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Thread: Rudder and centreboard construction

  1. #1
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    Default Rudder and centreboard construction

    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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    Default Re: Rudder and centreboard construction

    Lots of options, and many, many threads on this forum. If your boards are going to have some shape, try a search using "foil" and you should get a long night's reading and education.

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    Quote Originally Posted by paxtonm View Post
    Lots of options, and many, many threads on this forum. If your boards are going to have some shape, try a search using "foil" and you should get a long night's reading and education.


    Thanks for that. I'll take a look.


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    Default Re: Rudder and centreboard construction

    You are quite correct in your original post that skimming down one inch ply is not a good idea.In fact ply generally is a poor choice for foils,I have seen literally dozens break because the ply only has a proportion of the veneers running in the right direction.A laminated blank is a much better idea and not that difficult to do.Is there an aspect of laminating a foil that causes you concern?

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    Quote Originally Posted by John Meachen View Post
    You are quite correct in your original post that skimming down one inch ply is not a good idea.In fact ply generally is a poor choice for foils,I have seen literally dozens break because the ply only has a proportion of the veneers running in the right direction.A laminated blank is a much better idea and not that difficult to do.Is there an aspect of laminating a foil that causes you concern?


    Thanks John.

    I don't think I'm worried about laminating foils. (How hard can it be?!! &#128528 I am learning as I go along. I'm mid gaff jaw shaping at the moment, and they're laminated, albeit on a smaller scale!

    What wood should I go for?


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    Default Re: Rudder and centreboard construction

    I would use a 6mm ply core and laminate 8mm x 50mm Douglas Fir strips each side – perhaps in practice 10mm thick strips to allow for cleaning off, especially if you are going to shape up as a foil.

    But at 22mm it's probably not really worth shaping it up a foil as it is too thin to be any better than a flat plate. Really centreboards and rudders are better something like a NACA 0012 - NACA 0018. I would just radius the leading edge and taper the final 100mm or so of the trailing edge to, say, 6mm. Foils that don't follow known principles are often worse than no foil at all.

    Three or four coats of epoxy, with graphite in the final coat, should be good. You can incorporate a light glass cloth in the second coat if you wish but woven cloth doesn't add any strength to speak of.

    Cheers -- George
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    Quote Originally Posted by debenriver View Post
    I would use a 6mm ply core and laminate 8mm x 50mm Douglas Fir strips each side – perhaps in practice 10mm thick strips to allow for cleaning off, especially if you are going to shape up as a foil.

    But at 22mm it's probably not really worth shaping it up a foil as it is too thin to be any better than a flat plate. Really centreboards and rudders are better something like a NACA 0012 - NACA 0018. I would just radius the leading edge and taper the final 100mm or so of the trailing edge to, say, 6mm. Foils that don't follow known principles are often worse than no foil at all.

    Three or four coats of epoxy, with graphite in the final coat, should be good. You can incorporate a light glass cloth in the second coat if you wish but woven cloth doesn't add any strength to speak of.

    Cheers -- George


    Hi George

    Thanks for that advice - this forum is great. Like having a boatbuilding mentor with you. 😏


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    Default Re: Rudder and centreboard construction

    Racundra when searching this forum use google and enter "wooden boat forum" along with what you're looking for.

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    Quote Originally Posted by andrewpatrol View Post
    Racundra when searching this forum use google and enter "wooden boat forum" along with what you're looking for.


    Ok, cheers Andrew. (I use Tapatalk on my phone most of the time, which has a ropey search function....)


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    Quote Originally Posted by Racundra View Post
    Ok, cheers Andrew. (I use Tapatalk on my phone most of the time, which has a ropey search function....)


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    Oh, are you saying I should keep researching, and not go with a plywood core approach??


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    Default Re: Rudder and centreboard construction

    No just saying about searching. Just for your info I used to have a rudder blade from 18mm ply, rounded front edge and a trailing to about 2 or 3 mm but I had to keep repairing it from getting eaten by outboard prop, otherwise seemed to work well.

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    Quote Originally Posted by andrewpatrol View Post
    No just saying about searching. Just for your info I used to have a rudder blade from 18mm ply, rounded front edge and a trailing to about 2 or 3 mm but I had to keep repairing it from getting eaten by outboard prop, otherwise seemed to work well.


    When you say it was getting eaten, do you mean the turbulence was damaging it, or the prop was actually chomping into it?!! 😮


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    Default Re: Rudder and centreboard construction

    Yeh prop was eating it. I had a linkage between main helm and outboard tiller but it'd jump off sometimes and also when helm was hard over the rudder blade would touch prop even with linkage. It was quite a big rudder blade. Hartley TS16.

