Propeller pitch Lombardini 30 hp

Discussion in 'Props' started by radier, Dec 1, 2012.

  1. radier
    Joined: Dec 2012
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    radier Junior Member

    I'm about to install one Lombardini 30/28 hp diesel in SB hull of my 37 ft sailing catamaran. Untill now I have used one 20 hp 4-stroke Yamaha OB in each hull cruising at ca 6,5 knots by one OB, top speed 10 knots by both. I'll keep the 20 hp Yamaha in port hull for harbour use, cruising will be by the 30 hp diesel only and I hope for ca 7 knots cruising at a reasonable rpm, top speed not so important.
    Hull LWL is 11,0 m, weight ca 3,3 t cruising, easily driven hulls with recorded sailing top speed of 25 knots.
    Lombardini 30 hp at 3600 rpm, gear 2,6:1.
    I have been recommended a 3-bladed Flexofold prop 16"x9" but feel perhaps 16"x11" should be better. Any experts out there (I have no experience with diesels, just jet engines:)?
     
  2. tunnels

    tunnels Previous Member

    exsplain why you think the 16 x 9 is to small and you should use a 16 x 11??? if the motor will rev its max rpms with little effort then yes the 16x9 could be a fraction small but in a heavey seas with head winds etc etc it could be better suited than the 16x11 that will make the motor work all the time at 99% Of its efficentcy in calm seas without head winds and rough seas and your gallons per hour will go up . I am a firm believer having a little in reserve and never running small motors at the max all the time !!
     
  3. jonr
    Joined: Sep 2008
    Posts: 721
    Likes: 11, Points: 0, Legacy Rep: 57
    Location: Great Lakes

    jonr Senior Member

    Engine and prop in sailboats has more to do with worse case needs than best economy cruising. But a two speed transmission could give you both.

    With the second engine, your case is different and you could select for best economy (should be around 2000 rpm). This suggests a higher that normal pitch prop.
     
  4. radier
    Joined: Dec 2012
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    radier Junior Member

    Thank you very much tunnels and jonr for your reply. Have bought my prop by now and final desicion became 16"x12" 3-bladet Flexofold folding prop. Have to wait some time to see how this will work out, winter in Norway just now.
     
  5. masalai
    Joined: Oct 2007
    Posts: 6,818
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Hi radier,
    What is the gearing between engine and propeller? - I run a pair of 20hp kubota/nanni saildrives and push my loaded 6.5 tonne cat at 7knots with 2500engine rpm through a gearbox that rotates the propellers at close to 1000rpm (15 x 12" - 3 fixed blade) - - The boat readily surfs to 19knots in a good following sea (plus 4M swells)...
     
  6. radier
    Joined: Dec 2012
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    radier Junior Member

    Hi masalai,
    Gearing is 2,5:1
     
  7. masalai
    Joined: Oct 2007
    Posts: 6,818
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Hi radier,
    That should get close to your assessment, as at 2500 rpm my engines are producing around 10hp each - best of luck come spring thaw - 28 deg C plus water temperature at present - in the mooring...
     
  8. radier
    Joined: Dec 2012
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    radier Junior Member

    "28 deg C plus water temperature at present - in the mooring..."
    Please don't mention this masalai, I was ca 1 hr drive from here earlier this week, - 25C air temp in the morning (yes minus 25C) and ice on water :-(
    But in a few days it's downhill to a new summer and sailing season :)
     
  9. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 53, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    I think you should get higher than 11" pitch. 16x11 is quite standard for 20-30 hp monohulls with about the same shaft rpm but top speed of 6.5-8 knots. I have a VP MD2020 SD (20 hp 3600 rpm 2.47:1) in a 35 feet 6 tn monohull. I used to have a VP 3-bladed 16x11 and it was a good fit. Top speed was about 7.4 kn at 3400 rpm. Cruising speed something like 6.5 kn 2500 rpm.

    Now you have 10 hp more, faster boat and a bit higher gear ratio (is it 2.6:1 or 2.5:1, you have given both???). Certainly you will need a higher pitch!

    A quick calculation shows something like 16x13 could be OK. 16x9 is definitely not OK! Who would recommend that?
     

  10. radier
    Joined: Dec 2012
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    radier Junior Member

    Thank you very much Joakim.
    Gearing is 2.6:1
    The prop I have bought is 3-bladed 16"x12". Seller told with more pitch I could get into problems maneuvering in harbours on a windy day as the boat has some windage. I still have plenty of time to switch to 16"x13" if that would be better to start with. Seller has no experience with such a boat and I'm offered to switch prop later on at some cost if I want to. If you want to have a look the boat is here http://home.online.no/~uthaug/gallery/norgallery.htm
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.