Daniel, the most important thing to remember when working on and repairing the poly boats is... they aren't fiberglass. The advantages of a poly boat are they are practically impervious to chemicals and they are strong but flexible so they fend off most impact damage. Both of these positives are also negatives when it comes to repairing this material. I have used a duct tape bandage on my Sunfish too but the same type of fix won't last very long, underwater, on a poly boat, especially at the back of the centerboard trunk.
When I got your reply I went out to my boat and checked a few things and took some pictures for you. If the pictures don't show up in this post you can look at the album SailNet Community - Propertydoctor's Album: Escape Rumba crack repairs
Yes, you can get a 5" deck access plate in the spot where you need to make the hole. From my quick look I think you will be within easy reach of the backside of the crack. These plates are made to be walked on so they will take sitting on them fine. When you buy a plate, make sure it has an o-ring seal, otherwise you will have water leaking into the hull from the plate.
Silicone is an excellent product when it is used as intended. That being said, silicone was never intended to be a very strong adhesive and because of the impervious properties of poly, the silicone can't make a chemical or mechanical bond with the plastic. BUT! If you use silicone under the flange of your deck access plate and install it loosely until the silicone cures, then tighten all of the fasteners against the backing plate, the silicone makes an excellent watertight gasket.
For my backing plate I used a cheap poly cutting board from Wal-Mart. The cutting board was about 3/8" thick and was rigid enough to stabilize everything when the stainless flat head machine screws with nuts and lock washers got tightened down. You will have to cut the backing plate so it will fit the flange of the deck access plate and I recommend clamping them together and pre-drilling the holes. Mark a reference point on both so you keep the backing plate oriented correctly in case any of the holes are off. You will also need to make a cut across the ring of the backing plate so you can twist it through the hole like you would put a key on a key ring.
Another thing to remember is, you do not want to install your deck access plate before you make your repairs. You want as much room as possible to get your arm into the hole and do the work, as it is you will be working blind inside the hull. You also do not want to get any epoxy on the threads or the o-ring, or you will be buying another deck access plate.
Here are some pictures of the epoxy/glass repairs on the inside. The dark marks you see are sharpie marker points I used as reference to make sure the glass tape was over the crack. I did use 2 layers of fiberglass cloth tape, the first layer was 2" wide and the second was 4". I used the G-Flex epoxy and followed the instructions for "flaming" the plastic for better adhesion.
You will find the PVC boom is not strong enough to handle the loads imposed by the main. You can definitely use the PVC as a way to work out the dimensions and then use that pattern to fabricate your aluminum to. Keep in mind, the boom is sheeted at the mid point, there is enough stress at that point to fold a PVC boom in half. Maybe you could make the PVC boom and then insert a straight section of aluminum tubing to reinforce the length. Huh... interesting idea. I might just play with that on another boat I have.
I don't know how much you know about the SmartRig but it is a variation of a product called the Hoyt Jib boom http://www.forespar.com/pdf/techTips...t-Jib-Boom.pdf
. Gary Hoyt is definitely a smart man.
Hope all of this helps and isn't too confusing. Let us know how your making out.