[Rhodes22-list] Cockpit Bench Seat update
Lou Rosenberg
steadilsr at verizon.net
Sat Aug 28 10:31:55 EDT 2004
Roger,
I must have exaggerated the amount of fibers I observed which were
loose underneath.
The cracking was coming from the underside of the stb seat hitting the
liner by the laz.
I m confident it needs no work except for replacing the small amt of
wd along the inside of the seat running the entire length.
thanks so much for your advice.
Now I am onto the replacement of the portlights as duly noted
yesterday.
peace
Lou
On Thursday, August 26, 2004, at 01:16 PM, Roger Pihlaja wrote:
> Lou,
>
> If there are visible cracks or other flaws in the FRP composite
> underneath
> the cockpit seat; then, you will have to do something ASAP. Failure
> of an
> FRP composite structure is a cumulative damage sort of phenomena.
> Every
> time you step on the seat & hear a "crack", that is the sound of some
> glass
> fibers snapping or the bond between some of the glass fibers and the
> polyester resin substrate failing. FRP composites do not "heal"
> themselves.
> Under moderate overload, they just progressively fail one fiber or one
> fiber/substrate bond at a time until the remaining structure is too
> weak to
> support the load. Then these materials fail catastrophically in a
> sudden
> brittle manner. If the cracks are visible on the surface; then, you
> may be
> able to reinforce the structure by injecting unthickened epoxy into the
> cracks with a syringe. Have someone stand on the weak spot while you
> inject
> the epoxy as deeply into the opened surface cracks as possible. Then,
> have
> your assistant get off the seat. Repeating this several times will
> work the
> epoxy up into the FRP composite structure out to the tip of the
> developing
> crack. After the epoxy hardens inside of the FRP composite structure,
> this
> should arrest the crack development. However, you will still need to
> reinforce the cockpit seats or this sort of failure will start occuring
> again.
>
> Roger Pihlaja
> S/V Dynamic Equilibrium
>
> ----- Original Message -----
> From: "Lou Rosenberg" <lsr3 at MAIL.nyu.edu>
> To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
> Sent: Thursday, August 26, 2004 12:13 PM
> Subject: Re: [Rhodes22-list] Cockpit Bench Seat Reinforcement - The
> RestOf
> The Story
>
>
>> Roger,
>> I knew I should have kept studying my advanced math in college
>> instead of playing the tromone!
>> I am planning to add the stiffener in form of wood, running the
>> entire length of each seat as well as the two supports from this wood
>> stiffener to the floor. The entire assembly will resemble a
>> truncated triangle . I am planning to use epoxy each support ( which
>> will inevitably be a different size) to the floor to underneath .
>> I dont discount Stan's previous post about why the creaking is Ok its
>> just that I dont really have a creak as much as a loud CRACK when I
>> step on my stb side seat in one location. Looking underneath i found
>> the glass beginning to come apart. I could just add support to this
>> weakened part as I feel comfortable doing that type of repair but I
>> am NOT comfortable with the amount of FLEX in my seats now. The
>> cushions I inherited are not good and will be replaced and I dont
>> believe the seats would crack . I just want more support and the
>> additional wood on the floor will allow me to add a netting for
>> storage of sailing items like PFDs and the like. THis is not a major
>> repair alteration for me. My next job is to take out my portlights
>> and re seat all with new acryllic (sp?) and a re seal. thats a job.
>>
>> MUCH THANKS FOr the physics of the seat situation I will review it
>> later in detail with my step son who is studying at Cooper Union.
>> peace
>> Lou
>>
>>
>>> Lou,
>>>
>>> I see the list already forwarded my cockpit seat reinforcement
>>> solution
> to
>>> you.
>>>
>>> Your proposed method will also stiffen the cockpit seats somewhat.
> However,
>>> what you are proposing to do may not sufficiently stiffen the seat to
> stop
>>> the creaking. I believe the creaking sound is caused when the
>>> inboard
> edge
>>> of the seats deflects under load, thus causing certain structural
> components
>>> to move relative to one another while pressed hard together. For the
>>> purposes of this discussion, let's simplify the physical situation &
> model
>>> the cockpit seat as a simply supported beam, point loaded in the
>>> center.
>>> This model is not perfect; but, it's good enough to illustrate my
>>> point.
>>>> From structural mechanics, the deflection of our model beam is
>>>> given by:
>>>
>>> d = P * (L^3) / (16 * E * I)
>>>
>>> Where:
>>> d = beam deflection (inch)
>>> P = point load in center of beam (lb)
>>> L = unsupported beam length (inch)
>>> E = modulus of elasticity of beam material of construction
>>> (lb/inch^2)
>>> I = moment of inertia of beam cross sectional shape (inch^4)
>>>
>>> In my solution, I added two structural columns per cockpit seat,
>>> equally
>>> spaced down the length of the cockpit. The picture link that was
> forwarded
>>> to you illustrated one of those four structural columns. Adding
>>> these
>>> support columns had the effect of reducing L in the above equation
>>> down
> to
>>> 1/3 of the original length. All else being equal, this also reduced
>>> the
>>> deflection down to (1 / (3^3)) = 1/27 = 0.03704, or only about 3.7%
>>> of
> the
>>> original deflection. i.e. no more creaking under load! I don't
>>> know how
>>> much the seat would have to be stiffened in order to just eliminate
>>> the
>>> creaking. But, for the sake of discussion, let's assume my method
> provided
>>> just enough stiffening.
