RUDDER - Number, Position - New Old Idea

[mydauphin]

I have three design.

A. 4 rudders - 2 on each side of prop, focuses prop wash to back, more resistance
B. 2 rudders - on inside of rudders, out of prop way, some low speed
C. 1 rudder - Big enough to get some propwash for lower speed, less resistance

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Here is idea, with twin engines and bow thruster. Are two rudders really necessary or would one bigger rudder - work ok at low speed and allow for less drag. I remember some of the older destroyers had one big spade rudder. Remember with one rudder - the prop flow is untouched by rudders.

Check out drawings attached.

The drives are surface piercing out the transom about six feet out, transom 45deg. Looking for most efficient at 15 to 20 knots with adequate low speed.

[Gilbert]

The reason most rudders are in the prop wash is because they produce more turning reaction than if they are not, which means you have better control.
Of course, with surface drives, having the rudder in the prop wash is somewhat of a problem to say the least. No doubt that is why some of them have active shafts (or whatever you would call them).

[messabout]

When you say surface piercing props it is suggests that you are referring to Arneson drive/super cavitating props. You also mention 15 to 20 knots. That kind of prop layout is usually applied to very fast boats, not 15 to 20K. What am I missing here?

If you are talking about the "dragon tail" type then you'll want to get the prop immersed with a cavitation plate over the prop. Dragon tails do not need a rudder, they are usually articulated.

[mydauphin]

I should have said, "Like Surface Drives".
Also not in drawings are the ventilation plates reaching from transom over propellers.

This is a design for large but light ship, not too much horsepower. Trying to be efficient. I am stuck with two engines, two props, semi-planning hull. Transom was setup for straight shafts out bottom of transom. I have to add rudders.

With two engines - I dont really need two rudders for slow speed manuvers.
I can help to think that 1 rudder will produce less drag than two. I just have to move it as far back as possible so that when it turns it gets some of the propwash.

If you look at some navy ships like Carrier Intrepid and Older Destroyer they have 4 or 2 props and only one big rudder