70 cm rudder for DF920

[johp]

I made a 70 cm deep rudder for my DF920 to make sailing in the shallow water better possible. I sail a lot at the Waddensea in the North of the Netherlands, what is shallow water in general and is dry at low-tide.
At least two persons have asked me about this rudder and here a post to share all my information I have.
General: It is home made. I calculated the strength (thickness of the glass skin) to a level the rudder will hold at 20 knots speed and rotated 90 degree in the water (very artificial, can never happen). I bought some wooden multiplex plates in different sizes what fits about the NACA profile. Glued the plates together with epoxy on the correct position. Shaped the NACA profile by cutting away the plate edges with a grinder and a sander. Put epoxy and a thin glass sheet as first layer and than a thick layer of polyester_glass to match the required thickness of the glass.
I'm extreme happy with the rudder:
- theoretical not more resistance, so speed is (not noticeable) reduced,
- at low speed the rudder is much more effective (due to extreme thick profile and long profile),
- Sailing is much easier in shallow water. I can sail up to about 70-80 cm depth till rudder kicks up. But than you really feel the boat is slowing down due to the ground effect and it is time to creep in stead of sail.
- the play in the rudder is far smaller than the (worn out) old rudder
It is used now for 3 years and still fine. After a few months I made a stainless steel U-plate at the position where it hits the stainless steel head, because it started to worn out like the old one had already far more.
The balancing was guide good. The first sailing it was more of less over- balanced and I had to push back a bit the rudder in stead of pulling. With the stainless steel U-shape this was gone (rudder rotated little bit backwards) and I never tweaked it again.
Calculations, pictures, offer of the wood, offer of polyester I will make available. All in Dutch, but not that difficult to understand. The calculations are not reviewed and I’m not responsible for the calculations, although I’m pretty sure it is far strong enough: it survived 18 knots, several hits to the (sand) ground, some hitting by the propeller of the motor when pulled up half way, ...

Download the calculations and cost:
https://1drv.ms/u/s!Agr6u6iy1fFRhjFGHcI ... G?e=SL6p17
(the underneath link-connection is not complete,
copy the full line, so from https://....17 and post in your browser adressline)
Download pictures:
https://1drv.ms/u/s!Agr6u6iy1fFRhjKoEvn ... R?e=86J69C
(copy the full line, so from https://....9C and post in your browser adressline)