Pondering a question on propeller performance - SailNet Community
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post #1 of 14 Old 04-14-2014 Thread Starter
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Pondering a question on propeller performance

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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post #2 of 14 Old 04-15-2014
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Re: Pondering a question on propeller performance

Reason for the wide blades on many props is the increase the surface area due to limitations on the diameter of the prop. The narrower blades on yours would probably result in less drag when sailing. As to the situation where you had a problem, powering in those conditions would be tough for any boat.
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post #3 of 14 Old 04-16-2014 Thread Starter
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Re: Pondering a question on propeller performance

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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post #4 of 14 Old 09-01-2014
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Re: Pondering a question on propeller performance

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Originally Posted by ReefMagnet View Post
I'm thinking a wider blade, reduced diameter prop is more efficient.
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!
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post #5 of 14 Old 09-01-2014
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Re: Pondering a question on propeller performance

Reefmagnet, go look at Cambell Sailor props. It appears that may be what you have. Even if it isn't, there is quite a bit of info on the site that you may find useful.
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post #6 of 14 Old 09-01-2014
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Re: Pondering a question on propeller performance

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Originally Posted by ReefMagnet View Post
Now the thing as, I don't recall seeing too many thin bladed props in use.
They are very common, at least in these parts. I see them every day.
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post #7 of 14 Old 09-01-2014
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Re: Pondering a question on propeller performance

RE: prop efficiency, I once had it explained to me in a way that really made sense. I'll botch the details for sure, but here is the version I remember.

The most efficient (thrust per hp used) prop is something like what you would find on a Cessna. They could have a 4 blade prop, but they don't need to because they have enough room for a large diameter, thin, two blade (efficient) prop.

Once you start getting into bigger planes, the wing size, clearance issues and the need to service more HP requires adding more blades to the props. This is a compromise, and if the planes had the room for it, they'd all have huge 2 blade props.

On sailboats clearance is almost always the issue and drag when the prop is not being used (not an issue on an airplane) is also an issue. So, we tend to have 3 blade props, or if you have a heavy powerboat with huge engines, maybe you can't transfer all that thrust with only 3 blades, so you're forced to add a 4th blade.

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post #8 of 14 Old 09-01-2014 Thread Starter
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Re: Pondering a question on propeller performance

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Originally Posted by SheldonP View Post
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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Re: Pondering a question on propeller performance

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Originally Posted by ReefMagnet View Post
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?
Essentially yes, although cavitation is not caused directly by the speed of the blade, but the local pressure in the fluid surrounding the blade. When this pressure drops below that of the vapour pressure of water, the water effectively boils to form the bubbles. The tip creates a vortex from its trailing edge, and at the centre of this is an extreme low pressure, but by increasing the diameter of a similar propeller, you decrease RPM demand and increase the torque required to transmit the same power.

A well designed propeller will probably have a 'lightly loaded' tip, both to reduce blade stress and reduce the possibility of tip vortex cavitation. I should also say that the vortex is a necessary part of the blade's lift generating capacity, and is always present at any RPM, but the pressure may not be low enough to cavitate.

Quote:
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.
I think the story you are thinking of is the development of the first propellers, which were originally a screwthread type of two full turns. full story at the link below.

https://en.wikipedia.org/wiki/Propel...rew_propellers

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post #10 of 14 Old 09-03-2014
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Re: Pondering a question on propeller performance

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Originally Posted by MedSailor View Post
.....The most efficient (thrust per hp used) prop is something like what you would find on a Cessna. They could have a 4 blade prop, but they don't need to because they have enough room for a large diameter, thin, two blade (efficient) prop.

Once you start getting into bigger planes, the wing size, clearance issues and the need to service more HP requires adding more blades to the props. This is a compromise, and if the planes had the room for it, they'd all have huge 2 blade props......
This generally correct, but there is much more to it. Drag is also a factor vs. the hp of the engine trying to turn more blades. The biggest reason, however, that more blades are used is noise. The prop tip travels through the air at a faster speed than the root. Faster is louder. In fact, there are some early aircraft where the blade tips reach the sound barrier. They make a very distinctive and extremely loud sound. Shortening the blades quiets down the ride, but you need to add blades to get the power back. Then you need to have enough hp to overcome the drag of more blades.

Boats and aircraft have this exactly in common...... everything is a compromise.
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