Rudder change – please advise, who can

[BarryG]

The boat is a Bruce Roberts “Mauritius”, a centre cockpit, cutter rigged sloop, 43ft LOA, 33 ft waterline, 17 tonnes laden for cruising.
The keel is long, starting just fore of the mast, and the aft end is angled as per the rough sketch. (not to scale). The rudder stock is also angled and is supported by a nylon-bushed pintle at the lower end.
The boat is heavy on the helm, partly because the wheel is relatively small, being bulkhead mounted on the port side of the companionway, an arrangement which I shan’t change.
She doesn’t hold her course well even when hand steered, and though the Aries wind vane can keep her on an average course, it works hard to do so, and she progresses in very long sweeps. She is also sluggish and slow to respond to her rudder, making her awkward to manoeuvre in tight spots. I suspect the rudder area is too small, but the angle at which it’s set doesn’t help either.
I have in mind to change the angle of the rudder stock to make it hang vertically and to balance the rudder by building about 15% of the total area forward of the pivot line. I hope that would lighten the helm. It would also push the rudder aft and I hope that would improve her directional stability. Since the rudder will have to be built anew, I would like to ensure it is as big as it should be.

Please could somebody advise:
1. Will I be creating an unforeseen problem by making this change. The mechanics I can cope with, but I’m referring to handling, performance or safety.
2. What guidelines might I use to arrive at a suitable rudder surface area.
3. Will the foil shape of it be very critical, bearing in mind she is a heavy cruiser, and not racing?

[Nordic Cat]

Can you post the file as a jpeg for example? Not many of us use corel draw..

Regards

Alan

[MikeJohns]

Barry

You can make the rudder larger without much undue effect providing you beef up the bearings shaft and steering gear accordingly. Also any auto-helm drive. It does sound as though the rudder is undersized.

For a sailboat you are better shifting that proposed balanced rudder furtherl aft you can still have a lower support which reduces the shaft diameter. It is also worth adopting a decent foil shape.

A refit we did that may be of interest; the 2m tall rudder was fabricated from 6mm mild steel the LE was terminated well short of the max curvature with a flat plate and the nose was carved from wood and sikaflexed and bolted. It worked very well and no pre-forming was required, it has been in service on a 70 foot fishing boat for 2 years now and is perfect. Fabrication overall was a lot quicker.

Cheers

[BarryG]

Thanks, I appreciate your advice.
Bruce Roberts thinks my boat may have been built from pirated plans and I have no idea how to find out. He may be right. He designed a solution two years after my boat was launched, but is jealously guarding them and won't help unless I can prove she was built from bought plans. I can't as I'm the third owner and it seems the boatyard faded with it's aged founder. The hell with him, I might try your suggestion. She might be a bastard child in his eyes but to me she's a princess, and I'll try to fix her.
Regards
Barry Garnett