Structural GRP question about temperature.

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jonjo

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When beefing up a GRP hull section with extra epoxy laminate how important is the maintain at 20 degrees C for 24 hours rule?

The extra laminate will be internal, 5mm to 10mm in depth and comprise of 3 sections each 3 ft long x 1 ft x 90 degree turn up the sides of a floor grid to a height of about 8".

I was hoping a few kw of internal electrical heating would be enough to keep a chill off the laminate overnight during a UK September haulout.

This is just a round about way of saying can I avoid additional mast off/in-shed expenses?

And by the way before some ask, the keel is fine thanks I am just musing over the option of beefing up the keel bolt support to increase reserve strength in the area.
 
The glib answer is what do you recommend? In practice I will ask a pro to implement a scaled down version of the J&J designed fix that Bavaria applied to the larger Match 38's and 42's.

My hull is flow coated polyester. I need to dig out details on the J&J spec but from memory it was about 8 layers including woven rovings.

The Match 42 has 12 to 13 mm of hull skin laminate where the 2.5 ton keel hangs on 7 keel bolts on a very small root. My keel is about 1.8 tons attached via 11 bolts. My keel root is 1 meter long and narrow in section.

One of the best small boat surveyors in the UK studied my keel and hull as the boat was repeatedly lowered onto the keel quite aggressively. He pronounced the keel attachment and bilge reinforcement remarkably solid for a fine keel boat and said he could not detect any movement.

My objective is to bring the hull thickness up to 20mm to lessen any tendency of the hull to deform locally around keel boats when under tension, which could then transmit high stress onto a sharp 90 bend where CSM tabbing bonds the floor grid to the hull. I believe the process of failure that lead to the loss of the Match 42 keel started with fracture of the grid/hull tabbing near the keel bolts which in turn allowed the keel to flex in a lumpy seaway while racing, which in turn led to an accelerating fracture of the grid to keel bond.

I also plan to introduce much larger metal plates to distribute keel bolt stress. Currently I have one 8mm plate about 30cm square on the centre 4 bolts and thick 3 to 4 inch washerson the other 7 bolts for and aft.
 
Yes I think keeping it warm is important but yes I think heaters will do the job. You might consider insulation around the outside of the hull to stop heat escaping.
To do the job really well the proffessionals would try to get a vaccuum cover over the new resin. ie polythene sealed around th edges with an absorbant layer underneath and also a porous layer (also allows the vaccuum to spread) The vaccuum is connected to the bag such that atmospheric pressure presses the resin and glass into the base material. This reduces air bubbles and the absorbant layer removes excess resin. This is most important in aircraft repair to reduce weight. The whole patch is then covered with a blanket type heater with temp sensors.

I don't imagine you wan to go to those extremes and of course you are not worried about excess weight. But the pressing together of the resin and glass onto the frame with exclusion of air bubbles especially on tight corners could be beneficial.

Incidentally beware of the problem of stiffening one area so that any bending action is magnified to another area. To explain... take an aircraft wing. You get cracks say 1 metre from the root. You put reinforcing .5 metre wide across the crack. Now when the wing bends there is extra bend near the root and also more bending beyond the patch. Inevitably it cracks just beyond the reinforcing.
The answer to this problem is to try not to over reinforce. If you do choose to reinforce you taper the reinforcing either by thinner material progressing toi the edgees or by puting in finger shaped edges to the reinforcing rather than a straight line. (like a zig zag edge)

I can only imagine your reair scheme but consider if possible tapering your reinforcing edges to avert a sharp delineation between reinforced and original.

Sorry just waffling and don't mean to worry you but something to think about. Cos every part of a F/G hull has to flex in some way. Stop the flex in one place can magnify the flex elsewhere. Good luck olewill
 
If you're an engineer or similar then ignore the following as one assumes you have then thought it all through. Also, I recognise that Olewill has made introductory comment to the following based on experience with airframes.

Personally I would be very wary of stiffening up any part of a lightly built boat's structure without good advice from a yacht designer or naval architect experienced in composite yachts. It would seem, if you are anticipating going from 12mm to 20mm thickness (I assume that is plating thickness) that you are adding a hell of a lot of stiffening.

The problem is that stress is only relieved by dissipating it as energy through deflection in the structure and diluting it by spreading amongst structure so in the case of a keel grid, if it were perfectly rigid (which it won't be) all stress will be transferred out of the grid to the surrounding hull plating and structure (if any). The stiffer it is the more stress that will be transferred to the rest of the structure and that may not be what the designer intended.

This can be most frequently seen in damage to light fast big MoBo's when they have had a large slamming type experience such as a big slam off a big wave at speed. Even though there may be no damage to the bottom plating and structure, there may be damage elsewhere. The stress is transferred off out through the structure, gradually being dissipated as energy through deflection and dispersion into wider structure but that which is left may be sufficient to fracture remote structure when it meets a discontinuity in it or else the structure there was just not designed for such added stress. Some years back I had a naval architect who was very good at this as he had worked for the navy investigating the effect of underwater explosions on composite hulls and we worked on a number of such damage based insurance claims together.

