Over the last few years my 15 hp Johnson seems to run about 3-4 mph faster right after ice-out when the water temps are in the upper 30's to low 40's compared to the 70's of summer. I have heard that outboards may run faster in colder water because the water is more dense, but I did not expect such a jump in speed. Has others experienced anything like this or am I overlooking something?
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outboard speed and water temp
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Charger25
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I noticed the same thing years ago when I was fishing bass tournaments. I asked my mechanic about it and his reply was "Its because in the winter the air is cooler thus more dense and contains more oxygen per square foot. In the summer air is less dense and has a higher humidity , less oxygen per sq. foot. "
Seemed to make sense to me as most newer cars these days have a "cold air intake" where they draw outside air and not hotter air off the engine compartment. just my 2 cents
Seemed to make sense to me as most newer cars these days have a "cold air intake" where they draw outside air and not hotter air off the engine compartment. just my 2 cents
I am a bracket drag racer...and I watch the weather almost as closely as a pilot does, and for different reasons. A 10 deg temp change can affect how my car runs down the track-as much as a half of a tenth (I try not to vary ET by more than .005-.015 second). Weather plays a big role.
Cooler air is good for an engine. It is denser. Meaning more oxygen molecules per cubic foot of air, which translates into more power (assuming there is enough fuel flowing to support a little more air). The downside is that on very fast boats (and cars, planes, etc) cooler denser air takes more power to plow through. With aircraft, drag goes up, but so does lift and thrust (usually). But some hulls are designed to use the air for lift (hydro's are an example) and they "like" denser air. Some fast bass boats are similar.
The hotter the air temp, the less HP the engine will make.
The more humidity, the less HP the engine will make.
Less humidity means a little more power.
Cooler air means a little more power.
The density of the water doesn't change a lot until around 37°. Above that and the density falls off. Below and density falls off-which is why ice floats (it is less dense). A boat on plane running across 37° water will be slightly faster. The density of the water is near it's highest point, therefore your boat will float a little higher in the water (less hull drag). Also typically if you have water that cool, the air near it will also be cooler...so the motor will make slightly more power (normally).
We use a calculation to find the "density altitude" of air. It's (DA) a calculation of dewpoint, relative humidity, actual air temp, current altitude above sea level, and a few other things. Basically DA is the quality of the air expressed in feet above seal level (MSL). DA right now is around 3700'. That means our air quality as of right now is approximately the same as someone who lives on a hilltop at 3700' above seal level. I've raced in DA around 5300' before. The car is a slug-relatively speaking. I've also run in -350' DA. That is what we call "mine shaft air", basically the same as racing below sea level. About the same thing as very mild supercharging. The difference in those two DA's on an 1/8 mile track at my current horsepower level (about 700) is roughly 4 tenths ET, or 50 HP give or take. A big difference when you're watching thousandths of a second. I was doing the calc's the hard way using paper and pencil, however nowadays I've got software and a weather station that does it for me.
Outboards are the same way. 2 stroke seems to be more sensitive than 4 stroke for some reason. On a cool day in the spring (air temp 45, water temp upper 30's), I managed to get mine up to 31.3 mph, on the rev limiter (bouncing 6300 RPM). This past monday (air temp 104, water temp 85) I was out hunting crappie and the highest I saw was 29.2.
Cooler air is good for an engine. It is denser. Meaning more oxygen molecules per cubic foot of air, which translates into more power (assuming there is enough fuel flowing to support a little more air). The downside is that on very fast boats (and cars, planes, etc) cooler denser air takes more power to plow through. With aircraft, drag goes up, but so does lift and thrust (usually). But some hulls are designed to use the air for lift (hydro's are an example) and they "like" denser air. Some fast bass boats are similar.
The hotter the air temp, the less HP the engine will make.
The more humidity, the less HP the engine will make.
Less humidity means a little more power.
Cooler air means a little more power.
The density of the water doesn't change a lot until around 37°. Above that and the density falls off. Below and density falls off-which is why ice floats (it is less dense). A boat on plane running across 37° water will be slightly faster. The density of the water is near it's highest point, therefore your boat will float a little higher in the water (less hull drag). Also typically if you have water that cool, the air near it will also be cooler...so the motor will make slightly more power (normally).
We use a calculation to find the "density altitude" of air. It's (DA) a calculation of dewpoint, relative humidity, actual air temp, current altitude above sea level, and a few other things. Basically DA is the quality of the air expressed in feet above seal level (MSL). DA right now is around 3700'. That means our air quality as of right now is approximately the same as someone who lives on a hilltop at 3700' above seal level. I've raced in DA around 5300' before. The car is a slug-relatively speaking. I've also run in -350' DA. That is what we call "mine shaft air", basically the same as racing below sea level. About the same thing as very mild supercharging. The difference in those two DA's on an 1/8 mile track at my current horsepower level (about 700) is roughly 4 tenths ET, or 50 HP give or take. A big difference when you're watching thousandths of a second. I was doing the calc's the hard way using paper and pencil, however nowadays I've got software and a weather station that does it for me.
Outboards are the same way. 2 stroke seems to be more sensitive than 4 stroke for some reason. On a cool day in the spring (air temp 45, water temp upper 30's), I managed to get mine up to 31.3 mph, on the rev limiter (bouncing 6300 RPM). This past monday (air temp 104, water temp 85) I was out hunting crappie and the highest I saw was 29.2.
