Hobie Forums :: View topic - Nylatron rudder pins?

I purchased a set of Murray's Stainless Steel Rudder Pins .... They are hollow .... therefore MUCH lighter .... and stiffer. There are some veeeery long engineering formulars to explain this fact but it is true .... a "hollow" column is less likely to deflect then a "solid" column ....

As I said ... It involves some very long engineering formulars to prove my point, that a hollow column is stiffer the a solid column. Now I didn't say a "thin walled" column ... just hollow. Now I'm trying to remember stuff that I studied 30-35 yrs ago in college studying engineering ... and I've forgotten alot over the years. But this is based on the theroy that the "load" is carried on/in the surface of the pin/column when it somes to shear/bending moments ... now a solid pin/column has only one surface .. the outside surface ... a hollow pin/column has two surfaces the outside and the inside ....

....Blah Blah Blah...Now you think that just the fact that the "low grade steel" pin is solid it is automatically stronger??? ....Blah Blah Bla.....

In the case of hollow versus solid beams of like materials, the only variable is I, the area moment of inertia. Again, the area moment of inertia of a hollow beam is smaller than a solid beam of equal outside diameter (since the inside area is deleted from the overall area) and therefore the deflection of the hollow beam will be greater.

Why did the “Hobie” pin fail when the “Murrays” pin didn’t? I don’t know. Perhaps there was significantly higher load applied to the Hobie pin. Perhaps the Murrays pin was made of a stronger grade of stainless. Pin geometry is not the only factor to consider.

How did you post the formulars???? My Keyboard doen't have a 3.14 (ie: "pie") symbol on it ....

Author:	Fa1321tx [ Wed Nov 03, 2010 8:12 pm ]
Post subject:	Nylatron rudder pins?
Anyone ever use these? I sail in the gulf and bought these because they seem like a good idea but somebody else told me they can break just from the boat speed and if one breaks the other one will follow shortly and there is a chance could lose my rudder system. Any thoughts or opinions?

Author:	MBounds [ Wed Nov 03, 2010 8:53 pm ]
Post subject:	Re: Nylatron rudder pins?
I used them for many years . . . until I lost a whole rudder (an EPO), castings and tiller arm that now lie on the bottom of East Traverse Bay (Traverse City, MI). They can save your transom in surf, but fixing the transom is just about expensive as loosing the rudder.

Author:	srm [ Thu Nov 04, 2010 6:39 am ]
Post subject:	Re: Nylatron rudder pins?
Stainless steel pins don't break, plain and simple. Every other type (including aluminum) can break. You play in the ocean and at some point, the ocean's going to get the better of you. But, would you rather bust your hull 70 feet off the beach, or lose your rudder system 2 miles from the beach? Personally, I don't think I'd ever use anything but stainless, even in the surf. If you do use non-stainless pins, cary some spares (although re-attaching your rudder system offshore is going to be no easy task). sm

Author:	Fa1321tx [ Thu Nov 04, 2010 6:57 am ]
Post subject:	Re: Nylatron rudder pins?
I guess it a personal choice but losing a transom offshore would suck! I had a daggerboard slip into the well and knock a big hole on a Nacra once about 3 or 4 miles offshore on a calm summer day and we made it back in but if it had been a rough day there would have been no way we could of kept the boat upright it was a real struggle on a calm day with one hull full of water. I wonder if a leash could be made that could attach the tiller arm to to one of the gudgeons so you do not loose your rudders it might hold till you could stop pull it up onto the boat. My main concern was heading out thru the surf and getting pushed back into a sandbar sometimes the wind just drops out at the wrong time.

