Twin Engines - Rudders Required?

[Cottontop]

If I can dock using throttle differential, why can't I steer using throttle differential?

Advantages of rudders:
Steer the boat

Disadvantages of rudders:
Heavy
Expensive
Drag
Potential big hole in boat
Linkages, etc. clutter up spaces
Maintenance
Fouling
Hard working AP

Let's stipulate two things:
Both engines are used all the time
If an engine goes down, there will be a way to steer (get home rudder?)

 

[hmason]

Steering by engines would pretty much negate the ability to use a synchronizer and would make for rough going. I think.

 

[GFC]

I honestly have never heard anyone pose that question before, but suppose you're trying to do that and crab across a current. I doubt you would ever reach your destination, or if you did you'd be so worn out from constantly adjusting the throttles that you'd be exhausted by the time you got there. Especially if you were running in any kind of heavy sea.


If you try to run using throttle differential to steer the boat, at least one big thing will happen. Engines running at different rpm's will cause a vibration. You can hear it (it sets up a harmonic imbalance that will make a pulsing sound) and feel it in the boat.


Over time that vibration that comes from the harmonic will shake everything loose and cause premature wear in many areas.


There's a reason why boats/ships have had rudders since boats were first invented. They work and they are necessary.

 

[Baker]

I honestly have never heard anyone pose that question before, but suppose you're trying to do that and crab across a current. I doubt you would ever reach your destination, or if you did you'd be so worn out from constantly adjusting the throttles that you'd be exhausted by the time you got there.

All good points except this one. You do not have constant pressure on the wheel when you are "crabbing across the current". You set up your course/heading and you steer it...everything in equilibrium. Same would apply if you were steering with throttles. Once you set your heading to steer then everything would be in equilibrium. The boat has no clue if there is a current. It is all just part of the "fluidmass"....like an airplane is part of an airmass. You steer a heading to counter the effects of the current to achieve your course.

 

[Ski in NC]

You can steer a boat with just power differential. I've tried it on a couple boats, it works. But under certain conditions, the rudders give a quicker response, and in tough conditions that can be very helpful.

Try it.

 

[SCOTTEDAVIS]

I hope you and I don't meet in a narrow curving channel some night with you needing to make a quick course change.

Bad idea.

Want to get rid of your rudders, get pods.

 

[Capt.Bill11]

You can steer with just the engines for miles and miles, and hours and hours. I've done it.

I don't recommend it on a regular basis. For a lot of reasons. Like, it can be tiresome, your autopilot won't work very well, you end up running the engines out of sync much of the time, etc.

 

[jwnall]

I think that you deserve some sort of an award for asking a boat-related question which has never been asked on the Forum before!

 

[SCOTTEDAVIS]

You can steer with just the engines for miles and miles, and hours and hours. I've done it.

I don't recommend it on a regular basis. For a lot of reasons. Like, it can be tiresome, your autopilot won't work very well, you end up running the engines out of sync much of the time, etc.

You can also steer by shifting weight, perhaps a couple of water tanks and some large hoses and a large two way pump....hmmm.

 

[sunchaser]

Having been on stern, outboard, Arneson, and jet drive twins I can only add that the question was asked and answered well back into the last century - control the direction of trust and rudders are not necessary.

Pods anyone?

 

[Capt.Bill11]

I hope you and I don't meet in a narrow curving channel some night with you needing to make a quick course change.

Not a problem. You can turn just as fast or faster with just the engines. If fact the quickest course changes can come when using both engines and rudders.

 

[psneeld]

Not a problem. You can turn just as fast or faster with just the engines. If fact the quickest course changes can come when using both engines and rudders.

...plus to the OP...most of those things listed as negatives like weight, holes, cost....etc...are really not a big issue in the overallscheme of a boat....

 

[kthoennes]

Coincidentally, just last weekend I was trying to return to the slip on a very windy day. I always had a rule or practice that once I'm in the yacht basin or a marina fairway, you maneuver by shifters and throttles only, not the wheel (rudders) because ideally you're going too slow for it to have much effect anyway. That's just the way I was taught when we got our first big boat and I never varied. Well, because the wind was shoving me sideways so hard that the dead slow range of the throttle arc was too wimpy, but the higher rpm part of the arc pushed too hard and made the boat go too fast, just for kicks I tried a rudder/throttle combo. Worked like a charm and I was able to come in even on a windy way without whacking the side posts at all. (I do have combo shifter/throttles though, two sticks in other words instead of four.) I don't expect to make a habit of it when it's dead calm, but I might moderate my rule about never touching the wheel inside the breakwater.

 

[rardoin]

I might moderate my rule about never touching the wheel inside the breakwater.

I suggest that you do.

"Hard and fast" rules are rarely just that...there are usually exceptions. You proved that for yourself. Now, try using ALL the control elements of your boat in even calm conditions and you may 'poo-poo' that rule

 

[cafesport]

If you've ever have to cross a current such as the gulfstream on a twin engine boat with a broken damper plate your gonna be wanting that boat to have rudders. The rudders are there because there are times when they are needed they are not like an appendix. Try docking on one engine without a rudder as well, then decide for yourself.


