Steering with twin engines: Differential Throttle Control

H

hardee

New member
As some of you know, I am a firm proponent of twin engines. One of the reasons is because I thoroughly enjoy the pleasures of low speed, intricate directional control, in confined spaces.

On my summer trip on Vancouver Island, I had some steering fluid leak and as a result have been researching some SeaStar info here on the CBRATs site. One of the threads I ran across had some comments about steering, loss of, and the possible results.

The thread is here:
http://www.c-brats.com/viewtopic.php?t= ... ks&start=0

And one of the comments, speaking of hydraulic steering system on twin OB’s “So if it does fail entirely, I would be screwed.”

On that trip, I had the opportunity to watch dozens of twin engine out board vessels, in lots of different situations, and rarely saw where the operator used the twin engines in other than a single engine steering mode, turning the wheel as both engines rotate accordingly. On one occasion, I watched a skipper struggle for several minutes, taking 5 or 6 trys to get into a berth. On each attempt there was at least one exchange of gel coat and more than a little blue air. In another instance, there was a telltale long dark streak on the side of a big white sail boat.

Early on in my experience with my twin engine C-Dory, I had a chance to hear from a couple who just returned to Everett, I believe it was, from Friday Harbor, with a twin engine C-Dory 22 who had suffered a loss of their steering system. It was not a big concern for them, and he introduced me to the concept of differential throttle control. It was shortly after that, I was told to take my hands off the wheel when I was having difficulty getting into a slip at John Wayne Marina. That gentleman came over and explained how it works in detail. It was “life changing”, and has become a standard practice once I’m inside the breakwater at the marina – any marina.

It is a simple process, based on simple physics. If one side goes forward and the other side goes in reverse, the boat will pivot. AND that pivot will be around a point very close to the point half way between the engines, and probably slightly forward of that. The point will vary depending on the relative RPM of each engine. If they are both in forward but at different speeds, the boat will turn in the direction of the slower engine.

It is worth trying, practicing and perfecting. Be sure your OBs are lined up midline or centered. That will make life much easier.

Have you tried it? Do you use it?

Harvey
SleepyC:moon

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Yup Roger, You are right. I even had someone tell me it would not work on a 22 because the engines were too close together. Guess my boat doesn't know that.

Harvey
SleepyC :moon

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OK, this is one of the advantages (steering control) of twin engines
over a single engine vessel. As with most things, there are limitations.

Some are

* Wider is better for propeller separation (cats > monohulls)

* In higher wind or current situations it still can be difficult to control
your bow since the center of rotation is so far aft

* It is most effective when the props (OB & I/O) are centered dead ahead
at low speeds. The more off center the twins are, especially if hydraulic
steering failure occurs then, and at higher speeds the less differential
throttle steering works.

* Twin inboards with the propeller shafts fixed straight ahead works best

Aye.
 
I've messed around and practiced it, but to be honest most of the time I don't bother. I know your probably thinking I don't know what I'm missing. That could be true, but I really don't see a need to fix what's not broken. Maybe its just stubbornness or a resistance to learning new things!

Over the years there's been a couple of times getting back into a slip with very high winds and I have to retry, but its doubtful that messing around with the controls would have helped me.

Growing up with inboard ski boats with a rudder, anything is better than that at low speed. If anything the twins handle better at low speed at least on the 22, compared to any single outboard boat I've ever owned or driven. I think that may be because of the flat bottom and how easily its drawn from side to side. I've never driven a single C-dory.

I might try it again this weekend if we decide to go out :wink:
 
Paul, just get out, away from everything and start with both engines at idle and one forward and one astern and watch what happens. (Steering straight ahead). Then after a few minutes switch both. You'll find that the boat will react differently because both props turn the same direction and tend to "walk" the stern. Then, do the same thing but CHANGE THE THROTTLE on one engine, bet it will surprise you. Then run through the same drill with the wheel hard over, first one way, then the other. The hard thing is remembering what will happen with each action.

I've always had twins except on a CD 16, my current CD22, an FFG (1 screw, 350' long), and a CV (4 screws, 1080' long).

Just always remember never to approach a stationary object (or one that's moving for that matter) any faster than you would like to hit it :roll:

Have fun and let us know how it goes! :smile

Charlie
 
Alright you guys talked me into it. If I'm out this weekend Ill most likely get bored of slow fishing (like normal) and give it another try. Ill let you guys know what happens
 
Counter rotating props also help. There is more "prop walk" with the larger wheels, but even with outboards you can use it to your advantage.
With inboards I was able to "walk" a boat sideways, although it can be done with the Tom Cat, it is more difficult than larger monohulls.

