How much performance loss at altitude?

[RkyMtnNewJack]

Hello all,

I am in the process of buying a new boat, and it will be my first. I've enjoyed reading some of the threads on iBoats, so I thought I would see if you all could answer a question that is starting to cause me concern.

I am buying a 23' bowrider with a 5.7 Merc BIII from a guy in California. I've already asked about the seawater issue, and see no reason (yet) to be concerned. I'll inspect in person before I actually purchase.

The question that I have is around performance at altitude. I live in Colorado, and intend to bring this California / Nevada (it ran on Lk Mead) boat back with me to Colorado - getting a good deal, and I get to take a quick weekend vacation to Las Vegas.

After reading some of the posts, I am now wondering how this boat will perform at altitude. Most lakes that I will be using are a mile high. This boat is probably set up for lower altitudes. I have read that I might need to change out the props for the right props for altitude. As this is a BIII drive, I will be looking at big $$ change out. I am not sure what to expect when I get the boat back to Colorado.

Anyone have some thoughts on this, or questions I should be asking?

 

[puddle jumper]

Re: How much performance loss at altitude?

Depending on the carburation you have you may have to re jet the carb or at least reset the air fuel setting. If its fuel injection probably nothing.

 

[Lakester]

Re: How much performance loss at altitude?

hello,
there r formulas for figgr'n it out, but i will tell u from experience driving both stock and high performance vehicles at sea level, gulf coast, and at altitude, denver to vail... a loss of up to 33% can easily be expected. if u r in the denver area, id say and ez 20-25% loss due to the thinner air.

good luck, sounds like u got a swell deal.

regards
lakester

 

[jaxnjil]

Re: How much performance loss at altitude?

rmnj; i used to mechanic on heavy equipment and trucks the rule of thumb was 3% to 5% power loss per 1000 ft elevation for turbo and super charged motors and 5% 10% for natural. with all the newer fuel and igniton systems available this could be out the window but i still use them for a idea of where you are.
when i tried to find how to re jet the carb on my mercuiser iL 165 last year mercuirser didnt recommend changing jet sizes or adavancing timing to compensate for eleation. there are how ever lower gearing package recomadtions and lowering prop pitch to stay with in wot range.
live in notrh west co and boat on powell and flaming gorge. 3625 and 6020. if if i go to steamboat lake that is around 8000ft, wont take this boat up there. it killed our bayliner up there-took for ever to get on plane with the prop we had
our boat came with a 1:32 to 1 ratio and i changed it to 1:62 to 1.
we run a 15-21 prop at powell and a 15-19 at the gorge. these are both mercury mirage pluse props and were far better than any aluminum we tried
i installed a fuel flow meter this spring and can tell you that the most fuel flow at powell was 22.7 gal per hr and when we moved to the gorge the most fuel we could flow was 20.3{the difference being elevation}wot speed at powell 43mph and the gorge 40 with 19 prop and 41 with 21 checked with gps
i wish i could tell you the difference in rpm. checked out tack 2 week ends a go with a photo tach and found after 2000 rpm tack will steady and point like thats your rpm but can be off 300 rpm plus or minus what actual reading should be and is'nt the same every time. i have to fix that before i do any more proping.
its pretty hard to hold a photo tach steady at wot so now i'm not sure of my wot figures. they can be skewed 600 rpm and till i checked with photo tack, wouldnt have known

 

[RkyMtnNewJack]

Re: How much performance loss at altitude?

Hi all,

Thanks for the advice. I ended up getting the boat, and I just got it towed back to Denver from Las Vegas (lots of gas / $). The boat is performing decently at Denver altitudes. The engine is fuel injected, so I didn't have to make any adjustments. I "test drove" the boat on Lake Mead, and I have definitely lost some power due the altitude. Top speed at Lake Mead was about 48, now in Denver with a similar load of gear and people, I am getting around 42 measured by the speedometer. I am definitely seeing some delay in getting the boat on plane with a full load of people. We haven't done any skiing / wakeboarding with a fully loaded boat yet, but I am thinking we're going to have some problems with the hole shot time.

I am going to be reading up on this site about how to improve my hole shot by changing propellers. I'll post a picture of the new boat soon. Thanks for all of your help!

Rocky Mountain New Jack

 

[Hitech]

Re: How much performance loss at altitude?

I'd suggest trying some Smart Tabs first. They will help the boat get on plane faster and even get you a couple of MPH top end.

 

[jaxnjil]

Re: How much performance loss at altitude?

if you have a chance to try the mercury mirage II try one. i'm not sure there is a dealer there in denver tha will let you test props.
i tried 4 differnt props on mine and could tell the biggesst difference with the mirage, just works well with my set up and has vent holes that you can tune to get on plane quicker. i havent tryed any of the different vents as mine workes so welll now. the nearest place i can find the vent pulgs is buck board on flaming gorge. if i get up that way this summer i'm going to buy some and play around with them and see if they make any difference.