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14-02-2022, 02:08
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#46
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Registered User
Join Date: Feb 2022
Location: Tasmania
Boat: Simpson 11 mtr
Posts: 15
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Re: Moving Rudder Position
Rudders where built with a ply web in between the tangs glassed and then glued cedar on and shaped and glassed
The balanced rudder design was a naca design of the web which worked out a 65 mm extension on the front of the rudder from the centre line of the stock and shaped
The rudders then controlled the boat 95% better
Over the weekend I will take some pictures of the outboard slides
I have noticed easy cat have a under bridge set up but the motors don’t drain and the ones I know of have a lot of corrosion problems
Mine always drain when lifted
Yes they do cavitie a bit in tide against wind situations at sea in big seas they don’t have a problem
Mine are adjustable up and down so in some situations I adjust up a bit and gives better speed which is interesting
Yes the outboard supports at the bottom do bang a bit but not bad in some seas (angle)
The outboard propeller is about level with the bottom of the boat
3500 rpm on 2
20 hp Honda gives a good 6 knots at just under 3 litres a hour
Any thing else you would like to know I am happy to share
With outboards you don’t have that horrible diesel smell happy wife
If going with outboards when looking the distance from the bottom of the mount to the cave plate is the one that matters Hondas have a big mount but I like Honda outboards
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14-02-2022, 05:20
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#47
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Registered User
Join Date: Oct 2011
Boat: KH 49x, Custom
Posts: 1,765
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Re: Moving Rudder Position
Planfour4:
Very nice job with your mods to the original design. The profile looks great!
Cheers.
Paul.
__________________
If you can dream it; with grit, you can do it.
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14-02-2022, 06:06
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#48
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Writing Full-Time Since 2014
Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 10,278
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Re: Moving Rudder Position
Quote:
Originally Posted by CMNZ
Wingsail, sounds sensible, thanks,
Grit/Franziska
If I were to put winglets on would you suggest something along the lines as below? A or B or something else?
Thanks
Craig
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Niether. Like this (from Ian Farrier, the designer of the F-boat trimarans). Obviously, the dimensions vary with the boat.
The location is forward, not aft, because the purpose is different (anti-ventilation, not anti-vortex). I know they work because I ventilated at about 15 knots before, and now I don't.
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14-02-2022, 06:12
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#49
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Writing Full-Time Since 2014
Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 10,278
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Re: Moving Rudder Position
I extended my PDQ 3 feet, as well as some other changes to the sailplan and keels, without moving the rudders. I don't think moving them would have made things better, though in retrospect I might have gone a few inches deeper.
I'm sure it all depends on the total design.
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14-02-2022, 07:55
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#50
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Registered User
Join Date: Oct 2015
Location: Beaufort, NC
Posts: 739
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Re: Moving Rudder Position
Quote:
Originally Posted by CMNZ
I am completing the building of a Simpson 11m which has been lengthened to 11.6m by adding sugar scoops. I want to move the rudders further aft from the original design but it will mean that the tip of the rudders will (using existing design - lengthened to achieve same depth) extend above the waterline.
Is this likely to cause any deleterious effects and if so should I modify the top of the rudder design so that it is entirely below the waterline and how far? This will mean the rudder is no longer "hugging" the bottom of the hull.
Below is the original plan showing the original waterline and I have drawn on where the new waterline will be (approximately) when the rudder is moved aft. the circle indicates the amount of rudder likely to be above the WL.
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I am assuming the scoop is out of the water. Therefor why move the rudder aft? Second keep the top of the rudder a close to the bottom of the boat. That will help with the laminar flow thus increase your speed.
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14-02-2022, 08:27
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#51
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Registered User
Join Date: Oct 2011
Boat: KH 49x, Custom
Posts: 1,765
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Re: Moving Rudder Position
Quote:
Originally Posted by CMNZ
I am completing the building of a Simpson 11m which has been lengthened to 11.6m by adding sugar scoops. I want to move the rudders further aft from the original design but it will mean that the tip of the rudders will (using existing design - lengthened to achieve same depth) extend above the waterline.
Is this likely to cause any deleterious effects and if so should I modify the top of the rudder design so that it is entirely below the waterline and how far? This will mean the rudder is no longer "hugging" the bottom of the hull.
Below is the original plan showing the original waterline and I have drawn on where the new waterline will be (approximately) when the rudder is moved aft. the circle indicates the amount of rudder likely to be above the WL.
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Wow.
I'm usually more perceptive than this, and I obviously didn't open the photo before now.
You don't have to do anything, you're fine. You still have an endplate, and your stern wave will fill that spot.
I'm always bitching to myself about folks not reading the OP, and I'm certainly guilty of that in this thread.
In that photo, the top corner of the rudder is exposed in flat water. If I understand the photo correctly, you're saying you have the back 20 cm or so above the water, that's nothing.
My apologies. Don't do anything to your rudders. No fences, no lengthening, nothing. You're absolutely fine the way it is.
Cheers.
Paul.
__________________
If you can dream it; with grit, you can do it.
