Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 23-02-2014, 15:52   #31
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,153
Re: Welding fiberglass.

Fiberglass and plastic melt at very different temperatures. I don't see how one could melt individividual strands of fiberglass back together at the melting point of fiberglass without burning the hell out of the plastic, which has a much lower melting point.
__________________

__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 23-02-2014, 16:15   #32
Resin Head
 
minaret's Avatar

Cruisers Forum Supporter

Join Date: Aug 2007
Location: Seattle WA
Boat: Nauticat
Posts: 7,201
Images: 52
Re: Welding fiberglass.

Quote:
Originally Posted by David M View Post
Fiberglass and plastic melt at very different temperatures. I don't see how one could melt individividual strands of fiberglass back together at the melting point of fiberglass without burning the hell out of the plastic, which has a much lower melting point.


Poly resin does not melt at any temp, it burns. I've fixed a lot of burn boats, very familiar with what happens to FRP at high temps.
__________________

__________________
O you who turn the wheel and look to windward,

Consider Phlebas, who was once handsome and tall as you.
minaret is offline   Reply With Quote
Old 24-02-2014, 03:05   #33
Senior Cruiser
 
s/v Beth's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Location: Coos Bay, Oregon
Boat: Valiant 40 (1975)
Posts: 4,066
Re: Welding fiberglass.

Yeah, we should probably make a new thread. Since I am not currently doing anything....
__________________
s/v Beth is offline   Reply With Quote
Old 24-02-2014, 11:46   #34
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Welding fiberglass.

Quote:
Originally Posted by Delancey View Post
<SNIP> an anecdote about one of the Gougeon Brothers <SNIP>

After pointing out the fact that stiffness and the ability to resist deflection is what you are really looking for in a boat building material, he proceeds to pull several materials samples out of a briefcase. They all have the same weight and measure the same length and width and include samples of Western Red Cedar, aluminum, steel, and a piece of carbon fiber composite.
<SNIP> let's say the steel sample was .060" thick, the aluminum sample was .180" thick, the carbon fiber sample was 1/4" thick, but the lowly WRC sample was 3/4" thick.

Can you guess which were the hardest and easiest samples to bend? If you guess the strongest and densest material was the easiest you are right!

<SNIP>.
Thanks for that; it's a great illustration of something which is not well understood by the general public.

The relationship between *stiffness* and panel thickness (all other things being equal) is a third power one.
In other words, if you double the thickness of a panel, you make it double x double x double the stiffness, or eight times as stiff. (In other words, you could put eight times the load at the centre and still only have the same deflection under a given load [#*#])

Whereas the relationship between *strength* and panel thickness is a second power one: if you do double the thickness of a panel, you make it double x double the strength, or four times as strong. (In other words, you could put four times the load at the centre before it would fail)

In both cases, naturally, the panel will weigh twice as much, but it will be four times as strong and eight times as stiff.

Separately from considerations of the panel dimensions, the material itself brings its inherent stiffness to the table, and also its inherent strength. These are independent variables, and (as I mentioned recently in another context) steel is an interesting example, because all steels, regardless of alloying constituents, and hence over a huge range of strength variation (nearly an order of magnitude), have pretty much unchanged inherent stiffness.

I don't know much about wood, but I imagine the picture is very different there. And certainly plastics have quite diverse values for inherent stiffness. I don't recall what Starboard is composed of, but it seems remarkably stiff (I agree with delancey that this is probably what Newt's dodger is built from)

PLEASE NOTE IF YOU READ THIS FROM THE EMAIL NOTIFICATION:
[#*#] : in my original unedited posting, this sentence was different, and WRONG
__________________
Andrew Troup is offline   Reply With Quote
Old 24-02-2014, 12:06   #35
Registered User

Join Date: Oct 2010
Location: Duluth, MN
Boat: Morgan 383
Posts: 129
Re: Welding fiberglass.

I'd like to know how stiffness and strength are defined, and what units of measure are used. Thanks.
__________________
Dale Hedtke is offline   Reply With Quote
Old 24-02-2014, 13:07   #36
Registered User

Join Date: Dec 2013
Posts: 209
Re: Welding fiberglass.

Quote:
Originally Posted by Dale Hedtke View Post
I'd like to know how stiffness and strength are defined, and what units of measure are used. Thanks.
By applying force and measuring deflection, plotted on a chart, until failure. Deflection is stiffness and force at failure is strength. Units depends on where you are. Engineers usually use millimeters and newtons. I use a hammer and a vice.

Sent from my SCH-I415 using Cruisers Sailing Forum mobile app
__________________
wellin is offline   Reply With Quote
Old 24-02-2014, 13:30   #37
Registered User
 
Delancey's Avatar

Join Date: Oct 2011
Location: Miami, FL
Boat: sunk by irma
Posts: 3,466
Re: Welding fiberglass.

