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Thread: Noosacat economy

  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.

  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.

  4. #49

    Re: Noosacat economy

    I've recently put Suzuki 140s on my 2300 walkabout and they are awesome. I changed from 19 to 21 props and get 30knots at 5000 for around 25l per engine. I was worried about the bullcrp about them being glorified 115s but I wouldn't swap them for anything. Especially when they are nearly 40kg lighter each than the yammy 150s.

  5. #50

    Noosacat economy

    Quote Originally Posted by gofishin View Post
    ...Some recent performance figures in glassy conditions, which from other trip stats, counteracted the reduced crew weight anyway as she was sticking to the glassy calm water a bit .

    These figures were with just me on board, only 15 hrs up on the donks, ~250L fuel & ~120L water, ~ 125kg gear. On other days with 4 or 5 adults and a lot more weight/liquids, the figures at 25 – 30kn don’t change much from below even in 15-20kn Moreton Bay conditions – at least from what I have seen anyway.

    All the readings below were from the gauges except for the last column [km/h], which I calculated as I didn’t write them down. Fuel data is the combined Total for BOTH donks.

    rpm, kn, L/nm, km/L, L/h, [km/h calculated]
    3000, 15, 1.8, 1.0, 26.4, [28]
    3200, 18, 1.5, 1.2, 28.6, [33]
    3400, 20, 1.5, 1.2, 31.2, [37]
    3600, 22, 1.5, 1.2, 33.8, [41]
    3800, 24, 1.6, 1.1, 38.6, [44]
    4000, 25, 1.6, 1.1, 42.8, [46]
    4200, 26, 1.8, 1.0, 47.2, [48]
    4400, 27, 1.8, 1.0, 49.0, [50]
    4600, 29, 1.8, 1.0, 54.7, [54]
    4800, 31/2, 1.8/1.9, 1.0, 60.4, [57]

    Sent from my iPad using Ausfish mobile app
    Just to update above...
    I have since changed to 3x16x21.5 Zuke Watergrip props, and the consumption & economy shown on the gauges is even better.

    However, I have also confirmed, via two calibration checks over ~1500 litres thru the donks, that the figures aren’t quite so good! Economy figures are about 7% worse than above figures (which were with 3x16x20 props).

    With the 21.5 props I now regularly see 1.6-1.8L/nm “on the gauges” between 25-28kn, with ~10kn wind and a good load. However, 7% worse are the real economy figures.

    Still bloody good for just over 3t empty, just not ‘phenomenal’

    Sent from my iPhone using Ausfish mobile app

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