How Critical is Rudder Perpendicularity? What Effect if Not?

[sailingcolin]

On a yacht (40ft racing yacht), how important is it that the rudder is in line with the keel? Lets say the horizontal is 0 degrees and the keel is 90 degrees what effect would it have if the rudder blade was a few degrees out?

 

[Triassic]

It will be more effective and efficient on one tack than the other...? Look at the vessels equipped with twin rudders, they are a fair bit more than a few degrees off the keel line........

 

[NormanB]

The key question is how did it get to the state where it is not perpendicular?

If it was a build error then I would not worry about it because you cannot do anything about it without spending Lost's of £££££.

If it is a result of grounding and the rudder shaft is bent or the the rudder support has been damaged internally then this may be a cause for concern.

If the rudder is free to move from hard over port to hard over starboard it would however indicate there is not much wrong.

 

[savageseadog]

It will be more effective and efficient on one tack than the other...? Look at the vessels equipped with twin rudders, they are a fair bit more than a few degrees off the keel line........

I was thinking about the twin rudder boats, does the keel cant too? But where does that leave the other rudder?

 

[sarabande]

Don't naval architects use some form of Ackermann principle ? (DIfferent angles of turn from P to S, and vice versa, to take account of 'skid')

 

[wotayottie]

On a yacht (40ft racing yacht), how important is it that the rudder is in line with the keel? Lets say the horizontal is 0 degrees and the keel is 90 degrees what effect would it have if the rudder blade was a few degrees out?

I doubt that 2,3,maybe 4 degrees makes much difference. For a start does the boat float level - mine has a small permanent heel and so do many others. And is the hull symetrical - doubtful even with mass produced boats like Bevs. Then of course is the mast perpendicular?

In an ideal world everything would be bang on but the world isnt ideal . Yes it will have some effect but unless the misalignment is gross, I doubt it will matter much

 

[Tranona]

And is the hull symetrical - doubtful even with mass produced boats like Bevs.

Why would you think that when the moulds are made using CNCs? Far more likely for a boat made the old fashioned way to be asymmetrical given that the plugs were usually made from wood by hand. Hulls from the yard that I worked at in the 1980' were all asymmetrical, and considered quite normal. Gave the shipwrights fitting out the interior something to do to make everything fit. Of course they were trained to do it.

As you say lots of other things can be out of alignment, some of which can be adjusted out but others are structural.

 

[LittleSister]

How often is your keel vertical when sailing?

 

[gmkjr]

Current thinking is that the keel and mast should be parallel, and the rudder will work better as an element of the lateral plane of the boat if it is parallel with the keel. However, one boat I sailed on was tuned so that the mast leaned a few degrees to starboard.

That made the boat marginally faster on starboard tack. which was important in the first few minutes of a race when starting on starboard tack.

That said, I don't think a few degrees of error would make much difference, and I would note that on many boats, the top bearing on the ruddder stock can be moved relatively easily.

 

[wotayottie]

Why would you think that when the moulds are made using CNCs? Far more likely for a boat made the old fashioned way to be asymmetrical given that the plugs were usually made from wood by hand. Hulls from the yard that I worked at in the 1980' were all asymmetrical, and considered quite normal. Gave the shipwrights fitting out the interior something to do to make everything fit. Of course they were trained to do it.

As you say lots of other things can be out of alignment, some of which can be adjusted out but others are structural.

In the nature of practical engineering Tranona. No doubt your beloved Bav will be way better than the old boats you mention but even if the plug is distortion free, which it wont be, the installation of components and uneven loading will cause distortion afloat. It isnt a question of whether but of how much

 

[savageseadog]

How often is your keel vertical when sailing?

Hopefully never

 

[Tranona]

In the nature of practical engineering Tranona. No doubt your beloved Bav will be way better than the old boats you mention but even if the plug is distortion free, which it wont be, the installation of components and uneven loading will cause distortion afloat. It isnt a question of whether but of how much

Nothing to do with being a Bavaria. Any boat built with modern techniques is more likely to be accurate to the drawings. We are not talking about distortions afloat (although that again is largely a thing of the past) but a rudder structurally out of line with a keel, which can only be caused by poor build or damage.

Suggest you look at the videos of how modern boats are made (both Jeanneau and Bavaria are on You Tube) to see that what you suggest is unlikely.

 

[Ingwe]

Assuming that it is the rudder that is off axis then it does have some effect as the rudder will end up being either slightly toe in or toe out depending upon which tack that you are on which slightly changes the hydro dynamics at the top compared to the bottom of the rudder and will induce progressively more drag (and weather helm) the faster you go on one tack (it might actually create slight lift on the other tack), at six knots of boat speed the effect is probably negligible but at fifteen knots it would be noticeable.

This only normally gets thought about when it comes to boats with twin rudders as they are deliberately designed so that each rudder is at an optimal angle when heeled for its tack but equally it has to not fight the other rudder when the boat is flat which means that they have to be calibrated to within a couple of mm of optimal or you induce significant drag at speed - I have a Sunfast 3200 and the designer has produced really exact measurements so that owners can realign the rudders correctly as it is pretty critical.

But if you find that your rudder and keel are no longer in line check your keel first!

 

[Skylark]

Nothing to do with being a Bavaria. Any boat built with modern techniques is more likely to be accurate to the drawings. We are not talking about distortions afloat (although that again is largely a thing of the past) but a rudder structurally out of line with a keel, which can only be caused by poor build or damage.

Suggest you look at the videos of how modern boats are made (both Jeanneau and Bavaria are on You Tube) to see that what you suggest is unlikely.

Slight thread drift. My 2013 Beneteau has asymmetry of the D Shroud hull chain plates. The port side shroud touches the lower guard rail, the starboard side is clear by about 5 to 10mm. I only found it because I had a failed LED navigation light and this put 12v into the rigging. Overnight sailing I noticed a glow where the two wires touched. There was quite a lot of erosion of the shroud. The dealer said by email "not possible with modern boat construction". Come and take a look, I replied. "Oh dear, we had better replace the shroud and insulate the new one from the guard wire in case another electrical short ever happens again".

Personally, I think that the odd 5mm asymmetry in a 41ft hull is perfectly acceptable.

 

[wotayottie]

Nothing to do with being a Bavaria. Any boat built with modern techniques is more likely to be accurate to the drawings. We are not talking about distortions afloat (although that again is largely a thing of the past) but a rudder structurally out of line with a keel, which can only be caused by poor build or damage.

Suggest you look at the videos of how modern boats are made (both Jeanneau and Bavaria are on You Tube) to see that what you suggest is unlikely.

Everything to do with being a Bavaria. As someone who has run mass manufacture engineering plants, i am very confident that quality control in the Bav plant ( or indeed any other mass manufacturer like Benny) will be way better than it ever was in say Moody.

But at the same time I know that rudders will be out of line and hulls asymetric - the only question is how much. Could be microns, could be mm but it will be there.