Surface Propulsion, long-tail-emprovements.

Discussion in 'Surface Drives' started by tom kane, Feb 9, 2016.

  1. powerabout
    Joined: Nov 2007
    Posts: 2,913
    Likes: 62, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    I watched a 53 Magnum in a Marina once in a blow and it needed lots of room with lots of froth and bubbles at the stern
     
  2. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    I am not sure what you mean when you say "them" in regards to driving and watching them, powerabout.
    I doubt if you have seen any retractable shaft drives in any Marina, or many long tails or mud boats being maneuvered.
    Shaft drives set with fully submersed propellers at 18 to 20 degree shaft angle should have no trouble with reverse they have been going a long time but fixed surface propulsion with small shaft angle will especially on deep v hulls.
    I doubt if I/O drives or outboards would have trouble maneuvering unless run in tilted position.
    I agree with Gerr when he says that performance relating to shaft angle is very difficult to measure.
    Most boats that I see are always to heavy in the stern and have a too acute plane angle unless they have a lot of extra power to overcome the extra drag of greater draft caused by deep draft at the stern and they never reach optimum flat plane for comfort and vision and making use of hull shape.
    You can only purchase and compare the products that are on the market but if you want something better suited to your purpose you have to build it yourself and find out the merits or otherwise of what you have built.
    There are always some one who will tell you "that won`t work" but can not come up with better alternatives.
     
  3. powerabout
    Joined: Nov 2007
    Posts: 2,913
    Likes: 62, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    Tom
    its the propshaft angle to the hull that is the issue and if that is in the same plane or higher when parallel you have the reverse problem. Countering that you get low drag so good performance.
    When a prop thrusts the water wants to come to the surface so now look at the surface drive prop and your transom and when parallel to the hull at least half the reverse prop wash is facing the transom.
    If you trim up to direct that under the hull you may well be lifting the prop out of the water but certainly putting blades very close to the surface.
    If you trim down you are just directing more water to the transom.
    As soon as someone invents a propshaft that can move up and down in the level plane we will have the ultimate. ( racing outboards have this built into the transom bracket)
    Cheers
    PS High mounted sterndrives have this issue so you trim up when you need reverse but propshaft plane under the hull so its more effective and if you run stand off box the tilt pivot is well behind the hull
     
  4. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    It is so easy to have a prop shaft that can move up and down and also steer the boat and remain horizontal to the water surface, but I won`t enlarge on that as it should be easy to work out for your self. It probable would not "look right " to some people or suit traditional boat design.

    Having a propeller that has rake to change the angle that the prop thrusts the water and also lifts the water closer to the surface (surface propulsion) to speed up ejecting the water from the prop eliminates steeper shaft angle inefficiencies if there is really any.

    Surface propulsion is using the descending blade to scoop up the water and ejecting it to the rear into the less dense air rather than down in the water.

    I do not find prop shaft angle as a major problem but more of an advantage in surface propulsion as less drag from less metal in the water, and no rudder drag like many surface propulsion. Just use the prop for everything, nothing else in the water to cause drag.
     
  5. powerabout
    Joined: Nov 2007
    Posts: 2,913
    Likes: 62, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    Love to see the drawing of a system that can move the propshaft whilst keeping it in the same plane.

    Propshaft angle is a critcal factor in performance and prop design
     
  6. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,969
    Likes: 608, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You mean like an outboard on a jacking plate ?
     
  7. Barry
    Joined: Mar 2002
    Posts: 1,314
    Likes: 151, Points: 63, Legacy Rep: 158

    Barry Senior Member

    To your comment "most boats that I see are always too heavy in the stern and have a too acute plane angle"

    Assuming a constant deadrise of the the wetted surface of a planing powerboat, the optimum trim angle changes with different constant deadrise angles and speed. ( should probably include with specific thrust angles due to the drive, ie trimable outboards
    Optimum meaning least thrust required for a given speed

    A 30 degree deadrise has an optimum trim angle of about 10 degrees, say at normal cruising speeds above 20 knots.

