Power Boats: Sizing the Engine

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Hull speed horsepower and LWL

Displacement Boats

For a given displacement power boat the LWL is constant (not true for sailing boats with big overhangs: when they heel, their LWL becomes longer and they can move faster then when upright; that is valid until the excessive heel causes too much loss in the effective sail area, spilling the wind). The initial acceleration is always easy and effective. But the closer the speed approaches the hull speed, the greater increase in power is required for a given increase in speed. As an example, a 32-foot LWL displacement boat can be driven in calm water close to its hull speed by an engine of less than 20 HP. At around 75% of hull speed, its optimal cruising speed, the boat is very efficient. Beyond this point, as the second wave approaches the stern, the drag increases and the fuel consumption becomes increasingly disproportionate to further increase in speed.

The graph shown above right gives the approximate horsepower required to drive a displacement boat at its hull speed in calm water. Starting from the boat's LWL, take a vertical line to the intersection with the red graph. At that point, move horizontally to the left, you can read the required power. Now, that is for calm water. From experience, it takes about 35% more power to cater for adverse conditions. And if the engine is to run a generator, a fridge and similar, add some more.


High-Speed Planing Boats

Expected speed prediction: Relationship of the displacement (in pounds), SHP (shaft horsepower) and <br />predicted speed (in knots) for a planing hull.

Like most people, you might have bought your power boat already fitted with an engine. All you do is turn the engine, and wroooooooom, off you go. Well, nothing wrong with that. You might just not be technically inclined, you just want to enjoy your boat, trusting that whoever put the boat and the engine together knew what they were doing. And that is probably true. Yet, on a calm water, you still occasionally rev the engine high, towards its upper limit of some 5500-6000 RPM, and you watch the speedometer to see how you are going. Now, would it not be nice to know that the engine really matches the boat, and that you are runnung the best you can?

Or, if you are considering to buy a new engine for your boat, how do you make the right choice? If you ask the boat dealer, would they give you the advice that is the best for you, or would they just try to sell you the engine they've got on stock? Would it not be good to know how to make your choice?

Choosing the right engine power for a boat is not a super-precise process. There are approximations that can not be avoided. There are calculations that at the end just double the engine power to cater for the possible adverse conditions out there. And when the final choice is tested on calm water and fits within 10% up or down of the predictions, that is an excellent match.

Many different methods are used to predict the speed of a boat of a known displacement with different engines. The picture to the right is a graphical one, simple to use and yet accurate enough. It is valid for high speed boats capable of planing (speed/length ratio 3 or higher).

The red line represents the Platypus parameters, with two adults on board, full fuel tank 120 lit, and the usual gear. Tested at full throttle on calm water and with not much wind, we are just there at about 32 knots. As it is really difficult to know the actual displacement of the boat, we are satisfied with the accuracy of this simple method.

The drawing is deliberately somewhat bigger to be easier to use. If you wish, just right-click on the image, save a copy to your disk and print it from there for your own use. Just be careful if you scale the image up or down to keep the width and the height in proportion, as the relative position of the three vertical lines matters.

LWL and C-values for planing hulls

Another relatively simple method to predict a planing hull speed for a known displacement (in tons, 1 ton = 2,240 pounds) and the engine's BHP (brake horsepower) uses the simple experimental graph shown to the left. Starting with the boat's LWL, from the graph you determine the boat's C-value. You then calculate the predicted speed by using the formula:

 Speed (kn) = C * Sqrt(BHP/displacement)

Once again, all this is valid for high speed planing hulls (speed/length ratio 3 or more). Be careful to express the displacements in tons. (1 ton = 2,240 pounds) (If you are more a metric-type, so are we. But when we talk boats, knots, feet, inches, pounds, long tons and similar units are used by tradition. For other boaties to understand you, you best use these units too.)

Again, the bright red lines show the example of our boat Platypus: At LWL = 15 ft, the determined C-value = 2.85, estimated displacement 3,000 pounds = 1.34 tons. When you calculate the predicted speed, you get 24.63 kn. We believe that this C-value based method is rather conservative, or our displacement estimates are too inaccurate, or both :-) The Platypus actually achieves about 32 kn, and we use a GPS-based instrument to measure our actual speed.

The differences in the predictions achieved by the two above methods only confirm that all the results are only rough estimates. We have come across tables, where you estimate the engine required BHP based on the LWL, the displacement and the type of stern (canoe, transom and moderate-flat bottom, transom and very flat bottom with hard chine); as you estimate the BHP, you correct it according to the stern of your bout, and then just double it, just for the sake of those occasional rough conditions out there. And then, there are other methods that use experimentally developed coefficients in mathematical formulas. You will understand why we have shown here the above two simple methods that do produce fairly close results (if you use both methods, and their actual result is somewhere in between, even that is a good achievement).

You may wish to comment on the above methods and our conclusions. You may have some other views and experiences. Why not sharing them with the rest of us? With your agreement, we will publish your text under your name (nickname). Together we do know more.
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Last upd: 19-Aug-10 F150806