I'll Bet Anything...

[k3nlind]

Hmmmmm...

Well the engines don't sound like they are lugging... and both tachs read about the same... is their an independent baseline tach that can be easily attached somewhere in order to check the helm tachs

And gee... I gotta go back to school so I can re-learn math just to fully understand all those equations Joe wrote up Thanx Joe. I am looking forward to fully understanding these figures. But it would appear that from the calculations I might be running 5K RPM and not the 4.5K that the tachs are showing.

Concerning the water speed, I just want to mention that I am reading it off a Lowrance 335 for both the GPS and the water speed indicator. But the water speed indicator is not a pitot tube it is a mechanical spin wheel.

thanx again,

[Sea Wolf]

Ken-

Larry H would know for sure, but I'm guessing that an outboard mechanic would have a hand-held tach that could be used to verify engine/tach rpm.

I'm pretty sure we're right about the 5000 rpm being your current max, or at least something like that.

OK on the paddlewheels- most really good fishfinders have them. Probably more accurate than a pitot tube at higher speeds, and definitely more accurate a lower ones.

I wonder what others think about all this?

Joe.

[helesh]

Ken,
I remember Yellowstone John had a formula whereby you lost 3% of your horsepower for every 1000ft of altitude. If you're boating on Great Salt Lake you could expect a 10-12% loss over those sea level guys.
Les

[Sea Wolf]

Les-

Good point!

I think Larry H also said that it was -3% for every 1000 feet over the first 1000, if I'm not mistaken.

So an elevation of 4330 would be 3330 over 1000 ft, and

3.33 x 3% = 9.99% or 10% of your hp out the window!

This might account for maybe (guessing here) of 500 rpm lost on the top end?

Joe.

[Hunkydory]

All,

On Yellowstone Lake in Yellowstone National Park we run max speed of 20 mph at 5000 rpm with propulse adjustable props set at one notch below normal marks used for adjusting pitch. This is the lowest setting possible. Elevation is approximately 8000 ft and I figured we lost 24% of our Honda twin 40's power. This is with a medium light load. Jo-Lee and I, full fuel and water, food and gear for a long weekend.

At sea level with heavy load cruising for extended period, adjust props two notches up with approximate top speed 24 mph at 5200 rpm. We live at 6200 ft. and most local outings are between 6000 and 8000 ft with a trip to the sea hopefully once a year. The propulse props have made things a lot easier for us.

Les, if the the 5000 rpm is correct and with that much weight up front the speed sounds about right to me. Agree that you do need to find out for sure about the rpm.

Jay

[k3nlind]

thanx guys... very interesting

I was going to ask about the altitude thing, but somewhere I had read that the Honda 40 doesn't need any adjustment for altitude. Mentioned this to a couple of friends and one mentioned another aspect I haven't seen yet in this thread... according to my friend there may be more prop "slippage" pushing water at a higher altitude. Has something to do with the O in the H2O being less dense than at sea level Anybody ever heard anything like this

[thataway]

I don't think that you will find that prop slip is greater at elevation--it is decrease HP. I figured 2" pitch for each 3000 feet.

[k3nlind]

Here's a bit more... and it seems to me that the elevation thing seems to fit right in with this:

Spent the weekend at Flamming Gorge Reservoir (fishing for Kokanee). Elevation was reading just over 6K. Lost another 100RPM and top speed dropped to just under 20MPH. Interesting.