Rudder Angle and Ackerman

I've always heard the prevailing wisdom that planing hulls typically have a 70* rudder sweep (35* in either direction). And that with twin rudders you typically want some Ackerman in the steering geometry so the rudder towards the inside of a turn streers to a higher angle than the rudder on the outside of the turn.

I was doing some work on my steering today and decided to check how far my rudders actually swing. Despite my boat being a planing hull (top speed around 25 kts, fast cruise at 17-18), I have over 90 degrees of rudder swing. And oddly, it has some reverse Ackerman. The rudder to the inside of a turn gets to 45 degrees, the outside rudder gets to about 50 degrees at full travel.

This certainly explains why this boat turns so tightly, but I'm a bit confused by the reverse Ackerman. It also explains why 5 turns lock to lock doesn't feel slow.

What's the group's thoughts on such high rudder angles on a faster boat and especially the reverse Ackerman?

If you have a large rudder swing you’ll need a knowledgeable and experienced helmsman that does’nt go “hard over” at 25 knots.

I had a 45 degree rudder deflexion each way on my Willard 7 knor boat w 3 turns LtoL.
Worked beautifully.
I view the “Ackerman” is like the front wheel steering on a car in the interest of near equal rudder loading in turns.
rslifkin please correct me if I’m incorrect.

Nomad Willy said:

If you have a large rudder swing you’ll need a knowledgeable and experienced helmsman that does’nt go “hard over” at 25 knots.

I had a 45 degree rudder deflexion each way on my Willard 7 knor boat w 3 turns LtoL.
Worked beautifully.
I view the “Ackerman” is like the front wheel steering on a car in the interest of near equal rudder loading in turns.
rslifkin please correct me if I’m incorrect.

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I'm not actually sure if it's possible to get full rudder deflection at fast cruise. I think it takes enough force that you're into the hydraulic relief valves before full deflection at that speed. Either way, all the rudder you can get at 17 kts gives a very tight, steeply banked turn, but no sketchy handling.

For Ackerman, that's my understanding as well. The inboard side follows a tighter track in a turn, so it needs more angle for equal loading. But my rudders are configured opposite of that, which I've never heard of on another boat. It's definitely the factory configuration on mine though.

Does the boat hold course without helm input at wot? How about at hull speed? How does the helm feel? I would guess for your hull, 7 degrees toe in (trailing edges out, leading edges in) is a good starting point. It’s an error and trial adjustment process to find out what works best.

cafesport said:

Does the boat hold course without helm input at wot? How about at hull speed? How does the helm feel? I would guess for your hull, 7 degrees toe in (trailing edges out, leading edges in) is a good starting point. It’s an error and trial adjustment process to find out what works best.

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It tracks like it's on rails at pretty much any speed and the steering is appropriately responsive. I haven't measured the toe setting, it's at whatever the factory set it to 37 years ago (I know for sure it hasn't been touched). I'll have to see if I can get a good measurement for it (rudders are wedge shaped, so it might be hard to measure accurately due to the thickness of the trailing edges).

I don't have any complaints with the steering, I just found the amount of rudder angle and especially the reverse Ackerman to be surprising and wasn't sure if anyone had a reason it may have been built that way, as it just strikes me as sub optimal. I'm thinking of dropping the rudders and going over everything next winter, so that might be the time to assess it all in more detail.

Rob,

I'm not sure how your current rudder travel and toe in/out adjustment compares to what my former 1981 381 was set to but I will agree with you that these boats respond to helm very easily. I found with my 381 if I had any forward motion it would respond to rudder. Relative to other planing boats their size, the 381s have large rudders. The rudders on my 47' Commander appear no larger. As I understand it Chris J. Smith was responsible for dialing in the rudder setup on Chris-Craft boats during development until he retired in 1986 and he did it based on trial and error. If your setup works, I don't think I would mess with it. Another thing I recall reading years ago about toe in vs toe out beyond the Ackerman concept was that leading edge toe out could result in additional lift at speed. If you have the time, why not record the existing settings as a baseline and then see if you can adjust for better performance at speed.

thedeltaskipper said:

Rob,

I'm not sure how your current rudder travel and toe in/out adjustment compares to what my former 1981 381 was set to but I will agree with you that these boats respond to helm very easily. I found with my 381 if I had any forward motion it would respond to rudder. Relative to other planing boats their size, the 381s have large rudders. The rudders on my 47' Commander appear no larger. As I understand it Chris J. Smith was responsible for dialing in the rudder setup on Chris-Craft boats during development until he retired in 1986 and he did it based on trial and error. If your setup works, I don't think I would mess with it. Another thing I recall reading years ago about toe in vs toe out beyond the Ackerman concept was that leading edge toe out could result in additional lift at speed. If you have the time, why not record the existing settings as a baseline and then see if you can adjust for better performance at speed.

