Rudder stock turbulence

[Paul H (admin)]

Here is a photo of the Lynx 16 (left) and Lynx14 (right) rudder designs; waterlines line up, same scale:



Notice the L14 stock sides drag through the water, but on the L16 only the rudder blade is in the water. Is this an error in scaling one hull to the other?

Down the Thames I have always been aware of turbulence around the rudder case, visually and audibly. Measuring the tide mark of the stock about 80-90mm could be in the water, causing precious drag for the electric drive.
EDIT: The stock is 50mm wide, so there is potentially an area 90mm x 50mm causing drag; although I used a 12mm rounding bit in a router to curve all edges

My quick fix is to mount the rudder hinges as high as possible, +40mm. I'm tempted to shave 10mm off the stock sides, or make them more stream lined.

In Chichester harbour with speed she turns on a sixpence, sometimes tacking I oversteer and have to correct. So I don't think these changes will impact sailing in a bad way, and drag reduction is good in all conditions. But we don't race do we?

Any other observations? Could I trim the stock height, so that it just touches the waterline?

cheers
-Paul

[Timmo]

The stock for the 14 does seem overly tall compared to the 16. Actually seems taller despite the shorter transom, unless that's a illusion created by the photography. The 16 drawing looks right with the transom kissing the waterline and the stock sitting at that same height.

Logical approach seems to be to match the 16 layout by shortening the 14's stock (taking it off the top and remaking the tenon for the tiller.)

Tim.

[Paul H (admin)]

Tim,

My thoughts exactly.

The original photo has the L14 plan rotated very slightly, but it does look like the L14 rudder is deeper below the waterline!

I'm going to move the lower transom pintle up 90mm (which is the case depth on Paul Fisher's 1:10 plan), the upper pintle can go about 35mm up, then I'll  move the upper rudder gudgeon down to fit.

Then as you say trim the top of the stock - it's really heavy (and looks long) so a haircut will be good. Although I might wait for confirmation of her turning circle in the water first.

-Paul

[Paul H (admin)]

I'm trimmed the stock down, saving 0.8kg in weight:



My Record BS250 bandsaw made a very clean cut, but it's tricky feeding the heavy stock by hand. If there was time I would have built a large plywood table with sled to fit around the cast iron table.
But I just did my best and used a hand plane to trim the high spots.

There is no time to mix up the grey Jotun 2-pack undercoat, so I rounded the edges with a router and sealed the cut areas with epoxy for now.

I use a hinged tiller to allow the rear locker to open. I just need to add a stop block for the tiller and refit the up/downhauls.

Paul

[Timmo]

Very neat!

On 16 May 2018, at 13:43, Paul (admin) [via UK HBBR Forum] <[hidden email]> wrote:

I'm trimmed the stock down, saving 0.8kg in weight:



My Record BS250 bandsaw made a very clean cut, but it's tricky feeding the heavy stock by hand. If there was time I would have built a large plywood table with sled to fit around the cast iron table.
But I just did my best and used a hand plane to trim the high spots.

There is no time to mix up the grey Jotun 2-pack undercoat, so I rounded the edges with a router and sealed the cut areas with epoxy for now.

I use a hinged tiller to allow the rear locker to open. I just need to add a stop block for the tiller and refit the up/downhauls.

Paul

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