propeller selection

[hillmaster]

hi

I am currently an undergraduate university student and I am during my final year during which I have to do my final year project (thesis). my thesis is the design and construction of a human powered boat.

i am currently trying to determine the propeller that i should use. for my project i will not design the propeller but i will look for what is available and optimize the propeller that best fits my human powered boat.

i have some queries regarding the selection...

regarding diameter: it is stated that a larger diameter will be more efficient therefore i will go for a propeller with a large diameter as possible. space is not a problem so i may even opt for 15" up to 17" propeller.

regarding pitch: the propeller will be rotating relatively low (about 320rpm) therefore the pitch has to be quite large if i want to achieve the required cruising speed of about 2m/s. therefore pitch will have to be larger than 15".

regarding number of blades: my main problem is regarding number of blades on the propeller. i have read elsewhere on this forum that two blades will perform better with human power. but why? is it because the same thrust can be achieved by a larger diameter prop and hence more efficient than a three bladed prop which would be small in diameter but three blades? am i correct to say that the thrust developed depends on the blade area?

regarding torque and thrust: of course I would need to calculate the thrust, torque and efficiency of the propeller. I studied the blade element theory to be able to calculate these values however a lot of data is required to be able to use this approach (blade element theory). any alternatives or suggestions??

i really appreciate any comments and suggestions!!

THANKS A LOT FOR THE HELP

CLIVE

[hillmaster]

any suggestions from where can i buy the propeller? most commercially available propellers that are used on boats are 3 bladed or even more and usually with a relatively small diameter. two bladed propellers are relatively difficult to find. one of the options is props used for sail boats but these are almost all feathering props and therefore two bladed props are almost impossible to find!

another option that I am considering is props used for small (radio controlled) airplanes. the problem with these are that the prop blades may flex under the thrust generated in the water since it would be designed to operate in air. although this may not be a problem since the prop will be running with greatly reduced revs than it would in air. again any suggestions from where to buy the air props.

currently the best option that I found is a propeller made from carbon fiber reinforced polymer from Bolly propellers and which was specifically designed for human power boats. the size is 15" diameter by 25" pitch.

NB ideally the prop may include also the propeller performance curves although i am aware that these would be two expensive due to the testing involved to obtain the data and hence almost impossible to find unless it it a very specific propeller built where the cost would not be the main concern.

ONCE AGAIN THANKS A LOT FOR THE HELP

CLIVE

[FAST FRED]

"i have read elsewhere on this forum that two blades will perform better with human power. "

Actually a 1 bladed prop would be best, but unless the power is very low , and a large diameter is used the blade area gets too low and it cavitates.

Each prop blade has its own induced DRAG , a power loss, so the fewer the better , IF you can.

Notice the lack of bi of tri planes in the air? Induced drag.


Skenes Elements of yacht design , or Dave Gerr's Propeller handbook , will get you within a few percent of Optimum.

In reality , after the boat is built "Third time Lucky" is best chance at great prop matchup.
FF

[marshmat]

Hi Clive,

You've come to the right place.... most of the questions you raise have been discussed on here, in extreme gory technical detail, by many of the people who are actually doing the experiments to find out the answers.

Take a browse through the archives- use the 'advanced search' feature to look for HPV, human powered boat, high efficiency propeller, etc. and you'll find a wealth of interesting discussion on this topic.

[Rick Willoughby]

Clive
If you track back through the archives on this blog you might get some useful information:
http://www.adventuresofgreg.com/HPB/2008_09_15_archive.html

As Matt said you can find some useful ideas here amongst a lot of irrelevent stuff:
http://www.boatdesign.net/forums/boat-design/pedal-powered-boats-23345.html

If you want to do your own prop design then JavaProp is a good place to start. You can easily play with parameters to get the best result. You need to set options page for water. There are other threads explaining how to use JavaProp.

Rick W

[Rick Willoughby]

The Bolly prop is not too bad. It will be OK if you have a very low drag light hull. The blade area is a little small relative to pitch and diameter for most applications. Rounding the leading edge might improve its range of operation.

JavaProp will produce the prop performance curve and you can adjust the parametrs to produce a shape very similar to the Bolly prop so you will be able to predict performance quite accurately.

Rick W.