Surface drives

Discussion in 'Surface Drives' started by marios55, Dec 13, 2014.

  1. marios55
    Joined: Nov 2014
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    marios55 Junior Member

    Hi guys,
    was looking for a pair of second hand surface drives but cant seem to find any.
    Will try make my own.
    What determinates the height of the shaft center, over the bottom of the hull transom?
    Thank you.:
     
  2. tom kane
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    You would be very wise to build or have your drive so it is easily adjustable so you will not have to use a gas axe to make alterations to the shaft angle later. A fixed surface drive does not work well in adverse water and weather conditions such as following seas and different loading conditions.

    Surface drives are great for such work as big fast ferries, high speed boating and and racing. Using rudders with surface drives means you need deep rudders for good steering like the race boats do and that cancels out all purpose boating in shallow water and re- introduces the problem of the increase in drag which is why S/P drives may seem attractive.
     
  3. gonzo
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    gonzo Senior Member

    First, the term "surface drive" doesn't really mean anything. There are propellers designed to run ventilated. Whatever you attach them to that allows them to work in that mode is fine. Most of the rest is sales hype. An outboard installed at the proper height will work. The key to a proper design and installation is to have the forces balance out. That is, the thrust from the propeller, the center of gravity, lift of the hull are all properly aligned. For example, if the line of thrust is too high, the boat will tend to bury the bow. Conversely, if it is too low, it will get the bow pointing up.
     
  4. marios55
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    marios55 Junior Member

    Thanks to both of you....those were interesting comments.
    What worries me though is that the boat wont have the same L.C.G. due to the fact that it will be used as a passenger boat, which means different load, different L.C.G.
    The only reason I want to use surface drives is because of gaining 20-30 % on speed therefore consumption.
    Otherwise under-hull tunnels with fixed shafts would be better maneuvering but slower.
    I have tried to find details on tunnel plans but no luck. Any ideas where to get tunnel plans would be much appreciated.
    Thanks Mario.
     
  5. gonzo
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    gonzo Senior Member

    How did you calculate those gains? They seem rather excessive.
     
  6. marios55
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    marios55 Junior Member

    Hi there,
    well I didnt, its what I read out of all the company's sites that manufacture them.
    Would be very interested to hear your comments about this.
    Thank you
    Regards Mario
     
  7. gonzo
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    gonzo Senior Member

    Ask the company if they are willing to give you a contract in writing to guarrantee the claims. I doubt they will though.
     
  8. marios55
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    marios55 Junior Member

    You are very right,
    but then again what propulsion to use?
     
  9. gonzo
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    gonzo Senior Member

    The drive system should not be considered alone, but in relation to the rest of the design. Are you retrofitting or is this a new build?
     
  10. JSL
    Joined: Nov 2012
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    JSL Senior Member

    some more info on your boat, power, etc would really help.
    I have done several boats with surface drives (Arneson, SeaFury) and they have worked well. As to maneuvering, twin Arnesons are good... a skipper could even 'walk' the boat sideways from the dock.
    Tunnels work too but better suited to slower boats (under 25 knots), depending on the depth of the tunnel.
     
  11. marios55
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    marios55 Junior Member

    Hello again,
    as I said the boat is under construction and has the following dimensions.
    > L.O.A 11.95 mtrs
    > L.W. L 9.5 mtrs
    > B.O.A 4.00 mtrs
    > B.W.L 3.80 mtrs
    > Draft 0.45 mtrs
    > Deadrise at transom 13 dgr.
    > Deadrise at bow 52 dgr.
    > Maximum weight 10 tons

    > The hull has two reverse steps which will be ventilated by forced
    > (blower) air from the deck.
    > Desirable cruising speed over 35 knots.
    two Volvo penta D6 370 hp will be used , with no gearboxes yet.
    Thank you very much for your time.
    Best regards Mario.
     
  12. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    Gonzo & Tom have some good points.
    Generally, your numbers look okay but I will query:
    - the Cb looks fairly high.
    -fairly flat bottom (low deadrise) - I guess you are operating in calm water
    -chine beam at transom, and maximum???
    - what is the deadrise at max. chine beam.?
    - what is the LCG for'd of transom
    - details of the reverse steps.(whatever these are?) This may induce cavitation. most steps are vented from the side.
    - Not sure if a stepped hull is warranted for the speeds you are after.
    - if you plan to use a name brand drive (surface, or other) the manufacturer will have an application form for you to fill out.
    - there are probably more questions and input from other members.
     
  13. marios55
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    marios55 Junior Member

    Hi Gonzo and Tom,
    Forwarded your reply to the builder and this is what he replied.

    -Flat Bottom - at 14 degrees with variable service loads. Higher deadrise at transom not useful. Just waste of HP.
    -Transom 14 degrees developing forward at 54 degrees.
    -Steps looking aft will contain air ventilation distributing it aft.
    -No cavitation is anticipated; a) air vents at 4 metres
    b) surface drives with air anticavitation props will not be affected.
    -Estimated angle of attack 4 degrees.

    Thank you and Merry Christmas to all the team.
     
  14. gonzo
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    gonzo Senior Member

    I am not sure what a) and b) mean. Surface propellers are ventilated to prevent cavitation. The low pressure on the forward face gets air which prevents a steam explosion (cavitation)
     

  15. marios55
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    marios55 Junior Member

    Hi Gonzo, do you have an email so I can forward you some sketches and more details please.
    Regards Marios.
     
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