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FEATURE ARTICLE - “What a drag!” - Trim angle & engine height can reduce drag & increase speed.
To get the most from your high-performance powerboat setup, motor placement and motor trim angle are just as important as horsepower and propeller selection. Hydraulic/electric power trim and power lift accessories have made the fine tuning of high performance powerboats a uncomplicated reality for everyone.
The hydraulic drag generated by the motor lower unit or outdrive appendage can make or break your hulls performance. If you can raise the motor higher, so that there is less unit in the water, then there is less drag – and more speed! The thrust that propels your hull to the checkered flag is most efficient when if is aimed exactly in direction of boat – so motor trim angles of 1 degree up or 1 degree down can hurt performance.
Trim Angle
The most efficient trim angle – as far as engine thrust and drag goes – is exactly neutral. So, if we can design and/or set up our hull so that it runs properly with the motor/drive unit trim at ‘zero’, then we will have the ideal situation.
The difference is not trivial. Let’s consider the effect of even small trim angles. First, a trim angle of positive 2 degrees – meaning trim creating a ‘bow-up’ condition of the hull – which means that the lower unit torpedo is angled down at a 2 degree angle. For a 200hp outboard engine, the difference in forward thrust lost is about 7hp!
The same type of problem can occur with a trim angle of negative 2 degrees – meaning trim creating a ‘bow-down’ condition of the hull – which means that the lower unit torpedo is angled up at a 2-degree angle. In this case the power is not being fully applied in the ‘straight-ahead’ direction, but rather, some of your power is actually being used to raise the aft end of the hull (vertical). Both of these conditions can be avoided with proper hull design and/or setup.
And that’s just power loss – there is also increased profile and induced drag of the lower unit. At 60mph with this engine, this will amount to another 1 hp for this lower unit angle.
So total, we have experienced a power penalty of 8 hp (4%) resulting from our trim angle. Translated into speed loss, for a typical performance ModVP hull this will mean a reduced top speed by 2 mph!
Motor height
Adjusting the height of the motor (lower unit) has a big influence on total hydrodynamic drag. In an outboard, this is often done with a manual or hydraulic jack plate. On an IO, this is adjustment is referred to as the ‘X-dimension’. In all cases, when the drive unit is lower in the water, there is more drag generated.
A common starting point for adjusting the setup lower unit height for high-performance hulls is for the torpedo centerline to be level with the trailing edge of the planning surface. Too high and the propeller can over-cavitate, too low and the setup will generate excessive drag.
Lower Unit (or torpedo) heights, called “X-dimension”, can be adjusted on inboard/outdrive setups using different-height lower drive units. These are available from high performance suppliers. It’s not as easy as using a hydraulic jack plate on an outboard, but it now makes the height adjustment a possibility, where it was almost impossible a few years ago.
Let’s consider a setup where the torpedo centerline is one inch (1”) below the trailing edge of the planning surface (sponson or pad). At 60mph, and our 200hp outboard engine, this will generate an additional 9.0 pounds of drag – equivalent to another 1.5hp loss.
But, there’s more. If your hull setup is using a surfacing propeller (most performance hulls can benefit), then there is an additional propeller drag increase by submersing more of the lower unit (and thus, the propeller) under the water surface. In the case of an additional 1” below the planning surface, this can affect the thrust transmission efficiency – which can lower your available engine RPM by as much as 5%. This can easily represent 300 RPM – or another 6hp loss!
Translated into speed loss, for that typical performance ModVP hull this means a top speed further reduced by 2 mph!
In summary, by being able to trim to the optimum angle, and having the lower unit set at the optimum height, we can make a significant improvement on performance!
** This article was originally published in September 2004 issue of HOT BOAT Magazine - "What a Drag" - Trim Angle & Engine Height Can Reduce Drag and Increase Speed", by Jim Russell, author of "Secrets of Tunnel Boat Design book [editor-HB]. Or get your own copy of the full-color feature article at:
http://www.aeromarineresearch.com/ad..._Sept2004.html
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