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post #11 of 14 Old 09-03-2014
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Re: Pondering a question on propeller performance

Its not the number of blades that has this effect, its the blade/swept-area ratio. A propeller could have a huge number of narrow blades and have the same blade area as a 2 bladed prop with wide blades. the reason fewer but wider blades are preferred is to reduce the surface area of the propeller (viscous drag of spinning it in water) and to evenly distribute the pressures over as large an area as possible (reducing cavitation and reducing root stress)

I suspect that for your application (sailing yacht) wider blades would detract from sailing performance, however if you were looking only at propeller efficiency... there are several methods of optimising all the parameters of a propeller (and there a quite a few of these, see link below!)

If you ever wanted to know more about the powering calculations that designers and engineers use, I would suggest you read some of this (it is a huge resource written by several of our professors):

Anthony F Molland_ Stephen R Turnock_ Dominic a Hudson-Ship Resistance and Propulsion _ Practical Estimation of Ship Propulsive Power-Cambridge University Press (2011)

Chapter 12 onwards relates to propeller design
Its not aimed at the general public, and it gets very technical so if you have questions, PM me

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post #12 of 14 Old 09-03-2014
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Re: Pondering a question on propeller performance

Originally Posted by ReefMagnet View Post
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?
I'm wrapping up my first extended cruise with a Campbell Sailer prop, a popular thin-blade design similar to yours, with a 'cupped' blade to maximize thrust. I've had a self-pitching Autoprop for years, but I was a bit nervous about a cruise to a more remote destination where any real services are unavailable, and the water is far too cold for me to be able to dive on... So, I swapped it out for the simpler Campbell for the time being...

I think it's a fine fixed blade prop, and the performance is decent in smooth water... But, my experience has been very much in line with what you've described - almost a shocking loss of drive when attempting to power into any significant head sea, or strong breeze... I'll certainly keep it as a spare, but when I get back home, I'm pretty sure the Autoprop will be going back on the shaft... :-)

One thing I will say for the fixed blade Campbell, my transmission literally 'slips' in and out of gear, I can barely feel it... With props such as the Autoprop, you really are throwing a massive amount of weight and torque around whenever engaging forward or reverse, and there's a very heavy clunk as a result... The fixed blade has got to be much kinder to my gearbox over time, I know I'm gonna be grimacing every time engage gears with the Autoprop, again...
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post #13 of 14 Old 09-06-2014
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Re: Pondering a question on propeller performance

Keep things simple. I am in to fixed props. My USC Polaris 36 has a big twin blade fixed prop that I can align with the keel to reduce drag.
It has served me for 22 years and probably has served the boat for all of her 37 years.
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post #14 of 14 Old 09-06-2014
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Re: Pondering a question on propeller performance

Something I don't understand. With the 75 hp Yanmar turbo the 3 blade and 5 blade max props work fine but the 4 blade doesn't. Why?

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