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Definitive thread on modifying the Centerboard for 17mk3 and 20mk3

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Below, copied from this google docs. https://docs.google.com/document/d/14d9ZdooVy53R75y2mquZRJl8XymKDjYJaVbmyjAJ-is/edit?usp=sharing


Centerboard Modifications to the 17 and 20 Mark 3 designs. Jan 2023


Note that if you are purchasing a kit or plans today, none of this is relevant to you as you have the most up to date version. If you have a Core sound 20mk3 or 17mk3 prior to the hull number below then you might consider this modification.


Shortly after painting and relaunching Graham’s 17mk3 ‘Carlita’ in late 2016, Graham posted a thread called ‘Carlita goes under the knife’ where he detailed lengthening his centerboard after feeling that he could improve her upwind performance with a longer centerboard. This was just in time for the Everglades Challenge in 2017. This change was then added to the 17mk3 kit and plans in early 2017 (longer trunk and centerboard) for all hulls after hull #12. Then shortly after this a major overhaul of the entire design was completed around April 2018 which included Hull #15 and boats afterward. The overhaul included raising the cabin slightly, redesigning the trunk to move the centerboard further forward and other improvements to the kit. 


For the Core Sound 20 Mark 3, the centerboard location and length was updated for hull #21 and all hulls afterward. 


You can easily identify the difference in either model if you are still unsure as the original centerboard location was such that no part of the centerboard or trunk extended forward of the companionway bulkhead. This was done to keep the board out of the cabin. We found however that the boat had a touch of lee helm though we originally thought the mizzen location (being further aft than previous Core Sound designs) would account for this. This was especially evident with the 17mk3 due to its shorter waterline. Interestingly the Core Sound 20 Mark 3 does not seem to exhibit lee helm and we’ve yet to have the opportunity to sail the original and relocated board 20mk3 back to back. (perhaps at the next messabout) in order to see the effect. Perhaps after that it will be more clear if the 20mk3 would benefit from the modification. 


Ok, how do I do it?

For those interested in making the modification to an existing boat, a very helpful video explaining the process (specific to the 17mk3) should be watched here. Pictures of Graham’s original modification which he did just before his trip around the Delmarva Peninsula can be seen here. The modification for the 20mk3 is done in the same way


Don Silsbe completed the third known modification of an existing 17mk3 board and trunk on hull #6. (#2 and #6 also have the modification) Here is a link to his thread with good step by step shots of the process starting from the removal of the existing king post.


We’ve created full size printable templates (seen below, each comes in 4 sheets of letter size paper) to use for the modification. See below to download the files for the 17 or 20 mk3 and be sure to read a few additional notes below. 


17 mk3 Modification Dimensions and Files: Download Here


20 mk3 Modification Dimensions and Files: Download Here


Do I have to extend both the trunk and the board?

Yes, if you do the trunk, you should also extend the board. 


Extending the board: 

To extend the centerboard itself we are recommending doing this from the top of the board by adding a “crescent” shaped piece (shown below)  to the top of the board and fiberglassing it on. This preserves the majority of the shaped board and means that redoing the lead tip is not necessary. Some of the existing board will need shaping into a foil shape as it will now be below the hull when rotated down. Also note that some additional uphaul purchase may be needed since the board’s lead tip has additional leverage on the uphaul line. The current centerboard rigging diagrams can be viewed here. 

17mk3 rigging diagram and 20mk3 centerboard rigging diagram





Below shows the extent of the difference on the 17 mark 3. 



With the extended trunk and board there will be a slightly larger gap behind the board in the trunk but probably not enough to worry about doing anything about.





Below shows the extent of the difference on the 20 mark 3. 



Pictured below the lengthened centerboard in the original trunk with the trunk extension. Note that with the 20mk3 there will be a gap of approximately 7” behind the lengthened centerboard when it is rotated into the original trunk. We recommend cutting a block of blue insulation foam that will slide up into this space and form a “filler” that can be glassed over and painted. Dow Blue foam is epoxy safe and the primary reason for doing this is to reduce the amount of water turbulence in the trunk which slows the boat down. 