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14-02-2022, 08:30
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#52
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Registered User
Join Date: Feb 2017
Location: Washington DC
Boat: Hunter Legend 405
Posts: 67
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Re: Moving Rudder Position
CMNZ -- kudos for undertaking an amazing project! Having surface-piercing rudders, though, is an extremely bad idea. You might be able to compensate by using ventilation fences as proposed by other posters, but chances are good that without experimentation, the fences will have many disadvantages and may not work. I did my PhD thesis on surface piercing lifting surfaces. Would be glad to talk to you if you like. Don't know if emails are allowed on this forum but my email is richard@rothblum.org
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14-02-2022, 08:36
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#53
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Registered User
Join Date: Oct 2011
Boat: KH 49x, Custom
Posts: 1,765
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Re: Moving Rudder Position
Quote:
Originally Posted by Rothblum
CMNZ -- kudos for undertaking an amazing project! Having surface-piercing rudders, though, is an extremely bad idea. You might be able to compensate by using ventilation fences as proposed by other posters, but chances are good that without experimentation, the fences will have many disadvantages and may not work. I did my PhD thesis on surface piercing lifting surfaces. Would be glad to talk to you if you like. Don't know if emails are allowed on this forum but my email is richard@rothblum.org
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Id like to suggest that everyone read the OP, and look at the photo. I just did it myself. I think we're all barking up the wrong tree. The circled part is above the static waterline, that's all. Not even worth thinking about. That rudder will be fully immersed at 5 knots of boatspeed, and the hull is the endplate.
Boy do I feel silly for not properly reading the OP, and actually opening the photo.
Cheers all.
Paul.
__________________
If you can dream it; with grit, you can do it.
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14-02-2022, 09:02
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#54
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Registered User
Join Date: Dec 2011
Location: Fremantle
Posts: 560
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Re: Moving Rudder Position
Rothblum. Surface piercing rudders ?
Get Real. What ********.
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14-02-2022, 09:03
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#55
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Registered User
Join Date: Dec 2011
Location: Fremantle
Posts: 560
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Re: Moving Rudder Position
What Bull excrement!
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14-02-2022, 09:17
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#56
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Registered User
Join Date: Jan 2015
Posts: 571
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Re: Moving Rudder Position
Better too far aft than forward. Moving aft increases the fulcrum of the lever arm from center of lateral resistance to rudder. This give more power (longer lever arm), and reduces helm angle because of the longer arm. So X degrees of deflection will not turn quite as much in calm condition (may take a little more helm for same results. But when the wind and seas start to (bully) the boat around, you will have more power and authority to change the boats course.
I know people that had Alberg 35’s, a great boat but did have a rudder quite far inboard. It made the helming less authoritve and not able to track as well in a seaway. (Still great sea boats but did take a good amount of steering) so some owners install a second rudder further aft on the rather long overhanging stern. She was 35 feet but only a 24 foot waterline length. They liked the results and just used the old rudder to trim weather helm. I thought of doing this but never did.
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14-02-2022, 13:26
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#57
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Registered User
Join Date: Jan 2022
Location: Auckland, New Zealand
Boat: Simpson 11.6
Posts: 26
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Re: Moving Rudder Position
Quote:
Originally Posted by catsketcher
CM - I have been through many iterations with outboards. If you are okay with losing some interior space I can send you some pics of my outboard setup. I like it as the pods lower only when needed and rotate out of the way and flush with the bridgdeck when sailing. I lose some space inside but am much happier with the lack of banging and wetting the outboards.
The pods are made from 9mm ply - ply is a good fit here. I removed the hydraulic tilt mechanism from the Yammie 25 and bolted it with the large tilt bolt and lashings down low. Works really well. The tilt mechanism is in the shed if needed anytime.
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That would be great, thanks
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14-02-2022, 20:26
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#58
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Registered User
Join Date: Aug 2009
Location: Lake Macquarie NSW
Boat: Chamberlin 11.6 catamaran
Posts: 991
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Re: Moving Rudder Position
Gday Richard,
I would love to know more about the issues with surface piercing foils. I think there should be lots of data on them as the Americas Cup boats all have surface piercing rudders and other foils. I see that AC boats don't have fences, so maybe the idea of fences is a little old, although I did see some whale-like protuberances on one set of foils (American Magic perhaps?).
I remember talking to Andy McDougall - the designer of the Mach 2 moth. He said that the tolerances needed for the Moth foils could only be produced in steel molds formed by CNC machines. So I get that foils are tricky business. I tend to only worry about using NACA 00 series - 0012 is a good choice for my boats and to be truthful I have difficulty getting to fine tolerances with this simple shape.
Can you give us a short precis on what your Phd was about and what you found out? I know that fluid dynamics is really complicated and Navier Stokes is non-linear and all that. But are there some simple concepts that us non-engineers/students can learn about our foils?
cheers
Phil
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14-02-2022, 20:26
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#59
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Registered User
Join Date: Aug 2009
Location: Lake Macquarie NSW
Boat: Chamberlin 11.6 catamaran
Posts: 991
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Re: Moving Rudder Position
Quote:
Originally Posted by CMNZ
That would be great, thanks
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I will make a video when I next go to the boat - send me a PM and I will email the whole shebang going up and down.
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26-02-2022, 02:08
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#60
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Registered User
Join Date: Feb 2022
Location: Tasmania
Boat: Simpson 11 mtr
Posts: 15
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Re: Moving Rudder Position
Hi some pictures of our outboard slide
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