Boat Parameters

Here is a list of definitions for the various parameters used to describe boats.

As a general rule, parameters and ratios listed on the M. B. Marsh Design site are for the boat at its fully loaded design displacement, including full tanks, full crew, and typical cargo. The one exception is the light-ship displacement figure, which is (roughly) the weight of the fully fitted out, but unloaded, boat as it leaves the construction shed.

For multihulls, most of these parameters (with the exception of waterline beam) apply to the boat as a whole. In the case of trimarans and proas, waterline beam and length are stated separately for the outriggers, and wetted surface includes as much of the boat as would be in the water in normal operating conditions.

If you'd like to read more on where these parameters come from, a good place to start is Eric Sponberg's article "The Design Ratios", which can be found at http://www.sponbergyachtdesign.com/Articles.htm .

Topic: 

Displacement

Definitions

Displacement is the truest measure of just how much boat you're dealing with. In a nutshell, it's the actual volume or weight of the vessel.

Of course, there are more than a few ways to state displacement, but only two- the volume displacement and the displacement mass, both at the design waterline- are fair points of comparison between different boats. The rest, although important and helpful when used properly, can be deceptive when seen alone. Always ask which weight is really being discussed, and what's included in it!

Units: Volume measure (cubic metres or cubic feet) for volume displacement, or mass (kilograms/tonnes or pounds/long tons) for the others.

Volume displacement ($\nabla$ or D)

The total volume of water displaced by the boat when she's sitting on her design waterline. Or, equivalently, the below-waterline volume of the hull and appendages.

Displacement at design load, or displacement mass (mLDC)

The weight of the boat when she's loaded to her design waterline. By Archimedes' principle, displacement mass is equal to displacement volume multiplied by the density of the water (nominally 1000 kg/m3 or 62.4 lb/ft3 for fresh water, 1025 kg/m3 or 64 lb/ft3 for seawater). This is the figure that should be used for all performance ratios and comparisons.

Displacement light-ship, or empty boat displacement mass, or dry weight (mLCC)

The weight of the boat when she leaves the build shed, before there's any personal gear brought aboard and before any fuel or water are put in the tanks.

Although it's important to determine mLCC when designing and building a boat, it'll never be sailed this light. Performance ratios calculated using the dry weight are worse than useless; nevertheless, some marketing materials list only this weight and numbers based on it in an attempt to trick the buyer into thinking the boat is faster and sportier than it really is.

Light load displacement mass (mMOC)

The weight of the boat with (usually) half-full tanks, a standard complement of essential gear, and a small crew. This is the absolute lightest weight the boat is ever likely to be sailed at, and performance ratios based on mMOC can be deceptively racy.

Topic: 

Length

Definitions

There appear to be no less than six commonly used ways to measure the length of a boat. Most of the discrepancies have to do with whether stern platforms and bow pulpits are "integral parts of the hull" or are bolted on separately.

Units: Linear measure (usually metres/centimetres or feet/inches).

Length overall (LOA)

The length from the forward tip of the stem to the aftmost point on the hull. Usually includes any bow or stern appendages that are a structurally integral part of the hull (eg. a swim platform or anchor pulpit moulded as part of the hull), but excludes bowsprits, boomkins, bolted-on pulpits and stern platforms, or other appendages that aren't an integral part of the hull itself.

Length waterline (LWL)

The length of the design waterline, i.e. the length of the part of the boat that's in the water when she's loaded exactly to the waterline shown on the plans.

Length between perpendiculars (LPP)

The length from the forward point of the design waterline (same as for LWL) to the centre of the rudder stock. Used mainly for large ships, and rarely mentioned aboard anything under a hundred feet or so.

Length on deck (LD, LOD)

The length from the foremost point of the deck to the aftmost point of the deck; it leaves out reverse-sloping transoms and most types of integral pulpits.

Length of hull (LH)

The overall length of the hull itself. Defined similarly to LOA, but usually excluding bow pulpits.

Rated length

Depends on which rating rule your boat is subject to. This is usually some combination of LOA, LWL and various parameters describing the shape of the bow and stern overhangs.

Which one to use?

For marina fee purposes (they're measuring)

Marina administrators often like to use the total length of the boat, including bowsprits, boomkins, pulpits, protruding sterndrives, the trailer tongue, and other length-extenders that would be considered "cheating" if you used them to describe your boat to a friend. That 18-foot runabout? 22'6" to the boatyard, once you add the tilted-up outboard and the trailer tongue.

