The prop blades are shrouded to protect against any nicks and to avoid giving away any information about the acoustic characteristics of the boat's drive train.
"Ceterum censeo 0bama esse delendam."
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)
You could be right. I wrote some comments to HosePipe that touched on this. You may want to check it out.
http://www.freerepublic.com/focus/news/3275989/posts?page=51#51