Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 01-09-2015, 19:35   #1
Registered User

Join Date: Jun 2015
Location: Nova Scotia
Boat: S&S Loki Yawl 38'
Posts: 89
Sailboat rudder design

I would like to get some input on three different rudder designs, attached to the keel, skeg hung and free standing spade. Based on the research I have done so far, along with my own personal experiences, here is what I have so far.

Attached (to aft end of the keel) rudder: Works similar to the flaps on an aircraft by altering the camber of the rudder/keel assembly. The advantages I see: Best rudder protection. Unlikely to catch lines, pots etc. The rudder shaft angle can be set up with the bottom of the rudder forward and the tope aft which reduces effectiveness while upright, but makes the rudder more effective when the boat is heeled and the rudder is highly loaded and more control is needed. The rudder shaft angle of the other two types are rudders seem to be either vertical or raked aft at the bottom, which looks nice but does reduce the rudder effectiveness when heeled. This kind of a rudder can be operated at large angles if needed (with a drag penalty) so little chance of losing control so long as you are strong enough to turn it. Disadvantages are pretty well known, cannot be easily balanced so has high torque loads as compared to a spade, lower efficiency and the largest turning radius of the three rudder types.

Skeg hung rudder: Again, the moveable rudder and the skeg act together as in the keel hung rudder providing variable camber. Advantages: Improved manuverability as compared to a keel hung rudder if properly designed. If a full skeg is used, the rudder cannot be balanced but retains much of the protection that the keel hung rudder has so long as the skeg is substantial. From the research I have done so far , these types of rudders stall between 24-27 degrees however it appears that while the drag increases significantly when stall is reached, the rudder apparently remains effective so control is maintained despite reaching stall according to the data I have seen. If anyone feels that this is incorrect please let me know since I don't have much personal experience with the skeg mounted rudder design and I have been looking at a boat with this type. Disadvantages: This type of rudder is apparently roughly 30-40 percent less efficient than the spade, hence the rudder/spade combination results in more overall wetted surface to generate the same turning moment as a spade rudder.

Spade rudder: This is the full flying foil. It is the most efficient of the three rudder types and provides the best manuverability both going forward and reverse. This control is further improved since it is relatively easy to position the prop wash close enough that it is quite effective. The spade is structurally the most challenging to build but the reliability seems to be getting better with time. This free standing design is the least protected and tends to catch lines, pots etc. It is sometimes possible to catch something between the top LE of the rudder and the hull which can potentially lock and disable the rudder. It appears that depending on the foil used and the thickness ratio that the stall angle is between 14-17 degrees for these rudders which is about 10 degrees less than the skeg mounted rudder. Stalling this rudder type per the data I have seen and also from my personal experience can result in a loss of control, it seems that there is no free lunch in boat design!

Any input appreciated!

James
__________________

__________________
Lokiyawl is offline   Reply With Quote
Old 01-09-2015, 20:55   #2
Registered User
 
Snowpetrel's Avatar

Join Date: Jul 2012
Location: Hobart
Boat: Alloy Peterson 40
Posts: 3,071
Re: Sailboat rudder design

Hi, I've seen this done a few times, on boats simmilar to yours. If it was me I'd go a strong, well engineered spade rudder. And maybe keep some of the old rudder as a trim tab. Trying to add a skeg to an old wooden boat is asking for problems. What is it you are trying to achieve? More speed, control, or maneuverability? Or all three? Another approach would be to add an auxiliary rudder suystem like a hydrovane or scanner autohelm.

This mod looks very well done

http://www.sponbergyachtdesign.com/CopernicusRud.htm

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app
__________________

__________________
My Ramblings
Snowpetrel is offline   Reply With Quote
Old 01-09-2015, 21:10   #3
Moderator
 
Don C L's Avatar

Join Date: Sep 2014
Location: Channel Islands, CA
Boat: 1962 Columbia 29 MK 1 #37
Posts: 4,370
Images: 34
Re: Sailboat rudder design

