Fresh Water Resistance

Hi all,

I've notice in an electric motor web about presizing the engine a boat needs. And they give a enormous importance to the water the boat is floating on: if it´s salty the power is twice than if it is fresh.

You can see it here: http://www.kraeutler.at/motor_details_en/items/26.html

Lets see, the viscosity of the sea water is going to increase around 1.04 times, increasing the viscous resistance but the volume displaced by the hull is going to decrease (reducing the wetted surface a little bit, around the 0.991 times for that in fresh water reducing the viscous resistance).

For a sailing boat: (V 6 knts and Lwl = 7.7 m)

Reynolds in fresh water = 20848537
Rn in salt water = 19972548

ITTC 57 frictional resistance coeficient

fresh water = 0.002651
salt water = 0.002669

now taking the added wetted surface:
fresh 1 * 0.002651
salt 0.991 * 0.002669= 0.002646

The resistance in salt water is 0.998 times less!!!

Well Hullspeed from Maxsurf software gives for fresh water 1.78 kW (100 efficiency) and 1.83 kW for sea water what makes it 1.02801 times higher!

this with a Puma 29 feet (Spanish sailboats shipyard, designed by fomous Primrose) for 6 knts and a displacement of 4 tonnes.


On other hand the propeller is going to work in a fluid of a greater density, absorbing more power...

Well my question is wich of this conclusions is wrong, and why this electric motor manufacter gives to it so much importance!

Thanks,

Good winds

 

For all practical purposes the resistance in fresh and salt water is equal.

Krautler states that a particular 10 KW electric motor is suitable for 4 tons displacement in fresh water and 2 tons in seawater because the sailing conditions at sea are much more demanding (current, wind, waves, distance to shore).

Also at sea there will be much more marine growth increasing drag and reducing propulsion.

 

Hi CDK,

Thanks for your reply. That is an importatnt factor that I didn´t realize. Take it into account since now!

Cheers!

 

Look for the article "Performance differences in fresh and salt water" in the October 2005 Marine Performance Technology Exchange newsletter. You can find it at:

www.hydrocompinc.com/knowledge/library.htm

Don MacPherson
HydroComp, Inc.

 

HydroComp

Thanks! This is a really interesting web that you have!

I have enjoyed myself reading that article as it says that not a final conclusion can be taken for all cases.