As the boat turns...

[Brian D]

The other day I moved my boat. It was in the slip stern first. I had it this way so I could work on the outboard. Some will remember that debacle.

As I was moving forward from my slip into the fairway, I noticed the boat started to drift to the starboard. I had a medium port wind but did not have enough headway to counter it. I started to drift into the opposite slips. Time to take action, so I put the boat in reverse, turned the wheel so the boat will back up to starboard. This swung the bow to port and I was lined up to exit. Like a pro. LOL

Coming back into my slip I had the same problem. Wind pushing me to port. As I turned to starboard to enter my slip I saw that I was not going to be lined up. Again, put the motor in reverse this time let prop walk do its thing. Lined right up and in I went. Like a pro. LOL

So this brought me to the question, how does a keel boat turn. I know the answer, but we get a lot of new sailors here so lets hear you thoughts. Don't be overly technical.

Is the bow turning? Is the stern turning causing the bow to appear motionless? Is the boat pivoting on the keel?

Assume: no current, no wind, flat water, no sails up, just keel boat and motor. If you want, you can describe both full keel and fin keel. Even center board or dagger board.

 

[Tom J]

The trick seems to be using bursts of power. I have watched single engine fishing boats pivot by applying burst of power in reverse and then forward. I tried this on my sailboat (winged keel) to turn her around at the slip. I got her out of the slip, then applied a burst of power in reverse and let prop walk pull the stern over, then a burst in forward, moving ahead a few feet, then another burst in reverse, etc. She didn't exactly pivot, but close enough!

 

[jon hansen]

it's called 'backing and filling' 800' ships use this technique also.

 

[Joe]

Unless the prop and rudder are at the bow (as in "bow thruster") all boats turn from the stern. That's why so many new boat owner's get out of kilter... they think they're steering their car into a parking space.
If your partner has trouble with tiller steering... tell her/him to point the tiller in the direction they want the stern to go.

 

[agprice22]

Keels and rudders are wings. They create lift. As the leading edge of the rudder is pointed to port, the trailing edge experiences a low pressure vortex on its port side that “lifts” it to port, causing the stern to swing to port, and the bow swings to starboard. The keel, during this action actually experiences lift in the opposite direction. Think in terms of a horizontal stabilizer/elevator as the rudder and the wing as the keel. As the elevator pulls the tail downward, the wing generates upward lift due to the change in attitude.
An easier way to think of the rudder is that it “slows” one side of the boat like using a canoe paddle to steer a canoe by dragging it, but this picture only works with transom hung rudders.

 

[Will Gilmore]

Are we talking about turning in place or with minimum headway by engine power or with full steam under sail? The dynamics are a little different, prop-walk, prop-wash across the rudder, backing and filling (as Jon puts it), vs agprice's lifting keel. It's all there to some degree, but the primary dynamic changes for each situation.

Bow thrusters aside. Boats turn by their stern. Water is, of course, fluid, so there's always some slippage. A fin keel provides the best pivot point so the bow swings with the stern, in the opposite direction. Turning, in reverse tends to drag the bow inside the turning radius like the rear wheels of a car, but sort of the opposite when turning in forward. Then, the bow goes wider the more headway you have and the stern is trying to run around to get behind it (slippage). Various bottom configurations act more or less like this, so get to know your boat.

-Will (Dragonfly)

 

[shemandr]

The trick seems to be using bursts of power. I have watched single engine fishing boats pivot by applying burst of power in reverse and then forward...

I think that's because the first thrust from a propeller is sideways. And the keel and rudder don't have flow over them and therefore little lift or steering until the boat starts moving forward. So you want that burst of power to move the stern sideways without a lot of forward motion.

 

[jssailem]

I think @Will Gilmore nailed it.
Not that while the prop force is pushing or pulling the boat the keel is trying to make the boat go strait so the boat pivots around a location just behind the mast.