Via iPhone.

 

[Nomad Willy]

Think of following seas and possible broaching.

Rudders are essential.

 

[Giggitoni]

If I can dock using throttle differential, why can't I steer using throttle differential?

Docking can be more efficient if you use the rudder. You'll discover that to be true over time and in different docking conditions.

 

[psneeld]

I have found splitting the plants more effective the rudders in a broaching situation....

The whole reason you are broaching is the loss of rudder effectiveness at some point.

Being able to reverse and twist to act like dragging a drogue if you aren't already.

 

[Marin]

I have observed that most people who maneuver a twin using thrust alone do so because they are afraid of using the rudders. It's another variable to deal with and they feel--- possibly rightly so in some cases--- that they will become confused, won't be able to deal with it, and will "lose control." So they stick with the simplistic method they were first taught or read about.

That's fine, it's human nature to be afraid of the unknown, but it puts that boater at a huge disadvantage and can very well lead them into getting into trouble at some point with their boat while maneuvering.

The rudders provide a major advantage in maneuvering a twin, calm conditions or not. Fortunately we were "taught" to maneuver using rudders as well as thrust at the outset of our running a twin so it became second nature almost from day one. I cannot conceive of wanting to do it any other way unless the rudders were disabled in some way, in which case it's good to know the boat can be maneuvered using thrust alone, albeit it not as quickly. But we use the rudders in almost every maneuver we do.

What we were a little later getting to the party with is using differential power during maneuvering. It took the most experienced boat handler I've ever met to teach us that that one as a result of watching him and his wife maneuvering their big LCM-6 landing craft (twin Detroit 6-71s), but once we got used to using differential power that's another tool we frequently use now.

You have to be mindful of what you're doing with differential power so you don't rev up one or the other engine up to move the boat where you want it and then shift without bringing that engine back to idle and letting the prop shaft stop spinning. If you start shifting too soon it can be very hard on the transmission.

Not long after acquiring our boat we lifted the engine room access hatch in the main cabin sole and timed how long it takes each prop shaft to stop spinning when the transmission is shifted into neutral at idle rpm. So we know how long to pause in neutral before shifting into into gear again.

 

[shufti]

timed how long it takes each prop shaft to stop spinning

Interesting - and a bloody good tip. I'm very mindful of letting the prop/shaft spin down - but must admit to not knowing exactly how long that takes. Out of interest, what time did you get (yes, I realise this number will vary boat-tboat and engine-engine)?


Oh, and to answer the OP question - I had a steering linkage break earlier this year and was forced to use differential throttle to steer [at displacement speed and luckily in relatively calm water] until we made it to the anchorage (~3.5hrs at end of a 8hr trip). Was never a problem, but it was a royal pain and a very tiring couple of hours. Not only the constant changes in throttle thanks to gusty side-winds - but also the constant feeling (not to mention that horrible noise) i should be synching the motors kept me somewhat 'on edge' for the entire ordeal. I was surprised at how tired I was when we finally got to anchor -mentally and physically.
Rudders are a WAAAY more relaxing way of doing things. You don't even notice you're making small corrections after awhile with a wheel - but you have to think through every correction when using throttles.

 

[Max Simmons]

Years ago, the chain gear for turning the rudders on our old Trojan broke while cruising in the San Juans, 5 hours from our home port. Dad cruised all the way back turning with just the throttles. Not ideal, but worked in a pinch.

 

[Marin]

Psneeld-- On our boat, assuming the boat is operating at idle and is moving through the water at idle speed, not decelerating from a higher speed, it takes about 3 seconds for the shafts to stop. A major factor in this is the cutless bearings--- how tight or loose they happen to be. Ours are pretty tight.

However, most of our shifting is done with the boat not moving through the water much at all. So the shafts stop a wee bit sooner than they do with the boat established at idle speed which for us is just under 3 knots indicated.

So we allow at least 3 seconds in neutral when going from one gear to the other.

The reality is, however, that we almost never find ourselves having to shift all that quickly from one gear to the other. So the transmissions generally remain in neutral for a lot longer than 3 seconds between a shift out of gear and the next shift into it. In fact during a docking or whatever the shifters are in neutral probably 90 percent of the time and we just "nudge" it with thrust and rudder as necessary and let inertia do most of the work.

 

[Cottontop]

I appreciate the many thoughtful comments.

Some responses:

I am thinking about a power catamaran. The most I'll need is perhaps 125 hp per hull.

I started out looking at pods, but can't find any that small. I don't want a science project trying to cobble together something.

Steering will not involve shifting; just small amounts of differential will accomplish most turns. I don't really turn very much until I get where I'm going. Go straight for a while, maybe make a little correction, then straight again.

The AP will have nothing to do but adjust throttle; nowadays this is an electronic function. I appreciate that programming here might be a science project. Maybe no AP is needed.