Author:	Harry Murphey [ Thu Nov 04, 2010 3:33 pm ]
Post subject:	Re: Nylatron rudder pins?
Hi, I tried Nylon rudder pins once on my H18 for launching in/through a surfbreak ..... they just slowly deflected/bent until the rudder casting became detached. They may work on the H16 since that lowwer rudder casting has (3) points that the rudder pins passes through effectively .... the H18 only (2). Therefore the loading on the rudder pin is more concentrated on a H18. I wouldn't even mess w/ aluminum rudder pins .... I sail in salt water .... and have had to cut out aluminum pins on my boat and several of my friends over the years, w/ a hacksaw blade specially modified to fit between the rudder casting and the transom .... tedious work ..... I recommend Stainless Steel Rudder Pins (w/ new bushings) ..... after bending a "Hobie" SS Rudder Pin ... (yes, I was quite surprised) .... I purchased a set of Murray's Stainless Steel Rudder Pins .... They are hollow .... therefore MUCH lighter .... and stiffer. There are some veeeery long engineering formulars to explain this fact but it is true .... a "hollow" column is less likely to deflect then a "solid" column .... I do have the "Upgraded" H18 Rudder system .... (same casings as the H17) I haven't had any problems since ... but I do make sure that my rudder system releases properly ... (I lubricate w/ "dry lube/teflon")

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Author:	srm [ Thu Nov 04, 2010 6:27 pm ]
Post subject:	Re: Nylatron rudder pins?
Harry Murphey wrote: I purchased a set of Murray's Stainless Steel Rudder Pins .... They are hollow .... therefore MUCH lighter .... and stiffer. There are some veeeery long engineering formulars to explain this fact but it is true .... a "hollow" column is less likely to deflect then a "solid" column .... Not sure I agree with the suggestion that hollow rudder pins are superior to solid pins. When it comes to rudder pins, we're concerned with the pin's ability to resist the shear force and bending moment applied upon it. This is a function of the pin's modulus of elasticity (an intrinsic material property) and the area moment of inertia (which is defined by the pin's geometric shape). A hollow pin is going to have a smaller area moment of inertia (since the hollow area is effectively subtracted from the overall area) and therefore it has less ability to resist both the shear forces and bending moment. I believe the only case where a hollow beam is benificial would be in the case of long, slender beams where the inertial forces of the beam begin to come into play (i.e., the weight of the beam begins to induce significant bending in the beam). In any case, if you want to insure that your rudders stay attached to the transom, use stainless pins of some sort. Whether or not the transom stays connected the the rest of the hull may be another issue. By the way, if you're at all handy, you can same a few bucks by ordering yourself a 2-foot length of 3/8" diameter stainless steel round bar for about $10 online. Then all you have to do is cut the bar to length and drill a hole in the end of each pin. It costs about 1/2 the price of a retail pin. sm

Author:	Fa1321tx [ Thu Nov 04, 2010 7:01 pm ]
Post subject:	Re: Nylatron rudder pins?
I am getting the point SS pins it is and if my rudders dont unlock I have insurance to cover the boat! Thanks guys I just had to hear from a few sources because the Nylatron sounded like a good plan for the surf. I guess I could keep them to use on light wind days if the surf is rough that is when your most likely to get pushed back onto a sandbar.

Author:	Harry Murphey [ Fri Nov 05, 2010 5:20 pm ]
Post subject:	Re: Nylatron rudder pins?
Hi, As I said ... It involves some very long engineering formulars to prove my point, that a hollow column is stiffer the a solid column. Now I didn't say a "thin walled" column ... just hollow. Now I'm trying to remember stuff that I studied 30-35 yrs ago in college studying engineering ... and I've forgotten alot over the years. But this is based on the theroy that the "load" is carried on/in the surface of the pin/column when it somes to shear/bending moments ... now a solid pin/column has only one surface .. the outside surface ... a hollow pin/column has two surfaces the outside and the inside .... Now you can just do a simple experiment .... buy a set of Hobie SS rudder pins and a set of Murray's "Hollow" SS rudder pins ... clamp them in a vise ... and take a hammer ... wind-up and take a good "swack" at the pins .... which ones bend the easiest???? I already know which will bend easiest .... you want to bet $20.00 on it???? (With a last name of "Murphey" do you think I make bets often ????) PS: I HAVE bent solid Hobie SS rudder pins .... I have NEVER bent a Murray's SS rudder pin ....

