dimensions for kick up rudder

Discussion in 'Fiberglass and Composite Boat Building' started by seanseward, Dec 15, 2013.

  1. seanseward
    Joined: Dec 2013
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    seanseward Junior Member

    The article on rudder and centerboards, your explanations and images are all helpful.

    Watson suggests redwood...wow I have some nice 2x10 old redwood out back...what is the opinion of my guiding experts?

    Watson appears to have a different number crunch than you, Par. He states that board requires reinforcement if chord thickness is at or less than 4% of unsupported span (length from waterline to tip of blade). My rudder will have 30" from waterline to tip of blade.

    1"/30" = .0333333333333333333333333 ....which is less than 4%

    This suggests that 1" board is too small and would require significant graphite reinforcement.

    1.5"/30" = .05 = 5%...therefore no reinforcement.

    Your photo of router confirms my fantasy/imagined thoughts (fed by your earlier descriptions) of running a template male "bridge" out to front and back edges.

    Running tape between laminate on front and back edge is also an interesting idea offered by Watson.

    Thanks to you guys I feel like I got the concepts down and I can see how the options begin to multiply with creative thought....tape laid into skill saw cut trough along center-line of bottom edge with strong epoxy mud in final 1/4" diameter trough....then cut off the bottom forward corner, seal connecting faces and reconnect to create water membrane to prevent suck up...etc.
     
  2. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Is that Wellingtonia? Now that is light, if fully dry, and personally I would definitely sheath it and use 28-30mm (1 1/8" - 1 3/16") thickness. If you go this route be very careful attaching uphaul/downhaul system ropes, it may require a little local reinforcing. Some local Wellingtonia (in UK) was somewhere between WR Cedar and Balsa for weight and the yard cutting and selling it said it got used for kitchen carcassing!. More use on a light racing blade. Although some references claim it to be the same density as Sitka, my own experience is that it is much lighter, I would estimate (after handling quite a few pieces in a yard) it to be around 300 -350Kg per cubic meter. Of course that may be an aberration, you just have to 'feel' what you actually have. Drier the better for encapsulation with epoxy.

    With your boat I would feel safer with the D Fir, it will take many years of use without any problem. If properly edge glued it would take massive loads before breaking, even at 1", so just thicken it to 1 1/8" (or 1 3/16") if you feel more comfortable. The D Fir will be much more tolerant too of any serious damage. It will not suddenly break because of a small ding where water has penetrated, and of course you can repair it later. If you follow PAR's suggestion of a 0012 section (good choice) you should also have enough 'meat' in the blade to maintain great stiffness. Go with the D Fir especially if you intend any serious sea sailing, it won't let you down.

    PAR and others have a good 'feel' for foil thickness and chord section. He knows the Neptune better than I, but it is not a performance craft. Some boats shorter than the Neptune can exert greater loads so need thicker foils. It depends on the 'operational condition'. Personally I expect small boats to take 40mph wind without stuff breaking. After that I don't expect to be out there or racing will be abandoned. If cruising, you can get yourself to shelter or reef. That 'feel' is about experience and learning, PAR has plenty of that and you should trust his instinct on this one. Especially if thickened at the stock end, no 1" D Fir blade would break. Hell, I'd even taper it(towards bottom), but advise you to keep it even thickness on your first one.

    Let the racing boys push the limits....I always note blades that break and examine them carefully. Last one was carbon/foam core ($800 foil) rudder on a Merlin Rocket (14') in 32Kn off Hayling Island. My own unsheathed Sitka blade on a slightly less powerful boat handled the same conditions, no problem and that blade was 21 years old!.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I agree with Watson on the 4% rule, with low density species, but good looking 1/4 sawn or rift Douglas fir or SYP will be stiff enough at 1", without reinforcement on this boat, especially if the portion inside the rudderhead (between the cheeks) is fattened to 1.5" and transitions to 1" as it exits the bottom of the cheeks.. This boat's rudder isn't highly loaded and could live with a simple plywood blade or even an aluminum plate. If I remember the original version of this correctly (Newport 16, built from a cut up Whittaker or Capital mold), it had some rudder issues, mostly related to not enough area (I think). This one has about 30% more area then that of the Newport's. I'm pretty sure the originals had just an aluminum plate blade.

    It's pretty easy to over think and/or engineer this thing. This is one reason I like inert materials for this sort of thing, you cut it and forget it. Yet another blade method might be a plywood core, say 1/2", cut to the plan form, then covered with foam, that gets shaped to a foils section. Over this you can bond an epoxy/cloth skin as a sort of pre-peg. By this I mean you make a flat sheet of a couple of layers of biax, maybe on a Formica counter top and when dry, but still in the green stage, this is wrapped around the foam/plywood sandwich, of course on a bed of fresh goo. A little trimming and you're done. You could substitute the plywood core HDPE or aluminum plate, making it fully inert. The idea with this method is to have the core (plywood, aluminum, HDPE, etc.) participate with some of the stiffness, so you don't need as much laminate on the outside.

    I wouldn't go crazy with it myself, just focus on a nice symmetrical set of shapes and splash this old gal. On a boat like this, you could just double plank the blade from 1/4 sawn solid stock, say1x4's edge glued with staggered seams, in a two layer laminate. Varnish up after shaping and call it pertty.
     
  4. seanseward
    Joined: Dec 2013
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    seanseward Junior Member

    A regular summer afternoon on the San Francisco Bay is usually a steady 20 knots gusting up to 25. On occasion during the summer it can blow 25 knots with gusts up to 30 and 35. No long ocean swells unless you are out near the Golden Gate. Instead the water can be choppy with some intense short and steep swells...usually funner when on a sailboard or light, go fast, dingy. I will sail the boat in these conditions and I want to be extra sure that it will withstand the stress.

    Leaving the cheeks at the 1 1/2" spread will require the design modifications that I was excited to avoid with the 1" board; inlaying the arms of the pintles into the inner walls of the 1/4" aluminum cheeks with counter sunk allen head screw/bolts (a la Laser/Bahia design). This brings us full circle back to the 1 1/2". I will ponder the blade width...fairing a blade that drops from 1 1/2 " above water to 1" in the water raises more big design questions that are over my head....I drift back to the easier 1 1/2" d fir blade head to tail with no sheathing.....4-6 coats of epoxy. My heart is starting to warm just thinking about it. Pack some peanut butter around the vulnerable edges and presto!

    I will also look into a 1" d fir blade from top to bottom with some longitudinal inset epoxied carbon ribs...(I saw some guy do it on a you tub video)...similar to Watson's graphite reinforcement......sounds expensive. One continued gut response is "cross grain strength" which Watson suggests to reinforce with the one layer of sheathing. I just need to be extra sure that it will endure conditions stated above.

    Something I embrace and will pursue to "attempted perfection" in all options is the NACA00xx foil shape. This will take some technique development to fair it well and borrowing a few tools from my brother, but I am interested in the challenge. I may have to reload my old CAD program I used when playing in the world of architecture...

    I'm excited about the work...I may even throw in an extra redwood board depending on how much epoxy 6 coats takes. Switch them out ...more room for error and learning.

    thanks to your sharing of knowledge I feel like I see my options and will have fun doing it.

    Smooth sailing to you all ...... I'll probably have a new bag of questions when I actually do the work... I hope you guys are around to answer my questions...until then.....if you want to go fast, a gentle luff is better than stalling!
     

  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The performance difference between a 1" and 1.5" board will be very difficult to notice on your boat. If the 1.5" board solves a few issues for you, then this is the reasonable route to take.
     
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