Hi all!
Since the efficiency is a key element in a solar/electric boat, I was intrigued by the propellers used on Turanor. They are using surface piercing propellers (also called "surface drive"). Some pictures here:
http://www.planetsolar.org/fileadmin/user_upload/images/gallery/events/event-unveiling-feb2010/dsc_1145.jpg
http://www.planetsolar.org/fileadmin/user_upload/images/gallery/events/event-launch-mars2010/dsc00147.jpg
We generally see such propeller on fast patrol boat (40 kts +), but on a slow displacement boat?
I found this nice article on the subject:
http://www.well.com/~pk/SPAprofboat.html
Extract:
The important operating feature is that each propeller blade is out of the water for half of each revolution. And here is another reason for skepticism. Surely a propeller blade is more efficient if it operates continuously in the smoothest possible flow, rather than splashing through the water surface twice with each revolution. But nature can play tricks on our intuition. Sometimes an unsteady process is actually more efficient than its continuous counterpart.
Wikipedia is also nicely summarizing the concept:
http://en.wikipedia.org/wiki/Surface_piercing_propeller
Extract:
The surface piercing or ventilated propeller is a propeller that is designed to intentionally cleave the water and entrain atmospheric air to fill the void, which means that the resulting gas layer surrounding the propeller blade consists of air instead of water vapour. Less energy is thus used, and the surface piercing propeller generally enjoys lower drag than the supercavitating propeller.
I also read a few discussions in the boat design forum:
http://www.boatdesign.net/forums/surface-drives/surface-propeller-low-speed-displacement-boat-25284.html
http://www.boatdesign.net/forums/surface-drives/low-speed-surfacing-propeller-9633.html
So after all, Planet Solar's choice of using such propellers on Turanor seemed to make sense... until I read something puzzling on the blog of the Sun 21 builder (http://www.transatlantic21.org), the first solar boat that crossed the Atlantic. FYI, the original builder (http://www.mwline.ch) went under and is now doing business under another name (http://www.grove-boats.com/en). The Planet Solar project initially started as a joined venture with MW-Line but apparently, both parties had different views on the ideal solar boat. To make a long story short, the Planet Solar team finally decided to go their way and ignored MW-Line recommendations. Here are links on the MW-Line ex-director's blogs (Sorry...french only...):
http://solarly.canalblog.com/archives/2010/04/22/17654342.html
http://cleanboat.blogspot.com/
Basically, they are saying that Turanor has many beginner's mistakes. Their main concerns are about the weight of the boat (60 tons), the position of the propellers (aft of the tubes), and that the top of the propeller will not receive a laminar flow of water, hence a low efficiency.
So, is using a surface piercing propeller on slow displacement boat good or bad? Who is telling the truth?
Sorry for the long post…
Pierre
No comments:
Post a Comment