Sounds like you understand rudders very well. Now are you asking for what we prefer? Racing, a spade rudder is a must. But since I have seen a big spade rudder snapped clean off of a big racing boat I now look at all spade rudders with suspicion. I like my keel hung rudder. The matter of balance is important though so windward helm is not forcing you to yank on the tiller and effectively put on the brakes. Skeg hung is probably the best design for cruising.
__________________
DL
Pythagoras
1962 Columbia 29 MKI #37
Don C L is offline   Reply With Quote
Old 01-09-2015, 21:45   #4
Registered User
 
neilpride's Avatar

Join Date: Jul 2009
Location: in the world
Boat: csy 44 tall rig.
Posts: 3,099
Re: Sailboat rudder design

For me,, i can live with a strong , well engineered spade rudder , but oh well, the market is full of substandar stuff and probably those hig tech spade rudders are on expensive boats, skeg hung rudders are my favorite choice if they are build it for the task,,, i can live with the lack of maneuverability v spade rudder, after all its a small Price to pay ...
__________________
neilpride is offline   Reply With Quote
Old 01-09-2015, 21:54   #5
Registered User

Join Date: Jun 2015
Location: Nova Scotia
Boat: S&S Loki Yawl 38'
Posts: 89
Re: Sailboat rudder design

Thanks for the response. I don't have any intentions of modifying my existing boat, I think she is an excellent design as is. I am considering a larger boat for extended cruising so the purpose of opening this discussion was to get some opinions on rudder design. Thanks for the link. James

Quote:
Originally Posted by Snowpetrel View Post
Hi, I've seen this done a few times, on boats simmilar to yours. If it was me I'd go a strong, well engineered spade rudder. And maybe keep some of the old rudder as a trim tab. Trying to add a skeg to an old wooden boat is asking for problems. What is it you are trying to achieve? More speed, control, or maneuverability? Or all three? Another approach would be to add an auxiliary rudder suystem like a hydrovane or scanner autohelm.

This mod looks very well done

Copernicus Rudder

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app
__________________
Lokiyawl is offline   Reply With Quote
Old 01-09-2015, 22:13   #6
Registered User

Join Date: Jun 2015
Location: Nova Scotia
Boat: S&S Loki Yawl 38'
Posts: 89
Re: Sailboat rudder design

DL, I have also seen a foam filled skeg snapped off just from sitting on the bottom at the dock but it looked like a construction issue rather than design…lots of foam and almost no glass. While I do feel that the keel hung rudder has the potential to be the most protected and reliable, I think that the engineering is available to make any of these rudders reliable structurally. There seems to be quite a bit of information online regarding spade rudders ,but I am not finding a lot of data on the skeg hung despite there being quite a few designs that use that type. The idea of having a very manuverable (spade rudder) boat is certainly tempting to me but having reliable steerage under all conditions is probably higher up on my list. Most of my sailing has been with keel hung rudders and I have never experienced a loss of control despite some pretty bad conditions. It would be great to hear others experiences, especially under heavy conditions with the spade and the skeg hung rudders. Thanks, James

Quote:
Originally Posted by Don C L View Post
Sounds like you understand rudders very well. Now are you asking for what we prefer? Racing, a spade rudder is a must. But since I have seen a big spade rudder snapped clean off of a big racing boat I now look at all spade rudders with suspicion. I like my keel hung rudder. The matter of balance is important though so windward helm is not forcing you to yank on the tiller and effectively put on the brakes. Skeg hung is probably the best design for cruising.
__________________
Lokiyawl is offline   Reply With Quote
Old 01-09-2015, 22:26   #7
Registered User
 
DaveC's Avatar

Join Date: Oct 2009
Location: Melbourne, Florida
Boat: Belliure Endurance 35
Posts: 124
Send a message via Yahoo to DaveC
Re: Sailboat rudder design

A couple of more benefits of a keel-hung rudder with an angled post is that:

1: It allows for more rudder in the deeper, denser water which is great when you're in a seaway and the stern is lifting over crests, as it lets the rudder maintain bite in the water.

2: In a grounding, the rudder moves up off the ground as it is lifted from below and turns, letting the keel take the punishment.
__________________
DaveC is offline   Reply With Quote
Old 02-09-2015, 06:22   #8
Registered User
 
Snowpetrel's Avatar

Join Date: Jul 2012
Location: Hobart
Boat: Alloy Peterson 40
Posts: 3,071
Re: Sailboat rudder design

I could be happy with a well engineered and designed boat with any of these rudder types. And iv'e owned and sailed offshore with all three types.