When Brian pulled out he was trying to get the bow to move, but it was the stern turning, and likely because the pivot point was not far enough out of the slip and in the fairway the boat was sliding towards the other boat.

Get the boats pivot point out into the waterway, then use prop walk to push or pull the stern around the pivot point. Of course Current and wind mike make you slip, so you need to add or reduce power to maintain control.

 

[Don S/V ILLusion]

At slow speed, a keel serves no purpose other than to counterbalance a mast. It has no effect on direction. At such speed, a boat is more analogous to a floating log.

Push on the rear end of a log and the pivot point becomes obvious as both ends turn in opposite directions.

 

[LeslieTroyer]

@Brian D with the title of the thread I was expecting a soap opera. So disappointed it was there was no cheating or suspense.

Les

 

[jon hansen]

momentum is an ingredient used to enhance the 'backing and filling'
. lets the paddlewheel effect creating the 'propwalk' to work better.

 

[Brian D]

@Brian D with the title of the thread I was expecting a soap opera. So disappointed it was there was no cheating or suspense.

Les

I was wondering if anyone would catch the subtlety of the title. There is suspense and drama within the turning of a boat, you know. Anyway, you made my day.

 

[CaptKimble]

I was awed in the Navy watching the coxswains pull the 40 ft. liberty launches up to the stern of the carrier in wind and waves that lifted the boat up and down and sideways. They would manipulate throttle and transmission along with the rudder to bring the boat to a halt right next to the gangway making it look somewhat easy most of the time. And yes I did say "most of the time". They developed a feel for how that flat bottomed boat was reacting to the wind, current etc. Never once did we "slam" into the gangway although I can't say it didn't happen. They used "prop walk" as a matter of course. As an aside, on those launches the throttle was on the steering shaft just below the wheel which was mounted flat on top like a plate. The shifter was on the deck. To shift you pivoted the shifter forward and back. It reminded me of a Ford Tractor. Good times. They had a crew of 3 plus a boat "officer" as well.

 

[shemandr]

Watch a yacht club launch operator (At the end of the Summer.) "We don't need no stinking' bow thrusters."

 

[jssailem]

Watch the crew at the Annapolis Boat Show... It is a sport after the end of the show. Up on the balcony, drinking beer and betting in the pool as to the time of the last boat/dock departure.

 

[Will Gilmore]

Push on the rear end of a log and the pivot point becomes obvious as both ends turn in opposite directions.

Don, that's not exactly true. If there is force in only one direction at one end, that end, of course moves with the force, the other end, however doesn't simply pivot by equal measure in the opposite direction. Try setting a pencil down on a table. It has a pivot point near its center, but if you only apply lateral pressure at one end, that center moves at almost half the speed at the end where the force is applied. It may not be so straight forward with a keel deep in a fluid, but that is essentially how the forces and movement react. The overall position of 'center of mass' and 'center of lateral resistance' will move in the direction of force. It will certainly pivot, just not while remaining stationary in the center.

-Will (Dragonfly)

 

[Brian D]

Interesting point on the "pivot" of the boat. When I am at the helm and start a turn, it "appears" that the bow hardly moves. It "appears" that the stern is moving faster than the bow, like the pivot point is near the bow. This is my perspective of being at the helm.

Conversely, when in reverse and turning I have to pay attention to the bow because it looks like it is speeding up to try and keep up with the turn. Sorta like playing "crack the whip".

Am I out in left field on this? Or do others see this and have to account for this when pulling into a slip.

 

[sailnoproblem]

I don't know about the technical aspects of backing down(sorry I just used a term from my sport fishing days).
I always pull my boat into my slip stern first, I really don't have a problem doing it.
I don't have the biggest prop either. Sorry I can't give advice I just do it.

 

[jon hansen]

keel, prop location, prop, rudder size-location hull shape, ....... are all variables. that said, on can figure that the turning/ pivot point will be 1/3 of the way back from the bow, at the waterline.

 

[jon hansen]

practice, practice, practice, and then some more practice.