# Understanding design and performance of Stepped Hulls

By Kobus PotgieterSo many readers have asked me about “Stepped Hulls” and with this in mind, I thought it might be a good idea to write a brief article about this concept and design feature on the hull bottom of a planing hull.

Stepped bottoms have been used for a very long time to improve performance. A very famous design was Maple Leaf, built in wood in 1912, and since then many successful racing hulls have had this type of bottom. One can say that it is a further refinement of the deep V-Hull...

**What is a step?**

Steps are breaks in the hull intended to reduce the amount of hull surface in contact with the water. Steps can run straight across the hull (although these are structurally weak and not often seen today), or they can be V-shaped, with the vertex facing forward or aft. They will have large apertures on the outboard side of the hull to allow air to be sucked down into and ventilate the step. In general, the speed increase of about 10 to 15 percent can be expected from a stepped hull over a non-stepped hull with the same power train.

The reason why the stepped hulls are more effective is that the wetted area is divided into several smaller areas each with a large beam compared to the length;

Short, wide (high-aspect) surfaces are more efficient than long, narrow (low-aspect) ones in terms of frictional drag on water. Lift generation is just far more efficient with a large beam-to-length ratio surface. So, the idea behind a stepped bottom is to reduce wetted surface by allowing the hull to plane on two or three high-aspect planning surfaces rather than one large, low-aspect surface.

And the popular notion that any added speed from a stepped bottom is due to a layer of bubbles undoubtedly reduce frictional drag to some extent, but the real saving is in minimizing the hull area in contact with the water, specifically by presenting two or three wide and short surfaces to the water instead of one long, narrow one.

**How its works!**

As I have explained in a previous article on hull shapes, the lift production is more efficient for a surface, with a small length to beam ratio. (The planning bottom is different from a wing, where it usually does not help splitting the surface into several tandem wings.) The increased lift generation capability means that the total wetted surface may be reduced, as well as the friction. My drawing shows that the region behind each step has to be ventilated. Air thus has to be sucked into this region in sufficient quantities. Normally this is not a problem since the pressure is very low, but it’s extremely important that the air supply is not cut. New air is continuously needed since the water entrains the air behind each step. This may be achieved most simply by extending the step sideways to the open air at the hull’s side.

**When does it starts to work?**

In general, data indicate that if a boat can’t cruise easily at close to 30 knots or more, it can’t go fast enough to ride up on hull steps, so steps would only add drag. More specifically, this means that a petrol-powered family cruiser with steps should be able to cruise fully loaded at 30 knots, not just reach this speed at full throttle. Otherwise, the extra cost of tooling and the added time and cost spent laying up a stepped hull is wasted, and the stepped bottom is just a marketing gimmick. Some runabout builders even carve out a little scoop at the chime amidships, which I suppose is meant to suggest that the bottom is stepped, when in fact the bottom is as straight as an arrow.

**Downside to the step hull**

This principle is somewhat dangerous.

since the openings may be closed temporarily (and momentarily) by waves. When the air supply is lost, a backflow occurs behind the step causing an excessive increase in resistance. The speed thus drops momentarily – a dangerous situation, which may even cause injuries to the crew. If the supply is cut only on one side the hull will turn abruptly, and possibly even capsize. To avoid this problem air is often sucked through openings well above the waterline, or it may be supplied through tubes in from deck level. Another possibility is to discharge the exhaust gases through the step. In this way the gases will be sucked out, improving the efficiency of the engine.

Builders often provide large inlets to the areas behind the steps, and a few even provide air paths through ducts that lead to the trailing vertical edge of the steps.

Since the lift is now spread to several surfaces along the hull (see drawing) the longitudinal stability becomes very large. It is difficult to change the trim. This is no problem in smooth water, but in seaway the hull may tend to follow the contour of the waves. Larger hulls may acquire a tendency to bump into the next wave, making the ride very uncomfortable. Smaller boats, which tend to jump between waves, are not so affected by this problem. Another effect of spreading the lift to several efficient surfaces, one after the other, is that the transverse stability may be put into jeopardy. The hull rides high on a very narrow set of wetted surfaces. At very high speed some designers have chosen to take advantage of the aerodynamics of the above-water part of the hull, using wing-like devices to keep the hull upright. Transom flaps may be fitted to the hull to control the trim. Temporary adjustments for correcting changes in the centre of gravity may thus be made easily. The flaps may also be used to adjust the trim when the hull is running at off-design speeds, for instance in restricted waters or when the hull is under acceleration. This reduces the fuel consumption and, even more importantly, the generated waves, which may be excessive at these speeds. It is also possible to sue the flaps for adjusting the trim in a seaway to reduce the bumpiness.

