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Old 17-06-2016, 05:27   #166
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Re: Skeg or Spade?

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Originally Posted by salticrak View Post
Yes spades are faster,but skegged rudders are stronger for sure.
This is the same confusion as discussed before. With a kayak, losing the rudder is a hassle but you can generally get by without it.

The kayak with a skeg was probably built by a manufacture trying to focus on ruggedness. They build it to take a grounding.

The kayak with a spade rudder, is probably focused on performance. Since it's not a mission critical item (if you hit the beach, you almost always have the option to get off and walk to shore), they sacrifice durability.

There is no reason the spade couldn't be built just as sturdily.

Also as discussed, sailboats are different animals always trying to fight leeway where as a power boat (or kayak), is generally not fighting leeway, so lift is less critical.
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Old 17-06-2016, 05:49   #167
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Re: Skeg or Spade?

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Originally Posted by valhalla360 View Post
This is the same confusion as discussed before. . . .

There is no reason the spade couldn't be built just as sturdily.

. . . .

Right. "THIS skeg rudder is stronger than THAT spade rudder" is something you might say in some specific case, but "ALL skeg rudders are stronger than ALL spade rudders" is nonsense. Either type of rudder can be built as strong as desired. It's not even clear that it's even more expensive, to build a spade rudder of X strength, compared to building a skeg rudder of the same strength.
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Old 17-06-2016, 06:41   #168
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Re: Skeg or Spade?

gentlemen, a seriously damaged skeg sure is worse than any calamity that can befall a spade, no?
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Old 17-06-2016, 07:23   #169
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Re: Skeg or Spade?

They are called ring bearings. Instead of having a shaft the rudders are monolithic (normally carbon) structures that penetrate the hull. Without the shaft they can be made much thinner, which is great for fast boats but not necessary for slower ones.

The major advantage is that they can be stiffer for the same thickness because of the size of the load bearing surface (the entire rudder head). And because they are monolithic structures there is almost no chance of water intrusion into the rudder, and because there is no stainless even if the water does get in it doesn't matter because there is no steel to corrode.

They also have the advantage that there is no slot between the rudder and the hull to catch lines and add drag, the rudder also gets a full end plate effect at all times.


Basically it's a really high end spade. But making them is expensive so they tend to be reserved for high end race boats.
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Old 17-06-2016, 07:32   #170
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Re: Skeg or Spade?

...be VERY easy to replace in case of damage though...
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Old 17-06-2016, 07:58   #171
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Re: Skeg or Spade?

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gentlemen, a seriously damaged skeg sure is worse than any calamity that can befall a spade, no?
That's what Dashew says. There are two problems:

1. a damaged skeg rudder jams
2. a damaged skeg can open a breach in the hull
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Old 17-06-2016, 08:59   #172
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Re: Skeg or Spade?

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That's what Dashew says. There are two problems:

1. a damaged skeg rudder jams
2. a damaged skeg can open a breach in the hull
Mind you opening a breach to the hull seems to also work on spade rudders attached to glued in plywood boxes... as in the whole ass end!
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Old 17-06-2016, 09:30   #173
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Re: Skeg or Spade?

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Mind you opening a breach to the hull seems to also work on spade rudders attached to glued in plywood boxes... as in the whole ass end!
Indeed. All bets are off if something is horribly shoddily built. Even keels have been known to fall off some boats
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Old 17-06-2016, 10:02   #174
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Re: Skeg or Spade?

Some back of the envelope force calculations on shaft bending loads and bearing side loads for a full spade rudder and partial skeg deisgn.


Common-
Area of rudder = 5 x 2 = 10 sq ft
speed through water = 10 kts (peak surge speed running or broad reaching)
Max side force on rudder with assumed lift coef of 3 = 8,500 lbs

For spade rudder:

Distance between lower bearing and rudder tip = 5 feet

Distance between lower and upper bearing (inside hull) = 4 feet

Bending moment at lower bearing (beam deflection force (not rotation torque) on shaft at exit of lower bearing=21,250 ft-lbs.

Upper bearing side load = 5,312 pounds

Lower bearing side load = 13,812 pounds



For partial skeg hung rudder with only two bearings:

Distance between lower bearing and tip = 2 feet

Distance between upper and lower bearing = 7 feet

Bending moment at lower bearing (beam deflection force (not rotation torque) on shaft at exit of lower bearing on skeg = 8,500 ft-lbs.

