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Hydrofoil lift-off occurs earlier in salt or fresh water ?

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SupaEZ
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Hydrofoil lift-off occurs earlier in salt or fresh water ?

Postby SupaEZ » Sat Sep 28, 2013 12:37 am

Also do you stay up longer and higher if the wind lulls in salt or fresh water ?

Before lift-off a certain minimum planing speed of the hull is required...what is it ?

Knots..miles per hour..meter per seconds..kilometers per hour

What is the lowest wind speed ever officially recorded to achieve lift-off and stay up riding ?

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davesails7
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Re: Hydrofoil lift-off occurs earlier in salt or fresh water

Postby davesails7 » Sat Sep 28, 2013 5:08 am

SupaEZ wrote:Also do you stay up longer and higher if the wind lulls in salt or fresh water ?


Looking at the equation for lift coefficient, lift is proportional to the fluid density. Also lift is proportional to the velocity squared.

The density of salt water is 1.025 x the density of fresh water, so you will get 2.5% more lift in salt water.

If you can lift off at 7.00 knots (board speed over water) in salt water, you would need to be going 7.09 knots to lift off with the foil at the same angle of attack. I don't think you'd notice much :thumb: drag on the foil will also be 2.5% more in salt water, so probably negligible difference in riding ability in salt vs fresh water.

No idea if 7 knots is accurate for board speed at lift off.

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edt
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Re: Hydrofoil lift-off occurs earlier in salt or fresh water

Postby edt » Sat Sep 28, 2013 5:16 am

I think supa is gunning for silliest question. But he failed here. This question is actually kind of interesting.

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davesails7
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Re: Hydrofoil lift-off occurs earlier in salt or fresh water

Postby davesails7 » Sat Sep 28, 2013 11:22 am

edt wrote:I think supa is gunning for silliest question. But he failed here. This question is actually kind of interesting.


Yeah, I wasn't sure if this one was serious or not since it was posted along side "my bro foil" :roll: but I thought it was kind of an interesting question, haha

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Peter_Frank
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Re: Hydrofoil lift-off occurs earlier in salt or fresh water

Postby Peter_Frank » Sat Sep 28, 2013 2:31 pm

davesails7 wrote:
edt wrote:I think supa is gunning for silliest question. But he failed here. This question is actually kind of interesting.


Yeah, I wasn't sure if this one was serious or not since it was posted along side "my bro foil" :roll: but I thought it was kind of an interesting question, haha


True - what a stupid way to ruin a forum for others, especially as most on this forum dont have english as their native language :o
But seems most has learned to ignore these posts..

-----------------

To the topic - of course the salt water will make the foil lift earlier.

Just like it will lift ANY board earlier.

Salinity varies a lot though.
Around here in our open sea to the east, it is only 1% giving an increased density compared to freshwater of 0.8%

At the westcoast open sea, and most open coastlines, salinity is 3.3% giving a density increase around 2.6% just like Dave writes :thumb:

In the mediterranean salinity can go above 4% easily though.

And as lift is squared with windspeed, you need 1,6% less speed to get the same lift, if in 3.3% saltwater compared to freshwater, because of the 2.6% density difference (just like Dave calculates).

This does not go for foils only though - but ALL sailing crafts, twintips to foils and everything in between.

8) Peter

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edt
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Re: Hydrofoil lift-off occurs earlier in salt or fresh water

Postby edt » Sat Sep 28, 2013 3:09 pm

salt makes crashes harder not so bad in ocean but great salt lake you can take a beating

SupaEZ
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Re: Hydrofoil lift-off occurs earlier in salt or fresh water

Postby SupaEZ » Sat Sep 28, 2013 3:10 pm

davesails7 wrote:
edt wrote:I think supa is gunning for silliest question. But he failed here. This question is actually kind of interesting.


Yeah, I wasn't sure if this one was serious or not since it was posted along side "my bro foil" :roll: but I thought it was kind of an interesting question, haha


:thumb: Glad North America gets Supa.....at times
Like D-me-3 says at times ....i can't please everybody all the time :lol:
It was a 2 part question....silly yes....with elements of seriousness
:cheers: to all the comedians of KF that also have a serious side when needed


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