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Agreed, the 381 rudders are pretty generous for the speed of the boat. We don't run ours up on plane all that much, all I'm not sure if I'd notice a speed gain, but I'll definitely make to see what I've got and whether it seems to merit any adjustment experimentation.

Hmmm, fixing something that's not broken?

backinblue said:

Hmmm, fixing something that's not broken?

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That's the issue. I'm torn between "this has worked fine for 37 years and still works fine" and "from what I'm seeing, this has to be wrong, I just don't know why or how much it matters or if I could improve it".


I'm definitely not going to do anything beyond some more measurements right now though, as we're only 2.5 weeks out from spring launch, so I don't have enough time to mess with anything significant.

rslifkin said:

That's the issue. I'm torn between "this has worked fine for 37 years and still works fine" and "from what I'm seeing, this has to be wrong, I just don't know why or how much it matters or if I could improve it".

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Or if you could make it worse! It's a fair question and I get it, but if it's handling fine and you are not trying to correct an observed issue, there may be a better use of your time. You're welcome to work on my boat any time!

backinblue said:

Or if you could make it worse! It's a fair question and I get it, but if it's handling fine and you are not trying to correct an observed issue, there may be a better use of your time. You're welcome to work on my boat any time!

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Exactly what I'm afraid of and trying to find out. From my searching around online, I haven't found a single mention of a boat with reverse Ackerman on the rudders. So it's definitely uncommon at the least. My hope is that someone else has seen it somewhere and might have a "it might be done for this reason".



Realistically, there's only 1 steering quirk I can think of that might be related. At higher displacement speeds (6 - 8 kts) going hard over on the rudders produces a good, tight turn, but one heck of a nasty shudder in the boat. I've always assumed it's prop cavitation and disturbed water flow from the stern moving sideways too quickly, but I wonder if it could actually be a water flow issue around the rudders. This only shows up at certain speeds, it's no problem at idle, and once you're a bit above hull speed you don't get as much of a stern sliding sideways effect in a turn, so it's all good and smooth.

rslifkin said:

Exactly what I'm afraid of and trying to find out. From my searching around online, I haven't found a single mention of a boat with reverse Ackerman on the rudders. So it's definitely uncommon at the least. My hope is that someone else has seen it somewhere and might have a "it might be done for this reason".



Realistically, there's only 1 steering quirk I can think of that might be related. At higher displacement speeds (6 - 8 kts) going hard over on the rudders produces a good, tight turn, but one heck of a nasty shudder in the boat. I've always assumed it's prop cavitation and disturbed water flow from the stern moving sideways too quickly, but I wonder if it could actually be a water flow issue around the rudders. This only shows up at certain speeds, it's no problem at idle, and once you're a bit above hull speed you don't get as much of a stern sliding sideways effect in a turn, so it's all good and smooth.

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So full disclosure, I was not familiar with the term "Ackerman". Spending a little time time on ChatGPT I now understand the theory and why it's important in cars. I asked a few follow-up questions relating to dual rudder boats. This was the latest reply:


The dual rudder system used in boats is typically not designed using the Ackerman geometry like in land vehicles. While the principles of the Ackerman geometry are based on the fact that the inside and outside wheels of a vehicle need to turn at different angles during a turn, this is not necessarily the case in boats with twin rudders.

The dual rudder system in boats is designed to improve handling and control, particularly at high speeds or in rough seas, by providing better stability and control. The two rudders can work independently or together to provide more precise control over the boat's heading and direction, but the design of the system is typically not based on the Ackerman geometry used in land vehicles.

That being said, some boats with twin rudders may have steering systems that incorporate some principles of the Ackerman geometry, but this is not a common design approach for dual rudder boats. Instead, the design of the steering system in twin rudder boats is typically customized to the specific requirements of the boat and its intended use, based on factors such as the size and weight of the boat, the type of water it will operate in, and the desired level of maneuverability and control.

rslifkin said:

Exactly what I'm afraid of and trying to find out. From my searching around online, I haven't found a single mention of a boat with reverse Ackerman on the rudders. So it's definitely uncommon at the least. My hope is that someone else has seen it somewhere and might have a "it might be done for this reason".



Realistically, there's only 1 steering quirk I can think of that might be related. At higher displacement speeds (6 - 8 kts) going hard over on the rudders produces a good, tight turn, but one heck of a nasty shudder in the boat. I've always assumed it's prop cavitation and disturbed water flow from the stern moving sideways too quickly, but I wonder if it could actually be a water flow issue around the rudders. This only shows up at certain speeds, it's no problem at idle, and once you're a bit above hull speed you don't get as much of a stern sliding sideways effect in a turn, so it's all good and smooth.