Author:	srm [ Fri Nov 05, 2010 8:13 pm ]
Post subject:	Re: Nylatron rudder pins?
Quote: As I said ... It involves some very long engineering formulars to prove my point, that a hollow column is stiffer the a solid column. Now I didn't say a "thin walled" column ... just hollow. Now I'm trying to remember stuff that I studied 30-35 yrs ago in college studying engineering ... and I've forgotten alot over the years. But this is based on the theroy that the "load" is carried on/in the surface of the pin/column when it somes to shear/bending moments ... now a solid pin/column has only one surface .. the outside surface ... a hollow pin/column has two surfaces the outside and the inside .... OK, that’s all well and good. I’ve also studied engineering, and so without any real equations to back up your “theory”, I have a tough time buying it. As far as I can see, there are really only two issues at play when it comes to rudder pins – shear and bending. The pin can basically be viewed as a cantilever beam extending from the termination of the gudgeon. So the shear stress is simply defined by the applied load divided by the cross sectional area of the pin. A solid pin has more cross sectional area than a hollow pin of equal outside diameter and therefore is capable of withstanding higher shear stress when all other material properties are equal. The bending moment is a function of the deflection of the pin which is defined by the equation D(delta) = P L^3/3EI where P is the applied load, L is the distance from the applied load to the fixed end of the beam, E is the modulus of elasticity of the beam, and I is the area moment of inertia of the beam. In the case of hollow versus solid beams of like materials, the only variable is I, the area moment of inertia. Again, the area moment of inertia of a hollow beam is smaller than a solid beam of equal outside diameter (since the inside area is deleted from the overall area) and therefore the deflection of the hollow beam will be greater. Why did the “Hobie” pin fail when the “Murrays” pin didn’t? I don’t know. Perhaps there was significantly higher load applied to the Hobie pin. Perhaps the Murrays pin was made of a stronger grade of stainless. Pin geometry is not the only factor to consider. sm

Author:	MBounds [ Fri Nov 05, 2010 8:41 pm ]
Post subject:	Re: Nylatron rudder pins?
Harry, you're going to have a hard time convincing me of your theory. It's been almost 30 years since my statics classes at U of Michigan, but there's nothing complicated about it, and srm's explanation is correct. I've destroyed hollow stainless pins in heavy air (on my 14 at Rehoboth) to the point where they needed to be cut out. They were the last ones I bought (5 years ago). I've also had hollow pins develop narrow sections where the action of the gudgeons / castings act like roller presses and elongate the pin, creating the "necked down" section. Hollow pins are also subject to buckling and collapse - solid pins don't do that. I've got stainless pins on all my boats and have never had a failure, or even significant wear.

