Cutting back the rudder stops?

[78puget-trawler]

On this 34' CHB of mine, the rudder stops consist of a wood board that sits so that the steering arm makes contact on both sides effectively stopping further swing. I have not measured the arc amount but was wondering if others had ever cut back on the stops to allow more rudder swing. There is still about 2 plus inches of ram showing before it hits the ram housing when hard over. Just curious.

 

[boatpoker]

35 degrees is a rule of thumb maximum useful rudder angle. After that you are in a stall condition.

 

[78puget-trawler]

Looks like I have about 30 degrees on it side to side.

 

[psneeld]

Rudder stops usually do 2 things. Limit the swing that is usable most of the time due to stalling and prevent overpowering the rudder at design speeds, and it also protects the ram from maximum throw all the time which I heard can be detrimental.

 

[78puget-trawler]

I tend to usually leave things alone which is probably what I'll do here. Adding on to the rudder some day would probably be more worthwhile.

 

[Nomad Willy]

35 degrees is a rule of thumb maximum useful rudder angle. After that you are in a stall condition.

Depends mostly on speed.
At 2 or 3 knots I could have a rudder the size of a regular man door and swing it 45 degrees. My rudder swings 45 degrees and the boats max speed is 6.4 knots. Of course it's smaller than a man door. With the CHB and more speed I'd swing the rudder about 40degrees.

My Willard has two holes in the rudder horn and I switched it to the hole closest to the rudder shaft. There is no rudder stop but of course the rudder stops when it reaches the end of the hydraulic cylinder stroke. It was a bit of an effort to mount the hyd cyl in the perfect sopt to get equal swing of the rudder both ways. The attach point fore and aft was optimized for best compromise of angles fully deployed each way. It has worked very well for years and there's never been even a hint of rudder stalling. But again 6.4 knots max operating speed.

 

[78puget-trawler]

I suppose I should clarify or ask for clarification. When we say rudder swing, are we talking hard over to hard over or just one side? When I said I had about 30 degrees, that's just one way. 40 would be better for sure. Typical I suppose CHB rudder, not too big.

 

[boatpoker]

Depends mostly on speed.
At 2 or 3 knots I could have a rudder the size of a regular man door and swing it 45 degrees. My rudder swings 45 degrees and the boats max speed is 6.4 knots. Of course it's smaller than a man door. With the CHB and more speed I'd swing the rudder about 40degrees.

My Willard has two holes in the rudder horn and I switched it to the hole closest to the rudder shaft. There is no rudder stop but of course the rudder stops when it reaches the end of the hydraulic cylinder stroke. It was a bit of an effort to mount the hyd cyl in the perfect sopt to get equal swing of the rudder both ways. The attach point fore and aft was optimized for best compromise of angles fully deployed each way. It has worked very well for years and there's never been even a hint of rudder stalling. But again 6.4 knots max operating speed.

That's why it's called "a rule of thumb"

PS. Years don't count, hours run do.

 

[boatpoker]

I suppose I should clarify or ask for clarification. When we say rudder swing, are we talking hard over to hard over or just one side? When I said I had about 30 degrees, that's just one way.

You are correct.

As mentioned 35 degrees is a "rule of thumb". There too many variable to define a specific number for general discussion. If you do go to 40 degrees, make sure you don't stress your ram by running it to the end of it's travel, you will shorten it's life.

 

[Larry M]

Our KK42 has a 40 degree swing as designed. It seems to work well but I have nothing to compare it to.

 

[psneeld]

Our KK42 has a 40 degree swing as designed. It seems to work well but I have nothing to compare it to.

For a full displacement trawler hull it makes sense.

For the semi displacement hulls run faster...it might be, might not be too much as speed increases.

Not sure exactly what the symptoms of a stalling rudder might be other than a tiny loss of effectiveness in the beginning with a bit of feedback depending on the control system.

 

[bayview]

Their might be a concern about overloading the ram when rudder is not at stops when in reverse.

 

[Nomad Willy]

If one is to change the rudder swing most importantly one should consider the rudder design as to it's strength. If one was to loose a rudder from structual failure it would probably be in big following seas. Under such conditions I have frequently used full rudder (45 degrees) to keep Willy going straight enough to suit me. Always makes me wonder what other skippers do w less. Said skippers usually say "she's a handfull" but to what degree scary or dangerous is unclear.

As to structual integrity there are no standards that I know of. I assumed my Willard was up to it from their good reputation and the fact that the inbd hole on the rudder horn was in fact there. The rudder shoe looks real stout as does the rest of it so I hooked up to the inbd hole and that gave me 45 degrees each way or 90 degrees total. The only other modification I made was to change from push pull cable to hydraulic rudder control and the conponents there are overbuilt overkill. Probably what most 40' trawlers typically have.

