How can I raise and lower the mast on my 23-foot boat so that I can travel through canal bridges?
The good news is that you're actually a member of a relatively large group of boat owners who have faced this problem before you. Through collective effort, they've developed a number of ways to step and unstep the mast on boats the size of yours. The best way to do it when you're afloat and on your own is by way of a system that most users refer to as a tripod. Now, before proceeding, we should interject that the tripod method is really only intended for masts that are deck-stepped by way of a tabernacle arrangement. For those that are stepped on the keel, you'll have to resort to other methods described below.
To use the tripod, you'll have to carry a few pieces of gear that you might not otherwise have on board. What you'll need are a six-foot section of aluminum tubing at least an inch and a half in diameter (some sailors use their spinnaker poles or whisker poles), a four-to-one purchase (like your vang but longer), and two, low-stretch bridles. You'll also need a way to attach the tubing or poles to the base of the mast on the forward side, and a way to attach one end of the bridles to far end of the tubing or poles and to the toe rails on either side of the mast step. You'll also need to attach one end of the purchase system to the top or outboard end of the tubing or pole and secure the other end of the purchase system to a fitting on the bow.
Start with the mast lying on deck (and the butt of it properly seated in the tabernacle) and attach your pole (or tubing) to the mast base and secure the bridles to the pole and to the toe rails or deck, and attach the purchase system. What you've done so far should look like a mini mast pointing skyward, stepped on the mast itself and intersecting with it at a 90-degree angle. Next, secure your purchase system to the upper end of the pole or tubing and secure the head or your headsail halyard to the end of the tubing or pole as well. Now make sure that the halyard is taut and the purchase system is as well.
At this point you're ready to raise the mast. Just begin hauling on the purchase system and the mast should start to incline up. Because of the bridles rigged to either side of your pole or tubing, you shouldn't need to worry about the mast wavering from side to side. It should simply trace a stratight line up as you continue to pull on the purchase system. Once you've hoisted the mast upright, your tubing or pole should now be parallel to the deck. Simply secure your headstay and then the shrouds and backstay (if your boat is rigged with one) and you should be ready to dispense with the tripod and fine tune the rig tension.
Now, if your mast isn't stepped on deck by way of a tabernacle arrangement, there are a couple of options open to you. One is investing in a gin pole. Most sailors consider this the secondary option because it's less safe than using a tripod, and takes about the same amount of time to set up and use. Usually gin poles are made from older mast sections that are at least a third to half as long as your mast. A block is affixed to the top of the gin pole and a line run through that block. Then the gin pole is stood up aft of the mast on deck and stayed with temporary shrouds (essentially three lines that run from the top of the gin pole to cleats—two aft and one forward). To unstep the mast, take the line that's run through the block on the gin pole and tie a loop loosely but securely around the mast and then hoist the line up until the loop stops at the spreaders. The key here is to still have enough lifting distance so that you can pull the butt of the mast up and clear of the deck.
Once you've pulled the butt of the mast up above the deck, you'll need to steady the spar so that the masthead doesn't swing around in an arc. You can see that it makes sense to do this in flat or protected water. At this stage, if you're by yourself, it gets a little difficult because you have to both lower the spar and guide it to the deck. It's really best with two or more people, but it can be done by one. To restep the mast, just rig the gin pole the same way and simply reverse the process.
Of course there are other methods for stepping and unstepping masts on small sailboats. You can always raft up to a larger vessel and use the mainmast and halyard on that boat like a gin pole. And some marinas or boat launching areas are equipped with a simple crane or mechanical arm that you can use in almost the same way that you employ a the gin pole, but these places aren't always handy near the bridges and canals that are requiring you to unstep your mast in the first place.
Of course if you have enough people around, you can simply muscle the spar up and down via joint effort. If you've ever seen a gathering of J/24 or Melges 24 sailors getting their boats ready for a regatta, you'll notice that many of these sailors step and unstep their masts by hand. The J/24 spars are made of aluminum, so they're heavier than the carbon-fiber spars on the Melges 24, but by using a coordinated effort, these sailors nonetheless get the spars up and down by hand. However, this is really a last-resort method because it's likely that your spar isn't tapered like the ones mentioned above and will be heavier. If you're sailing short-handed, I'd recommend you get the right gear and arrange to use the tripod method.
Here's hoping that this information is useful to you.