Power Boats

Under power

Many like it this way.

In our modern times, it is almost a natural extension of the busy daily life to have a power boat as your preferred choice. Get there fast, get back fast, run away from bad weather fast, do it all fast. Especially if it is a speed-planing boat, it is as close as it gets to driving a car on the water. Make sure there is enough fuel, turn the key, press a few buttons, rev the engine, and off you go. The power of your engine is an important factor determining how far and where you can go, under what conditions, how fast you will get there and how long you can stay... The boat design details are almost secondary. With a strong engine at the back, almost any flat bottom shape can slide and glide over the water. And they do come in an endless variety of designs, some of them literally dangerous to expose to the waves and wind.

Power boats are great, let us be clear on that. They do have their values and even some advantages, at least some of them do. My son and I built together in our home garage the Platypus, a 6.2 m LOA aluminum-plate mini-cruiser (a modified runabout; I call it that because of its reasonable cabin with two full size bunks and a fairly big and safe cockpit where another two people can comfortably sleep; with enough food and water, a family or a group of friends can spend several days on it pottering around protected bays. It is certainly not a boat for the open ocean.) With a four stroke 100 HP Yamaha outboard, it is capable of planing at about 35 kn with two adult people on board. Under power, it is quite good, safe and dry on reasonable waves that are regular on the wide and deep estuary bays around Sydney, Australia, where we live. But if you switch the engine off, it is like a dry leaf on the water, the wind and waves immediately take over.

So, I do own a very fine power boat and enjoy using it immensely. It is a great boat to take your party of friends (it is registered for up to 7 people, very reasonable for its small size) to a nearby beach, or to splash around the protected bays. It is also a perfect fishing platform, where up to 4-5 friends can cast their lines in any direction without getting into each others way. It is also a trailer boat, parked snugly on its trailer when not in use. It is a boat that serves well those basic needs.

But I still think there are not many things more out of place and out of sense than a small 3.5-4 m open-design speed boat with a huge 200-250 HP engine roaring at the back, skimming the tips of the waves in an attempt to fly. Or, seeing a short and stubby power boat capable of planing going at some speed and generating lots of noise and lots of wash, yet with its engine trim badly set, pointing its bow at some odd angle sky-wise, burning a lot of fuel on continuously pushing that big waste-deep wave aside, doing only a fraction of the speed it is capable of achieving if only the engine trim was properly adjusted.

But that is only my personal opinion. What do you think? What would you say?

One thing that comes with experience: Boats, and especially planing-design power boats, often look and feel entirely differently on dry land, especially in a glitzy showroom, then on the water. At first, the moment you lower them on the water, they all become much smaller. And if you dare to take your big boat out on the 2-3 m ocean swell, it becomes real tiny, you feel like riding in a nutshell. Yet, you will really know your boat only when you have run it against wind and waves, with some sea spray in your face.

And, whatever design is your power boat and in whatever conditions you are using it, I believe it is wise to always have a sizeable main anchor attached to an at least 2-3 m of heavier gauge chain followed by at least 30-50 m of rope (of course you know the basic rule that you have to lay out about 3 times more of the anchor line compared to the depth below your boat, for your anchor to have a good chance to safely grab and hold).

A boat is not like a car, where you switch the engine off, engage one of the low gears, pull the brake, and the car is safely parked. When you switch the boat engine off, the boat still moves around, especially if there is some wind and waves. It has absolutely no effect to engage a gear, and there are no brakes. All you've got is your anchor, and it is essential that it is selected and attached in a way that matches the task of holding your boat, and you and your party, at one place under the circumstances as they are. With a power boat, sooner or later you will be in a position that your engine stops for some reason. In a situation like this, it may be critical that your anchor setup is ready and up to the task.

If you walk through any marina, you will often see big, heavy and expensive power boats adorned with pathetically small main anchors neatly tucked in their bows. It sends shivers down my spine to even think about being on a boat like this in a moment of need, when it all depends on the holding power of the anchor.

Know Your Boat: Boat Design Parameters

Displacement power boats are generally far more seaworthy than the typical speed-planing boats. They move through the water pushing it aside at the bow. By design, their keel often runs the full length of the bottom. That gives them excellent directional stability and contributes to calmer ride through the waves, yet prevents them to lift up to a plane even with a strong engine. They are simply not designed for that.

