Everything You Ever Wanted to Know about Boat Building HTML version

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.
Displacement Hulls
All boats operating at low speeds are displacement hulls. This includes planing 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 at which the boat passes through the water is dependent upon the shape of the hull at the water-
Look at it this way: Imagine a kitchen knife. If you push it through water sideways, there's lots of resis-
tance 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 displace-
ment 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 in-
creases in speed. The speed where 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 receeding hill
Note how the waterline of this sailboat is canoe shaped in spite of the transom stern.