Planing hull efficiency
Posted on 31 July 2009
Written by Eric Sorensen
The overwhelming popularity of the planing hull is a direct result of its pure speed potential.
While the displacement hull is limited to the speed of an open-ocean wave of the same length, a planing hull has no such restraints. Add power, and you get more speed.
But how much speed you get for the horsepower is the crux of the issue. What impact do displacement, trim, deadrise, form and frictional drag, waterline length, and chine beam have on speed for the horsepower — in other words, efficiency? To answer that question, let’s take a look at how planing hulls work and what makes some more efficient than others, starting with the types of resistance the propulsion system has to overcome.
Propulsion and resistance
Two factors are at work when a planing vessel is in motion: the propulsion power, or force, that creates forward motion, and the resistance that opposes it. Independent of hull design acumen, a planing boat’s efficiency and optimum speed range is, to a large degree, determined by the propulsion system.
For example, a conventional inboard is quite efficient up to 25 knots or so, but since resistance increases as the square of hull speed, the running gear (shafts, struts, rudders, props), which has a lot of frontal area, starts creating substantial drag above that speed. A conventional inboard can achieve speeds of 40 or 50 knots and higher, but as speed increases, a greater percentage of the propulsion force is absorbed just driving the running gear through the water.
Sterndrives, outboards and pod drives are more streamlined and, therefore, more efficient than an inboard at speeds above 25 knots. This efficiency gap widens as speed increases. The waterjet is typically most efficient at 25 to 45 knots, while the surface-piercing drive is in its element above 35 knots, since the only propulsion equipment below the water is the lower half of the propeller, greatly reducing parasitic drag.
In addition to its natural operating range, each propulsion system generates forces and moments that affect the hull’s trim in different ways, and this has to be taken into account in the hull’s shape and weight distribution.
Working to oppose propulsion power are various kinds of resistance, or drag. Form drag results when the hull travels through the water, with its shape and frontal area relative to its direction of motion determining its total resistance. Hull beam at the chines and hull depth, as well as drag from appendages such as struts and rudders, create form drag.
Frictional drag is a product of the hull’s wetted surface, or the total area in contact with the water. Friction is created as the hull drags a boundary layer of water along with it. That layer increases in thickness as it moves aft and down along the bottom of the hull. As you would expect, a smooth, waxed fiberglass bottom creates less frictional drag than a painted and fouled bottom.
Form and frictional drag are proportional to the square of the speed, so a planing hull at 40 knots has a drag component 100 times greater than a displacement hull at 4 knots (16 vs. 1600). In a planing hull, form drag predominates at high displacement and low planing speeds, with the wake size serving as a reliable indicator of form drag at any given speed. At higher speeds, as the boat rises vertically to the top of the water surface and the wake flattens out, form drag (from the hull, not the appendages) decreases relative to frictional drag, which continues to increase. On high-speed boats, there’s also frictional drag from hull spray, which can be reduced by chine flats and strakes that break spray away from the hull.
Then there’s parasitic drag on equipment or parts, such as bottom-mounted transducers or wind resistance from a tuna tower. A 34-foot tower and a soft plastic flybridge enclosure can slow a 2,600-hp, 55-foot sportfisherman by 2 to 3 knots in a head wind. Wind resistance above also increases trim, because the resistance, though perhaps modest, is so high up, creating a considerable bow-lifting lever arm.
Every planing boat has a speed range at which it’s most efficient — a function of propulsion type, hull size, design and displacement, and the trim, or running angle. To get on plane, a planing hull has to climb over and pass its own bow wave; it has to get over the proverbial hump. This takes a great deal of power because of the hull’s high transitory angle of attack relative to the water surface. But once on plane, efficiency increases as the hull rises and trim decreases, flattening the wake and reducing form drag. With less hull bottom in contact with the water, frictional drag also decreases.
Form equals function
Planing hulls require a bottom form, or shape, that can develop the pressure needed to lift the vessel vertically to the surface of the water. The faster the boat goes, the higher out of the water it rises, and the more the hull is supported by hydrodynamic lift and the less by hydrostatic buoyancy. What makes a hull plane are 1) buttocks aft that are nearly parallel to the waterline when the hull is at rest, and 2) an immersed transom, as opposed to the displacement hull’s upswept buttocks with the transom above the waterline. Think of the buttocks as creating the wing surface that generates lift, with the immersed transom developing lift all the way to the stern, preventing squatting so the boat can climb over and pass its own bow wave.
