Updated 5 May 2022

Going fast in small boats (without a big motor)

How to make a boat fly—on a budget.

Plodding along at a snail’s pace can get a little boring. No matter how relaxed I am, nothing puts a smile on my face quicker than speed when on the water. With a bigger motor (almost) anything will go fast, but there are ways around it.

To move fast with a short boat (that isn’t skinny like a catamaran), it needs to plane. Skimming across the water instead of pushing through it. But getting on the plane with a small motor is a challenge.

If you ask your marine dealer if the 10ft aluminium Tinnie he is selling will get up on the plane with a 3.3hp motor, you will send him into fits of laughter (it happened to me). He’ll tell you it’s impossible. But I had the last laugh…

Hull speed

As a hull moves through the water, resistance caused by the water sliding along its surface (skin friction) plus the energy needed to create the waves a hull produces (wave-making resistance), oppose the force of the motor (or sail) that is pushing the hull forward.

Air/wind resistance, the shape of the hull, and protrusions like rudders and daggerboards also play a role in the total drag, but skin friction and wave-making resistance are the most significant.

Skin friction increases linearly with speed, but wave-making resistance (WMR) increases exponentially. For a displacement boat (most boats and all ships), one that moves through the water, WMR rises rapidly with increasing speed until it hits a “wall”, its hull speed. The speed beyond which the increase in resistance and power required to go faster becomes uneconomical or even impossible.

The formula for hull speed (in knots) = 1.34 x √ hull’s waterline length (in feet). For example, the hull speed for a small boat with a 3.0m (10ft) waterline length, the hull speed = 1.34 x √10 = 4.23 knots (7.8kmh). Not quick…

Counting the waves

The wave sets created (2) as the boat moves forwards begin at the bow (front) and the stern (back) of the boat. As the boat moves faster, the wave pitch, the distance from one wave crest to the next, gets longer.

Next time you’re on a boat, observe what happens as it speeds up. Watch the pitch of the waves that form along the sides of the hull, starting off as mere ripples, get longer as you speed up. The distance between the crests depends on the speed of the boat. If the boat goes fast enough, the distance from the bow wave to the 2nd wave crest becomes so large it’s equal to the length of the hull, leaving a trough halfway along the length of the hull. In bigger heavy boats, i.e. a tugboat, it can be quite pronounced.

The speed that this occurs is at the boat’s hull speed. Without even looking at formulas, it’s easy to visualise a longer boat will have a higher speed when it reaches this condition. A short boat reaches this condition at a low speed of ~ 4 knots for the 10ft boat.

A container ship, in contrast, with a waterline length of 600ft entering the harbour at 10 knots (18.5 km/h) leaves barely a ripple behind as it is moving along at nowhere near its hull speed of around 28 knots (52 km/h). There are many wave crests and troughs along the length of its long hull.

Back to the small boat example: as speed increases the distance between the crests grow larger and the crest that was at the rear of the boat moves further behind, the trough between the crests moves further aft, and the boat sinks into a “hole” it is digging for itself. Watch a speedboat before it gets on the plane. The bow rises and the stern sinks.

Going faster than hull speed

The heavier and wider the boat, the deeper the “hole” it digs. To escape out of this “hole” onto the plane, extra power is needed. Once on the plane, resistance drops off.

To go fast with only a small motor, the boat needs to be long for its weight, so the hole it digs will be shallow, shallow enough that it can climb onto the plane with little extra power. Reducing the width also helps, as does a flat, smooth bottom, but length is key.

I experimented with many hull shapes during the time I was building my cat, and the difference was huge. Adding an extra metre (3ft) to the length of a short boat often made the difference between being able to plane or not. With really long, narrow flat-bottomed boats, there was almost no “hole” effect. The boat just accelerated straight onto the plane with no noticeable rise in the bow.

Longer is better

The longer and narrower the boat, the better. With Noosa, I could reach planing or semi-planing speeds with my son and myself onboard, including a small amount of gear with a 3.3hp mercury outboard. On my own, it flattened out completely on the plane. Flying!

If you are planning on building a boat, read my book “Wooden Boat Building”. It has all the info and tips you need to help you build a fast small boat, on a budget.

Happy boating…

Tim Weston

 

Some more info on planing hulls (Wikipedia).