Do you expect a keel to make the canoe more stable, as in being resistant to tipping? If that's your intention, I don't believe you'll perceive any benefit. It could even make it worse in the one situation where a sudden rolling action is most likely to occur and where it's also the most difficult to counteract, which is when suddenly encountering a swift cross-current for which you haven't properly prepared yourself by leaning the opposite way as the current wishes to roll you. In other situations, I don't believe a keel will supply enough surface area to resist a normal rolling action, and for simply "tipping over", as would be the action in normal lake paddling (getting cockeyed on the face of a steep wave, for example), a keel won't even "move sideways" enough in the water to help even if it had lots of surface area. The keel would have to stick very far down below the boat for the tipping action of the boat to generate enough "swinging" motion of the keel to provide resistance to that motion. That's just geometry. You can see this for yourself if you lay the boat on the floor and sight down along the proposed keel line while an assistant tips the boat, and watch how that line moves sideways relative to the floor during that tipping process. No lateral movement of the keel line relative to the floor illustrates no lateral movement of the keel line relative to the water that supports the boat. Of course, this demonstration isn't perfect because the rolling action on the floor won't be quite the same as when in water, because yes, when in water the keel line will move a very tiny amount to the left when the boat tips to the right, but the floor exercise will make it clear that this motion will be miniscule. After all, how much extra "swinging" motion from the keel can be generated when it's only embedded three inches below the water's surface?
Also, a boat with a keel can be pivoted, turned sharply and side-slipped pretty well, demonstrating that the keel isn't "grabbing" the water particularly well. I think a brace is your best tool for the job if you need to prevent tipping.
I second what guideboatguy said Additionally, without knowing more about the “fiberglass extrusion” I’d be concerned about creating localized stresses in the hull, which could result in cracking. Will this extrusion expand and contract with temperature changes at the same rate as the composite hull? If you strike a rock with the new keel, how will that effect the hull?
The Magic is a wonderful hull for it’s intended purposes.If lack of stability is an issue for you, perhaps some instruction would be helpful. Learn how to get the maximum out of the hull, as it is, rather than try to make it into something that it isn’t.
Another possibility is that the seat is mounted overly high. I’ve seen folks raise seats considerably, often for greater comfort, but at the expense of stability.
Keel If I were you I would forget all about adding a keel. It may make the boat track straigher but will never make it more stable. Learn to paddle better. That is your key. Not having a keel makes moving the boat sideways in a river much easier.
The creative possibilities are endless You don’t say how deep the T extrusion is. If it’s six inches deep, you will overcome GBG’s objections about insufficient depth to affect stability. However, with a 6" deep keel, you will no longer be able to sail in Assateague with Mike McCrea because the whole dang place is only 5" deep.
Contrary to Marc Ornstein’s fears, the T extrusion may strengthen the bottom of your canoe. The only way to test this will be to bash down a 60’ per mile descending shallow creek.
Of course you should attach it first temporarily with tape or velcro so you can remove it. This will also allow you to position the extrusion on top of your off-side gunwale vertically to present greater freeboard to an imminent tip. Alternatively, you could tape it on the side of your off-side gunwale to act as an anti-tipping shield.
Finally, you could just lay the enter 14’ section horizontally across the gunwales in front of you, with empty 5 gallon plastic jugs hanging off each end. This would act as a Polynesian double pontoon and allow any Magic to surf across the Molokai Channel.
Ahhh, the ideas that come to mind late at night when one is bored and is unable to sleep.
No, not the idea of adding a keel to a perfectly designed hull, but rather asking a question that is outlandish and then sitting back to see how thoroughly I am flamed.
You would think the boat alteration firm of Saults and McCrea could make a rudder that could do double duty as a rudder and a centerboard. That hole in the bottom of the Magic would take some clever engineering, no?