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Old 01-16-2012, 12:17 PM   #41
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Backing

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skipperdude wrote:
A twin walks to starboard in reverse both engines reverse, idle, rudder amidship?

SD


*No, because most twins have counter-rotating propellers.* So with both engines in reverse the propwalk of both props cancel each other out so the boat backs straight (assuming no wind, current, etc.)

This is one of the advantages of a twin.* If you put one transmission in forward and the other in reverse, both props will turn the same way and the propwalk force is, in effect, doubled.* So the stern will swing quite rapidly in the direction the combined propwalk is moving it.* Add rudder and this is why a twin can be pivoted in place.

Twins without counter-rotating props and both engines in reverse will swing their sterns in the direction the propwalk of both props is moving them.* But almost all modern (like 1960s and later) twins have counter-rotating props.

The PT boats of WWII had three engines and all three props rotated the same direction.* They were an absolute misery to maneuver at slow speeds according to PT vets I've interviewed.* And my wife and I have ridden on a restored one, and the effort it took to maneuver the thing back into a rather tricky slip was amazing.* Even though it was being driven by a fellow who had been a PT skipper during the war and knew how to do it.


-- Edited by Marin on Monday 16th of January 2012 01:23:07 PM
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Old 01-16-2012, 12:20 PM   #42
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RE: Backing

My Monk 36 single engine backs to stbd as did my previous boat a Camano 31, also a single.
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Old 01-16-2012, 12:23 PM   #43
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RE: Backing

Never driven a twin. That was why I asked reguarding the begining of this tread as to who's boat backs to starboard. Several people said thay back to starboard so I was wondering if twins did so.

It appears only the Coot has a left*hand *prop.

So he will back to starboard. Right hand prop backs to port. Most boats have right hand props.

SD
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Old 01-16-2012, 12:26 PM   #44
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Backing

According to the manuals, all Grand Banks single engine boats have left hand props.

If a signle-engine boat has a counterclockwise turning engine (viewed from the rear) like the Ford Lehman 120 and-- I think-- most other engines, and there is no counter-rotation gear in the transmission (as there is in the starboard transmission of most twins unless it has counter-rotating engines), this means the prop will turn the same way as the engine, so the prop has to be a left-hand prop.


-- Edited by Marin on Monday 16th of January 2012 01:30:46 PM
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Old 01-16-2012, 01:32 PM   #45
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RE: Backing

Quote:
Marin wrote:
According to the manuals, all Grand Banks single engine boats have left hand props.

If a signle-engine boat has a counterclockwise turning engine (viewed from the rear) like the Ford Lehman 120 and-- I think-- most other engines, and there is no counter-rotation gear in the transmission (as there is in the starboard transmission of most twins unless it has counter-rotating engines), this means the prop will turn the same way as the engine, so the prop has to be a left-hand prop.



-- Edited by Marin on Monday 16th of January 2012 01:30:46 PM
Most trawler style boats and cruisers have the steering on th starboard side as it makes it easier to watch the danger zone for crossing situations. *That puts the door on the starboard side also. *A left hand prop will back to starboard. *That makes it easier to bring along side the dock with a little burst of reverse to tuck the stern in.
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Old 01-16-2012, 01:33 PM   #46
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RE: Backing

Most of our trawlers have BW Velvet drive transmissions that have left hand rotation in fwd gear and prop walk to stbd in reverse.

Marin I do'nt think prop walk on a twin would be any more than a single as the props are usually smaller. Even when they are'nt (like a lot of trawlers on TF) the prop walk is proportional to the amount of power applied so prop walk should be the same (roughly) when backing.*

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Old 01-16-2012, 02:02 PM   #47
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RE: Backing

RE: Backing

I do'nt think prop walk on a twin would be any more than a single as the props are usually smaller. Even when they are'nt (like a lot of trawlers on TF) the prop walk is proportional to the amount of power applied so prop walk should be the same (roughly) when backing.*

______________________________________

With twins you have the same forces as a single plus a few more.*

The ability of the twin to rotate on its center point is the most obvious and the one we all focus on.* This is a result of applying forward and reverse thrust, running one*propeller in*forward and the other in reverse, as previously stated.**By applying*forward*propeller thrust on one side and reverse thrust on the other,*the boat will rotate on its center point.* The center point is not always constant and varies by boat design, thrust, momentum, wind, tide and other conditions.*

If you're really interested in the physics side of it, there are a number of subject papers online which go into great detail.

