Gerr Elements FRP Ambiguities?

Hello everyone,

I've been working up a spreadsheet based on Gerr's "Elements of Boat Strength", for cored FRP. With regard to the density of the final layup, and the percent content of mat and roving, it gets somewhat ambiguous. Densities, and fractional contents are provided for all mat, alternating mat and roving, and all roving, almost as if this is a choice left to the designer; however, these three layups would not be the same strength. Further more, the all-mat layup has the least amount of glass, by weight, and the all roving layup has the most. This is the opposite of what I would expect, if both variations are to be of similar strength. I've read, elsewhere on this forum, that Gerr's scantlings are conservative, and I don't want to make my boat unecessarily heavy. Are all three options valid and safe? If I use the all-mat weights (lightest) but use all roving instead of mat (strongest), will the resulting laminate be adequately strong? Any advice regarding the proper interpretation of Gerr's rules is most welcome.

Regards,

David

 

The ratio of resin to mat is lower than to roving. That makes the roving laminate have a higher percentage of glass.

 

If you read Geer's text you'll note he mentions this convolution briefly, but adds "it'll be strong enough". This coupled with the also openly admitted "wholesome" laminate schedules the book recommends, means you'll be working with a sufficiently stiff and strong end result. Also in the text is the suggestion that high preformance craft have careful load and structural analysis, to secure appropriately lighter, yet sufficiently strong schedules.

In short, if contemplating a build, based on Geer's scantling rules, you'll be developing a well founded, cruiser laminate schedule. A more preformance oriented schedule, can have some of the scantling suggestions reduced and a full up race boat, naturally will require you actually preform the math yourself, if safety and ultimate yacht abilities are primary SOR goals.

In the end, the Geer scantling rules are a general guide, that will produce a healthy laminate schedule. If you desire something other then this, you'll need to preform the calculations.

 

Thanks for the replies. I'm not attempting to achieve the absolute minimum weight, but I don't want to "pile-on" to an already conservative design. If Gerr's calculated laminate thicknesses provide an adeqate margin even for an all-mat layup, I cannot see why I should add more glass, and weight, because I'm using biax, which should actually allow me to reduce the amount of glass I use instead of increase it. I think Gerr's approach would be easier, and more logical, if it directly prescribed the per-unit-area weight of glass to use in each hull region, instead of the thickness, which can change with both the type of glass cloth and the layup technique.

 

Yep, but it would be an encyclopedic effort don't you think, instead of a handy reference volume? You could probably safely trim 10% off his suggestions in most places, though now you've ventured from the wholesome scantling schedule he's envisioned. Using his recommendations will not produce an overly heavy boat, but it will be heavier then it "can get away" with if looking for a real preformance edge or production line savings, while producing a harbor queen type yacht.

 

Not that I can see. The glass weight is linearly proportional to the thicknesses he already calculates. All of the engineering is built into that formula. Just re-scale his thickness formula to directly produce the weight of glass, presuming density and percent glass for the all-mat layup. Provide a table with an adjustment factor for each type of layup, which would provide the opportunity to actually save weight by going to a stronger reenforcement. The same table would also provide the weight-per-unit-area of the final layup, for the purposes of estimating the weight of the boat. Leave thickness out of the equation, altogether. I think it would actually be simpler.

Note that, elsewhere in the book, Gerr indicates that using vacuum bagging will result in a lighter (ergo thinner) layup that will actually be stronger for it, and that in that case the amount of glass is not altered. He is effectively acknowledging that the final thickness of the layup is not particularly of interest, from a weight or strength perspective.

All very interesting stuff, regardless.

 

G'day 'gonzo' - 'hay mate - where did you get that info from. If we take the loose term 'roving' which is ment to be 'woven roving' & still is & look at a more lateral definition of - woven rovings & cloths as in woven fabric - incl, glass, kevlar, carbon & dynel - then the ratio of 'completely & properly wet-out' fibers will be much in favour of woven fabrics (woven roving/cloth) over CSM (chopped strand mat).

Next question is - who in their right mind - would build any performance craft - using CSM in these days of such wonderful new technology??? That just looses me totally!! It's not cost/time/results effective in any way!!

Caio, james (semi-retired yacht builder for 45 years & m,ultihull builder/sailor/lover).

