[Rhodes22-list] Cockpit Bench Seat Reinforcement - The end of the
day..
Lou Rosenberg
steadilsr at verizon.net
Fri Aug 27 20:55:51 EDT 2004
Stan was right AGAIN folks!
I didn't need to do anything when I reviewed the situation again.
The small amount of fibre glass that is disturbed under my stb seat is
not a good reason to have to add any supports. In addition, the liner
is making the cracking sound as Stan pointed out. The flex of the
seats is nominal and doing nothing is better than doing a two day job
when time and energy are of the essence in my boat refurbishing career
now.
after I sail it for awhile I may change my mind if the seat deflection
really gets to me.
My boat is doing fine, I have removed a leaky portlight and need to
find replacement frames for the old style ('81 boat)
any suggestions???
thanks everyone.
Lou
another Everyday Miracle
i didnt go broke at West Marine
On Thursday, August 26, 2004, at 12:40 PM, stan wrote:
> you have got to learn to listen to god. the sound is from the cockpit
> liner
> rubbing against the seat where the seat cantilevers past the vertical
> end
> (usually the bow end) of the cockpit liner. it is cured by increasing
> the
> clearance so when the seat deflects it still will not reach the cockpit
> liner glass. should not be doing this for nothing - particularly for
> boats
> purchased elsewhere so, since I am a betting man, let' s place a bet.
>
> stan/gbi
>
> ----- 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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