Anyone running an F70LA at 5,000 foot elevation?

[guitodd**]

I have a 2013 18' Kingfisher Falcon (aluminum windshield boat). I live in Vancouver, Washigton and took the boat to Lapine, Oregon for a week to discover that this motor doesn't have a baro/oxygen sensor. RPM went to 3,800 and couldn't plane with the boat.

I don't have all the numbers in front of me but went from a 17" Yamaha K-series to a 15" on the suggestion of Ampro in Portland...with not much difference.

I've searched the forums, calculators and web and am just frustrated at this point. Way more complicated than I imagined. Slip? Shouldn't have skipped so many math classes in high school!! I can exchange the prop I bought, but only get to Central Oregon a few times a year. Could take me years to figure it out at this point.

Curious what the Central Oregon guys are running. Or any rules of thumb that will steer me in a faster direction. Dealer in Madras, Or. was no help... made the mistake of telling him I bought a prop from Ampro.

[boscoe99]

What is the WOT RPM, with the 17 pitch propeller, with the boat loaded, at sea level?

By the way, the F70LA does have a manifold absolute pressure sensor that makes the necessary air/fuel adjustments when the pressure is lower at altitude. However, the motor still won't make the HP at altitude that it makes at sea level.

No Yamaha four stroke uses an oxygen sensor.

Rule of thumb for HP loss due to altitude alone is ~ 3.5% per thousand feet. So, in your case that is a loss of 17.5%. Now what was the barometric pressure, temperature and humidity when you were up at the lake? Throw in higher than normal temperature and humidity, lower than normal atmospheric pressure, and the power loss will be even greater. All you can do is to prop the motor to get at or near red line RPM (6000) with the boat loaded as you normally use it. The power will be what the power will be.

[tmann45]

What is your maximum RPM and speed at sea level? With those numbers, your pitch and your gear ratio you can make a SWAG on what pitch you need to run.

Well Boscoe you beat me to it. Google gave me 3% hp loss for snowmobiles per 1000 ft, what do ya think, 3% or 3.5%? Compromise at 3.25%?

[guitodd**]

Thanks for the responses.

WOT for me at my home lake, Lake Merwin... showing 240' elevation is about 5,500 rpm. I understand I should be closer to 6300rpm

My first trip to Lapine was at lakes around the 5,200 foot level and I was only getting 3,800rpm. I changed the prop for the second trip (17" to 15") but we fished East Lake at 6,400 ft and I saw about 3,900 rpm. My assumption is I din;t go down in pitch far enough?

[boscoe99]

Thanks for the responses.

WOT for me at my home lake, Lake Merwin... showing 240' elevation is about 5,500 rpm. I understand I should be closer to 6300rpm

My first trip to Lapine was at lakes around the 5,200 foot level and I was only getting 3,800rpm. I changed the prop for the second trip (17" to 15") but we fished East Lake at 6,400 ft and I saw about 3,900 rpm. My assumption is I din;t go down in pitch far enough?

That seems to be a good assumption. The motor does not make rated HP until it gets to 5800 RPM.

[guitodd**]

I guess my confusion is... If I lived in La Pine and bought an F70 could I never expect it to be a 70hp motor? Do they have different ECU's for high altitude applications or is it ALL prop selection? Luckily most of those lakes seem to have speed limits that I'm not even close to violating

[boscoe99]

I guess my confusion is... If I lived in La Pine and bought an F70 could I never expect it to be a 70hp motor? Do they have different ECU's for high altitude applications or is it ALL prop selection? Luckily most of those lakes seem to have speed limits that I'm not even close to violating

Unless a supercharger is installed, no piston motor will or can make the HP at altitude that it can produce at sea level.

The ECU can make some adjustments for pressure and temperature but they are no substitute for the missing oxygen. Thankfully, your motor has an ECU. If the motor had carburetors then the jets would have to be replaced each time up and down the mountain. A real pain even if you can find the correct jets.