LB
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Old 01-16-2012, 02:03 PM   #48
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Backing

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nomadwilly wrote:
*

Marin I do'nt think prop walk on a twin would be any more than a single as the props are usually smaller. Even when they are'nt (like a lot of trawlers on TF) the prop walk is proportional to the amount of power applied so prop walk should be the same (roughly) when backing.*

*
Speaking only for Grand Banks boats because they are the only ones I'm this familiar with, the prop used on a single engine GB is the same as the prop used on the port side of a twin-engine GB of the same size and using the same type of engine as the single.* In the case of FL120-powered GB36s, the diameter and pitch is 24 x 18 for their original, three-bladed props.

The only other evidence I have is that we chartered a single engine GB36 before we bought our own twin engine GB36.* To yaw the stern to starboard as a single left-hand prop will do in reverse, our twin with both props rotaing clockwise (port reverse, starboard forward) swings its stern to starboard much more responsively and faster than the single ever did. There are reasons for this other than just propwalk of course, but the combined propwalk of two props turning in the same direction instead of just one does play a signficant role.

One potential factor in our favor is that we have four-bladed props where I believe the single we chartered has a three-bladed prop (but I could be wrong about that).* And a four bladed prop develops more propwalk than a three-bladed prop of the same diameter, one reason why the prop shop folks say "A three backs better than a four."* By which they mean develops less propwalk.* It's also more efficient, but that's a different topic.


-- Edited by Marin on Monday 16th of January 2012 03:12:35 PM
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Old 01-16-2012, 02:56 PM   #49
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RE: Backing

Marin,

It's not NORMAL for a twin engine boat to have twice as much power as the single engines counterpart. And the 4 bladed prop will NORMALLY have less dia than it's 3 blade counterpart. And less dia means less walk. Also it would NORMALLY have less pitch because of more blade area and hence even less walk. In some ways Marin the GB boat is not normal and can't be correctly used as an example.

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Old 01-16-2012, 02:58 PM   #50
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RE: Backing

Quote:
Edelweiss wrote:
The ability of the twin to rotate on its center point is the most obvious and the one we all focus on.*
*The Coot's bow thruster has the same effect.
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Old 01-16-2012, 03:08 PM   #51
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RE: Backing

So you are saying most boats have left hand turning propellars.

contrary to what I have always thought.

I am pretty sure most outboard's are right hand.

'SD
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Old 01-16-2012, 03:13 PM   #52
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RE: Backing

Quote:
markpierce wrote:Edelweiss wrote:
The ability of the twin to rotate on its center point is the most obvious and the one we all focus on.*
*The Coot's bow thruster has the same effect.

*Rotate*the boat in a circle, now that it will do.***Right on!!
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Old 01-16-2012, 03:49 PM   #53
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RE: Backing

When "threading the needle" to get to available dockspace at one of our YC outstaions, I frequently do it in reverse, in order to get the favoured starboard landing. Steering is done by propwalk and forward power with rudder. In reverse, especially at low speeds, the rudders have no effect on direction. So if there is enough wind or current to present a predictable direction of drift, I set up to use the side that is in the direction of drift as the reversing engine, and the other side with the rudder kicked outboard to add a little bit of correcting force. Only if there isn't enough propwalk to counteract the wind/current, will I need to change the ruddder. If there is little or no current/wind, I alternate engines to keep centered between boats tied on either side of the passage.
My engines are counter-rotating, with starboard propwalk in reverse on port and port propwalk in reverse on starboard. My engines are identical, mated to BW Velvet drive trannies, one is 1.9:1 with no reversing of rotation in forward, the other is 2.0:1 reversing. I don't know why the gearing is different, as the props are very close to identical pitch.
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Old 01-16-2012, 04:10 PM   #54
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Backing

I usually back my car or truck into a parking space or driveway as well.

Why, you might ask? Well, when I get ready to leave a few hours or days later, I can easily see what's in front of me. Traffic, pets, small children, etc.

Other than the social aspect and the fact that it's customary at my marina, backing into the slip is much the same. Leaving, you're going forward and have the best visibility and control.


