How many blondes does it take to run a motor?

[kali]

My boyfriend and I just purchased a 14 ft deep-V Tracker boat with a 25hp EL 4-stroke EFI Mercury outboard. The next day we took it out on Lake Mead, Nevada and it ran like a champ. It topped out at about 25 miles per hour. The next week we hauled it up into the mountains at approxiamately 8500 feet and we could't get it above 15 mph. The engine didn't miss or make funny noises, just wouldn't go any faster. We called the boat dealer and were told we needed to regear the engine. The million dollar question is...is that necessary? We will still be running it at vastly different altitudes depending on the season. What can we do to make the boat perform better at higher altitudes without compromising it's performance at the lower altitudes? Thanks for all your help.

 

[Don S]

Re: How many blondes does it take to run a motor?

You can "Help" the problem by changing props. If you were to run at that altitude all the time, then a gear change in the lower unit would be best. But the prop change to less pitch will make it a little better.
You might try posting in the prop forum, they may be able to help a little more with size and pitch for what you need.

 

[kali]

Re: How many blondes does it take to run a motor?

Thanks, I will try the prop forum.

 

[fishrdan]

Re: How many blondes does it take to run a motor?

Hey Kali,

I'm from Vegas also and deal with the altitude problem when going to the mountain lakes. I have a 7.5HP Force on a 14' jon boat that will do 20-25MPH locally, but only 12MPH up at Fish Lake (9000') and that was after re-jetting the carb leaner. I'm looking for a prop with less pitch to help raise the RPM of the engine.

My 140HP Mercruiser also suffers from similar problems up at 9000'. So far I have leaned the carb and swapped from the 17 pitch prop to a 15 pitch prop, but still not satisfied with it's performance at 9000'.

From researching the problem there are several things that can be done;
1- go to a lower pitch prop,
2- jet the carburetor leaner for the thinner air
3- advance the engine timing a few degrees (not easily accomplished on an outboard)
4- drill vent holes in the prop (I'm going to try this on my 15 pitch Mercruiser prop, but this is experimentation and has the potential of ruining the prop)
5- Change gears in the lower unit (this is expensive and not easily reversed, it's something done if the engine is going to be run most of the time at high altitude)

If the outboard ran well at altitude (well, besides being under-powered) I would first swap the prop for a lower pitch prop. IE: if stock is a 19 pitch prop try and find a 17 or 15 pitch prop. This is something that's easily changed back and forth, opposed to the other difficult modifications.

You can also use the search function on Iboats to look for other threads on this subject, there are a few other threads on this subject.

Edit: looks like Don beat me to the punch... Day late and a dollar short I guess

 

[j_martin]

Re: How many blondes does it take to run a motor?

Engines have trouble getting enough air at altitude just like we people do. They won't put out as much power, so the best solution if you're changing back and forth to different altitudes is to use a different prop to more efficiently use the lesser power. Just carry along both props and a plastic prop wrench, and maybe a spare prop nut and keeper, just in case you get clumsy.

It can go to extremes. I change the prop on my bass boat if my partner is over 200 lbs.

hope it helps
John