All you or I can do (short of adding a supercharger) to get the best performance is to reprop the motor. Here is some verbiage from Mercury Marine about the loss of performance due to temperature. With the increase in altitude (loss of air pressure) it is much worse.

[99yam40]

Less O2 available at higher up so you cannot burn the fuel needed to produce the rated HP.

I am sure they just buy bigger HP motors that get de-rated to the HP they need to push the boat properly

[zenoahphobic]

humidity

It is interesting that mentioned above that higher humidity reduces available HP.
In the past, less efficient engines performed better with some humidity. This was because the water in the air aided in more complete combustion. And I think humid air expands at a slightly greater rate than dry air when heated. Remember the water bottles that had hoses attached to a port on carburetta . It was probably the cooling affect that was at play- like methanol fuel (less energy than petrol but methanol engines can be stressed more because they run cooler and therefore produce more HP).

[tmann45]

Thanks for the responses.

WOT for me at my home lake, Lake Merwin... showing 240' elevation is about 5,500 rpm. I understand I should be closer to 6300rpm

My first trip to Lapine was at lakes around the 5,200 foot level and I was only getting 3,800rpm. I changed the prop for the second trip (17" to 15") but we fished East Lake at 6,400 ft and I saw about 3,900 rpm. My assumption is I din;t go down in pitch far enough?

Yep, using available data, calculations show you should be using a 13.5" pitch prop. at 5000 ft elevation. This will get your 58 HP F70 up to 6300 RPM range at 5000'. This assumes the 13" pitch prop is same model as your 17" pitch prop.

Is your WOT speed at home around 36 MPH? A 13" prop should get you +/- 30 MPH at 5000'.

[guitodd**]

Great info guys! Thanks!! I did get an answer from the place I buy my parts from this afternoon. He felt that based on the info I provided I should look at a 12-13" also.

One thing I never did was try running the 15 (instead of my stock 17) at my home lake. He thought I should see better RPM and top speeds too.

I'm seeing 30mph top with two fat guys and a cooler full of beer. He was thinking I should see 5mph better than that and closer to 6,000rpm. Think I'll put the 15" on and give her a run. Looks like I gotta drop another $150... but it might be worth not exchanging the 15" I bought.

[tmann45]

I'm seeing 30mph top with two fat guys and a cooler full of beer. He was thinking I should see 5mph better than that and closer to 6,000rpm. Think I'll put the 15" on and give her a run.

With this new info I doubt any of my calculated numbers are very reliable, 5500 RPM, 30 MPH, 17" pitch gives 21% slip. Do you have the motor trimmed all the way down?

Yes, you should try the 15" at home and get some exact numbers with the motor trimmed up for maximum RPM and speed.

[guitodd**]

I do play with the trim to get that top speed. I don't have an indicator or know that angle I'm trimmed to. Have connected my Lowrance HDS-5 and can now see trim on the screen.

I agree what you guys are saying.... I'm going to give the 15" I have a run at the home lake and see what I get. I'm going to call Ampro and hopefully they have something in the 13" range in stock that I can try.... Heading to La Pine again Sunday - Wednesday. We'll be on low speed lakes but I'd like to give it a run so I know. Cultus is close.. maybe I can give her a run there.

Thanks for all the help guys!! I really appreciate it and learned a lot. I'll post my findings as I'm sure this will help others.

[boscoe99]

Yamaha offers an aluminum propeller in an eleven pitch.

With the Pontoon series in SS they go all the way down to a nine pitch. Here is a selection of Yam props for your F70.

http://yamahaoutboards.com/sites/def...e-F115-T50.pdf

In this document you can read further from Yamaha about propping at altitude.

http://yamahaoutboards.com/sites/def...DT_4%20WEB.pdf

[guitodd**]

Ampro in Clackamas had a used 13 pitch that's been 'sand blasted' but didn't have any chips out of it and was in balance. Was able to pick it up for $75. Will try that this weekend.