According to boat-lore, longer boats are faster. You'll find formulae on the interweb that will work out the theoretical hull speed, but not many explanations about how the formulae were derived. So, I'll give it a go, based on my understanding of hydraulics.
The formulae are based on wave theory, but what has that got to do with boats? Firstly, sailing boats are displacement boats, meaning that they sit in the water, rather than skimming over the top of it like planing boats (or the ultra-modern sailing boats with foils). Displacement boats have to push the water out of the way to move forward. Most importantly, a displacement boat will sit in the trough of a wave, with the front peak at the bow, and the rear peak somewhere behind. The front peak is always at the bow, but the position of the rear peak determines the magical number - the theoretical hull speed.
The other important fact to bear in mind is that the speed of the wave is a function of the distance between the peaks (the wavelength). So, the faster the boat goes, the longer its wave becomes, until the rear wave peak falls behind the stern of the boat.
Try to picture three cases in your mind, looking at a boat from the side as it sails past;
- Front wave peak at the bow, rear wave peak half way along the boat. Both the front and rear of the boat are supported by the wave peaks, so the boat is sailing on the flat, lengthwise. The boat is simply overcoming friction.
- Front wave peak at the bow, rear wave peak at the stern. Again, both the front and rear of the boat are supported by the wave peaks, so the boat is still sailing on the flat, lengthwise.The boat is simply overcoming friction, again.
- Front wave peak at the bow, rear wave somewhere behind the stern. Things now change because the stern is not supported, so it drops. From the point of view of the boat, it now needs to climb uphill to keep up with the front wave peak. The boat needs to expend extra energy to climb the up the trailing face of the wave. Not only is the boat overcoming friction, it is climbing against its own weight, which tends to push if back, away from the front wave peak, thus slowing it down.
The additional energy needed to climb the trailing face of the wave becomes a case of diminishing returns, the faster we go, the longer the wave, and the steeper the climb up the trailing face of the front wave peak. The theoretical hull speed, then, is not a fixed limit, but it becomes harder to overcome both friction and the climb up the wave as the speed of the boat increases.
That's why it is possible to drive a sailing boat beyond its theoretical hull speed. You can do it with a motor boat, which will push it all the way to the top of the front wave peak but, then, you'll be planing.
And so, we can get to the formulae, which are a mix of theory and empirical observation. A useful one relates theoretical hull speed to the square root of the length of the water line
V = 1.34 x √LWL
Where
V = Theoretical hull speed in Knots
LWL = Length of water line in Feet
Note that this relates to LWL (length of waterline), not LOA (length overall).
Applying this to an eclectic selection of boats yields the following table. Apart from the last two boats, I have not included boats above 32 foot. This is because they become much more expensive above 30-35 feet, and they will likely need crew. Bigger might be better, but there is a trade-off in dollars and the ability to sail off on your own.
| Boat | LOA (ft) | LOA (m) | LWL (ft) | LWL (m) | Theoretical hull speed (knots) |
|---|---|---|---|---|---|
| Cygnet 20 | 19.3 | 5.9 | 17.7 | 5.4 | 5.6 |
| Austral 20 | 20.0 | 6.1 | 17.0 | 5.2 | 5.5 |
| RL 24 | 24.0 | 7.3 | 19.6 | 6.0 | 5.9 |
| Noelex 25 | 25.5 | 7.8 | 22.1 | 6.8 | 6.3 |
| Ross 780 | 25.6 | 7.8 | 23.3 | 7.1 | 6.5 |
| Austral Clubman 8 | 26.7 | 8.2 | 25.3 | 7.7 | 6.7 |
| RL28 | 28.1 | 8.6 | 23.6 | 7.2 | 6.5 |
| Hanse 315 | 31.6 | 9.6 | 28.5 | 8.7 | 7.2 |
| Cavalier 32 | 32.0 | 9.8 | 24.0 | 7.3 | 6.6 |
| Suhaili | 44.0 | 13.4 | 28.4 | 8.7 | 7.1 |
| Ranger (J Class) | 135.0 | 41.1 | 87.0 | 26.5 | 12.5 |
This table indicates that my Austral 20 has a theoretical hull speed of 5.5 knots. However I have pushed it to 6.5 knots and just touched 7.5 knots briefly, with a crew, coming down off a wave. It depends on which point you're sailing, how many spare hands you have, and the state of the sea. For instance, if I'm closed hauled and climbing upwind, I'd be happy with 3.0 knots, but if I bear away on a broad reach in the same conditions, I could do 6.5 knots. In the light of these experiences, I look at the theoretical hull speed as an indicator of the average upper speed of your boat. My Austral 20 might do a circuit of Peel Island at an average of 5.5 knots, but it would be prudent to allow for 5.0 knots, or even 4.0 when planning the day's sailing. If I were to upsize to an Austral Clubman 8 or Cavalier 32, I might be able to increase these allowances to 6.0 or 5.0 knots. The gain of 1 or 2 knots is not insignificant when you consider that tidal currents in Moreton bay could get up to about 1.5 knots against you.
Compare Sir Robin Knox-Johnson on Suhaili, who was the first person to sail solo around the world non-stop, covering 30,123 Nautical Miles in 313 days. His average speed was 4.02 knots (A World of My Own, Robin Knox-Johnson, 1969), which was much less than the theoretical hull speed of the boat (7.1 knots). However, he did not have the luxury of picking his conditions or scampering to port when the winds were either too light or too fresh.
Finally, it seems the boat manufacturers have done their sums, and know that most new boat-owners are likely to be fair weather sailors. The older designs, after the likes of Suhaili, have a considerable shorter LWL than LOA. The reason is that the boat is more "sea-kindly" with a sloping bow and counter (overhang) at the back.
So, for instance, an older design like the Cavalier 32 has a lot of boat above the water at the bow and stern. This makes it more comfortable in heavy weather. It slams less, when the front of the boat drops off a wave and hits the water below, because the bow is more knife-like. Its the difference between slapping water with the palm of your hand, and doing a karate-chop on it. It is also less likely to get pooped, when a trailing sea dumps a large volume of water in the cockpit, because the approaching wave will pick up the counter before hitting the cockpit. However, these sea-kindly features come at the expense of accommodation. Pinched ends generally mean smaller bunks and cramped cockpits.
The other factor is that although LWL gives you speed, LOA is what you actually pay for. Marinas tend to charge per foot (or metre) of the overall length, per berth. So, the difference between LOA and LWL becomes an "overhead". If you want to reduce the overhead, you need to reduce the difference between LOA and LWL, which brings us to the newer boat designs, such as the Ross 780, Austral Clubman 8 or Hanse 315.
These boats have plumb ends, which give you more theoretical hull speed per foot or metre of LOA. They also have larger accommodations; the Hanse 315 as a stern wide enough to place a double bed width-ways across the boat, which makes good use of the available space. They might be less sea-kindly than the older designs, but the manufacturers probably reckon that their owners would have tied up safely in their marinas than put to sea in rough conditions, anyway.
In conclusion, then, the theoretical hull speed is a guide, rather than a limit. Also, there is a trade off between speed, sea-kindliness and accommodation. Generally, the bigger the better, but bigger boats are more expensive and might be difficult to sail single-handed.
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