**My conclusion**

Stepped hulls should be used by experienced drivers who know what the hull is likely to do in a seaway and in hard cornering, and who know how to react to the unexpected.

# How to Evaluate Boats?

By Kobus PotgieterSo you are in the market for a boat. Hundreds of boats are sold every year and every salesman has his or her own sales pitch trying to perswade you in the brand they are selling. If you have the time on your hand to comparison shop, I have a few of useful computational tools to help making a comparison between boats...

These numbers are nondimensional and can be applied to any size of boat. If using these tools, don’t let the spraed between boat sizes get too large otherwise the comparison will be distorted. Try to keep a maximum range of about 2.4 – 3m between the smallest and largest boat you compare.

The speed-length (S/L) ratio is derived from the Froude number.

Froude Number? William Froude done a number of tests on thin planks in the 1890’s. Froude discovered that a boat’s speed tops out when the wave it is creating has the same length as the boat’s waterline in a displacement mode. I have touched on this a bit in my previous article of Leisure boating explaining the different modes of planing.

The equation for S/L is:

An S/L of less than 1.5 shows that the boat is in displacement mode.

Between 1.5 and 2.5, the boat is operating in a semidisplacement mode.

Above 2.5 and sometimes even higher, the boat is in planing mode.

Calculate the boat’s displacement-length ratio by dividing the boat’s displacement in pounds by 2240 to get long tons. Divide this figure by one-hundredth of the waterline length (ft) cubed. In other words, the ratio is:

If the weight of the boat is 8500 pounds(3856 kg) and the waterline is 31ft (9.5m), the displacement length ratio equals:

In general, the higher the number, the heavier the boat for its length and the slower it is. At sea, the heavier the boat, the likely to handle waves better than a similar lighter boat. Planing hulls are in the 130 to 220 range whereas trawler hulls are above 300. Semiplaning boats are typically between 225 and 300.

Don’t fall into the trap of lightweight numbers. Many buyers fall for the trap that “heavy” boats are a draw back and so many salespeople use the comparison to sell their product using the words “our boat is so much lighter than our opposition. You as a buyer need to ask yourself the question –

How much boat do I need in terms of displacement?

Simply take the total weight of crew and stores you’ll carry and multiply it by 7.5. This is the reciprocal of 8 percent times 60 percent loading – [ (1/0.08) x 0.6] = 7.5. The answer is displacement you’ll need plus or minus 10 percent.

Let’s calculate the displacement required for a Cruiser, Crew of 4 on an ordinary 10 day vacation:

4 x 72kg = 228 Crew

4 crew x 10 days x 6.6kg/day x 1.5 reserve = 396kg food and water

4 crew x 10 days x 2.3kg/day = 92kg personal gear

TOTAL: crew, food, water, personal gear = 776kg

776kg x 7.5 = 5820 kg displacement

Plus or minus 10% = 5240kg – 6040kg displacement boat required.

The length-to-beam ratio gives an indication of how long the boat is relative to its beam and allows you to compare two boats of different size.

For example, comparing a 50ft (15.3m) cruiser with a 12ft (3.7m) Beam to a 40ft (12.21m) cruiser with a 10ft (3m) beam.

We find that the larger boat has a length-to-beam ratio of 4.167 whilst the smaller boat has a ratio of 4.

This just show that for it’s length, the smaller boat has more beam. A smaller ratio indicates a boat with greater transverse stability, making it a better for trolling or drifting in beam seas.

POWER-TO-WEIGHT RATIO

When I design a boat, I use the power-to-weight ratio to indicate whether the boat has sufficient horsepower for its weight. The ratio is:

This is merely an indicator of the amount of horsepower a boat needs to push its own weight through the water. When comparing boats, make sure that you use the same horsepower number whether its is brake horsepower (bhp) or shaft horsepower (shp).

CUBIC NUMBER

This is a good way to compare two boats of different size.

Multiply the waterline length by the boat’s beam and depth (from bottom of the hull to the deck edge), you get a cubic number (CN).

For example:

Boat 1 is 30ft (9.14m) on the waterline and has a maximum beam of 10 ft (3.04m) and a depth of 6 ft (1.83m) and therefore a CN of 1800 ft³ (50.9 m³)

Boat 2 is 34ft (10.36m) Lwl, a beam of 11ft (3.35m) and a depth of 7ft (2.13m) and therefore has a CN of 2618 ft³ (73.9m³).