I think getting rid of all the disadvantages adds up pretty big. There are lots of threads about steering issues and the occasional thread about rudder disasters. I can say that I am frequently being told, "That's where the rudder post has to go, that's where the AP mounts, that's the space for the cross bar, " etc.

And this way, I never have to sort out the spade rudder debate.

 

[Ski in NC]

Marin- It's not a big deal to shift before shafts stop windmilling. The clutch plates engage from relative motion, and some shaft rotation just means more relative motion. It's oil that gets sheared to engage the clutch, no significant wear.

Still it's good to let the shaft stop, but it is not a big deal. No different than shifting with idle 100rpm higher.

On my boat I can stall the engine if I jerk it straght from fwd to rev. That's why I pause!!!

 

[GFC]

Disadvantages of rudders:
Heavy
Expensive
Drag
Potential big hole in boat
Linkages, etc. clutter up spaces
Maintenance
Fouling
Hard working AP

Let's stipulate two things:
Both engines are used all the time
If an engine goes down, there will be a way to steer (get home rudder?)

As I reread through the OP's first post and, considering each of his listed disadvantages both separately and then as a whole, I fine I'm questioning several things:
Heavy--really? The bigger the boat the bigger the rudder but, comparatively as a percentage of the overall weight of the boat, the weight of the rudders is tiny.
Expensive--same thing here. Really? When compared to the cost of a boat?
Drag--similar thing here. We're on a trawler forum where they move slowly through the water and we're even considering the drag caused by rudders? Really? It's that drag through the water that makes them work.
Big hole in the boat--yup. I'll give you that one. Probably two big holes.
Linkages--aren't those usually tucked way back in some inaccessible space in the stern? What else would you use that space for, 'cause on most boats you can't really get to it easily.
Maintenance--yup. Just one more thing to check on and lube when doing routine maintenance.
Fouling--if whatever is going to foul on the rudders makes it past the prop(s) I guess it might foul on the rudders, but all it's going to do is drag through the water and possibly make the rudders a little harder to turn.
Hard Working AP--maybe, but how are you going to use an AP if you don't have rudders. Is your AP going to adjust your throttles to keep you on course. If you didn't have rudders you probably wouldn't have an AP.

I recently had one set of hydraulic shifters go out and that prop (starboard side) was stuck in reverse. Because I do have rudders on the boat I was able to go into a lock and get tied up, then exit the lock and make it about 15 miles back to the marina. Without rudders I'd have been stuck with one engine and no way to use differential anything to get anywhere.

Thanks OP, but I'll keep my rudders.

 

[Ski in NC]

I appreciate the many thoughtful comments.

Some responses:

I am thinking about a power catamaran. The most I'll need is perhaps 125 hp per hull.

I started out looking at pods, but can't find any that small. I don't want a science project trying to cobble together something.

Steering will not involve shifting; just small amounts of differential will accomplish most turns. I don't really turn very much until I get where I'm going. Go straight for a while, maybe make a little correction, then straight again.

The AP will have nothing to do but adjust throttle; nowadays this is an electronic function. I appreciate that programming here might be a science project. Maybe no AP is needed.

I think getting rid of all the disadvantages adds up pretty big. There are lots of threads about steering issues and the occasional thread about rudder disasters. I can say that I am frequently being told, "That's where the rudder post has to go, that's where the AP mounts, that's the space for the cross bar, " etc.

And this way, I never have to sort out the spade rudder debate.

If you build this boat, leave provisions available to add rudders later if needed. You might not like the way it runs without them.

 

[psneeld]

Marin- It's not a big deal to shift before shafts stop windmilling. The clutch plates engage from relative motion, and some shaft rotation just means more relative motion. It's oil that gets sheared to engage the clutch, no significant wear.

Still it's good to let the shaft stop, but it is not a big deal. No different than shifting with idle 100rpm higher.

On my boat I can stall the engine if I jerk it straght from fwd to rev. That's why I pause!!!

Especially with the BWers...on the assistance tow boat my tranny sees a workout in one day that the trawler guys give them in a lifetime.

I am only on the second tranny in 13 seasons with no end in sight on this one.

 

[Marin]

Especially with the BWers...on the assistance tow boat my tranny sees a workout in one day that the trawler guys give them in a lifetime.

Like most everything else to do with boating, it all depends on which "expert" one believes has the most credibility.

Transmissions are like all mechanical things--- the easier you make their lives the longer their lives tend to be.

 

[Ski in NC]

Like most everything else to do with boating, it all depends on which "expert" one believes has the most credibility.

Transmissions are like all mechanical things--- the easier you make their lives the longer their lives tend to be.

So which "expert" is telling you that you will damage your gear by shifting with shaft still rotating?

 

[Capt.Bill11]

Think of following seas and possible broaching.

Rudders are essential.

At least until the following seas over run the rudders and then the only way to save the boat from broaching is with the engines.