Have a go at my prop sizing?

Discussion in 'Props' started by DennisRB, Apr 4, 2016.

  1. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 38, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Prop size check. What do you guys come up with? 2 blade folders for a 43 foot sailing cat of 20000lbs

    Flexo fold first told me 2B 17x13 now they are saying 2B 16x13!

    3GM30 27hp @ 3600rpm

    SD20 2.64 reduction

    The thing is I have 3B 17" props already (well I just measured one blade from my 3 blade volvo at 8.5" so I assume that is a reliable way to check diameter). Seems very odd they would go down in blades and size? I would be under propped no?

    The 3 blade 17" Volvo props can push the boat to 9K+ in calm air with both motors (3500rpm) so only very slightly over propped, which is how I would like it. With only one engine I can get 8K but the RPM is lower.

    Using an online calculator I also get something like 16.7x12.8, for a 2B so a 17x13 would be a good fit and slightly over propped.

    Thoughts?
     
  2. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 38, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Thread bump please. I am about to spend up big on props and want a second opinion. Seems that flexofold should know what they are talking about but they said that with the 17x13 2B props I may only reach a max RPM of say 3000. They say if I go down to 12 pitch it will not give enough speed. The thing I cant figure out is how this makes sense when my current volvo folding props are 3 blade not 2, and they are 17", yet I can reach 3500 with them @ 9K. The 3 blade props are either 11 or 12 pitch.

    So would a 16x13" 2B really be a good swap for a 17" 3B? I am only looking at 2 blade pops. The choice is 17X13, 17X12, 16X13, or even 16x14.

    What do you think? Maybe I just get the 17X13 and if its too propped I can get a bit taken off, but I cant see how it would be given that I have 17" three blade props now, even if a flexofold 2B has similar blade area compared to a volvo 3B as suggested by flexofold.

    According to my engine speed and gear ratio a 12 inch pitch gives me 33% slip at 9K which seems a about right?

    http://www.rbbi.com/folders/prop/propcalc.htm
     
    Last edited: Apr 9, 2016
  3. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 38, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Anyway I found my volvo folding props are 3B 17x12. So I think I will get flexofold 2B 17X12. Flexofold state only slightly less blade area, so I think the performance should be similar to my 3B volvos with narrow blades and unload the engines a little. Note I am more interested in punch into wind and speed on one motor than outright top speed in optimal conditions. So in this case I think the 17X12 will be better than 16X13.
     
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