Rudder design

Hi guys I am building a crackerbox http://s753.photobucket.com/albums/xx179/fergalbutler/crackerbox/?albumview=slideshow and am thinking of going with an outboard rudder like the one below I would have to get it made but have know idea of what the profile should be or what length below the hull to come down. It's a 15ft boat with a V8 but I will only be using it for messing about on lakes and not racing, so if you have any input I would love to hear it no mater how big or small as I know nothing about this sort of thing
Thanks.

 

Congratulations my friend has done a great job What is this horsepower motors;

 

Have a very close look at that rudder in the picture and tell me what you actually see !!:?:
Draw out close to scale what is there !! and really take note of it!!:idea:
Look at the pivot points and look at where the actual leading edge of the rudder blade is !! think about what is there !!
Do you understand the action of the leading edge of the rudder blade what is there ??

There is a reason for that and there is a action because of how it works !can you tell me what the action is ??
Personally i woudnt touch a transom hung rudder like that on for a pleasure boat !!.

 

For shape You might investigate the Boesch Bulge rudder. Its used on single screw ski boat to nuetralize prop wash pull.

 

From the back is a interesting shape would like to see a side view and see where the centre line of the shaft is . !! Some where is a whole subject about rudders for small power boats . Might be Proffessional boat builder mag but a few years back .

The flat steel blade works ok but give it a semi foil shape and works much much better !The deepth and actual height are things that all matter and when all works together its a pleasure to drive .

 

Thanks for the reply, from what I can make out it looks like it has a small cavitation plate inline with the bottom of the hull where it also looks to be curved to go under the hull a bit and down to about 3/4 of the prop.
I know nothing about the action of leading edge of the rudder accept maybe it should be an aerofoil shape
The reason I was thinking about the outboard rudder is so I can lower the prop shaft angle by moving the prop back and I thought this rudder kept the classic look of the boat and as my boat has a barrel back this would be easy to fix to it.
Thanks again.

 

The rudder in the photo is a "semi equalized " rudder and designed to have the most authority on plane,at speed, if your plan is to decrease speed but equalize steerage pressure( power steering) then put your pivot closer to the center of the rudders vertical mass,but keep more then 50 percent of the massaft of the rudder shaft, but if you adding a semetrical surface keep your "axis" of the rudder shaft forward of the thickest part of the chord of the semetrical surface, this will keep you from having "flutter" at higher speeds, equalized rudders take the work and steerage pressures off the operator but increase loads on the rudder axis

 

The rudders are also used on large cruisers like this.

 

Thank you CHASE -B !!! YES AT LAST some one that has been there and done that !!
All thingS being equal and working like they should if in a turn at a moderate speed you should be able to let the wheel go and it should gradually start to straighten all by its self .
Its whats infront of the line of pivot thats quite important .(semi equalized )To much lead and it will pull the wheel out of your hand and wants to over steer ,. To little and it takes two hands to turn the wheel into a tight turn!!!
Also having the blade under the hull stop the blade from becoming airated in a tight turn and the rudder not having any bite Its bit like a aircraft wing stalling and lossing its lift !
Took awhile but it had to come !!

 

So in layman's terms if the rudder is 10" wide below the water line will the pivot point be ok about 1.5" from the leading edge and 8.5" behind or is there a standerd % that I should work off.
Thanks again, I know I must be like talking to a donkey

 

And it looks really gorgeous.

 

Yes Aphrodite is a real looker - splendid boat.

 

the closer your pivot ,.the easier it will be to turn at speed if your not exceeding the speed where cavitation occurs on leading edge of a flat rudder ,..( the thicker it is ,..the slower the cavitation speed will be)
personally I would use a semetrical rudder,.. and 1 to 1.5 inch in front of the thickest part of the chord (semetrical foil) is where I would put the pivot,.. the width and or length of the rudder (semetrical chord) will determine the right mass to your pivot/axis shaft,..a tappered chord/ rudder will put less load on the bottom tip of the rudder,.. you can do some quick checks on the material your using for your rudder shaft for side load capability at any website that lists tolerances for that material,..
example ,..the strut connect bolts for my airplane are 7/16" diameter ,..list a 75000 lbs shear and 150,000 lbs breaking load,.but ,..that is at a tightened state,..without extended side load
you'll have to factor in the length of your rudder as to the shear load capability of the material you choose,..I would use a stainless steel,..316 series or better ,..do not use any 500 series,..it will rust,.. or aluminum,..use a 70xx series ,.. 60xx series will bend easier then 70xx,.. both polish up nice,..7075 T-6 is used for spring landing gear on aircraft ,..so if it does get bumped ,..it will spring back to original config,..

I would list some websites for tolerances ,..but Im a bit underthe weather due to the medication the VA put me on for back surgery ,..and to be honest I cant see straight

Chase

 

Thanks for the reply Chase B, hope the back gets better soon I know how you feel. " If it's not one thing it's a rudder"

 

ha ha ha it hurts to laugh ,..but that was funny