Elevation vs Speed

[Craig]

I seem to recall before there being a post about elevation having an effect on top speed I suppose on carbuerated boats.

Anyone remember or know what effect is?

Example: boat goes 55mph at 2500 feet above sea level.

Expect ??? mph at sea level.

I think I recall it may have to be a significant altitude difference to notice much change, but seems I've heard people "in the mountains" say they get a bit more speed at sea level.

[MOP]

I dug up two past interesting posts that will help you out.

http://www.donzi.net/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005016#000000

http://www.donzi.net/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=003898#000004

[Craig]

Thanks MOP -- I copied them for reading later. I gotta a busy day today!!

[wytmike]

As a general rule of thumb not taking into account temp., you lose approx. 3% of horsepower for every 1000 feet you go up. So at 2500' you have lost +/- 7%. It holds true for carb or F.I.. Even supercharged and turbos lose some depending on the system design.

[Craig]

Cool! So I'd gain 7% hp coming "down to" Sea Level? How's that equate to an anticipated speed gain. Still haven't read the above posts. Looks like I actually started one of them. They say the mind is the first to go.

Hey, does this hold true moving even "further down?" I mean the Dead Sea's elevaltion is 1349' below sea level. There's at least another 4% increase!

Kind of an expensive way to gain horsepower; moving to the Middle East and all...

...I mean just for 11 horsepower.

I could see maybe 25.

[wytmike]

It sure does. Dig a deep hole and gain more h.p. I boat up at Lake Tahoe and we loose about 18% (6200') and that is really noticable

[TuxedoPk]

So this would mean that a stock 18 2+3 would have to be how many feet underwater to pass GEOO? :D Matty, what do you think- most compelling reason for a windshield we've heard so far? wink

[BigGrizzly]

In my job we use to test car, motorcycle and power equipment engines for everything from performance to emissions. And what we found is that there is not a real issue untill you hit 3000 feet and higher except around Reno Navada. there is a pocket on the mountain that units start to run rich at 1500 feet and above. Yet on the other side of the same mountain in Cali the same symptum does not exhist on the same unit. The change on your boat engine from 2500 feet to sea level bearly be noticed on a engine that produces under 1 horspower per cubic inch. Now at 6000 feet that is another ball game.

[Craig]

Aw man Randy! You're blowin' all my fun! I've been harassing my friend in Salisbury MD who runs his 20' Yamaha jet boat (at sea level) telling him all the reason's I could blow his doors in if I brought the boat down there. On top of alreay being 5 mph faster, I was using the elevation discussion to sling some more baloney and exaggerate just a bit more.

I'll weigh your trusted experience into my "real" knowledge of the facts, but I'm not going to tell him!

We took his jet boat out at Ocean City MD down to Assateaugue a few weeks ago. I got a healthy respect for you guys that play around in the ocean. That's a whole new world for me. Lots of fun.

We also ran his boat in the Pokomoke River (near Salisbury, MD). It ran 48 mph @ WOT in the ocean and 50 mph in the river.

He's new to boating and thought his boat was super fast. Actually what it is, is "super quick". With 2 135 hp jets it accelerates very fast. Also handling is very impressive (when you're accelerating). It is a fun boat, just different.

Anyway, I have to take my boat to the ocean some time. Even if it's not faster "down there" I'd like to experience the different environment in my own boat.

Thanks for the input guys.

[BigGrizzly]

I thint the Ocean os neet in a little Donzi, Even thoe your boat isn't a real Donzi bit a great copy, it will do really well in the ocean a lot better than the yamaha. Our 16 has spent more time in the Ocean than the bay and only in fresh water 3 times.

[Craig]

Ooo, I gotta copy and paste that part about "better than a Yamaha" and share it with my good, but overconfident friend; and I'll be sure to tell him this came from experienced "ocean folk". Thanks for the additional "ammo".

My boat's not a real Donzi? Oh yea I forgot. You know, by now, I'm dilusional about this.

[Blewbyu]

I have a 74 X-18 with a 355 in sbc/385 HP at crank.Run 5600 RPM 60 ft above sealevel on Lake Washington near Seattle.1 week later at Sandpoint Idaho, on Lake Pend Oreille (2455 ft)-5250 RPM at same ambient temp.Same every year/same prop every year.Same brand gas (Chevron Supreme).Chevron DOES tailor their gas for different altitudes however-and I run Idaho gas in Idaho,Seattle gas in Seattle.Have never tried to use Idaho gas in Seattle, or vice versa.Could be too expensive a mistake.Burned one engine down with bad gas already!Slower hole shot in Idaho too.Sooooo, 2455 ft does make a difference!