Dagger rudder idea on an aft raked transom. Would it work?

[Mike1902]

Hello All,
I have been playing around with rudder options and ideas.
As the title suggests, this is one of the ideas, will it work, I'm unsure.
So I'm putting it out there.
First a diagram that is not to scale but shows what's up.

IMG-7466.jpg

This particular idea is to make a cassette that looks a little like the top of a barn door rudder, it will house the dagger rudder.
Now here's the thing, because of the transom rake, dropping the foil in vertically means that the leading edge of the foil will be about 165mm ( 61/2") away from the transom at the waterline.
My questions are, would this gap be to much? Would it affect the efficiency of the rudder? Would an end plate at the bottom edge of the cassette be effective?
Any comments or thoughts would be appreciated.
Thanks in advance.

Cheers,
Mike.

[Peerie Maa]

You could make it stronger by taking the cassette down to the bottom of the skeg.
Forward sloping rudders also work.
P1060150.jpg
Less likely to break if you touch bottom.

[Edward Pearson]

Heavily raked rudder heads like the one you've drawn are poor design and best avoided. At moderate to large rudder angles you get enormous change in the perpendicularity of the rudder foil. Your tiller will unlikely clear the gunwale if you wanted it to either. 'Big boats' with forward raked rudders are a distortion to lower wetted area under the waterline. The cobles are unique in using the the deep aft rudder and the immersed forefoot to combine to give a CLR somewhere normal in the middle but while the forefoot immersion keeps the boat to the waves when backed into the beach. Unique and clever but they also had a bad reputation on a run due to the immersed forefoot!

You want to extend your transom out to nearly or plumb vertical. Have the the rudder cheeks out of the water to avoid resistance. Have the geometry of your foil so that when it's raised its level with the bottom of the boat so it won't hit but and you've still got steerage. To get extra width to do that, choosing an elliptical planform gives you a wider chord width at the root (more space for machine guns in the spitfire) over a straight rudder, so you can get the pivot point a bit away from the bottom of the rudder head. Reinforce the pivot through a rudder strap.

Coming into a beach you don't want to be fart arsing over the back of the transom lifting a rudder along with everything else. Pivoting rudder unless some other compelling reason like building a GIS.

You want the rudder as close to the skeg as possible for max efficiency. Your's is sitting in turbulent wake off the skeg. If you do that, choose an especially fat 4 digit NACA section to help reattach flow.

[Woxbox]

I made one like this, but with the rudder blade parallel to the raked transom. It works well on a 15' Redmond Whisp.
Photos in my thread "Whisp B&B"

[johngsandusky]

It should work.

[TerryLL]

The transom rake that's shown in the OP is less than the rake typically found in sailing dories, so a raked rudder post is not a problem. A better option than the dagger rudder would be something akin to the kick-up rudder used in the Oughtred double-enders, which have an angled rudder headstock that encloses a blade that drops to a vertical position.

[P.I. Stazzer-Newt]

That design gives a large distance between the centre of effort of the rudder blade, and the pivot axis.

I've sailed boats with that design error and they are shockingly hard mouthed.

[John B]

King of the mockups Mike, you might get an unfair advantage in the light with that there mirage drive esq setup.

[John Meachen]

I had one on a Moth in the late 70's and wouldn't have another.Worst of both worlds in that you could hit something and lose a rudder-or transom,and there wasn't the positive feeling of a fixed rudder.A well built pivoting rudder is a much more practical proposition.I would keep the blade slender and not far from the pivot axis.A lot of sweep back causes a massive increase on the loading of the gudgeons and pintles but doesn't give a correspondingly increased turning moment because the slight increase in distance to the CLR is of much less consequence.Edward's suggestion of an effective NACA section is a nudge in the right direction as it will generate a greater force than a board with the corners removed.

[Mike1902]

You could make it stronger by taking the cassette down to the bottom of the skeg.
Forward sloping rudders also work.
P1060150.jpg
Less likely to break if you touch bottom.

Thanks for the input Nick.
I tried the forward sloping option.
IMG-7470.jpg
With the current configuration the balanced percentage worked out at about 22%. Alright going slow but could get a little squirrely off the wind, adjustments can be made of course.
The foot shape of the rudder in this photo would not be used if going this way.
If using a dagger rudder, it would mean that the pull would be up and away, something that would feel awkward I think, so a pivoting option would be better.
As far as the cassette goes, I would like to keep most of it out of the water to reduce the drag.

Cheers,
Mike.

