Fitting a centerboard trunk

I'm working on a 15' Phantom class dinghy, and I'm having some issues trying to fit a centerboard case into the hull. The plans I'm working from give me the assumed curvature of the floor of the boat at the CB trunk, but my boat has come out somewhat flatter. I've tried trial and error planing the bottom of the CB case to fit the hull, but I'm still not getting a good, tight fit. Does anyone have any tricks or tips for getting this right?

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RE: Fitting a centerboard trunk

   get yourself some stiff card and rough cut approx curvature. then get a small block and attach a pencil on top. so you have a fixed marker that is approx 1in above the floor.   place the stiff card at 90% to the floor in the area that your wanting the measurement for and then draw the marker along the card thus leaving a curvature mark against the card.

 this will give you an exact copy of what the floor curvature is.  last thing to do is cut the card along the given marker line and use as pattern for bottom of the centreboard case.  sorry no pictures but i hope tis helps.

 

RE: Fitting a centerboard trunk

All lumber yards sell door skin.  It is very thin plywood and cuts easily with a razor knife.  It also planes or sands quickly.  When I have an odd shape piece to make, I cut door skin into 2" wide strips.  Then, using a hot glue gun, I shape a piece for each surface and glue them together as I go.  The result is a full size pattern.  Transfer that to your wood of choice, add bevels where needed and you end up with a perfect fit...   ~BRUCE~ 

RE: Fitting a centerboard trunk

Alternatively, make the trunk a bit too long so it extends through the slot, mount it in place and remove the extra that sticks out the bottom.

Laszlo

 

RE: Fitting a centerboard trunk

I second Capt pugwash suggestion. a set of scribes will work out best . making it longer and pushing it through decreases the strength of the joint.

RE: Fitting a centerboard trunk

>> making it longer and pushing it through decreases the strength of the joint.

I respectfully disagree.

If the trunk is supposed to pass through the slot in the bottom anyway, how can it make any difference to the strength of the joint if the excess is removed before or after the trunk is mounted?

There's other reasons to use one method over the other (ease of flipping the boat, when in the build sequence does the trunk need to be mounted, ease of access, etc.), but relative strength certainly isn't one of them. Either way will result in a strong centerboard trunk.

Laszlo

 

RE: Fitting a centerboard trunk

Similar to the capt.,  I have fit trim right against a not so straight wall by using a small washer to trace the wall on to the wood with the inside edge of the washer.  The washer will give an even line down the whole length.   

RE: Fitting a centerboard trunk

thank you Garrett!!  Some of us build items while others assemble.

RE: Fitting a centerboard trunk

model t,

I'm still puzzled about what you said about the joint being weaker. Could you share your experience, please?

Laszlo

 

RE: Fitting a centerboard trunk

Hi there Laszlo, I too am puzzled by model t's remarks, both of them. I'd be very interested in hearing more from model t. on this.  SEEYA Jack

RE: Fitting a centerboard trunk

Not to muddy the water regarding whether the trunk should penetrate the bottom, but this has made me curious.

I would think the widest glue line would be best even if there is to be a fillet there as well. If the trunk material is thicker than the bottom material (not sure if it is, but wouldn't think it would be thinner), that would suggest not penetrating the bottom.

And I don't know what the material is, solid wood or plywood, but if plywood, is there any risk to exposing the edge grain thu the bottom, even though I assume it has a layer of glass covering it?

However, I can see how a closer joint could be made more easily by having the trunk pass thru.

Never built a boat with a centerboard, so this is just speculation.

RE: Fitting a centerboard trunk

Not being familiar with the details of the Phantom dinghy's construction, I hesitate to speculate. But there are often solid timber logs that reinforces the joints between the board case sides and the thin bottom planking. Whether the actual sides of the case pass throught the planking or not, those logs must be trimmed to fit the inside curvature of the planking. That might be what Model T meant by the joint being weaker if the case just passed through the bottom, with no reinforcing case logs.

Grant

RE: Fitting a centerboard trunk

Thanks guys. That all sounds plausible and that's the kind of thing I was looking for.

My thought is that is all depends on what the designer specified. If the trunk was specified to pass through the bottom anyway, there's no difference in terms of strength whether it's cut before or after the fact, except that cutting after makes a tighter joint.

If the designer specified that it stays above the bottom, then you have the issues that Grant & Garland mention and there it seems to me that they have a point. It might actually make a difference as they say.

I do wonder, though, if modern stitch and glue construction might ease those requirements? A full 45 degree fillet with the proper glass tape is usually stronger than a wooden log or chine (though more expensive and possibly heavier).

I saw one boat being built which was a combination of the 2 techniques. The trunk passed through the bottom, but the edge of the hole was built up with solid framing for strength. But the framing was simply pushed down onto the bottom and fastened in place. It was flexible enough to take the curve without any shaping.

Anyway, I don't have anything against the scribing method, I've used it myself many times. Just wanted to throw another option out there in case it was applicable.

Have fun everyone,

Laszlo

 

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