Da rudder please

Several TF posters from time to time have complained that their rudders are too small and too non responsive. Some have increased the size of the rudder but it sounds like a trial and error process or maybe just personal preference.

I have found our rudder is not as responsive as I would like it to be. I assume it is due to the rudder being too small.

How is optimal rudder size determined? Why do manufacturers install undersize rudders if this condition does in fact exist?

Standard disclaimer - there is no intent to turn this into a "Your rudder is too small, it is too unresponsive and it leaks" discussion.

Here's a link from a former TF member and what he did to get better control at the helm. The boats a DeFever/PassageMaker 40.

Rudder

Larry M said:

Here's a link from a former TF member and what he did to get better control at the helm. The boats a DeFever/PassageMaker 40.

Rudder

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Nice article. Unfortunately, I couldn't find the results after his installation and sea trial.

I assume the issue I am having is my rudder was designed for a twin engine boat and it's rudder area is closer to 2.5% of the underwater profile instead of 5% which a single engine boat would require.

He brings up a good point on the rudder shaft. Obviously. if you increase the rudder surface area, there will be more loads on the shaft.

So it turns out to be more complicated and precise than I thought it might be and there might be other considerations since my rudder has a bottom skeg.

Dave Gerr's book, Boat Mechanical Systems Handbook, covers this subject very well. And its just in general a good book to have around.

Donsan said:

Nice article. Unfortunately, I couldn't find the results after his installation and sea trial.

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Larry stole my thunder but search the forum and you'll find that Mike had a comprehensive thread about the subject. I'm pretty sure it included the follow up results his blog post missed.

I sure do miss Mike.

Show us your rudder. My rudder is quite sufficient, but I don't expect much if any from it moving slowly while not in forward gear.

CPseudonym said:

Larry stole my thunder but search the forum and you'll find that Mike had a comprehensive thread about the subject. I'm pretty sure it included the follow up results his blog post missed.

I sure do miss Mike.

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What happened to Mike? Did he have an adverse event or just drift away? Do you know his handle? A search on rudders yields all kinds of unrelated posts. I need some better words to search on or his handle. He seems to have been from my hometown area. I certainly envy his machine shop.

Donsan said:

What happened to Mike? Did he have an adverse event or just drift away? Do you know his handle? A search on rudders yields all kinds of unrelated posts. I need some better words to search on or his handle. He seems to have been from my hometown area. I certainly envy his machine shop.

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Sorry to say Mike passed on. His wife Mellissa, has been gracious to keep his blog active. There's some good stuff there. He was a great contributor to TF.


http://www.mvfullstep.com/index.html

I cannot recall his exact monicker now, perhaps someone else does. To narrow your search in the advanced mode set the date for +/- 6 months of the date of his blog post Larry linked to. That should get you the right rudder thread.

Once found search threads started by him for more good stuff. Trust me, Mike new very well how to use his machine shop and offered excellent advice.

CPseudonym said:

I cannot recall his exact monicker now, perhaps someone else does. To narrow your search in the advanced mode set the date for +/- 6 months of the date of his blog post Larry linked to. That should get you the right rudder thread.

Once found search threads started by him for more good stuff. Trust me, Mike new very well how to use his machine shop and offered excellent advice.

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Tried that and found his handle was "pull" and he only made about 39 posts. Nothing was posted on how his rudder design turned out. I have little doubt it worked well if he got it installed and trialed prior to passing but it would have been interesting to read it firsthand.

Hmmm, I'll poke around a bit when I get near a real computer instead of this phone but that doesn't sound right at all. This forum used to be formatted with Sparklight software, when we switched to the current V-bulletin software some data corruption happened. It sounds like Mike's profile may have been affected by that, dammit.

While I have not changed the rudder on a trawler, I did build with a friend, a new rudder for my dive boat (Downeaster). The original rudder was too small and did not have enough leading edge which made it difficult to steer at higher speeds as the boat has mechanical not hydraulic steering. From talking to people in a couple of boat yards where rudders are occasionally fabricated, we came up with the following parameters.

1. Rudder should be as tall or taller than the diameter of the prop.
2. Top of the rudder should come close to the hull so that water following the hull curve doesn't miss the rudder.
3. The leading edge (part in front of the shaft or pivot point) should be 25 to 28% of the trailing portion (part behind the shaft or pivot point).
4. Clearance between the leading edge and the propeller shaft is important. General rule of thumb is to have enough clearance to be able to remove the propeller with the rudder hard over. The more clearance between the propeller and the rudder, the less effective the rudder will be.

What we ended up doing was to make measurements and lay out a drawing that allowed for the biggest rudder within the above parameters with out moving the rudder and stuffing box or shortening the propeller shaft.

Don't have the height dimmension, but the rudder is 12" long behind the shaft and 3.25" (28%) long in front of the shaft. My new rudder is perfect! Boat turns on a dime, I can back it in a straight line, and at 15 knots you can steer the boat with 2 fingers. Increasing the leading edge made a huge difference in the steering effort and more water hits the rudder when turning which dramatically increases the rate of turn at slow speed.