Within these parameters, planing hulls can have different shapes. A round-bilge planing hull can get up on plane, but it’s not as efficient as a hard-chine planing hull for two reasons. First, the chines extend the size, or surface area, of the hull’s planing area all the way out to the hull sides, while the round-bilge hull starts to curve upward well inboard to meet the hull sides. The chines create more lift-generating bottom surface, which acts to reduce bottom loading per square foot and helps the boat plane more easily — and therefore more efficiently — and at lower speeds.
Second, hard-cornered chines allow the water to break clean from the hull surface, creating flow separation, which reduces the wetted surface — the amount of hull in actual contact with the water (a function of boat weight, speed, deadrise, trim and chine beam) — thereby reducing drag and increasing speed.
Length-to-beam, et al
The hull’s length-to-beam (l/b) ratio is a very important efficiency factor for several reasons. But consider first a disturbing trend during the last 25 years — boats getting wider and wider, making the hulls less efficient and harder riding to boot. In part, this is because shorter/wider boats run with higher trim angles (bow up), which increases form drag as well as vertical accelerations from wave impact (it’s called pounding when extreme).
Thanks to market demand for condo-sized accommodations afloat, we have production 42-footers with 15- or 16-foot beams, for an overall length-to-beam of 2.8 or less. If you build the same-sized boat at 46 feet by 13 feet, 6 inches, it will be more efficient, less susceptible to trim change as speed and weight distribution vary, will run at more moderate trim angles (improving both ride and efficiency), stay on plane at lower speeds, and be more comfortable in a seaway — in short, a superior all-around boat.
Let’s look at the length-to-beam ratio’s effect on trim. As I pointed out, while short, wide boats run bow high, narrower, longer boats run with more modest bow rise, since the boat’s weight is spread out over a longer waterline and that greater length better resists trim-changing weights and forces.
To achieve maximum efficiency, one wants to minimize the form drag created by stern immersion and buttocks angle of attack by lowering the bow, but not so far that frictional drag increases to counterproductive levels with the additional wetted surface forward. The longer/
narrower boat runs more naturally at this optimum angle, without wedges and with less use of trim tabs and, therefore, with less drag. Trim equilibrium is reached when the center of dynamic lift of the water flow along the bottom of the boat is balanced by the vessel’s longitudinal center of gravity, which is determined by the weight of the vessel and everything in it.
Another efficiency factor is deadrise. A flatter hull develops lift more efficiently, but bottoms that are flatter in the forward half of a 35-knot hull pound mercilessly, so that’s not a solution. There are, however, many of these boats being sold today, so beware. And I doubt the difference in efficiency between hulls with 20 and 22 degrees of transom deadrise can even be reliably measured, though the difference between 15 and 24 degrees can be. Unless you’re in a 70-knot boat, it’s the deadrise farther forward and in the hull’s midsection that determine ride quality, not transom deadrise.
Weight matters most
Even when on plane, a hull displaces water; that’s where the wake comes from, and the wake is both the result and the measure of form drag. So for a planing hull, displacement (total vessel weight) is the single most important part of the efficiency equation, because the hull is correspondingly more deeply immersed, plowing a deeper trough through the water.
If you add weight, a boat will slow down very predictably — so predictably that performance prediction curves produced during the boat’s design phase are remarkably accurate. Weight change has more effect in a planing hull than in a displacement vessel because form drag goes up faster as weight is added. It takes a lot of energy to keep a hull skimming along the top of the water (recall how fast a planing hull comes off plane when you chop the power.)
Think of it like this: The hull has to run along at an angle of attack to the water surface — typically from 2 to 7 degrees, depending on the shape of the hull and its weight distribution — to develop the lift needed to plane. The more weight, the more power it takes to maintain a given forward velocity against the increasing pressure of the bottom meeting the water surface at this angle of attack.
The pressure on the bottom of the hull on plane at a given trim, or angle of attack, increases directly with the boat’s displacement. We call this pressure bottom loading, and it can refer to the static pressure of buoyancy when the boat is tied to the dock, and to dynamic pressure when on plane. Bottom loading is a very important concept, and overlooking its significance is a big reason some production planing boats today are so inefficient.
