Masthead30_June2014 H.pub - page 14

The Masthead
Know It All Contest Solution
to the March 2014 Question
On what methods of planning-hull trim control
This issue’s new Know it All question is on page 13
The Know It All questions and correct answers are important design tips for students as well
as other marine professionals. We suggest that you file them away for future reference.
The Question Was:
There are four methods of trim control for planing hulls—methods for controlling or reducing the planing angle. Can you
name and describe all four?
The Solution is:
Though we received several answers to this question, and the answers submitted included all the correct methods,
apparently no one person had the combined sagacity, gumption and pure brainpower to list all the correct methods
of trim control. The methods are:
1) Trim Tabs
Trim tabs are the most common method today. They are flaps mounted along the bottom edge of the transom. These
flaps or tabs can be raised or lowered by the operator while underway. Operation is usually electric-powered hydraulics.
Lowering the tabs deflects water down and so creates a lifting force at the stern, raising the stern and depressing the bow
and thus reducing trim angle. Effectively, this is lowering the effective buttock angle of the hull underbody aft.
2) Wedges or hook
Another approach to modifying the effective buttock angle is wedges or shims added by gluing, screwing, bolting or weld-
ing them to the hull underbody at the transom edge. These are not adjustable, obviously. They have the benefit of no mov-
ing parts and the drawback of giving you less flexibility of operation. Still another method of modifying the effective run-
ning angle is to build or mold in hook to the buttock lines. In other words sweep the buttocks down slightly as they ap-
proach the transom. This has the same effect as wedges or trimmed down tabs, but is also not adjustable. I’ve used very
slight hook to reduce planing angle in a few designs.
On metal boats, hook is often arranged by extending the hull bottom plate a bit aft of the transom. The extended plate is
then manually bent down to the appropriate angle by trial and error during haul-outs between test runs. Once the proper
angle is set, it is left fixed unless subsequent changes in loading or operation indicate that a further adjustment should be
made.
3) Interceptors
A very different and less well-known approach is interceptors. These are sliders mounted on the transom, where they raise
and lower vertically like vertical siding gates. The interceptors build up pressure under the transom aft and so have the
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