My current boat has what is basically a spade rudder with a vestigal skeg/bustle. I think the biggest advantage of the spade is the excellent control while reversing, and the tight turning circle. None of the other types come close, This makes a huge difference to manoeuvring in a tight marina. Another benefit is simplicity, no underwater fittings like pintles to give problems, but of course it lacks the brute strength of a very well built skeg or keel hung rudder.

But from an engineering perspective a spade is very simple, and its hard to properly engineer and build a skeg to transfer the load properly into the hull (rather it's not hard, but often not done properly). A fair few steel boats and glass boats have had issues at the skeg root. Attached rudders tend to be more vulnerable in a grounding, due to the fact that the rudder is often the full depth of the keel.

The balance of a spade also gives a light helm as long as the bearings don't bind at speed due to the rudder flexing. A hybrid rudder (semi balanced), half skeg/ half spade works well but is more complex overall.

Another Pro for the spade is the ability to engineer sacrificial bottom section, and front to absorb impact without totalling the rudder. The Dashews seem to engineer them well.

If I was going to pick my ultimate rudder it would be some sort of super strong transom hung spade rudder (with crush zones) in a lifting cassette like some of the open 60's and catamarans are going for. I'd carry a small spare, and have the stern with a watertight bulkhead forward of the rudder so it can't flood the boat even if it gets ripped off. Something like a centreline version of these.



But I dislike twin rudders due to having sailed to Antarctica with them. Every bit of ice clunks into them and they seem way more vulnerable than a single centreline rudder. Also you get no prop wash unless you have twin engines or 3 rudders. The steering linkages also get complex.

But really its hard to separate the rudder question from the rest of the design. Two of my favorite boats are a Atkins Eric (basically a wetsail 32) and a Young 88 (Lightweight, fin keel, spade rudder, dinghy hull). Both poles apart, and both very good boats in very different ways.
__________________
My Ramblings
Snowpetrel is offline   Reply With Quote
Old 02-09-2015, 06:57   #9
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,027
Re: Sailboat rudder design

Skeg hung and keel hung rudders can be balanced, I don't have a picture but all you need of course is a "horn" forward of the hinge.
Spade rudders can be both reinforced with a lower attachment and extremely well protected as well, Island Packets although full keels, have spade rudders for example
__________________
a64pilot is online now   Reply With Quote
Old 02-09-2015, 07:53   #10
Registered User

Join Date: Aug 2010
Posts: 74
Re: Sailboat rudder design

Quote:
Originally Posted by Snowpetrel View Post
I could be happy with a well engineered and designed boat with any of these rudder types. And iv'e owned and sailed offshore with all three types.

My current boat has what is basically a spade rudder with a vestigal skeg/bustle. I think the biggest advantage of the spade is the excellent control while reversing, and the tight turning circle. None of the other types come close, This makes a huge difference to manoeuvring in a tight marina. Another benefit is simplicity, no underwater fittings like pintles to give problems, but of course it lacks the brute strength of a very well built skeg or keel hung rudder.
.

I agree that reversing with a keel-hung rudder is challenging, especially with all the spade-rudder guys watching as you try to back into a slip. I can almost hear them thinking, "wow, this guy doesn't know what he is doing". You want to scream, "It's a full keel! You have know idea how tricky this is!", but of course you just smile and wave.

However, under power, I can turn a very tight circle with my full-keel/ keel hung ("barn door") rudder. Having previously owned boats with spade rudders (with and without skegs), I was shocked how tightly I could turn this new (well, new to me, it's a 48 year old) boat. With the right amount of thrust and careful shifting, I can spin it around virtually within its own length, almost like a skid-steer forklift. I guess it has something to do with how far over I can throw the rudder, and how the propwash gets deflected out of the aperture.

Admittedly, under sail, I would have to agree with you about maneuverability. I have done a little PHRF racing with this boat, and the tactics are very different, especially at the start line and mark roundings. You really have to plan your moves ahead and be aware of the other boats around you because you simply can't spin the thing around as fast as the rest of the fleet (and, of course, the more experienced racers know this, and fully exploit my limited maneuverability).
__________________
Yukon Cornelius is offline   Reply With Quote
Old 02-09-2015, 08:16   #11
Registered User

Join Date: Jun 2015
Location: Nova Scotia
Boat: S&S Loki Yawl 38'
Posts: 89
Re: Sailboat rudder design

You raise some good points. I would be curious to know if the vestigial skeg/bustle at the top of your rudder might cause that portion of the rudder to behave as attached or skeg mounted? Have you ever encountered a situation where your rudder stalled and lost effectiveness? I am thinking that the upper portion might not stall until a higher AOA than the lower portion? There might also be some sealing benefits to the arrangement at the top of your rudder as well. Rudder ventilation (especially on transom hung rudders) seems to be an issue that comes up quite a bit.

