Curious rudder balance problem

[barnakiel]

Flow from the prop is different than flow from sailing. This is the why.

Now how. I think by experimenting.

Possibly adding area aft of the chord BUT only where prop wash hits the blade.

b.

[Cadence]

Originally Posted by Kim Klaka

I have a Van de Stadt 34 with a spade rudder. The rudder was nicely under-balanced when sailing ( i.e. just a bit of feel), but overbalanced under power (i.e. tiller would go to its stops if let go). I modified the rudder by adding chord length aft (retaining its NACA section, just going from about 0012 to about 0010). This reduced, but didn't eliminate, the overbalance under power , but has made the helm rather heavy when under sail. I do not understand why the rudder is overbalanced under power but not sail. The stern gear is conventional p-bracket, and the overbalance problem has not changed since swapping from 2-blade folding prop to 3-blade folding prop.
Any suggestions as to what is going on, and how to get the balance acceptable under both power and sail?

My guess would be the amount of leading edge was designed for sailing. Prop wash is causing the rudder to go to the stops. I'd say don't fool with it. You may screw up your sailing balance.

[Pauls]

This is how I see the dynamics:
1. Under sail the rudder is seeing a constant water flow velocity over its entire surface (yes this is a simplification but it is adequate for our purposes here).
2. Under power the prop is delivering a column of high velocity water that is approximately the diameter of the prop. This high velocity column flows over the rudder. When the rudder is turned the entirety of the balance portion of the rudder is exposed to the high velocity flow. But the large part of the rudder behind the shaft centerline is only partially impacted by the high velocity prop flow, with much of the aft portion seeing only the lower velocity flow from the boat speed thru the water. This will move the effective balance point of the rudder forward, in your case far enough to create an unbalance.
3. This would suggest that adding extra area at the aft end of the rudder will be inefficient, as the aft end is operating in the lower velocity water flow.
4. Reducing/eliminating the balance in the area of the prop wash, and adding more balance to make up for this loss in the area outside of the prop wash, should give you a rudder with good balance under both power and sail. (Assuming that this theory is correct).

[Cheechako]

Hard to understand how the rudder in that pic can turn against the water force when motoring. That's a huge rudder and very far aft. Seems to me it must be bent or non assymetrical...

[topmast]

When the prop is close to rudder leading edge forward of chord is very sensitive but that ugly slab is well back. Is it possible that somebody backed into a rock and bent the rudder shaft forward ?

[deblen]

My Mascot 28 motorsailer acts exactly as you describe.
Fingertip,well balanced,hands off steering while normal sailing & brutal torque that immediately turns boat to stbd while under power.This torque increases with RPM but is always there at some level while powering.
My prop turns CCW (Left Hand) 17D LH 14P 3 blade Volvo Penta 28HP.
My prop is well spaced ahead of rudder-at least a meter- & I have always thought that having prop CLOSER to rudder leading edge may alleviate the problem. But comments from other posters seem to dis-prove that theory
This link should show a pic of my underwater:

https://www.dropbox.com/s/7vaho28jcg...stop2.JPG?dl=0

My solution was to inst. an autopilot,with a Raymarine M81131? linear ram & use it,especially while powering.
IMHO-modifying the rudder to correct the problem would ruin it for sailing-unless you had an adjustable "trim tab" or some such.
I am convinced the problem is caused by the prop wash,which leaves the prop face at an angle off the fore/aft centerline. This angled wash applies more pressure to one side of rudder,thus turning the rudder.
This angled stream is the same cause of so called prop walk in reverse,except in reverse,the angled wash acts on one side of the hull.

Just my opinion. Hope you find an easy solution.
Cheers/ Len

[Cadence]

Originally Posted by deblen

My Mascot 28 motorsailer acts exactly as you describe.
Fingertip,well balanced,hands off steering while normal sailing & brutal torque that immediately turns boat to stbd while under power.This torque increases with RPM but is always there at some level while powering.
My prop turns CCW (Left Hand) 17D LH 14P 3 blade Volvo Penta 28HP.
My prop is well spaced ahead of rudder-at least a meter- & I have always thought that having prop CLOSER to rudder leading edge may alleviate the problem. But comments from other posters seem to dis-prove that theory
This link should show a pic of my underwater:

https://www.dropbox.com/s/7vaho28jcg...stop2.JPG?dl=0

My solution was to inst. an autopilot,with a Raymarine M81131? linear ram & use it,especially while powering.
IMHO-modifying the rudder to correct the problem would ruin it for sailing-unless you had an adjustable "trim tab" or some such.
I am convinced the problem is caused by the prop wash,which leaves the prop face at an angle off the fore/aft centerline. This angled wash applies more pressure to one side of rudder,thus turning the rudder.
This angled stream is the same cause of so called prop walk in reverse,except in reverse,the angled wash acts on one side of the hull.

Just my opinion. Hope you find an easy solution.
Cheers/ Len

Your leading edge seems to be about 30%. I'll bet it works well. A bit more than I would have imagined. People that think the trailing edge does all the steering are nuts.

