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Long Hull Theory

They won’t have called it 'long hull theory' of course, but humans probably became aware of “long thin hull theory” shortly after they first took to the water. It would not have taken much to find out that longer, narrower pieces of fallen-over tree can be pushed along more easily than shorter, wider ones. And when your only source of power is yourself that’s an important discovery… if you choose the right shape “boat” you can go further or faster, catch more fish, etc, for the same effort.

That’s why canoes and kayaks, whaling boats, pilot gigs, and countless other forms of muscle-powered watercraft developed into the long, lean shapes we recognise today. If you put the same load into a craft with a longer waterline, it needs less effort to get it to where you want to go.

The advent of mechanical propulsion of course made it easier to add more power, to compensate for less efficient hull shapes. You can cram more accommodation into short fat motor boats and they’re cheaper to moor. But it’s a vicious circle: more power costs more money to install, weighs more (necessitating more power to compensate), and uses more fuel (so you have to carry more fuel, necessitating more power to compensate).

That’s why large commercial ships such as oil tankers and container ships are still relatively long and narrow, even today. A shorter, wider shape of equivalent carrying capacity would be easier to moor and probably cheaper to build, but those factors are outweighed by the lower lifetime running costs for a longer hull.

Aerial photo of cargo ship at sea

It’s quite simple: all else being equal, a longer hull is more efficient.

Long Hull Theory
Technical Stuff

Some Technical Stuff

Most conventional hulls obey a basic rule of thumb: maximum displacement speed will be 1.4 times the square root of the waterline length. So a displacement hull that’s 16 foot long on the waterline can be driven up to about 5.5 knots relatively easily, but to go any faster than that requires a huge increase in power.

Planing hulls can be made to exceed their displacement speed by applying lots of power to get them “over the hump” (effectively their own bow wave). But they do require quite a lot of power to achieve that, because quite a high proportion of the power goes into creating lift rather than forward motion. And planing hulls are much less efficient than displacement hulls, when they’re not planing.

Long narrow hulls inherently have a higher displacement speed (because they have a longer waterline length). But also, once you exceed a length to beam ratio of about 4:1, the conventional rule about maximum displacement speed starts to break down. Less power is needed to achieve speeds higher than their theoretical displacement speed.

Portability benefits

Portability benefits

We’ve been selling folding and inflatable boats for many years and one particular problem comes up all the time. The largest motors that most people are happy to pick up and carry are 6hp (10hp at the very most). 6hp will get one person on the plane in most conventional inflatable dinghies, but not usually two people. Flat bottom boats might plane with 6hp and two people on board, but they’re horrible to use as soon as you encounter any waves. (SpearFish are not flat-bottomed.)

So with conventional dinghy shapes you need either an unmanageably heavy motor, or (with smaller motors) will be restricted to relatively slow speeds.

That’s because most conventional dinghies, even at 3-4m long, only have a length to beam ratio of 2:1. In fact some of the shorter dinghies on the market are nearly square! That is not very efficient, so we decided to start playing with longer, narrower inflatables… and SpearFish was born.

The SpearFish 450 has a length to beam ratio of just over 4:1, and on the 550 the ratio is higher still (ie even more efficient). Both boats are much, much, much more easily-driven than conventional short fat inflatables.

SpearFish boats are long and narrow so they can make better use of the small, lightweight engines that can realistically be called portable.

What's It Like in the Waves?

But What’s It Like in Waves?

Everything above hopefully sounds quite logical, but a lot of people then question the SpearFish range’s behaviour at sea. The answer might not be what you think. We’re not saying these boats are for crossing the Atlantic… but we’d much rather head into a chop aboard a 4.5m SpearFish than a 2.5-3m “fat inflatable”. That’s because a SpearFish has a much finer entry (less frontal area), so it tends to slice through the top of waves rather than bounce over them. This makes for a smoother ride. Side waves also have less effect than they would on wider boats, because there is less distance from the outer edges of the hull to the centreline (less tipping leverage).

The initial stability is a little less than it would be on a wider boat, but as soon as you start trying to immerse those side tubes (at angles of a few degrees) the “push back” from the immersed buoyancy is huge. There is more than adequate stability to stand up and cast a fishing line for example, even in the 450 (the 550 is more stable).

Man stood up on SPearFish boat fishig

We’ve really tried to do some quite stupid things in SpearFish boats, in conditions we would not recommend you go out in, and their behaviour has always been impeccable.

Is There a Downside?

Is There a Downside?

It’s probably obvious, but SpearFish boats have less space inside them than conventional (wider) inflatables of the same length. Think of the SpearFish 450 (15ft) as having the capacity of a 2.7m (9ft) dinghy, but with the hull stretched into a more efficient shape; and similarly the 550 (18ft) model as having capacity more like a 3.3m (11ft) dinghy.

Considered as boats alone they are consequently a little more expensive on a “Pounds per person capacity” ratio, but that is compensated for by the fact that they can use smaller engines to push them along at equivalent – and often greater – speeds. The total package price is therefore similar, while longer term running costs (fuel and servicing) will be lower.

Comparison of similar performance boats (UK prices at Feb 2018):

  • SpearFish 450 plus Suzuki 6, £2450 vs Honwave T27 plus Honda 8, £2750.
    Not forgetting the Suzuki 6 only weighs 24kg, whereas the Honda 8 is 42kg.

  • SpearFish 550 plus Tohatsu 8, £3250 vs Honwave T32 plus Honda 15, £3323.
    But the Tohatsu 8 is 37kg, while the Honda 15 is 47kg (over 100-lb!)

Remember the whole point of the SpearFish project was to make more efficient use of smaller, lighter, more portable engines, ie make it easier for you to go boating.

If you can’t lift the motor on and off the boat without hurting yourself you won’t want to do it, so then you either won’t use the boat, or you’ll end up keeping it set up on a trailer… a further cost (not included above), and a storage problem.

In the two comparisons above the SpearFish option is lighter, faster on the water, and more comfortable in waves.


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