Skeg or Spade?

[Ericson38]

"Assuming the skeg is attached to a hefty 1" thick hull skin and doesn't penetrate the hull and get extra support from interior bulkheads, that will result in forces in the fiberglass skin in excess of 12,600psi."

That would depend on the mating circumference, but anyway, why would someone prepare this in a sub-standard manner ?

Yes, the loads will be with you always. Its the bending moments that change with placement. Hold a broom with two hands at the end 1 foot apart, then hold it with one on end and one in middle of handle.

It is a choice as whether to assign them all to the rudder through the bearings as transferred via the rudder post, or split the dynamic loads between the fixed skeg and the post-bearings, in the example where the lower bearing is lower and therefore closer to the mid point of the rudder's sideload center.

If the skeg is a structural support member, then the glass work and integration into the hull has to be done with some though as to what the intention is.

But that is true of chain plates, stem head fittings bolted on pointy bows that are always highly loaded, mast partner areas, motor mounts, and keel attachments.

[Sandero]

I am quite sure that the skeg on the 36s is quite strong... and not glued on to the hull... that is absurd.

And the prop shaft is quite well protected as well.... more wetted surface than a sled... you betcha... Neither will likely be damaged... and the 29'+ LWL does 150 nm in the ocean... which is fast enough for me.

[valhalla360]

Originally Posted by Ericson38

"Assuming the skeg is attached to a hefty 1" thick hull skin and doesn't penetrate the hull and get extra support from interior bulkheads, that will result in forces in the fiberglass skin in excess of 12,600psi."

That would depend on the mating circumference, but anyway, why would someone prepare this in a sub-standard manner ?

The same question could be asked with the design of a spade style rudder. Why would someone prepare the design in a sub-standard manner?

I never suggested you couldn't design a skeg to handle the loads, just that the seat of the pants calculations without considering the structure supporting it can lead to false results.

Then again, I think a flaw in the initial assumptions is that you would continue to hold the rudder at the same angle of attack regardless of the side loads. 8500lbs side load on the rudder would likely spin most cruising boats so fast you would be thrown from the cockpit. In reality, you would turn the rudder back a bit to relieve the load or the boat would broach taking the load off the rudder.

[Ericson38]

Originally Posted by valhalla360

The same question could be asked with the design of a spade style rudder. Why would someone prepare the design in a sub-standard manner?

I never suggested you couldn't design a skeg to handle the loads, just that the seat of the pants calculations without considering the structure supporting it can lead to false results.

Then again, I think a flaw in the initial assumptions is that you would continue to hold the rudder at the same angle of attack regardless of the side loads. 8500lbs side load on the rudder would likely spin most cruising boats so fast you would be thrown from the cockpit. In reality, you would turn the rudder back a bit to relieve the load or the boat would broach taking the load off the rudder.

A 8500 lb side load is with a 10 sq ft rudder with 10 knot velocity of water flowing over it, with a lift coef of 3.0. A 50 ft sailboat of 45,000 displacement has that size rudder plane. This is what it is capable of generating before stall, in terms of lift to windward.

On the example attachment with 1 inch fiberglass skeg layup to the hull layup, with a 12,000 lbs/sq inch yield strength (or breaking), lets look at this.

A skeg for a 50 ft boat may have a form circumference where it meets the hull of, say, 40 inches. That would be 40 sq inches for the tension side, and 40 sq inches for the compression side. On the tension side, 12,000 x 40 = 480,000 lbs.

Not sure where the deficiency is, with 1 inch layup.

[Dockhead]

OK, here's the science:

http://eprints.soton.ac.uk/43260/1/001-78new.pdf





Ted Brewer is not quite right about the hydrodynamics of skeg rudders.

It is true, as Ted says, that skeg rudders tolerate higher angles of attack before stalling.

HOWEVER, they REQUIRE higher angles of attack to generate the same lift force.

So net net, a skeg rudder stalls at about the same lift force as a spade, just at a much higher angle of attack. Needless to say, the skeg rudder will be producing far more drag.

The only good thing you can say is that the stall behavior might be more "forgiving", but if you look at the curves, it looks to me like that could be corrected right out with gearing of the steering mechanism.

The linked article will tell you probably more than you ever wanted to know about the hydrodynamics of rudders. I enjoyed it very much.

[Snowpetrel]

Thats a very interesting article. Thanks.

A random thought occured to me the other day that faster boats are less likely to suffer from wave induced rudder stall than slower boats due to the angle of attack not changing as much due to the boat yawing.

Sent from my SM-G930F using Cruisers Sailing Forum mobile app

[TeddyDiver]

Originally Posted by Snowpetrel

Thats a very interesting article. Thanks.

A random thought occured to me the other day that faster boats are less likely to suffer from wave induced rudder stall than slower boats due to the angle of attack not changing as much due to the boat yawing.

Sent from my SM-G930F using Cruisers Sailing Forum mobile app

Very true, but fast boats don't go fast allways and then they have a problem...

[Tomaz473]

Originally Posted by Ericson38

To be neutral, here is a good example from a another designer (H&R)

https://www.bing.com/images/search?q...2o0&ajaxhist=0

As you mentioned H&R: They are definitely designing cruising boats, not racing boats. They are not build to cost point (like by Bene is) but to quality and safety.
Yet, they used skeg rudders in all of their old designes, then they moved to spade rudders.
Their new flagship model HR64 have a spade rudder.
I think this tells us a lot about what is better for a cruiser.

[Snowpetrel]

Originally Posted by TeddyDiver

Very true, but fast boats don't go fast allways and then they have a problem...