 

A small percentage of csm between clothe layers IS beneficial and does count in a high performance lay up !!,any percentage of unreinforces resin layer is a potentual weak place ! so add a little glass makes a differance when it comes to peelabilityof layers !!
sorry you never heard about peel ??

 

Sinjin,

If you would like a better engineering description of laminates and how to design and engineer them, I recommend getting a copy of Robert J. Scott's book "Fiberglass Boat Design and Construction" which gives you a lot more detail about the characteristics of mat and woven roving. It is available from Amazon.com.

For even more information, get a copy of Eric Greene's "Design Guide for Marine Applications of Composites" which you can download as a pdf from his website: http://www.ericgreeneassociates.com/articles.html. Go down to the bottom, it has a link in the bottom right-hand corner.

In general, mat is the weakest material, woven rovings are much better, and the knitted materials are best. They all have useful purposes. Mat soaks up the most resin and it is difficult to get it higher than 30-35% glass content by weight. Woven rovings come next where you are able to get up to 40-50% glass content by weight for all roving laminates. Using mat-roving pairs, glass contents are in between these ranges--30-40%. Strength and stiffness are directly proportional to glass content within small ranges. So the more glass content you have, the more strength and stiffness you have. Also, the higher the glass contents, the thinner the laminates. This is universal for all composite laminates.

Unidirectional materials can get upwards of 60-70% fiber content by weight, but you need to be really good, use impregnators or infusers, and use vacuum bagging techniques. Typically, fiber contents over 70% usually mean you just don't have enough resin to hold the laminate together. Also, it is nearly impossible to achieve fiber contents this higher than 50% with hand lay-up.

I hope that helps.

Eric

 

SNAME has it at a considerably lower price, even for non-members.

 

James,

No desire to build and all-mat boat, here! It was part of our discussion because Gerr includes an entry for the density of such a layup in one of his tables, presumably as a corner case. In fact, part of my original question was whether that layup would be strong enough, given Gerr's calculated thicknesses.

Eric,

Yes, very helpful. I'll definitely use your references.


Regards,

David

 

Eric, great links. I would also add ISO12215-5 that is available in the net in validation version, or can be purchased from iso.org. That standard covers properties of laminates and contains simplified engineering method for scantlings. Better study this standard instead of playing with Gerr's book...

 

Every glass has its use, every resin has it use . Its the combinations that are the difficuly part, and how much resin is enough ,and how much resin is not enough ,there are a million combinations all comes to percentages , right ?? sorry it comes to exsperiance , it comes to know how !!,crunch numbers on a computer as many times as you like but unless you have put the right information in the computer how do you know you getting the right answers ?? how do you know what the answers are anyway ??.
Its all unknown ! its all just guessing , there is no perfect answer becasue every situation is differant . write and read as many books as you can and have so much information you will be confused . There is no black and white yes and no answers that are 100% possitive Just a huge array of mights and maybes and lots a shades of gray .
I been doing my job for 30 years and worked with glass all that time and i know i will never know everything even if i do that job till im 100 years old . If you think you have all the answers sorry you dont , and never will because there are new materials coming on the market all the time !:?:

 

I've already been investigating this, indirectly, using the VectorLam spreadsheet created by VectorPly. Still trying to decide if I should invest in a copy. Does the method in this standard account for out-of-plane (or through-plane) impact resistance? My understanding is that these dominate laminate thickness for small boats (<30'). The required layups predicted by VectorLam, according to ISO12215, seem awful thin, compared to those calculated using Gerr's method. I suspect I'm not using/interpreting the spreadsheet correctly.

 

Thin ! like all things have you feed the beast (computer ) ?? cruising boat , family boat ? what speeds ?? this makes a huge differance !! off shore ? inboard ? out board ? If you want answers you got to feed it right !! thick or thin also has to so with whats inside in the way of frame work , stringers and girders frames etc etc . with a core (balsa or foam) or solid glass ??what best ?? The list of glasses given is just a part, what about resin ?? what resin you going to use of the 3 systems there a great long list of alternatives When building a boats its whole a package deal !! there are no black and white answers just a million shades of gray . the laminate given is also only as good as the people laying the glass as well! the schedual of the stack has a bearing on its performance as well and you can gain simply where and how its all put together .
??