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Old 08-31-2015, 03:21 PM   #1
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Twin Engines - Rudders Required?

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?)
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Old 08-31-2015, 03:25 PM   #2
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Steering by engines would pretty much negate the ability to use a synchronizer and would make for rough going. I think.
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Old 08-31-2015, 03:28 PM   #3
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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.
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Old 08-31-2015, 03:49 PM   #4
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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.
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Old 08-31-2015, 04:09 PM   #5
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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.
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Old 08-31-2015, 04:21 PM   #6
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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.
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Old 08-31-2015, 04:27 PM   #7
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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.
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Old 08-31-2015, 04:28 PM   #8
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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! :-)
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Old 08-31-2015, 04:34 PM   #9
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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.


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Old 08-31-2015, 04:37 PM   #10
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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?
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Old 08-31-2015, 04:40 PM   #11
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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.
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Old 08-31-2015, 04:44 PM   #12
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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....
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Old 08-31-2015, 04:46 PM   #13
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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.
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Old 08-31-2015, 05:23 PM   #14
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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
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Old 08-31-2015, 05:28 PM   #15
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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.


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Old 08-31-2015, 05:34 PM   #16
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Think of following seas and possible broaching.

Rudders are essential.
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Old 08-31-2015, 06:09 PM   #17
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Quote:
Originally Posted by Cottontop View Post
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.
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Old 08-31-2015, 06:29 PM   #18
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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.
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Old 08-31-2015, 06:45 PM   #19
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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.
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Old 08-31-2015, 07:00 PM   #20
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Quote:
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.
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