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Steering with twin engines: Differential Throttle Control
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hardee



Joined: 30 Oct 2006
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PostPosted: Thu Oct 15, 2015 1:28 am    Post subject: Steering with twin engines: Differential Throttle Control Reply with quote

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=18297&postdays=0&postorder=asc&highlight=seastar+seals+leaks&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
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Dreamer



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PostPosted: Thu Oct 15, 2015 1:36 am    Post subject: Reply with quote

All the time Harvey! And, it's even easier when the engines are 4-5 feet apart and counter-rotating Laughing
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hardee



Joined: 30 Oct 2006
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PostPosted: Thu Oct 15, 2015 1:50 am    Post subject: Reply with quote

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
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hardee



Joined: 30 Oct 2006
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PostPosted: Thu Oct 15, 2015 2:25 am    Post subject: Reply with quote

Here is a link to an article with some active demonstration on how relative differential throttle control affects a boat. This is from "Boating How To"

http://www.boatinghowto.com/content/maneuvering-twin-engine-boat-part-1-principles-194/

Harvey
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rogerbum



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PostPosted: Thu Oct 15, 2015 5:57 am    Post subject: Reply with quote

I use this technique quite often and practice driving and docking with no hands on the wheel in case the steering fails.
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Foggy



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PostPosted: Thu Oct 15, 2015 7:59 am    Post subject: Reply with quote

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.

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PaulNBriannaLynn



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PostPosted: Thu Oct 15, 2015 11:41 am    Post subject: Reply with quote

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
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Captains Cat



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PostPosted: Thu Oct 15, 2015 12:03 pm    Post subject: Reply with quote

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 Rolling Eyes

Have fun and let us know how it goes! Smile

Charlie

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PaulNBriannaLynn



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PostPosted: Thu Oct 15, 2015 12:07 pm    Post subject: Reply with quote

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
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thataway



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PostPosted: Thu Oct 15, 2015 12:24 pm    Post subject: Reply with quote

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.

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Kushtaka



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PostPosted: Thu Oct 15, 2015 1:59 pm    Post subject: Reply with quote

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.
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hardee



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PostPosted: Thu Oct 15, 2015 3:41 pm    Post subject: Reply with quote

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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beermanPDX



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PostPosted: Thu Oct 15, 2015 4:21 pm    Post subject: Reply with quote

Kushtaka wrote:
In forward your boat pivots from near the stern, in reverse it pivots closer to the bow


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

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Foggy



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PostPosted: Thu Oct 15, 2015 4:23 pm    Post subject: Reply with quote

hardee wrote:
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



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.
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hardee



Joined: 30 Oct 2006
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Photos: SleepyC
PostPosted: Thu Oct 15, 2015 4:41 pm    Post subject: Reply with quote

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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