Rudder mount design for small outrigger

I have a small tacking double-outrigger I'm adapting from a fibreglass surf ski. It uses 40mm OD aluminium round tubes as akas.

I'm wanting to use a cassette quarter rudder similar to that shown by Gary Dierking (after Michael Storer etc). Instead of using two posts, one above the other, for a mount, I'm hoping to use only the aka as a mount point (adding a second post to the surf ski is tricky without cutting more holes in it, which I'm trying to keep to a minimum, also other reasons...).

Slid over and strongly pinned through the aka is a "telescopic" fibreglass tube (ie same ID as aka OD).
The 200mm length of this tube is strongly bonded to the inverted base of an equilateral triangle of 7mm marine ply, flattened(cut-off) extending about 200mm downward.
The vertical centre length of this triangle provides the "transom" or mounting points to which the rudder cassette is attached.

Strong side forces from the unsupported bottom of the "transom" are transmitted through the ply webbing to the tube and thence to the aka. Weaker fore-and aft forces try to rotate the tube over the aka, resisted by two large diameter (9mm) aluminium pins each passing through both tube and aka.

The rudder blade is fairly small, about 170mm wide, max sideways lever arm around 800mm.

Has anyone seen anything like this that works reasonably well? The obvious downside risk is that waves might hit the inverted triangle head-one with enough force to either seriously slow the boat or shear through the pins/aka, or that the cassette might not release the blade sufficiently in a grounding.

 

Googled Gary Dierking but couldn´t find what you mean, still curious.
"A picture is worth a thousand words". I use MS-Paint for sketches, it´s good enough.....

 

I assume you are going to offset the rudder. You could use either light aluminium tube or fairly stiff PVC tube. Clamp a length over and under the transverse beams. You'd need them lashed tight or make clamps.

Atttach your pintles to the end of each tube and hang your rudder assembly off that.

If the boat is demounted for trailering it should not take too long to disassemble. Offset would make steering from a central seat more comfortable. It wold be cheap enough. It would also be the start of an outboard bracket if you decide to go that way later,

 

Pictures or sketch.
Too many assumptions above for me to understand.

 

Here is the kick up rudder I use on the Ulua. The bracket clamps on to the beam and doubles as a motor mount. You could mount a cassette rudder on it too. You would have to pin the bracket to your round beams to prevent it from rotating.

 

Yes that was what I was thinking, only to save drilling holes take it forward to the front beam. Because we don't know anything much about the boat I thought piercing or welding on the beam might compromise it.

Incidentally I found your account of dealing with cavitation with your little outboard very interesting. Thank you for posting that.

 

This is a rough diagram to scale from side on and from the front. The grey cylinders are the aka sections, the green is the fibreglass outer tube and faired glass mounting to the backing ply. The pins shown are aluminium 9mm, with holes drilled in the ends for shockcord connection (I considered 6mm al pins but though that they might shear too easily, and that 9mm gives a wider load-bearing surface on the thin aka tube). I left out the tiller for simplicity.

The mount is somewhat similar in principle to the Ulua kickup rudder shown in Gary's pictures, which I hadn't previously seen (thanks Gary). However I don't want to add an outboard (purely personal dislike of motors!) so the mount is simpler but probably more fragile and has an element of structural risk due to pins going through the akas (which are 40mm x 2mm Al tube - don't know the temper but probably not the highest structural 6061, I think they are 6063 T5. The span from aka mount to ama is about 1250mm, it's a two aka design (ie fore aka and aft aka) and the volume of the amas is about 70 litres each - so not huge forces).

 

If I used a single outrigger config (tacking proa) that idea might work really well, as I could take a beam forward from the rudder mount to the front aka (1900mm forward) and use it as the basis of a sitting position. If I stick to a double outrigger (with a centre-hull seat like a standard surf ski) I think taking a mount support forward to the front aka would add too much weight and complexity - it would probably end up being simpler and lighter to add a lower rudder support to enable a more standard setup.

 

You could use very light tubes, but whatever makes you happy.

Personally I think the pins are a bad idea. The tubes are probably far stronger than you need but drilling a hole through the vertical axis is weakening them where they most need strength. I might instead use a really strong clamp, possibly in combination with something gritty for grip.

2c..

 

Yep, light tubes would be possible, and plenty adequate for the rudder mount, but kind of makes for messy intrusive infrastructure unless they are used for something else as well.

Re pins: I've avoided them for years for exactly the reason you mention, instead using ss hose clamps tightened with thin strips of rubber under them. Finally got sick of them - you wouldn't believe how much clamping pressure you need to avoid rotation on an AL tube, never got it to work well in any high load situation. The clamps are also fiddly - need to be tightened with wrenches etc. Another option: bond something directly onto the tube - can get a good epoxy bond with aluminium by wet-sanding through the wet epoxy to avoid oxidation, works very well. I was avoiding this to preserve removability but you have constructively reinforced my doubts about pins

 

Option one: use clamps (for comfort something custom instead of hose clamps) and install one or two anti torque lines from the bottom of your "transom" to the front aka. This will resist rotation without you having to drill holes into the vaka.
Option two: make the beam rectangular in the rudder area by adhesive bonding a shaped piece of wood/fiberglass. Attach the rudder apparatus to this part by removable means.

 

Option 2 sounds good.

 

I would not have used hose clamps. Too time consuming and as you say not enough clamping force.

If it were me I'd either permanently attach a rectangular section as mentioned above and have holes or studs to attach the rudder

or

get a block of timber. Bore a hole. Cut it in half. Bold the halves together. Make the beam, the inside of the hole or both abrasive by whatever means you prefer. Attach the rudder mechanism to that. Clamped with 4 bolts it should be quick to assemble and not move. It should be straightforward to make.

2c.

 

Hi Peteyak,
Maybe a little radical, but I would dump the complicated rudder box and integrate the rudder with the pintles. Instead I would go for a friction coupled clamp that lets the rudder swing back if you hit something or need to lift up. The locking bolt(s)
could be one or two of the eccentric type you use on bike saddle posts. To simple?