Rudder Design

Bill

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not anywhere near this phase of project but looking for feedback regarding stainless rudder design.. Im planning on cutting the glass off the barn door I have now and reusing the shaft and going with 3/8" stainless plate for the rudder. cutting the shaft into 2 halves, having the "3/8" slot machined into the shaft halves ( about 6") and then welding them on to the stainless plate..

What is the general rule of thumb for the measurements of the leading edge ( forward of the shaft) the radius of the corners, and the clearance of the rudder from the top of the rudder to the bottom of the boat and the bottom of the rudder to the skeg?

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Blitzen

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That is a loaded question for sure. Rudder design is all about the intended purpose, some aspects are good for one thing and bad for others. Are you going to be idling around lobster pots all day or are you going to be speeding off to the tuna grounds? Depending on what you are trying to do with the boat will have a big effect on the design of the rudder.
 

Bill

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right, should have said that.. looking for an "average" rule of thumb..not going to be lobstering with this one.. speed is more of a concern
 

captainlarry84

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Bill, I would leave the stock rudder intact & use it as a model to build the new rudder. The amount of money you save by reusing the original stock is pennies. I ALSO PERFER BRONZE OVER SS IT GETS ALONG BETTER WITH THE BRONZE WHEEL.
 

Toolate

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What is in place on the keel to support the bottom of the post? Just curious.

Also, why a new rudder?
 

Bill

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Bill, I would leave the stock rudder intact & use it as a model to build the new rudder. The amount of money you save by reusing the original stock is pennies. I ALSO PERFER BRONZE OVER SS IT GETS ALONG BETTER WITH THE BRONZE WHEEL.

Ok Larry.. dont you think this guy is a little big?

Did I hear "SPEED"???
yes , if I get a trailer it may come up to maine during the summer to partake

What is in place on the keel to support the bottom of the post? Just curious.

Also, why a new rudder?

the shoe at the bottom is gone, needs to be replaced.. the rudder has seen better days I do not see it lasting a lot longer
 

Bill

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also while we are at it.. why dont most boats use this config ?

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Keelboater

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Bill - I was just reading about rudders the other day because I am considering going to larger ones in my Bertram. You will be limited in the leading edge length due to prop removal clearance. Have you operated this boat at all, or was it strictly a project to begin with? If you did operate it, was it a ballanced feel at the helm or was there excessive torque required to steer? The rule of thumb for balance is to get 17% of the blade area ahead of the pivot shaft (leading edge in your photo) and 83% behind it. In the first photo it looks like you may have an unbalanced rudder which may require a bit of torque at the helm. You may have to add to the leading edge, but too much "leading edge area" creates a situation where the center of pressure is moved ahead of the pivot and it can lock up the rudder hard over. Be careful there. As Blitzen suggested, speed plays a big part in the rudder design. Being a DE work boat however, the speed is most likely not that fast. "Wing shaped rudders" are more efficient than the flat blade type which is sometimes the reason for glassing them over.
 

BillD

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Bill,

Here's a picture of the rudder on my 25 T Jason.
I put about 12 hours off "sea rial" time on the boat before starting the rehab/repower project.

The rudder on my 25 is fiberglass but a little different in shape than the one on your 25.

I'll be keeping my rudder for now. The boat tracks like it's on a rail in any sea and the boat WILL back up L or R with the rudder in river current.

I'm very pleased with how the 25 T Jason handles.
Needs more hp than 124 hp but that is on it's way later ;)

I think you'll be pleasantly surprised on how well your 25 T Jason handles when you getting spiffed up, repowered and in the water !

Bill D

IMG_1269.jpg

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Bronze rudder

Bill. Ideally you try to have 20% on the leading edge. For your new rudder, I would consider one cast of manganese bronze with stainless shafts pinned in place. Manganese has a high tensile strength, the blade thickness for a rudder of your size would be approx. 3/8 +/-. The transition where the blade and stocks are inserted can be very fair allowing for good water flow and a hole can be bored in the rudder so the propeller shaft can be removed with out having to drop the rudder. I definitely would not reuse the stainless shaft that is in your current rudder
 

Bill

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Boils down to money, time, and effort.

thanks, it just seems to be a lot of stress on the packing gland of the rudder..I may explore this option if I have "time"..

Bill - I was just reading about rudders the other day because I am considering going to larger ones in my Bertram. You will be limited in the leading edge length due to prop removal clearance. Have you operated this boat at all, or was it strictly a project to begin with? If you did operate it, was it a ballanced feel at the helm or was there excessive torque required to steer? The rule of thumb for balance is to get 17% of the blade area ahead of the pivot shaft (leading edge in your photo) and 83% behind it. In the first photo it looks like you may have an unbalanced rudder which may require a bit of torque at the helm. You may have to add to the leading edge, but too much "leading edge area" creates a situation where the center of pressure is moved ahead of the pivot and it can lock up the rudder hard over. Be careful there. As Blitzen suggested, speed plays a big part in the rudder design. Being a DE work boat however, the speed is most likely not that fast. "Wing shaped rudders" are more efficient than the flat blade type which is sometimes the reason for glassing them over.
I will use that 17/83 as a rule of thumb thanks for that info..
Bill,

Here's a picture of the rudder on my 25 T Jason.
I put about 12 hours off "sea rial" time on the boat before starting the rehab/repower project.