Author:	Harry Murphey [ Thu Nov 11, 2010 10:13 am ]
Post subject:	Re: Nylatron rudder pins?
Hi, I believe that you guys are not carefully reading what I posted and getting to hung up on theory and not "practical knowledge". First I said ... "lighter and stiffer" .... so "lighter" is the first fact ... ... and then stiffer is the second ... To prove that the Murray's rudder pins are in fact lighter ... just wiegh them. You will find that they are indeed lighter .... much lighter. As for stiffer .... yes, I do not know exactly what SS alloy is used in either rudder pins. And I never said that both rudder pins are made of the same alloy/material. But, my observation/experience is that the "Hobie" rudder pins are of a softer alloy and cut w/ a hacksaw much easier then the "Murray's" rudder pins. Now, since we have all studied "engineering" we have all seen this in our homework problems .... "assume the wieght of the beam is negligable ... " if this where true in the real world then we would be using "solid" beams" and not "box beam" structures which are what ??? HOLLOW !!! If a cantilevered beam was solid at some point it would not be able to support it's own wieght as it's length increases. (This does sound like a exam problem .... ) Now, (ref: Mechanics of Materials/Popov, Edition 2, 1975) as the distance from the origon/axis, increases so does the shear forces .... therefore a conclusion can be demonstrated that the shear forces are low and very little strength is gained by the material at or near this axis ... but this material does have mass = wieght .... so you are adding wieght but not really gaining a corrisponding or equal amount of strength. The shear force increases as the distance from the origon/axis increases So (an extreme example) lets make two different pins, same "OD", Ok??? ... one is solid and made of low grade steel .... the second is hollow and made of a steel/titanium alloy. Now you think that just the fact that the "low grade steel" pin is solid it is automatically stronger??? This is why engineering problem/solutions are complicated .... there are alot of factors that must be considered to arrive at a correct solution. Some of them are "physics" ie: shapes/structure to be used .... and some are "material properties" ie: the strength/hardness/molecular structure/resistance to fatigue .... of the materials to be used. It's a balance .... Example: Airplanes .... steel does not "fatigue" like aluminum does which only retains it strength for some many "low percentage" (of it's overall strength) vibration cycles .... then the aluminum fractures and fails at loading that is only a small percentage of it's origonal rated strength. Now steel withstands a higher percentage (of it's overall strength) loading for many more vibration cycles ... retains it's strength and will fail at a much higher percentage of it's origonal rated strength. So airplanes made of steel would last longer .... but would be too heavy ... requiring larger wings for increased lift ... bigger engines ... which means more fuel .... now the airplane doesn't fly .... ( I still have most of my engineering class books ... and I spent the last several days reviewing ... DiffEq ... Mohr's Circle ... (Second) Moments of Enertia ... Poison's Ratio ... Hooke's Law ....etc, etc. so it took me some time to post a reply. If you want equations ....Vector Mechanics for Enginneers:Statics 2nd Edition Beer&Johnson, Mechanics of Materials 2nd Editon Popov, Marks Mechanical Handbook ..... I don't have a book specifically on "Materials" in my personal library which have lists of alloys/molecular structures/strengths/characteristics for proper material selection .....

Author:	srm [ Thu Nov 11, 2010 1:42 pm ]
Post subject:	Re: Nylatron rudder pins?
Harry Murphey wrote: ....Blah Blah Blah...Now you think that just the fact that the "low grade steel" pin is solid it is automatically stronger??? ....Blah Blah Bla..... Ummm..obviously NO and I thought I made that pretty clear in my previous post: Quote: In the case of hollow versus solid beams of like materials, the only variable is I, the area moment of inertia. Again, the area moment of inertia of a hollow beam is smaller than a solid beam of equal outside diameter (since the inside area is deleted from the overall area) and therefore the deflection of the hollow beam will be greater. Why did the “Hobie” pin fail when the “Murrays” pin didn’t? I don’t know. Perhaps there was significantly higher load applied to the Hobie pin. Perhaps the Murrays pin was made of a stronger grade of stainless. Pin geometry is not the only factor to consider. I still have yet to see in any of your longwinded and misguided explanations any proof, preferably in the form of an equation, as to why a hollow rudder pin would be stronger than a solid rudder pin OF LIKE MATERIALS. I provided the two simple equations in my previous post that I believe to fully explain the mechanics of the situation. See ya. sm