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During my sea trial, my boat was put over to full port and startboard rudder at WOT. The port turn produced a noise and slight "shudder". The boat was hauled and everything verified with the running gear and as you described it was determined that it was turbulence of the water in the prop tunnel at that speed and rudder angle. I've never had an issue in normal use as I'm not normally trying to evade a torpeedo.

backinblue said:

So full disclosure, I was not familiar with the term "Ackerman". Spending a little time time on ChatGPT I now understand the theory and why it's important in cars. I asked a few follow-up questions relating to dual rudder boats. This was the latest reply:

The dual rudder system used in boats is typically not designed using the Ackerman geometry like in land vehicles. While the principles of the Ackerman geometry are based on the fact that the inside and outside wheels of a vehicle need to turn at different angles during a turn, this is not necessarily the case in boats with twin rudders.

The dual rudder system in boats is designed to improve handling and control, particularly at high speeds or in rough seas, by providing better stability and control. The two rudders can work independently or together to provide more precise control over the boat's heading and direction, but the design of the system is typically not based on the Ackerman geometry used in land vehicles.

That being said, some boats with twin rudders may have steering systems that incorporate some principles of the Ackerman geometry, but this is not a common design approach for dual rudder boats. Instead, the design of the steering system in twin rudder boats is typically customized to the specific requirements of the boat and its intended use, based on factors such as the size and weight of the boat, the type of water it will operate in, and the desired level of maneuverability and control.

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Yes, plenty of twin rudder boats keep the rudders parallel through a turn. Unless it's an extremely wide boat (like a catamaran), that's not a bad compromise. Turning the inside rudder (relative to the turn) slightly further (Ackerman) can reduce drag in low speed turns depending on how far apart the rudders are (I don't think it helps much at higher speeds where the boat isn't pivoting so tightly).

However, my boat has the exact opposite of beneficial Ackerman. The outside rudder in a turn is turning 5+ degrees more than the inside rudder when at full deflection. That's the bit I find odd, as I can't imagine any situation where that's beneficial. If it just had no Ackerman at all I'd probably be happy to call it good enough.

Thinking about it, this just gave me a thought... I need to take a close look at the tiller arms on the rudders. If they're toed out slightly relative to the actual rudder position, it's possible that they were just installed on the wrong sides originally and switching them would give a more normal Ackerman effect. That won't be a project for this year though, as the rudder installation requires dropping the rudders to remove the tiller arms. And any work on the steering gear is slow and unpleasant, as it has to be done hanging upside-down over the aft end of the fuel tanks...

rslifkin said:

That's the issue. I'm torn between "this has worked fine for 37 years and still works fine" and "from what I'm seeing, this has to be wrong, I just don't know why or how much it matters or if I could improve it".


I'm definitely not going to do anything beyond some more measurements right now though, as we're only 2.5 weeks out from spring launch, so I don't have enough time to mess with anything significant.

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Do you have specs from 37 years to compare to current rudder position?
Steering is something I am working on. Bleeding air has made the wheel more responsive, but it still wants to wander, requiring corrections. I am used to when prop rpm is balanced the boat goes straight. That is not happening, so next is rudder adjustment.
A while back TF had a thread of a tech setting rudders toe out, mistaking toe for heel. How can you be certain in 37 years a well meaning person saw they were not parallels so made them so.
IMO the shudder you feel is from prop wash and air in hydraulic lines.

SteveK said:

Do you have specs from 37 years to compare to current rudder position?
Steering is something I am working on. Bleeding air has made the wheel more responsive, but it still wants to wander, requiring corrections. I am used to when prop rpm is balanced the boat goes straight. That is not happening, so next is rudder adjustment.
A while back TF had a thread of a tech setting rudders toe out, mistaking toe for heel. How can you be certain in 37 years a well meaning person saw they were not parallels so made them so.
IMO the shudder you feel is from prop wash and air in hydraulic lines.

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I know the history of the boat pretty well, and from everything I know, the only work that's ever been done on the steering prior to my ownership was new hydraulic lines. There are no tool marks on any of the hardware around the rudders or steering cylinder save for the packing nuts on the rudder posts, so that further points to nothing being touched. And while I'll be able to measure it this afternoon, I don't think the toe setting is wrong, as the steering behavior near the center point is as good as I could ever hope for. It tracks like a car when on plane, and at slow cruise (6.5 - 7 kts), as long as the sea state is calm, I can take my hands off the helm (no autopilot) and go 3 - 5 minutes without touching the wheel and not end up more than about a degree off course.



