A Trim Tab for Centesimal

Jul 14, 2018 | Technical

I was in Suva in 1980 and noticed a nice little H28 from Whangarei sitting in the cockpit was the owner and creator of a very clever trim tab rubber system. Some weeks later we sailed up the coast in tandem and I noticed then how well she sailed and the wind vane was obviously doing the work!
This got me thinking and a bit more enquiry led me to discover the owner had made it himself and now I have an apology! – I have totally forgotten his name and that of the boat, but the germ of an idea was started and now some years later whilst chewing through batteries to the great god of all boats, – the electronic marvels of this and that .. it struck me that once before I had seen a solution.

Then with the recession in full swing I had a bit of time so I set to on CAD and drew up and modelled the options until I thought it MUST work! I have a small workshop and so I see to and got some parts laser cut to make the job easier and welded and machined up the rest to suit my drawings. At this point I should credit the following sources for parts of the design; To the unnamed but very successful originator from Whangarei – many thanks!

To Fay marine yacht plans – your advice was too most valuable.

To the many yachties and engineers who all in their way contributed to the outcome many thanks – I am sorry if I was slow with all your
collective advice – but for some reason one learns best if one also learns through doing it twice!

Probably the single most important lesson was that if the trim tab would not smoothly operate with the pressure of one little finger with no hiccups from any point on the arc of movement and from any control surface be it below water or from the wind vane then it will not work at sea!
And the second one was the degree of movement of the trim tab was small in comparison to the capability of it to move the main rudder….so little and often = a straight line, big and infrequent = a course like a one legged spider!
The third – well feedback from the water blade to the wind vane is critical and less corrective weight is better than more, thus design and placement of fulcrum points and shape of linkage is paramount for success.

Back to the job now the trim tab is best manufactured from a piece of 20mm 316 Stainless steel tubing with a very thin sheet of stainless steel folded in half lengthways and profiled to suit the desired shape then tig welded to the tube and finally filled with foam to create a very light strong former for a 6mm fibreglass surfacing layer.

The manufacture of pintles is easy by comparison and the only trap for young players is to use a grade of nylon bushing that does not expand in salt water to make absolutely sure there is no friction in the alignment.

Drive for the trim tab is best addressed by making a cap plate for the rudder to mount the universal joint that alters the angle of drive to the trim tab stock from the horizontal motion at the top of the rudder and placement of a pivot link shaft exactly 20% of the distance aft from the main pintles to provide a fulcrum from which to swing a reverse lock bail selector and finally a link lever that provides the power stroke from both the wind vane and the tiller pilot.

A small modification to the main rudder tiller shaft is necessary to make a piggy back type .303 shot bolt unit to ensure that when going astern the
trim tab is securely fixed in the same plane as the main rudder.

Because I wanted to have both a tiller pilot and a wind vane work the same trim tab a countershaft was necessary to convert and consolidate all the links and allow selection of the drive from wind to electric as well. This unit has to run very freely but with minimal slack as any slack here creates ongoing sea way chattering and noises!

I should mention here that the countershaft and all the wind vane components are tig welded onto the pushpit for simplicity. From the countershaft unit to the wind vane proper is a simple box type power transmission link with a thrust section point on the bottom of the wind vane vertical drive tube to allow for coarse setting up of the unit.

Fine trim settings are done at the lever on top of the trim tab itself.

The course selector disk latches and wind vane counter weights are all aligned In conventional fashion around a twin wall glazing panelblade with a fibreglass edge support for strength and mounted in servo vane type configuration at 15 deg to the horizontal to provide a little more actuation stability. This part of the design seemed to give least rouble and has worked pretty much without issue.

A 6mm 455 grade SS push rod drives the lower fulcrum lever via a self centring type swivel joint at the lower end and a commercially available SS ball joint (helicopter type control rod end) at the upper end.

The whole thing has now been rebuilt twice with small design modifications mostly around the placement of the rudder cap controls, linkage points, and most importantly the shape of the trim tab has seen 3 revisions but now it works!!

It is simple to use – works in winds from about 4 knts and steers a course which I would be proud of! It also on electric uses a fraction of the previous amount or power and does not get overpowered in sloppy seas.

Peter McBride – “Centesimal”