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Old 25-11-2019, 03:41   #1
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Emergency turn under engine

Assume you are motoring, both engines, 3/4 ish throttle, 7knts ish

You see, dead ahead, a log / whale / shipping container / kraken, and want to turn to avoid.

This is the titanic problem. Will the boat turn better with propwash.

What is actually more effective

Rudder Hard Over, Both Full Fwd
Rudder Hard Over, One Full Fwd, One Full Rev (Hopefully the right ones)
Rudder Hard Over, Both Full Rev
Rudder Hard Over, One Full Rev, One Neutral.
Don't Turn. Both full rev. (Titanic might not have sunk if hit head on)
Some other combination.

Now - here is the tough bit - it's not only how quickly the boat rotates heading, but how quickly the vector changes.
It will sideslip. And yes, daggerboards down would help. Assume no time for that. You are motoring with daggerboards up.

Comments?
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Old 25-11-2019, 03:51   #2
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Re: Emergency turn under engine

i think helm hard over would be a good start at 7 knots,no brainer really.
probably want to stop engines if there is netting ,ropes or rubbish in the water.
and you have not mentioned sea state? 2m waves ,10m waves ,flat calm?
gale from ahead, gale from behind?
is the auto pilot on or off?
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Old 25-11-2019, 04:16   #3
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Re: Emergency turn under engine

Quote:
Originally Posted by atoll View Post
i think helm hard over would be a good start at 7 knots,no brainer really.
probably want to stop engines if there is netting ,ropes or rubbish in the water.
and you have not mentioned sea state? 2m waves ,10m waves ,flat calm?
gale from ahead, gale from behind?
is the auto pilot on or off?
Titanic situation
Flat calm, poor viz, autopilot on
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Old 25-11-2019, 04:22   #4
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Re: Emergency turn under engine

get out of armchair,put coffee down,run to helm,turn off auto pilot,helm hard over ,stop engines.
asses situation.engage brain.........

probably don't want to be motoring at 7 knots in poor vis with the AP on in the first place
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Old 25-11-2019, 04:26   #5
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Re: Emergency turn under engine

Flat calm, poor visibility, auto pilot, 7 knots. Sounds irresponsible if 7 knots exceeds the distance required to avoid obstacles.

In THE Titanic situation the vessel was traveling too fast to turn within its field of vision. As with 70mph cars in fog leading to hundreds of cars in a pile up...

Maybe learn not to travel faster than sight allows correction time.

The best tactic is to try to avoid being in the situation.
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Old 25-11-2019, 05:21   #6
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Re: Emergency turn under engine

The big most people forget to do because it's counterintuitive is to turn into the obstacle as it comes abeam your pivot point (somewhere amidships) which will kick your stern away from it. Continuing to turn away could actually cause you to hit something you would otherwise clear.
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Old 25-11-2019, 06:21   #7
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Re: Emergency turn under engine

Iím going back to original example
You see something in the water
Best rate of turn is....
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Old 25-11-2019, 06:41   #8
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Re: Emergency turn under engine

Quote:
Originally Posted by nfbr View Post
Iím going back to original example
You see something in the water
Best rate of turn is....
epirb
don life jacket
inflate liferaft
check insurance is current
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Old 25-11-2019, 06:52   #9
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Re: Emergency turn under engine

Back to the original question. "You see an object dead ahead ...."

When you move the rudder to one side, it causes the STERN to move in the opposite direction from where you want to go. So turning "away" from the object actually moves your stern towards the object, exposing the side of the boat. Thatís apparently exactly what happened with the Titanic.

In the OP question, Iíd think that both engines in reverse would be the best "general" solution, with the various conditions of the specific incident modifying that.
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Old 25-11-2019, 06:54   #10
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Re: Emergency turn under engine

At 7kts, what you do with the engines is unlikely to have any significant effect...assuming it's questionable if you can avoid it or not. Just crank it hard over. At 7kts, you rudders should be very effective. Playing around, we could do 90 degrees in just over a boat length at cruise speed or above. Once the wheel is hard over, dropping back to neutral won't hurt to bleed off a little more speed but not likely a make or brake part of the maneuver.

One of the risks with the throttle is you panic and throw it full forward to full reverse and shred the transmission. If you throttle back, give it a couple seconds in neutral and then gradually increase power in reverse...you already hit the object. At best you slowed down a little bit.
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Old 25-11-2019, 07:24   #11
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Re: Emergency turn under engine

From a twin engine powerboat perspective, there are a few choices. They'll all work on a twin engine sailing cat as well.

If you see the object a couple boat lengths ahead and have nowhere to turn to (narrow channel, etc.), you can probably stop. I know I can stop from 7 kts in less than 2 boat lengths. Slam throttles to idle. Transmissions to reverse with a brief pause in neutral. As soon as they engage, apply a fistful of throttle. Mind you, this is on a planning hull powerboat that's powered to do a lot more than 7kts. So on a sailing cat or something, it may take WOT in reverse to stop quickly.

If you need to turn, rudder hard over at 7 kts will make for a pretty fast turn. Some extra throttle on the outboard engine will increase the rate of turn. On a sailboat with big rudders, you can reduce throttle to slow down in the turn. On a small rudder-ed boat, you can reduce throttle on the inboard side, but maintain or increase throttle on the outboard, as reducing throttle will noticeably hurt rudder effectiveness.

At low speeds, the tightest turn will be rudder hard over, outboard engine forward, inboard in reverse with throttles adjusted as needed to maintain either slight forward movement or an in-place spin.
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Old 25-11-2019, 07:36   #12
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Re: Emergency turn under engine

Just turn fast (and then counter steer when abeam). I've dodged a lot of crab pots as well as bigger objects with a cruising cat for many years. At 7 knots, a small dodge will happen faster than you can do anything with the engines. What Valhalla said.



At speed, reversing engines will just make a shallow draft cat crab. Low speed marina maneuvering is a separate conversation.



If the object is really big, consider full reverse. Cats are light and stop a lot faster than monos.
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Old 25-11-2019, 07:52   #13
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Re: Emergency turn under engine

Quote:
Originally Posted by thinwater View Post
...

If the object is really big, consider full reverse. Cats are light and stop a lot faster than monos.
Then gentle moor up to the container and collect your winnings
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Old 25-11-2019, 08:06   #14
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Re: Emergency turn under engine

I think the answer is “it depends”... on the characteristics of your boat (among other things).
So the real answer is spend enough time practicing a full range of manoeuvres in risk free circumstances that you KNOW exactly what will work and what won’t for your boat.
Pilots regularly practice a wide range of situations in the specific plane they fly. Simulators are a god send, but if you don’t have one then practice in low risk situations. You don’t practice stall or spin recover at 1,000 ft you do it at 6,000+
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Old 25-11-2019, 09:29   #15
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Re: Emergency turn under engine

Rudders hard over , cut the inboard engine , once turned throttle up again to drive forward and hopefully avoid object
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