GB 36 rudder authority

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Caballero II

Caballero II

Senior Member
Joined
Mar 12, 2017
Messages
394
Location
USA
Vessel Name
Caballero II
Vessel Make
Grand Banks Classic 36
I have a single screw GB 36, with a Lehman 120 hp. I'm happy with it and do the best I can with the tools at hand, which are what I can do with propeller "walk" and "back and fill" when maneuvering in tight spaces. But, is there a well accepted rudder modification which allows more rudder authority at low speeds while docking, and/or allowing some reverse control? It doesn't matter where I put the rudder in reverse, the boat backs to starboard. "Back and fill" is about all I've got to pivot the boat. I'm looking for a tried and true rudder modification that would improve low speed maneuvering.
 
The rudders on my twins GB36 were small and only have affect at speeds higher than docking.
Many DIY mods seen over the years, however I do not know of any rudder mods to make a signifcant difference at slow movement. Thrusters?
 
Caballero II said:
I have a single screw GB 36, with a Lehman 120 hp. I'm happy with it and do the best I can with the tools at hand, which are what I can do with propeller "walk" and "back and fill" when maneuvering in tight spaces. But, is there a well accepted rudder modification which allows more rudder authority at low speeds while docking, and/or allowing some reverse control? It doesn't matter where I put the rudder in reverse, the boat backs to starboard. "Back and fill" is about all I've got to pivot the boat. I'm looking for a tried and true rudder modification that would improve low speed maneuvering.
Click to expand...
On my smaller 50’ single screw commercial fishing vessel I made the rudder as large as possible. I also welded a tapered plate both sides on the aft part of the rudder, top to bottom. Not sure of angle, enough so flow would hit it and add more force. I will try to add a pic.
 

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The biggest thing that will add more rudder authority in reverse is a bigger rudder (most likely requiring beefed up mounting). Chances are, the current rudder will steer some in reverse, but only at higher speeds than you'd want while docking.

To get more than a small improvement there you'd probably be getting up towards a sailboat sized rudder. And the amount of keel a GB36 has will also limit how tightly it'll want to turn with the limited turning force available in reverse (even with a big rudder).
 
Different boats will just have different backing characteristics. I have a good friend who says that no matter where the rudder is pointed his boat backs straight. He wishes it backed to one side or the other so he could count on that for docking.
Be careful what you wish for.
 
I have a GB36 single. We have a bow thruster and it makes a tremendous difference. Can't imagine how nice a stern thruster would be.

An articulating rudder would help a lot.

Boat does prop walk but your really need perfect conditions.
 
SteveK said:
The rudders on my twins GB36 were small and only have affect at speeds higher than docking.
Many DIY mods seen over the years, however I do not know of any rudder mods to make a signifcant difference at slow movement. Thrusters?
Click to expand...
You hit it on the head. The tiny rudders on these boat require a fair amount of prop wash to have any authority, too much for docking speeds. I've tried to avoid thrusters (cheaters) because they're not sexy and I hate the way they sound. However, I must admit that they probably are likely the best answer.
 
I can back straight with my wheel turned all the way to port, I can back slightly to starboard with my wheel centered, I can back around a corner to the starboard with my wheel turned all the way to starboard.
No backing to port at all for us.
 
There have been some past threads on this topic, if I have some time I will try searching for them.

I feel your pain. Coming from sailboats that had rudders that actually worked, it was a tough transition to a power boat. The articulating rudders apparently can make a difference at slow speed and I think would be very nice to have.

Articulating rudder 101

I need a lesson on articulating rudders! I saw a boat for sale where the owner claims he spent 7k on it and it works better then a stern thruster?? Can y'all elaborate on this? I'm trying to learn. Never really knew what the pros or cons where on this, for in all my boating years, I never really...
www.trawlerforum.com www.trawlerforum.com
 
Look at SideShift thrusters. They are relatively inexpensive, about $5K, and are simple to install. No holes below the waterline. Ours work very well. We have bow and stern.
 
In close quarters, remember, neutral is a gear, use it. Get the stern going in the desired direction, neutral and coast, same for the bow.
If desired, use spring lines, they are magical
 
I have this rudder hanging under the boat, it works very well!
Advantage of the rudder, it gives little resistance when you sail straight.

Greeting,

Pascal.

E1i4IOw.jpeg
 
mvweebles said:
There are two possible modifications. A fishtail trailing edge of the rudder; and an articulated rudder.

en.m.wikipedia.org

Schilling rudder - Wikipedia

en.m.wikipedia.org en.m.wikipedia.org


I don't know whether the GB36 single's rudder would accommodate either or both - requires a skeg.

Thruster would be cheaper and more effective. And would also have a modicum of resale improvement.

Peter
Click to expand...
Peter, I didn't notice this attachment about the Schilling rudder before. I think it maybe could work on a GB 36. We (the single screw models) do have a skeg. Whether there is enough room ahead of the pivot axis to accomodate a 1/3rd length I am unsure. I don't know why it would be more expensive than a thruster, but maybe so, or close enough to make it a moot point. I just hate thrusters. I need to know more about these. Seems like a cool concept.
 
Comodave said:
Look at SideShift thrusters. They are relatively inexpensive, about $5K, and are simple to install. No holes below the waterline. Ours work very well. We have bow and stern.
Click to expand...
Yeah, but. All that out in the stream? I don't get it. It looks like a lot of "windage".
 
Pascall said:
I have this rudder hanging under the boat, it works very well!
Advantage of the rudder, it gives little resistance when you sail straight.

Greeting,

Pascal.

E1i4IOw.jpeg
Click to expand...
Yeah, but what is it. Got any details?
 
OldDan1943 said:
In close quarters, remember, neutral is a gear, use it. Get the stern going in the desired direction, neutral and coast, same for the bow.
If desired, use spring lines, they are magical
Click to expand...
Pretty hard to use spring lines when you're not even in yet. They're great for leaving, but not so much for entering.
 
Caballero II said:
Ja, maar wat is het. Heb je nog meer informatie?
Click to expand...

The rudder is very effective.
Sailing on autopilot, the hydraulic pump has to work very little, the movement of the hydraulic cylinder is only a few millimeters, even in bad weather!
The rudder transversely and with the bow thruster on you sail sideways.
With rudder transverse, the ship rotates on its axis.

Pascal.
 
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The prop walk while backing is what is pulling the stern to starboard. (left hand wheel) Get the boat moving then pull it out of gear and the rudder will respond as intended. Personally I like the fact that I know exactly how the boat is going to respond and use it to my advantage.
 
Pascall said:
I have this rudder hanging under the boat, it works very well!
Advantage of the rudder, it gives little resistance when you sail straight.

Greeting,

Pascal.

E1i4IOw.jpeg
Click to expand...
I love this design. It looks very effective.
 
Caballero II said:
Pretty hard to use spring lines when you're not even in yet. They're great for leaving, but not so much for entering.
Click to expand...
Ah, did I forget to mention I have a Dockmaster system?
I still put the helm full port and back out at ‘crawl’ speed.
Spring lines can be used (to dock) if someone is on the dock to assist or you can corral a cleat
 
Last edited:
Caballero II said:
Pretty hard to use spring lines when you're not even in yet. They're great for leaving, but not so much for entering.
Click to expand...
I have been able to use spring lines when coming into a dock solo. It involves coming up with a way to snag a cleat with a spring line when coming into a slip or along side a dock. It can work well in certain situations but the requirements are specific and doesn't work in all situations. That is the biggest downside.
 
Pascall said:
I have this rudder hanging under the boat, it works very well!
Advantage of the rudder, it gives little resistance when you sail straight.

Greeting,

Pascal.

E1i4IOw.jpeg
Click to expand...
I really like the way that looks. It seems like it would be very effective.
 
Cigatoo said:
The prop walk while backing is what is pulling the stern to starboard. (left hand wheel) Get the boat moving then pull it out of gear and the rudder will respond as intended. Personally I like the fact that I know exactly how the boat is going to respond and use it to my advantage.
Click to expand...
Cigatoo,

This is what I do to minimize prop walk, and I too enjoy using the prop wash (or not) to be able to predict what's going to happen. I do have a left hand prop (and 4 blades). However, on a GB 36, even when out of gear after backing down to get her started, the boat will not respond much to rudder angle at all but simple backs straight, or nearly so. So, I use the prop wash as needed then coast as far back as needed. I think the rudder is so small, that it just can't cause much deflection of heading. It is predictable, but limiting.

I sure like the look of the Schilling or Becker Flat rudders. Are they essentially the same thing?
 
Caballero II said:
Cigatoo,

This is what I do to minimize prop walk, and I too enjoy using the prop wash (or not) to be able to predict what's going to happen. I do have a left hand prop (and 4 blades). However, on a GB 36, even when out of gear after backing down to get her started, the boat will not respond much to rudder angle at all but simple backs straight, or nearly so. So, I use the prop wash as needed then coast as far back as needed. I think the rudder is so small, that it just can't cause much deflection of heading. It is predictable, but limiting.

I sure like the look of the Schilling or Becker Flat rudders. Are they essentially the same thing?
Click to expand...
Schilling and Becker do essentially the same thing, but there are some significant differences. The schilling is a one piece rudder. Imagine it as a Y shape when looking straight down at it. The trailing edge is fairly wide so that when the rudder is off center it will deflect the water more to the side than a flat rudder will. If it’s balanced, even better, as more of the prop will be covered by the rudder. But too much rudder in front of the pivot point causes trouble, so careful balancing is required.
Becker flap rudders actually have a hinge in the back portion on a separate pivot point than the main rudder post. As you turn, the hinged portion in the back deflects more to the side. Kind of like an airplane wing flap. (I know, not really, but it’s easy to visualize)
The advantage of the Becker is that the trailing edge is thin so less turbulence is seen at the trailing edge.
 
Caballero II said:
Yeah, but. All that out in the stream? I don't get it. It looks like a lot of "windage".
Click to expand...
Not really. At slow speeds it doesn’t matter and at planning speeds it is out of the water. The stern thruster is in the shadow of the stern.
 
Comodave said:
Not really. At slow speeds it doesn’t matter and at planning speeds it is out of the water. The stern thruster is in the shadow of the stern.
Click to expand...
Yeah, well ... It's a GB36. It don't plane with my single 120 hp Lehman so I wouldn't be able to get those thrusters out of the stream. I am not interested in doing so. I am interested in a modification that would allow better control at low docking speed. It may be a thruster, but I am looking for others options, if there are any.
 
So, I just started to read Dave Gerr's Boat Systems Book. Great read if you like that sort of thing. He details a "fish tail" rudder that he says significantly improves slow speed performance. My question, has anyone done this on the West Coast, and how did it work out. I'm looking for anyone that has experience with this mod. I bet it might be cheaper than a thruster.
 
If the question is how to modify a rudder to have greater "authority" when the boat is moving backwards, I'm having trouble imagining any modification that will have much effect. When moving astern, it doesn't matter whether or not the propellor is turning. There is no prop wash across the rudder. The maximum achievable water flow across the rudder is the velocity of the water moving at the boat's own velocity through the water.

My experience with a single engine GB 36 is that in stock form, with no thruster or gimmicks, it's a handy boat to maneuver. Regardless of rudder position, it backs straight unless you really get some way on in reverse. But that exercise seems unnecessary. In close quarters (where most slow speed maneuvering happens) it's quick and easy to put the rudder hard over in the direction opposite to that you want to aim the stern toward, goose the throttle in forward just long enough to re-orient the stern, and then resume backing toward your destination. (I guess that's the so-called "back and fill" procedure that the OP refers to). A bow thruster can come in handy to reorient the bow while moving slowly in reverse, and I have run a GB 36 so equipped. It makes the boat a total piece of cake to maneuver.

When it comes to the GB 36 rudder, I'd be averse to messing with success.
 
We put both bow and stern thrusters on our current boat. I was going to put in Sidepower thrusters but when we took possession of the boat I found there wasn’t room to fit them inside. So I went with SideShift thrusters. The cost difference was dramatic. The SidePower thrusters were going to cost $25K. The SideShift thrusters cost about $10K. But I had to do the install myself. My wife and I did the bow thruster in about 1.5 days. My neighbor and I did the stern thruster in a day. It makes it so easy to handle the boat. We are both in our 70s and not nearly as agile as we used to be so with the thrusters docking is a much easier and relaxed time.
 
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