The rudder on my 25 is fiberglass but a little different in shape than the one on your 25.

I'll be keeping my rudder for now. The boat tracks like it's on a rail in any sea and the boat WILL back up L or R with the rudder in river current.

I'm very pleased with how the 25 T Jason handles.
Needs more hp than 124 hp but that is on it's way later ;)

I think you'll be pleasantly surprised on how well your 25 T Jason handles when you getting spiffed up, repowered and in the water !

Bill D
sounds good Billy. Ill problally just keep it about the same size if thats the feedback im getting.. I wasnt sure if this ones size way way overkill or not
 

brianj

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Bill - I was just reading about rudders the other day because I am considering going to larger ones in my Bertram. You will be limited in the leading edge length due to prop removal clearance. Have you operated this boat at all, or was it strictly a project to begin with? If you did operate it, was it a ballanced feel at the helm or was there excessive torque required to steer? The rule of thumb for balance is to get 17% of the blade area ahead of the pivot shaft (leading edge in your photo) and 83% behind it. In the first photo it looks like you may have an unbalanced rudder which may require a bit of torque at the helm. You may have to add to the leading edge, but too much "leading edge area" creates a situation where the center of pressure is moved ahead of the pivot and it can lock up the rudder hard over. Be careful there. As Blitzen suggested, speed plays a big part in the rudder design. Being a DE work boat however, the speed is most likely not that fast. "Wing shaped rudders" are more efficient than the flat blade type which is sometimes the reason for glassing them over.

Keelboater/SouthShore have it right (15-20% Chord)...the center of pressure for a symmetric airfoil is 25% from the leading edge...it is desirable to be forward of this location to have a positively balanced rudder with some feedback. Being too far forward will cause excessive load.
 

Beamie

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Like others said i would go with the bronze over stainless. I have to ask on your original plan of slotting the shaft and welding.....I think it would be better to keep the shaft whole unslotted and put a full penetration weld on the plate and tie it in with multiple passes.
 

Bill

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Bill. Ideally you try to have 20% on the leading edge. For your new rudder, I would consider one cast of manganese bronze with stainless shafts pinned in place. Manganese has a high tensile strength, the blade thickness for a rudder of your size would be approx. 3/8 +/-. The transition where the blade and stocks are inserted can be very fair allowing for good water flow and a hole can be bored in the rudder so the propeller shaft can be removed with out having to drop the rudder. I definitely would not reuse the stainless shaft that is in your current rudder

Keelboater/SouthShore have it right (15-20% Chord)...the center of pressure for a symmetric airfoil is 25% from the leading edge...it is desirable to be forward of this location to have a positively balanced rudder with some feedback. Being too far forward will cause excessive load.

Like others said i would go with the bronze over stainless. I have to ask on your original plan of slotting the shaft and welding.....I think it would be better to keep the shaft whole unslotted and put a full penetration weld on the plate and tie it in with multiple passes.

Ok so doing a little research on galvanic corrosion .. what I guess I am not getting is if we are worried about dissimilar metals ( the prop and a stainless rudder) why put them together to make a rudder out of bronze and stainless? According to what I read, the more far apart the metals on the noble chart, the more they corrode..so for example if you were to put tin and platinum together its gonna be a problem.. but if you put bronze and type 410 stainless steel together you may be ok because they are so similar..

this was an issue why are so many DEs built with stailness rudders? could coating a stainless rudder solve this problem? asking because I dont know.. this is my first rudder..


Read this...

http://www.nickelinstitute.org/~/Me...sGalvanicCorrosionandit PreventionAEBauer.pdf
 

Toolate

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Prop is on a SS shaft.... think bonding and anodes/zincs would more than handle these concerns?
 

Beamie

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I am probably a bit of a traditionalist and would go with bronze keeping with the same or closest material to the wheel.

That being said yes many many newer boats have welded up ss rudders and there is nothing "wrong" with this that good anodes shouldn't handle. Also fabbing out of ss will most likely be cheaper and faster. I'd rather bronze though.
 

Blitzen

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Either way you choose to go, you will still be putting on a zinc.
 

backman

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Who im Mass can help me this winter w/ either steel or bronze for a Northern Bay 36. I currently have a fiberglass rudder; need to pull the shaft this winter anyways and wanted to go stainless before reading this thread.

Tuna boat usage - the boat cruises at 20 knots; I'm hoping to eek out another 1/2 knot of speed moving to metal.
 
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