Building a rudder

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Re: Building a rudder

Postby GreenLake » Mon Feb 04, 2019 6:37 pm

Still using the same rudder.

The lamination between the two sheets of plywood in the blade was starved of glue and had to be reworked after the first season.

I refinished the blade once after repairing some minor dings from ground contact and also had to repair some cracks after the rudder was hit by another boat at the dock (hard enough to wrench out pintles and gudgeons).

Other than that, has worked fine for me.

I picked plywood because it has differently colored layers that make a pattern when you shape the blade. That was a great help in roughing out the foil shape. Also, plywood dimensions are in full / half inches, not some weird intermediate lumber sizes.

I used birch plywood with many more than the usual number of layers (5 or seven).

Did not use marine grade. You want some grade that has no voids, but once you seal in epoxy and encase in glass, there's really not the exposure that makes marine ply critical.

Since I have the factory rudder head and blade, I originally conceived of this as an experiment: can I create something with reasonable effort that is lighter and has better foil shape? I think I succeeded and find that it's standing up to some long-term use.

I use epoxy. Almost any epoxy (other than the H/W store stuff in the dual syringes) will work pretty well. I use SystemThree. Their "GelMagic" is the one formulated for gluing. You can get it in a caulk gun cartridge with self mixing tip -- makes it very convenient.

There's no point in gluing the nylon bushings other than to seal the drilled hole (the latter is important!). Doubt the glue will permanently bond with the Nylon.
~ green ~ lake ~ ~
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Re: Building a rudder

Postby tc53 » Mon Feb 04, 2019 7:12 pm

Thanks, Greenlake. As you could probably tell, I had not yet seen page two of this string when I posted my questions. For mounting the pintles, I think I may try your idea from that second page, cutting out "slots" for the brackets but applying layers of glass over these slots and an inch or two of the cheeks above and below them. From your drawing of that idea, it looks like your intention would be to taper down into these "slots" leaving a layer or two of the cheek ply in place, then applying glass across and beyond these tapered slots to maintain cheek strength. How would you suggest cutting or creating these slots?

Thanks again!
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Re: Building a rudder

Postby tc53 » Mon Feb 04, 2019 7:14 pm

Also, am I correct that you used 1/4" ply, sandwiched between layers of fiberglass, for the cheeks?

And looking at you photos from page two, it looks like the cheek portions begin approximately halfway between the two pintles and extend down from there, with the transition to the upper part of the head being fared to a gradual slope. Is that correct? I'm only asking because on my old rudder head, the cheeks extended almost all the way to the top of the head, only leaving room for the tiller attachment and rotation.
Last edited by tc53 on Mon Feb 04, 2019 8:20 pm, edited 1 time in total.
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Re: Building a rudder

Postby GreenLake » Mon Feb 04, 2019 8:20 pm

Yep, 1/4" ply.

You may find that the brackets for the pintles may not span more than 1". In which case you may have to use a coping saw to cut a slot into the cheeks, then use a router or chisel to take some material off the top of them main head; then do the glass thing to create a new continuous "skin" over the depression. If they span more, e.g. 1.25" of can be bent a bit to open, you can assemble first, then use your favorite tool to make a fairly wide depression about 1/8" deeper than needed for the bracket and fill that 1/8" with glass.
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Re: Building a rudder

Postby tomodda » Thu Feb 07, 2019 3:13 pm

GreenLake:

Two "curiosity killed the cat" questions:

-For your rudder cheeks, did you build a new set from scratch or saw down your existing cheeks? In other words, can I saw off the last 3 inches of the cheeks (below the bolt), seal it with some epoxy and paint it?

-Likewise, could I just take my existing rudder and round off the top 3 inches of the frontside using a router? Then seal and paint? This is a quick-and-dirty solution, not as elegant as making a new rudder with a much better foil shape.

What do you think?

Tom
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Re: Building a rudder

Postby GreenLake » Thu Feb 07, 2019 3:52 pm

Tom,

the "seal & paint" part, unless you do that very localized is the most labor intensive part of any project - the rudder is no exception.

You can certainly cut away part of the cheeks of an existing rudder head. I think most of the support for the rudder blade is provided from the pivot to the top edge of the blade portion. The outer end of the cheeks, being not supported will probably "give" a bit under load anyway and therefore not contribute much. But that's just my thinking on it. As it happened, I started with fresh 1/4" plywood with a sheath of fiberglass cloth to make the new cheeks.

Modifying the front end of the existing blade portion of your rudder is dicier - not sure what you will find (including mild-steel rebar) when you dig into it. (My factory rudder has rust stains).

But you are losing sight of the biggest advantage of building a new rudder: weight at the stern. I did an experiment recently (you can duplicate it if you have a GPS) of trying to measure the effect of boat balance at moderate speeds. I used my new ElectricPaddle for that, which drives the boat at 3.1 knots in calm flat water conditions. Shifting weight forward aggressively (by moving onto the cuddy top) I was able to increase the top speed to almost 3.5 knots.

Replacing a heavy factory rudder by one that has slight positive buoyancy - that is, it weighs "nothing" when deployed, reduces weight at the most critical point.

The improved foil shape should decrease drag, possibly even require slightly lower rudder deflection - if true, that would add another component of drag reduction (would be cool to see whether the effect is measurable under motor).

The added drag reduction from not exposing a bit of flat profile at the top and not dragging the cheeks through the water is unlikely the major component in all of this, so let's be charitable and say it makes up 1/3 of the improvement.

But because it involves "sealing & painting" it's not as quick as you might think compared to the full project. Roughing out the blank, cutting and assembling the pieces was at best 1/3 of the work I did. Another 1/3 went to fairing the blade - I should have used the notched spreader technique to speed this up - and the final 1/3 (and somehow the largest third :) ) was spent on final finishing and painting.
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