Upper bearing side load = 1,214 pounds

Lower bearing side load = 9,714 pounds


This used equations posted at -

https://www.pacificcup.org/kb/emerge...ign-guidelines
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Old 17-06-2016, 12:58   #175
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Re: Skeg or Spade?

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...be VERY easy to replace in case of damage though...
It depends on the design. Some the entire rudder is slotted down then the ring in which case replacement is trivial. On others only the front 1' or so is slotted thru. The VOR's actually carry a spare rudder and can change them at sea (they can also scavenge the others side if they need to).

Apparently on the VOR's it takes about 10 minutes to drop in a new rudder.
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Old 17-06-2016, 13:13   #176
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Re: Skeg or Spade?

Interesting thread.From my way of thinking though, a skeg would absorb a helluva lot of the force of an impact before being transferred to the rudder.A spade would have to absorb thee impact/grounding completely.
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Old 17-06-2016, 13:24   #177
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Re: Skeg or Spade?

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Interesting thread.From my way of thinking though, a skeg would absorb a helluva lot of the force of an impact before being transferred to the rudder.A spade would have to absorb thee impact/grounding completely.
False assumption:

A skeg is only as strong as it's design. A skeg attached only to the hull skin may look strong but could be much weaker than a spade supported by the interior bulkheads.

In reality, it doesn't matter if it's the skeg or the rudder that takes the impact. If either fails, you have an issue. For most practical purposes, without the skeg, a skeg hung rudder has zero strength.
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Old 17-06-2016, 13:39   #178
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Re: Skeg or Spade?

John Rousmaniere discusses the tradeoffs between skeg and spade rudders in his book, "Desirable and Undesirable Characteristics of Offshore Yachts" in the 'The Rudder' section starting on pg 92 of the 1987 edition.

https://books.google.com/books?id=pe...rudder&f=false
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Old 17-06-2016, 13:50   #179
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Re: Skeg or Spade?

Quote:
Originally Posted by Ericson38 View Post
Some back of the envelope force calculations on shaft bending loads and bearing side loads for a full spade rudder and partial skeg deisgn.


Common-
Area of rudder = 5 x 2 = 10 sq ft
speed through water = 10 kts (peak surge speed running or broad reaching)
Max side force on rudder with assumed lift coef of 3 = 8,500 lbs

For spade rudder:

Distance between lower bearing and rudder tip = 5 feet

Distance between lower and upper bearing (inside hull) = 4 feet

Bending moment at lower bearing (beam deflection force (not rotation torque) on shaft at exit of lower bearing=21,250 ft-lbs.

Upper bearing side load = 5,312 pounds

Lower bearing side load = 13,812 pounds



For partial skeg hung rudder with only two bearings:

Distance between lower bearing and tip = 2 feet

Distance between upper and lower bearing = 7 feet

Bending moment at lower bearing (beam deflection force (not rotation torque) on shaft at exit of lower bearing on skeg = 8,500 ft-lbs.

Upper bearing side load = 1,214 pounds

Lower bearing side load = 9,714 pounds


This used equations posted at -

https://www.pacificcup.org/kb/emerge...ign-guidelines
Your example is a little extreme as a 5' tall rudder is pretty large unless you are talking extreme performance boats or very large boats.. 3' to 4' seems more typical for your 30-45' cruising boats but let's go with it:

The issue is where the rudder/skeg meets the hull in your example, the bending moment is identical. It's just a question of which parts support it.

If the rudder is only picking 8500ft-lbs with the partial skeg design, that means the skeg must take up the remainder of the 21250ft-lbs. 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.

None of this really means much until you take into account the strength of the supporting parts. Presumably the bearings for the spade rudder have been beefed up to accommodate the extra force and presumably the skeg has been strengthened to accommodate the extra force on the skeg.
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Old 17-06-2016, 14:06   #180
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Re: Skeg or Spade?

But , how many skeg rudder boats are sunken by rudder loss and how many with spade rudders???
To me is a matter if the builder pay attention to the skeg layup and built the skeg with a hig safety margin, lots of bad skeg rudders boats out there same as lots of bad spade rudder boats....
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