RE: [bolger] Oldshoe - rudder turbulence
*That is, a proper “fat” rudder…
Wayne Gilham
From:bolger@yahoogroups.com [mailto:bolger@yahoogroups.com]
Sent:Saturday, October 21, 2017 3:04 PM
To:bolger@yahoogroups.com
Subject:RE: [bolger] Oldshoe - rudder turbulence
Turbulence is KEY. I have direct experience – on a much different boat. My Irwin 43 center-cockpit sloop has semi-balanced rudder just aft of (and extending well-below skeg, for balanced portion) a 5” (approx. width) skeg.. and after a Bahamas reef-issue, I had to have rudder rebuilt. Yard “did me a favor” by rebuilding to modern practice of a long thin higher-aspect-ratio blade (really screwed up because no area ahead of pivot-point, so the resulting “barn-door” was REALLY hard to “put the helm over”) – but since blade was SO MUCH THINNER (just a pinch more than width of rudderpost, maybe 2-3” at most) than the skeg ahead of it, there was - I am assuming – ‘flow separation” such that the boat completely lost that “sense” of “being in the groove” – the helm was just DEAD, no feedback at all. I had Ted Irwin call back some of his retired workers and re-cast me a proper rudder* from the original molds, and Lo! the boat felt “alive” again…
So I think option “b” below will answer many of the existing problems on your OldShoe
Wane Gilham
Tacoma WA
Prior owner, Black Skimmer
Current owner, Idaho
From:bolger@yahoogroups.com[mailto:bolger@yahoogroups.com]
Sent:Friday, October 20, 2017 1:42 PM
To:bolger@yahoogroups.com
Subject:Re: [bolger] Oldshoe
I've been giving some more thought to the rudder of the aluminium Oldshoe...
Although I haven't found a/the pictures to see the keel and rudder currently on the boat, it occurred to me that if the rudder is indeed centred behind a 7" wide keel box, it must be working in absolutely horrendous turbulence.
The workarounds I can see to this are:
a) Try using parallel rudders (one each corner of the keel) to approximately original size. This won't solve the turbulence between the rudders, but should direct it somewhat and noticeably improve the direction of the flow of water as it runs past the sides of the keel an then the rudder. Those rudders could have endplates on the bottom, but my tendency would be not to overdo it on the inside edges (i.e. in the space between the rudders) as I reckon the turbulence in that area will still be pretty brutal. However, the parallel rudders will serve to effectively direct whatever flows through in approximately the right direction (and possibly also allow space for the outboard to clear properly in reverse - although this hasn't been a problem for me since I simply use the outboard trimmed up 2 stops).
b) Use a solid rudder, approximately the fore/aft and vertical dimensions as originally designed, but wedge shaped when viewed from above, so the width at the rudder post is similar to the width of the keel. You will need some sort of rubberoid flap or blocking to reduce turbulence around the rudder post (due to its disproportional size) and it would almost certainly also benefit from a bottom plate. The rudder only swings around 45° each side, so you don't (necessarily) need to use a 7" diameter rudder post (and the 3.5" gaps that would imply) although you will still need to allow clearance. You could make make the rudder body free flooding/draining, using relatively small flood holes to avoid it floating up.
c) Use parallel rudders clear of the sides of the box keel so they work in the flow of the water both sides of the keel. This has the advantage of clearing the turbulence at the aft end of the box keel but leaves unprotected rudders (and the question of how/where to mount them...
My inclination would be to try (a) first. Any thoughts anyone?
David
Turbulence is KEY. I have direct experience – on a much different boat. My Irwin 43 center-cockpit sloop has semi-balanced rudder just aft of (and extending well-below skeg, for balanced portion) a 5” (approx. width) skeg.. and after a Bahamas reef-issue, I had to have rudder rebuilt. Yard “did me a favor” by rebuilding to modern practice of a long thin higher-aspect-ratio blade (really screwed up because no area ahead of pivot-point, so the resulting “barn-door” was REALLY hard to “put the helm over”) – but since blade was SO MUCH THINNER (just a pinch more than width of rudderpost, maybe 2-3” at most) than the skeg ahead of it, there was - I am assuming – ‘flow separation” such that the boat completely lost that “sense” of “being in the groove” – the helm was just DEAD, no feedback at all. I had Ted Irwin call back some of his retired workers and re-cast me a proper rudder from the original molds, and Lo! the boat felt “alive” again…
So I think option “b” below will answer many of the existing problems on your OldShoe
Wane Gilham
Tacoma WA
Prior owner, Black Skimmer
Current owner, Idaho
From:bolger@yahoogroups.com [mailto:bolger@yahoogroups.com]
Sent:Friday, October 20, 2017 1:42 PM
To:bolger@yahoogroups.com
Subject:Re: [bolger] Oldshoe
I've been giving some more thought to the rudder of the aluminium Oldshoe...
Although I haven't found a/the pictures to see the keel and rudder currently on the boat, it occurred to me that if the rudder is indeed centred behind a 7" wide keel box, it must be working in absolutely horrendous turbulence.
The workarounds I can see to this are:
a) Try using parallel rudders (one each corner of the keel) to approximately original size. This won't solve the turbulence between the rudders, but should direct it somewhat and noticeably improve the direction of the flow of water as it runs past the sides of the keel an then the rudder. Those rudders could have endplates on the bottom, but my tendency would be not to overdo it on the inside edges (i.e. in the space between the rudders) as I reckon the turbulence in that area will still be pretty brutal. However, the parallel rudders will serve to effectively direct whatever flows through in approximately the right direction (and possibly also allow space for the outboard to clear properly in reverse - although this hasn't been a problem for me since I simply use the outboard trimmed up 2 stops).
b) Use a solid rudder, approximately the fore/aft and vertical dimensions as originally designed, but wedge shaped when viewed from above, so the width at the rudder post is similar to the width of the keel. You will need some sort of rubberoid flap or blocking to reduce turbulence around the rudder post (due to its disproportional size) and it would almost certainly also benefit from a bottom plate. The rudder only swings around 45° each side, so you don't (necessarily) need to use a 7" diameter rudder post (and the 3.5" gaps that would imply) although you will still need to allow clearance. You could make make the rudder body free flooding/draining, using relatively small flood holes to avoid it floating up.
c) Use parallel rudders clear of the sides of the box keel so they work in the flow of the water both sides of the keel. This has the advantage of clearing the turbulence at the aft end of the box keel but leaves unprotected rudders (and the question of how/where to mount them...
My inclination would be to try (a) first. Any thoughts anyone?
David