Protective skeg

Discussion in 'Multihulls' started by HakimKlunker, May 24, 2011.

  1. HakimKlunker
    Joined: Aug 2009
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    HakimKlunker Andreas der Juengere

    Someone said:

    "Although my 49ft cruising sailboat is NOT a beach cat, it may accidentally touch the ground in shallow waters, or unexperienced yard personnell may put my boat incorrectly on the hard; or some day I will be forced to beach the boat due to an emergency. In the end my rudders or the sail-drive will be damaged."
    Someone now thinks that a protective skeg is a suitable solution.

    In general an idea worth to think about, we are aware that there are a hundred and more different ways and opinions.
    i.e.: Some arrangements may induce more drag,
    others may reduce manoeuvrability,
    also the strength of the attachment to the hull will have effects.

    We would be very much interested to see how others see the ideal solution.
     
  2. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    Kickup rudder with a skeg.. :)
     
  3. Boston

    Boston Previous Member

    kick up drive unit to go with that with that kick up rudder and skeg
     
  4. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Stainless "shoe" on the bottom and the bottom of the skeg, to go with the kick up drive unit, kick up rudder and skeg....
     
  5. s v ugly sister
    Joined: Jun 2004
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    s v ugly sister Junior Member

    No skegs - cassette rudders with kick-up feature - - replace sail drive unit(s) with standard gear box(es) going thru an RPS(s) (retractable propulsion system(s) ala Volvo 70 - see www.amartech.nl) there are Amartech copies out there using less exotic materials - search "sailing anarchy retractable propeller" for article & picture of the RPS in Bakewell-White 11m twilight racer - - in both cases - (rudder & prop unit) - its hard to damage something that isn't there Dale Miami
     
  6. mikereed100
    Joined: Jan 2007
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    mikereed100 Junior Member

    A kickup rudder would be a difficult modification on a boat that wasn't designed for it. I suppose all rudders and drives are "kick-up" if you kick them hard enough. A small skeg ahead of the drive unit would not be too hard to do and could be made very strong. I think Outremer, among others, does this in the factory.
    A skeg large enough to protect the rudder would affect the handling qualities of the boat. If it were my boat I would wait until I ran into something and destroyed the rudder, then would rebuild with a skeg hung rudder.

    Mike
     
  7. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Kick up rudders in a 'box' are one option of the original design.
    In my (our) present case the boat is yet to be built, although we have already started and the hulls and/or bulkheads cannot be modified without major effects all around.
    Perhaps the sail drive can be replaced by a V-drive. A conventional shaft would leave no space for the rudder/s I reckon.
    As the rudders aren't large anyway, I see a risk to place them further aft: In rough and short waves they may 'come out' too much and lose efficiency. Presently they go 25 cm deeper than the lowest point of the keel line.
    The aft section of the hull would allow for a kick-up version.
    Anyhow, a saildrive looks very attractive to us because of easy and compact installation.
     

    Attached Files:

  8. tspeer
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    tspeer Senior Member

    If you want the rudder to be steerable when it kicks up, you won't be able to have as long a sugar scoop as shown in the drawing. The hull must be inside the radius from the transverse pivot axis to the top of the movable rudder.

    Chris White solved this problem on the Hammerhead 34 by putting a fixed skeg between the rudder and the hull. The distance between the pivot and the bottom of the skeg is the same as the distance between the pivot and the bottom edge of the transom.

    [​IMG]

    The steering cables pass through turning blocks that are colinear with the pivot axis, so the length of the cables does not change as the rudder kicks up. The allows the rudder to still function when the rudder is partially retracted.

    On this boat, there is enough rocker that the folding prop is behind the belly of the hull. I wouldn't want to be operating the prop at the time, but the boat can be beached without damage.

    [​IMG]

    The only downside of this arrangement is, because of the taper of the rudder planform, there is more rudder area ahead of the pivot axis at the top of the rudder than at the bottom of the rudder. When sailing, the rudder is well balanced. However, when under high power, the propwash blowing on the top of the rudder, where the balance area is proportionately greater, results in the rudder being way over-balanced. You can't let go of the rudder for a second when under power, or the boat will diverge rapidly. It's easily handled once you get used to it, as the tiller loads are low when the rudder is near center. But if motoring for a long distance, the autopilot is a definite convenience.
     
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  9. Spiv
    Joined: Jun 2007
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    Spiv Ancient Mariner

    I was paranoid about being able to go into shallows and feel safe.

    I had box kick-ups and found them generally ok. The only slight problem was that with all the swivels and linkages they were sort of wobbly. Mine were designed and built 11y ago, I am sure that it could (maybe it has already) be improved with better design.

    Sail drives aft are a bad choice, a friend tried to motor into 40kn and the cavitation made the task impossible; engines run hot and had to be turned off.
    Fortunately he was hundreds of miles from the nearest coast....

