Centerboard ply core


Paul Macloy
 

Does anyone know why Ian designed the swinging centerboard with a 4mm ply core and then laminate sides? Looking at building a new one. My rudder I rebuilt I just used all timber laminated together without the ply core. 


dennis@...
 

I don't know what he was thinking. However, ply does have equal strength in both directions. Also, it's probably a lot less work to use the material that you have at hand. Laminating strips is a lot more work and would require UD wrapped around to compensate for twisting forces. Otherwise the core might split lengthwise. Just my thoughts.


Alex Caslow
 
Edited

Hey Paul,

I would wager laminated ply is far more rigid than timber. The forces exhibited on the centerboard are quite extreme when going to weather, any flex at all could cause a catastrophic failure.

I'm not an engineer, but Ian was, so as I refit my 720 I am following the plans to a Tee.


Paul Macloy
 

Dennis, I rebuilt my rubber by laminating strips of hardwoods together without the ply core and it was surprising easy. 

Alex, you are right. Ian really knew what he was doing with his designs. 
The centerboard that I took out has a ply core and was warped so badly that it was flat on one side and very foiled on the other. At first I thought maybe it was designed that way to counterbalance the centerboard being slightly off center. But when I finally saw the plans I see it should be symmetrical. The timber was still solid and in the early days of my rebuilt I reshaped it. But now I'm realizing it must be too thin with the timber that's been removed and I want to build a new one. 
Really struggling with the idea of using the ply core instead of just building strip laminate hardwood. 

Photo. Rudder before painting. 


Try Flying
 

My original c/b was ply and twisted plus heavy as all hell. I made a new one that is carbon an foam core as per the later designs and is far superior... plus it floats so releasing the pull down and it pops straight up. Forces on a TT are way less than what you get on say a F85sr or F22 as the foils are much shorter and greater chord so thicker.


Try Flying
 

Also .. a very worthy mod to the original design is to enlarge the pivot hole to say 30mm (or whatever the diameter of a rod of delrin is easy to get). You then make a delrin bushing that is free to turn in the c/b and just shy of the case width and has a thru hole for the pivot bolt. That way when you tighten the c/b pivot bolt it squeezes teh bushing not the board and makes a water tight seal to the case


Camtron0@...
 

Hi I just broke my centerboard on my 720.  Could you send me the plans?  I'm going to make a new centerboard and this would really help.  Thanks
Camtron0@... 


Try Flying
 

You can find the plans in the files section the outline for the C/B is on sheet 10.

Where are you situated?


Camtron0@...
 

Thanks Try
I'm in Three Fathom Harbour, Nova Scotia, Canada. Close to Halifax. 
We just bought our TT 720 in July.  If the boat number is on the main sail we're number 54.  Where are you? 


Try Flying
 

Little bit further south.... just north of Sydney (the one in OZ) 

Was asking as I have a CNC and might have been able to help you out with the blank.

I assume you still have the head of the broken C/B.... keep it as it will be a useful template for the pivot hole location.. Also check out the thread on how to get a good "centerboard seal"  as its a good method.

Cheers

Mark


Camtron0@...
 

Ahh well thanks anyway! 
Do you have any pictures of the core you used for your centerboard?  I'm still picking materials for mine.  I'm leaning towards western redwood because the internet told me to and I'm in north America lol.  The extra weight might be good for us tho.  It often gets really choppy here on the north Atlantic and commonly overpowers us.  That's partly what happened when we broke our centerboard, we got caught in 1 to 2 meter waves for 4 hours.  
I've even been musing about a 12V waterbalast system  in the bilge to add mass when I want, to get knocked around less.  
Thoughts? 


Try Flying
 

I've done a Tramp C/B in cedar with carbon uni reinforcements where mine is Divinycell 80kg foam and carbon wrapped HD spine both work and are appreciably lighter than ply... I think there are some pics in the rudders etc photo album..  I'd vote no on adding weight or water ballast as I don't feel our hull shapes have enough flotation to have them add any real benefit.  Short steep chop is the worst and like you say can beat you up a bit. Keeping powered up by dropping down a few degrees and cutting thru it is the best but not always possible/practical.

Sheet on, travel out and hold on for the bumpy ride :)

On our boats if your burying the leeward float then its time to reef , not add water ballast IMHO. The only time I wished I had more weight was running before 20+ knots in 3m swell and foolishly had the full rig up. Holding the back of the boat down was an issue with a big square top main. Reefing was the solution....while I convinced my son to sit as far back as possible (ie get out on the sugar scoop..) while I went fwd.

