Sail Water vs. Fresh Water Sailing

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Ray Bowles

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Oct 6, 2008
857
Hunter, Island Packet, Catalina, San Juan 26,38,22,23 Kettle Falls, Washington
I read in one of my sailing mags that racers that have raced in salt water verses fresh water can notice a difference. I've sailed both but I'm truly not good enough to feel that difference.
Is there one and what is it?
Ray
 
RichH

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
At normal temperatures (20°C/70°F) salt water has about 5% more frictional characteristics (viscosity) than fresh water. The viscosity is the controlling 'energy' / friction factor when comparing between salt and fresh water and a boat moving through (skin friction).
The density of salt water is also about 2% greater than fresh and causes a boat in salt water to be less immersed than in fresh water (projected impact friction or 'stagnation')...

Rx: - (VERY approximate) Net difference is ~3% better 'resultant' performance in fresh water than salt water (without adding a lot of other complicating factors, etc.)

http://ittc.sname.org/CD 2011/pdf Procedures 2011/7.5-02-01-03.pdf
 
Joe

Joe

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Jun 1, 2004
8,260
Catalina 27 Mission Bay, San Diego
Ray Bowles said:
I read in one of my sailing mags that racers that have raced in salt water verses fresh water can notice a difference. I've sailed both but I'm truly not good enough to feel that difference.
Is there one and what is it?
Ray
Click to expand
Based on RichH's comment, which I'm sure was a validated resource....

Rx: - (VERY approximate) Net difference is ~3% better 'resultant' performance in fresh water than salt water (without adding a lot of other complicating factors, etc.)
Click to expand
...... a boat making 6kts on salt water would achieve 6.18kts on fresh water, all other conditions the same and the speed is within the hull's design limitations. Wow, that's significant.

I wonder why we don't here more about that phenomenon.
 
RichH

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
We do ... for those who use towing tanks to prove out or test various hull designs.

The reason we dont, as per racers is probably since all of one fleet is racing in the same medium, the 'ratings' differences for 'viscosity and density' correction would apply to all (and 'almost' equally), .... just like differing base wind strengths don't apply correction to the base rating when racing PHRF; but does when racing under Portsmouth Yardstick rules.
Plus, 'important' racing venues dont 'handicap' as in PHRF.

The best answer to all this is to race: 'one design' - where the boat that crosses the finish line first is the winner. ;-)
 
J

John R

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Oct 9, 2012
110
Catalina 36 Emeryville
But since salt water is denser, wouldn't a boat that is loaded the same float higher in the salt water, meaning there would be less surface area in the water meaning less friction/less drag? Wouldn't that cancel the other factor out?
 
RichH

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
John R said:
But since salt water is denser, wouldn't a boat that is loaded the same float higher in the salt water, meaning there would be less surface area in the water meaning less friction/less drag? Wouldn't that cancel the other factor out?
Click to expand
Salt water density is about 2% higher, making a boat 'ride higher' or less immersed; and, that affects 'projected' or (frontal) area impact/stagnation 'drag' ... and was the 2% deducted from the 5% skin friction = 3% advantage for fresh water (posting #4)... all VERY approximate, of course.
 
Jackdaw

Jackdaw

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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
RichH said:
Salt water density is about 2% higher, making a boat 'ride higher' or less immersed; and, that affects 'projected' or (frontal) area impact/stagnation 'drag' ... and was the 2% deducted from the 5% skin friction = 3% advantage for fresh water (posting #4)... all VERY approximate, of course.
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As Rich says, VERY approximate! ;^)

And beyond the mentioned factors that might come close to nulling this out on any boat/conditions/etc, some things to think about that might help you from losing sleep over this.... ;^)

Remember this is talking about DRAG, not SPEED. Your max potential speed stays the same. As the wind builds and reaches the point where the sails are fully power up, you should hit max hull speed. It might take slightly more breeze.

As it is a percentage that adds drag, at very low speeds (0-3 knots), the effect (as a % again) on actual boat speed is very small.

If you are racing, it effects all boats the same!
 
J

JohnShannon

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Jan 4, 2010
1,037
Farr 30 San Francisco
I thought near hull speed drag was mostly due to wave formation, skin friction is only important in light air at slow speed.
Still interesting notion in tidal areas do you look for the relatively saltier water or the relatively fresher water?
I bet currents winds and such are far more important factors.
 
RichH

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
hull speed is because of bow and wave formation, not the other way around. Hull speed is actually how fast the WAVES are moving and propagating. Since salt water has higher viscosity, the waves created in salt water would be moving/propagating slower and with less amplitude due to the higher internal friction of the salt water; hence, a ≈5% slower 'hull speed'.

Imagine sailing in 'motor oil' @ about 500 centipoise, instead of water @ 1 centipoise !! ;-)
 
Jackdaw

Jackdaw

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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
RichH said:
hull speed is because of bow and wave formation, not the other way around. Hull speed is actually how fast the WAVES are moving and propagating. Since salt water has higher viscosity, the waves created in salt water would be moving/propagating slower and with less amplitude due to the higher internal friction of the salt water; hence, a ≈5% slower 'hull speed'.

Imagine sailing in 'motor oil' @ about 500 centipoise, instead of water @ 1 centipoise !! ;-)
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Rich,

I know what you are saying regarding hull speed and waves. But here is where I was coming from:

Like you said, hull speed is a function of the wave created; a bow wave and a stern wave form at the hull speed, creating a 'hole' that the boat cannot lift out of. But this wave, regardless of the slight increase in viscosity, SHOULD be the same size and span, even if it took more work on the boats behalf to create. And that should mean hull speed stay the same, just more power. And the boat has extra power, it just never did any good.
 
RichH

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Disagree. What would the amplitude be for waves generated in 'vaseline' or uncured epoxy resin? :)
 
Rick Webb

Rick Webb

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Jun 2, 2004
3,607
Hunter 23.5 Fort Walton Yacht Club, Florida
Never Noticed a Difference Sailing

I have noticed a huge difference waterskiing. You get up much quicker in salt water but can't make the same cuts in freshwater. I thought I had gotten pretty good at barefooting till I tried it in fresh water like learning all over again.
 
Kermit

Kermit

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Jul 31, 2010
5,714
AquaCat 12.5 17342 Wateree Lake, SC
Rick Webb said:
I have noticed a huge difference waterskiing. You get up much quicker in salt water but can't make the same cuts in freshwater. I thought I had gotten pretty good at barefooting till I tried it in fresh water like learning all over again.
Click to expand
Tastes different, too.
 
J

JohnShannon

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Jan 4, 2010
1,037
Farr 30 San Francisco
fluid dynamics numbers Reynolds or Froudes or something like that, have as part of it, fluid viscosity and for water waves gravity.

so I guess on Jupiter your boat would be faster, good thing too cause Jupiter is much bigger.
 
mark_swart

mark_swart

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Sep 25, 2008
385
Harpoon 5.2 Honolulu, HI
If nothing else, I think you don't have to do as much brain work in fresh water because you generally don't have to compensate for tides/currents!
 
kloudie1

kloudie1

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Nov 6, 2006
10,152
Hunter 34 Mandeville Louisiana
John.S., Herr. Ludwig Prandtl would be sad if we left his name out of this discussion too !
 
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