Minimum propeller centre distance?

Discussion in 'Props' started by PetterM, Aug 10, 2009.

  1. PetterM
    Joined: Sep 2004
    Posts: 211
    Likes: 9, Points: 28, Legacy Rep: 201
    Location: Norway

    PetterM Senior Member

    Looking at a long very slender monohull with twin propellers, on this particular design the optimum propeller diameter meter is limited by having sufficient tip clearance between the propellers.
    It there any rules of thumb how close I can put the propellers?
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 7,077
    Likes: 1,010, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    If the flow into the props is identical and clean and they are basically inline....shouldn't make any difference
     
  3. apex1

    apex1 Guest

    Did not understand clearly. Do you ask for tip clearance to hull or between props?
    To hull is min 15% of prop diam. (better 20), between props is min. 1,5 prop diam. better 2. On a very slender hull, you will have a problem or you use a deadwood.
     
  4. PetterM
    Joined: Sep 2004
    Posts: 211
    Likes: 9, Points: 28, Legacy Rep: 201
    Location: Norway

    PetterM Senior Member

    Thanks for your replay.
    I should have expressed myself clearer. What I meant to ask for is the tip to tip clearance. So minimum .5 of propeller diameter?
     
  5. apex1

    apex1 Guest

    1,5, read: 1,5 or deadwood!
     
  6. PetterM
    Joined: Sep 2004
    Posts: 211
    Likes: 9, Points: 28, Legacy Rep: 201
    Location: Norway

    PetterM Senior Member

    What would happen if the propellers were mounted too close? Is there any literature on this?
    The propellers are designed to run relatively low slip at design point.
    Thanks,
    Petter
     
  7. Ad Hoc
    Joined: Oct 2008
    Posts: 7,077
    Likes: 1,010, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Not much literature that I know off. However having designed several boats in the past, with props all in line and very close together (ie just several cm's apart tip to tip) no losses were noted boat did the expected speeds and all data as predicted on sea trials. It is of course a different matter if the prop's are offset against each other.
     
  8. Frosty

    Frosty Previous Member

    I agree with ad hoc if the flow to the props is clean it shoud'nt make a difference. However the whole point of having 2 props is lost if it is for maneuverability.
     
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 7,077
    Likes: 1,010, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Good point Frosty, manoeuvrability comes into play too...unless the boat is not meant for slow speed manoeuvring.

    Beg's the question then..why have 2 that close, any particular reason?
     
  10. apex1

    apex1 Guest

    I´m happy to agree with Frosty (thats quite rare) in case of maneuverability. The distances given are (as asked for) rules of thumb, above 1,5 prop diam. there are no measurable rotational interferences which CAN occur definetily below such distances under certain conditions. I can´t find the papers, but am sure to recall "Hucho" and "Fresenius" on that subject. Maybe John has? I know "Hamilton Sundstrand" once made some related analyses on air props.
     
  11. powerabout
    Joined: Nov 2007
    Posts: 2,925
    Likes: 66, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    on a race boat they could be as close as 30mm tip clearance or less.
    You didnt mention fast or displacement.
    Under the hull, surface drive, stern drive or what or the diameter or prop rpm
     
    Last edited: Aug 16, 2009
  12. Ad Hoc
    Joined: Oct 2008
    Posts: 7,077
    Likes: 1,010, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Apex/Richard

    I'll look through my old files on prop's. I know I have lots of old research reports from VT, KMW, US Navy and others, but not sure about those you have mentioned. Need to get my duster out...they are old papers!!
     
  13. apex1

    apex1 Guest

    Archive of applied mechanics (here 4th Int. congress for applied mechanics 1935 Cambridge, Flügge, Lotz et al.)
     

  14. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    Is it not the engine width, plus mountings etc. that determines the distance between props?
    Unless you have some sort of Siamese twin engine, imho the prop size is always smaller than the required engine space.
     
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.