godevil on canoe

propulsion unit

 

Gareth
Not as much as I thought initially after doing the numbers. Depending on loading condition the efficiency for the prop shown could go from 58% to say 62%.

The shrouding in this case provides a measure of safety as well as the thing would be quite a weapon.

Rick W.

 

Rick,

Thanks for that, do you have any references for this calculation, or do you use a piece of software?

Thanks

Gareth

 

Gareth
I have my own prop analysis software only because I can do all sorts of manipulation with different foils. It does not give results much different to JavaProp:
http://colaco.freeshell.org/mhepperle/javaprop/jp_applet.htm

As downloaded, the options are set for air but you can change them to suit water.
Density for saltwater is 1025
Viscosity for water is OK at 0.0000013

You have to select the type of foil. Most boat prop blades are thin so I usually use the MH 9.8% 500,000 for bigger/faster boats and the E193 100,000 foil for smaller boats. You can adjust the AoA to get a suitable chord.

I have attached the screen dump from one of the calculations I did for the Whipella. Note that there is a checkbox that can be ticked to have with and without shrouding.

Effective shrouding will eliminate induced drag so it is particularly effective in heavily loaded applications.

As far as the boat goes conserning speed and power, I just guesstimated the speed and drag by looking at the video. I was only interested in the relative performance of the shrouding.

I have seen loading conditions where shrouding has had a dramatic increase in efficiency. Better than 10% gain in efficiency. This gain is offset a little by the extra drag of the shroud.

JavaProp provides a primarily anylitical result based on the underlying physics rather than relying on empirical data so it gives good understanding of what is going on if you care to work through the various analysis tabs.

Rick W.

 

And he hasn't even cleaned the weed growth off