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Laker SC12 Tunnel Race Boat Plans
Page Eleven
Jackplate and Trim

A word about motor height.

On a typical fishing boat, the motor's cavitation plate is about even with the bottom of the boat, or up to an inch below the bottom.

Motors on racing boats are mounted much higher.

On a tunnelboat you can generally jack your motor up as high as performance dictates, to the point where the entire "bullet" is actually above the deepest part of the boat. There will still be plenty of spray coming through the tunnel to feed the cooling water intake, and you will have reduced engine drag to the barest minimum.

However, if you wish to take advantage of this sort of engine setup on your tunnelboat, you will surely need hydraulic trim. Without the ability to trim your engine down, pushing your prop deeper into the water, and directing thrust downward somewhat, you will never get your boat on plane.

Jack plates can be purchased. But they are a simple device that you can build yourlself.

Please consider my Jackplate Plans.

Trim is almost a necessity on a tunnel boat, particulurly one expected to excede 50 mph.

The engine angle that works best going upwind will likely be too low when you turn around an head downwind. On a breezy day, the best downwind angle may be uncontrollable into the wind, and in the worst case could lead to a blow-over.

The system shown here uses a 1980s vintage Mercury trim cylinder. The cylinder is attached at the foreward end to a bracket that is actually part of the jackplate. The aft end is connected to the motor by a homemade bracket made from a short section of aluminum channel.

These Mercruiser type pumps generally work very well. Probably any pump will do, though you generally want one that moves the cylinder quickly.

Another development in homebuilt trim systems: Using a mechanical actuator in place of a hydraulic cylinder.

Brent Compton (from Customer Projects) tested his system on his Dillon Mini Vee with a 30 hp OMC motor and reports that it "worked great!"

I've never tried this myself, so I can't recommend it personally. But it seems worthy of further experimentation, at least with smaller motors.

A new Lenco actuator can be purchased for under $200. A pair of solenoids is all that's needed to complete the system.

See also this discussion on Boat Racing Facts.

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