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Controllable Pitch Props on FPBs

Hi Steve … I just read the Propellers section on the Offshore Cruising Encyclopedia, and got wondering: given the FPBs are such efficient machines, wouldn´t there be a case for controllable pitch props on them?

Hi Alex:

CP props work great where the loads vary a lot, as with a sailboat motorsailing, or a fishing boat pulling a trawl, and then racing home with a catch. However, the FPBs operate in a narrow load range and would not benefit enough to make the cost, length, and efficiency drop under average conditions worth while.


Posted by Steve Dashew  (April 3, 2011)

This entry was posted on Sunday, April 3rd, 2011 at 1:51 pm and is filed under Cruiser's Tech Talk. Both comments and pings are currently closed.




6 Responses to “Controllable Pitch Props on FPBs”

  1. David Guest Says:
    April 3rd, 2011 at 2:27 pm

    Steve,

    Did you mean “not benefit enough”?


  2. Steve Dashew Says:
    April 3rd, 2011 at 4:05 pm

    That’s it David – the tradeoffs do NOT work for the FPBs with CP props.


  3. Matt Marsh Says:
    April 3rd, 2011 at 8:39 pm

    I’ll trust that Steve has done the math on this one; the conclusion that fixed props have the edge in a near-constant-load, near-constant-speed scenario appears logical.
    It may also be worth noting that I’ve never come across a CPP gearbox that would be compatible with the FPB’s engine-aft layout, at least not without a large, probably expensive custom V / offset gear.


  4. Steve Dashew Says:
    April 3rd, 2011 at 10:12 pm

    Hi Matt:
    We have used Hundested CP props on three boat with aft engine rooms, one of w hich was our own Beowulf (78 foot ketch). She had a straight drive, no V, and the Hundested gear box and muff coupling added about three feet to the required length of the engine room. Along with the usual feathering/motorsailing benefits, the reversing blades allowed us to walk the stern in either direction, on or off a dock from either hand. The negative for a performance oriented sailboat was the drag of the hub – huge – and the cambered prop blades which were big contributors to drag as well. We figured that on the days we were doing 300 NM in 24 hours in the trades that the Hundested drag was costing us between 12 and 15 miles. But we liked the maneuverability and ability to motorsail at 12.5 knots in light airs.


  5. Rick de Castro Says:
    April 6th, 2011 at 11:37 am

    Steve, what are the advantages/disadvantages of a folding prop on the ‘get home system’ hydraulic motor you described as a possibility?

    Would a normally installed folding prop, and a fixed-pitch spare (to be installed as able) be reasonable?

    Oh, glad you’re getting back to the ‘test bed’….

    Thanks,


  6. Steve Dashew Says:
    April 7th, 2011 at 8:58 pm

    Hi Rick:
    The main reason for the folding wheel is reduction of drag. We envision the fixed wheel be put on for a longer passage.