Longer rudders

Hi,
I hope you are all doing well. I'm writing to ask your opinion about the posibility of installing new rudders on this boat. The boat in unestable and doesn't response very well at high speeds, therefore we want to to try with longer rudders. Is there any reason why you wouldn't install new rudders that go past the propeller shaft?
Thanks in advance.
Cheers.
https://lh3.googleusercontent.com/-...AAAAGbs/xZ-YWhakyzY/w758-h569-no/IMG_3691.JPG

 

The rudders look about the right size. What is the cross section? If the plate is flat, you may need to go to a wedge shape with the thick part aft. Flat plate rudders often cavitate at higher speeds.

 

the further aft the rudder the more effective it is. I think it is considered desirable to not have the rudder in the plane of the prop blades, so in case of failure of the prop it does not harm the rudder, but it has been done.

You might consider making the rudder foil shaped (like an airfoil in cross section) rather than a flat plate, and make it a bit larger. this should increase effectiveness and improve stablity.

It is best to try and diagnose the problem before you try and fix it. You may put larger rudders in and it is still hard to control. A fixed skag in front of the rudder might be what you need, or perhpas it is something else about the hull design that causes it to be unstable at high speed. Usually larger rudders will work, but best to know before you spend a lot of money and time that it will actually work.

 

You'll want high speed rudder close, not far away from the props. Also offsetting the rudders behind the props is so you can remove the shaft, not any preferred location, unless talking about some serious high speed, which judging by that hull, it's likely possible..

They appear a tad small, but not overly so. You should define the boat a little better. What is "unstable" about her? When you say high speed, what is this 30 knots or 70 knots (for example)? How much HP, what's the reduction, prop diameter and pitch, etc., etc., etc.?

 

Did you see that meaty looking skeg ? I doubt the boat is a speed demon.

 

Yeah, this is my thinking as well, a hefty skeg, two exposed shafts, 4 struts, questionable tip clearance and a round bilge. A pretty draggy arrangement, so "high speed" is a relative term on this boat.

The plan form on those rudders, isn't the way I'd do it, but they should work, though this is also relative, as it's the absolute worst arrangement (well except a single fixed screw) for maneuverability and speed.

 

Since the props aren't extremely light pitch I would think they (the rudders) should be arranged a little pigeon toed to accommodate the outward skew of the prop wash from the upper part only.
May be fly stuff though.

 

Considering the likely performance of this hull (lots of huge assumptions here), the rudder needs only to be far enough aft, to let the prop get removed and far enough offset to let the shaft be removed, both without removing the rudder.

 

with flat plate rudders like you have you might just cobble up some larger rudder plates and screw them to the extinguishing rudders and see if it helps, as an experiment. It if solves the problem than you can spend the money to have larger and better rudders made.

Something else that might be simple to try is to adjust the linkage to give a little bit of toe-in on both rudders and see if it makes a difference.

considering the size of that center skag the hull might indeed be directionally unstable making it hard to keep in a straight line when at speed. larger rudders, or fixed skags in front of the existing rudders should improve the directional stablity, and give you some more control authority.

Post pictures of the whole hull, and give size and displacement of the boat, and size of those rudders, might help us give you some useful suggestions.