Quote:
Originally Posted by Andrew Troup View Post
Separately from considerations of the panel dimensions, the material itself brings its inherent stiffness to the table, and also its inherent strength. These are independent variables, and (as I mentioned recently in another context) steel is an interesting example, because all steels, regardless of alloying constituents, and hence over a huge range of strength variation (nearly an order of magnitude), have pretty much unchanged inherent stiffness.
Not to completely distract from the subject at hand, but I find this comment to be a particularly interesting observation in light of the fact that there is at least one forum member here who is obsessed with the notion that anchor shanks should be designed and built to be "unbendable" however impractical or unrealistic that may be.

I am not an engineer, and only know what I know through hands on experience making and breaking things, so it is something of a revelation for me to learn that, in fact, alloy type has a limited influence on stiffness. Also uncertain of how temper factors into this equation of variables?

In your opinion, is there a significant performance benefit to the use of tool vs mild steel is manufacturing anchors to justify the cost? I seem to recall this individual went so far as to propose using ballistic armor plate to achieve their goal.
__________________
Delancey is offline   Reply With Quote
Old 24-02-2014, 14:11   #38
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Welding fiberglass.

Quote:
Originally Posted by Dale Hedtke View Post
I'd like to know how stiffness and strength are defined, and what units of measure are used. Thanks.
Stiffness is measured in the lab by pulling on a (usually round) test bar and measuring how much it extends. This is called "strain", by both scientists and engineers.
Strain does not have any units: it's a simple, dimensionless ratio: the change in length divided by the original length.

The applied stress at a given load (see below) is then divided by the strain at that same load to arrive at a generalised measure of the inherent stiffness of the material.

The result is called the "Modulus of Elasticity" or "Elastic Modulus"

The units are load per unit area, and while they share the same units as stress, set out below, they measure something quite different

Strength is measured by continuing to pull on the test bar until it starts to deform permanently (eg, the lowest load at which it will no longer revert to the original length if you remove the load). Scientists call this the "Elastic Limit"; beyond this point, the metal undergoes "Plastic deformation"
The engineering term for the elastic limit is the "Yield Point" (there are nuances I won't go into) and this is one criterion for assessing strength.

Further load will eventually break the test bar, and the load at this point is called the "Ultimate Tensile Load"

The effect of loading a material, when we're interested in strength, is to create 'stress', whereby the individual atoms are being pulled further apart, or pushed closer together, than their natural distance.

The current units of measure for stress (and consequently, Elastic Modulus) are Newtons per square meter for scientists, MPa for engineers (a similar but more convenient measure) -- or in US customary units (used by engineers in the US but not scientists): pounds per square inch.

The 'Newtons' captures the force or load applied, and the 'per square metre' captures the cross sectional area of the test bar. If the area is twice as great, the material can handle twice the load.

During the elastic phase (at loads too low to permanently deform the metal) stress is proportional to strain, more or less perfectly depending on the material. Some non metals are quite imperfect, in this regard.
__________________
Andrew Troup is offline   Reply With Quote
Old 24-02-2014, 14:14   #39
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Welding fiberglass.

Anyone whose eyes have not yet glazed might have noticed the units for stress happen to look like a 'pressure'

... and it seems superficially as though stress is analagous to pressure, but there is a lot more nuance to stress than there is to pressure: there are tensile (or compressive) stresses, and shear stresses, both of which vary throughout the material in intensity and direction.

These in turn may arise not simply from pulling or pushing (tension or compression) but instead or as well they may arise: from twisting loads, bending loads, hoop stresses (in a closed vessel subjected to internal pressure), local stress concentrations etc etc.

Whereas pressure is a uniform, directionless, simple construct.
__________________
Andrew Troup is offline   Reply With Quote
Old 24-02-2014, 14:34   #40
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Welding fiberglass.

It might be confusing that stiffness is measured in tension, because most commonly we are interested in stiffness for sideways loads, like how much pressure a panel can withstand on one side, or how much downwards load a horizontal beam can handle.

But bending actually gives rise to tension and compression within the material.

Lets make it simple and consider a steel rule, overhanging a table and clamped to it with one hand (you know, the way you get it to make that wonderful noise like a knife thrown hard at a wall!)

If you pull down on the free end, the graduations on the top side of the rule will move further apart, and those on the underside will move closer together (imperceptibly, unless you have a toolmaker's microscope handy). Halfway through the rule, there will be no change in length, and hence no strain, and hence no stress.

Provided you don't overdo it, when you let go, the rule will revert to its original dimensions and straightness.

But the resistance to flexing is caused by the atoms on top of the rule resisting being pulled apart, and those on the bottom being pushed together. (Which conveniently is almost exactly the same degree of resistance, for metals within their elastic mode)

So the stiffness measured in a lab in pure tension is useful and directly applicable when working out stiffness in sideways bending.

