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Living the dream
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Discussion Starter · #1 ·
Our new to us boat has a three blade fixed prop with narrow blades as per the attached picture. I was concerned about the performance compared to the wider "Mickey Mouse Ear" blades, but have found the boat motored well on our delivery trip home (about 50 hours running time). This included some motor sailing in very rough seas and motoring to windward in both protected and more exposed water.

Only one time so far, when going directly into the wind of about 20 knots sustained velocity in the face of a horrible chop (wind opposing tide with nearby reef) did the performance drop alarmingly. In fact at one point I'd thought we'd lost our transmission as the boat slowed from 6 knots and struggled to maintain 2 as a succession of larger waves pushed the bow through large vertical arcs. During that particular 2 mile or so passage (to dodge an islet and it's extended reef) we struggled to maintain 3 or 4 knots. This boat has power to weight of about 4+ hp per ton so there's plenty of engine power available.

All in all, the performance of the prop has exceeded my initial expectations (which were really just guessing) that a thin bladed prop will lose performance very quickly in adverse conditions compared to a more conventional unit. It seems it will, but only once conditions get a little hairy. This could be a good thing or a bad thing I suppose although the pro is that there is/should be less drag from it when sailing.

Now the thing as, I don't recall seeing too many thin bladed props in use. Would there be good reason for this?
 

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Living the dream
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Discussion Starter · #3 ·
Ok, thanks for that. I'm thinking a wider blade, reduced diameter prop is more efficient. I've also got my suspicions having thought about it that prop walk increases with the thinner blades and bigger diameter as well.

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Living the dream
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Discussion Starter · #8 ·
Actually quite the reverse, a larger diameter propeller will usually be more efficient, as the swept area is one of the major factors in propeller design. the power is transferred over a larger swept area reducing the load per blade area. a blade thats having to work hard will often come close to cavitating, which is one of the largest losses a propeller can have!

Another is the rotational momentum imparted to the water from the spinning 'disc', which is a function of the blade lift/drag coefficients and RPM. an ideal blade is one with a high Clift and a low Cdrag, but this is nigh on impossible! lift cannot be created without drag, so a highly loaded propeller will have a high Clift, but also a high Cdrag, which decreases rotational efficiency. you also have to turn a smaller propeller faster, which results in more rotational losses.

another way of thinking of it is this: A large merchant tanker will often have a very large propeller turning at less than 100 RPM. They will often design the stern frames of the vessel to accommodate the largest propeller possible! vessels like this are all about efficiency!
Source: Currently a masters student in marine engineering :)

ps. I know this is my first post, but I have been reading sailnet for some time and thought I could help!
Makes sense. Would cavitation limit maximum tip speed as is there a point where increasing the prop diameter for a given working RPM starts to reduce the efficiency? I recall reading that during the initial development of the screw propeller way back when that the boat speed increased when the tips of the blade broke away during trials, and I've always wondered why this was so.
 
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