Everything You Ever Wanted to Know About Boat Building
Building Boats – Selecting a Boat to Build – The Style of Boat for Your Needs Before you ever start building a boat, you should first consider what type of boat you want and/or need.
I say and/ or, because a lot of people think they want a certain type of boat, due to current styles or some fanciful dream, when they actually should be considering an entirely different design.
Let’s discuss some of the basics of boat hulls so that you’ll be able to look at a hull and figure out how it will perform.
All boats operating at low speeds are displacement hulls. This includes planning hulls going slow. What defines a displacement hull is that the boat displaces the weight of water equal to the boat’s weight (including the weight of the people and cargo inside.)
Sailboats, canoes, kayaks, most dories, rowboats, trawlers, and cargo ships are all examples of displacement hulls. For a displacement hull to move through the water it must push water aside as it passes, then after it passes water comes back together to refill fill the space taken up by the hull.
The ease with which the boat passes through the water is dependent upon the shape of the hull at the waterlines.
Look at it this way: Imagine a kitchen knife. If you push it through water sideways, there’s lots of resistance to it moving, but if you push it through water edge first, it moves quite easily.
Displacement hulls work the same way. If their waterline shape is long and skinny, like a knife, they move through the water with ease.
However, if the shape is short and wide, they have lots of resistance to motion. The ideal shape for a displacement hull is a canoe or kayak shape: long, narrow, and pointed at both ends.
Believe it or not the stern shape is just as important as the bow shape when it comes to minimizing the drag. If you take a look at displacement hulls, like racing sailboats or commercial fishing boats, you’ll see that the waterline shape is nearly always double ended, even if the boat has a transom stern.
As a displacement hull moves through the water, it creates a wave both at the bow, where the water is being shoved aside, and at the stern where the water is rushing back together. These waves get larger as the boat moves faster.
As a displacement hull approaches the speed where these two waves interact, the waves actually start to push each other apart. Since the bow wave cannot move forward, as it is being created by the bow of the boat, the stern wave actually separates from the stern and begins to move aft of the stern as the boat increases in speed.
The speed at which this stern wave separation takes place is called the “hull speed.” If the boat goes faster than the hull speed and the stern wave separates from the stern, the hull “squats” or lowers in the stern and begins having to not only move forward, but also move upwards, climbing a continually receding hill of water.
Trying to push a displacement hull faster than its hull speed becomes very inefficient power wise. If, for example, a 10 hp outboard pushes a certain boat to its hull speed of 6 knots, putting on a 20-horse motor (Doubling the power) may only get it up to 7 knots.
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