plywood keels and gussets

[Al Stead]

After leafing through the sunday ads, I was taken by the idea that 1x12's vs 3/4 MDO was heavily weighted in favor of the MDO cost wise. I have read that most folks on this forum don't favor plywood keels for some very good reasons, but let me run this by you guys and see what you think. 

I am going to start assembling my Weekender in the spring when it warms up enough to set glue.  If I go with the 3/4 MDO, I will apply a white ash shoe to the bottom of the keel to head off any wear and inflitration problem.  I will also apply a most likely, 1 1/2 inch gusset to both sides of the keel joint instead of the bondo.  I figure that by gluing both the sides as well as the top of the keel to the bottom of the boat which I intend to make out of 1/2 MDO, I should achieve a bond that screws cannot compare with.

Right now I am favoring the MDO for a couple reasons.  One, I can get some pretty nice stuff at my local big box lumber retailer.  There are virtually no voids that I can detect and the plys are as nice as anything I have used.  It is also double sided and when I checked it out, I found that even very old epoxy will bond with it beautifully.  Because it is double sided, it should not check inside the cabin and so far I haven't been able to wear the overlay off of anything I have built. 

So any comments?  Any experiences with MDO in boat applications will be greatly appreciated.

Al Stead

Duluth Mn

[PAR]

MDO use to be good stuff, but the last ten years has seen it decline as much as other sheet goods. The rosin faces take epoxy well, but not as well as the raw wood under them, so in areas where you need a structural bond, grind away the rosin face to get at the wood.

The plywood keel idea has some merit, if it was a box keel, but as a solid laminate of plywood it would be considerably heavier then needed and weaker longitudnaly. Considering Weekender's need for additional capacity, particularly aft, adding a bunch of weight to the deadwood assembly wouldn't be the best choice.

If the deadwood assembly was "boxed", you'd have a lighter keel, that also adds capacity to the hull. Okay, you wouldn't get much where you need it (aft), but you gain some, which is better then none or subtracting from none.

From a technical stand point, if Weekender was built as a true taped seam (stitch and glue) boat, the keel isn't necessary for hull integrity. It's just there for lateral resistance (which it does a poor job at, in my opinion). As a true taped seam build, you could remove a large percentage of the separate "boat parts", like chine battens, butt blocks, etc. This further lightens her up as well as means you don't have to buy, cut and install these piece too.

[Al Stead]

Thanks for the response Paul.  I hadn't considered the box idea at all.  To expand on that, my thought was to make large circular holes in the center layer like struts on an airplane wing.  It shouldn't reduce the overall strength and could reduce the weight to whatever degree I felt comfortable with.  When I make my bent shaft canoe paddles, I hollow out the bend before laminating them up to save a few onces and make the bend easier to make.  So far no probs.

Right now my thought is to at least use the MDO on the bulkheads, rudder box, and bottom if the quality is satisfactory at the time I go to buy it.  What I have seen lately is superior to the ACX that I was shown at my local specialty lumber yard.  If nothing looks up to snuff late next winter, I will go with marine plywood.  I want very much to go sailing next summer, so most likely I will stick to the plans, other than using what I have on hand in my lumber rack vs what is called for. 

If I get hooked on building these things, I will get more creative the next time around.  That little schooner of yours looks good to me for that one.

Al Stead

[Frank Hagan]

The large holes in the center lamination are probably a bad idea ... any area where moisture can collect is bad, which is why "voids" are such a problem in a sheet of plywood.  Water will collect there and rot it from the inside out.

On the Weekender, the keel is what the boat rests on most of its life on the trailer.  Its a pretty big part of the structure of the boat.  I think there have been a couple of plywood keels done, but I haven't heard how they stood up over time.  Plywood still has wood movement, but less so than the solid lumber used in most keels, so you might be able to glass it without it cracking (a problem on most every glassed solid lumber keel, I think). 

A box keel is usually open to the inside of the boat, right PAR?  You could make it wider, hollow in the center and open to the cabin so it wouldn't collect moisture and rot.  The dimensions of the front of the keel are important: I've always thought that the forefoot of the keel acts like a large rudder in front (Bolger had a really ugly big boat that used this principle, with a huge centerboard right up at the bow).  So you should try to keep the depth and shape about the same as in the plans.

[bdann]

For what its worth... My Weekender has a plywood keel.  Plywood wasn't our first choice, but we want to build this boat without breaking the bank and use it as a learning experience.  The cost of suitable 1x12 stock was many times what the plywood cost and we elected to go this route.  If one of the things we learn from this project is, "Don't use plywood for the keel"...then so be it. 

Here is a photo:

I put stringers the full length to attach the hull bottom too in order to avoid putting screws down through the end grain of the ply. In hindsight, this probably wasn't necessary, as driving the screws down at an angle would have held and I could have added a filet on each side later if I'd wanted.  Lesson learned I guess. 

[Al Stead]

Yeah, I'm on a fixed income so I would like to keep the cost down to the minimum.  That is why I am going to make as much as I can out of the white ash I've been harboring for 25 years.  I've done quite a bit with all kinds of plywood and left it outside to go to heck.  I really am impressed with the durability of the MDO I've used even when it is left outside uncoated for several years. 

As far as the boat goes, I sure don't want her to fall apart right away because of some construction faults, but I'm old enough that she doesn't have to last forever either.  I need her to be sturdy and reliable so I can take her into the wilderness and not worry about her falling apart.  But if she doesn't outlive me, that's okay. 

I intend to sail the daylights out of my weekender.  She'll sit on the trailer ready to get wet at the drop of a hat.  I have a brand new safe harbor and launch facility only a few miles from the house.  On the days when we get a nice stiff NW off shore wind, I will probably push her pretty hard once I get some skills with her.  There are many summer days on the north shore here when you could sail in a 15 to 20 mph wind on a beam reach until the sun goes down with no chop at all.  One day I'm going to find out how far that is. 

Al

[Konrad]

I intend to sail the daylights out of my weekender.  She'll sit on the trailer ready to get wet at the drop of a hat.  I have a brand new safe harbor and launch facility only a few miles from the house.  On the days when we get a nice stiff NW off shore wind, I will probably push her pretty hard once I get some skills with her.  There are many summer days on the north shore here when you could sail in a 15 to 20 mph wind on a beam reach until the sun goes down with no chop at all.  One day I'm going to find out how far that is. 

That's my kind of sailor, right there.