Boating’s Great Debates … Settled

The answers to boating’s most-argued questions are not as debatable as you might think. Or are they?

February 17, 2010

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.

Stainless Steel: Will it Rust or Won’t It?
Let’s settle this one by calling it “stain-less” steel. Yes, it can rust, but not easily.

This alloy gets its corrosion resistance by having at least 10.5 percent chromium — and up to 18 percent for marine applications. The chromium reacts with oxygen in the air or water to form a thin layer of chromium oxide that protects what is underneath from corroding. Scratch the surface and it immediately heals itself as long as oxygen is available.

The most common staining from this steel is “weeping,” or streaks of rust around a fitting. This means that water has seeped into the fastener’s hole because of poor or no bedding. The water becomes stagnant and oxygen-depleted, the metal corrodes because it is no longer protected by chromium oxide, and rust seeps back out. When you see this, it means that the fastener is failing.

Marine-grade stainless steel also has nickel added to improve corrosion resistance and tensile strength, and the carbon is removed to make it nonmagnetic. Type 304, often used in sinks, is most common and is best above the waterline. Type 316 is better, and 317 (rare) is best.

Adhesives: Should We Trust Them or Fear Them?
The typical reaction to a boat held together by glue is one of mistrust, if not fear — which is just another example of how little knowledge there often is in common knowledge.

An increasing number of major components in our boats is being held together by adhesives, usually methacrylates. Rivets, screws and bolts concentrate mechanical stress loads; adhesives evenly distribute stress.

The most popular of the adhesives is Plexus. Wellcraft was the leader in the use of Plexus back in the mid-1990s with its coastal 26. Today, most builders use it to bond parts such as internal liners, stringer grids and decks to hulls. It’s based on methyl methacrylate, which acts like a solvent to fiberglass laminates. Think of it as welding without heat. The process is called cross-linking, joining the parts chemically and physically at the molecular level. This is in contrast to conventional adhesives such as epoxies, which grip only the surface of cured fiberglass.

So don’t be put off if the boat you’re about to buy is held together with “glue.” If it’s the right kind, you’ll be getting a better product.

Anchors: Is it Size or Weight That Gives Holding Power?
Most folks will tell you it’s all about weight. That’s why we buy anchors by the pound, right? Wrong.

If you depend on weight alone, you’ll need a tremendous amount of it. Let’s say a 2-cubic-foot block of concrete weighs 300 pounds. however, for every cubic foot of volume, an object is buoyed up by 64 pounds when put in salt water. So, our 300-pound block is now doing the job of only 172 pounds. The only hope it has of holding is if one of its edges digs into the bottom. And it’s the concept of “digging in” that led ancient seafarers to make the transition from a killick (a large stone with a rope) to the first anchor with flukes — albeit small ones. This, in turn, has been refined to the high-fluke-to-weight-ratio anchors we have today, such as the plow, Bruce and Fortress.

In the end, it is fluke area, not weight, that determines holding power. The fact that a 40-pound Fortress holds more than a 20-pound model is not because of its weight, but its greater fluke area. The weight is only a byproduct of using more metal.

Diesel Engines: Are They Money Savers or Wasters?
Diesels can save some boaters money, but for most of us, they’re just wasted dollars.

The controversy is based on the fact that diesels use less fuel — a lot less. For example, a 300 bhp diesel might burn 17 gallons per hour when wide open, while a similarly powered gas engine would use about 30 gph. Another saving is that a diesel can typically go 5,000 hours before needing a major overhaul, compared with a gas engine, which might last only 1,500 hours. This is because diesels are made to closer tolerances and built heavier. With proper maintenance, good installation and long, steady running times (rather than a lot of short runs), a diesel can give you a lot for your money. However, while diesels use less fuel, the fuel they use is often more expensive than gas. Diesel averages about 20 cents more per gallon in most states. Diesels also cost more. For example, on a Formula 43 cruiser you’d pay about $55,000 more to get twin 300 bhp diesels over the standard twin 375 hp gas engines. Gasoline engines are also lighter, and less weight means less power needed to get on plane.

So unless you’re fortunate enough to use your boat a lot, as in hundreds of hours per year, diesels turn out to be a waste of money.


More Boats