Power Boats: Boat Design Parameters, part 2

Previous page: Boat Design Parameters, part 1

The speed/length ratio is used to group boats into three categories used in designing the boat:

  • Low speed - speed/length ratio of up to 1.6 (remember, hull speed is achieved at 1.34)
  • Medium speed - speed/length ratio between 1.6 and 3
  • High speed - boats capable of planing, speed/length ratio above 3

For example, a boat with 30 ft LWL doing 20 knots has a speed/length ratio of 3.6, so it is classified and designed as a high speed vessel. A 300 ft LWL ship at the same speed of 20 kn is running at a speed/length ratio of 1.15, so it fits in the low speed category and will be designed as such.

The design of the boat's bottom at the stern is the most obvious consequence of the speed category the boat is built for:

  • Canoe stern - for low speed boats
  • Transom and moderately flat stern bottom - for medium speed boats
  • Transom and very flat stern bottom, hard-chine form - for high speed boats

Why are all boats not built for speed? Why would someone deliberately build slower boats? Well, it is not just that simple. It is not only the question of speed. The move into a higher speed category requires not only stronger engine or larger sail area, it brings compromises in comfort and seaworthiness.

Take an example of a hard-chined, very flat bottom high-speed planing boat. In calm waters, throttle up, it goes as a dream. It has been designed for that. But, the moment it hits the waves, it can beat itself to pieces trying to plane over them. It has to slow down and even seek shelter. Going at low speed, a boat designed for high speed planing, with its wide flat transom partially submerged and causing lots of turbulence, and its flat not-much-keel bottom that skids over the waves in all directions like a dinner plate, simply behaves like a badly designed displacement boat. It is skittish, requiring constant adjustment of direction at the helm, giving the people onboard a feeling of discomfort and seasickness.

In contrast, a low-speed category cruiser with a canoe stern and deep long keel will keep meeting waves and wind coming from any direction with confidence, steady on its course, dry and comfortable on the inside.

So, boats are designed with specific conditions of use in mind, full of great solutions, but also with many compromises. It is important that you, as the owner and skipper of your boat, know well its good features and its capabilities.

In the quest for speed beyond the hull speed, to bring the bow down and improve the efficiency, one often seen approach are wider and fuller sterns. The high-speed boat designed for planning is again on the extreme of that. With a wide, hard-chined stern, flat transom and flat bottom, at speed its stern acts as a strong wedge pushing against the oncoming water, bringing the bow down (the same concept can be seen in many racing sail boat designs). That is what enables the high-speed boat to lift out of the water, to nearly level up (the bow raised at some 6-80 higher than the stern seems to be an optimal high-speed planing angle) and plane barely touching the water. But the wide flat transom and flat bottom are very uncomfortable, even risky, at low speeds and with the following sees.

Boat's deadrise, chine, spray rail

Deadrise is the angle the boat's bottom makes with the horizontal drawn from the centerline. An increased deadrise contributes to a more comfortable riding on rough seas. Both power and sail boats may have an average deadrise of about 150 midships, increasing substantially towards the bow (a deep vee of 250 or more is not uncommon), flattening somewhat towards the stern (to maybe 120). A power boat designed for higher comfort may have some 220 midships and about 180 aft. With a strong engine, this could be a fast power boat, but its chines will not lift up as much as with boats with flatter bottom (lesser deadrise).

Another designer's "trick" to increase the speed of the high speed boats, those with the speed/length ratio above 3, are spray rails. They are typically triangular in shape, with their bottom faces parallel to the water surface (in other words, horizontal). The first (and possibly the only) set of symmetrical (left and right from the centerline) spray rails is fitted running from the bow (where the profile may be tapered down) to the stern along the chine. One or more additional sets of spray rails may be fitted closer to the centerline (the above picture shows a secondary set). The bigger is the deadrise, the stronger is the effect of the spray rails. Beam vs. Speed At high speeds, the spray rails deflect the rushing water keeping the boat drier, but also provide additional lift and narrow the effective beam of the boat (the effect of the secondary set), all very desirable effects.

On slower boats, those that can not plane (speed/length ratio less than 3), spray rails have little effect. They may look cool, but all they do is increase the wetted area and thus slow the boat down.

The graph to the left shows the ideal planing beam (achievable with flat bottom and carefully positioned spray rails) for a boat with 25 ft LWL.
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Last upd: 19-Aug-10 F150806