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Thread: Removing propeller shaft casings

  1. #1
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    Default Removing propeller shaft casings

    Hi. I wonder if someone could help me with information about Catalac hull construction.
    I have a 9 m Catalac with twin diesels and saildrives. Just beside the saildrive on the bottom of each hull there is a wedge shaped construction. I assume it is intended for exit of straight propeller shafts. It is about 50 cm long and 20 cm deep in the aft end. As I have saildrives I donít need these and expect they create unnecessary drag. Does anybody know if this is a normal Catalac 9 m construction? Are they only external additions that can be removed, without revealing holes or weaker parts of the hull behind? They do not show from the inside.
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  2. #2

    Default Re: Removing propeller shaft casings

    Hi Baste,

    here is a picture of my twin diesel straight propeller shafts.
    The shaft and P-bracket are exactly in the middle of the hull.
    I have nothing like the smalls fins you have next to your stern drives but I don't think they are in the correct position to be former straight shaft exits.
    No idea whether they are structural or add-ons...

    Puck

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  3. #3
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    Default Re: Removing propeller shaft casings

    Hi Puck
    Maybe these structures could be intended for something else than propeller shafts, but I can't imagine what.
    I found some pictures of catalacs on the internet having the same "fins" in the same place.
    For example the 8 m catalac in the rudder replacement project displayed on the Catamaransite: http://www.catamaransite.com/skeg_rudders.html.
    So mine does not seam to be the only one having these "fins", on the side of the centre line.

    Is there anyone out there who knows anything about the purpose of these Catalac stern "fins"?

    Baste

  4. #4
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    Jul 2010
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    Default Re: Removing propeller shaft casings

    Yep, they are the former applications for the propshafts. In my Catalac 9, (no 49) they are still present. I have also twin saildrives but these are located in the rear part of the hulls so I can easily see those wedges from the inside. I guess you can easily remove them but wonder about the benefits it will bring. Total estimted surface under water = about 20 - 24 m2 I recall. Those applications bring about 2*2*0,5*0,50*0,20 = 0,2 m2.

  5. #5
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    Island living on the Outer Banks
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    Default Re: Removing propeller shaft casings

    You want my opinion?
    Go sailing and quit focusing on stuff that doesn't matter! You're going to get more drag from barnacles than what these two wedges will produce. Life's short. Take advantage of it while you have your health...
    KNOWLEDGEABLE SENIOR CAPTAIN CHIEF OF THE EAST COAST

  6. #6
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    Default Re: Removing propeller shaft casings

    Thank you for confirming my assumption Jan and thank you for your advice Capt.Terry.
    On the other hand, pondering improvements and testing ideas is what makes sailing and owning a boat fun. I believe it contributes to keeping me healthy, just as much as the sailing does. Furthermore, I live in a part of the world where boats are on land during winter and repainted with antifouling each spring. That provides time to pursue updating projects on the boat and keeps barnacles away.
    I think you may be right that the added surface of these wedges may be neglectable, compared to the whole bottom area, but they are 5 cm wide and may cause more drag by turbulence that by friction of wetted surface. I got inspiration from Talbot last year who explained that windward performance of a Catalac is very dependent on boat spead, and that he achieved improvements by cleaning the bottom from anything creating drag.

  7. #7
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    Default Re: Removing propeller shaft casings

    If these wedges/fins do exist on several Catalacs can it be that no one has ever tried to remove them when they are not used for anything?

  8. #8

    Default Re: Removing propeller shaft casings

    Baste

    You would not see any measurable improvement in your boat speed if you remove these 'wedges'. Your Catalac isn't built for speed. It is primarily a very safe cruising multihull cleverly designed to get you from A to B without any trouble.


    If, despite this, you continue to look for gain in terms of fractions of a knot then look to your rig. Sails in good condition, not stretched beyond their original shape and always a clean hull. Much more than that doesn't in my opinion produce any discernible benefit. Sailing vessels are always 'at war' with either induced or parasitic drag.

    The square root of the waterline length of your hull dictates the maximum speed attainable by your boat to windward.

  9. #9
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    Default Re: Removing propeller shaft casings

    One very beneficial improvement for my boat was replacing the flat plated rudders by profiled ones.

  10. #10
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    Default Re: Removing propeller shaft casings

    That is interesting Jan. What profile did you use and what improvement did you experience?

  11. #11
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    Default Re: Removing propeller shaft casings

    I appplied a NACA profile, width abt 5cm, length some 30 cm. Added endplates on the downside to improve rudder efficieny and to suppport the boat-aft side when drying out. Glued plywood with lots of epoxy around a welded frame of scaffolding tube. Then epoxied thick glass fiber layer at the end. As a result the boat is more responsive to steering, could even head up a little against the wind, I guess at an angle of (2*) 60 - 70 degrees when not too choppy. The boat is lifted much more on the aftside due to the floating characteristics of wooden rudders compared to the steelplated ones. I stored the latter in my basement for just in case... But told them lately,when seeing them, that they will be there forever .. One drawback is that steering in reverse become more difficult compared to the flat steelplates, which is somewhat annoying with manouvring, in locks etc. On the other hand, 99.5% of cruising appears to be in the forward direction so who cares.....