-- Edited by rwidman on Monday 16th of January 2012 05:11:14 PM
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Old 01-16-2012, 05:08 PM   #55
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Backing

Quote:
nomadwilly wrote:
It's not NORMAL for a twin engine boat to have twice as much power as the single engines counterpart. And the 4 bladed prop will NORMALLY have less dia than it's 3 blade counterpart.
I guess it depends on your definition of "normal."* The more modern production boats I'm familiar with that are offered in single and twin configurations--- and there are*not all that many of them besides GB although a number of the "Taiwan Trawlers" could be had either way--- all used the same engine in the twin as they did in the single.* I know you think that's wrong, but regardless, that's what they did.* Even into the 2000s with the GB36 before they discontinued it.

Second, according to the prop people we've talked to about our props--- at Kruger & Sons who I know you don't like despite virtually everyone we've talked to in this area telling us they are the best around, and a couple of others---* the answer we always got was that*for a given boat a three-bladed or four-bladed prop (or props if a twin)*should be the same diameter but the four bladed prop*would*normally be*pitched down one inch.

So our boat is very "normal" for what the production boat*companies have been doing since the mid-1960s.* Whether you think they should have been doing that, I suspect, was irrelevant as far as they were concerned :-)


-- Edited by Marin on Monday 16th of January 2012 06:09:57 PM
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Old 01-16-2012, 05:22 PM   #56
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RE: Backing

*

Sidebar:**

If your marina allows transients, does your harbor master/marina jefe

tell them how to pull*in?***** KJ*
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Old 01-16-2012, 05:25 PM   #57
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RE: Backing

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KJ wrote:
*

Sidebar:**

If your marina allows transients, does your harbor master/marina jefe

tell them how to pull*in?***** KJ*
*Rarely unless it's up to a bulkhead and shallow water.
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Old 01-16-2012, 05:29 PM   #58
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RE: Backing

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koliver wrote:
My engines are counter-rotating, with starboard propwalk in reverse on port and port propwalk in reverse on starboard. My engines are identical, mated to BW Velvet drive trannies, one is 1.9:1 with no reversing of rotation in forward, the other is 2.0:1 reversing. I don't know why the gearing is different, as the props are very close to identical pitch.
*You sure you don't mean that your final ouput drives from the transmissions are counter-rotating with both engines turning the same way?

We have Velvet Drives in our GB with the similar ratios you describe-- 1.91 to 1 on the starboard side and 2.1 to 1 on the port side.* The reason they are different is that the starboard (1.91 : 1) has an extra gear in it to* reverse the rotation of the output shaft.* They could not fit this gear into the standard*transmission housing and keep the same drive ratio, hence the slight difference between the two.* But the engines--- in our case FL120s--- both turn the same way.* There are no counter-rotating FL120s.

GB accounted for the two different final drive ratios by pitching the port (slower) prop up an inch.* However the prop shops we talked to, including Kruger who we had rework our props completely, said they don't do this unless a customer asks as they don't feel it really makes any difference.* Our props are now*pitched the same and despite one turning a wee bit slower than the other at a given rpm,*whenthe helm is centered*the boat tracks straight.
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Old 01-16-2012, 05:58 PM   #59
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RE: Backing

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KJ wrote:
*

Sidebar:**

If your marina allows transients, does your harbor master/marina jefe

tell them how to pull*in?***** KJ*

Not that I am aware of.* There's a more or less transient boat (supposed to be here for a month but it's been two so far) near mine.* It's stern in.

Here's a photo:



As you can see, the fairway is pretty narrow.* Add to that the tidal current and it's pretty important to have good control as you're leaving the slip.* Most boats can't just pull out of a slip, we have to pull out, back and straighten out, and then leave.* Usually crabbing yo compensate for the current.
*
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Old 01-16-2012, 06:12 PM   #60
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RE: Backing

In the Pacific NW I think the norm is bow in. That certainly holds true for the majority of sailboat owners. For our 33' full keel sailboat, even when contending with tidal currents, we typically go bow in. Of course, being in the aft cockpit to back out means I'm right there and able to easily see into the fairway. Plus, as someone said, a single-screw full keel sailboat simply handles easier and is way more predictable going forward. However, if there is no wind and no current, and the slip next to me is open, and we want the vee-berth pointing away from a busy transient dock... It's a lot fun to back in! OK, so it might take two attempts sometimes, but it makes for good practice and helps keep me "one with the boat"...
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