So, in the keel structure situation for grounding, shock load from boat falling into a hole in the sea, in a knockdown (the keel being horizontal is the worst static case for keel root stress) or even just from transferral of cyclic deflection to areas not intended resulting cracking, one has to be sure that the whole structure into which stresses will be transferred is adequate. So this does not necessarily mean just strengthening the local area - if just that is done it may then mean that at a structural discontinuity elsewhere, or even just at the surrounding plating and structure, they will have stresses transferred into them for which they were never designed resulting in later failure or cracking.

It may not be a problem (and generally never is in more heavily constructed boats) and what you propose may be perfectly adequate but if was me I would take expert advice for a lightly constructed boat. Such advice may be from the original designer (if willing), appropriately experienced yacht designer/naval architect, or from marine composite design experts such as High Modulus in Hamble (there are others, I just mention them as they are HO'd in my home country). A surveyor is not capable of offering appropriate advice.

Personally, I would never buy a second hand light vessel that had added structure not certified as appropriate by an appropriately experienced yacht designer or naval architect.

As I said, if you have sufficient engineering knowledge to have already considered all that, then disregard it - is not my intention to teach anyone to suck eggs /forums/images/graemlins/smile.gif.

John
 
I think Ships-Cats caveat would be well to heed. Given the issue with these particular boats, I don't know if temperature of the laminate during re-inforcing should be your primary consideration - have you expert advice on exactly how this repair should be undertaken?

Having said the above, the simple anwaer to your question would be to ring the epoxy manufacturer and ask them. They may well have a formulation that will cure at lower temperature to meet your requirements.
 
You said "additional mast off/in-shed expenses" so I assume you will be keeping the keel in place. If so then the keel will determine the temperature of the area. Heating the surface might mean just the surface is warm.

Also making GRP so that it is the correct ratio of resin and glass (and no air) around the bolt heads will be key. If the keel bolts are not removed then getting that right will be hard. A wrong ratio and the area will be weaker in compression than the original. Spreading a load as the others have pointed out requires no sharp discontinuations of strength. Larger thick metal plates might just move the stress nearer the weak point of the internal frames and cause more problems. (like Ships_Cat said)

If the issue is with the framing parting from the hull, then there are other ways to avoid this. One suggestion would be extra framing (vertical fillets) between the bolts and outside the first line of the structure. This would soften the hard edge formed by the frame. If the problem is the frame joint, then concentrate on it. Making the keel area more rigid will just means more stress arrives at the weak bit.

I would love to know what aggressive lowering is, as opposed to dropping! Most boats can be checked for hull shape change with a straight edge and a few marks. For some you only need your eyes! Others you have to arrange spreaders and a few acro props.

However in the end it is a design job and depends on how the sideways forces in the keel were meant to be distributed.
 
Like all reaction, the hardening of epoxy is temperature dependant. So you need to talk to the epoxy maker to establish the performance of any particular resin / hardener combination at a particular temerature. I seem to remember that West have hardeners that will work even at zero centigrade.
 
Agree with all Cat and others say But would add... If it ain't broke why mend it?
If you are determined to do it then why use epoxy? Standard Polyester or perhaps Vinylester will be more tolerant to lower temperature (usually 16deg) and be much easier to use. Wetting out using epoxy is very difficult. If you clean and then abrade the area thoroughly before laying up the first CSM layer you will get a good bond and a structural strength of the new laminate equal to the original hull layup. Frankly I wouldn't be doing this at all but if I did I would want a resin similar in properties to the original layup. Also make very sure you "fade out" the thickness over a very big area to avoid creating local stress points. Ignore the amateur pundits who suggest you will not get a "chemical" bond using polyester. Most people that witter on about "chemical bonds" don't understand what they mean by the term anyway! Its a common fallacy. If you don't believe me just try bonding a small area somewhere unimportant and try chiselling it off again afterwards! Epoxy does certainly have superior adhesive properties but grinding the area with a flap wheel to remove all surface crap and then degrease well will achieve the bond you require. Also remember that temperature is only part of the picture. Humidity in this case is a bigger problem. I would NOT layup ANYTHING below the waterline before drying out first as any residual moisture in the hull will cause problems far worse than slightly low temperature. A check with a moisture meter would be worth doing.
 
You make a good point here Neil. The keel should be supported by frames, not just the skin. And I also agree that the keel bolts should be removed and the new laminate drilled through. Any local increase of stiffness that did not include the keel to hull attachment could be a disaster.
 
Re: Photo and Diagram

The following links might help paint a better picture of the proposed reinforcement. The first is a photo of the bilge area with a shoe and drinks can to provide a sense of scale (previously linked to in a YBW thread last year).

http://i14.photobucket.com/albums/a340/BavariaMatch35/KeelGridWhole.jpg

The second is a fuzzy copy of the J&J spec for the mod to match 38's and 42's.

http://www.bavariayachtbau.de/typo3/typo3temp/pics/f2f45e4496.jpg

I propose to reinforce the 3 bilge wells into which the 3 groups of keel bolts emerge. This involves laminating in 3 unconnected high sided trays with the bond taken high up the sides of the floor grid.

