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Thread: Rudder and centreboard construction

  1. #36
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by debenriver View Post
    I would simply radius the leading edge and taper the final 100mm
    That's got my vote, but if had the glass handy I sheath it too.
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  2. #37
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    Default Re: Rudder and centreboard construction

    I've never had any problem with wood movement on stave constructed foils. However, if you have concerns with stability of the wood over time you might consider a couple of "belt and suspenders" approaches. The first is using several bronze drifts, the second is routing in a spline around the circumference. I use a slotting bit set to it's maximum depth (about 1") and create about a 1/4" width slot that is filled with thickened epoxy. The bottom of the boards are much more resistant to abrasion and dings with the spline. You can see the spline on this partially finished rudder.

  3. #38
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    Quote Originally Posted by bheys View Post
    I've never had any problem with wood movement on stave constructed foils. However, if you have concerns with stability of the wood over time you might consider a couple of "belt and suspenders" approaches. The first is using several bronze drifts, the second is routing in a spline around the circumference. I use a slotting bit set to it's maximum depth (about 1") and create about a 1/4" width slot that is filled with thickened epoxy. The bottom of the boards are much more resistant to abrasion and dings with the spline. You can see the spline on this partially finished rudder.


    That's clever. Probably above my (limited) experience level though!!


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  4. #39
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    Default Re: Rudder and centreboard construction

    Thanks, Mik -

    So what that Pollock math boils down to - for a simple builder, who's no ambition to be an engineer - is that we can do a thinner section, leave the middle bits flat/parallel, do a NACA-type nose and tail... and end up with most of the performance of a full-NACA foil. And that taking the time to do that bit of shaping will pay dividends - compared to a flat plate, or flat plate with a radiused nose shape. I've seen this several times, as people have replace their dead simple flat-plate first attempts with a Storer/Pollock semi-NACA foil (can we call it that?), and seen even beginning sailors notice the difference.
    David G
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  5. #40
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by David G View Post
    Thanks, Mik -

    So what that Pollock math boils down to - for a simple builder, who's no ambition to be an engineer - is that we can do a thinner section, leave the middle bits flat/parallel, do a NACA-type nose and tail... and end up with most of the performance of a full-NACA foil. And that taking the time to do that bit of shaping will pay dividends - compared to a flat plate, or flat plate with a radiused nose shape. I've seen this several times, as people have replace their dead simple flat-plate first attempts with a Storer/Pollock semi-NACA foil (can we call it that?), and seen even beginning sailors notice the difference.
    Experience in the NS14 class which is a popular and technical two person body hiked class in Australia is interesting.

    For club racing there is a handicap system which adjusts by a moving average system that operates over time to apply an appropriate time handicap to every boat.

    So it is really noticeable when boats suddenly improve.

    The existing "state of the art" in the early seventies is that we would read articles about different sections, rough them out with flat tangential surfaces and then match up the surfaces planed into the blanks.

    This is much, much better than just "rounding off the nose and tapering the tail". They were highly polished and finished.

    And we found that they were slow when the next evolution happened.

    Classic LOOKS GOOD vs IS GOOD experience.

    When carefully made templated foils started appearing in the class they were good for a 4 minutes in a normal race compared to the existing state of the art.

    That might not sound like much but it works out to around 10 percent difference in the upwind part of the race.

    To get a bit of a feeling for it ... a nice sailboat might do about 10 feet a second upwind in moderate conditions

    Ten percent works out to an extra foot every second.

    If you think of what an extra foot a second looks like ... it is not vague at all.

    Ten seconds, ten feet. Minute, sixty feet.

    And that is comparing something that looks pretty good with a templated foil. For a roughly shaped and finished centreboard it is much more.

    So if you want to make your boat go ... spend the extra half day using templates to make an accurate centreboard. And it will pay off every time you go sailing for the life of the boat.

    And the boat will feel completely different .. which is what I found when I made the switch.

    MIK
    Last edited by Boatmik; 05-15-2017 at 08:20 PM.

  6. #41
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    Default Re: Rudder and centreboard construction

    Thanks for the numbers MIK. My feeling has always been that the hull has a given amount of drag and putting a foil in the water can only add to it,so the less drag you add-the less harm you do to the performance.Its got a lot easier now with online foil analysis or freeware that we can play with at home to arrive at a more efficient solution.Its well worth remembering that even those of us who profess not to be interested in performance will have our sailing range extended if we can get to places a bit more rapidly.

