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I have just purchased the plans for the self steering device from Wharram and Friends. The plans call for a trim tab to be installed and  fared to trailing edge of my present rudder. The problem is that my rudder edge has been fired to approx 5mm diam whilst the leading edge of the trim tab will be approx 10 - 12mm diam. This would mean that I would have to take off my rudder and remake it...big job just removing it.
My question is will it effect performance of self steering if
1) I were to Set the trim tab back approx 5cm from trailing edge of rudder or
2) I was to install the trim tab as plans specified but not fair my present rudder.
Any advice would be most appreciated..cheers Pat

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Gidday Pat,

What about making the trim tab out of a piece of aluminium 5mm thick to match the trailing edge of your rudder. Round the forward edge, fair the trailing edge only slightly and round it too, surely it will be strong enough, and will also flow nicely with the foil shape of your rudder??

what size is the rod for your trim tabs? You could make that your max trim tab thickness and just run with that? or you could just bulk out one size of your rudders . I think having the trim tab aft of the trailing edge gives a more powerful effect from the trim tab. In my experience with trim tabs everyone tends to make them a bit short because once your are sailing the water is ofter flowing well above the water line(where everyone tends to end their height from the water)

There is one stainless steel grade that is stronger than 316 so you could use a slightly smaller diameter( I can't remember the grade now- it is more expensive but you're not using that much)

good luck

Thanks Frank and Brett. The recommended diam. Is 8mm. If I was to weld ss sheets 1mm thickness both sides say towards the back of the rod say at 6 mm width that may work . Even better if I could find a stronger ss alloy and decrease the diam as Brett suggests . Attached is a pic of the trim tab
Attachments:
Hi Brett . I have seen some trim tabs well separated from the rudder by as much as 10 cm . Not sure that I understand the dynamics of what is taking place. Why in one situation the trim tab is streamlined with the leading edge of the rudder and in another there is a large gap . Perhaps it has to be one way or the other not in between .






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Brett Parker said:

what size is the rod for your trim tabs? You could make that your max trim tab thickness and just run with that? or you could just bulk out one size of your rudders . I think having the trim tab aft of the trailing edge gives a more powerful effect from the trim tab. In my experience with trim tabs everyone tends to make them a bit short because once your are sailing the water is ofter flowing well above the water line(where everyone tends to end their height from the water)

There is one stainless steel grade that is stronger than 316 so you could use a slightly smaller diameter( I can't remember the grade now- it is more expensive but you're not using that much)

good luck

I think if it was me I would hunt out the different alloy and just run with that . Depends how much you love speed. Your boat will still have plenty of speed but if you have really gone to town and fared your hulls, and kept your weight down etc etc, then perhaps thickening up one side of your rudder might be an option . I don't think there will be a huge speed lose from the trim tab being a little thicker than your rudder trailing edge. Life is short!

Looking at the picture of the trim tab I noticed you'll have to cut back the end of the rudder so that the trim tab may be aligned with the end of the tiller. So that might give you a couple more millimeters.

Another possible venue would be to epoxy a hardwood veneer on both cheeks of the aft part of the rudder, then re-shape that part to the needed dimension.

In any case you'll have to take the rudder off the boat (but you'll have to do that anyway).

Hi Ricardobr /> The tiki 26 rudders trailing edge is straight unlike the rudder in the pic.I don't think I will take of the rudder as it would way too much work. Hopefully a solution can be worked out or I may go with a tiller pilot or a centrally mounted rudder . Cheers Pat




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Ricardo Aráoz said:

Looking at the picture of the trim tab I noticed you'll have to cut back the end of the rudder so that the trim tab may be aligned with the end of the tiller. So that might give you a couple more millimeters.

Another possible venue would be to epoxy a hardwood veneer on both cheeks of the aft part of the rudder, then re-shape that part to the needed dimension.

In any case you'll have to take the rudder off the boat (but you'll have to do that anyway).

Hi Pat, in case you are considering not going the Wharram plans way I was re-reading Bill Belcher's book, Wind-vane self steering, and he says that a horizontal axis vane coupled directly to the rudder should work well. Let me quote : "A Wharram catamaran of 14m overall also behaved quite well with a simple horizontal-axis vane directly coupled to the tiller for most of the time." (that would be a 42 footer). "From this it will be seen that a normal keeler of about 9m maximum, or a trimaran of up to 12m can be steered by a simple horizontal-axis vane."

So that might also be an option considering a horizontal axis vane is cheap and easy to build.

Cheers

Hi Ricardo. I checked on the net for "horizontal axis vane" but could not find a clear definition.  Do you have a photo or link? Cheers Pat

Here http://www.sailboat-cruising.com/self-steering-gear.html you may see a drawing of how would a horizontal vane work, and here http://wharram.com/images/self_steering_vane.jpg you can see a picture of one.

This last one is a wharram place so the plans you got probably have a horizontal vane in them.

But if you go for direct coupling the vane to the rudder I'd get Belcher's book, it has good info and plans of different self steering vanes (including the one you might need).

Cheers

 

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