Friday, 20 April 2012

What Is a Fast Boat?

So, my sailing boat will make 5 knots and your boat will make 6. Does it matter – really? And, if your boat is faster, is she a fast boat – and is mine therefore slow? Are we getting into the realms of philosophy? Am I losing the plot?

Well, if so, I blame it all on the recent posts

Boat design (1) Hull Shape’.


‘What the Brochures tell you’(April 2012)

Every question answered seems to pose another one. So here goes – if my boat were the only boat in the water, would she be a fast boat or a slow boat (see the link to philosophy – if a tree in the wood fell down and there was no one to hear it ……. )?

Time to consult the text books and here is a kind of answer. Speed over the water depends on a number of variables, the length and shape of the hull, the power of the engines. Obviously, a long waterline length and a shallow draft which allows the boat to rise above its bow wave is helpful but to get maximum effect you also need engines which can develop enough power to maximize the boat’s potential. But what happens if you are asked to compare boats of similar size or engine power, how can you define a boat as fast or slow for its length and engine power? Here’s one way used by boat designers who categorize boats as ‘low speed’, ‘medium speed’ or ‘high speed’ vessels – it is known as the speed/length ratio.

Boat Speed (V) is compared with Waterline Length (L)

(V) divided by the square root of (L) is the Speed/Length Ratio. If the answer to the equasion is 1.6 or less, the boat is a low speed vessel. Medium speed vessels will come out somewhere between 1.6 and 3. High speed vessels will score more that 3.

Here’s an example ---- a vessel capable of 6 knots (V) with a waterline length of 25ft (L).

Square root of 25 = 5. 

6 divided by 5 = 1.2

This would be defined as a slow boat

The same calculation applied to a vessel of 30ft waterline length capable of 20 knots would produce a speed/length ratio of 3.6 – this would be a fast boat

So, in searching for the ideal ‘Low Cost Cruiser’  what do I want fast or slow? Well there is a bit of pride in me which says I’d like to think that she can cover ground – but another part of me asks how much sacrifice in terms of comfort would I prepared to make to gain one knot over another vessel. Truth is …. probably not very much. A slow boat keeps you at sea for longer and if you don’t like being at sea – why go out there at all?