Salt vs fresh water

[JoRay]

This summer is the first time my boat has ever been in fresh water. It's been tuned up, running great, but just doesn't act the same as when it was in the Gulf. Used to be I could cruise on a plane nicely at 2800 rpm. Now it seems sluggish. (3000 rpm to stay on plane) The mechanic at my marina says it's cause fresh water is alot denser than salt water. Can this be true and cause such a difference in performance?

 

[spfortjohn]

He is correct, Ski in salt water and then ski in Fresh you will feel the difference.

 

[Bucit]

100% true. Salt water is more bouyant than fresh water.

 

[srt8mag]

Wrong. The opposite is true. Salt water is more dense.

 

[ChuckW]

Wrong. The opposite is true. Salt water is more dense.

..because Salt water is more dense it is more buoyant.

Objects (including divers) are more buoyant in salt water than in fresh water because salt water is denser than fresh water. Why is salt water denser than fresh? Because salt is denser than water and so if you add it to water the resulting solution is denser than pure water. By how much? The salinity (saltiness) of the ocean varies, but the generally accepted average amount is 2.5%. So salt water weighs 2.5% more than the same volume (a gallon or litre, for example) of fresh water. http://deepstop.wordpress.com/2009/02/26/buoyancy-salt-water-vs-fresh-water/

 

[dicor]

I never gave it any thought but it makes perfect sense. Taking it one step further I wonder if the prop gets a better bite and provides more thrust in salt water? Any physic teachers out there?

 

[floriduramax1]

100% true. Salt water is more bouyant than fresh water.

Don't know about the "dense", but 100% agree with salt water having more buoyancy than fresh. Salt water makes mine get up on plane faster and is just more fun!

 

[JimT]

I noticed this summer that my boat sat two inches higher in salt water than in fresh water.

 

[annie]

Don't forget about altitude change at your lakes also? I had a jet boat that lost 500rpm at 800 meters above sea level.

 

[comsnark]

All true. There is also differences due to elevation for the high mountain lake boaters ( lower elevation is better)

 

[Westie]

I get about 200 rpm more and 2 to 3 km more speed at WOT when I go to the coast vs the local lake. The lake is at around 3400 feet elevation so that also makes a difference. As well on the coast we were loaded up with food and gear for 3 weeks stay so alot heavier boat.
With my old boat that had a carb the difference was even greater.

 

[VinceR]

Engines without turbochargers lose manifold pressure at higher elevations. Lake Ouachita is not that high. I think around 1000’ ASL.

 

[Four Suns]

The mechanic at my marina says it's cause fresh water is alot denser than salt water.

I say your mechanic is the only thing denser than salt water.

Saltwater makes your boat float higher (less hull in the water) but I would be surprised if there is that much difference in performance... I see about .5 knot difference between salt and fresh at cruise RPM.

 

[JoRay]

Guess I'd better take the elevation into consideration and re-tune a bit. Never thought there'd be a difference like this. Several boats Ive owned of a smaller size (16 - 20 ft) Went back and forth from lake Tarpon to the gulf and never really noticed a change in performance.