Stern extension

[Tactic]

I am planning to modify the stern of my 20 foot trailer yacht by adding a extension to it. I want to make the addition as light as possible and keep the rudder in its existing location.
The extension will need to have a 'V' cut out of it to allow the rudder to pivot.
I have had some advice that this type of arrangment can be a bit "draggy"
I have attached some pics of a similar sized boat with this type of arrangment..Will my boat be faster with this addition?
My transom is fairly wide so the addition does not look out of place.

Cheers
Tactic
Edit..I have attached the before and after drawings for my boat

[Tim B]

I'm sure I've seen that design somewhere before. I thought then (and still think now) that it would be an awful lot more efficient to have the rudder mounted in a stern tube. this would allow you to get rid of the cut-out. This would probably give you more performance than adding a little extra length. however, you would then have to work out how to get it onto the trailer (if the rudder has to lift).

Out of interest, what is your thinking behind adding an extension to the stern? Do you want extra speed?

Tim B.

[Tactic]

Yes extra speed is the goal.
The boat is trailable and must remain so.I know that that I could mount the rudder on the back of a new extension,but then the weight etc of that structure would need to be considerable..hence the lightweight "swallow tail' idea.
Tactic

[Tim B]

I think you stand to gain a lot by putting the rudder below the hull and filling in the gap. If you can work out a good way to get it to fold up. It needn't be any great amount heavier than it is now, either. I think that would actually reduce the drag quite a lot.

Tim B.

[AYACHT]

Nope It will just add more drag and be slower.

[Mark 42]

Why not just build a new transom and move the rudder hinge point aft?

(or buy a longer boat).

[Tactic]

The swallow tail stern and leaving the rudder in its current position is the easiest option..Moving the rudder further aft will involve a lot more work hence my question on the drag of the "V" cutout.
I may just do the whole job..move the rudder aft and all,not sure yet
Will the "V" tail stern be faster than no stern modification at all?
There are quite a few boats around with this type of stern..would they be faster if it was cut off?
Thanks for your Advice Tim B, helpful.

Cheers
Tactic

[Chickadee]

See attachments

1. Observe how the water flows at this level. If no whirls, you won't gain anything.

2. You need to have flat bottom hull for an extension with a 'V' cut.

When sailing under variable conditions (heel angle, speed, up-down wind), look at the position of the waterline on the stern section. It should not cross the centerline, else you may get more drag and weight than speed.

In "rough" see conditions it's another story (solidity->weight, etc).

[Sketch]

One solution to maintaining trailerability while extending the stern and keeping the rudder in it's current position involves a bit of precise building - but then again so does extending the stern

I wish I could remember where I read about this configuration in order to give thanks and a link to more detailed info. The setup involves a hollow carbon tube, the diameter of which is greater than the width of the rudder chord. The tube sits in a bearing that allows it to rotate. The tiller, or other steering mechanism is attached directly to carbon tube. Now imagine taking your existing rudder, but adding a cylinder that fits the inside of the carbon tube, on top (where the rudder stock would be).

Here's the cool part - drop the rudder into the hollow carbon tube and voila, retractable rudder, with little drag (the base of the cylinder should be faired into the shape of the bottom of the boat), in the same position as the original.

If you have a spare, you can even change your rudder without getting wet, say, to experiment with different foil sections, lengths, etc...

Kevin

[Tim B]

In fact, there is a thread somewhere on the forum that deals with the idea of retractable rudders, it might be worth a look.

Tim B.

[Tactic]

http://www.socasailboats.com/VARA_Sy...ra_system.html

I think this is what you are refering to.
I have also attached my sections drawings to help with discussion.
Cheers
tactic

[Chickadee]

The waterline is fine. If you don't overload the boat - and you have some margin according to the drawing - the stern extension will be useless for better performance.

[Alik]

Hi Tactic,
System seems not very effective to me. You limit available rudder angle but still have rudder aeration. Note that available rudder angle should be 35 deg from centerplane (this is widespread requirement for ships), but over 40 deg can be recommended for small sail boats. Your sketch gives smaller number, am I right?

[Tactic]

Chickadee,
Do you belive the overhang at the stern of an ACC yacht is effective?
It is much longer than what I propose.
Maybe the ACC designers have it wrong to?

Alik,
Yes I need to make more rudder angle it seems..I think I need to take the stern extension right across with the rudder under now.

Tactic

[Chickadee]

Originally Posted by Tactic

Chickadee,
Do you belive the overhang at the stern of an ACC yacht is effective?
It is much longer than what I propose.
Maybe the ACC designers have it wrong to?

ACC is a totally different story. Their design depends on formula combining WLL, sail area and much more.

AC yachts waterline length is made shorter for better rating, it's a trick to have much sail area or weight in the keel, etc. The huge overhangs are only a consequence.

DO NOT try to compare your small boat with these horses !