I don't think the shudder is air related, as bleeding the system has no effect on it. And the shudder isn't felt in the wheel anyway, it's felt through the deck. So yeah, some combination of water flow over the props and rudders at that combination of speed and rudder angle is likely the answer. Simple solution is just to not use full rudder above 1000 RPM in most situations.

OK, look forward to your measurements.

I just crawled around and did some measuring. The rudders seem to be set for zero toe. If there is any, it's so little I can't accurately measure it with a tape measure. Being that the rudders are slightly wedge shaped it makes sense that they wouldn't need much toe to keep things happy.

As far as the tiller arms, they're exactly parallel, so they're not installed backwards or anything. But I did find why I'm getting reverse Ackerman. The way the joints in the tie rod and tiller arms are configured leads to a slight loss of travel on the extended side of the cylinder (inboard side of the turn). See the picture below for an example. Being a double ended cylinder I'm not sure what can be done about it other than new steering arms to give some Ackerman to offset this effect.

What the above discovery really means is that if the Ackerman issue were resolved I should have something like 50* of rudder swing either side of center.

Is the other end the same with the ram being fixed position?

Here is a shot of my setup. The crossbar from rudder tiller to rudder tiller and ram has one fixed point and the other of the cross bar. Measuring is another day.

Yes, the other end of mine is configured the same. It's a fixed mount, double ended ram that directly connects to both rudders with the same attachment method. Whatever is going on is specific to the pushing direction of movement, but it happens symmetrically on both sides.

I thought any angle over 35 degrees makes the rudder essentially a brake.

I had a misaligned rudder on a single screw downeast. When i bought it, it would turn to port fairly welll, but hard over to starboard, it was a pig. (Was going 25 degrees port and 45 degrees starboard). It made steering unpredictable. Would steer well to starboard until hard over then went dead.

The prior owner installed a bow thruster to help. Lmao.

After i made the adjustment (the ram was places in the wrong spot in the steering arm) it steered much better in both directions etc…

At planning speeds, your not going to use full over steering amyways. Its all about lower speed steering anyways

Alisske said:

I thought any angle over 35 degrees makes the rudder essentially a brake.

I had a misaligned rudder on a single screw downeast. When i bought it, it would turn to port fairly welll, but hard over to starboard, it was a pig. (Was going 25 degrees port and 45 degrees starboard). It made steering unpredictable. Would steer well to starboard until hard over then went dead.

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I think it depends a bit on the shape of the rudder (and things like whether the rudder has a keel in front of it, or a skeg) and also how the boat moves through the water. My boat has its pivot point fairly far forward, so the stern gets moving sideways fairly quickly in a tight turn. That likely increases the maximum usable rudder angle, as the angle of water flow over the rudder is reduced once the boat starts to turn. If I could swing the rudders to the stop quickly enough that the boat wasn't noticeably turning before I hit the stop, then there might be some risk of a rudder stall from excessive angle. My rudders are also spade rudders with just the props in front of them, but because it's a twin, there's no keel or anything else to impact the waterflow over the rudders.

Mine was single behind a keel. Semidisplacement round chine hull

After the adjustment, it steered better both ways and more predictably.

When i measured the actual rudder swing and saw the “lopsideded ness” to it, everything made sense.

Again, i believe a rudder i er 35 degrees becomes a brake. (Hydrodynamically)

Alisske said:

Mine was single behind a keel. Semidisplacement round chine hull

After the adjustment, it steered better both ways and more predictably.

When i measured the actual rudder swing and saw the “lopsideded ness” to it, everything made sense.

Again, i believe a rudder i er 35 degrees becomes a brake. (Hydrodynamically)

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From what I know, that 35 degree figure is pretty typical for a flat plate rudder. And any rudder is going to have a good bit of drag at higher angles. Foil shapes, etc. can have different angles before they stall or develop any bad behavior though, and whether there's prop wash across the rudder or not is a factor too. Some rudders can go beyond 35* without stalling (just some extra drag but no bad behavior or loss of steering) while others (particularly on sailboats) may stall before 35*.

The other big question becomes what the actual angle of the rudder is to the water flow once you're in the turn. The rudder may be turned 30* relative to the boat, but with the boat turning, the actual angle to the water flow can be significantly less.

Full rudder on my boat definitely slows the boat down noticeably at lower speeds, so there's certainly a lot of drag there. But no bad behavior, it just turns.

Good explanation (thx)

Yeah line was basically a flat 3/8 bronze plate. I guess nothing is that simple as one hard fast rule. (Lol)

Thx and be well