For marina fee purposes (you're measuring)

If you're being billed by length, you'll try to low-ball the measurement. For a powerboat, LPP is usually the shortest, so you might start there. That might seem suspicious, though, so LWL and LH are the next to be presented. Eventually, you and the marina manager will grudgingly agree to use LOA, at which point a long and drawn-out argument will ensue over whether that anchor pulpit really is "integral to the hull".

In a builder's marketing brochure (model number, etc.)

It could be any of a number of quantities of questionable origin. Popular ones include "Take LOA in feet, round up, append two random digits". Or "Take LOA in feet, add swim platform but not anchor pulpit, round to nearest inch, state as FtIn". Or, "Take LOD in dekametres, add pulpit but not swim platform, round to nearest multiple of five".

At Marsh Design, the model number is the LOA in centimetres, using the standard definition given above.

When comparing performance

That would be LWL. For speed and seakeeping, you care about how much boat is actually in the water. That is, after all, what the waves see, and it's the length used to compare Froude number or speed/length ratio.

 

 

Topic: 

Beam

Definitions

Beam refers to the width of a boat. As with length, there are a few ways to define beam, depending on what you need the number for.

Units: Linear measure (usually metres/centimetres or feet/inches).

Beam overall (BOA)

The maximum width of the boat, excluding rub rails or lifeline stanchions that may protrude past the edge of the hull.

Beam waterline (BWL)

The width of the waterplane when the boat is loaded to the design waterline shown on the plans. On multihulls, BWL refers to the waterplane width of a single hull.

Beam centreline (BCB, BCL)

For multihulls only, the lateral spacing between hulls.

Which one to use?

For most purposes

Beam overall is the most commonly used description of a boat's width. It gives some indication of how spacious the interior will be, and how wide a slip needs to be to accommodate the boat. BOA, plus a comfortable margin, is the dimension used when determining whether the boat can pass through a canal or fit in a slip.

When comparing sailboat performance

For monohulls, BWL is the quantity of interest. Wider beam implies greater form stability, within a given family of hulls. All other things being equal, the hull with more form stability will be less tender and better able to stand up to a powerful sail plan. Making a hull wider, though, will generally increase wave-making resistance, and often increases wetted surface area- thus increasing frictional resistance. Finding the right trade-off is one of those "no right answer" optimization problems that make good design tricky.

Multihull performance depends on both BWL and BCL. The former contributes to the resistance; the latter contributes to the multihull's righting moment and therefore its sail-carrying power.

The effect of beam on performance is better understood by looking at the length/beam and beam/draught ratios.

When comparing powerboat performance

BWL is, once again, the appropriate quantity to compare. Since a powerboat doesn't need to stand up to a full press of sail, initial stability (due mainly to form stability, or hull shape) is more a matter of preference- although there are long-standing guidelines for what works well in a given situation.

If a hull is capable of planing, BWL becomes a very important factor. A wider running surface gives more planing lift, but also presents more area for the hull to pound on when bashing through waves.

Topic: 

Draught

Definitions

Draught, normal (T)

The depth of water needed to float the boat, when it's loaded to its design displacement and sitting on its design waterline.

Draught, minimum (Tmin)

In a boat with retractable appendages (rudder, daggerboards, outboard or sterndrive engine), the depth needed to float the boat when all these appendages are fully retracted. Again, this measurement is based on the design waterline.

Units: Linear measure (usually metres/centimetres or feet/inches).

Which one to use?

Most of the time, the maximum (normal) draught, plus a comfortable margin, is the figure of interest. Tmin is relevant only for a handful of boat types, and then only in those specific conditions when you can safely run with the keel, rudders or engine retracted. Very few retractable keel monohulls, for example, can be sailed with the keel up (although many centreboard boats can run downwind with the board retracted).

Topic: 

Wetted surface area

Definition

Wetted surface area is the total surface area of the hull and appendages below the waterline. It should include both sides of the rudder and keel, and if the boat has multiple daggerboards, the given wetted surface area usually includes as many foils as would be used simultaneously.

Units: Area (usually square metres or square feet).

What it's used for

Performance

Any surface that's in contact with the water causes drag; frictional resistance depends on how much surface area is in contact with the water. Less wetted surface area generally means less frictional drag.