I have also seen some keel mounted rudders that were designed to be almost the same depth as the bottom of the keel but most are not. Often times keel hung rudders have a mid point bearing so that even if the heel bearing is torn off the rudder retains enough support to remain useable. I have seen a few boats that ground on the rocks or coral to the point of dropping the internal ballast that still had a serviceable rudder.

The construction of a strong skeg seems to be complicated by having an aperture cutout. Also the portion of the rudder aft of the aperture would not benefit from being skeg mounted.

James

I could be happy with a well engineered and designed boat with any of these rudder types. And iv'e owned and sailed offshore with all three types.

My current boat has what is basically a spade rudder with a vestigal skeg/bustle. I think the biggest advantage of the spade is the excellent control while reversing, and the tight turning circle. None of the other types come close, This makes a huge difference to manoeuvring in a tight marina. Another benefit is simplicity, no underwater fittings like pintles to give problems, but of course it lacks the brute strength of a very well built skeg or keel hung rudder.

But from an engineering perspective a spade is very simple, and its hard to properly engineer and build a skeg to transfer the load properly into the hull (rather it's not hard, but often not done properly). A fair few steel boats and glass boats have had issues at the skeg root. Attached rudders tend to be more vulnerable in a grounding, due to the fact that the rudder is often the full depth of the keel.

The balance of a spade also gives a light helm as long as the bearings don't bind at speed due to the rudder flexing. A hybrid rudder (semi balanced), half skeg/ half spade works well but is more complex overall.

Another Pro for the spade is the ability to engineer sacrificial bottom section, and front to absorb impact without totalling the rudder. The Dashews seem to engineer them well.

If I was going to pick my ultimate rudder it would be some sort of super strong transom hung spade rudder (with crush zones) in a lifting cassette like some of the open 60's and catamarans are going for. I'd carry a small spare, and have the stern with a watertight bulkhead forward of the rudder so it can't flood the boat even if it gets ripped off. Something like a centreline version of these.



But I dislike twin rudders due to having sailed to Antarctica with them. Every bit of ice clunks into them and they seem way more vulnerable than a single centreline rudder. Also you get no prop wash unless you have twin engines or 3 rudders. The steering linkages also get complex.

But really its hard to separate the rudder question from the rest of the design. Two of my favorite boats are a Atkins Eric (basically a wetsail 32) and a Young 88 (Lightweight, fin keel, spade rudder, dinghy hull). Both poles apart, and both very good boats in very different ways.[/QUOTE]
__________________
Lokiyawl is offline   Reply With Quote
Old 02-09-2015, 08:44   #12
Registered User
 
Ericson38's Avatar

Join Date: Aug 2012
Location: Virginia
Boat: Taswell 49 Cutter
Posts: 187
Re: Sailboat rudder design

Quote:
Originally Posted by a64pilot View Post
Skeg hung and keel hung rudders can be balanced, I don't have a picture but all you need of course is a "horn" forward of the hinge.
Spade rudders can be both reinforced with a lower attachment and extremely well protected as well, Island Packets although full keels, have spade rudders for example
"The balance of a spade also gives a light helm as long as the bearings don't bind at speed due to the rudder flexing. A hybrid rudder (semi balanced), half skeg/ half spade works well but is more complex overall."

Bill Dixon was doing this on the Taswell line. To counter this, the Tayana 52 (Perry) and Hylas 49 (Stephens) don't have any part of the rudder forward of the post, so the skeg goes down to the bottom of the rudder.
Attached Thumbnails
Click image for larger version

Name:	IMAG0251.jpg
Views:	294
Size:	367.4 KB
ID:	108297  
__________________
Ericson38 is offline   Reply With Quote
Old 02-09-2015, 09:10   #13
Registered User

Join Date: Aug 2008
Location: New Bern, NC
Boat: Holman & Pye Red Admiral 36
Posts: 498
Re: Sailboat rudder design

Our rudder is a partially hung spade rudder on a skeg.

