High Altitude

[Donziweasel]

Just bought my first Donzi! I am 6000 ft and it is only running 4000 rpm at 51 mph. It is a 1997 16 with a mercruiser 5.7L 250 horse with 19 pitch prop and a 1.47 gear in the drive. I know I need to drop in pitch on the prop and a gear in the stern drive, but how many drops in gear and prop pitch to still have top end at my altitude? Any help would be much appreciated. AND, yes I am pumped about my first Donzi!:beer:

[Rootsy]

17P should get you where you need to be... 4400 - 4600 rpm with that motor...

she'll turn 4400 - 4500 with that setup at 1000 feet above sea level...

[Donziweasel]

How fast do you think it would run and would I have to drop a gear in the Alpha?

[Donziweasel]

Also, the boat porpoises and chine walks some. How much would trim tabs help this as my boat does not have them?

[MOP]

Tabs are pretty much a must on the 16, I had the small Bennets they were great.

Phil

[tmdog]

How fast do you think it would run and would I have to drop a gear in the Alpha?

Leave the od gear as is. Changing props to a lower pitch will achieve your goal.

[Rootsy]

the 19 vengence that is probably on there isn't a good prop for that boat... very unstable... as is the mirage prop on that hull.. turbo 1 is better or a 4 blade will really smooth the hull out... but you won't have optimal top speed.. issue with a 4 blade is that you'll have to go at least 1 inch of pitch lower than a 3 blade to achieve the same rpm. And you don't find many if any performance 4 blades in 16 or 17 pitch...

at 1000 feet above sealevel on a summer day in the 80's actualyl it didnt matter a whole lot on weather to my 16 in stock form... i was a 54 - 55 mph boat..

[MOP]

Surprised no one has brought up the possibilty of re jetting! Your air is much thinner and your jets may be to fat for the thin air, I would do a about a 1/2 mile run at WOT do a WOT shut down and pull a few plugs. I bet they are pretty black! A few on the board have gone through getting their boats to run up high, do a search you may find some good info. Also PM some of the othe high up boys they will have good in put. Where is Don when we need him!

[Donziweasel]

I was going to re-jet as I have with other boats I have owned at this Altitude. I live in Jackson WY and will be running the boat between 5000 and7000 ft. Thanks for all the input. What I have gathered so far is that I need to drop a pitch on the prop, leave the gear in the Alpha alone, rejet and add some trim tabs. What happened to the days when you could buy a boat and have it run fine. I had a 21 ft bowrider with a 7.4 and had to rejet, drop a gear and pitch on the prop. Living up here can be a challenge on getting a boat set up. Can anyone recommend a brand of prop (3 blade) in a 17 pitch for my Donzi. What manufactures should I stay away from? By the way, the boat came from Iowa at 1000ft above sea level and ran 62 at 4600 rpm's before I bought it. Just to let you know, I called mercruiser and they recommended a drop in gear, but they probably did not understand this was a 16 ft boat that weighed only 2000+ pounds.

[Donziweasel]

Also, looking at the Bennett 12x9 tabs, any input you guys have would help.

[boatnut]

As mentioned 6000 ft is a whole different performance story than 1000ft.
With a carb you have to rejet. You will not achieve the same speed as you can get at sea level -- unless you ram more air in with a turbo or supercharger (and of course it will still be faster at sea level). You might try a call to Sierra Boat Company in Lake Tahoe California (google search it). They are very experienced on making boats perform at 6000+ ft. and at one time they were even a Donzi dealer. Ed

[Donziweasel]

One last thing on re-jetting. The boat ran great, started fine and idled fine. I also did not smell any fuel like I use to on my 21 ft with the 7.4L befoe re-jetting. I will check the plugs and any input would be helpful.

[Rootsy]

not so sure there is much if anything you can do easily inside of teh 2V on top of the 350... i never had mine apart and i don't know how readily available jets and such are.

if 62 was off the speedo, don't believe it... 1.47:1, 19 P at 4600.. ZERO slip is only 56 mph (and is physically impossible in a fluid)... if you add an inch for cup (20P) the zero slip speed is only 59.. 8 - 13% is about "average" for slip numbers on this hull depending upon what prop you are running...

[Donziweasel]

The speeds were off the speedo, not GPS. I will take my GPS next time out.