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Judging Displacement Vessel Hull Form For
"The Ideal Passagemaker"
Copyright 2000 - 2016 Michael Kasten
A Sense of Form
Perhaps the best treatise one can read in order to get a general sense of optimum displacement vessel shape was written by Howard Chapelle in the summary analysis at the end of his excellent book, The Search for Speed Under Sail. Of course the vessels under consideration were sailing vessels. Sailing vessels have two hull form features that are not required by a power vessel:
- Greater beam to contribute sail carrying power
- Deeper keels to carry ballast down low and to prevent leeway under sail.
A power vessel has the advantage of being able to eliminate much of the keel, reducing wetted surface area and therefore surface friction drag. A power vessel keel will preserve only what is needed to carry the vessel's structure, to house the propeller, and to carry the rudder. Since there are no sails, a power vessel requires less beam. With less beam, a power vessel can optimize flow characteristics around the boat (less beam = less drag).
Surface friction drag is the dominant drag component at slow speeds below S/L 1.0 or so (depending on vessel shape, smoothness, fouling, etc.). Ordinarily, at speeds above S/L 1.1, wavemaking begins to dominate, becoming much the greater drag component at hull speeds.
At speed, a vessel's underwater hull shape has the most effect on wavemaking, and therefore on drag. Whatever one can do to reduce wavemaking is ordinarily beneficial.
As Chapelle observed so many years ago among vessels built well before this century, the features of a relatively fast displacement hull are:
- A fine entry, provided that it is not so fine as to create a hollow forward which would tend to create a secondary bow wave at the "shoulder." This is why a chine should ordinarily be kept either entirely above or entirely below the water's "under-way" wave contour on a displacement vessel.
- An easy shape to the forward buttock lines.
- A balanced shape to the hull diagonals, in other words tending somewhat towards symmetry fore and aft. However, preserving some degree of asymmetry fore and aft is desirable for the sake of pitch dampening.
- A long, fine run, having straight or nearly straight buttock lines at a low angle of intersection to the surface as they approach the surface.
- It is a benefit to have a counter stern, which contributes waterline length at speed.
The idea is to part the water with as little fuss as possible, move through it easily, then leave it behind as gracefully as possible having created the smallest possible wave pattern. Lindsay Lord expressed this very well by saying, "According to Newton's Second Law of Motion, the resistance encountered is equal to the time rate at which the body changes the momentum of the fluid. That is, the rate at which the driving force does work is equal to the rate at which kinetic energy is imparted to the fluid." In other words, changing the momentum of the water abruptly with an un-graceful shape is very much a disadvantage in terms of resistance, and much more so at speed.
To the above very general characteristics observed by Chapelle, we are now able add the results of towing tank research that has quantified the optimum prismatic for any given operational speed. It has been shown to be less of a penalty to have a non-optimum prismatic at slow speeds than at higher speeds, therefore prismatic is ideally chosen to accommodate the expected upper cruising speed of the boat.
On larger boats (say above 80 feet) we also may use a bow bulb in order to help cancel some of the wake. The bulb, the general hull form, and the choice of the optimum prismatic coefficient are all aimed at reducing wavemaking resistance at speed.
A few added thoughts on "boat shape" can be found in our article on Shaping the Ship.
Non-Displacement Types
To say anything definitive about hull shape requires that the typical cruise speed, the vessel's "operational speed" be considered first. For example, many boats operate beyond ordinary displacement speeds.
A semi-displacement boat will ideally have a larger waterplane area in order to help support the weight of the boat in semi-planing mode. Therefore, the boat must generally be wider, and will have a relatively more broad transom. There will be a less fine entry in order to encourage riding more on top of the water. There will be a more shallow and fine run aft, and the buttock lines will be at a shallower angle to the water at the stern. This will often mean that the transom will be slightly immersed.
All of these are intended to reduce resistance at speed and to discourage squatting. Optimum prismatic will be much higher on a semi-displacement boat, showing that more displacement has been pushed into the ends. Chines are usually brought above the water at the bow, with the expectation that the vessel will ride more or less "to her chine" at speed. A chine "flat" will usually be present at the bow in order to reduce spray.
With a planing boat, these same features are taken yet farther: the run will become completely flat and as close to horizontal as possible; prismatic will be greater still; the bow will be less fine; and more power will be applied. Often, the vessel's mid-section shape will be carried right aft to the transom without change.