    I solved the problem with outboards in nacelles nearly amidship.
    When down the prop was just above the keel, so I could beach the boat to unload passengers and reverse out of it.

    Many people would frown at the idea of outboards, but modern outboards are very reliable. Pros:
    1- my 4 strokes Yamahas would run underwater ( or nearly....)
    2- cost a fraction of diesel inboards,
    3- weight a fraction,
    4- replaceable in a short time,
    5- serviceable almost anywhere
    6- out of the water when sailing etc etc.

    Cons:
    1- Gasoline storage and delivery requires additional design considerations
    2- people perceive it as being 'cheap'.

    My nacelles created an area between themselves and the hulls where waves would slamm; I corrected that by filling the gap later, but a design that took that into account would nearly eliminate this problem.
     

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  10. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Spiv, thanks for your input.
    Unfortunately in our case an engine position where you suggest it will interrupt or 'spoil' the interiour lay-out - the hulls are not that wide that you can walk around, and so (inboards and outboards the same way) there would be an obstruction in the corridor. Or do I misunderstand and you have your motors sort of out-centered?
    With the Diesel's consumption economy there is also an increased operation range under engine/s. The boat will (for a start) be operating close to the equator and if we like it or not, motor power is an issue here.
    Our present engine position is not only a question of space availability: We had at first some trouble to give the boat the right trim and to prevent it to be a 'nose-diver'. The sail drive will be mounted forward of the engine, so at least a cavitation risk is reduced to the minimum possible. But if someone has a better idea, I will have open ears.
    The 'wobbly' steering arrangement of a kick-up solution directed us to the fixed rudder shaft. It would be interesting to hear from someone who fixed this 'wobble-problem'. That perhaps would give us a point to follow up.

    From all responses until now: Can I resume that a skeg is rather considered to be a compromise? Or are there voices supporting fixed rudders, saildrives and skegs?
     
    Last edited: May 26, 2011
  11. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Thanks guys: You opened our hearts for a kick-up rudder.
    Remains the issue about a skeg.
    We found some samples as per attachments. May I ask for comments? (I'll add mine later)
     

    Attached Files:

  12. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Thanks guys: The overwhealming majority advocating kick-up rudders convinced us.
    May I ask for more comments from you?
    We now wonder which skeg will work best.
    There is
    1: The issue of not reducing propeller performance.
    2: Build a firm mounting structure

    Our ideas:
    A: A short skeg will produce less (sideways) forces at its hull joint, but perhaps disturb the prop. A longer skeg will rather place the trailing vortex away from the prop. (?????)
    B: A 'shoe' at the bottom is meant to reduce 'digging' into soft bottom in the case of (unintentional) grounding.
    C: The gap between skeg and sail drive will be bridged by thin laminate strips to prevent turbulences. These strips are touching the sail drive, but are not fixed there.
    D: The hull skin (sandwich) at the bulkhead bottom will be cored out to prevent compression issues. The bulkhead will take vertical forces from weight and from collision.
    E: A fore top hat stiffener distributes side forces (i.e. from boat swaying at anchor and touching rocks, corals, sea bottom)
    F: The skeg will be laminated to the hull; this is to prevent bolts.
    The bottom area will have a weaker laminate than the top. So, in case of heavy collisions (under way) the risk of leakage is minimized; the concept here is to make the skeg sacrificial in extreme situations.
     

    Attached Files:

  13. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    What size boat are you building?,...and what sort of power?
     
  14. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    I like skegs...they protect your drive train. A broken rudder equals a bad day...push your drive train into the engine room and you're facing a bad year.

    Your sketch looks logical....Hydro dynamically I dont know what shape of skeg would be best...how close to the drive leg do you mount it ?

    Perhaps seek guidance from boats already fit with skegs.

    A skeg looks like a mighty fine location for a refrigeration system keel cooler box.
     

  15. HakimKlunker
    Joined: Aug 2009
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    HakimKlunker Andreas der Juengere

    Results

    Thanks guys, for all input.
    We have gone for this solution: Kick-up rudders and a skeg right in front of the sail drive leg which goes a little lower than the prop when it is folded.
    The engines will be 53 HP Yanmars (a little high, but it is the owner's wish).
    The appointed boat weight has been 9000 kg but we end up ca. 9400 kg when the boat is fully equipped and loaded.
    The skeg foot is under the bulkhead so that if needed the boat can rest on it and has a little fin to act against sinking into soft ground.
    The profile follows the dagger board profiles and is a simplified NACA. (This detail could have been worked out more scientifically, but it is good enough for us - we do not intend to challenge Ellen...)
    The skeg's side laminates go through the hull and in- and outside there are
    L-laminates to transfer side loads.
    A big thanks goes to Nicholas Cossich at ATL in Australia who engineered the laminates for us!
     

    Attached Files:

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