I generally don't use round dowels. The range of wood species is very limited, usually birch. And they can be hard to find straight ones. And, except for the lowest masts of very large sailing ships, masts are perceptibly tapered. Birch dowels are hard and difficult to sand.
I make spars out of square pieces of wood that I cut on my table saw, which gives me the choice of any species of wood I can find. That includes the most workable wood I know of; basswood which is linden. Basswood strip can also be easily found from Midwest or other sources. It's sold as boards at Home Depot.
Holly, cherry or walnut also are good choices. I haven't tried mahogany, preferring to stain that color on lighter wood, but if Philo likes it, I'd say try it.
I don't use pine or oak. Oak is very hard, cutting it and sanding it is a task. Pine isn't very stable.
Try basswood strip longer than the finished part, plus whatever is needed to anchor the mast to the hull. To me, that means usually through the deck all the way down to the keel, or an inch or more in a solid hull. Take a square piece and mark on one side the taper you want. Sand the two sides flat to those marks. Flip the piece 90 degrees and mark it again on the other axis. Sand to those marks. Now you have a tapered piece, four sided. Sand the four corners so that it's an equal sided octagon in section, wider at the bottom than at the top.
Now hold the end with exceess length and start to draw sand it through paper held in your other hand. You'll quickly learn the feel of how to slightly rotate it and maintain a round section. I can't think of a better way to taper a spar.