Heavily raked rudder heads like the one you've drawn are poor design and best avoided. At moderate to large rudder angles you get enormous change in the perpendicularity of the rudder foil. Your tiller will unlikely clear the gunwale if you wanted it to either. 'Big boats' with forward raked rudders are a distortion to lower wetted area under the waterline. The cobles are unique in using the the deep aft rudder and the immersed forefoot to combine to give a CLR somewhere normal in the middle but while the forefoot immersion keeps the boat to the waves when backed into the beach. Unique and clever but they also had a bad reputation on a run due to the immersed forefoot!

You want to extend your transom out to nearly or plumb vertical. Have the the rudder cheeks out of the water to avoid resistance. Have the geometry of your foil so that when it's raised its level with the bottom of the boat so it won't hit but and you've still got steerage. To get extra width to do that, choosing an elliptical planform gives you a wider chord width at the root (more space for machine guns in the spitfire) over a straight rudder, so you can get the pivot point a bit away from the bottom of the rudder head. Reinforce the pivot through a rudder strap.

Coming into a beach you don't want to be fart arsing over the back of the transom lifting a rudder along with everything else. Pivoting rudder unless some other compelling reason like building a GIS.

You want the rudder as close to the skeg as possible for max efficiency. Your's is sitting in turbulent wake off the skeg. If you do that, choose an especially fat 4 digit NACA section to help reattach flow.

Thank you Edward for the feed back.
The transom on the dinghy has a negative rake of 20 degrees off vertical, so that is a given, hence the idea of moving the leading edge of the foil to vertical in the cassette, the negative being the gap created between the keel and blade. I did read about some practical experiments tried on seven different rudder configurations for a Finn class dinghy where the best performing rudder was mounted off the transom, only an inch mind you.
The NACA foil section for the rudder will be 0015, the dagger board below is a 0010.
IMG-4367.jpg
As far as fart arsing goes, I used to sail an OK racing dinghy, that had a fixed rudder, that made coming and going fun, they finally allowed pivoting rudders in 2020.
The tiller will have lift to clear my knees as well as compensate for the lateral movement.
Thanks again for your input Edward, duly noted.
Cheers,
Mike.

[Mike1902]

I made one like this, but with the rudder blade parallel to the raked transom. It works well on a 15' Redmond Whisp.
Photos in my thread "Whisp B&B"

I checked out your thread Dave, very cool.
I guess what I'm trying to achieve here is a classic look (Barn door style) with some performance.
That means a compromise between practicality and aesthetics. The bungy cords are a safe bet but I would rather they were not visible on this particular boat.
Good to know about the forward rake working out, will bear that in mind.
Cheers,
Mike.

It should work.

Thanks John. I think it just may as well.

Cheers.

The transom rake that's shown in the OP is less than the rake typically found in sailing dories, so a raked rudder post is not a problem. A better option than the dagger rudder would be something akin to the kick-up rudder used in the Oughtred double-enders, which have an angled rudder headstock that encloses a blade that drops to a vertical position.

Thanks Terry.
Most of those Oughtred designs have the tiller coming in over the stern. My intention is to have the tiller coming through the transom, which will limit the height of the rudder headstock.
In saying that, if sturdy enough, the headstock should offer enough support to the rudder.

Cheers,
Mike.

That design gives a large distance between the centre of effort of the rudder blade, and the pivot axis.

I've sailed boats with that design error and they are shockingly hard mouthed.

I hear you Doug.
The transom is 20 deg off vertical, the forward edge of the rudder moves from 90 deg to 87 deg with a 30 deg tiller angle. The foreward bottom point of the blade moves about 3 inches forward.
Moving the leading edge directly under the pivot axis just about eliminates all of that.

Cheers,
Mike.

King of the mockups Mike, you might get an unfair advantage in the light with that there mirage drive esq setup.

Hah! You may have just given me the answer I need John. Whack on one of those Hobie numbers, problem solved
IMG-7469.jpg
Here is a mock up of the original idea. It's that distance between the transom and the leading edge of the foil that has got me wondering if there will be any significant loss in steering performance.

Cheers.

I had one on a Moth in the late 70's and wouldn't have another.Worst of both worlds in that you could hit something and lose a rudder-or transom,and there wasn't the positive feeling of a fixed rudder.A well built pivoting rudder is a much more practical proposition.I would keep the blade slender and not far from the pivot axis.A lot of sweep back causes a massive increase on the loading of the gudgeons and pintles but doesn't give a correspondingly increased turning moment because the slight increase in distance to the CLR is of much less consequence.Edward's suggestion of an effective NACA section is a nudge in the right direction as it will generate a greater force than a board with the corners removed.

Thanks for the advice John.
There was always going to be a vertical foil incorporated into the mix somehow. What position that will be in is the answer I'm looking for.

Thanks again everyone for the feed back.
Currently working on the pivoting option.

Cheers,
Mike.