Some of the new control systems, use outboards which are independently steered as well as throttled. An example of this is the Mercury "Zeus" system.
 
I do this regularly (most of my boats at work are twins, but I prefer a single and a kicker on my CD, precisely because of the redundant steering the kicker allows).

On my twin engine boats I have had hydraulic steering failures that have caused both engines to remain fixed and not turn (they stopped while turning slightly to starboard, if you notice this happening to a boat with twins I suggest you use any last bit of ability to steer to center your engines!). I ended up running the boat back home with differential throttling, and up onto the trailer. No problem.

I have also had my steering fail such that the engines swung freely. In this case, if you can't lock the engines down straight, differential throttle doesn't work well. In this case a kicker with a tiller is a better backup.

I like using counterrotating props for docking so I can walk the stern left (starboard motor as you shift into reverse) or right (port motor as you shift into reverse). It can be very helpful.

An added, and often unrecognized benefit is the ability to turn your boat on a midship pivot point. In forward your boat pivots from near the stern, in reverse it pivots closer to the bow, and with a split throttle (fw and rev) it pivots from the middle. On larger boats this makes a bigger difference, but is still a factor on a 22.
 
From my experience (almost 10 years on a 2 with twin 40's) the pivot point when both engines are set centered, and both at idle with one forward and one in reverse, (and it makes no difference which one is which) the pivot point is about 3 feet forward of the props. This will vary slightly depending on side wind or current.

The easiest way for consistent docking for a side tie, like a fuel dock, is, in _ easy steps.

1. Approach dock in an arc, from pointing at the dock at a 45 degree angle, turning to be facing away at a 45.
2. Stop in that point, (distance does not really matter).
3. Both engines into reverse at idle.
4. Back slowly towards the dock.
5. When the stern corner is about 2 ft from the dock, put the opposite side engine in forward, with just a slight bump in RPM, Hold that RPM until the bow begins to swing towards the dock.
6. Hold that RPM until the boat is parallel to the dock.
7. Both engines back to neutral. Tie the lines.

If you have crew aboard, at Step 5, a stern line onto a dock cleat will initiate the securing of the vessel. If there is a cross wind or current pushing you off the dock, at this point Secure that stern line and then the engine opposite the dock side in forward will swing and hold your boat against the dock while a crew member secures a forward line.

Hope this helps.

Harvey
SleepyC :moon

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hardee":2von8wm2 said:
From my experience (almost 10 years on a 2 with twin 40's) the pivot point when both engines are set centered, and both at idle with one forward and one in reverse, (and it makes no difference which one is which) the pivot point is about 3 feet forward of the props. This will vary slightly depending on side wind or current.

The easiest way for consistent docking for a side tie, like a fuel dock, is, in _ easy steps.

1. Approach dock in an arc, from pointing at the dock at a 45 degree angle, turning to be facing away at a 45.
2. Stop in that point, (distance does not really matter).
3. Both engines into reverse at idle.
4. Back slowly towards the dock.
5. When the stern corner is about 2 ft from the dock, put the opposite side engine in forward, with just a slight bump in RPM, Hold that RPM until the bow begins to swing towards the dock.
6. Hold that RPM until the boat is parallel to the dock.
7. Both engines back to neutral. Tie the lines.

If you have crew aboard, at Step 5, a stern line onto a dock cleat will initiate the securing of the vessel. If there is a cross wind or current pushing you off the dock, at this point Secure that stern line and then the engine opposite the dock side in forward will swing and hold your boat against the dock while a crew member secures a forward line.

Hope this helps.

Harvey
SleepyC :moon

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Click to expand...

This is gospel when a stiff crosswind is blowing you away from the dock
when you desire a beam-to tie up. Great job!

Aye.
 
Thanks Foggy. For me it seems so easy I don't understand not doing it, but there are those.

For clarification in my use, Pivot is the point where the boat rotates around, and Swing is the movement that happens at the bow. The direction depends on which engine is in Forward and which is in Reverse.

Harvey
SleepyC :moon

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beermanPDX":2oti3wq0 said:
Kushtaka":2oti3wq0 said:
In forward your boat pivots from near the stern, in reverse it pivots closer to the bow
Click to expand...

Isn't it the other way around? Unless by "pivots from" you mean the end that swings more.
Click to expand...

Yup. Backwards. Sorry. I shouldn't have used the word pivot. In my defense, I do have a fever. The actual axis is usually about 2/3 of the way forward, with the stern swinging in forward, and about 1/4-1/2 of the way forward in reverse, Most larger boats will pivot from the center, or can if you adjust the throttles a bit as you rotate.