  12. #12
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    Default Re: Removing propeller shaft casings

    I have had plans to remould my rudders into NACA 0012 profiles, as I anyway do have to make some repairs on them due to a recent encounter with an under water rock. So thank you for confirming that it actually could be worth it.

    It would be nice if it could make the rudders more efficienct when turning to tack. That is a very slow manouver today, loosing boatspead down to at very best 1.5 knots, even in very good conditions. If I miss just a bit in preparations, attention or timing the boat may stop completely.

  13. #13
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    Default Re: Removing propeller shaft casings

    You might want to consider a fatter section than a NACA0012 if stalling during a slow turn is an issue. A 12% profile will stall at ~12* of rudder at around 3-4kts. Sounds like Jan used a 16-17% profile, and a NACA0020 isn't out of the question for a low aspect rudder on a boat doing <10kts.

    Mark
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  14. #14
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    Default Re: Removing propeller shaft casings

    Mark, I dont know if stalling is the problem. How can one tell?
    I read NACA 0012 was the most common on rudders, so I just assumed it would be the one to use.

    What do you say, Jan? Are you able to keep better speed through a tack with your new rudder?
    Which NACA profile number did you use?

  15. #15
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    Default Re: Removing propeller shaft casings

    Was afraid for that question. Recall something with 20 or 25. Found a cardboard profile though. Max. width 63 mm. Length along the profile is 405 mm. X-axis = 390 mm. Measured from the sharp cut-off on the rear-side, so corresponds to database values that will be somewhat higher.
    Measured in hte early morning, after well-celebrated christmas days, so only limited warranty on those observations...

    The length of the rudders was adapted to the draft of the boat in order to dry out horizontally. Therefore I added endplates, some 5 cm's bigger as the profile. But still have to experience that.

    Tacking goes much better, for sure. Boat is much more responsive. One could vaguely think it even has sailing capabilities :-)

  16. #16
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    Default Re: Removing propeller shaft casings

    If the complete profile lenght were 420 mm it could correspond to a NACA 0015. I assume 63 is the full width from side to side. But what is x-axis?
    Do you have skegs in front of you rudders? I assume skegs could reduce rudder effiency and turning ability.

  17. #17
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    Default Re: Removing propeller shaft casings

    If the complete profile lenght were 420 mm it could correspond to a NACA 0015. I assume 63 is the full width from side to side. But what is x-axis?
    Do you have skegs in front of you rudders? I assume skegs could reduce rudder effiency and turning ability.

  18. #18
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    Default Re: Removing propeller shaft casings

    The chord is 420, straight line between both sides is 390 mm. Yep, 63 is full width. No skegs, and the pivot of the rudder should be around 50 mm of the front of the profile. I'll try to find literature, it was as I recall now, an article in the Dutch CTC-magazine. Will try to find some photographs too, boat remains in the water this winter.

  19. #19
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    Default Re: Removing propeller shaft casings

    If your current rudders are flat plate, then they are stalling when tacking unless you carve through a long tack with very low rudder angle. The stalling is probably contributing a lot to making your boat tack very slowly - basically, you are braking the stern while letting momentum bring the bow around. After a tack, you could have almost no speed at all if this is large enough.

    12% is a common foil section for many higher aspect rudders, but it sounds like yours is a low aspect planform. Probably even 1:1 or less, given what Jan has written. For this aspect, a larger section will give more lift and stall much later than a thin one.

    Then there is the practical aspect of strength. I'm still confused about what Jan's numbers mean (x-axis, straight line between sides etc), but if the length of the rudder chord is ~400mm, then the width of a 12% section will be ~50mm, which isn't much "meat" around a rudder post unless your post is very small, and the blade itself will be thin and possibly weak toward the trailing edge. So if you have a 30mm rudder post, you will only have 10mm on each side of it for strength, and the back half of the profile will be getting down to razor thinness and difficult to manufacture with any strength or stiffness.

    Larger rudders can use smaller foil sections because the absolute size of those sections is larger even though the relative size of the section is the same.

    Mark
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  20. #20
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    Default Re: Removing propeller shaft casings



    Scaffoldingtube is 50 mm I guess. Construction was galvanized. But unfortunately had to be removed again by sandblasting because epoxy should be applied on the bare iron as the provider of it told me.
    The width of 63 mm of the profile is measured from the cardboard template that I found at home. Rudder-dimensions may/will deviate.
    I used a lot of epoxy and glass-fiber to get a solid construction. But the force is captured by the iron construction inside.
    I applied multiplex, (Bruynzeel) around the iron construction.
    Was quite a job I must say..

    (With x-axis I mean the distance from nose to tail. The chord is of course larger. (So, the y-axis is perpendicular to that and displays the width.) If not clear I will provide a visio-drawing
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    Last edited by Jan Versteeg; 27th December 2016 at 03:35 PM.

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