  7. #42
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    So I've been talking to my local timber merchant and they can get me 20mm sapele in the dimensions I need without completely breaking the bank (does 90 sound a lot for wood for a centreboard and rudder?), but the plans say 22mm... Do you think the missing 2mm will matter? (Assuming I can glue the blank up accurately and shape it without losing any more width...)

    Thanks


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  8. #43
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    Default Re: Rudder and centreboard construction

    If you glass the 20mm,you won't be too far from 22mm and it will be stiffer than just the wood.

  9. #44
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    Quote Originally Posted by John Meachen View Post
    If you glass the 20mm,you won't be too far from 22mm and it will be stiffer than just the wood.


    Ok cool. I wondered about that. What would I use to glass it? (Glass, I know, but I'm a beginner!!)

    Thanks


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  10. #45
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    Default Re: Rudder and centreboard construction

    My choice would be a plain weave cloth of 180 to 300 grams/sq. metre.The heavier the cloth,the less willing it is likely to be to wrap round tight curves.Its a pretty unpleasant job that pays dividends for stiffness and durability.I prefer to paint the foils as I have mentioned on this forum often.

  11. #46
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    Quote Originally Posted by John Meachen View Post
    My choice would be a plain weave cloth of 180 to 300 grams/sq. metre.The heavier the cloth,the less willing it is likely to be to wrap round tight curves.Its a pretty unpleasant job that pays dividends for stiffness and durability.I prefer to paint the foils as I have mentioned on this forum often.


    Ok, thanks John. Sounds like fun...


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  12. #47
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    Default Re: Rudder and centreboard construction

    I think a lot is made of glass and resin, its not that bad.
    wet the timber with epoxy especially the end grain, it may need 2 coats in quick succession. Lay on the cloth and wet that out with resin. Use some kind of squeegee to wet the glass, let the epoxy harden for a few hours. After that i use small 100mm rollers to put on a couple more coats to fill the weave. Leave time for the previous coat harden, but still be somewhat uncured - this is to get a chemical bond between each coat.
    let that harden completely and sand lightly, and recoat with resin - if a perfect smooth finish is important.
    Epoxy, its character building.
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  13. #48
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    Default Re: Rudder and centreboard construction

    Hi
    You can use a 3/4" Okume, and shape/glass with 10 oz cloth and flow coat epoxy finish...should get you to 7/8"


    Quote Originally Posted by Racundra View Post
    Hi all

    My next job (in the next few weeks I hope) in my Oughtred Guillemot build is the rudder and centreboard.

    The plans call for 7/8" (22mm) thick blades. It seems to me that using 1" ply is a non-starter because I'd have to remove the outer layers to get it to the right thickness and then it'd have no strength.

    So, should I be laminating these and coating them with a fibreglass/epoxy coat? What's the best way forward?

    Thanks in advance!

    Noel


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  14. #49
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    Default Re: Rudder and centreboard construction

    Australian racing dinghies have centred around staved construction with a single layer of 4 or 6oz glass as standard construction.

    This produces foils that are extremely reliable over lots of years with only minimal upkeep. It has been the standard method since the middle '70s when we were still using polyester resin, which didn't always stick to the timber. Epoxy availability solved that problem by the end of '70s.

    I'm not talking simple majorities ... I mean 99.9 percent of racing dinghy foils across a huge range of classes, from monos, cats and even the original few years of the full foiling moth development.

    Without the glass foils can be prone to the timber splitting along the grain. Which is the main reason for a single layer of glass becoming the standard.

    Sometime splitting can be from use, but more often its from dropping the foils or some hard knock from the side when not sailing.

    It also allows the use of lighter timbers like Cedar or Paulownia with a strip of slightly harder stuff on the trailing and perhaps the leading edges. Don't go afor rock hard timbers on leading and trailing edges as it makes fairing the foil more difficult.

    Until recently even carbon foils were timber staved. And the carbon did the job of the glass in preventing splitting along the grain.

