CATAMARANS -  PLANING V. DISPLACEMENT COMPARISON

Power catamarans, like monohulled vessels, can be divided into two basic types:- planing craft and displacement craft. Certainly there are the hybrids like the side wall hovercraft [or surface effect ship], which is a catamaran with front and aft skirts and forced air induction to lift the boat partially out of the water and thereby decrease drag. There are catamarans with hydrofoil assistance and some that become completely foil borne when underway; but the vast majority of vessels, particularly the pleasure boats, are either displacement or planing craft. So how do these two types differ ?

Basically they differ in the way each attempts to reduce the hulls resistance to motion and this is reflected in the different hull forms employed by the two approaches. The planing vessel typically has a low deadrise hard chine hull to develop lift and encourage the boat to plane on the waters surface to reduce wetted surface and wave drag. Whereas that of the displacement vessel is typically round bilge to minimise the wetted surface drag as its slim hull slices through the water virtually ignoring wave drag. These differences in hull form determine the differing resistances of vessels with similar parameters when moving at various speeds.

How much power is required to move any particular hull form will vary with speed and these variations are reflected in the shape, and magnitude, of their resistance curves. If you look at the resistance curve for a planing catamaran then it is virtually identical with that of a monohulled vessel with the same displacement.The catamaran may initially require more horsepower than the monohull because of its lesser planing surface but there is the same initial high drag hump which requires lots of horsepower to overcome. Once this hump is scaled there is a marked decrease in the hulls resistance to motion which only gradually begins to climb again as speed increases. What this means in theory is that once a planing vessel has overcome the initial wave resistance hump it will just go faster and faster as the available horsepower is increased. In practice of course this is not possible because such things as the sea state, and passenger comfort, intervene. Not only that but there is a physical limit to how large an engine can actually be fitted into the boat, and how much horsepower can be developed for a give engine weight, never mind the extra fuel requirements that the larger more powerful engine brings. This phenomenon is however exploited very sucessfully in the Formula 1 tunnel hulls and the large offshore racing catamarans where there is horsepower a plenty and the comfort of the passengers is relatively unimportant as long as they survive.

In the case of the displacement catamaran [ and this includes SWATH
vessels ] the resistance curve of the catamaran and the monohulled vessel of the same displacement is somewhat different. It is true that they are of the same general type, the smoothly increasing exponential curve, but the catamaran curve will be well displaced toward the high end of the velocity continuum. In other words; although the displacement catamaran will have a wave resistance determined hull speed just like the monohull, this hull speed will be very much higher. But unlike the planning catamaran, where the power to weight ratio is the major determiner of speed, the speed of the displacement catamaran is largely a function of the fineness ratio of the hulls.The displacement is relatively unimportant per se except for how it impacts on the fineness ratio of the hulls, the height of the wingdeck off the water and the longitudinal moments of inertia of the vessel.

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If we look at the graph of fuel useage plotted against speed it is obvious that at low speeds of around 7 knots there is not a lot of difference between the four different hull types that are plotted. In fact it was a bit tricky getting some figures down at these speeds because it is very difficult to get the displacement catamaran to go that slow!. However as the speed increases we get four rapidly diverging fuel useage curves [see fig.1.]

Let us then compare the resistance curves for a displacement catamaran and a planing catamaran of the same displacement . This comparison is based on two existing boats of very similar dimensions, including length and horsepower, that have been tested in New Zealand over the measured nautical mile by Len Gilbert of Diesel Craft Evaluations who use a computerised fuel metering system. Although the overall beam is 2.5m [8.2'] greater for the displacement boat, all this does is give a much larger interior space, it has negligible effect on the resistance curve as the displacements are still the same. If you look at the two curves you will see that at around 12 knts the planing vessel is using three times as much fuel as the displacement boat ie: its hull resistance is three times as great and it needs three times as much horsepower to move at this speed. At around 20 knts the planning boat is still using twice as much fuel but the curves are converging so that if we look at the speed of around 24 knts it is obvious that the resistance is now equal. Both types require the same amount of horsepower to push them at this speed. From here on up the speed of the planing boat can be increased by pushing in more horsepower if it is available. But of course the resistance curve will continue to steepen until even huge increases in horsepower will not increase the speed any further. The speed of the displacement boat can not as readily be increased much past this point. The reason for this is that this particular displacement catamaran has a hull speed of 27 knts beyond which it will not go unless some ridiculous amount of horsepower is applied. The displacement boat is getting close to its hull speed limit whereas the planing boat no longer has a wave drag determined hull speed limit in the same sense though even its resistance to motion will continue to increase exponentially until it to grinds to a halt.

Although not pertinent here, there is another factor that starts to
come into contention as the size of the vessel increases. This is the bottom loading of a planning vessel. For a vessel to plane the loading, or weight, on the planning surfaces of the bottom must not exceed so many kgís per square cm of surface area. If the loading is greater than this then the vessel will not plane and it will function as a rather inefficient displacement craft. This is the reason why the larger monohulled pleasure boats are displacement vessels. This principle is even more important with catamarans which may have a limited planning area in the first place. So at somewhere around 30mís or so the power catamaran has to become a displacement boat unless the evil hour is postponed by foil assistance or by very light weight [and expensive] construction.

The resistance curve for the displacement boat can be moved further to the right ie: it can be made to have a higher hull speed, by making the hulls longer to increase the fineness ratio and thereby achieve a higher maximum hull speed. It is obvious from the shape of the resistance curve for the displacement boat that it is advantageous in terms of fuel economy\horsepower requirements to keep your top speed, and particulary your cruising speed, as far down into the flat part of the resistance curve as is practically possible [in this case around 15 to 20 knots. Going much slower than this does not reduce the fuel consumption by very much]. But, just like with the planing vessel where you just canít continue to push more horsepower into the hulls, there will be practical considerations that will prevent the displacement boat being made extremely long to achieve good fuel economy and performance. Not the least of which is cost even though hulls are the cheapest part of the structure[around 10%] and an increase of a couple of metres [7'] is going to have very little effect on cost. Ultimately however the cost will start increasing exponentially. It may also become increasingly difficult to find an engine that will fit into these long slim hulls and somewhere where you can park them. But, within these various constraints, it can often be extremely advantageous to just make the hulls a little longer. It may in fact reduce the horsepower required to push the vessel at a given speed because you may now be on the flatter part of the resistance curve. This may actually reduce the capital cost of the vessel, as well as the operating costs, because of the lesser engine requirement, and will also probably increase the sea kindliness of a vessel which already treats its passengers very gently.

The conclusion to be drawn from this comparison is that, as expected, it is a matter of horses for courses. If you want to go fast in a short boat then you have to plane. If you can go a little longer then it is possible that you may be able to have the same performance but with much better fuel economy and comfort. It really is a matter of juggling all the variables of hull length, displacement, power requirements, accommodation, comfort and speed; to see which is going to be the best type of boat for you.

This article has been reprinted courtesy of POWER MULTIHULLS. 

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