake at top speed but speed is not the priority here. I do a fair amount of trolling and ability to function at trolling speed is as important as top speed.

I haven't had the chance to check the WOT rpm yet with the current 10X15 prop and the only tach I have access to is going to be difficult to use for that. But, I intend to try before the end of the summer if possible.

I would appreciate feedback/comments/advice from anyone with experience in the area of the effects of altitude on power and prop size.

TIA,

Mel

 

[Johnny Too Bad]

Re: Effect of altitude on pitch requirement?

I have been battling the effects of altitude for about ten years now, and I can assure you prop pitch is the single best way to combat the loss of horse power.

The prevailing rule of thumb is that you lose about 3% of hp for every 1000 foot rise in elevation. Therefore, I was losing about 15% hp when I moved from sea-level, and the prop I used at sea-level was just too extreme. As I'm sure you know, the pitch indicates the theoretical distance the prop would move forward (in inches) with one revolution. The higher the pitch, the farther it would travel, and the more power you would need to turn it.

I currently live in Denver (5280 ft above sea-level) and my old boat came with me from the gulf coast of Texas (25 ft above sea-level). I have a 9.5hp Johnson that would easily plane off a 14' jon boat with two adults in the boat. At Denver's altitude, with a 12" pitch, I couldn't turn enough rpm's at wot to push the boat more than about 3mph. I dropped down to a 9" pitch, but realized that you lose too much hp at altitude and that I would need a bigger motor. I ended up having to move to a 15hp Yamaha in order to get the boat on plane.

I also do a lot of fishing in the mountains, and I lose another 15% hp when I'm up there. After talking to quite a few people around here, I've come to the conclusion that I'll need to be on the high end of the hp range that my boat is rated for and use three different props based on where I'll be running the boat.

Hope this helps,
JTB

 

[hwsiii]

Re: Effect of altitude on pitch requirement?

Mel, buy your own tach, you don't even know how good your motor is doing right now with the prop you have. You certainly can't make adjustments without that knowledge. Sorry, but you can't make intelligent decisions without knowledge and without a tach you can have no knowledge of what your boat is doing right now.


H