  14. #14

    Default Re: Rudder and centreboard construction

    My understanding (may not be correct) is that plywood was generally Not a good choice for a centerboard because it isn't very stiff. I am doing a NACA0012 board using 35 x 35 mm douglas fir strips alternating the grain direction and all epoxied up. Once I shape the foil I will cover with a 6 oz cloth.
    ripping and gluing up the douglas fir strips was super easy and obviously could be used for your 22 mm board.

    After initial glue up:


    After initial foil shaping:

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    Quote Originally Posted by galleywench View Post
    My understanding (may not be correct) is that plywood was generally Not a good choice for a centerboard because it isn't very stiff. I am doing a NACA0012 board using 35 x 35 mm douglas fir strips alternating the grain direction and all epoxied up. Once I shape the foil I will cover with a 6 oz cloth.
    ripping and gluing up the douglas fir strips was super easy and obviously could be used for your 22 mm board.

    After initial glue up:


    After initial foil shaping:


    That looks good! I have read the Gougeon article on building foils and think it might be the way to go too...

    Thanks for your input.


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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by galleywench View Post
    My understanding (may not be correct) is that plywood was generally Not a good choice for a centerboard because it isn't very stiff. I am doing a NACA0012 board using 35 x 35 mm douglas fir strips alternating the grain direction and all epoxied up. Once I shape the foil I will cover with a 6 oz cloth.
    ripping and gluing up the douglas fir strips was super easy and obviously could be used for your 22 mm board.

    After initial glue up:


    After initial foil shaping:
    This.

    Cut the strips oversize and let them sit for a couple or three days. Cutting up a board relieves tension in the board that allows the strips to move, sometimes to large degree. Discard any strips that warp too much. Glue up the good ones with a rough alignment. I would also flip every alternate strip end for end. Flatten the board with a hand plane ( or huge jointer if ya got one ), send the other side through a planer, then shape the foil. I made a leeboard and rudder from sapele this way and covered them with glass. They have remained dead flat for over 15 years.

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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by galleywench View Post
    My understanding (may not be correct) is that plywood was generally Not a good choice for a centerboard because it isn't very stiff. I am doing a NACA0012 board using 35 x 35 mm douglas fir strips alternating the grain direction and all epoxied up. Once I shape the foil I will cover with a 6 oz cloth.
    ripping and gluing up the douglas fir strips was super easy and obviously could be used for your 22 mm board.
    The thing with a 22mm board is that the chord length for a NACA 0012 foil can only be 183mm. Even for a 22mm NACA 0009 foil, the chord length will only be 244mm. For a 35mm thick NACA 0012 foil the chord length should be 292mm. Once you depart much from the correct numbers for a foil you can easily make a foil that is less efficient than a simple flat plate.

    Generally rudders with their larger angles of attack want to be a fairly fat foil (like NACA 0018) so they don't stall so easily. A centreboard, with is narrower angle of attack can reasonably be a thinner foil (NACA 0009 to NACA 0012).

    Woven glass cloth does add some abrasion resistance – not significant strength however. But the board should be adequately strong at it's designed thickness of 22mm anyway.

    Cheers -- George
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    A C Grayling

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    Default Re: Rudder and centreboard construction

    Seems to me that a piece of 3/4 (19mm) BS 1088 hydrotek meranti with 2 layers of 6 oz. glass in epoxy would be approximately indestructible, besides being absolutely stable.

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    Default Re: Rudder and centreboard construction

    I pretty much always use staved construction for foils. Plywood really is a bad choice for a variety of reasons.

    I use lighter, rot-resistant species for the center staves. All but one at the nose and one at the tail. Western Red Cedar is my normal choice. For those fore and aft staves, I use something denser, harder, and tougher. Dark Red Meranti, Iroko, Black Locust, etc. That allows the foil to better stand up to the abuse from running into things, and the delicacy of a tapered aft edge. Then I glass and paint.

    Pretty much everything I've learned about foils, and a bunch of careful engineering thinking from Mik is available by spending $20 for plans for an Oz Racer. The instruction booklet that comes with... is a mini-course in beginning boatbuilding. And it includes specific instructions on the best and quickest way to make effective foils.
    David G
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    Default Re: Rudder and centreboard construction

    I would seldom disagree with debenriver,but on the subject of glass sheathing foils I will do so.I hate the job of glass sheathing and subsequent fairing and polishing and wouldn't do it if it didn't add a fair bit of strength and protection.The glass is performing the same function as the skins on a strip built canoe or a sandwich composite panel and in the direction that the fibres lay.

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    Default Re: Rudder and centreboard construction

    Do you have Ian's book about clinker plywood boat building? Lots of good advice for his boats and similar. I think he would recommend a solid wood foil, from a single piece of good stock or a lamination.

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    Default Re: Rudder and centreboard construction

    Would a steel CB and rudder be an option? The CB could be 1/2 or 3/8 thick and with battens on either side to keep it from rattling around in the trunk. The rudder would be maybe 1/4 th inch thick.