Dr. Bob is right on as usual. The coolest thing out there these days are the computerized helms that turn each outdrive independently. I've played with a Volvo Penta IPS system. If there were ever anything to make me start using i/o engines and drives, it's this. Good thing I can't afford it!!!
 
Having now operated our CD 22 with twins since spring 2003, I agree that twins do help maneuvering in tight places with moving the controls much like you would do when operating any tracked vehicle on land. For a long time I made it harder on my self wth not knowing the motors should be centered. In the last couple years, I have spent quite a few hours practicing doing this at remote docks & while underway. While it works well for me in very calm wind & current & can be a help at times when not, the steering control using only the prop controls with the close set twins & non counter rotating props on the CD 22 can be overcome by wind & current, that is not very strong or at least with me at the controls it can. I'm hoping to be proved wrong in this by Harvey or someone else by watching them sometime bring their CD 22 into the dock using only the controls with a strong wind blowing directly off it. In this kind of condition I've only been able to do this maneuver by using the helm & making the approach to the dock at a speed higher than I would liked to have hit it, at about a 45 degree angle & just before bow contact go into reverse while using the helm & momentum to bring the stern toward the dock. If done well the boat will ease up smoothly to the dock & square with it & you have a very short time to get from the boat to the dock with both lines in hand to secure before a strong wind or current takes the boat back away. Of course if not done well some new marks will be put on the boat or the distance from the dock to boat gunnel will be to far to get off & secure it & another try must be made or a better place to dock or anchor found. I have watched boats with wider spaced motors & counter rotating props such as the Glacier Bay cats at Elfin Cove, Alaska do amazing to me maneuvers when docking.

Bottom line is, I believe it best for a CD22 owner/operator with twins to be proficient in using both the helm & motor controls for steering.

Jay
 
Jay, you are right that one should be proficient in both processes. I have practiced the wheel rotation method and can do it, but the other is so much easier for me.

Your mention of wind and current making it difficult or impossible brings up a good question. How much throttle is safe to apply in the differential system, when, with a 22, the OB's are pretty close together. With my 40's, which are not counter rotating, I have limited it to about 15-1700 and that for only a very short burst. Most of the time idle speed or just above, maybe 1000 is usually plenty adequate.

Harvey
SleepyC :moon

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Does anyone know the technique called "walking" The vessel? It is a simple technique used mostly with conventional twin screw tugboat. As captain of a tugboat, I use this technique whenever I am in a tight spot. Walking the vessel refers to moving the vessel perpendicular from its fore and aft orientation with little or no forward or aft movement. It is done by using a combination of ahead and astern propulsion with hard rudder. You could use this technique to get into some really tight spots or between two other vessels. All you need is just enough room for your boat. Here is a quick step by step:

Port side landing:

1) approach the dock at a slow speed (about two knots) and about a 30 degree angle.
2) when close to perpendicular to your desired position, turn your wheel hard right and simultaneously go slightly ahead on your port throttle and astern on your starboard throttle. You will have to go a little harder astern to get the vessel to stop its forward motion.
3) once your bow starts swinging to the starboard, shift the position of the throttles. Ahead on the starboard throttle and astern on the port throttle. Keep the wheel hard starboard. You will now start to move perpendicular toward the dock.
4)adjust your fore and aft motion by using the throttles. If you are moving foward slow your starboard (ahead) and increase your port (astern). If your are creeping astern, slow your port (astern) and increase your starboard (ahead). You will have to practice this until you can find the right balance. Wind and sea condition will be a factor as well.
5) when you are a few feet from the dock, put your throttles in neutral and watch your friends faces as you drift slowly and perpendicular to the dock for a perfect landing in an impossibly tight spot.

For a starboard side landing, everything is opposite. Hope you get a chance to try it. It will take some practice, but I'm sure it will work just as well on a small scale as it does on my tug boat. Let me k ow how it goes.
 
Reading others steering techniques with twin engines makes
quasi boring reading and doesn't replace actual practice in
your vessel.

Not knocking well meaning contributors to this topic (myself
included), but it think it is well established actual in the water
practice trumps academia every time.

So, get out there and do it in every condition you can find.

I have a friend who never sailed until he bought a sailboat and
wanted to sail to Europe. He bought the boat and practiced
by repeatedly going out on the Great Lakes when most others
wouldn't or were coming in to port due to 'bad weather'. He
gained the confidence he needed and ended up doing 2 Great
Circles to Europe and back with his wife and 2 young daughters.

As we know, "We are the sum of our training."

Aye.
 
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