props

[txsurbrook]

Am thinking of changing from my 2 blade prop to a 3 blade. A 12" 5P 3 blade is the what I was told to use.  What has been the thoughs on this change.

Thanks David

[Craig Weis]

Why would you change if the two blade prop was 'sucking' up all the available hp that your engine has?
Wouldn't a three blade just have less pitch in order to keep the rpm of the engine in the same range as the two blade?
I can never figure this stuff out.

Anyone know why an automatic transmission that can shift through the gears are not used on any boats?
I mean as the hull through the water increases in speed why not shift gears to reduce engine rpm's...?

I remember one of my dad's houseboats having twin Chrysler 383 V-8's with Velvet drives and 4 blade props that would stand both Carter four barrels carburetors butterflies wide open and engines would turn maybe 3200 rpm, but we were on a plane and could go skiing quite well.

skip.

[Steve Ullrich]

Hello David,

As there is a two blade prop on the boat I just made an offer on I thought I'd do a little reading about it.  I may wish to replace the prop while she is still on the hard...  As you might expect, there are advantages and disadvantages both ways.  Either way, I always think it best to make informed decisions if possible.

http://www.sailnet.com/forums/general-discussion-sailing-related/4205-three-blade-prop.html
http://www.cruisersforum.com/forums/f114/two-blade-vs-three-blade-prop-24747.html
http://www.rhkyc.org.hk/files/Marine%20Article/Props.pdf
http://www.boatfix.com/how/props.html
http://archives.sailboatowners.com/pviewarch.htm?fno=0&sku=2002273163242.53&browse=T&g=%5Bg%5D&ptl=Prop%3A%202%20Blade%20or%203%20Blade

There is a lot more information out there. Just Google two blade vs three blade prop if you have time on your hands.  By the way: Welcome to the forum!

Am thinking of changing from my 2 blade prop to a 3 blade. A 12" 5P 3 blade is the what I was told to use.  What has been the thoughs on this change.

Thanks David

[cdreamIII]

Am thinking of changing from my 2 blade prop to a 3 blade. A 12" 5P 3 blade is the what I was told to use.  What has been the thoughs on this change.

Thanks David

David,
  I recently attended the Central NY boat show and was talking with Schyler Barnes from 'The Sailboat Shop"  (www.thesailboatshop.com) who mentioned that they sold a new 27 last year and ended up replacing the 2 blade prop with a 3 blade prop in order to get rid of a vibration problem.  You may want to contact them & "pick their brains" so to speak.

Bill E

[Nicolina]

In general, two blade props are more efficient but three blades run more smoothly and may require less diameter. I have a two blade on my CP-27 and the small Universal 10 hp engine. Vibration is not much of an issue, but efficiency is, so I plan on keeping it.

There is some good reading on propeller selection (which gets very technical very quickly). A good, accessible discussion is in the "Dummies Guide to Boating" (which I found in my local library) but many yacht design books will cover it too, if perhaps with more maths. I'd recommend reading up on it a little before deciding to switch the prop.