[Rhodes22-list] Cockpit Bench Seat Reinforcement - The RestOf
The Story
Lou Rosenberg
lsr3 at MAIL.nyu.edu
Thu Aug 26 17:02:27 EDT 2004
Ok,
I always listen to GOD. In fact only a few weeks ago the voice said to me:
"buy this boat NOW" I did and I love the experience so far!
I m not a betting man, Stan but I will take your advice as well as
all others who have offered advice on my decision to add or not add
supports.
I will first add some epoxy via syringe under the obvious cracked
area. This boat has not been covered during the summer months I
believe and it shows it. I realize the glass should not be coming
apart but it seems to be just in that one spot. If that does in deed
fix the issue I will not add the wood, and take Stan's word that this
is the way the boat was designed.
thanks again to all!
Lou
_______________
>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
>> >>
>> >>
>> >
>> >
>> >__________________________________________________
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>>
>> __________________________________________________
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>
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