    Best wishes
    MIK

  15. #50
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by John Meachen View Post
    Thanks for the numbers MIK. My feeling has always been that the hull has a given amount of drag and putting a foil in the water can only add to it,so the less drag you add-the less harm you do to the performance.Its got a lot easier now with online foil analysis or freeware that we can play with at home to arrive at a more efficient solution.Its well worth remembering that even those of us who profess not to be interested in performance will have our sailing range extended if we can get to places a bit more rapidly.
    all things being equal it seems that eventhough sail boats sail on both sides of their foils a foiled shape still reduces drag... the question becomes a bit more dicey though when frontal area of a body moving through a liquid is considered.... often times a plate (flat sided with fairing for a short percentage of Chord at nosed and tail) especially a steel or aluminum plate has a much smaller frontal area and displaces far less water than wooden and fiber glass foil.

    if an 16' sailboat is carrying a metal plate board the width of the board might be 1/4 inch or 3/8ths.... where that same boat might have a wood and glass foil an inch thick! a factor of 4 times thicker, meaning the foil must displace 4 cubed times as much water as the plate when traveling at high speed through the water column.

  16. #51
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    Quote Originally Posted by Boatmik View Post
    Australian racing dinghies have centred around staved construction with a single layer of 4 or 6oz glass as standard construction.



    This produces foils that are extremely reliable over lots of years with only minimal upkeep. It has been the standard method since the middle '70s when we were still using polyester resin, which didn't always stick to the timber. Epoxy availability solved that problem by the end of '70s.



    I'm not talking simple majorities ... I mean 99.9 percent of racing dinghy foils across a huge range of classes, from monos, cats and even the original few years of the full foiling moth development.



    Without the glass foils can be prone to the timber splitting along the grain. Which is the main reason for a single layer of glass becoming the standard.



    Sometime splitting can be from use, but more often its from dropping the foils or some hard knock from the side when not sailing.



    It also allows the use of lighter timbers like Cedar or Paulownia with a strip of slightly harder stuff on the trailing and perhaps the leading edges. Don't go afor rock hard timbers on leading and trailing edges as it makes fairing the foil more difficult.



    Until recently even carbon foils were timber staved. And the carbon did the job of the glass in preventing splitting along the grain.



    Best wishes

    MIK


    Hi Mik

    So should I be going for, say, 1" or 2" wide strips of 1" thick softwood (Douglas fir?) for these, and glassing them afterwards? I was about to go down the sapele route....

    Cheers

    Noel


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  17. #52
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    Default Re: Rudder and centreboard construction

    Douglas Fir is fine for the job. Within reason it's not particularly important what the timber is as long as it's reasonably clean and clear. Big box store DF lumber, carefully selected, is often good for the job if you can re-saw and plane it.

    Here's the centreboard from our Design 077 a 15' Pocket Cruiser OK rather a different sailboat and not a raceboat, but the principle is the same. This particular board is a full foil because it is thicker, and it has a 9mm ply core with DF staves each side. The rectangular profiled part remains in the case.

    The second photo shows the finished board, epoxy/graphite coated, with the uphaul and downhaul lines attached. The ply core will prevent splitting etc. If you are just going to use just staves, then it is best to glass them as many posts have detailed.

    You do need to start a bit thicker, even if you are going to glass cloth, because you will inevitably use some thickness in the cleaning up and fairing process. 25mm should do it OK.

    Cheers -- George



    Last edited by debenriver; 05-25-2017 at 07:09 AM.
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  18. #53
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by Daniel Noyes View Post
    all things being equal it seems that eventhough sail boats sail on both sides of their foils a foiled shape still reduces drag... the question becomes a bit more dicey though when frontal area of a body moving through a liquid is considered.... often times a plate (flat sided with fairing for a short percentage of Chord at nosed and tail) especially a steel or aluminum plate has a much smaller frontal area and displaces far less water than wooden and fiber glass foil.

    if an 16' sailboat is carrying a metal plate board the width of the board might be 1/4 inch or 3/8ths.... where that same boat might have a wood and glass foil an inch thick! a factor of 4 times thicker, meaning the foil must displace 4 cubed times as much water as the plate when traveling at high speed through the water column.
    I think your math is a bit off here. A shaped foil that is 1-inch thick can only displace a max of 4 times the volume of the same flat plate board 1/4-inch thick. All other dimensions (length and width) are the same. In addition, the shaped foil is not 1-inch thick for its whole width; assume it is on average 3/4-inch thick, it displaces only three times as much as the flat plate, but has much less drag from turbulence as the flat plate, and is far less likely to stall.

