View Full Version : Who knows Trim tabs?

02-06-2015, 10:50 AM
Getting ready to build the rudder for my 32' Kahuna. The drawings call for a trim tab directly attached to the aft end of the rudder, intended to be controlled by cables and connected to an auto pilot kept dry in the lazerette. I understand how it is supposed to work while using the auto pilot but am trying to figure out how it functions when sailing routinely/manually using the tiller. Is it locked in-line with the main rudder or allowed to swing freely?
My plan all along has been to have a trim tab in conjunction with a wind vane as on the Pardey's boats.

Here's a picture of Smaalder's Kahuna's Plans:


And here is a pic of a trim tab\wind vane combo modeled after the Pardey's



02-06-2015, 11:27 AM
Why not contact the designer and tell him what you would like to do.

The pictured tab looks like an add on. Best to design the tab and its control mechanism into the rudder before construction .

02-06-2015, 01:10 PM
My main question is how it's not used....what to do with it when not in use. I do have my questions sent to Mark also,...just looking for a discussion here also. thanks

Ian McColgin
02-06-2015, 01:12 PM
This sort of trim tab works by countersteer - twist it one way so it will by water pressure push the rudder the other. Works in motion, not at zero-low speed maneuver. The picture I'll try to attach below shows a quite different sort of trim tab that turns with the rudder, adding power to the turn. As you can see, it's permanently geared by a slider at the bottom.

With your desired rig, when steering by hand, lock it in line with the rudder if you're maneuvering about. If you're on a steady course, want a lighter helm, and don't want to engage the wind or auto, dial in a bit of trim to make the rudder do what you want.


Here we go. This is more common on Pacific Northwest Coast fishing boats, which Island Maid was once.
https://scontent-a-lga.xx.fbcdn.net/hphotos-ash2/v/t1.0-9/1044710_10200394060349204_812667137_n.jpg?oh=690ae 2e96b3eb26a738293264de417c1&oe=554E68C7

02-06-2015, 01:22 PM
Thanks Ian. I had seen your posts with this picture on an older thread but didn't notice the mechanism at the bottom. Pretty ingenious. Reminds me of flaps and dive brakes from WWII. That piano hinge looks like it would go a long way for decreasing turbulence but beyond by wood-working skills :p

Phil Y
02-06-2015, 03:22 PM
Id guess if you have a bit of weather helm you could use the trim tab to manage that. otherwise Im not sure that locked or free would matter either way. But thats just a guess, no experience.

wizbang 13
02-06-2015, 04:26 PM
one can think of it as a tiller mounted backwards. Thus, a well balanced lightweight boat may have a small trim tab nicely faired into the rudder itself.
While a heavier full keel may want the tab larger and further aft, as in the doubleender above. The metal brackets that hold the tab out are the equivalent of a long tiller.

02-06-2015, 06:54 PM
I've used them on aircraft. Did not know boats have them too. Thanks for enlightening me.