Challenge: Captain's License Questions

[cwyckham]

Even if the rudder isn't behind the props (twin prop, single rudder in between them), just getting that big old board turned so that it presents less lateral resistance should make it easier to spin the boat.

[nigel1]

Quite a few of the US tug boats were designed this way, need inward turning props, with no nozzles around the prop.
Driven a couple of Jay Ray McDermot boats, handled very nicely

[wsulasgeir]

If you turn the vessel using one engine ahead and one engine astern, rudders amidships , the boat will describe a circle around a point between the propellers, the equivalent diameter of two boat lengths.
If you use helm to complete this manoeuvre the vessel will turn `short around`and describe a circle slightly over its own length.
Beware of kinetic energy whilst attempting these, and all close manoeuvres. Close quarters handling is a bit like roller skating on ice.

[Lodesman]

Originally Posted by wsulasgeir

If you turn the vessel using one engine ahead and one engine astern, rudders amidships , the boat will describe a circle around a point between the propellers, the equivalent diameter of two boat lengths.

The pivot point isn't between the props - it's about a third of the boat's length aft of the bow.

[FSMike]

Originally Posted by Lodesman

The pivot point isn't between the props - it's about a third of the boat's length aft of the bow.

OK - How do you figure that? I've spent the past ten years +/- on a 45' sailing catamaran with twin Yanmars and I can pretty well guarantee you that when I spun that boat around the pivot point wasn't anywhere near where you indicate.

[wsulasgeir]

I agree with you ,if you have helm on the vessel,but as you reduce the helm the pivot point moves aft to between the propellers.

[barnakiel]

Originally Posted by GordMay

If there's no way on, the rudder has no effect.
(...)

Sure?

b.

[Lodesman]

Originally Posted by FSMike

OK - How do you figure that? I've spent the past ten years +/- on a 45' sailing catamaran with twin Yanmars and I can pretty well guarantee you that when I spun that boat around the pivot point wasn't anywhere near where you indicate.

A catamaran is a very different beast than a monohull. Regardless I can't imagine the pivot point is that different when the cat is turning at rest - probably on the centreline, somewhere between 1/3 and 1/2 LWL back from the bow. The pivot point (or the axis of yaw) is not actually fixed. When making headway the pivot point moves aft. But in the context of this discussion, we're talking about turning at rest. Where do you think the pivot point in your cat is?

[Captain Bill]

I've got to admit that I pivot my cat using the rudder amidships method. I have feathering props that reverse pitch when put into reverse and I don't have much trouble with asymetrical thrust issues. I have not actually done a test of throwing the helm hard over to see if that speeds up my turn. When I'm pivoting the boat I am pretty much always trying to maneuver in tight quarters and have one hand on each throttle/gearshift. I can do some pretty fine control with this method and cannot see why I would give up that control to throw the helm hard over. I guess I could use the helm hard over method to pivot in open water, but I hardly see why I would want or need to pivot the boat in open water.

On the question of the pivot point I find my boat tends to pivot about the stub keels which are near the center of the boat. My cat pivots in its own length, it definitely does not pivot around the props. I can definitely see where boats that don't have significant keels would tend to pivot around the props. I used to captain an Allmand 34ft sedan cruiser and it definitely tended to pivot close to the props. I'm not sure it was right at the props, but the bow defintely had a lot more motion than the stern. As has been said before this all would seem to depend on the design of the boat.

[Lodesman]

You know why they sell bow-thrusters? Because you can't move the bow from the screws!
If the props are in the middle of the boat, like they are in some tugs, then it's possible for them to pivot around a point between the props, but for most boats where the props are well aft of midships, it is simply impossible for the props to stay fixed in position (in a liquid) and move the bulk of the mass of the boat, and virtually all of its deadwood around them like a barn door. In any shiphandling text, you will find the term "prop-walk" used frequently, because that's what they do. "Prop-walk" is also called "paddlewheel effect" if that helps you picture it.

[Captain Bill]

Ok, guys I did the test on my Cat today. Rudder amidships 360 degrees in 90 seconds, rudder hard over in the direction of turn 360 degrees in 70 seconds both tests done with forward and reverse engines at 1500 rpm. The boat pivoted around its center not the props. I would expect a different pivot point if I didn't have the stub keels.

[David M]

1/3 from the bow is typical for a monohull that is not making way. Other hull designs and making way can be different.

[Adelie]

Originally Posted by over40pirate

The first time (1989) I got my ticket, I took a prep course. Books and teacher. No computers. I passed the first try.

When I renewed 5 years later, I took a course to get the towing endorcement, and went to a sea school, where all I did was sit in front of a computer and took a test. Did it a few times until each time I knew the answers.
When I went to the CG to test, I knew the answers, before reading the whole question! Word for word.

This open book test has some killer questions. And you need a 90% to pass.

One I can't find.
Dayshapes must be shown:
a. daylight hours
b. sunset to sunrise
The other 2 answers were wrong and I didn't show them.

All I find on Lights and shapes says during the day. But for lights, states from sunset to sunrise.

If I was going for a license first time, I would taks a course that had computers with all the questions.

All the arguements I've seen for answer a) are logical semantic arguements. The logical TEST arguement (ie the test is logically consistent with itself) is for b), if you have the have a light on sunset to sunrise, then when the light doesn't have to be on you have to have the required dayshape up if one is required. You say you found the hours requirement for lights.

The other arguement for sunrise to sunset is that they are both very bright lines, for any lat/lon you can look up the exact time for sunrise and sunset, there is no arguing about when during twilight it stops being daylight. The folks that enforce rules like sharply drawn lines, daylight hours is too open too arguement. There is probably some wording that gives them leave to penalize you under extraordinary circumstances, but in general the bright lines will be the ones enforced.

[Adelie]

Originally Posted by GordMay

If there's no way on, the rudder has no effect.
Twin screw boats should generally be close-maneuvered with gear shif & threottle only, no rudder.

This page disagrees with Gord
Page Title

I haven't looked at you link yet, but I would expect the rudder to have an effect even if the boat is not moving forward or aft, the boat will pivot around a point near amidships, a bit in front or a bit behind depending on the exact shape of the underbody(s). That means that the rudders will see transverse flow at the boat pivots, assuming the rudders are near the aft end of the boat.

I would expect that rudder with the turn would speed up the turn.

I would expect that rudder against the turn would slow the turn but also decrease the radius of the pivot.

It's been decades since I got to play with a double screw boat and I don't have access right now so I can't go out and try any of this.

[David M]

Depends...

Small rudders like what is typically found on a powerboat will not work unless engine wash is passing it, regardless of whether or not the boat is making way.

Larger rudders like what is typically found on a sailboat will work when the boat is making way or when there is engine wash flowing past it. A sailboat can be both not making way and have engine wash flowing past the rudder such as when you are stopped and then hit the throttle.

Twin screw boats can turn in their own length while not making way with engine wash alone. Moving the rudder from zero degrees to hard over has the effect of making for a faster turn.