But the key concept is that the "Stiffness" concept is only applicable up to the point where a permanent change takes place (the Yield point, or Elastic Limit, mentioned in an earlier post)

So Delancey's question about anchor shanks needs unpicking.
(I'm ducking into a trench and donning flak jacket, because recent marketing practices have rendered anchor discussions highly emotive, bordering on irrational)

Stiffness is not strictly applicable: it tells us how many degrees of bend a given shank will adopt, given a particular sideways load, for any load insufficient to cause the material to yield (deform permanently). You might think of stiffness as the predictor of behaviour during flex, up until the point where the shank becomes bent. (Not engineering language, but perhaps more familiar)

So stiffness data tell us nothing about how much load it takes to BEND the shank.
For that, we need to consider strength data.

And that's why high tensile plate is a good choice for the shank of a fabricated anchor, even though it is no stiffer, it's WAY stronger than mild steel --

provided this does not compromise the welds (the higher the tensile strength, the trickier the welding) and provided it does not unduly compromise ductility (very high strength steels can be prone to brittle failure, particularly after galvanising or other electrochemical challenges -- or fatigue -- or etc-- )
__________________
Andrew Troup is offline   Reply With Quote
Old 24-02-2014, 14:56   #41
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Welding fiberglass.

Quote:
Originally Posted by Delancey View Post
<SNIP>

I am not an engineer, and only know what I know through hands on experience making and breaking things, so it is something of a revelation for me to learn that, in fact, alloy type has a limited influence on stiffness. Also uncertain of how temper factors into this equation of variables?

In your opinion, is there a significant performance benefit to the use of tool vs mild steel is manufacturing anchors to justify the cost? ...
I had a crack at answering your second question above, but as for the first (how does temper factor into stiffness?), here's my best shot (from a standpoint of relative ignorance)

For all practical purposes, temper (degree of hardness, arrived at through heat treatment) does not affect stiffness, only strength, for all steels.

I'm guessing this is because stiffness is a measure of how the bond forces between individual atoms change with distance from their neighbours.

Whereas strength is a measure of the force is takes to rupture the bonds, NOT between atoms, but at the interfaces between large collections of atoms, in what it is sometimes appropriate to call a crystal.

It might be helpful to think of crystals as like individual floes or bergs in pack ice.
The material from which the individual bergs are made -- ice -- is much more resistant to being pushed or pulled apart than the larger-scale congregation.

It is the latter that an icebreaker cares about, it is the former that the bartender on said icebreaker has to deal with if he's trying to serve up Pisco Sours with multi-year ice, which he has to crush.

When we heat-treat steels, it's (to a very poor approximation) like warming up a region of pack ice to encourage stronger bonds between the icebergs when it refreezes.

It makes things much harder for the skipper, but not much changes for the bartender.
__________________
Andrew Troup is offline   Reply With Quote
Old 24-02-2014, 16:00   #42
Senior Cruiser
 
hpeer's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: Philadelphia
Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters
Posts: 3,902
Re: Welding fiberglass.

Porte vote

Material Buffered Copolymer Polypropylene

What does that mean?
__________________
hpeer is offline   Reply With Quote
Old 26-02-2014, 03:46   #43
cruiser

Join Date: May 2007
Location: Brisbane AUS
Boat: Cowther 43 - Hunter 40.5
Posts: 1,006
Re: Welding fiberglass.

When the waterpump broke on my gen set (no exhaust cooling) the plastic muffler melted. I was able to cut an unused mounting tab off it and use it as a welding filler and fixed the hole with it. That was 2 years ago and the repair can be considered permanent.
__________________
dennisail is offline   Reply With Quote
Old 26-02-2014, 05:20   #44
Marine Service Provider
 
boatpoker's Avatar

Join Date: Nov 2008
Location: Toronto, Ontario or Bahamas
Boat: Benford 38 Fantail Cruiser
Posts: 2,593
Re: Welding fiberglass.

Quote:
Originally Posted by dennisail View Post
When the waterpump broke on my gen set (no exhaust cooling) the plastic muffler melted. I was able to cut an unused mounting tab off it and use it as a welding filler and fixed the hole with it. That was 2 years ago and the repair can be considered permanent.
On another thread I had warned about the dangers of melting plastic mufflers and potential for sinking .... I was soundly trounced.
__________________
That hysterical laughter you hear as you sail a way in your "new" boat ..... is the seller.
boatpoker is offline   Reply With Quote
Old 26-02-2014, 19:10   #45
cruiser

Join Date: May 2007
Location: Brisbane AUS
Boat: Cowther 43 - Hunter 40.5
Posts: 1,006
Re: Welding fiberglass.

Well they WILL melt once the water pump fails. If the boat sinks or not depends on the installation. It would not have been an issue in my circumstance since both the muffler and exhaust outlet is above the water line.
__________________

__________________
dennisail is offline   Reply With Quote
Reply

Tags
fiberglass

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 12:25.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.