Before gaining sufficient speed to start lifting their bow out of the water, even boats capable of planing move initially as displacement boats. So, as any boat starts moving ahead through the water, it forms a set of pressure waves starting from the bow and moving diagonally out away from the boat, staying behind. Another similar set of waves, similar in pattern yet far smaller and less noticeable, forms at the stern. But there is another set of waves that runs along the vessels side, starting with the first wave at the bow, followed by another some distance behind, and another at the same distance after the second, and so on. It is important to understand that the distance between the crests of any two of these waves are the same for all displacement boats moving at the same speed, irrespectively of the size of the boat or the shape of the hull. A small dingy and a huge supertanker both make waves of the same wavelength at the same speed. It is only the wave height and depth that alters with the displacement and the hull shape.

Obviously, the distance between two wave crests is dependent on the boat speed. As the speed increases, these waves fall further and further apart. At some speed, aptly named the hull speed, there is only the bow wave, followed by a trough running alongside the hull, and then the second wave crest at the stern. That is the maximum speed for a displacement boat, and an important element in determining the required engine power. A displacement boat can exceed its hull speed only in the most exceptional circumstances, such as when it surfs down the face of a wave.

If the engine has excess power, you can try to push the boat to go faster. With flat bottom boats, the second wave crest will start leaving the stern, staying more and more behind as the boat accelerates further. But now the engine is pushing the weight of the boat up the bow wave, with the bow raising higher and the stern dropping into the trough.

For a boat designed for planing, this is a desired effect. Add more power and the boat will start lifting to plane, breaking free of its own wave formation. And as the boat lifts, the bow will drop down again. Once on a plane, the boat will only skim the surface with its stern end of the bottom, with only the engine leg and the propeller submerged.

But for a displacement boat, pushing the heavy boat up the bow wave is like trying to accelerate a car, yet at the same time pressing on the brake and gradually disengaging the clutch. It is a very uneconomical ride, as lots of the engine power and fuel is burned (wasted) pushing the weight of the boat "uphill". For her skipper, it is much wiser to decrease the engine revs and to return back to about the 75% of the hull speed, the optimal cruising speed for a displacement boat.

Generally, sail boats are displacement boats. The same maximum speed principle applies to them also, irrespectively of the hull shape and the sail area. Some designers try to break this limitation with extra light, flat-bottomed hulls and multihulls (catamarans, trimarans) capable of achieving planing speeds under big sails.

The table to the right shows the lengths of the waves formed by the movement of a displacement boat. Notice how rapid is the increase of the wave length with the increase in speed. And that relates directly to the LWL. To increase a displacement boat's hull speed from 5 knots to 10 knots, the LWL should be four times longer.

The simple conclusion is, the longer is the boat's waterline, the higher is its hull speed, the faster it can go. As a design element, the boat speed given in knots divided by the square root of the LWL expressed in feet is the speed/length ratio. The hull speed occurs at the speed/length ratio of 1.34.

Now, some people may say: That is really very nice! But, what good is it to me to know all that?!

Your Boat's Speed Is Determined by Its LWL

Displacement Boats
Boat Speed
Wave Length
(feet) (m)
1 0.56 0.17
2 2.23 0.68
3 5.01 1.53
4 8.90 2.71
5 13.9 4.24
6 20.0 6.10
7 27.2 8.29
8 35.6 10.85
9 45.0 13.72
10 55.6 16.95
11 67.3 20.51
12 80.1 24.41
13 94.0 28.65
14 109.0 33.22
15 125.2 38.16
Hull speed vs. LWL

Let us put this to some practical use. As LWL and the displacement boat's hull speed (it is fair to call it maximum speed) are so directly related:

Hull Speed (knots) = 1.34 * Sqrt(LWL (feet))

it is easy to put together the graph shown on the left.

Take as an example that you plan to buy, or already own a displacement boat, power or sail, with 25 ft LWL. It is certainly good to know that your maximum speed is 6.7 kn (you can read that from the graph). That is your boat's hull speed, your for all practical purposes maximum speed. Rev your engine beyond that, and you are burning excessive fuel on pushing your bow skyways, with probably no gain in speed. Your best economy cruising speed is at about 75% of the hull speed, just a touch over 5 knots.

Now, before you abandon your 25-footer and dash out to buy a faster displacement boat, just consider that to double your maximum speed to 13.4 knots, you need a boat with 100 ft LWL. And that will cost way more than four times the cost of your humble 25 foot "by-boat". Is it worth it? You tell...

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Last upd: 19-Aug-10 F150806