Note the small 4-5 inch gap between the aft keel bolts and the grid. If the hull locally deforms due to a snatch load from the keel in a seaway then 100+ kg of pull will be transmitted onto the tabbing at the weak 90 degree turn. The leading 4 bolts are similar.

This thin tabbing should transmit tensile forces from the keel bolts into the very substantial grid but is the Achilles heel of the whole Bavaria keel attachment design. I am not an engineer but my gut instinct is that if all keel forces are transmitted into the grid there is no doubt onward forces will be gracefully transmitted out into whole central third of the hull.
 
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Yes I think keeping it warm is important but yes I think heaters will do the job. You might consider insulation around the outside of the hull to stop heat escaping.

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I had not considered this and as another poster commented the keel could conduct way the heat from any internal heating. Since the keel is shaped like a large dinghy dagger board it would be quite easy to wrap it up in a roll of loft insulation. I must research the specific heat capacity of 1.8 tons of lead!
 
John,

The risk of creating a weak point elsewhere has troubled me. I hope the new pictures posted at the top of this thread will help.

Assuming the mod eliminates any flex between keel bolts and floor grid I think the question then becomes will the grid flex a bit across its whole structure or could it be oversized and too rigid? If too rigid we then have to imagine how will forces dissipate at the upwind outside perimeter of the whole grid structure? Hopefully the area of concern is now so large there is no chance of the whole grid trearing a 10ft x 10ft hole in the hull.
 
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Having said the above, the simple answer to your question would be to ring the epoxy manufacturer and ask them. They may well have a formulation that will cure at lower temperature to meet your requirements.

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Ok thanks I had not realized the epoxy formula could be adjusted to suit different working environments. The top priority has to be excellent adhesion to the existing polyester structures particularly the area of the new laminate that will run up the sides of the floor grid.
 
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Also remember that temperature is only part of the picture. Humidity in this case is a bigger problem. I would NOT layup ANYTHING below the waterline before drying out first as any residual moisture in the hull will cause problems far worse than slightly low temperature. A check with a moisture meter would be worth doing.

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Hopefully I am in the clear here, the hull is not quite 2 years old and has spent 20 months in the water. Two coats of epoxy paint were applied to the hull before launch.

But anyhow an internal bilge moisture check is now on my list.
 
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You said "additional mast off/in-shed expenses" so I assume you will be keeping the keel in place?

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Yes. Bavaria mate their keels to the hull using a rigid epoxy glue and I have heard that persuading an undamaged keel to separate from the hull is a major battle.

I can imagine a frustated yardhand trying to break this seal by applying high leverage to the bottom of my 2.2m keel and causing the hull skin to break away from the internal grid which is exactly what I am tryinig to preempt with the reinforcement.

I take your point that the extra laminate moulded around insitu keel bolts would be 2nd rate but if limited to say 2cm around the bolts I would not expect this compressible laminate to affect the security of the clamping action between a much larger 8mm plate of say 25cm x 50cm and the keel below.
 
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to answer a previous post polyester doesnt stick well to cured polyester thats why epoxy is used for repairs

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Not true if the surface is well prepared. I agree with everything Boatmike said on the subject.
Epoxy is a superior adhesive but has totally different stretch and thermal properties and used structurally may increase local stress.
Provided that the existing polyester surface is thoroughly cleaned and roughed up to the point of exposing fibres and then soaked with acetone to soften the resin a good mechanical and chemical bond will be made.
 
Starred at it for a while and I see your point. The mast base tray looks a lot better glassed than the rest. Also the nuts and lack of obvious load spreading.

The diagrams in German look like they have more than just glassing added. What are the 3 structures added? Two have 3 frames and 1 has 2 frames. They seem to be spreading the load to half way out to the longitudinal main frames. I think they are the key, not the glassing of the frames alone.

The new frames seem to be either side of the bolts and glassed down to the fresh thick glass going under the bolts. I think these are load spreaders to stop the nearly flat hull from distorting. If the new frames are glassed down to new glass layers then the whole structure would be quite strong and delamination from the existing boat would not impact its strength. Since you would be relying on the existing frame to hull glassing.

The resulting structure is 5 U shaped sections with the bolts going through their bases and transferring the load to half a frame distance sideways.

Other structures built out of G10 of varying thicknesses and widths, layer on layer, could also be a solution.

However, I am not that sort of engineer and leave it to my brother to do those calculations.

For a strong bond to old fibreglass the best method it to take it back to the laminate structure so that there is a multitude of glass strands exposed on the surface. The sort of surface that would leave you with a severe fibre glass rash on your arms. Then when you laminate, it ties itself into the first layer. I have done it on the outside of boats successfully but never inside.

PS it looks like I agree with "mono"
 
8mm plates of metal would have too sharp an edge to distribute the loads. It has to be distributed over quite a distance. Even those new frames on the drawings have a longitudinal piece and then what looks like a pad of extra reinforcing underneath
 
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