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:
- The drives are trimmed down full at start. This maximizes lift of the stern.
- As the vessel speed increases, and plane is achieved, the drives are slowly trimmed level.
- 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.
