REPLACEMENT RUDDER

[JIM_TEAL]

Having decided to replace my pivoting rudder blade with a vertical lifting blade the leading edge of the new blade will be approximately one inch further aft than the pivoting blade due to the position of the transom hinges. It is a 23 foot ketch and I feel that one inch further aft will not make any noticable difference to the feel or weight on the helm.
Would the forum experts agree or should I re-design (difficult) before committing to wood and metal?
Jim

 

[KenMcCulloch]

The experts will be along shortly sir. In the meantime I suggest that it might move the CLR slightly aft and make your helm lighter. Sheet in your mizzen a bit.

 

[bazobeleza]

Is there any balance area (thats area of rudder in front of the pintles) on the old swing down rudder blade? if there was (usually such rudders are rounded in side on shape allowing part of the rudder to swing in front of the pintles) and its not allowed for in the new I would have thought the rudder would be heavier, There was an article in PBO last year I think on the effects of adding or subtracting balance areas to rudders a search of their archive might help.

If there isnt any balance area then I'd agree with ken. all in all an inch will not make a huge difference if noticeable any at all.

 

[ex-Gladys]

Removing an inch of the leading edge (in effect) will not materially affect boat balance. It will however make the helm heavier, because you have effectively moved the rudder further away from the pivot point.

 

[Blueboatman]

..which could theoretically be countered by changing the angle at which the new drop down rudder slides up and down.
That is, when fully lowered, the front lowest point of the rudder would be an inch or two forward of the top formost point, when the boat is viewed from the side.
Yorkshire Cobles did this radically, traditionally in an attempt to make the rudder an effective 'keel',being shallow keelless beach boats.

 

[thalassa]

Additional remark: make sure the blade is as symmetrical as possible. After a breakage of the lifting rudder blade (at night, in the German Bight, during the CAM race -... it was a substantial log...) an emergency one was made up in bare bankiraï in Denmark. It worked so well, that it was built up afterwards by glassing it in. However, the profile was more curved on one side, creating lift. This resulted in an ever-present helm pressure to starboard, very pronounced when motoring. I've partially corrected it now by hacking of the glass on one side, but it is not the most pleasant job you can do on a boat.

 

[aitchw]

Shape, area, profile and depth will all have far more effect than a 1" difference in position of leading edge.

I would be really interested in seeing your design for the support of the new blade and how it works mechanically. I have been looking at a not dissimilar solution on my little bilge keeler if I move the rudder to transom hung.

 

[oldsaltoz]

G'day Jim,

A drop down needs to be very strong in it's mounting and fixing to the transom, to reduce the risk of ending up with a heavy helm you should look into adding up to 12% balance; that's 12% of the total area ahead of the pivot point.

The rudder will end up looking a bit like an inverted 'P'.

Hope this helps.

Avagoodweekend......

 

[William_H]

My particular boat type is available in drop rudder or swing rudder.
I prefer the swing rudder because I think it gives better handling.
However you can not use a swing rudder in shallow water ie with rudder partly lifted because it goes aft away from the transom by a long way before reducing draft. But that doesn't concern my with my kind of sailing. ie not in shallow water. If you have a lift keel and want to sail in shallow wateer you need a lift rudder cos it is usable partly raised.

What is dramatically noticeable on my boat is if the rudder is not fully home (forward) the rudder loads are huge because rudder balance is lost and rudder centre is way aft of the pintles.

So yes I reckon by moving the rudder blade aft by one inch you will be surprised how much heavier the rudder is to steer with.
I have reshaped my swing rudder some time back to increase the area in front of the pintles to reduce helm loads. I also increased the depth to improve control with shy spinnacker. ie give me more control of the weather helm which occurs with heel. It sails a lot with heel.
As suggested if you tilt the drop rudder so the bottom of the blade is well forward of the pintles you can get some rudder balance. You need i reckon about 15% area forward of the pintles line which equates to reducing the area you have to push against the water by 30%.
Bear in mind however that the position centre of lateral resistance of the rudder as a distance aft of the pintles as related to the distance forward of the pintles where your hand is gives the leverage you have over the rudder. So closer rudder centre of lateral resistance is to the pintle the lighter the force you apply to the tiller to turn the boat.

Here aree a couple of photos to show what I have found to be very successful. regards olewill

 

[JIM_TEAL]

Thanks to all especially the Aussies for advice and ideas.
Almost all the design of the support frame is in my head at present, not yet on paper.
The frame wil be mainly s/s tubing with hardwood guides. That way I have a little scope to adjust the balance factor if it turns out too heavy. When I get it drawn and welded I'll try and post some pictures.
Jim

 

[geoffatstanpit]

Two rudders on the same pivot bolt is a good solution to the shallow water problem. The grey plastic rudder is the same depth as the bottom of the motor skeg.