The best example of excessive bottom loading that comes to mind is a popular 36,000-pound, 40-foot, 1,600-hp, 35-knot sportfisherman that gets a hair above 0.4 nautical miles per gallon. This boat’s bottom is so small for its displacement that it needs more and more power (and more fuel) to make its 30-knot cruise, creating a sorry state of affairs. Much better that the boat should have more beam and waterline length to lower the bottom loading and increase efficiency. Rather than reducing the hull’s bottom loading and cutting some weight out of its structure, the builder — and there are many like this one — took the easy way out and just added horsepower. Oh well, the boat rides better with more weight, anyway, they rationalize, and their customers buy into it.
Now if one boat weighs 10,000 pounds and has 250 square feet of bottom area, then bottom loading is 40 pounds per square foot. If the next 10,000-pound boat has 300 square feet of bottom, then bottom loading drops to 33 pounds per square foot. This weight per unit area of bottom makes a huge difference in how easily a boat can get up on plane and on the minimum speed at which it can stay on plane.
The deeper the transom is immersed (this is a great visual indicator of bottom loading) when the boat is at rest, the more power it will take to get on plane, the more water it will displace as it planes, creating a big wake, and the less efficient it will be. The more lightly loaded 10,000-pounder with a bigger bottom can stay on plane with a flat wake astern at 12 knots, while the other one, like our 40-footer above, starts wallowing and falling off plane at 17 knots. Guess which one is more efficient?
This ability to plane at lower speeds also lets a boat come home in rough weather at 12 to 14 knots, still efficiently on plane, while the more heavily loaded boat will be in semidisplacement mode at the same speed, burning a lot more fuel.
One lesson that presents itself in this discussion is that all the speed/length guidelines about how fast a boat has to go to semiplane or fully plane are only very rough rules of thumb. A lightly loaded 40-footer can be on plane, defined as a rise in the center of gravity with a clean wake astern, at 11 knots. The same 40-footer with 10,000 pounds of fish in the hold might not be on plane until it’s making 16 or 17 knots.
Trim matters, too
If weight matters the most, then trim is a very close second, because trim determines lift as well as drag. Compare two 30-footers of the same displacement running along at 30 knots. One runs at 6 degrees of trim and the other at 3 degrees. The transom on the first boat will be more deeply immersed, so it will displace (push aside) more water, create more form drag with its deeper hull, burn more fuel, and pound more in a chop than the next boat running at 3 degrees of trim. In fact, there is a direct correspondence between trim and both efficiency and ride quality/vertical accelerations. And, yes, even a fast planing hull displaces water — that’s what creates wake.
Getting the right trim is a balancing act in search of a sweet spot. Trim the bow up with the drives, and form resistance increases as the hull plows a deeper trough through the water, though frictional drag decreases with less hull in contact with the water. Drop the bow down with drives and tabs, and form drag decreases as less water is displaced by the less deeply immersed stern, but the increased wetted surface forward adds frictional drag. The trick is to find the precise trim that produces the optimum balance of lowest combined form and frictional drag for greatest efficiency. To get a hull to run naturally at the optimum trim, hull shape, weight distribution and the force vectors created by the particular propulsion system all have to be taken into account.
The biggest factor in planing hull efficiency is weight, specifically the amount of weight per square foot of hull bottom. Reducing weight using cored construction is a good start, but just as important is reconsidering whether you really need the icemaker, large-screen television, vacuum cleaner system, washer and dryer, teak decks, 30-foot tower and the 12-foot dinghy. Simplicity can be its own reward.
And do you really need to cruise at 30-plus knots? If a 22- to 24-knot cruise will do, you might cut your power requirements in half, which reduces both engine and fuel weight. And it is so much more pleasant, in terms of both noise level and boat motion, to run at 20 knots rather than 30.
A 20-knot boat also can use conventional inboard power efficiently, since running gear drag doesn’t become a major issue until well above these speeds. A longer, narrower boat runs more efficiently and comfortably than a shorter, wider one in part because its unaided trim angle is more moderate. It will stay on plane at a lower hull speed, so it will run efficiently in semidisplacement mode as well.
The very best way to go about designing a boat is to settle on the size you need — say a 40-footer — then make it 15 percent longer, with nothing added other than length. This will make it more efficient and faster with the same power, and it will be more comfortable to boot.
Keep it simple and light, with fewer things to break and add stress, and you just might find yourself having more fun on the water.
See related articles:
- Time to throttle back
- The folks who did it first still do it best
This article originally appeared in the August 2009 issue.