It's amazing how many boating “facts” seem to be up for debate, especially when it comes to used boats. You’d think that a fact is a fact, end of discussion. But we’re bombarded with bad information that sounds good enough to believe, and good enough to stir heated conversation. Think about it. The facts once were: The earth is flat; no man will ever run a mile in under four minutes; four-stroke outboards weigh too much to be useful to recreational boaters. As it turns out, those were opinions. For the last few months, I’ve roamed the docks listening to boatyard arguments about boats, both used and new. You’ve heard them all. Now here’s the way it really is. End of discussion. Probably.
Propellers: Do They Push or Pull?
We call them “screws,” which conjures up an image of a wood screw twisting its way through water, pulling the boat with it. But the image is way off.
Propeller blades are foils, like sails or airplane wings. As a prop’s blade passes through the water, it builds up a high-pressure area on the trailing side as it tries to push water aside, while simultaneously creating a low-pressure area on the leading side. This causes the prop to “lift” — forward — from the area of high pressure to an area of low pressure. Because the prop is connected to the engine, and the engine to the boat, all of it goes along for the ride.
This pressure differential also causes water to be drawn in from the front of the prop and accelerated out the back at a higher velocity as it is funneled past the blades. The change in momentum of the water, called thrust, gives an added boost to the push/pull.
So the short answer to the original question is “both.” There’s a combined force that pushes and pulls the prop forward, along with the boat to which it’s attached.
Stepped Hulls: Do They Produce Lift or Aerate the Water?
You see the churned-up water streaming aft from your transom and figure it’s the same flow coming out from behind each step. This leads you to assume that aerated water under your boat is reducing skin friction and helping you go faster. It’s an intuitive deduction, but a wrong one.
Water flowing along the bottom comes off a step’s aft edge, providing lift and leaving an air pocket behind the step. The water soon rejoins the hull just before hitting the transom, where it also provides lift. Between the step and transom, the bottom is relatively dry, and that’s what reduces skin friction. If the water along the bottom of your boat were all bubbly and aerated, it wouldn’t provide much support.
Steps not only reduce skin friction but also provide their lift in a more efficient way. Ideally, a boat’s bottom should meet the water at about a four-degree angle. To achieve this “angle of attack” in a conventional V-bottomed boat, the bow is raised by positive trim to the drives, by shaping of the hull, or by the fixed down-angle of an inboard’s shafts, the latter reducing forward thrust since thrust is no longer level. In a stepped hull, this ideal angle is built into each step. There is almost no bow rise. The trim is built in.
Bottom line? Steps give you more speed by reducing skin friction and improving lift.
Strakes: Are They for Lift or Stability?
Don’t call those flat strips running along a hull’s bottom “lifting” strakes. Their main purpose is to prevent spray and water from riding up the hull, thereby reducing wetted-surface resistance. While there is a minimal lift and stability generated by the deflected water and from the strake’s flat underside, these are secondary benefits.
A strake’s edge must be sharp to divert water away from the hull lest they create speed-reducing drag. This is a problem, because fiberglass likes to be molded with nice rounded edges. Want to see what strakes should look like? Check out the ones on a Skater racing cat. They’re sharp and straight.
So the answer is a little of both. But strakes’ main purpose is to help reduce surface friction.
Displacement: Is It a Boat’s Weight or Its Size?
While you can think of it as being only about weight, designers understand displacement as the true measure of a boat’s size — and you should too.
Around 220 B.C., the Greek mathematician Archimedes figured out that a boat would settle into the water until it has “displaced” (taked up the same space as) a volume of water whose weight equals that of the boat. Seawater weighs 64 pounds per cubic foot (fresh water about 62 pounds). As long as there’s enough hull to push aside a cubic foot of water for every 64 (or 62) pounds of boat weight, it will float.
Displacement, then, refers to both weight and volume. Knowing this, you can get a better idea of a boat’s true size. For example, a 10,000-pound 26-footer will be a lot bigger (have more volume) than a 6,000-pound model. This is a good thing to keep in mind when shopping, because it will help you compare boats by their true sizes.