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Thread: Skeg Design Query

  1. #1

    Default Skeg Design Query

    My boat (small cat) is an early model with just a spade rudder and no skeg. The builders quickly realized there was not enough directional stability/tracking and later boats all had a skeg fitted. The rudder area would be about 3sq ft and the area of the skeg about 2 sq ft.
    The common improvement for these boats is to add a skeg similar in size to the factory skegs (2ft 9" long x about 14" at the top tapering down to about 6" at the bottom and the rudder having a bottom pivot attached to a fitting at the skeg bottom).

    Retrofitting a skeg as described above is not is without its challenges although not a difficult job to complete I think. However the original skegs were just "glued on" with poyester resin as I think the builders were more concerned about an impact removing the skeg and a section of hull with it than losing a skeg in a minor ipact. It is not an impossible job to strengthen the hull but access in the area required is not too easy and it would be time consuming to do this but not an enormous job.

    My question is whether I could achieve the same tracking/windward improvement by fitting a skeg which rather than having a profile similar to the rudder was much shallower in depth but longer say 2ft 9" wide and say 9" deep. There is a convenient "slot" to put this in and utilising a redundant stub at the forward end designed for an inboard prop shaft bearing.

    So I would have the same area added as the factory designed skegs but running along under the boat rear hull (flat in this area so would be like a shallow fin) ) rather than behind the rudder. The advantage of this over the factory built skegs would be that the skeg would not be vulnerable to ripping off and possibly breaching the hull in the event of hitting a rock or a hard grounding and the disadvantage would be that the opportunity to improve flow over the rudder skeg combination would be missed out on. The fabrication and fitting would also be substantially easier.

    I would love to hear any views on this. I am pretty conversant with wood/epoxy and laminating glass cloth so I am comfortable with the practicalities of doing this just unsure about the design aspect?

  2. #2
    Join Date
    Sep 2014
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    Too far north to be comfortable.
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    135

    Default Re: Skeg Design Query

    No, it wouldn't have the same effect. But it would surely be better than none at all. The best stabilizers, or rudders, or daggerboards... will be longer in the direction perpendicular to the water flow. The best ratio seeming to be near 3:1, meaning 3 times taller than wide.

    Sticking with 3:1 can make the appendage a little "flimsy" so most will flare out the topsides to meet the hull over a longer area. This also helps with shedding debris.

    I hope this helps.

    Paul.

    PS Which manufacturer puts a skeg "behind" the rudder?

    Paul.

  3. #3

    Default Re: Skeg Design Query

    PS Which manufacturer puts a skeg "behind" the rudder?

    Paul.[/QUOTE]

    Not very well described it is of course forward of the rudder but it is also behind the rudder if you are looking from astern!

  4. #4
    Join Date
    Sep 2014
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    Too far north to be comfortable.
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    135

    Default Re: Skeg Design Query

    haha, that's a good one. Ok, now it makes sense.

    Good luck with your skeg build, I hope you find a good compromise.

    Do you have kick up rudders? If not, a deeper skeg would help to protect your rudders, as well as improve directional stability.

    Cheers.
    Paul.

  5. #5

    Default Re: Skeg Design Query

    Thanks for your input Paul. The boat (Catalac 8M) originally had kick up rudders but these were rapidly replaced by fixed rudders on later models and with a skeg. When I got the boat 11 years ago the rudders were shot so I made new ones from 3x18mm marine ply glued together and sheathed with glass cloth and epoxy. A bit heavier than the factory (foam covered with GRP) but I suspect much stronger although I have no real evidence of this. Anyway the boat has been on a variety of drying and swinging mooring over the years and the rudders have been fine despite a certain amount of pounding on hard sand, particularly at Beaumaris. The rudders are attached to gudgeons/pintles mounted on the rear back lockers. In the event of a major impact with a rudder it would be probable that the rudder would rip out of the locker at the mountings (unless the rudder broke apart which may be aa good reason for not making the rudder too strong!) and this locker being separate from the main hull there would not be a sinking situation. However a skeg being torn off at the root could cause a hull breach, which is my concern. Strangely people with mono hulls with rudder and skeg do not seem to have the same concern about the result of a skeg being torn off and damage to the hull resulting. Perhaps the answere is to make the skeg weak by design so that it fails rather than being so strong that it can damage th hull?

  6. #6
    Join Date
    Sep 2014
    Location
    Too far north to be comfortable.
    Posts
    135

    Default Re: Skeg Design Query

    I guess I'm in the "stronger is better" camp. I built mine quite strong; they're primarily to protect the prop shaft and prop, but they are the deepest part of my sterns. I had them penetrate the hull and installed gussets on either side, to resist side loads, and impact loads. Certainly overbuilt, but we all have our quirks.

    Cheers.
    Paul.

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