Balsa core problems

Dan,
I just finished repairing my tank hold-downs. I would disagree with you about the reason the screws are pulling loose. At least on my boat (2002), the screws are pulled loose by the nylon retaining strap across the top of the tank. When the tank deforms with temp changes, fuel absorption or whatever reason, if the top pushes up on the strap which is firmly atch at the top of the vertical cover board this puts an almost vertical pulling force on the screws. The screws holding the 1x1 cleat to the floor are only held by the thin veneer of fiberglass over the balsa. An expansion of the tank in a horizontal plane would put a sheer force on the screws, in a direction they would be relatively strong. As a side note the tanks don't actually rest against the transom. The rear edge of the tank rests against a SS strap-eye which is screwed to the floor. I do agree the the fix from the factory, at least based on what I have read here, will solve the problem. I think the pressure from the strap theory explains why some people have not had a problem while others have. It just depends on how much you tank deforms against the top strap. Just my $.02 FWIW.
 
Dan, I was at the C-Dory factory Thursday, and it appears the new tank mounting cleats, which are glassed onto the cockpit floor, are absolutely firm against the tank, with the tank hard up against the transom. However, the bulkhead will now be fastened on the forward side of the cleats, rather than aft. So, the tanks will fit tight to the cleats at their base, but there will be a 1" (or so) space for expansion between the tanks and the bulkhead anywhere above the height of the cleat. This is what Jeff meant by allowing clearance for expansion. That clearance is in the center of the tank, but not at the very base. I think the assumption is that the tank expands by bulging in the middle, not by growing in length along the edge. If you follow me so far, do you think the tanks need some clearance at their base, to avoid stress on the cleat which might now crack the laminate, as opposed to loosening screws? Thanks.

Chuck
 
If you have floorboards in the cockpit there may not be room enough between the cabin bulkhead and the cleats to allow flipping the covers to the front side of the cleats and use the original screw holes but it would sure be worth a look see.

Jeez, I'd been under the apparent delusion that engineers always looked for complicated solutions!
 
Dan,
CAVU is a military aviation acronym from my previous life, Ceiling And Visibility Unlimited. I suppose it really doesn't make any difference why the screws pull out, what matters is that you fix the problem. In the case of my boat, with the strap securely fastened to the floor and to the top of the cover board, it just seemed logical that a force in the center of the strap would apply an equal upward force at the both ends. Seems it would be just like a giant U-bolt? Anyway I hope every owner takes a good look at the tank cleats. I love following this site because I learn so much about the C-Dory.
 
By the way, what does CAVU stand for.

It's a weather term mainly used by aviators: Clear Air Visibility Unlimited.

My licence plate used to read CAVU 2U!

And glassy seas...

And tight lines...
 
Hello again,
Mark on 3rd Byte has made an important and accurate point. If one has the factory deck option there isn't enough room between the cabin bulkhead and the tank cleat to move the tank bulkhead to the front of the cleat. My solution was to chisel a one inch wide and quarter inch deep chunk from the front of the tank cleat and install a 1"wide and 2" high piece of eighth inch aluminum through which I made two holes; one near the bottom and one near the top. the bottom hole accepts a stainless wood screw which secures the aluminum piece to the front of the cleat in the slot previously chiseled out and the top hole accepts a machine screw which corresponds to a whole drilled through the tank bulkhead. At the rear of these holes through the tank bulkhead are installed three pronged, inside threaded, bolt receivers. What am I talking about? Well sorry, but I don't have the correct name for them thingies but the three prongs dig into the back of the StarBoard tank bulkheads and keep the threaded fitting in place with no worries about dropping a nut or threading a nut onto the bolts. Anyway, the result is a tank bulkhead that rests on top of the cleat and is well secured yet removable. It is important to add that the tank bulkheads need to be shortened by the height of the cleat for a perfect fit. Its a bit late now but tomorrow I will post a picture as I fear my description has added little clarity to the issue.


To The 'Nancy H',
The tanks will easily slide over the now permanent cleats.

Mike 'Levity"
 
Hi All,
The factory decking option limits the available space one has to make corrections to the tank bulge problem. I have posted some pics of one solution to the problem in the Levity album, page 4. Included is a picture of the three pronged threaded bolt receiver thingy.
Mike 'Levity'
 
Levity,
I checked out your pix. That fix looks like it should work fine. I have always called those T-Nuts or is it Tee-nuts? That should give you an extra 1/2" clearance for bulging. Did you change the height of the strap attachment point? Moving the cover board up on top of the floor cleat would make it about 3/4" to 1" higher?
 
Ken,
Thanks for giving those thingys a proper name. The strap attachment point is at the same height because the tank bulkhead was shortened a measure equal to the height of the cleat upon which the bulkhead now sits.
Mike 'Levity"
 
Dan, Ken, et al,

I bought a new 2004 three weeks ago. It had been with a broker in Wisconsin for a year. Not sure if the boat had ever been in the water or not, I think for a brief engine test. I wrote to Jeff Messmer to say hello and he answered a couple of questions. He also said check this board, and the C-Dogs as my boat was in the vintage that was prone to fuel tank bulge problems. He said March 04 was just about the time they switched to foam in the transom instead of balsa so my boat could be either. He also said the stem guard screw issue was fixed in 2003 and did not apply to my boat.