By dividing the CN of the first boat into the CN of the second boat, you can see that the second boat is 2618 / 1800 = 1.45 (73.9 / 50.9 = 1.45) as large as the first boat. In other words it is 45 percent larger and all other things being equal, should cost more to buy and maintain.

COMFORT RATIO

This is a measure of motion comfort between boats of a similar size and type. It is based on the fact that the quikness of motion or corkiness of a hull in a choppy sea is what cause discomfort and seasickness. The corkiness is determined by two factors:

(1) The beam of the hull and (2) the area of the waterline. The formula is as follows:

Displacement is measured in pounds and the Lwl and Loa in ft.

Lightweight boats and smaller yachts that have a higher Beam/length ratio will rate poorly on the comfort scale while as we would expect, heavy oceangoing cruisers rate more favorably. The ratio ranges from 10 or less for lightweight day cruiser to thehigher 50-60 such as a old sailing pilot boat. Average ocean cruisers come up somewhere in the mid 30’s.

PRISMATIC COEEFICIENT (Cp)

The prismatic coefficient is the ratio of the largest underwater section of the hull

multiplied by the hull’s waterline length, to the volume of the displacement of the boat.

To simplify, if you took a block of wood, the length of the waterline and shaped to the underwater portion of the midships section, then carved it away to model the ends of the boat, the Cp is the remaining percentage of the original midships-shaped block. See fig.1

The optimum Cp ratio varies in direct proportion to the hull resistance and the boat speed. Designers use their experience and knowledge of other designs to select the best Cp for the style and speed of the boat they design.

The Cp of a powerboat hull should become higher as boat speed increase. Obviously, the fastest boats is not a barge which have a Cp of 1.

A typical displacement boat has a Cp of around 0.55 – 0.65.

A high speed deep V hull can have a Cp as high as 0.75, Put in another way, the planing hull needs to be fuller at the ends – especially aft – to develop dynamic lift.

Now you have some tools to evaluate boats of more or less the same lengths. Hope this make your decision easier.

**SPEED – LENGTH RATIO**The speed-length (S/L) ratio is derived from the Froude number.

Froude Number? William Froude done a number of tests on thin planks in the 1890’s. Froude discovered that a boat’s speed tops out when the wave it is creating has the same length as the boat’s waterline in a displacement mode. I have touched on this a bit in my previous article of Leisure boating explaining the different modes of planing.

The equation for S/L is:

An S/L of less than 1.5 shows that the boat is in displacement mode.

Between 1.5 and 2.5, the boat is operating in a semidisplacement mode.

Above 2.5 and sometimes even higher, the boat is in planing mode.

**DISPLACEMENT – LENGTH RATIO**Calculate the boat’s displacement-length ratio by dividing the boat’s displacement in pounds by 2240 to get long tons. Divide this figure by one-hundredth of the waterline length (ft) cubed. In other words, the ratio is:

If the weight of the boat is 8500 pounds(3856 kg) and the waterline is 31ft (9.5m), the displacement length ratio equals:

In general, the higher the number, the heavier the boat for its length and the slower it is. At sea, the heavier the boat, the likely to handle waves better than a similar lighter boat. Planing hulls are in the 130 to 220 range whereas trawler hulls are above 300. Semiplaning boats are typically between 225 and 300.

Don’t fall into the trap of lightweight numbers. Many buyers fall for the trap that “heavy” boats are a draw back and so many salespeople use the comparison to sell their product using the words “our boat is so much lighter than our opposition. You as a buyer need to ask yourself the question –

How much boat do I need in terms of displacement?

Simply take the total weight of crew and stores you’ll carry and multiply it by 7.5. This is the reciprocal of 8 percent times 60 percent loading – [ (1/0.08) x 0.6] = 7.5. The answer is displacement you’ll need plus or minus 10 percent.

Let’s calculate the displacement required for a Cruiser, Crew of 4 on an ordinary 10 day vacation:

4 x 72kg = 228 Crew

4 crew x 10 days x 6.6kg/day x 1.5 reserve = 396kg food and water

4 crew x 10 days x 2.3kg/day = 92kg personal gear

TOTAL: crew, food, water, personal gear = 776kg

776kg x 7.5 = 5820 kg displacement

Plus or minus 10% = 5240kg – 6040kg displacement boat required.

**LENGTH-TO-BEAM RATIO**The length-to-beam ratio gives an indication of how long the boat is relative to its beam and allows you to compare two boats of different size.