[Edward Pearson]

One issue with a forward raked rudder is at hull speed the water flow is coming up 'out of the trough' towards the transom along 'the run'. Back in the 70’s the yacht design books then positioned the rudder axis perpendicular to this angle. You see alot of IOR age yachts with the skeg for the rudder raked perpendicular to the hull bottom ‘run’ for this reason.



Anyhow, a boat worth taking inspiration from at your size and shape: the 14ft Western Skiff by the esteemed Nigel Irens/ Ed Burnett also has a heavily raked transom like your boat (its a pilot cutter inspired sail and oar dinghy in outline) and has its rudder arranged thus. It may be worth a close perusal. I’m pretty sure that rudder was in the online download planset for Ed Burnett’s chosen sailing charity donation. You can get them printed abd its all full size parts. You should be lining up ‘the next one’ at this point in the build. Well if you have the rudder, the rest if the boat would be an option later. Or call it ‘research’.

Nice job on your foils. This company sells ally naca nose sections or naca extrusions if you need them. I have a 29er rudder and daggerboard ready for the right project stored away in the back of my wardrobe gor the right project one day...

http://www.windknife.com/

[Mike1902]

One issue with a forward raked rudder is at hull speed the water flow is coming up out if the trough towards the transom. Back in the 70’s the yacht design books then positioned the rudder axis perpendicular to this angle. You see alot of IOR age yachts with the skeg for the rudder raked back for this reason.



Anyhow, a boat worth taking inspiration from at your size and shape: the 14ft Western Skiff by the esteemed Nigel Irens/ Ed Burnett has a heavily raked transom (its a pilot cutter inspired sail and oar dinghy in outline) and has its rudder arranged thus. It may be worth a perusal. I think that rudder was in the online download planset for Ed Burnett’s chosen sailing charity donation.

Nice job on your foils. This company sells ally naca nose sections or naca extrusions if you need them.

http://www.windknife.com/

Thanks for the info Edward.
I will check out that Western Skiff.
Below is a link to the site that gave me some instruction about foil builds.
How to Build Rudders or Dagger Boards | Small Trimarans

Otherwise have a look at my posts in Building and Repair from page 8 onwards.
http://forum.woodenboat.com/showthre...clinker-dinghy

Cheers,
Mike.

[Max F]

I changed to a forward raked rudder on my Sooty Tern some time ago and are so happy I did.
https://www.flickr.com/photos/seaand...57704454366205
The strain on the gougeons and pintles decreased extremely.
The rudder became next to ballanced and the boat turns much quicker on a much tighter curve.
I had no negative effects on the run or anything else at all.
My new Cape Henry will have for this reason a quite similar kick up rudder.

Cheers
Max

[John B]

I checked out your thread Dave, very cool.
I guess what I'm trying to achieve here is a classic look (Barn door style) with some performance.
That means a compromise between practicality and aesthetics. The bungy cords are a safe bet but I would rather they were not visible on this particular boat.
Good to know about the forward rake working out, will bear that in mind.
Cheers,
Mike.


Thanks John. I think it just may as well.

Cheers.



Thanks Terry.
Most of those Oughtred designs have the tiller coming in over the stern. My intention is to have the tiller coming through the transom, which will limit the height of the rudder headstock.
In saying that, if sturdy enough, the headstock should offer enough support to the rudder.

Cheers,
Mike.



I hear you Doug.
The transom is 20 deg off vertical, the forward edge of the rudder moves from 90 deg to 87 deg with a 30 deg tiller angle. The foreward bottom point of the blade moves about 3 inches forward.
Moving the leading edge directly under the pivot axis just about eliminates all of that.

Cheers,
Mike.



Hah! You may have just given me the answer I need John. Whack on one of those Hobie numbers, problem solved
IMG-7469.jpg
Here is a mock up of the original idea. It's that distance between the transom and the leading edge of the foil that has got me wondering if there will be any significant loss in steering performance.

Cheers.



Thanks for the advice John.
There was always going to be a vertical foil incorporated into the mix somehow. What position that will be in is the answer I'm looking for.

Thanks again everyone for the feed back.
Currently working on the pivoting option.

Cheers,
Mike.

The distance isn't the issue , Mike, it's the action of the heel of the rudder because it's hinge point is on an angle. A pencil attached to the front lowest edge of the blade will scribe a line instead of staying put on the axis. It's not like a raked rudder like we have on Janet or Waione.
How important that is, I don't know.

[Mike1902]

I changed to a forward raked rudder on my Sooty Tern some time ago and are so happy I did.
https://www.flickr.com/photos/seaand...57704454366205
The strain on the gougeons and pintles decreased extremely.
The rudder became next to ballanced and the boat turns much quicker on a much tighter curve.
I had no negative effects on the run or anything else at all.
My new Cape Henry will have for this reason a quite similar kick up rudder.