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    Default Re: Rudder and centreboard construction

    Considering that there are a lot of fiberglass foils with foam cores, I have trouble believing that a fiberglassed plywood one is going to be much a failure risk. Plywood foils break for the same reason plywood boats delaminate and rot. Cheap, simple materials attract people who are typically more interested in kludging something together vs. making it properly. Marine plywood is stable, and makes for a quick to build, easy to shape foil, that when glassed, isn't going to break. Remember that people make hollow masts, because most of the strength comes in the outer layers. The failure mode for plywood comes when the foil is pressed into a sharp edge on the centerboard or daggerboard case. This can crush the fiberglass and outer layer of the ply, and then you really have lost a lot of strength. All it takes to avoid this is tapering the edge of the case to a slight angle.

    That being said, since Guillemot's centerboard is a swinging foil, it will be easy and cheap to make out of solid wood. I used plywood, as per the plans for my build, but if I was building Guillemot I would make one like in post #14.

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    Default Re: Rudder and centreboard construction

    Half the wood in plywood is going the wrong way, doin nuttin.

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    There is clearly more than one way to skin this particular cat!
    I'm inclined to go with what galleywench suggested. I think Iain suggests plywood in his book, but how do you make it to 22mm?? (The plans also show minimal profiling...)


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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by debenriver View Post
    The thing with a 22mm board is that the chord length for a NACA 0012 foil can only be 183mm. Even for a 22mm NACA 0009 foil, the chord length will only be 244mm. For a 35mm thick NACA 0012 foil the chord length should be 292mm. Once you depart much from the correct numbers for a foil you can easily make a foil that is less efficient than a simple flat plate.
    .....
    Cheers -- George
    Just to be clear, I wasn't suggesting that Racundra attempt to get a NACA0012 foil out of a 22 mm wide board; 183 mm is a pretty short chord indeed. I was trying to focus on the douglas fir stave construction and was just saying what I had done to get a NACA0012 foil for my particular build.

    I used the Gougeon Brothers article 000-448 for guidance on how to arrange the cut staves prior to gluing up. Jamestown distributors has it online here:
    https://www.jamestowndistributors.co...ds_000_448.pdf

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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by galleywench View Post
    Just to be clear, I wasn't suggesting that Racundra attempt to get a NACA0012 foil out of a 22 mm wide board; 183 mm is a pretty short chord indeed. I was trying to focus on the douglas fir stave construction and was just saying what I had done to get a NACA0012 foil for my particular build.

    I used the Gougeon Brothers article 000-448 for guidance on how to arrange the cut staves prior to gluing up. Jamestown distributors has it online here:
    https://www.jamestowndistributors.co...ds_000_448.pdf
    I wasn't thinking you were ... I didn't express myself very clearly. What I was really saying is that it probably isn't worth making a foil from a 22mm thick board. Though for a centreboard where the angle of attack is generally low, a NACA 0009 would work and that the chord length would be 244mm – I don't know what the actual chord length of this centreboard is ...

    For a rudder blade you need a fatter foil and you can't really get this out of a 22mm thick blade.

    Sorry for any confusion. George
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by John Meachen View Post
    I would seldom disagree with debenriver,but on the subject of glass sheathing foils I will do so.I hate the job of glass sheathing and subsequent fairing and polishing and wouldn't do it if it didn't add a fair bit of strength and protection.The glass is performing the same function as the skins on a strip built canoe or a sandwich composite panel and in the direction that the fibres lay.
    I was thinking of quite light woven glass cloth – like 200g (6oz). With this, just like plywood, only half the fibres are going in a useful direction and there is so little glass in comparison to the timber, that it doesn't really add a lot to the strength; it does however add abrasion resistance. I agree absolutely that you can add significant strength to a timber structure with glass cloth – but the rovings really need to run in the direction that you need the strength in; hence the use of significantly heavier bi-axial cloth as the diagonal strength member in a strip-planked cold-moulded hull.

    Structures that use a glass skin over a fairly weak core (like foam etc.) have a much heavier glass lay-up than I was thinking of with this centreboard or rudder blade.

    I often use a ply core on my designs for rudder blades and centreboards – because it is the outside timber laminates that are taking the load – the core basically just keeps them apart, so it doesn't matter if half the veneers aren't really contributing. But all solid laminates are as good or better, and is certainly probably a better way to go for a thin board like this.

    Cheers -- George
    To be truly free to live, one must be free to think and speak.

    A C Grayling

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    Just for information, the Guillemot centreboard and rudder chords are about 12".


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    Default Re: Rudder and centreboard construction

    So 22mm is about 7% of the chord. You can't really get a useful foil at 7%. NACA 0009 (9%) is about the thinnest. A flat plate should be as good for both.