Displacement Types
Naturally, many of these Semi-Displacement or Planing Vessel features are incorrect for an efficient Displacement craft.... Although a displacement boat is much slower, it will be much more efficient in terms of fuel use at the displacement speeds for which it has been designed.
On the other hand, a planing or semi-planing boat operated at displacement speeds is relatively slow and inefficient due to having a non-optimum shape for displacement speeds.
On close inspection, one will observe within Beebe's Voyaging Under Power, and also in Leishman's more recent re-write of the book, that most of the example vessels are not optimum long range ocean voyagers, but are instead optimum coastal cruisers. Most of them tend toward being semi-displacement types. In other words, the majority of the example vessels, presented as true "passagemakers" have been given hull forms for which high speed has been given greater importance than sea keeping or low fuel consumption...!
While there are a number of very fine exceptions in both editions of VUP, the majority of the boats presented are surprisingly not optimum for long range displacement speed Voyaging Under Power!
Questioning Our Design Goals
Given the incentive to do so, one can very certainly design an excellent displacement vessel that is both very efficient, and readily able to exceed the normally considered "limit" of S/L 1.34. Given the right approach to hull form, this can be achieved with relatively low power, and without having to resort to some of the less desirable features of a semi-displacement hull form.
How shall we do this?
Long length, light displacement, narrow beam, a fine entry and a long fine run are the primary ingredients, along with the other usual requirements such as having the correct prismatic.
Of course, you'll not find many "modern" power boats like this.
Why?
Furniture, picture windows, features, gadgets, spare staterooms, systems, backup systems, etc... all within the shortest possible boat length to fit at the dock. In order to achieve vast accommodation space within the shortest possible length, the boat must then become overly tall and wide in order to contain all the desired features. The inevitable result is that the vessel must then have an enormous hull volume for her waterline length, or an overly large superstructure.
The unfortunate consequence of this trend is that a great deal of extra power and additional fuel are required to propel all of this through the water, often with relative inefficiency. On top of that, we are expected to believe that bow bulbs, squirrel cheeks, maintenance strakes and all manner of other appendages will magically recover some of what has been lost to an over-bloated shape.
By now, you may have discerned that I find this trend somewhat distressing...
What to do?
First, a boat can usually be made to require less power and less fuel at the same speed and displacement by simply making the boat longer.
With some hull types, one can use the same power and get greater speed just by lengthening the boat, and without losing actual displaced volume or giving up accommodation space. If by proper design the boat can also be made lighter in the process, the overall result while voyaging is that the lighter, longer, narrower boat will reach her destination in fewer days, having used much less fuel.
In other words, the relatively lighter, longer, leaner boat can be fitted with a smaller engine, will require less fuel for the same passage, and will usually allow more boat for the money.
Interestingly, this does not imply a reduction in luxury! Quite the opposite!
Current Trends
What does all of this mean?
It seems the majority of boats we see on the market have been designed not for being at sea, but instead for the "moment of sale" where the couple may stand in the galley, look around, and say, "Yeah, we could live here, and we could probably afford the moorage..." In terms of hull form, this means that no small amount of new boats now being offered are designed for "efficiency" while sitting at the dock!
I should apologize I think for my lack of understanding at times... Am I out of place to think it odd that moorage fees and sales people have dictated hull form to such an unbelievable extent...??!!
The Ideal Passagemaker Hull Form...?
Our original question concerned optimum hull form for displacement vessels in terms of reducing resistance. With some study of the subject, we quickly observe that the real culprit regarding resistance at speed is primarily wavemaking...
Thus, if a boat is shaped so that she is slender; pushes up as small a bow wave as possible; does not have a marked "shoulder" in the forward waterlines which would then encourage a secondary bow wave; and does not pull along a big stern wave, we will have done the best job in terms of reducing resistance under power. The ideal passagemaker will therefore have a long waterline; an easy entry without creating a hollow forward; there will not be a 'shoulder' forward where for example a chine may cross the waterline; there will be as long and straight a run as possible; and the boat will not be overly wide or tall. If the vessel is large enough, say with a waterline length of 80 feet or more, then one might consider adding a bow bulb.
The net result of these characteristics will be a vessel that is more spacious due to her length; more gentle in terms of rolling accelerations; and one that will reach her destinations more quickly and efficiently. In other words, the boat will not only travel faster, but will do so at less cost and in greater comfort.
In a displacement vessel intended for blue water, is this not the trend that we should pursue...?
49' Trawler, MONSOON
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