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  1. #46

    Noosacat economy

    Quote Originally Posted by Smithy View Post
    The torque curve on the 4.2 litre Yammies is weird. The shark net contractor here repowered original F250s for new 4.2 litre ones and had to dick around with props heaps even though it was same horsepower for same horsepower and they took a lot of getting used to. 10-15% better economy on that and a spanner crabber here who did the same on a Kevlacat, just they did totally different things at different revs with different speeds they were happy etc.
    Yeah completely different props required Smithy, and dicking around for sure. Actually the hp increased a lot between ‘250hp’ 3.3L and 4.2L variants. The 4.2s had ~24 odd hp more, but a massive ~70N.m more torque - albeit at higher revs (from memory, per EPA stats). Despite this substantial increase in both specs, the increase in prop speed via a change from 2:1 to 1.75:1 meant a 2” drop in pitch was reqd (and sometimes this was not quite enough). Also, per the theory, a slightly smaller diameter was beneficial. PITA when most of the good props around for them were originally designed with lower gear ratios in mind, and ‘past experience’ was out the window!
    Cheers Brendon


    Sent from my iPad using Ausfish mobile app

  2. #47

    Re: Noosacat economy

    Question to the guy's that have tried a few different props - I have found with the props on mine that the blade geometry has been the single biggest facilitator in economy change. Going from 3 to 4 with a similar blade shape produced an improvement but nowhere near what going to a large blade area 3 blade did - even though the pitch variation required was a 2 inch drop (and it still dropped revs) - have you found similar results on a cat? Essentially the prop has much lower slip numbers at mid range RPM thus generating the required mid range cruise speed to make the hull do it's thing with less revs. It's effectiveness falls off as the RPM's climb though - WOT boat speed is down a few knots. I also found that the vibration level at "just on plane" was a bit higher - this could simply be this prop though - I haven't had it balanced as it's not really bad enough to worry about IMO.
    "I soak the worms in rum. The fish love em and the worms die happy"
    "Alcohol is not the solution to your problems...................but then again, neither is milk"

  3. #48

    Re: Noosacat economy

    Well trialed 4 blade 17's and what a pig.
    Had a P66 transducer mounted directly in front of prop with good readings, 2 test runs on the 4 blades with same weight same fuel etc in calm conditions. 1 run with tranny in front of prop, 1 run with it mounted to the side of hull.

    Tranny mounted in front of prop..
    Hit 6100 rpm (topped out) at 40knts. Boat felt weird and burning excess juice and not to mention the hull rolled from side to side.

    Tranny to the side.
    Hit 5200rpm full tilt at 35knts and fuel burn was ridiculous but the hull felt more stable and solid with tranny moved to the side.

    Who would have thought a tranny would make so much difference,900rpm in fact.
    My thoughts are now that the tranny was aerating water and letting the engine spin easier on the port which then made the starboard run at higher revs due to speed being picked up. Take away the tranny and the water is cleaner and possibly creates more drag.
    Going back 17 3 blades and try ozfoils from powertech. Same as permatrims i believe.

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