    Personally, I would simply radius the leading edge and taper the final 100mm to maybe 6mm thickness at the trailing edge, no thinner.

    Cheers -- George
    To be truly free to live, one must be free to think and speak.

    A C Grayling

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    Quote Originally Posted by debenriver View Post
    So 22mm is about 7% of the chord. You can't really get a useful foil at 7%. NACA 0009 (9%) is about the thinnest. A flat plate should be as good for both.

    Personally, I would simply radius the leading edge and taper the final 100mm to maybe 6mm thickness at the trailing edge, no thinner.

    Cheers -- George


    Cool. (Perhaps it would have helped to post some dimensions in my first post! 🙄&#128518

    I will keep it simple then. I still need to get to 22mm though, so will probably have to do some laminating to get that thickness...


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    Default Re: Rudder and centreboard construction

    A flat metal or plywood plate gives poor performance. Some NACA foil shape is ideal. But Storer has a scheme for making it easy to fabricate a 'near-NACA' shaped foil. Much easier to build, and it returns a large percentage of the performance of a true NACA shape. That technique is shown in most/all of his building manuals, including the very inexpensive OZ Racer plan. I highly recommend this approach.
    David G
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  33. #33
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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by David G View Post
    A flat metal or plywood plate gives poor performance. Some NACA foil shape is ideal. But Storer has a scheme for making it easy to fabricate a 'near-NACA' shaped foil. Much easier to build, and it returns a large percentage of the performance of a true NACA shape. That technique is shown in most/all of his building manuals, including the very inexpensive OZ Racer plan. I highly recommend this approach.
    Not really true for a centreboard or daggerboard. At low angles of attack (which they have) the difference between a foil and a plate is quite small, specially if the foil is very thin (as 7% would be). And sometimes a "near-NACA-foil" can be worse (in terms of lift/drag) than a flat plate.

    Rudders are different because a nice fat foil won't stall so early on as a flat plate – but plenty of flat plate rudders work perfectly well ...

    George
    To be truly free to live, one must be free to think and speak.

    A C Grayling

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    Default Re: Rudder and centreboard construction

    Quote Originally Posted by Racundra View Post
    Cool. (Perhaps it would have helped to post some dimensions in my first post! &#128518

    I will keep it simple then. I still need to get to 22mm though, so will probably have to do some laminating to get that thickness...


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    Now that ply is moving to 8mm and 10mm (instead of the old awkward 9.5mm) you could laminate two 8mm and a central 6mm.. But maybe hardly worth buying two sheets of ply. Ply isn't ideal as Bruce so succinctly says in #24 because half the grain is going in the wrong direction. But there are a hell of a lot of ply centreboards and rudder blades out there ...

    Cheers -- George
    To be truly free to live, one must be free to think and speak.

    A C Grayling

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    Default Re: Rudder and centreboard construction

    Howdy,

    In Australia the standard method for centreboards and rudders for racing boats is staved construction with 4 or 6 oz glass.

    Across the thousands of boats that are raced every weekend and over decades of use it has been found to be the most trouble free long term option.

    I can only concur with most of the posters above on Plywood foils. They are problematic as the foil gets older. Almost all breakages we saw in my Duck Flat days were departures from the "stave construction with 6oz glass" recipe. And in particular ply boards.

    WEST did a retrospective on all the boats that they show scantlings for in Appendices of "Gougeon Brothers on Boat Construction" some 30 odd years after the book was published. They found that the boats had fared very well.

    Mostly deterioration, if any, was linked to alterations that had broken the epoxy sealing.

    But plywood rudders were consistently a weak point.

    As far as a range of NACA thicknesses, racing dinghies sailing at normal speeds range between NACA0008 and NACA0012 for centreboards. Sometime you see rudders at NACA0014, but it is relatively rare. Larger boats can do quite different things, but these are very established sizes for racing dinghies which are proved again and again on the racecourse. The last two numbers are the thickness in percentage terms. 0008 is 8percent. 0012 is 12percent. 10 percent is certainly the most common you see on the racecourse.

    For foils that are thinner than that range there is a family of front and rear shaped foils with a parallel section between ... rather than full foiled.

    Unlike flat plates, which they resemble, the computer optimised shapes provide maximum lift at 10 degrees which we have found very adequate. Additionally they are very easy to make accurately, even if tapered because the leading edge template and trailing edge template remain the same.

    All the descriptions of foils are overtly mathematical ... anyone with a college math or a bit of computer programming background should be able to extract the widths from the formula. Relatives friends if you are not in that group yourself.

    Zoot down to where it starts talking about "Pollack" (sic). Actual name is Pollock.

    http://www.paulzander.biz/centerboards.html

    Best wishes

    MIK

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