Cheers
Max

Very nice Max, that's some pretty funky hardware you have on that rudder head.

This is the latest version in the scheme of things.
As you can see I'm determined to have a dagger arrangement, but now leaning toward the balanced version.
IMG-7472.jpg
Made the chord 300mm (above), then realised that in the horizontal it extended between 20-30 mm, so reduced the chord to 280mm (below).
IMG-7477.jpg
This made for a 22 percent area forward of the axis, which is a bit on the high side.
Shaping the foot could alter this area and reduce the percentage somewhat, highlighted by the scribbled lines.
There is a line drawn for a 250mm chord as well (270mm in the horizontal), a work in progress. That makes for a 42mm thick board at the 30% mark for a 0015 NACA.

Then I checked the height above the transom when lifted to clear the bottom keel line.
IMG-7476.jpg

That worked out at 300mm, all good there.
Lifting the rudder will be interesting as the direction is up and away, which may feel a little awkward but it will be hollow which should help.
An interesting exercise.

Cheers,
Mike.

[Mike1902]

The distance isn't the issue , Mike, it's the action of the heel of the rudder because it's hinge point is on an angle. A pencil attached to the front lowest edge of the blade will scribe a line instead of staying put on the axis. It's not like a raked rudder like we have on Janet or Waione.
How important that is, I don't know.

Thanks John.
The front edge you mention does move around a lot.
As you can see , I have just posted the latest version which reduces that movement quite considerably.
The balance % is maybe a little high but looking at the Sooty Tern in comparison makes me think it should be ok.
Chord length is the next decision.

Cheers,
Mike.

[Max F]

Hey Mike

At first thought your rudder seems to be pre-balanced 22%
That is what most folks (inkluding me once) think.
But the important one to meassure from is the turning axis instead the perpendicular.
In real the rudder on that picture is not pre- balanced at all but the area is very near the turning axis.
The force on the tiller is greatly reduced by that.

Glad you liked the hardware.
That was good fun building. https://www.flickr.com/photos/seaand...57626033830344

Cheers
Max

[Mike1902]

Hey Mike

At first thought your rudder seems to be pre-balanced 22%
That is what most folks (inkluding me once) think.
But the important one to meassure from is the turning axis instead the perpendicular.
In real the rudder on that picture is not pre- balanced at all but the area is very near the turning axis.
The force on the tiller is greatly reduced by that.

Glad you liked the hardware.
That was good fun building. https://www.flickr.com/photos/seaand...57626033830344

Cheers
Max

AHA! Yes, thank you Max. Also what JB was alluding too in regard to the hinge point at the base of a raked rudder, it's not actually a balanced rudder at all.
Now I can sleep

Thanks for that Max

[skuthorp]

My Outred Macgreggor has a forward facing rudder now, in a cassette held by a tied on rubber wedge. If I hit bottom it just kicks up, and I sail it in very shallow water. But I have 2 0ther rudder arrangements, and often sail it with out one at all relying on weight distribution and a 6ft paddle if I need one. I can make no comment re efficiency or not.

[Mike1902]

My Outred Macgreggor has a forward facing rudder now, in a cassette held by a tied on rubber wedge. If I hit bottom it just kicks up, and I sail it in very shallow water. But I have 2 0ther rudder arrangements, and often sail it with out one at all relying on weight distribution and a 6ft paddle if I need one. I can make no comment re efficiency or not.

Thanks for the pointers Jeff.
If the dagger version doesn't work out, then the next option would be the kick up version.
The dinghy has a beam over twice that of a MacGregor and potentially 2 1/2 times the sail area, I will definitely need a rudder
Nice looking boat those MacGregor's.

Cheers,
Mike.

[Gary Dierking]

You can have a dagger rudder that kicks back and it works really well. https://www.storerboatplans.com/foil...dagger-rudder/

[Mike1902]

You can have a dagger rudder that kicks back and it works really well. https://www.storerboatplans.com/foil...dagger-rudder/

Thanks for link Gary.
I can see how that setup would work well and certainly minimize any damage in the event of a collision.
It's the exposed bungy cords, they are just not in keeping with the look I'm after for this boat.
I may rue the day for not going with a more forgiving concept but hey...you can't live in fear.

Cheers,
Mike.

[Mike1902]

This is a thank you to everyone who contributed to this thread with suggestions, observations, opinions etc, it has been very beneficial.
A plan of attack has been made, two plans actually but only one will be implemented initially, the results will be posted on the "believed abstraction "thread.
Thanks again.
Cheers,
Mike.