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Why does drive trim matter?

There are two basic types of surface drives on the market; those with adjustable trim and those with fixed trim.

Surface drives with a fixed trim are more appropriate for very small craft, under 8m, or racing boats. Adjustable trim models are non-optional for most vessel applications, in particular, vessels with military or government use. Why is trim so important?

The answer lies in the physics behind surface drive propulsion. In addition to providing thrust, surface drives produce lift; they “lift” the hull out of the water in order to plane. The diagram below illustrates the principle:

As one can plainly see, Delta (displacement), is counter-acted by the force N (buoyancy).  Df (drag) is added to resistance. Thrust counteracts resistance (“overcomes” it), and you get the speed of the vessel. What happens if the wetted keel is lessened by lifting the hull out of the water?

The answer is that buoyancy increases, resistance and drag decreases, as does displacement. Therefore, speed increases. If we can lift the hull out of the water and get it on plane faster, the speed will increase. So, by being able to adjust the trim of the drive, we can control how much lift is produced, and thus, the speed at which the vessel moves. Of course, too much of a good thing becomes negative. If too much of the hull is out of the water, the vessel becomes difficult to control.

When France Helices designed the SDS, it was designed to allow a trim angle of +/- 7 degrees. So, when operating the SDS, in general, the following procedure is used, from dead stop:

  1. The drives are trimmed down full at start. This maximizes lift of the stern.
  2. As the vessel speed increases, and plane is achieved, the drives are slowly trimmed level.
  3. When approaching top speed, the drives are trimmed up, decreasing the amount of lift on the hull.

The ultimate goal of trimming the drives is to achieve a level, planning operation, much like skipping a stone across the water.

Of course, trimmable drives have more practical, non-theoretic advantages, too. The drives can be trimmed up to avoid obstacles in beaching situations (like in landing craft), or in shallow draft environments. The drives can also be trimmed up in order to raise the propellers out of the water for safety, as with a rescue boat. Finally, when reversing, trimming the drives down allows the water to flow unimpeded below the vessel transom, allowing operation in reverse, and dispelling a myth about using surface drives in reverse.

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More Stories By John Miele

I am currently Sales and Marketing Manager, Asia / Pacific for France Helices. Created in 1977 by Paul BEZZI, FRANCE HELICES is today an international company. French marine propulsion leader, FRANCE HELICES is also one of the international leaders. Research and development are a very important part of FRANCE HELICES program, the CAD's department is equipped with the latest computer technology, using most up to date programs. Our engineers and technicians determine from the customers specifications and designs, the type of propulsion system to suit the boat's application. They can also advise on the ideal choice of power and optimum gearbox ratio to obtain the maximum thrust. There are many FRANCE HELICES' inovations and patents such as Surface Drive System (SDS) which enable boats to obtain very high speed with excellent handling capabilities and high quality Controllable Pitch Propeller systems (CPP) for both professional use and pleasure application. The constant research using cavitation tunnel testing, guarantees high efficient propeller blade shape for our customers. More than 20 000 propellers per year are manufactured by FRANCE HELICES workshops. In all sizes from small sailing boats to large fishing vessels to navy boats or mega yachts. The FRANCE HELICES workshops are equipped with modern foundries capable of casting propellers up to 3.5 tons in NiBrAl. They are also equipped with CNC milling machines and CNC lathe machines. FRANCE HELICES has four sites which cover the complete range of production. The development of FRANCE HELICES, supported by shareholders places our company as a leader in the international market and insures a constant development, worldwide, in order to be close to the end user.