[Rhodes22-list] Cockpit Bench Seat Reinforcement - The Rest Of
The Story
Roger Pihlaja
cen09402 at centurytel.net
Thu Aug 26 14:16:05 EDT 2004
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
>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>
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