    With shallower deadrise angles, the optimum trim angle seems to approach around 4 to 5 degrees.


    To your comment " never reach optimum flat plane"
    This is not correct. You always need some trim angle of presentation to the direction of travel to generate a DYNAMIC pressure distribution on a planing hull to create lift.
     
  8. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Surface Propulsion,long-tail-emprovements

    You should not need drawings of a mechanical set up that is used by the many millions all around you. There has even been a steam driven twin contra rotating model with shaft parallel to the water surface. There has been a similar device on this website which was derided by many so I guess the poster took it elsewhere. Harassed by shaft angle and efficiency purists.

    Most of the top selling surface drives run at steep angles along with the basic long tails and mud boats and do a great job so shaft angles are no problem.
    It seems shaft angles can become an obsession like efficiency and performance rather than pleasant boating.
     
  9. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Thank`s for those comments Barry, but your figures don`t necessarily accord with many other peoples opinions and actually boats do not perform to hopeful design targets taking into consideration loading of passengers and gear and propulsion.
    Suggesting that a 30 degree dead rise hull can ever be anything other than a displacement hull seems out of context to me.
    All fast planing boats traveling at speed should travel parallel to the water surface most of the time.
     
  10. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Surface Propulsion,long-tail-emprovements

    Similar, but a lot more convenient you do not need an outboard you can use an inboard and use really big power.
     
  11. Barry
    Joined: Mar 2002
    Posts: 1,314
    Likes: 151, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Tom
    I am not suggesting that a 30 degree dead rise hull can be anything other than a displacement hull. I don't know how you can formulate this statement
    I did state that an optimum bow up trim of around 10 degrees is close to optimal for a 30 degree constant deadrise and modest speeds.
    But there are many papers done in controlled tanks that show this to be accurate.

    Your understanding of things technical are weak. Certainly a boat travels parallel to the water but the bottom of the hull is at a trim up attitude. Otherwise the boat will not produce dynamic lift.

    Please spend some time looking over the papers, as even a layman can understand the graphs, pressure distribution of dynamic pressure acting on a hull.
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,969
    Likes: 608, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Planing hulls with 30* deadrise do exist, however impractical it might be. And some planing cats have hulls with more than that even. While the most efficient trim angle is the one that has the least resistance, it is usually desirable to have a lesser trim angle for comfort.
     
  13. Barry
    Joined: Mar 2002
    Posts: 1,314
    Likes: 151, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Agreed Mr E

    As several of the tables that I was looking at had optimum trim angles for deadrise up to 30 degrees, I just used that for an upper figure to illustrate that the higher the constant deadrise, the higher the optimum trim angle.
    I directed this to Tom as I got the idea from his post that he believes that having a constant deadrise hulls of various degrees, that the boat should run flat to the water surface for optimum efficiency. Ie when he sees a boat travelling that looks like it is stern heavy he assumes that this is inefficient while in fact depending on the deadrise, might be running at its best trim angle

    Another facet, though perhaps not pertinent in this discussion is that a hull will not automatically find its optimum trim angle without adjustments. Outboard or I/O trim, trim tabs or weight distribution can help but then the angle can change as speed increases or decreases.

    For our previous 40 foot, 30,000 pound boat , Volvo duoprops, at 24 knots totally trimmed in and tabs up, I could, on those very special mirror water days, tweak out another knot to knot and a half of "free speed" with changes in leg trim and the trim tabs
     
  14. powerabout
    Joined: Nov 2007
    Posts: 2,913
    Likes: 62, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    Tom
    are their any drive systems that can move the prop shaft parallel to the water surface or not?
     

  15. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Surface Propulsion,long-tail-emprovements

    Thank`s for the previous posts and a couple of images of mud boats with drives parallel to the water surface.

    Just imagine the impact on those drives and the tilting of the motors and difficulty turning the drive.
    There must be a lot of drag with those shaft drives and mechanical shock.

    A drive with a cam like action removes shock with out tilting the motor.
     

    Attached Files:

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.