Effect of Water Temperature on Speed - Moderated Discussion Areas

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Author Topic:   Effect of Water Temperature on Speed

meridian posted 05-28-2010 02:17 PM ET (US)
Can water temperature have an effect on boat speed? I know air temperature can have an effect on engine performance.
sosmerc posted 05-28-2010 03:52 PM ET (US)
All I can say is that I sure swim a lot faster in cold water !
gnr posted 05-28-2010 04:14 PM ET (US)
The density of water does change with the temp. It achieves its highest density at around 39 degrees f. This is why cold country lakes "turn over" a couple times a year when the surface temp rises/falls to 39 and sinks to the bottom.
It can however reach the same density at points on either end of the spectrum.
I suppose theoretically a hull might float higher in water at 39 degrees giving it a bit more performance then at 70 degrees but in our real world applications I don't think any gains/losses in performance would be noticeable.
sosmerc posted 05-28-2010 04:34 PM ET (US)
It has always been my understanding that boats are somewhat faster on salt water than fresh water due to changes in the density. If the water is cold it probably means that the surrounding air is at least cool......and cool dense air makes for higher horsepower.
dgoodhue posted 05-28-2010 04:46 PM ET (US)
Saltwater has more buoyancy than fresh water. It's easier to swim in salt water than fresh water for this reason. Using the same reason a boat with sit higher in Saltwater. I don't know if Boston Whalers are affected all that much by the difference but I know Offshore Performance boats (ie Cigarette) are often 3+mph faster in salt water vs fresh water.
cgodfrey posted 05-28-2010 05:02 PM ET (US)
The density of water changes with temperature, but a very, very, very small amount, especially considering the relatively narrow range of water temperatures your hull will encounter
http://en.wikipedia.org/wiki/Density#Density_of_water
I think fish flatulence probably has about as much effect on you hull's drag and speed as the water's temperature :-)
CG
PeteB88 posted 05-28-2010 05:26 PM ET (US)
I think your lower unit sucks up in the belly of the boat in cold water especially Lake Michigan in early spring, Lake Superior and Oregon Coast anytime.
WT posted 05-28-2010 05:35 PM ET (US)
If the water is frozen your hull will get dinged up.
elaelap posted 05-28-2010 05:45 PM ET (US)
"I think your lower unit sucks up in the belly of the boat in cold water especially Lake Michigan in early spring, Lake Superior and Oregon Coast anytime."
You got it, Pete...shrinkage ;-)
Tony
Add to your list the California coast north of San Francisco. This year the water temperature has stayed lower than 50 degrees later in the spring than any other year I can remember. It will work its way up to maybe 55 f. by the end of summer, then back down to the high forties during the winter. I get shrinkage just thinking about going overboard fishing alone in that stuff and watching my boat trolling away at an unreachable 2.5 kts. Brrrr...
Liteamorn posted 05-28-2010 05:59 PM ET (US)

quote:
All I can say is that I sure swim alot faster in cold water !
That's because we have less drag in colder water! (see Sienfeld episode 85 "The Hamptons")
ScooterCO posted 05-28-2010 06:40 PM ET (US)
"It has always been my understanding that boats are somewhat faster on salt water than fresh water due to changes in the density. If the water is cold it probably means that the surrounding air is at least cool......and cool dense air makes for higher horsepower. "
Almost correct.

Faster on salt water vs fresh water might equate to the generally accepted fact that salt water is generally at sea level and fresh water is generally a higher altitude than sea level. Air density is generally accepted that it is at its highest point at sea level for a normally aspirated engine. Any thing up from there in altitude will produce a lower air density charge. Thus making less horse power. Thus a slower top speed.

I live at 5280' ;-)
Perry posted 05-30-2010 06:49 PM ET (US)
A boat's motor runs better in cold air because it is more dense causing the boat to go faster. It can't hurt to have the cold water keeping the motor cool as well once it's to its operating temp.
jimh posted 05-31-2010 10:21 AM ET (US)
[This discussion has been moved to PERFORMANCE, where almost all discussions about boat speed optimization occur.]
jimh posted 05-31-2010 10:23 AM ET (US)
Water temperature can affect boat speed due to variations in the density of water with temperature. When the water is denser, the amount of hull immersed in the water needed to displace the weight of the boat will be less. This tends to reduce the wetted surface. When the water is denser, the thrust produced by the propeller should tend to be higher. A combination of reduced wetted surface and higher thrust should tend to improve boat speed.

