Rudder assembly

Discussion in 'Fiberglass and Composite Boat Building' started by Karl01, Jan 26, 2014.

  1. Karl01
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    Karl01 Junior Member

    hi I have a 26 ft trimaran (no I'm not in the wrong forum) I'm looking for advice on building anew rudder and or rudder box. I need to make it as strong and as light as possible, question. Is the most suitable material fiberglass ? I've used chop strand in the past but it seemed a bulky material would I be better using woven material. The other question is for the rudder itself if I were to make the rudder in 18 mm ply shape the foil can i cover in fibreglass . Does anyone have any links or pictures to help with this project many thanks karl.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Plywood is a poor choice for appendages, except in very small or disposable boats. Solid wood, usually sheathed is common, though I prefer inert materials, such as alloys and plastics.

    You're correct in the CSM isn't the best choice. In fact is is as you've observed and a bulking malarial, used to thicken laminates quickly. It has little strength, unless quite thick (heavy). Biax and regular cloth are what you want.

    WestSystem.com has a good PDF on building strip planked and foam foils (boards) and is worth a look see. Also check with SystemThree.com for information.

    Do a search here with the appropriate key words, as there are lots of previous threads on the subject (and related).
     
  3. Karl01
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    Karl01 Junior Member

    Rudder

    Hi parr. The original design has a rudder box that the actual wooden rudder drops through. If I were to use this configuration could I make a form from foam cover in fibre glass then remove to create a hollow box. How will it be strong enough to withstand forces front to back and side to side also have to mount pintails and keep height to a minimum thanks karl
     
  4. PAR
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    PAR Yacht Designer/Builder

    Your describing a cassette style of rudderhead, which is fairly common. As to the scantlings, well you'll have to work through some numbers or have this done for you. Of course, you could just take a guess, error on the side of slightly heavy, just in case you guessed too light.
     
  5. Karl01
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    Karl01 Junior Member

    Rudder

    Parr what I'm basically asking is is fibreglass alone strong enough to form the box if not does anyone know the best method of reinforcement keeping it light and not too bulky thanks karl
     
  6. messabout
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    messabout Senior Member

    I have made several rudders, daggerboards, and even some foils, that are stiff and light with a stress skinned method. If you have ever built model airplanes you will be familiar with the method.

    I start by laying up some GRP sheets. A waxed sheet of Formica type laminate is the base for the layup. That'll make a perfectly smooth side of the layup. Use a wood center spar, leading and trailing edge, and a few ribs. You get to select the foil shape that you want. Bend the glass sheet around the internals and epoxy in place. I have sometimes used a small stainless rod to make the leading edge less subject to damage. This is actually easier and quicker than the description might imply.

    The only problem is that these constructions will float which can sometimes be an annoyance. A slug of lead at the tip of the board will fix that problem but then it defeats the purpose of making the item light.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Yes, 'glass alone is strong enough to make your rudderhead (box, cassette, whatever) and the blade too, if desired. I have a 65' yacht made of all 'glass and I'm sure it's much more highly loaded then your rudderhead will be.

    You're asking for a design and appropriate scantlings, to which none currently exists. You can work out the math yourself, take a guess or get some help as the options I see you have. Maybe one does exist, what is the year, make and model of your multi?
     
  8. SukiSolo
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    SukiSolo Senior Member

    I would be concerned about 18mm thickness + glass for the rudder! Say 20-21mm, that is way too thin for a 26' tri, only suitable for an 11-12.5' mono dinghy. Have a look at some 24-26' mono rudders, transom hung ones, they will not be far away especially of the Sportboat type. As purely a personal guess (without seeing boat) somewhere around 45mm +/- would be more realistic. As PAR says you are looking for scantlings, what dimensions are the existing rudder? Presumably it still exists?

    Hollow Glass rudders/boards are common, even the Access dinghies have them with up to 43Kg lead inside at the bottom. However they are two part moulded and stuck together on edge flanges which are rather poor. Ideally the layup is bonded over a wider area at both ends or alternatively the last front end layup is put on after bonding the halves together. More work but more structurally sound. Good option if painting.

    For a one off, again personal, I'd just do a laminated light timber or foam core and wrap that, then sheath and paint.

    The cassette box isn't too hard to make. Good return flanges are important to stiffen it. The 49 er uses velcro (or similar) as an internal sleeve/slide face so the blade is not scratched. Some National 12s have similar arrangements, for their T foils. A slightly larger version would suffice for your requirements.
    Might be worth thinking about the clamping mechanism so it stays really solid if you are making long sea passages.
     
  9. Karl01
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    Karl01 Junior Member

    Regarding make size of boat Ect. The boat is a mid seventies telstar 26 trimaran the current rudder is approx 38 mm thick. 340 mm at widest point water level to tip 900mm. Mane from solid hardwood. When I said makeing a new rudder from 18 mm ply I was thinking more in terms of joining two pieces together giving 36 mm and wrap with woven mat. If required let a piece of 316 sis between the ply pieces.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Again, plywood is a poor choice, especially on this boat, even if of the appropriate thickness.

    [​IMG]

    In the mid 70's they made several model run changes. Do you have the MK I or MK II?.

    [​IMG]

    Is this the type of cassette you have?
     
  11. Karl01
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    Karl01 Junior Member

    Mk II. The cassette is similar to the one in the photo except the one I have is made entirely from stainless steel ( poorly) I should add.
     
  12. Karl01
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    Karl01 Junior Member

    Looking at the middle of the outline drawings it looks like my rudder is also wrong the drawing shows the rudder tip moving forward as it lowers I'm sure mine stays straight
     
  13. Karl01
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    Karl01 Junior Member

    If I'm to make a rudder box in fibre glass what material and method would be best. What would be the best way of securing the hinges to the new box
     
  14. SukiSolo
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    SukiSolo Senior Member

    Surprisingly they are not called hinges! They are rudder fittings, if you have a lower one (fastened to hull) with a longish pin it is a pintle. The upper one if it is a fitting with a hole is a gudgeon. Some systems use all gudgeon with a full length pin that drops through all four fixtures. I personally do not like this for a number of reasons, however it is quite popular. It is OK but ensure you have a retainer....I have known them come out whilst sailing...

    You can either glass in the fittings into the rudder cassette or bolt them to it later. Do not ignore alignment so if you glass them whilst making them do it after it is primarily manufactured and put it on the transom fittings. You will not get alignment easily afterwards. If you bolt them on later, again put it on the transom fittings and 'tack' them ensuring easy on/off before hard fixing. Note that trying to get recesses for bolt heads and access inside the cassette will be limited. You may need to pre fabricate room for them. If you glass in the fittings, my preference is a full mechanical key such as a few holes though the fitting side or grooves (semi circles) in the side cheeks to prevent the fitting from ever working loose.

    If Seasure do large enough fittings for your rudder, they would be a good choice.
     

  15. Karl01
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    Karl01 Junior Member

    Hi suki solo I did know the correct terms I just had a senior moment whilst rushing to ask the question before bed. (The senior moments are becoming a bit more frequent lately ) I have two gudgeons fitted to the hull with a 1 inch between the jaws. Where would I acquire the brackets to fit ? My gudgeons have 2loose pins that are secured with split pins. What's the best wood to make the tiller from ?
     
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