  19. #54
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    Default Re: Rudder and centreboard construction

    Thanks for all the input everyone.

    Pivot holes... Do I drill them before or after glassing??

    Thanks

    Noel

  20. #55
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    Default Re: Rudder and centreboard construction

    I drilled mine oversize before glassing, then filled with epoxy to make a bushing for the pin, and then glassed. I drilled the final hole after glassing when the epoxy bushing was already in place. That seems like the neatest sequence with least clean-up needed to me.

    Edit to say: Nope, I think I glassed first, then drilled. That makes more sense.

    Tom
    Last edited by WI-Tom; 05-25-2017 at 02:22 PM.
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  21. #56
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by WI-Tom View Post
    I drilled mine oversize before glassing, then filled with epoxy to make a bushing for the pin, and then glassed. I drilled the final hole after glassing when the epoxy bushing was already in place. That seems like the neatest sequence with least clean-up needed to me.

    Tom
    Ah,OK.
    Did you 'wrap' glass into the oversized hole? (How much oversize did you go?)
    Thanks

  22. #57
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    Default Re: Rudder and centreboard construction

    No need to wrap glass into the hole--the oversized hole will be totally filled with epoxy. Then when you drill the smaller hole for the pin, that hole goes through epoxy, and no wood is exposed at all. The epoxy also makes a nice tough bushing for the pin with no wear on wood surfaces.

    I think I probably drilled a 2" diameter hole in my rudder blade and centerboard with a hole saw, filled it with thickened epoxy, and cleaned it up after it cured. Then a 1/4" hole for the pin in the center of the epoxy.

    But now that I'm thinking back, I almost certainly glassed the board first, then drilled the oversized hole--not the other way around the way I first posted. Glassing first, then drilling, makes the glass lies cleanly right up to the edge of the hole and doesn't need any more work.

    Tom
    Last edited by WI-Tom; 05-25-2017 at 02:22 PM.
    You don't have to be prepared as long as you're willing to suffer the consequences.

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  23. #58
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    Quote Originally Posted by WI-Tom View Post
    No need to wrap glass into the hole--the oversized hole will be totally filled with epoxy. Then when you drill the smaller hole for the pin, that hole goes through epoxy, and no wood is exposed at all. The epoxy also makes a nice tough bushing for the pin with no wear on wood surfaces.

    I think I probably drilled a 2" diameter hole in my rudder blade and centerboard with a hole saw, filled it with thickened epoxy, and cleaned it up after it cured. Then a 1/4" hole for the pin in the center of the epoxy.

    Tom


    Thanks Tom, that's good info.
    Will hopefully get onto this next week...


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  24. #59
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by Daniel Noyes View Post
    all things being equal it seems that eventhough sail boats sail on both sides of their foils a foiled shape still reduces drag... the question becomes a bit more dicey though when frontal area of a body moving through a liquid is considered.... often times a plate (flat sided with fairing for a short percentage of Chord at nosed and tail) especially a steel or aluminum plate has a much smaller frontal area and displaces far less water than wooden and fiber glass foil.

    if an 16' sailboat is carrying a metal plate board the width of the board might be 1/4 inch or 3/8ths.... where that same boat might have a wood and glass foil an inch thick! a factor of 4 times thicker, meaning the foil must displace 4 cubed times as much water as the plate when traveling at high speed through the water column.

    Although it might be counterintuitive,the volume of water displaced isn't the sole determinant of drag.the following illustration is quite well known and shows a round section next to a NACA section with the same drag.




    On a different note,now that we have got round to discussing pivot holes and bushes,what are the details of this particular vessel?

  25. #60
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    Quote Originally Posted by John Meachen View Post
    Although it might be counterintuitive,the volume of water displaced isn't the sole determinant of drag.the following illustration is quite well known and shows a round section next to a NACA section with the same drag.




    On a different note,now that we have got round to discussing pivot holes and bushes,what are the details of this particular vessel?


    It's non-existent at the moment! I'm working on small items first, so I can fit everything in a small build space and so I'm not horribly frustrated when the hull is done!

    I have a yard and boom (the jaws aren't attached yet - something else I'm still thinking about...) and I have the wood for the mast. Hopefully it'll become a mast next week.

    The boat will be an Oughtred Guillemot.... when there's something to post, I'll post!

    😁


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