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Author	Topic: Effect of Water Temperature on Speed
meridian	posted 05-28-2010 02:17 PM ET (US) Can water temperature have an effect on boat speed? I know air temperature can have an effect on engine performance.
sosmerc	posted 05-28-2010 03:52 PM ET (US) All I can say is that I sure swim a lot faster in cold water !
gnr	posted 05-28-2010 04:14 PM ET (US) The density of water does change with the temp. It achieves its highest density at around 39 degrees f. This is why cold country lakes "turn over" a couple times a year when the surface temp rises/falls to 39 and sinks to the bottom. It can however reach the same density at points on either end of the spectrum. I suppose theoretically a hull might float higher in water at 39 degrees giving it a bit more performance then at 70 degrees but in our real world applications I don't think any gains/losses in performance would be noticeable.
sosmerc	posted 05-28-2010 04:34 PM ET (US) It has always been my understanding that boats are somewhat faster on salt water than fresh water due to changes in the density. If the water is cold it probably means that the surrounding air is at least cool......and cool dense air makes for higher horsepower.
dgoodhue	posted 05-28-2010 04:46 PM ET (US) Saltwater has more buoyancy than fresh water. It's easier to swim in salt water than fresh water for this reason. Using the same reason a boat with sit higher in Saltwater. I don't know if Boston Whalers are affected all that much by the difference but I know Offshore Performance boats (ie Cigarette) are often 3+mph faster in salt water vs fresh water.
cgodfrey	posted 05-28-2010 05:02 PM ET (US) The density of water changes with temperature, but a very, very, very small amount, especially considering the relatively narrow range of water temperatures your hull will encounter http://en.wikipedia.org/wiki/Density#Density_of_water I think fish flatulence probably has about as much effect on you hull's drag and speed as the water's temperature :-) CG
PeteB88	posted 05-28-2010 05:26 PM ET (US) I think your lower unit sucks up in the belly of the boat in cold water especially Lake Michigan in early spring, Lake Superior and Oregon Coast anytime.
WT	posted 05-28-2010 05:35 PM ET (US) If the water is frozen your hull will get dinged up.
elaelap	posted 05-28-2010 05:45 PM ET (US) "I think your lower unit sucks up in the belly of the boat in cold water especially Lake Michigan in early spring, Lake Superior and Oregon Coast anytime." You got it, Pete...shrinkage ;-) Tony Add to your list the California coast north of San Francisco. This year the water temperature has stayed lower than 50 degrees later in the spring than any other year I can remember. It will work its way up to maybe 55 f. by the end of summer, then back down to the high forties during the winter. I get shrinkage just thinking about going overboard fishing alone in that stuff and watching my boat trolling away at an unreachable 2.5 kts. Brrrr...
Liteamorn	posted 05-28-2010 05:59 PM ET (US) quote: All I can say is that I sure swim alot faster in cold water ! That's because we have less drag in colder water! (see Sienfeld episode 85 "The Hamptons")
ScooterCO	posted 05-28-2010 06:40 PM ET (US) "It has always been my understanding that boats are somewhat faster on salt water than fresh water due to changes in the density. If the water is cold it probably means that the surrounding air is at least cool......and cool dense air makes for higher horsepower. " Almost correct. Faster on salt water vs fresh water might equate to the generally accepted fact that salt water is generally at sea level and fresh water is generally a higher altitude than sea level. Air density is generally accepted that it is at its highest point at sea level for a normally aspirated engine. Any thing up from there in altitude will produce a lower air density charge. Thus making less horse power. Thus a slower top speed. I live at 5280' ;-)
Perry	posted 05-30-2010 06:49 PM ET (US) A boat's motor runs better in cold air because it is more dense causing the boat to go faster. It can't hurt to have the cold water keeping the motor cool as well once it's to its operating temp.
jimh	posted 05-31-2010 10:21 AM ET (US) [This discussion has been moved to PERFORMANCE, where almost all discussions about boat speed optimization occur.]
jimh	posted 05-31-2010 10:23 AM ET (US) Water temperature can affect boat speed due to variations in the density of water with temperature. When the water is denser, the amount of hull immersed in the water needed to displace the weight of the boat will be less. This tends to reduce the wetted surface. When the water is denser, the thrust produced by the propeller should tend to be higher. A combination of reduced wetted surface and higher thrust should tend to improve boat speed.