The problems with it were the lower rudder bearing, which fit into a shoe on the skeg. From the factory they were poorly attached and the shoe would fall off causing the rudder to break.

Other problems were the upper rudder stock was solid 1-3/4" stainless, while the lower rudder stock was 1-3/4" & 2" stainless tubing. This was done to save weight.

The fiberglass rudder blade was made in 2 parts and basically "glued" to the rudder shaft tabs, and the blade would come "unglued".

These were generic problems made when the boat was originally designed.

Plus with the rudder stock within the blade design you can't inspect the stock or tabs or the fiberglass joint to see how everything is survival over the years.

Since we are going to be going to very out of the way locations, we had a new solid stainless steel rudder made - much like on fishing boats and power boats. It is made out of 1/4" plate SS - no foil section. It does have re-inforcing ribs that foil sections could be added to at a later date, but we have found that it works fine as is.

The SS rudder shape has the same exterior outline as the fiberglass one.

We extended the solid 1-3/4" rudder stock to the full length of the rudder blade and changed the design of the lower rudder bearing into a bolt on 2-piece bearing. Since the overall rudder assembly is extremely long, we installed a shaft coupling right above the rudder arm so the rudder can be removed in 2-pieces - the blade from below and the upper shaft from above.

Since we have a SSB, I was able to use the rudder plate as out ground plate and the SSB performance has greatly improved. Since the rudder is 2-piece, we were able to weld a emergency tiller fitting directly onto the upper rudder bearing and rudder shaft.

The new rudder steers very easy and controls the boat better than the old fiberglass one. Plus it is extremely easy to work on/remove/maintain the new rudder.

Since everything on the new rudder is solid 316 SS, it wasn't cheap, but compared to other rudders was not to outrageous - and it will never break. Design, material and fabrication came to about $10,000.
__________________
Doug Brown is offline   Reply With Quote
Old 02-09-2015, 09:26   #14
Registered User

Join Date: Aug 2008
Location: New Bern, NC
Boat: Holman & Pye Red Admiral 36
Posts: 498
Re: Sailboat rudder design

Solid SS rudder picture - didn't attach to first post.
Attached Thumbnails
Click image for larger version

Name:	new rudder 002 (2).jpg
Views:	621
Size:	434.9 KB
ID:	108299   Click image for larger version

Name:	100_2004.jpg
Views:	243
Size:	410.9 KB
ID:	108300  

Click image for larger version

Name:	100_2137.jpg
Views:	195
Size:	400.1 KB
ID:	108301   Click image for larger version

Name:	100_2164.JPG
Views:	180
Size:	235.5 KB
ID:	108302  

__________________
Doug Brown is offline   Reply With Quote
Old 02-09-2015, 10:23   #15
Registered User
 
Ericson38's Avatar

Join Date: Aug 2012
Location: Virginia
Boat: Taswell 49 Cutter
Posts: 187
Re: Sailboat rudder design

Quote:
Originally Posted by Lokiyawl View Post

Spade rudder: It appears that depending on the foil used and the thickness ratio that the stall angle is between 14-17 degrees for these rudders which is about 10 degrees less than the skeg mounted rudder. Stalling this rudder type per the data I have seen and also from my personal experience can result in a loss of control, it seems that there is no free lunch in boat design!

Any input appreciated!

James
I used to push my Ericson 30 and later Ericson 38 to stall quite often, broad reaching from Santa Cruz Island in the afternoon winds in the Santa Barbara Channel. In other words, not really over-canvassed, boat balanced for the most part, but coming down some larger ones boat would round up, and therefore would have to let the main sheet go completely to get the rudder back. Winds in the high 20s. But a regular wind and wave pattern in that area. And these boats did not have wide sterns by any stretch. Boats both had the west coast deep fin keels, which were somewhat short lengthwise at the root (hull attach point).
__________________

__________________
Ericson38 is offline   Reply With Quote
Reply

Tags
rudder, sail, sailboat

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Rudder Design Dave6330 Monohull Sailboats 16 28-07-2013 04:17
Rudder Design for Beneteau mudness90 Construction, Maintenance & Refit 2 24-11-2011 08:06
Keel & Rudder Design GordMay Construction, Maintenance & Refit 1 16-08-2005 15:04



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 07:11.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.