High Altitude Boating Power Loss
02-01-2004, 03:20 PM
#1
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High Altitude Boating Power Loss
I know the majority of people here boat at or near sea level but I was wondering what the HP rating (Loss) and prop change would be for a set of N.A.496ho at 6228 ft. Elevation, ( Lake Tahoe Ca. ) this is my primary boating area. Are there any calculator's on line for this? Thanks....Whipple or not to Whipple?
02-01-2004, 06:43 PM
#2
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Charter Member
Most of what I have read about high altitude performance boaters involved the installation of a superchargers on their boats.
Last edited by RLW; 02-01-2004 at 06:48 PM.
02-02-2004, 01:56 PM
#3
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High Altitude....
Welcome to the nose bleed seats 
This should help although you may not like what you see.
The supercharger is a good equalizer.
http://wahiduddin.net/calc/calc_hp.htm
This should help although you may not like what you see.
The supercharger is a good equalizer.
http://wahiduddin.net/calc/calc_hp.htm
02-02-2004, 08:57 PM
#5
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Altitude HP Loss
My 300 HP airplane will put out a little over 80% of its rated HP at 5,000 feet. At 7500 feet the book says I can get 73% power. The air density goes down as you go up. This is in a normally aspirated engine..turbos are cool...as they will boost to whatever the waste-gate is set at. The turbo version of my plane will maintain full HP up to about 20,000' which results in about a 228 mph cruise speed!
Any normally aspirates engine will act this way at altitude, so whatever the rated hp of your boat is, it will be putting out about 75-80% of its sea level hp at 6,000 feet. As there is less air, you need to lean out the fuel mixture to keep it at stoichiometric ratio.. about 16:1 air: fuel. Less Air, less fuel. This is why the plane has an adjustable mixture, with a vernier control, and a fuel flow and exhaust gas temp gage. You lean as you go up, and watch the EGT to determine the right mixture for peak EGT.
I hope this explanation helps. Fortunately, in a plane, you pick up about 2% airspeed per 1000 feet of altitude due to the decreased density of the air. I don't think that would be the case with a boat..you will just go slower thru the water, and the carb will run too rich. If you are coming from sea level, you will need a jet change most likely. A FI engine is automatically adjusted by the ECM, and should lean itself out automatically. It will still lose the same amount of hp, it just won't run over rich like a carbed engine.
Good luck, I do all my boating on the bay, at sea level so I got whatever my tired 5.7 can put out at full RPM.
Dave Leonard
Any normally aspirates engine will act this way at altitude, so whatever the rated hp of your boat is, it will be putting out about 75-80% of its sea level hp at 6,000 feet. As there is less air, you need to lean out the fuel mixture to keep it at stoichiometric ratio.. about 16:1 air: fuel. Less Air, less fuel. This is why the plane has an adjustable mixture, with a vernier control, and a fuel flow and exhaust gas temp gage. You lean as you go up, and watch the EGT to determine the right mixture for peak EGT.
I hope this explanation helps. Fortunately, in a plane, you pick up about 2% airspeed per 1000 feet of altitude due to the decreased density of the air. I don't think that would be the case with a boat..you will just go slower thru the water, and the carb will run too rich. If you are coming from sea level, you will need a jet change most likely. A FI engine is automatically adjusted by the ECM, and should lean itself out automatically. It will still lose the same amount of hp, it just won't run over rich like a carbed engine.
Good luck, I do all my boating on the bay, at sea level so I got whatever my tired 5.7 can put out at full RPM.
Dave Leonard
02-02-2004, 10:04 PM
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force 320, Supercharger all the way. I boat at 3200-4000ft and know all to well, the effects of hp loss in a boat. I can't imagine how much loss there is at 6200ft.
When I bought my boat at 500ft it ran great, then when I brought it home, it was a total dog out of the hole. You will be absolutely amazed with a whipple. It idles and starts just like factory, but when you squeze the throttle, it has the power to get cruising very quickly. I have seen guy's put loads of money into their motors only to gain 7-10 mph and always had idling and reliability problems, but everyone around here that has installed a supercharger has gained 13+ mph.
Good luck!
When I bought my boat at 500ft it ran great, then when I brought it home, it was a total dog out of the hole. You will be absolutely amazed with a whipple. It idles and starts just like factory, but when you squeze the throttle, it has the power to get cruising very quickly. I have seen guy's put loads of money into their motors only to gain 7-10 mph and always had idling and reliability problems, but everyone around here that has installed a supercharger has gained 13+ mph.
Good luck!