[Juho]

Originally Posted by Pauls

This is how I see the dynamics:
1. Under sail the rudder is seeing a constant water flow velocity over its entire surface (yes this is a simplification but it is adequate for our purposes here).
2. Under power the prop is delivering a column of high velocity water that is approximately the diameter of the prop. This high velocity column flows over the rudder. When the rudder is turned the entirety of the balance portion of the rudder is exposed to the high velocity flow. But the large part of the rudder behind the shaft centerline is only partially impacted by the high velocity prop flow, with much of the aft portion seeing only the lower velocity flow from the boat speed thru the water. This will move the effective balance point of the rudder forward, in your case far enough to create an unbalance.
3. This would suggest that adding extra area at the aft end of the rudder will be inefficient, as the aft end is operating in the lower velocity water flow.
4. Reducing/eliminating the balance in the area of the prop wash, and adding more balance to make up for this loss in the area outside of the prop wash, should give you a rudder with good balance under both power and sail. (Assuming that this theory is correct).

The high velocity water column from the propeller is the best explanation I could find. When you turn the rudder, this column of water hits only the front part of the rudder, and that is enough to make the balance of the rudder change.

[Olddave]

Originally Posted by Kim Klaka

I have a Van de Stadt 34 with a spade rudder. The rudder was nicely under-balanced when sailing ( i.e. just a bit of feel), but overbalanced under power (i.e. tiller would go to its stops if let go). I modified the rudder by adding chord length aft (retaining its NACA section, just going from about 0012 to about 0010). This reduced, but didn't eliminate, the overbalance under power , but has made the helm rather heavy when under sail. I do not understand why the rudder is overbalanced under power but not sail. The stern gear is conventional p-bracket, and the overbalance problem has not changed since swapping from 2-blade folding prop to 3-blade folding prop.
Any suggestions as to what is going on, and how to get the balance acceptable under both power and sail?

Hi Kim,
I am wondering if you could get by with a smaller rudder like a Spitfire wing type elliptical balanced spade.
As you know the tip of the rudder doesn't do a lot of work when sailing because flow is lost under the rudder blade. I suspect that the lower part of your rudder may be causing the over balance problem when under power. The angled engine installation may be adding to the problem.
If your rig is nicely balanced and the boat can virtually be steered by adjusting the sails, perhaps you do not need so much area in the lower part of the blade.
Dave

[topmast]

Back when I designed my rudder, my research in design gave me a figure of approx 8% leading. As it was a plate of 1/2 steel the construction of 16 sqft the calculation was easy and turned out very successful The propeller was very close to the edge and a hole allowed the removal of the shaft when hard over .When I changed to a Max prop I had to trim an arc of about an inch over 20 inches on the leading edge. No noticeable difference under sail but a huge change under power. More force to steer while docking/manuevering was required .Nothing like the ass backward you describe but shows a little design change can be effective.or negative.

[Olddave]

Kim,
This link is to an over simplified example of what I am suggesting. The diagram even shows how the flow is turned as it crosses the tip of the rudder. If the NASA foil is carried all the way to the tip then an end plate should be fitted to prevent loss of flow(but end plates aren't perfect either).
Your shallow bilge mounted engine results in the prop aiming down towards the rudder tip and I imagine that is magnifying the uneven flow. The elliptical blade would reduce surface area forward of the rudder post towards the end of the rudder and improve flow over the trailing edge.
Design Tips | Competition Composites Inc.
Dave

[Pauls]

Olddave - If you are trying to maximize lift to drag, as for a race boat, then the elliptical planform you describe is best. However, if maximum lift (ie turning force) is desired, then the squared off bottom will be more effective. Partly this is due to more area, but also because of vortex induced lift at large angles of attack, ie large rudder angle. The squared off (and slightly raked down is best) tip will allow much larger rudder angles to be used before the rudder stalls. Marchaj explains this clearly in his "Seaworthiness" book, which I highly recommend.

[Olddave]

Originally Posted by Pauls

Olddave - If you are trying to maximize lift to drag, as for a race boat, then the elliptical planform you describe is best. However, if maximum lift (ie turning force) is desired, then the squared off bottom will be more effective. Partly this is due to more area, but also because of vortex induced lift at large angles of attack, ie large rudder angle. The squared off (and slightly raked down is best) tip will allow much larger rudder angles to be used before the rudder stalls. Marchaj explains this clearly in his "Seaworthiness" book, which I highly recommend.

Paul's,
Thanks for the suggestion. I was not trying to maximise lift, I was trying to help the OP to reduce the over balance his rudder has under power.
I may not have explained myself clearly enough. I do not believe that his rudder is benefitting from having the NASA foil at full size all the way from top to bottom.
An ellipse may help by reducing the area that the prop wash is affecting.

[Pauls]

Thanks. I can see the logic in that. Although if his rudder is steel, which I kind of assume it is being a Van DeStadt, elliptical would be challenging to build.

[Olddave]

Originally Posted by Pauls

Thanks. I can see the logic in that. Although if his rudder is steel, which I kind of assume it is being a Van DeStadt, elliptical would be challenging to build.

Paul's,
I was interested in the thread because I have built an elliptical rudder to plans by Ian Farrier. Good design throughout the boat resulted in one finger control at speeds upto 18 knots. You could fully let go of the tiller at that speed but it felt kind of risky!
That rudder was massive in its construction which is surprising on a multihull that is very weight conscious. Breaking the rudder at speed might not end well.
It has a heavy stainless shaft and web with a laminated ply blade that was foiled and glassed and foiled again. Farrier provided progressively smaller NASA foil templates to follow and the whole job was pretty easy, after having built the trimaran hulls etc.
I reckon Kim could do something similar and improve the Vandestat considerably.
Kim and I are both West Aussies and I am familiar with some of his achievements in design and building. It would be a walk in the park for him and the rudder would be perfectly at home on a steel vessel.
Dave