For example, comparing a 50ft (15.3m) cruiser with a 12ft (3.7m) Beam to a 40ft (12.21m) cruiser with a 10ft (3m) beam.

We find that the larger boat has a length-to-beam ratio of 4.167 whilst the smaller boat has a ratio of 4.

This just show that for it’s length, the smaller boat has more beam. A smaller ratio indicates a boat with greater transverse stability, making it a better for trolling or drifting in beam seas.

POWER-TO-WEIGHT RATIO

When I design a boat, I use the power-to-weight ratio to indicate whether the boat has sufficient horsepower for its weight. The ratio is:

This is merely an indicator of the amount of horsepower a boat needs to push its own weight through the water. When comparing boats, make sure that you use the same horsepower number whether its is brake horsepower (bhp) or shaft horsepower (shp).

CUBIC NUMBER

This is a good way to compare two boats of different size.

Multiply the waterline length by the boat’s beam and depth (from bottom of the hull to the deck edge), you get a cubic number (CN).

For example:

Boat 1 is 30ft (9.14m) on the waterline and has a maximum beam of 10 ft (3.04m) and a depth of 6 ft (1.83m) and therefore a CN of 1800 ft³ (50.9 m³)

Boat 2 is 34ft (10.36m) Lwl, a beam of 11ft (3.35m) and a depth of 7ft (2.13m) and therefore has a CN of 2618 ft³ (73.9m³).

By dividing the CN of the first boat into the CN of the second boat, you can see that the second boat is 2618 / 1800 = 1.45 (73.9 / 50.9 = 1.45) as large as the first boat. In other words it is 45 percent larger and all other things being equal, should cost more to buy and maintain.

COMFORT RATIO

This is a measure of motion comfort between boats of a similar size and type. It is based on the fact that the quikness of motion or corkiness of a hull in a choppy sea is what cause discomfort and seasickness. The corkiness is determined by two factors:

(1) The beam of the hull and (2) the area of the waterline. The formula is as follows:

Displacement is measured in pounds and the Lwl and Loa in ft.

Lightweight boats and smaller yachts that have a higher Beam/length ratio will rate poorly on the comfort scale while as we would expect, heavy oceangoing cruisers rate more favorably. The ratio ranges from 10 or less for lightweight day cruiser to thehigher 50-60 such as a old sailing pilot boat. Average ocean cruisers come up somewhere in the mid 30’s.

PRISMATIC COEEFICIENT (Cp)

The prismatic coefficient is the ratio of the largest underwater section of the hull

multiplied by the hull’s waterline length, to the volume of the displacement of the boat.

To simplify, if you took a block of wood, the length of the waterline and shaped to the underwater portion of the midships section, then carved it away to model the ends of the boat, the Cp is the remaining percentage of the original midships-shaped block. See fig.1

The optimum Cp ratio varies in direct proportion to the hull resistance and the boat speed. Designers use their experience and knowledge of other designs to select the best Cp for the style and speed of the boat they design.

The Cp of a powerboat hull should become higher as boat speed increase. Obviously, the fastest boats is not a barge which have a Cp of 1.

A typical displacement boat has a Cp of around 0.55 – 0.65.

A high speed deep V hull can have a Cp as high as 0.75, Put in another way, the planing hull needs to be fuller at the ends – especially aft – to develop dynamic lift.

Now you have some tools to evaluate boats of more or less the same lengths. Hope this make your decision easier.

# Planing Hulls

By Kobus PotgieterIn my previous article, I briefly discussed and touched on the different hull shapes available, how they work and how to distinguish these hull forms from one another.

In this article, I will be focusing in greater detail on planing hulls - such as why they ride on top of the water, how to evaluate a planning hull and provide you with some tools to assist you in choosing the right planing boat for your needs...

**How do Hydrodynamics relate to planing of a boat?**

A planing hull uses hydrodynamic lift to rise up and out of the water to reduce resistance.

In order to plane the hull must achieve an appropriate angle of incidence to the water flow, trimming up by the bow to generate lift.

This is a similar lift principle that an aircraft use to get aloft. As the generated lift approaches the weight of the boat, the hull rises from the water and start to plane.

The speed-power curve below shows how much resistance a boat generates as speed increase. As the boats speed increase in displacement mode, the bow trims up and the stern squats. At a speed roughly equal to 1.5 times the waterline length, if the hull is designed to plane, it will move into a transitional region where it is neither planning nor operating in the displacement condition. In this semi planning or hump region, the boat will have pronounced bow-up trim. When it breaks through the hump to a true plane (thanks to hydrodynamic forces), its speed increases and trim levels out.