In a sailboat, frictional drag usually dominates at low speeds, so minimal wetted surface is important to light-air performance. (This is better understood in terms of the ratio of sail area to wetted surface.)

In faster powerboats, and even some sailboats, the hull will climb on plane as it accelerates. Once on plane, wave-making resistance drops off dramatically, while frictional resistance increases. Wetted surface is once again an important factor, but since part of the hull is now out of the water, it's not the same wetted surface as in the at-rest condition!

Maintenance

Wetted surface area is, not coincidentally, the area that needs to be covered by antifouling paint. It's therefore a useful quantity to know when figuring out how long it will take, and how much it will cost, to paint the boat.

Topic: 

Length-beam ratio

Definition

L/B = length divided by beam.

Units: Dimensionless.

Usually, the waterline dimensions LWL and BWL are used for monohulls, or for a single hull of a multihull.

What it's used for

Performance

Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a given length.

If a boat can plane, smaller L/B often suggests more efficient performance at low planing speeds. The balance generally tilts in favour of high L/B for fast boats.

Typical ranges of L/B are:

2 to 4 - Small to mid-size planing powerboats.

3 to 4 - Most small to mid-size sailboats and motor yachts, the longer ones generally having higher L/B. Some "skimming dish" racing sailboats also have L/B in this range; their wide beam gives them more initial stability so that they can fly larger sails.

4 to 6 - Fairly long and lean for a monohull. Some large, efficient long-range cruisers fall in this range, along with many racing monohulls.

6 to 10 - Large freighters; main hulls of cruising trimarans; a few very portly cruising catamarans; the lightest and slimmest of large sailing monohulls.

10 to 16 - Fast cruising cats and tris; a few racing multihulls.

Over 16 - Racing multihulls. Such high L/B is conducive to very light, low-drag hulls for race boats, but makes it very hard to get enough room inside the hulls for equipment or living space.

Living Space

If a boat is going to spend most of its time in a marina or at anchor, relatively low L/B implies a larger, more spacious interior and increased carrying capacity when compared to slimmer competitors of the same length. For a boat that must entertain guests at the dock but will rarely be used in rough weather or at high speeds, this may be a significant advantage. The slimmer boat, though, will generally have the advantage when fuel is expensive or when the weather picks up.

Topic: 

Bottom loading

Definition

Bottom loading is the displacement mass divided by the area of the planing surface (usually somewhat less than the total wetted surface).

Since the area of the planing surface constantly varies, it's common to just use BWL * LWL as the reference area.

Units: Usually kilograms per square metre, or pounds per square foot. One might think of bottom loading in terms of pressure, but that can lead to confusion- so I prefer to avoid using units of pressure (Pa, or N/m2) to describe bottom loading. Just think of it as another arbitrary ratio that can be used to compare boats.

What it's used for

Planing performance

Bottom loading is a very useful figure when trying to determine how well a hull will plane. Higher bottom loading indicates that more lift must be produced by a given size of planing surface, so the boat will have to run faster or at a steeper trim angle to get on plane. We must confirm, of course, that the hull shape is amenable to planing- straight buttocks, roughly prismatic afterbody, etc.- before this ratio will be of much use.

In general, lower is better, within reason. A boat whose bottom loading is much less than her competitors' will be quicker to pop onto plane and better at low to moderate speeds, but may have a bouncy ride in rough weather.

Less than 100 kg/m2 (20 lb/ft2) - This implies a very light hull that should pop up on plane quickly at low speeds.

About 200 kg/m2 (40 lb/ft2)- For boats in the 4 to 7 metre range, this is the point where getting up on plane needs a bit of effort, and there may be a range of "no-go" speeds where the boat plows around with its bow in the air. Once up, planing can be easily sustained.

About 300 kg/m2 (60 lb/ft2)- For a boat in the 4 to 7 metre range, overloading is indicated, and the boat probably won't plane very well. Given enough power, it may run OK at high speed, but will be a dog from 8 to 20 knots or so. In boats from 10 to 15 m LOA, bottom loadings in this range are typical and indicate reasonable performance.

About 400 kg/m2 (80 lb/ft2)- Bottom loadings in this range on a 5 to 10 m boat indicate almost no ability to plane. On a 10 to 15 m boat, this bottom loading would indicate slight overloading and poor low-to-moderate speed performance, and on a 20 m boat would imply typical, but unremarkable, planing performance.

Topic: