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Old 03-22-2014, 02:50 PM   #1
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Rudder Size

Hi All,
This is my first post to the forum, so please forgive me if this is placed in the wrong location.

Last fall my wife and I traded in out planing hull for a 40 year old 32' home built steel hull trawler which we are currently in the process of restoring. The boat is powered by a Perkins 4.270 (60hp) single diesel.

The problem that I am having trouble with is the size of the rudder. A few years back the boat fell off the craddle and damaged the keel. The second owner (never had in the water we are the third owner). In his wisdom, he cut off the keel just past mid ships and removed the keel height rudder (Approx. 3' high and 3' long flat plate secured with a shaft at top and on the keel). This was replaced by a SMALL rudder (approx 1.75' x 1.25' in size). While this is suitable for steering the boat at 6knots, it is not suitable for getting into and out of the dock. The person we got the boat from said that was the size the rudder should have been in the first place. He is used to twin engine trawlers and tugs.

When coming into the dock, I need to be able to manouver much better! Just in gear is 4 knots, and at 15000 pounds, coming in at 4 knots is really not a good idea. At slow speed there might as well not be a rudder on the bottom of the boat as it does nothing, even at hard over! Unless I apply power, which as I had stated before must be done in moderation. I can currently turn a harder circle in reverse than in forward with the prop walk. Problem is the prop walk is to starboard and the dock that I have to come into is a turn to starboard with the dock to port.

Eventually (hopefully next year as it's out of the budget for this summer), I would like to have the keel extended back to the stern again for prop protection and better handling. I am trying to find a solution to the handling for this year that can also lend itself to being adapted into the full keel configuration with a shoe on the keel for the rudder at that time.

What I am wondering is how big of a rudder I really should have on the boat, and also how big I can go with just supporting the rudder from the shaft on the top for this year? I understand that the larger the rudder the more forces that are acting on it, and the last thing that I want to do is to turn the wheel hard over and snap the shaft all together.

I have attached a couple of photos of the boat and also the offending tiny rudder.

I am new to trawlers, but grew up sailing so displacement hulls are in my blood, and I know that sailboats have BIG rudders for their slow speeds.

Thanks in advance.
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Old 03-22-2014, 03:03 PM   #2
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Kinda like anchors ... bigger is better.

Unless you want some speed. Say over 8 knots.
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Old 03-22-2014, 06:18 PM   #3
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Looks very small to me. Mine is a similar size boat and much bigger rudder. Old picture but hasn't changed.
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Old 03-22-2014, 06:26 PM   #4
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Looks big enough to me. Probably will need to snort some power against rudder when maneuvering. Lots of singles barely steer at dead low idle.
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Old 03-22-2014, 06:29 PM   #5
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Looks way too small to me.
Sounds like the Second owner knew not to put it in the water, so he sold it.
At idle, my boat steers relatively well and I have big rudder
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Old 03-22-2014, 06:40 PM   #6
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Why is there a chain from the top of the rudder to the transom? I have never seen that on a boat before.
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Old 03-22-2014, 06:49 PM   #7
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Why is there a chain from the top of the rudder to the transom? I have never seen that on a boat before.
Keeps rudder out of prop if linkage inside boat comes loose.
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Old 03-22-2014, 07:01 PM   #8
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You now have approx. 2.2 sq feet of rudder area. A couple of rules of thumb suggest your boat should have 5-5.5 sq feet of rudder area, that would be a minimum.
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Old 03-22-2014, 07:11 PM   #9
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Rudder is way to small, needs to come up close to the hull and extend down past the bottom of the prop. If the old keel had a skeg that the old rudder tied into, there may not be enough strength in the hull where the rudder goes through to handle the side loading of a much larger rudder without it being tied to a skeg.

Too much of the water goes by without hitting the rudder. When you apply power, some of the water hits the rudder and gives you modest turning. What you have lost is most of the water that flows down the hull at slower speeds.

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Old 03-22-2014, 07:22 PM   #10
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With the rudder post connected to the boat at only one point, I'd be concerned whether the post would be strong enough to handle a much larger rudder.
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Old 03-22-2014, 07:29 PM   #11
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Second that about strength at top of rudder; be it the rudder itself or the hull. That thing is cantilevered downward a good bit and that really builds stress.

Regarding size: I have been around a couple boats where owner complained about poor handling. Larger rudders were installed and there was no or very little improvement. Downside was a reduction in speed, as the rudder being in prop wash eats a proportionately large amount of energy.

If boat handles well at sea, and only complaint is maneuvering at low speed, that can be corrected by using sharp bursts of fwd throttle, followed by another snort in reverse to slow the vessel. Walking back and forth like that is standard practice and you do need to use the throttle.

If boat can't steer at sea, than that IS a problem.
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Old 03-22-2014, 07:53 PM   #12
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Your rudder is way small. Looks like the boat would be "blowy" too with windage and no keel.
Great thing about steel. add can add easily.
First I would add to the height of the rudder running it right up close to the hull, this increases the aspect ratio (good) and the hull acts as a dam on top. Then I would add a dam at the bottom. Finally I would make a trim tab, 1/2 the finished height that can be bolted on while the boat is in the water for fine adjustment of relative power port & starb'd. Make this tab turn out 10 degrees or so and drill it and the rudder so it can be applied top or bottom port or starb'd to the rudder.
Make sure the rest of the steering gear is up to the additional power of the rudder.
While you are at it make more tabs because you may benefit from tabs on both sides or even 4 tabs.
My boat although 10% shorter is 26000 lbs. 50hp so I know what you are talking about.
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Old 03-22-2014, 10:32 PM   #13
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My rudder is generally ineffective at low speed (even though fairly large) unless there is forward thrust. When maneuvering while maintaining minimum speed, I'm often in neutral unless making a turn at idle speed. If I cannot afford more forward momentum, the bow thruster contributes to the turn.

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Old 03-22-2014, 11:00 PM   #14
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Here's a little rudder. I think it is 11"x17", barely extends below the shaft CL. Prop is 24x23. According to some rules of thumb, it is WAY too small. Even looks too small. When I did the refit on this boat, I decided to try a small rudder, but designed for an up-size if it turned out to be necessary. The intent was to minimize drag, and rudder drag is HUGE.

This little rudder is plenty. No up-size needed. Following seas on AP, it holds just fine at hull speed. Holds just fine at 20kt too. My small rudder experiment worked.

It is not necessary for the rudder to be in the entire shadow of the prop. Remember that the rudder does not need to deflect all flow, but merely create a side force when turned.
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Old 03-22-2014, 11:49 PM   #15
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Quote:
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With the rudder post connected to the boat at only one point, I'd be concerned whether the post would be strong enough to handle a much larger rudder.
His rudder could be stronger than yours Mark if the shaft was strong enough. Some planes have no wing struts but the wings don't fall off too much.

My rudder is responsive (useable to steer with) below my stabilized speed at an idle of 900rpm. I can steer Willy almost to a stop in a fairway. Before entering my slip in Thorne Bay I'd cut the throttle and go into neutral. While the boat was slowing down I'd steer w the rudder. Then when Willy was at just the right spot and almost stopped I'd go full rudder and 1200rpm for a moment to turn 90 degrees. Then I'd go full rudder the other way to overcome the inertia (of the stern swinging) for just a moment and I'd be coasting straight into my slip at the proper angle so when I reversed about 1500rpm or a bit more Willy would land alongside the float. Didn't always work out perfectly but when it didn't it wasn't the fault of the rudder.

As you can see in this pic I don't pay much of a price drag wise (due to the large rudder) as my propwash flows only over the flat plate part of the rudder.
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Old 03-23-2014, 01:08 AM   #16
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His rudder could be stronger than yours Mark if the shaft was strong enough. ...
I doubt it. I purposely purchased my boat because both rudder and propeller were keel-protected.
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Old 03-23-2014, 01:01 PM   #17
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The size (area) of the rudder vs the rudder stock should and can be calculated. I need to know the stock size (diameter), material, and how its supported (distance between bearings) inside the boat. Also your top speed, and horizontal distance from the rudder leading edge to center of the stock.

Turning force (exerted by the rudder) is a function of area, angle, and speed. A larger rudder certainly will make a difference, whether you are satisfied with that difference is a very different thing.

Boat handling, our impression of how well or poorly a given boat handles, is a purely subjective opinion. Unless a boat misbehaves atrociously one person may say "its just fine", while another can say, "completely unacceptable". This opinion is based (usually) on what our previous experience is. I grew up on big heavy displacement boats with huge rudders. I like the handling of boats that steer even with the engine in neutral. Light semi and planing boats drive me batty, directionally unstable and requiring what seems to me too much speed when maneuvering. But that's just me......
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Old 03-23-2014, 01:27 PM   #18
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Our previous boat, an Ennos Sapphire 32 apparently had slow speed handling issues according to the original owner. So on advice from Bill Garden he attached a small delta shaped piece (fishtail?) to the aft end of the rudder and he said it improved handling. I still wasn't impressed, although slipping the transmission in and out of gear while making turns at the dock is a huge benefit.

Best photo I could find is attached. (Taken before bottom paint!)
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Old 03-24-2014, 02:44 AM   #19
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Maybe you could just add a bolt on stern thruster.


If you are transitioning from a sailboat, keep in mind it is normal to be shifting from 'in gear' to neutral much more often in a powerboat when maneuvering. Use the in gear bursts along with rudder angle to provide steerage, then get out of gear to coast. Get familiar with the back and fill technique.

It may be easier to back into your slip as well.
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Old 03-24-2014, 10:19 AM   #20
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Use the in gear bursts along with rudder angle to provide steerage
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