Best prop size and pitch please

Discussion in 'Props' started by jumbuck, May 14, 2010.

  1. jumbuck
    Joined: Apr 2010
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: CAIRNS

    jumbuck Junior Member

    I am having trouble finding information on this and was wondering if I can politely ask for advice please.

    Trailer sailer full cruise weight: 1,250KG (not including persons onboard)
    Length: 22.5' (just shy of 7meters)

    What would the recommended propeller size and pitch be?

    I am also struggling with motor choice. Reasonable weight 4-strokes stop at 6HP, then they suddenly increase in weight by over 30% in anything above 6HP. It is hard to justify going from 27KG to around 40KG for only 2 HP more, so I'm totally stumped.

    I don't really want to have to hang a 40KG motor off the transom of a 22' trailer sailer but need more than 6HP, or do I?

    I don't want to go 2-stroke either because I genuinely fear the Great Barrier Reef Marine Park Authority will ban 2-strokes shortly (there's talk in the air of this).

    Any advice would be much appreciated.

    Thank you
     
  2. tunnels

    tunnels Previous Member

    When you buy a outboard it comes with a standard prop , this prop is sorted to be a good starting point for the hp of the motor in most applications . so start with the standard and work away from it . bigger diameter and lower pitch gives more push . smaller dia and more pitch usually gives more speed this is just a general rule of thumb !:D
     
  3. Çemberci
    Joined: Apr 2007
    Posts: 127
    Likes: 6, Points: 0, Legacy Rep: 133
    Location: Istanbul/Turkey

    Çemberci Senior Member

    size

    Let say hull waterline 6.30 metre waterline beam 2.064 metre draft 0.38 metre hull displacement 2.450 ton engine hp 6 hp rpm 3600 rpm 1.0:1
    reduction ratio hull speed 6 knot hull thrust 162 lb two bladed propeller
    propeller drawing is attached.
    oktay çemberci
     

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  4. Guest625101138

    Guest625101138 Previous Member

    6HP is adequate for that size boat in most conditions. Look at specific sail drive outboards like this one:
    http://www.tohatsu.com/outboards/6_4st.html

    The long shaft is important to keep the prop in the water. The sail props are usually the biggest diameter that can be fitted and are slightly lower pitch with larger blade area than standard. The charging capability is important if you want to do a bit of overnight cruising.

    I am not sure if you can get electric start on 6HP but it is handy if the outboard cannot be located easily for pull start. You can get a Honda BF8 with electric start. Likewise remote controls add a bit of class but only necessary if the outboard is difficult to reach. It is more expensive and more weight.

    A 4HP will push a boat this size well in light weather. Fine for getting home if the wind dies.

    A 6HP will keep you moving in most conditions but it is usually more effective to sail in heavy conditions. It is a matter of being able to reduce sail to suit the conditions rather than relying on the engine.

    Rick W
     

  5. jonr
    Joined: Sep 2008
    Posts: 721
    Likes: 11, Points: 0, Legacy Rep: 57
    Location: Great Lakes

    jonr Senior Member

    You can also consider fuel usage - a little over propped is better for that.
     
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