**Common Features of Planing Hulls:**

The need to generate hydrodynamic lift places constraints on planning hull designs such that all true planning monohulls share a number of features in common.

**1. Chines**

Look at many power boats from the side and you will see more or less a sharp corner on either side where the hull bottom meets the Topside. This is the chine. Because life is not as simple, chines comes in different forms – Hard chine (angular), Soft chine (rounded) or a reverse chine.

A hard chine is intended to throw spray to the sides of the hull and to prevent water from rising up the hull sides where it will increase drag. Chines with a wide flat area (called chine flats) contribute significantly to create lift in the moving boat.

Soft chines describe a sharp turn in the hull section but not a hard corner. The main characteristic of a soft chine boat is the smoother ride it creates in seaway. Much softer than a hard chine but top speed on soft chine boats is however not as high as hard chine boats.

Reverse Chine actually turn downward towards the water surface. The ultimate in reverse chine hull is the classic Boston Whaler (not regularly seen on the waters in SA), in which the chine forms a tunnel on either side. When the boat is underway, water thrown out by the center hull is deflected downward by the reverse chine to provide additional lift and gives an extremely dry ride. In extreme reverse chine design, one could almost say that the hull for is a cathedral hull (see previous article of leisure boating).

For most planing hulls the chine should be immersed below the waterline from midships (more or less the mid section of the boat) towards aft at a depth roughly 1.5% to 4% of the maximum chine beam. The chine should run parallel to the DWL (Design Waterline), from the transom forward to about midships. From Midships fwd to the stem, the chine sweep up higher, to the height above the DWL about equal to a distance 0f 20% - 25% of maximum chine beam.

**2. Deadrise**

Deadrise is the angle a hull bottom makes with the horizontal plane viewed from ahead or astern. The right amount of deadrise gives a boat directional stability, a softer ride and reduces wetted surface drag as the boat rises on a plane. Deadrise is said to be “constant” if it stays approximately the same from midships to the transom. Deadrise is “variable” if it changes from a deep angle at midships to a shallow angle at the transom.

For

**inshore crafts**, deadrise can be about 10 – 12 degrees from the midships aft, increasing from midships as you go forward towards the bow.

For

**coastal craft**, deadrise should be 15 to 20 degrees from midships aft, increasing as you go fwd towards the bow.

For

**offshore boats**, deadrise should be 20 to 25 degrees from midships aft, increasing as you go forward. Some very high speed offshore boats use deadrise in the afterbody as high as 26 – 30 degrees. This is to soften the impact of reentry when the entire boats jumps clear of the water and slams back down, at speeds in excess of 50 knots

In general, the deadrise angle determines at what speed and seastate a planing boat can best power.

**3. Lifting, Running strakes or Spray rails**

Spray rails provide additional lift for high speed hulls. They are usually triangular in cross section with the bottom face parallel to the water surface.

The number and location of spray rails, as well as their run along the hull, is a subject on which there isn’t clear agreement. Different designers and builders each have their favored system and each is sure that their system works best.

In the earlier years, many designers ran the spray rails along the buttock lines.

*(Buttock Lines is a set of lines designers use to define the hull underbody. These lines are the curves that result from slicing the hull from top to bottom and front to back – like slicing your loaf of bread from end to end along the side. Experience designers can tell much about a boats potential performance from studying the buttock lines).*

In other words the spray rails were dead straight if you look from them from directly beneath the hull. This caused the spray rails to curve up in profile and intersected with the chine. The reasoning was that water low straight aft along the spray rails which generated added lift with minimal added resistance. A few designers still prefer this method.

Generally, the modern thinking is the spray rails are dead straight (follow the buttocks) aft of station 4 to 5 (5 is generally midships), but curve in (in plan view) as well as sloping gently up, rather that following the buttocks as they run forward. In this way, the spray rail doesn’t cause an intersection with the chine.

Within reason, the more spray rails the better, however more than four per side is overkill. The same hull without any spray rails will, through, be a little wetter and a little slower, and will have less dynamic stability.

**My “Rule of thumb” for this issue:**

Need to know how much fuel to carry in order to meet the range required? An easy calculation is:

For petrol engines fuel consumption can be estimated as:

Litres/hr = 0.508 x KWprop

For diesel engines fuel consumption can be estimated as:

Litres/hr = 0.274 x KWprop

Where: KWprop = Kilowatts from propeller power curve