Tips on docking stern in with twins

[Hampton]

The very first statement is FALSE. A force on a boat doesn't have to TURN the boat - around its pivot point or otherwise.

I have to admit though, I hate canoes.

Agreed - For a canoe, where the only forces considered are a paddle stroke or wind, both coming from the side, it makes sense. For many other boater's consideration, not so much.

 

[Turtlesboat]

From Chapman Piloting and seamanship

"With forward motion, when the helm is put over to one side, a boat pivots around a point about one-third its length from the stem, Note that this causes the stern to swing in a wider arc than the bow.

When backing down with the rudder to port, the pivot point is about one-quarter of the boat length from the stern. The bow, therefore, describes a wider arc than the stern."

 

[gerryb]

I guess I need more convincing.

...pretend mercruiser has a new 'tractor drive' to propel the boat, very similar to a land-based tractor drive... if you have driven a tractor (or a tank!) you know the pivot point is NOT the front of the vehicle... as long as the driving mechanism is driving force front or backwards, the pivot point will be closer to the driving mechanism itself (not necessarily on top of it though).. propellors with rudder set straight, or stern drive in fixed straight ahead position are 'working' like a tractor drive as far as how force is applied to displace the item being moved. A boat is somewhat different because of the resistance the water gives against the hull, as well as a host of other reasons but the principle is the same.

If the driving force was side to side (port-starboard) well THEN you would have a pivot point opposite to where the side-to-side energy was being sent.

Bow thrusters (side-to-side type propulsion) work so well in the FRONT of a boat because they give you more control, not because the pivot point is up there.

(This is getting too deep...head starting to hurt now..someone call Mythbusters or something...)

 

[teekster]

Tonight is the night I will turn my boat around to have the stern in. I'll let you know how it goes. I do know that I have had very little luck trying to get the bow to move in the desired direction. I think Jim is on to something. Let the experiments begin.

FWIW, for you stern drive guys, I used to use the wheel and the outside engine all the time. I could achieve great precision that way. The drives were too close together and too far back in the 280 to try to control the boat well with just the throttles.

 

[douglee25]

Sweet. Let us know how it works.

Doug

 

[Woody]

As long as we're talking tractors. Anyone drive a forklift? Want to turn left, the rear swings right as the forks swing left. The pivot being at the drive wheels. On a boat going forward you see were you want the boat to go, turn to port, the stern swings to starboard as the bow ahead of the pivot point swings to port. I also would think there is some degree of slip or slide to the outside on most boats. My guess would be more times than not a boats centerline is not parallel to the heading your trying to hold.

 

[Gunn]

The very first statement is FALSE. A force on a boat doesn't have to TURN the boat - around its pivot point or otherwise.

I have to admit though, I hate canoes.

If the force is applied directly towards the center of mass of the boat, then it'll go straight. Otherwise, a torque will be generated (force x distance from center of mass) that will TURN the boat around its center of mass (is that equivalent to center of buoyancy on a boat?). That's hard not to do, so they'll always be some torque trying to turn the boat. Now, it may be counteracted by other forces to stabilize it (wind, currrent, etc.), but...

This "pivot point" we're all talking about is this center of buoyancy, or mass. We know where the force is applied (at the stern of the boat, at the propeller(s)). SeaRay engineers know where this other point is. That would explain everything! :smt001 The text pasted above from that link article mentions the pivot point moving; I can only reason its not -really- moving, but seems so because of the added friction forces from other forces (besides the propeller(s)).

Tom

 

[Presentation]

Jim, I think I figured it out.

Today I made some videos docking keeping the drives straight. I also did some 360’s in both directions. If I was doing a 360 and had more power applied to the reversing engine then in addition to spinning I was backing up. This gave the appearance of stern sideways movement from within the boat however from the video I was not moving sideways I was spinning as I was backing.

I’ll start a new thread since my videos will get lost by this point in this one.

I hope you are up to acknowledging, well, I just have high hopes for you.

I’ll post the the videos.

 

[Turtlesboat]

I hate it when that Newton guy gets everything wrong.