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Thanks for this, Alan.  I’m not lengthening the c/b this year, for a couple of reasons.  If I store her next winter, I’ll probably do that.  One question— We usually stand on the c/b, when righting after a capsize.  Are we confident that this modification will sustain that load?

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Gee Don, exactly what came to my mind.

I was pondering today about what the joint would need to be like if lengthening the centerboard from the top.  How could a joint duplicate the strength of all the full length wood pieces in the original?  Then I started thinking of how a butt joint COULD be reinforced… or made sufficiently strong… or making a scarf on a curved joint… or reinforcement rods… or some goofy ideas that came to mind. 

Of course, given my lack of genuine knowledge on these kinds of things my brain-musings are just to pass some time while driving, thinking about creative ideas (that I likely won’t do) just for the fun of it. 😁

On the plus side, I haven’t capsized Avocet or even started approaching anything close.  So, I haven’t had the need to stand on the centerboard. Maybe never? (Maybe not realistic.)

I guess my solution leans toward a new full length centerboard at some point. I really like the cnc shaped board B&B built for my CS15… that would be my choice solution.  We’ll see what the engineer-guy thinks next year. 🙂 That’s a ways off and there is plenty of skiing ⛷️ and sailing for me to do first. 

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Thank you Alan.


My early model 20.3 sails so well that I'm reluctant to do this modification.  The helm is very balanced on all points of sail.  The only time I have struggled to go upwind that wasn't due my poor technique (I learned how to sail on this boat), was an occurrence in the Chesapeake Bay when the strong wind died out, but the steep waves remained, so almost no wind with relatively big waves made it impossible to go upwind.


Generally speaking, I don't get 90 degree tacks in the Chesapeake Bay, but I think that is mostly due to a light boat responding to wave action and/or my former poor upwind sailing skills.  Using the upwind sailing tips Graham provided on this forum really helped and I can point a little higher using them.  I did have to make new longer sprits so I could make my sails flatter, and I'm more attentive to my tell tails now.


Graham did a quick informal upwind test with my boat and was able to easily get 90 degree tacks (light wind in smooth conditions).


I'm sure the boat would point higher with the modification however, so it is probably worth doing.  It really doesn't look that hard after you gain the courage to make that first cut, although a much easier solution would be to loan the boat out to Don next year!

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Our boats are the same configuration, and I agree, in light wind they are fantastic as is. My issue is reefing. I sailed up against a stiff breeze in Sodus Bay this fall, the strongest breeze I've sailed upwind in. Skeena has a very balanced helm with 2 reefs in the main and 1 in the mizzen under those conditions. It would have been better with weather helm. When I put the last reef in the mizzen, she had a good amount of lee helm at the time it is least desirable. I've got some good voyages planned this year and next, so I think this is worth doing.


I hate to have the centerboard case egress into where I sit most often down below, but I just went out into the barn to get a feel and it wasn't bad.  I am going to start this project mid-March. I am grateful for the pictures. I mentally was having a hard time seeing myself cutting into Skeena. but I'm over it now. 


As for the joint strength, having the case trapped between the berth should keep it pretty strong, and of course I'll be looking to tab it good.  I'm the only one I know who turtled a mark 3 and it was easy to right other than getting the C-board out of the case. I'm stoked to have a downhaul to lock it and some weather helm which may have helped prevent it. 


It's pretty easy to slide the boat off the trailer and roll her on her side to glass the bottom joints. Maybe I'll stand on the board and test it!



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My CS20.3 is getting a mini refit and paint…..would be a perfect time to do this mod.  But…….I really haven’t had a circumstance that I felt lea helm that I couldn’t balance out.  A longer board might help the windward abilities…. I am quite sure it would be worth it, that may be my next winter project.  
As you sea……Indecision may or may not my problem.  

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Good feedback Steve.  


Concerning centerboard strength after adding an extension, I would try to use a full length piece of glass that extends from the lead tip all the way to the pivot end (assuming your centerboard slot allows for the additional thickness).

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