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Date Posted: - Friday - 06/30/06 - 10:30am
Author: Mike Cox
Author Host/IP: cache-dtc-aa09.proxy.aol.com / 205.188.116.13
Subject: Seems too low
In reply to: Doug Wilson 's message, "Perkins 4-107 RPMs" on - Friday - 06/30/06 - 1:09am

I fully understand that a 4-107 or 4-108 can be cruised (5.5 to 6 knots) at a lower that recommended rpm and save some fuel but I would be concerned that if the engine is set up (prop size and pitch and xmission ratio) to obtain a good cruising speed with a low rpm in no or light wind conditions, it will not be able to obtain or maintain its optimum power output when you really need it - powering into a strong head wind or strong current, or once the prop and bottom began to get fouled up. It will also load the injectors with carbon build-up. An additional problem with having a boat set up (prop size and pitch with xmission) to reach cruising speed at low rpms is that you sacrifice slow speed control needed for docking or other maneuvering. One of my prior boats would cruise nicely at low rpms in light wind conditions but would overload (heat-up and black smoke) when pushed to higher rpms in stronger wind conditions. I also had to kill quite a bit of speed when docking as it would make 2.5 to 3 knots when running at the lowest rpms. I personally rebuilt the engine and saw the effects of running at too low an rpm. The boat was carrying too much prop pitch.

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