Don't have any pics of the old rudder, but here is the boat and the new rudder:







Ted

My rudder is large but it also moves through 90 degrees of deflection (45 each way). Since my speed is less than 7 knots this big rudder responds very well to that much deflection. And w/o the prop turning she responds well from a slow walking speed up. The side view gives an idea of how big the rudder is compared to the profile of the hull.

I know there was a discussion here some time back about "fishtail" or "Schilling" rudders. The general concept is to create a rudder that is more of a foil than the flat plate so many rudders are. A search on either term might turn up the thread. As I recall, retro fitting can be done for a not so exorbitant cost. Rudder efficiency is substantially increased.

They even have an app for this now on Google play

Boat Rudder Calculations
Nod Technologies - March 8, 2015
-
Everyone

Tools

I have not used it but for $2.75 it might be worth checking it out.

One big hassle is the boats attempt to be too many things from the same hull build.

A displacement boat will need a big rudder , mostly to be handier when docking or slow manuvering.

A semi plaining boat would be handy capped dragging a big rudder thru the water at speeds , and an autopilot with good authority might have a difficult time at speed..

O C Diver said:

2. Top of the rudder should come close to the hull so that water following the hull curve doesn't miss the rudder.

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Interesting that your rudder appears to come so much closer to the hull than markpierce or manyboats. Seems like the gap on the Coot is substantial.

Pull was apparently working on a Schilling type rudder. Your approach appears to ignore the loads on the shaft but those loads may be higher on a Schilling rudder which looks heavier and is not support by attaching at the bottom to a skeg.

Most rudders can be improved by drilling a hole somewhere near the trailing edge.

This gives a place to add a shackel , to which lines can be attached for emergency steering.

"Be prepared".

O C Diver said:

1. Rudder should be as tall or taller than the diameter of the prop.
2. Top of the rudder should come close to the hull so that water following the hull curve doesn't miss the rudder.
3. The leading edge (part in front of the shaft or pivot point) should be 25 to 28% of the trailing portion (part behind the shaft or pivot point).
4. Clearance between the leading edge and the propeller shaft is important. General rule of thumb is to have enough clearance to be able to remove the propeller with the rudder hard over. The more clearance between the propeller and the rudder, the less effective the rudder will be.

Ted

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Interesting.
My rudder fails rule #3. I haven't measured it but from this old photo I have, but the rudder stock appears to be only about 10% back from the leading edge.

I'd assume this would require more force to turn the rudder, although my steering seems light, even though its cable driven and the rudder is quite large.

AusCan,
Looks like you have a perfect rudder for your boat. So does Diver's Dive boat.
My hydraulic steering is not as light as the cable steering it replaced. Seal friction me thinks.
Don't back up very fast w that rudder (applies to me as well) because the water load on the rudder may snach the helm from your hands and slam the rudder against it's stops or tear out your mounting bracket loose or worse. I back 2 or 3 knots but hold on to the helm real tight and the rudder on hydraulic systems has far less feedback.

The Coot's rudder stays where it is set in normal conditions and adjusts with little effort.

AusCan said:

Interesting.
My rudder fails rule #3. I haven't measured it but from this old photo I have, but the rudder stock appears to be only about 10% back from the leading edge.

I'd assume this would require more force to turn the rudder, although my steering seems light, even though its cable driven and the rudder is quite large.

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The force required to turn the rudder greatly increases with speed. At hull speed it would be much less of an issue. If you think about a rudder with no leading edge, only about half of the water coming off the propeller would hit the rudder. As you add more leading edge, a greater percentage of the water off the propeller hits the rudder. At slow speeds under power, the water from the prop hitting the rudder is what pushes the stern over. More leading edge equals more force on the rudder equals more steering force. Some boats such as sail boats use a much larger rudder to use the flow of the water over the hull as the force against the rudder. Your boat may fall into that category when moving at cruising speed. But when going slow or docking, it is the water coming off the prop hitting the rudder that gives you steerage.

Ted

In the thread call a "rudder to be thankful for"around post 33 I posted a video and some spect on the schilling rudder I built. I could not be happier.


Here is a video on how well a fishtail rudder can work.

Thanks for the rudder lesson guys. I'm happy with my system, but it's nice to know the how's & why's.

I've never had an issue with reversing, but it's something I always do at low speed. I do feel a bit of pressure on the rudder when surfing waves, but the hull shape generally handles the situation very well.

Ditto what everyone has said about Mike Thompson. I never met the guy, but I think I would have really liked him. His generosity in sharing his experience was matched by his insightful write-ups.

Found this about Great Harbors with testing to confirm results: Fishtail Rudders Great harbour - Great Harbour Trawlers

bglad said:

Found this about Great Harbors with testing to confirm results: Fishtail Rudders Great harbour - Great Harbour Trawlers

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Interesting. This is good information but at the same time it is confusing. They tested by doing circles and turns. My issue with the rudder is in docking a single engine boat. The Great Harbours are twin engine boats with bow thrusters and I would have thought docking would be a non issue compared to us single engine guys. The fish tail rudder does significantly improve low speed manueverability so I guess the operator can rely more on steering and less on his thruster and engines when docking but it does sound like they solved a problem that didn't exist with the Great Harbours.