87MagnumII

Let's say for instance you were to take a given boat and run it at sea level and top speed is 70 mph. now take same said boat to 3000 feet above sea level. will this boat still run 70 mph?? what can be figured as a loss for higher elevation boating? i understand that we have to adjust our A/F mixture depending on altitude, but i dont see how its possible for an engine to perform the same with a 3000 ft change in elevation. even with AF adjusted correctly there is still more air in the air at sea level which allows for more fuel and produces more power than a mixture for 3000 feet above sea level which in turn would be much more lean. sorry if this doesnt make sense but i have had a few beers and this is the best logical reasoning that i can come up with as to why boat doesnt run the way i think it should, lol. im hoping for a few experts to jump in with some answers on what can be expected when changing elevation. i understand that temp and humidity play a part as well but elevation i think also plays a big part.

the deep

iTrader: (8)

Have one on me brother........by the way i think you meant there is more oxygen in the air at sea level ........anyway some of the experts will be able to explain it better than me . It's not as complicated when sober ....

triple 300s

Swap gearing I would think

Sydwayz

iTrader: (1)

I sold my 500EFI powered boat from sea level to the new owner at a high altitude lake in Montana. It slowed down ~10mph. He eventually went to a Whipple to get the speed back. Swapping gearing is not going to do much. The air is thinner, not the water! The air is less dense, which means less compressed air in the cylinders; less explosion, less power. You need to make up for the thin air by compressing more air into the cylinders; ala forced induction.

FIXX

iTrader: (2)

get a scuba tank and a nos solenoid and plumb it into the intake

RiverDave

What if I told you there is a method/device that you could do just that? Doesn't work real well on larger displacement motors aka boats, but for smaller 1.8 liter and below motors it works quite well.


As far as elevation, it was pretty much explained above. Air is thinner so 1 cubic foot of air at sea level is more dense then a cubic foot of air at altitude as there is less pressure being exerted upon it from the atmosphere above it. The higher you go, less pressure, less dense. The denser the air charge, the more oxygen it contains per cubic foot or whatever your preferred measurement. More oxygen = more fuel = bigger bang = more HP. Temperature always plays a factor, the hotter the air charge the more it has expanded, and again less dense.

RD

grads2112

I had a 70 m.p.h. boat at sea level and took it to Tahoe(6200'). I had to go down one prop size if not two and dropped about 10 m.p.h.

My twin engine daytona was a 115 m.p.h. boat with 32p props at sea level. At Tahoe I would run 28p props and they were perfect, not too big not too small, and it had a top speed of 95 m.p.h.

87MagnumII

I did mean oxygen I know that the air is less dense at higher elevation and the engine can only burn what is provided. Is there a formula that can be used to calculate a loss for higher elevation? For example, for every 1,0000 ft above sea level would it be safe to say that you lose 3-5%? Best my boat has run at 3000 feet is 63-64 mph. Would 70 be achievable at sea level if the AF mix was adjusted correctly?

Raylar

My experience and examples with normally aspirated engines over the years says use a formula of 5% loss of power for every 2000 ft. increase in altitude from sea level and that will get you pretty much where you need to put the resulting power on the engine.

Best Regards,
Ray @ Raylar

bobl

Just for the heck of comparison I took an HP500 dyno run I recently made. I can specify altitude on my weather data page. With all other parameters staying the same (temp, humidity and corrected barometric pressure) this is what it gave me. Sea level was 1.0334 correction, 1000 ft. 1.0703, 2000 ft 1.109, 3000 ft 1.149, 4000 ft. 1.190, 5000 ft. 1.234 correction. This should give you a rough idea of the changes. Of course one thing to consider is with altitude the temp usually is lower so it's really difficult to get a direct comparison. It not uncommon to find density altitude at low elevations of 3000-4000 ft during the summer