>>>
>>> In order to get a similar amount of stiffening by your proposed
>>> solution,
>>> you are going to have to increase the product of (E * I) in the above
>>> equation by a factor of 27X. Now, E is a material property. Here
>>> are
> some
>>> values of E for some common materials:
>>>
>>> Al 7075-T6: 10,400,000 lb/inch^2
>>> 1040 Steel: 30,000,000 lb/inch^2
>>> Stainless Steel Alloy 316: 28,000,000 lb/inch^2
>>> Nylon 6,6: 400,000 lb/inch^2
>>> High Density Polyethylene: 200,000 lb/inch^2
>>> Thermoset Polyester: 300,000 lb/inch^2
>>> Epoxy: 350,000 lb/inch^2
>>> Wood Douglas Fir parallel to grain: 1,600,000 lb/inch^2
>>> Wood Douglas Fir perpendicular to grain: 80,000 lb/inch^2
>>> S Glass: 10,000,000 lb/inch^2
>>> E Glass: 10,500,000 lb/inch^2
>>> High Modulus Graphite: 58,000,000 lb/inch^2
>>> FRP Composite Longitudinal: 6,000,000 lb/inch^2
>>> Epoxy/High Modulus C-Fiber Composite Longitudinal: 32,000,000
>>> lb/inch^2
>>>
>>> At the moment, the beam in your cockpit seat is made of some
>>> combination
> of
>>> wood & FRP composite. Since you said the wood is pretty much gone,
>>> let's
> be
>>> generous & say your beam is all FRP composite at the moment. With
> current
>>> materials technology, the stiffest material you could reasonably use
> would
>>> be Epoxy/C-Fiber composite. This will improve the E value by
>>> (32,000,000
> /
>>> 6,000,000) = 5.33X, leaving the rest of the stiffening to be
>>> achieved by
>>> increasing the I value by adjusting the beam's cross sectional shape
>>> &
> size.
>>>
>>> OK, you still need (27 / 5.33) = 5.06X more stiffness. The actual
>>> beam
> in
>>> the cockpit seat has a very complex cross sectional shape. To
>>> completely
>>> analyse the moment of inertia of this shape would require finite
>>> element
>>> analysis using a computer. However, for the purpose of this
>>> discusion,
> we
>>> don't need to get so sophisticated. Let's model the beam's cross
>>> section
> as
>>> a rectangle of width, w = 0.5 inch and height, h = 1 inch and assume
>>> the
>>> beam is bending about the neutral axis running down the centerline
>>> of the
>>> beam. From calculus, the moment of inertia of this beam is given by:
>>>
>>> [I]now = w * (h^3) / 12 = (0.5) inch * (1.0^3) inch^3 / 12 = 0.04167
> inch^4
>>>
>>> [I]reqd = 5.06 * [I]now = 5.06 * 0.04167 = 0.2108 inch^4
>>>
>>> Let's assume we will leave the width, w = 0.5 inch & calculate the
> required
>>> height of the new Epoxy/C-Fiber beam:
>>>
>>> (h)new = [[I]reqd * 12 / w]^0.3333 = [0.2108 * 12 / 0.5]^0.3333 =
>>> 1.72
> inch
>>>
>>> If you wish to use a beam with a round cross section; then, the
>>> required
>>> diameter, d, would be:
>>>
>>> [I]round = pi * (d^4) / 64
>>>
>>> (d)reqd = [[I]reqd * 64 / pi]^0.25 = [0.2108 * 64 / 3.1416]^0.25 =
>>> 1.44
> inch
>>>
>>> So, to get the sort of stiffening required, you would have to
>>> laminate a
>>> rectangular cross section beam of Epoxy/C-Fiber composite of about
>>> 1/2
> inch
>>> width X 1-3/4 inch height or a round beam of about 1-7/16 inch OD
>>> down
> the
>>> underside for entire length of each cockpit seat. You can't get the
>>> required amount of stiffness using just wood for this beam.
>>>
>>> As you can see, cutting the unsupported length of the cockpit seats
>>> by
>>> adding structural support columns is a much simpler solution.
>>>
>>> Good luck!
>>>
>>> Roger Pihlaja
>>> S/V Dynamic Equilibrium
>>>
>>> ----- Original Message -----
>>> From: "Lou Rosenberg" <lsr3 at MAIL.nyu.edu>
>>> To: <rhodes22-list at rhodes22.org>
>>> Sent: Tuesday, August 24, 2004 11:07 AM
>>> Subject: [Rhodes22-list] cockpit bench seat reinforcement
>>>
>>>
>>>> RHODIES;
>>>> Seeking advice on reinforcing the cockpit seats in my R22 (
>>>> 1981) .
>>>> The boat is now secure on is trailer in its winter home in Bklyn
>>>> thanks to Rick Centalaonza, a boat hauler ( Coastal)
>>>> and some good boat yard folks in Pt Jeff and Bklyn.
>>>> My stb side cracks when I walk or sit on it, but is not broken.
>>>> The glass work underneath is brittle and the wood stiffeners are
>>>> missing .
>>>> I have Casey's book on HULL & DECK repair and plan to stiffen and
>>>> support the entire length of both benches by epoxing a dowel in the
>>>> groove and then adding a triangular support made of pine ( since
>>>> its
>>>> not going to be seen much , teak is overkill, I believe). The
>>>> triangular support would need to be epoxied to the floor underneath
>>>> the seats. I plan to space the 2 supports on each side equally.
>>>>
>>>> Has anyone had weakening of their bench seating due to damage or
>>>> weathering and age of the glass?
>>>> I am wondering how the epoxied supports would react to the
>>>> flexing
>>>> of the boat later on as she is under strain. Would it make sense
>>>> to
>>>> not epoxy the supports and just wedge them down with rubber mat
>>>> under
>>>> the wood?
>>>> **
>>>> Any advice appreciated !
>>>> thanks
>>>> Lou
>>>> __________________________________________________
>>>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>>
>>>>
>>>
>>>
>>> __________________________________________________
>>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>
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>>
>
>
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