My boat's gas 20 gallon (nominally) tanks currently have 4 gallons on starboard side and 2 gallons on port side. They have never had more gas than this. The starboard tank has bulged enough that the retaining board builty of Starboard has bowed about two inches at the upper port corner toward the cockpit. You can see that the tank has bowed outward in the middle on the forward and port sides (probably all four sides). The tank seems to have "bulged" more than Dan's calculations figure.

I need some drier warmer weather and intend to move the Starboard panel on the starboard tank forward. I will reattach it to the front of the of the reataining block instead of the rear. I will put a 1/2 inch strip of aluminum along the bottom edge of the panel to act as a backing (fronting?) plate. If necessary I will put a piece of Starboard between the panel and tank if the space seems to wide. The retaining strap will have to be readjusted. I will check the screws and, if dry, reseat them with 4200, if wet or loose will use epoxy. Plan to use the same holes anyway.

I don't have the factory floorboards, I use carpeting over the top of rubber mats so coming forward with the panel is okay.

I am working on a transom "box." Grateful to Craig Bailey on "McNaughty" for his help. I hope to have a bulkhead to keep the seas out and a work table across the top of the front of the motor well. The top will be reinforced and stationary in the center to hold a downrigger. The side of the top will hinge forward instead of backward. This will be a box with two sides. I hope to store large round fenders in the motorwell. Since the top of the box is limited to being about 11 inches fore and aft for the motors to tilt up, the box will have the two sides. It will have a partial third side consisting of a bar or plank across the motor well to hold the fenders away from the motors and keep them from falling out, etc. I decided I wanted a space to store the big fenders more than the neat storage box like Craig made. When I finish I will put up some pictures and, with Craig's permission, share a couple of pictures he sent me.

Meanwhile I am "borrowing" ideas from Tyboo, Fishwisher, Anna Leigh, Moose, CAVU, and lots of other fine folks.

Thanks, and I hope to get the bottom of that boat wet soon.

Mark
 
Thanks, Dan, for your patient and thorough explanation of how the polyethylene tanks expand, and what results. The engineer at Moeller Marine who designed the C-Dory tanks essentially corroborates everything you've said. The one difference is that he indicated that the initial expansion due to absorption of fuel, combined with the cyclical thermal expansion, can be up to 3% in each direction. :shock:

I must admit, I was confused about the reason the tanks bulge. You're right, of course, Dan, that providing clearance in the middle of the tank is "neither here nor there," since it addresses the symptom rather than the cause. It is the plastic expansion we are concerned with (not the fuel inside the tank), and the plastic is just a series of edges (duh). The visualization that helped me is to think of putting a sheet of letter size paper in a box. If the box is 11" long, the paper will lie flat, but if the box is 10" long, the paper will have to curl one way or the other to fit. It does not matter which way, and so now I understand why some tanks have bulged out, and some have buckled in. This is for the benefit of any other owners who, like me, never did well on those spatial relations tests as a kid.

Chuck
 
Thank you for all of your input and discussion on this topic. As you know we have been working on a better solution for anchoring our fuel tanks on the 19 and 22. The current fuel tank anchoring system has been in place for many years and is an example of "this is the way that we have always done it". As many of you know Scot and I do not live by this adage. We strive to make improvements in our boats everyday. This is one of the things that makes C-Dory and our owners so special.

We also contacted Moeller manufacturting today regarding the expansion of the fuel tanks. Moeller is our vendor for the newer 25 gallon tanks. If there is going to be any fuel tank expansion the majority of it will take place in the first several months. This is due to the absorption of fuel into the plastic of the tank. There is also some expansion due to temperature changes. They have suggested that we leave a gap between the chocks and the base of the tanks. This will allow for any expansion that may take place. I will post some pics of Chuck B's boat when it is finished this next week.

We have been using the 25 gallon tanks for over 6 months and have yet to see much expansion at all.

Jeff
 
Is there an explanation as to why the starboard tank is the culprit? Has anybody had serious bulging problems on a port tank?
 
Good ideas, Dan. And for those who are refastening loosened cleats, or want to take an extra measure of precaution, one could use threaded inserts in the cockpit floor, as has already been described.

We should probably clarify that the 3% expansion potential is for NEW tanks that have not yet absorbed fuel. For those with older boats who are considering changing their cleats, the initial expansion due to fuel absorption has already occurred. Dan, do you think those tanks might "unbulge" or expand along their edges over time if the mounting clearance were sufficient? If not, then perhaps the additional clearance appropriate for an older tank--to accommodate the cyclical thermal expansion--would be smaller than 3%, and more in line with your original figures.

Chuck
 
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