 

[teekster]

The experiment is done. In MY boat (inboards), I have a very difficult time moving the bow with the throttles. Today was not very windy, yet it was still difficult to make adjustments close to my dock. When I went in stern first, I could swing the stern back and forth as I pleased. At least for inboards, Jim is right. The pivot point is about 1/3 back from the bow.

I will resume backing into my slip. Now to try the experiment on a windy day.

 

[CSR_Admin]

Pres, your vids are interesting. I would like to see you do a full 360 so we can see the pivot point of the boat. On my 300 I did about 8 revolutions, got the throttles exactly right so the boat spun within its length. While spinning like that the pivot point was absolutely not at the stern. I maintain that a stern drive boat should be operated like a single screw outboard or I/O. The wheel should be turned instead of using shifters as you will have more instant reaction to shifter inputs using less power. In your vids you show pivoting the bow away from the dock to pull away. This is a great way to teach people how to expose the most fragile part of their boat, the swim platform. Especially a new boater who is likely to bang the dock. When you move up to a twin screw inboard you will definitely not pull out bow first.

Great vid otherwise, really shows how an I/O boat will move the bow with shifter inputs. Most twin screw boats I operate are inboards and I operate them as such. When I am on an i/o boat I use the wheel, possibly I have been thrown off the whole time because they have a more aft pivot point and don't operate like inboard boats around the dock, so I have defaulted to turning the wheel for the same affect of moving the stern.

I may go out today to see what our 300 does against a bulkhead with just shifters to see if it behaves like yours. Assuming it will you will have made your case for an I/O boat. My argument all along has been that my comments apply to an inboard boat and that I/O boats should use the wheel. It's all right there in my first post :grin:

 

[Turtlesboat]

I think everyone(except a few) is missing the point of the pivot point of a boat moving through water. If a boat is standing still with no external forces, the pivot point is in the middle of the boat. If the boat is moving forward, the pivot point shifts to 1/3 from the bow. If the boat is moving in reverse, the pivot point shifts to 1/3 from the stern. These are the NATURAL pivot points of a boat. Sure you can "move" the pivot point anywhere you like but that doesn't mean you should.

 

[Presentation]

I maintain that a stern drive boat should be operated like a single screw outboard or I/O. The wheel should be turned instead of using shifters as you will have more instant reaction to shifter inputs using less power.

Jim we are in agreement on this.

In your vids you show pivoting the bow away from the dock to pull away. This is a great way to teach people how to expose the most fragile part of their boat, the swim platform.

This video is not intended to show the proper way to leave the dock. In my opinion the proper way to leave the dock is to turn the wheel as stated above, just like the second video shows at the end the proper way to pull the stern to the dock is to turn the wheel. Say if you are tied to starboard you can turn hard over to port and back away. Wind, current and obstacles may dictate other methods but this is how I would typically do it.

The purpose of the video was to show that you could not pivot the stern away from the dock, or back to the dock, by throttles alone on my boat with sterndrives.

My argument all along has been that my comments apply to an inboard boat and that I/O boats should use the wheel. It's all right there in my first post

Here I thought the question was the location of the pivot point. If you are spinning high rev 360’s you may get dizzy but you will not get to dock your boat. Actually, I know of no practical application for spinning 360’s.

The goal was to show that if you kept the drives straight and used only throttles the stern would not significantly swing to the dock like it does with a longer boat that has inboards with the propellers much further forward. I said the bow would swing but the stern would not swing over to the dock. This was the opinion difference as I understood it.

Anyways, I’m satisfied with the results. It reinforced what I have believed for a long time, and that is when a boater with inboards tells me I should dock my boat using only the throttles it’s OK for me to push them into the drink.

So do I at least win a free hat?

 

[RollerCoastr]

Mike, we really have a semantics problem here. The commonly accepted definition of "pivot" includes a stationary point of an object, whether it be a person, or data, or a boat, etc.

Doug did a great job of showing "pivot point" of a twin B3 powered vessel. Those dynamics can be explained without throwing out common sense.

Boat HANDLING is another topic. I have twin inboards that creates a fairly centered pivot point. I always leave a wall stern first, but I do so because of about 3 other reasons, including non-directional thrust, the shape of powerboats and use of springlines. That of course, is another discussion.