I assume you were getting slammed beam on to snap the c/b? Must of been a slog getting back without it if you needed to go to windward.


nelsolyn
 
Edited

Paul,
Aaah !   Sounds like the early days all over again.
There was quite a few broken in the hard sailing up in Queensland (Aus) where the biggest concentration of builder/sailors were concentrated.
Dennis in his reply hit on the main reason for Ian's original design - materials at hand and similar techniques as used during the rest of the build.
His design was well capable of the forces generated when 'hard on the wind' - the loading was in one direction for 90% of the time (to pick an illustrative number!)
The problem came on the reaches where the arch enemy of all structures _ SUDDEN and FREQUENT load reversals !
The board loads up first in one way then the other due to direction changes from, steering, chop, cross swell, wind gusts, spinnaker sudden deflation ... or who knows what else.
So the fact suddenly revealed itself, that most of the breakages happened on free legs or shortly after hardening up after such a leg

Once the word got around  -the sailing fix for those with still intact original design boards in fresh conditions, was to   RETRACT THE BOARD PARTLY SO THAT THE TRAILING EDGEOF THE TIP WAS JUST WITHIN THE CASE. !   So now it was supported by the case, but good steering was maintained - not only that, but the overall steering balance was improved with far less tendency to 'round up'.

For those who had a broken board, and had the money, materials, experience, skills and techniques available, the the obvious and proven choice was to make laminated / sheathed timber units.  Far stronger ( when fully down), and more rigid -  hence able to take many more load reversals than the ply core, which is only half strength in either direction at any given time.

Ian Farrier was very close  to the Queensland sailors and builders at the time and became aware at the time of the breakages experienced and the technique of partially retracting the board that experience TT sailors were using  - as did his agent Geoff Fineagan.  I understood at the time that Ian intended to add something, either to the plans or in the notes.  
Being so heavily committed in the design, production engineering, and building of the F27 at the time, probably put this good intention 'on the back shelf'
Secondly in his defence, many of the breakages occured on big rig, 'hot shot' boats that were always on the water and pushing hard !

A nice little anecdote was when in 1991,we hosted the TT Nationals on Lake Macquarie in New South Wales - for the time they were held out of Queensland.   Our great Southern Hope' a boat named Chrysalis,  unaware of the 'northerners' home grown technique,  broke his original design board. luckily it was the day before the lay day, during which they built another and resumed racing the day after.  He amassed enough points to win overall and take the Trailertri Tophy ('The Ashes') off the Queenslanders !

Cheers, Chris


Paul Macloy
 

Just for the record. I didn't end up make a new centerboard, just reshaped to old one. I had so much work to do on the rest of the yacht that my time was better used elsewhere. After first sea trials in a few weeks time it will go back on the list of jobs, after the full new rewire of all electrical system and solar panels, wind turbine, finish interior rebuild etc. 


nelsolyn
 

Well Paul at least you know how to extend the life of your standard board !
sorry I didn't pick up on this thread back in February - I don't always go into the TT digest as I do with the Tramp digest.  (Sorry Mark !)


nelsolyn
 
Edited

Camtron,
I agree totally with Mark (Try Flying)
DO NOT ADD BALLAST
Weight is the enemy of multihull and added weight would only subject the hulls/total platform and rig to more loads  beyond envisaged in the original calculations.
Like Mark says: in short steep chop/slop, keep the boat powered up - albeit with enough mainsail twist to allow the top to de-power early


Try Flying
 

Camtron (and anyone else interested)

I've just added 2 dxf files into the files/plans/centerboard  folder in case it helps. If you end up finding a local maker with a CNC then I can get you 3d models.

Centerboard.dxf is the foil as I made it showing reinforcements for Uni and pivot lift up/down holes. Note these may be different on yours so use as a guide not a given. Also added the NACA006 profile that I used.

Exports to DXF can do funny things sometimes so let me know if you have issues.


Cheers


dennis@...
 

I agree, these boats do not have reserve floatation in their floats. Adding weight to the main hull will therefore not increase stability or inertia in roll. Adding water ballast will only submerge the floats faster and the free surface effect, in case water moves sideways inside the hull, could well serve to roll the boat once the lee float is buried. Water ballast in the high side float could work though, as that would rely on the buoyancy of the main hull, which is more than adequate. But whether tge gains woukd be worth it. I have my doubts.


Camtron0@...
 

Thank you so much guys!  
I've downloaded the files and am exploring my options.  
Thanks for the suggestion of finding a CNC.  There's a big industrial park near me, I should be able to find something there.  We're getting our trailer back from repairs on Saturday so hopefully shortly after that I should be able to haul the boat and get to work.  This is going to be quite the project.  I'll keep everyone posted as I go.  I'm sure I'll have many more questions.  
I'm taking your advice and not bothering with a ballast system.  


Try Flying
 

Tip on finding a local maker..

Try posting on CNC zone forum also find someone who sells or has a hobby 3d printer.... Our local  gaming computer repair/sales guy is into that scene (geek demographic :) )

3D machining is tricky/slow so most  cabinet maker/kitchen mobs are generally not interested (or would charge a motsa)

End of the day its not hard to hand shape the C/B with a few tricks and a router. 


Cheers