But the strength of the system should be considered first. Sorry I didn't mention that in post #6.
And it should be clear that 45 degrees of rudder deflection may be too much for a typical SD trawler. May not be either. The 35 degree standard is probably for planing boats so more rudder deflection on a typical trawler may work fine. It may be because most NA's may be designing mostly by what's commonly done or refering to rules of thumb. There are so many variables re each individual boat that numerical standards are probably usless. However not being a NA or an engineer I don't know. I wonder how they address this on ships?

 

[Northern Spy]

The 35 degree thumb rule is due to hydrodynamic stalling and loss of efficiency.

The reality is though that you can generally use an increase in rudder angle greater than 35 degrees on a slow boat because as the stern starts to swing the dynamic angle of the rudder relative to the water likely won't exceed 35 degrees. My rudder turns about 42 degrees.

Now if you want to talk loading and rudder size, the Nordic Boat Standard, part C7 has some great calculations to determine rudder forces and equipment .

https://www.sjofartsdir.no/PageFiles/16383/NBS-eng-Del 1.pdf

 

[sean9c]

On powerboat rudders you're not really considering them as a lifting foil, heck most of them are flat plates, so they'd likely stall at even small angles. A powerboat rudder is simply deflecting the prop wash to vector the thrust.
On hydraulic steering the concern with increasing the rudder angle is that you are losing the mechanical advantage the more you turn. When your rudder is centered the cylinder shaft is roughly 90 degrees to the tiller arm so the max mechanical advantage. As you turn the rudder this angle is decreasing so the mechanical advantage is decreasing. If you turn the rudder far enough the tiller arm would end up in line with the cylinder shaft, so no mechanical advantage.
Can you design a system to go more than 35 degrees each way, sure, but, probably not more than 45 degrees.
You aren't likely to be turning your rudder to large angles while at speed when the load is the highest. The advantage of large angles is when no or low speed maneuvering where the rudder loads are low.

 

[Nomad Willy]

sean9c,
You're showing your sailboat past or perhaps present but you make better points than most of TF.
IMO the best non-power steering system is cable and sheave w a bit of chain. No mechanical advantage to be lost w the cable quadrant. Much more lineal feel probably too.
You said "A powerboat rudder is simply deflecting the prop wash to vector the thrust". That's true of fast boats but slow boats have props that have wash that frequently only goes over only about 1/2 of the rudder. I can steer my boat quite well even at very low speeds in neutral gear. But most SD trawlers are closer to planing hulls in this regard. However many are closer to FD than others.

I think you're selling "flat plate" rudders short. Stalling in water is much different than stalling in air ... air being a compressible fluid and water not. I have thought that drilling holes in the rudder near the leading edge would function much like "slats" that project from the upper leading edge of an airplane wing to to make the transition LE to TE more gentle. This should increase the usable range of deflection by increasing the effective angle of attack .. just like an airplane in "flare" just before touchdown. Just a backyard idea.

 

[sean9c]

First, back to the OP's question, before someone mentioned rudder stall, more rudder angle will give you better low speed maneuvering you'll likely notice no other benefit. If it was me I'd pull the bolt where the cylinder connects to the rudder arm and figure out how much extra cylinder stroke I had. Then figure out how many extra degrees of rudder travel you'd get using the extra stroke. If it was only a couple degrees each way don't bother. If it was 5 degrees or more I'd cut the stops but not so much that your cylinder bottoms out or goes fully extended. You'll likely not want to exceed about 40 degrees of movement in either direction.

Mechanical steering using a quadrant is probably the most efficient system, it gives you the same power no matter the rudder angle. Which is one reason it's about the only system used on sailboats. The disadvantage is that to get power you need a big quadrant so it takes up a lot of room. It's also expensive and more work to install than a hydraulic system.
Not sure what you're saying with this SD, FD stuff but I think you got it backwards, it's much easier to stall a rudder at high speed than low. Just because your boat turns at slow speed in neutral doesn't mean your rudder isn't stalled.
I'm not selling flat plate rudders short, they're crude and crap but they're cheap and they work and that's good enough.

sean9c,
You're showing your sailboat past or perhaps present but you make better points than most of TF.
IMO the best non-power steering system is cable and sheave w a bit of chain. No mechanical advantage to be lost w the cable quadrant. Much more lineal feel probably too.
You said "A powerboat rudder is simply deflecting the prop wash to vector the thrust". That's true of fast boats but slow boats have props that have wash that frequently only goes over only about 1/2 of the rudder. I can steer my boat quite well even at very low speeds in neutral gear. But most SD trawlers are closer to planing hulls in this regard. However many are closer to FD than others.

I think you're selling "flat plate" rudders short. Stalling in water is much different than stalling in air ... air being a compressible fluid and water not. I have thought that drilling holes in the rudder near the leading edge would function much like "slats" that project from the upper leading edge of an airplane wing to to make the transition LE to TE more gentle. This should increase the usable range of deflection by increasing the effective angle of attack .. just like an airplane in "flare" just before touchdown. Just a backyard idea.