Author:	MBounds [ Thu Nov 11, 2010 4:21 pm ]
Post subject:	Re: Nylatron rudder pins?
Let's boil this down to some hard math: Hobie 18 rudder pins are 235 mm long by 9.525 mm diameter (9.25" x 3/8") The hollow pins have a 1.651 mm (0.065") wall (per the Murray's catalog). The pins I get are 316 (austenitic) stainless steel - 70% of all SS made is of the 300 series, and I'll wager that the hollow pins are made from that as well. 316 stainless has a specific gravity (Sg) of 7.95 and an elasticity modulus (E) of 28,000 ksi. Even the strongest steels have an E of only 30,000 ksi. First, lighter. That's a no brainer. Yes, tubular pins are lighter than solid stainless pins: W = Lπ(ro²-ri²)Sg = 76.17 grams vs. 133.12 grams However, the aluminum pins they replace only weigh 46.88 grams. Now bending resistance: The second moment of inertia for a circular beam = πr^4/4 = 9.7072e^-4 in^4 for a 3/8" diameter rudder pin. For a 3/8" tubular section with a .065 wall = π/4(ro^4-ri^4) = 7.9386e^-4 in^4 Which means that a solid rudder pin has 1.22 times the bending resistance of a tubular pin with a 0.065 wall thickness. That's before you get into the gnarly non-linear stuff like buckling. For grins, let's compare a solid aluminum pin (E=10,000 ksi) with a hollow stainless pin: Assume L is the same, then Ps(Al)/3E(Al)Is = Ph(SS)/3E(SS)Ih Ps/29,212=Ph/66,684 So a hollow stainless pin is about 2.29 times more resistant to bending than a solid aluminum pin. On a practical matter - I've had to saw out both aluminum and hollow stainless pins. I've never had to do it with a solid stainless pin.

Author:	Harry Murphey [ Thu Nov 11, 2010 5:12 pm ]
Post subject:	Re: Nylatron rudder pins?
Hi Matt, How did you post the formulars???? My Keyboard doen't have a 3.14 (ie: "pie") symbol on it .... Yes, that does look correct .... In my ol' textbooks they call that "rupture" of a thin walled column/tube/vessel when it no longer retains it's shape and deforms .... I 've cut out w/ a hacksaw both aluminum and "solid" SS rudder pins ... but it took a Dremel tool w/ a cut-off wheel to "grind"/cut the Murray's pin as the hacksaw just dulled and didn't make any real progress .... Now w/ aluminum rudder pins I've observed that they can undergo "plastic deformation", as from the shear forces the pin takes on a "stepped" look, as the section that is in the gudgeon is slightly out of column w/ the rest of the pin ... when it get bad enough the pin can be no longer removed. Now I have never seen a SS rudder pin do that .... they do the "traditional" bending .... So the $64.00 question is what alloy are the Murray's pins made out of and what are it's characteristics .... and when is enough strength enough ..... and the rest is just extra wieght and overkill??? And Matt, I am impressed w/ your engineering and computer skills ... do you have a special keyboard w/ engineering symbols???? In the Mech Engineering studies at Drexel, what we are discussing here started in Statics l ... through Mech of Materials (Advanced Statics ll), Theory of Machines, and also continued in Advanced Materials Engineering in the Materials Dept which we were also required to take ..... heavy duty courses w/ alot of greek symbols/DiffEq .... and lots of long hours "doing books" ..... in one of those advanced courses we where taught that the loads are not distributed evenly through out material but where carried at the surfaces ... furthest from the central axis/center of gravity .... now I was just the lowly student ... the teachers where the ones w/ the Phd's ... There are many different SS and aluminum alloys ... w/ new ones being developed for new applications every day/year ....

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Author:	MBounds [ Thu Nov 11, 2010 5:48 pm ]
Post subject:	Re: Nylatron rudder pins?
Harry Murphey wrote: How did you post the formulars???? My Keyboard doen't have a 3.14 (ie: "pie") symbol on it .... There's a handy little program that's been on every Windows machine since Win95 called "Character Map" Start>All Programs>Accessories>System Tools>Character Map Choose your font (Arial is best) and you have access to typographical symbols (§,©, ®, ¶, ¿, æ for example), upper/lower case Greek and some superscripts and fractions. Depending on the font, it has the Cyrillic (Д), Hebrew (א) and Arabic (ظ) character sets, too.