Thats true. It was just more an observation.

Ive always thought its a challenge to keep a smaller cruising boat light enough to plane,

but once you start planing the rules change compared to displacement mode. I really think a lot of the issues occur in the transition between displacement and planing. I think its best to either keep the speed down, sub surfing, or you push hard and keep the speed mostly above displacement. But then I dont sail fast boats very often...

[TeddyDiver]

Originally Posted by Snowpetrel

Thats true. It was just more an observation.

Ive always thought its a challenge to keep a smaller cruising boat light enough to plane,

but once you start planing the rules change compared to displacement mode. I really think a lot of the issues occur in the transition between displacement and planing. I think its best to either keep the speed down, sub surfing, or you push hard and keep the speed mostly above displacement. But then I dont sail fast boats very often...

Except when running DW higher seas the transition often happens twice every wave. Needs brakes..

Tomaz, it's not actually not a choice one has to make. High aspect defineatly a spade, low aspect are mostly skeg hungs or even keel hungs. Between is a bit of grey area but in the end of the day it doesn't matter so much. Both have pros and cons, the main concern is the integrity of the structure. For me it's going to be double kickup rudders..

BR Teddy

[Ericson38]

Originally Posted by Tomaz473

As you mentioned H&R: They are definitely designing cruising boats, not racing boats. They are not build to cost point (like by Bene is) but to quality and safety.
Yet, they used skeg rudders in all of their old designes, then they moved to spade rudders.
Their new flagship model HR64 have a spade rudder.
I think this tells us a lot about what is better for a cruiser.

As far as what is better for a cruiser, I personally prefer this apprpoach:

http://www.caliberyacht.com/Download...r_Brochure.pdf

http://www.caliberyacht.com/Download...s_Brochure.pdf

[neilpride]

Originally Posted by Ericson38

As far as what is better for a cruiser, I personally prefer this apprpoach:

http://www.caliberyacht.com/Download...r_Brochure.pdf

Lol The caliber brochure is the same than the Beneteau .... now seriously what a good way to explain the details of the caliber , I miss those kinds of Brochures ....

[TeddyDiver]

Originally Posted by Ericson38

As far as what is better for a cruiser, I personally prefer this apprpoach:

http://www.caliberyacht.com/Download...r_Brochure.pdf

http://www.caliberyacht.com/Download...s_Brochure.pdf

Many well thought details I prefer too, but some shortcomings like anchor locker would be better further back in the bilge, no place for the dinghy, stupid pulpits and no seaberths.

BR Teddy

[Dockhead]

Originally Posted by Ericson38

As far as what is better for a cruiser, I personally prefer this apprpoach:

http://www.caliberyacht.com/Download...r_Brochure.pdf

http://www.caliberyacht.com/Download...s_Brochure.pdf

You know, my previous boat had an underbody just like that, with a full skeg and a long fin keel. She was a very strong, very seaworthy, very sea-kindly boat which punched way above her 38' weight.

The Caliber brochure explains the tradeoffs in underbody form very well. Those blind followers of fashion (we won't name any names) who worship everything more extreme, more wedge-like, don't understand these tradeoffs of motion against performance.

But my previous boat, to be honest, sailed like crap. Good thing she had a Perkins 4-108 which was only just barely broken when I got to 10,000 hours and never missed a beat in 20 years of hard usage.

Going to a much bigger boat but with much lighter D/L, and with a more aggressive underbody (bulb keel, semi-balanced rudder, higher aspect keel) I got all the seaworthiness and even much more, plus a totally different amount of speed, and mirabile dictu, the ability to make 100+ miles made good to windward per day.

My next boat will go even further in that direction -- longer waterline, lighter, narrower.

YMMV, of course.

[Ericson38]

Originally Posted by Dockhead

You know, my previous boat had an underbody just like that, with a full skeg and a long fin keel. She was a very strong, very seaworthy, very sea-kindly boat which punched way above her 38' weight.

The Caliber brochure explains the tradeoffs in underbody form very well. Those blind followers of fashion (we won't name any names) who worship everything more extreme, more wedge-like, don't understand these tradeoffs of motion against performance.

But my previous boat, to be honest, sailed like crap. Good thing she had a Perkins 4-108 which was only just barely broken when I got to 10,000 hours and never missed a beat in 20 years of hard usage.

Going to a much bigger boat but with much lighter D/L, and with a more aggressive underbody (bulb keel, semi-balanced rudder, higher aspect keel) I got all the seaworthiness and even much more, plus a totally different amount of speed, and mirabile dictu, the ability to make 100+ miles made good to windward per day.

My next boat will go even further in that direction -- longer waterline, lighter, narrower.

YMMV, of course.

Yes, my mileage certainly varied from your experience. Taswell has very similar underbody, except partial skeg and balanced rudder, and sails like a dream in nice as well as windy-choppy conditions. This with a scheel keel and cutter rig, 42 K lbs in the slings. Finger tip control at 35 kts on close and beam reach with 110 headsail and main (staysail furled). You can hear the roar of the bow wave in the head sail in these conditions, but she does not yaw and wander around. Autopilot current on actuator very low. You set course, easy to balance sails, and she goes right for it.

One of my acquaintances has a new 2014 Oceanis 45, and with a spade single rudder, but almost full 14.5 ft beam brought to the stern. They have broached in the bay several times, and wish it had twin rudders. When heeled over, the fore/aft trim shift is so much that the rudder is rotated close to the surface, as the boat is close to wedge shaped and floats nose down when heeled down.
Twin rudders would be a great upgrade for that condition, I think.