Rudder Design on KP 44

Discussion in 'Boat Design' started by marga, Apr 3, 2015.

  1. marga
    Joined: Apr 2015
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    Location: San Francisco

    marga New Member

    Hi all,

    I'm new to this forum, thanks for your help!

    My boyfriend and I own a new to us Kelly Peterson 44, which we bought as an in-process project from a friend. The previous owner removed the rudder and had a new one built. We are refitting the boat for a 2-year cruise and have some questions about the rudder.

    I have linked a plan drawing of the rudder. It is constructed of high density foam, epoxy, and glass. I don't know the exact laminate schedule but it appears universally about 1/8" thick. When I spoke to the builder of the rudder he said it probably has G10 tubing reinforcement around the shaft as well. The main rudder shaft is 1 1/2" 316 SS. It is skeg-hung with a bronze split gudgeon that attaches to a 1" pintle a few inches down from the aperture. (See drawing) [​IMG] https://www.dropbox.com/s/29sac4opjsug03v/rudderdrawing2.jpg?dl=0

    The KP 44's have an amazing owner's group online that I have consulted and I have seen pictures and descriptions of a lot of rudder rebuilds on that site. All those rudders seem to have substantially beefier construction than our rudder in terms of armature and laminate, ours is much lighter (I think it is buoyant, maybe 70 pounds?).

    Working at a boatyard, I have also consulted my bosses who have extensive boat building experience, as well as spoken to the builder of the rudder, who has an excellent reputation building high-end performance rudders. However, I have gotten some differing opinions. Since my boyfriend and I are planning to cruise this boat across the Pacific, it would be great to hear any thoughts on whether it is worth beefing up this rudder before installing it.

    Thanks so much!

    Marga
     
  2. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    I do not have any personal long distance cruising experiance, but know a lot that do. As an engineer I can appreciate your concern, long distance cruising into unfamilar waters has it hazards, particularly in remote areas. I know of many that cruise long distances with relatively light boats, and know of others that only will go to sea with everything very stout (unnecessarily it seems to me, but than I have not been in bad storms in deep water either).

    it seems to me that most successful cursing sailboats are very stoutly built to give that extra level of safety. That always strikes me as excessively heavy, but in heavy seas that extra weight improves comfort, and gives you more confidence when it gets rough.

    Likely for your typical use what you have is fine, but you can not always plan ahead for those unplanned mishaps. so it would matter on where you were going, your own experiance and skill level, your budget and comfort level with what you have.

    If you are already strapped, than a rebuild might be out of the question anyway, however you might see if just putting on stronger attachment hardware can be done for minimal cost and leave it at that.

    OTOH, if the budget is not a big problem, making it a lot stronger will not harm a thing. It will hold up longer in the long run, and better withstand the unplanned for mishaps. So the extra cost may seem like a bargain over the next ten years.

    Good luck.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I think your concerns are unfounded. A skeg hung rudder is inherently about as safe a setup as you can get, aside from a full keel protecting it. The skeg is full depth, so unless it fails (not unheard of), the rudder is quite safe. If you do bash into something hard enough to smash the rudder (after taking out the skeg), you'll probably have other major issues, such as holes in the hull shell.

    The buoyancy of the rudder blade can be fairly easily calculated with some simple math. Estimate it's surface area on one side, then multiple by 70% of it's thickness. This will give you a rough idea of it's volume. Subtract it's weight from the volume of water it'll displace and this is how much buoyancy it has. It's probable the blade is "neutrally buoyant" (after then kitchen table math problem) and will not detract or add to hull volume.

    Good luck, happy voyaging and welcome to the forum.
     
  4. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    Not all damage occurs when underway.

    I have read about cruisers damaging rudders (and props) when backing into a dock. In most of the Mediterranean ports it is most common to dock stern-in. often unseen obsoletes, anchor chains, etc lurk below in crowed anchorage, so I guess it depends where you plan to go. It is also not unusual to ground the stern to the beach if you drag an anchor while you are asleep.
     

  5. marga
    Joined: Apr 2015
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    Location: San Francisco

    marga New Member

    Thanks for the responses! We're more concerned with the rudder loads during a potnential storm or such since we are planning to do ocean crossings. Particularly I was concerned with the armature design. I understand that this is a low-load rudder and is generally very protected being skeg hung. If you all think it's fine, though, then I have gotten the advice I was looking for. I appreciate your input, thanks!
     
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