tag:blogger.com,1999:blog-266108914902271629.post2200149199970150962..comments2023-09-25T08:48:48.104-07:00Comments on John Vigor's Blog: All about propsJohn Vigorhttp://www.blogger.com/profile/02215080385571534292noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-266108914902271629.post-49666827323873081272011-11-18T16:15:27.413-08:002011-11-18T16:15:27.413-08:00Gary, Dave Gerr, well known New York naval archite...Gary, Dave Gerr, well known New York naval architect, in "The Nature of Boats" says: There should be a clear distance--a tip clearance--between the hull and the propeller blade tips of at least 10 percent. Ten percent is a bare minimum; 14 to 18 percent is the standard ideal distance., though the bigger the clearance the better ..."<br /><br />"Skene's Elements of Yacht Design" says: "The tips of the blades should clear the hull as follows:<br />"15 percent prop. diam. of clearance for auxiliaries.<br />"20 percent prop. diam. of clearance on motor sailers and powerboats."<br /><br />And many more credible authorities say the same thing.<br /><br />John V.John Vigorhttps://www.blogger.com/profile/02215080385571534292noreply@blogger.comtag:blogger.com,1999:blog-266108914902271629.post-64672418820642772552011-11-18T12:25:41.050-08:002011-11-18T12:25:41.050-08:00John, where do you get the 10% of prop diameter fo...John, where do you get the 10% of prop diameter for tip clearance from?<br />I'm curious because we just recently reduced the size of the aperture on my boat and just made it is small as possible around the prop.Garynoreply@blogger.comtag:blogger.com,1999:blog-266108914902271629.post-71955004030297749572011-11-18T10:44:39.985-08:002011-11-18T10:44:39.985-08:00Dear Anon: I have seen the experiment you refer t...Dear Anon: I have seen the experiment you refer to. I don't subscribe to the theory that it is correct for all boats and all props at all times. Each hull shape is different. Each prop is different. Different forces act at different speeds. The fact that serious experiments come up with conflicting answers, and the fact that trained experts differ in their views, is enough to convince me that nobody can give a definitive answer for all boats.<br /><br />I've had boats where the prop wouldn't freewheel at all, no matter what speed we achieved. But in any case I always lock the shaft because the noise of the spinning prop and gears drives me nuts and because I can imagine bearings wearing out prematurely for no reason at all.<br /><br />John V.John Vigorhttps://www.blogger.com/profile/02215080385571534292noreply@blogger.comtag:blogger.com,1999:blog-266108914902271629.post-76664797955369413082011-11-18T10:44:39.506-08:002011-11-18T10:44:39.506-08:00Dear Anon: I have seen the experiment you refer t...Dear Anon: I have seen the experiment you refer to. I don't subscribe to the theory that it is correct for all boats and all props at all times. Each hull shape is different. Each prop is different. Different forces act at different speeds. The fact that serious experiments come up with conflicting answers, and the fact that trained experts differ in their views, is enough to convince me that nobody can give a definitive answer for all boats.<br /><br />I've had boats where the prop wouldn't freewheel at all, no matter what speed we achieved. But in any case I always lock the shaft because the noise of the spinning prop and gears drives me nuts and because I can imagine bearings wearing out prematurely for no reason at all.<br /><br />John V.John Vigorhttps://www.blogger.com/profile/02215080385571534292noreply@blogger.comtag:blogger.com,1999:blog-266108914902271629.post-32954570486837542572011-11-18T05:58:38.890-08:002011-11-18T05:58:38.890-08:00I'm sure you've seen Maine Sail's expe...I'm sure you've seen Maine Sail's experiment on fixed/rotating props, and confirming results from other studies he refers to in his article. Do you disagree with those? And if so, why?<br /><br />No ill-feeling or backbiting, I promise. Just trying to make up my own mind. Thanks.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-266108914902271629.post-28758771194262722692011-11-18T05:18:15.384-08:002011-11-18T05:18:15.384-08:00Old Wotsisname... isn't he the one with a seiz...Old Wotsisname... isn't he the one with a seized injector, a gearbox full of sawdust, and four inches of barnacles on a lumpy ferro?<br /><br />Yes, I'm sure fine-tuning his prop according to water density will make a noticeable difference ;)<br /><br />Re. locking the shaft: If the gearbox manufacturer says not to let it freewheel, the choice is made for you (burned out clutches are not fun). Otherwise, I agree with you, testing it on your particular boat is the only way to be sure. Not all boats swing the same way.<br /><br />Re. helicopters: Unfortunately, autorotation doesn't give any insight into the boat prop problem. Autorotation isn't about spinning vs. fixed; it's about adjusting the blade pitch as the helicopter falls so that they're stalled near the hub, driven by the airflow in the middle, and lifting near the tips. Get that collective pitch even a tiny bit wrong and she'll fall like a stone, spinning or not.Matt Marshhttp://www.marsh-design.comnoreply@blogger.com