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Old 07-12-2011, 18:01   #31
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Re: Hull Extensions for Catamaran

From the schionning studyplans
Quote:
THE KIT
The Wilderness “kit” consists of 2400 x 1200 mm
sheets of balsa or foam DuFlex™ which are supplied
with factory cut scarf joins which create a
full strength flush join, no taping required.
Once
joined (in sheet form) and set, the holding tags
keeping the cutout pieces in sheet form are cut
to release full length panels and bulkheads.
http://www.schionningdesigns.com.au/...dyplansa4R.pdf
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Old 07-12-2011, 18:06   #32
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by 44'cruisingcat View Post
This is rubbish.
Thanks, I was hoping someone who has actually built a duflex or foam boat using current building methods was going to step in and comment.
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Old 07-12-2011, 18:09   #33
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Re: Hull Extensions for Catamaran

Thanks to both Cat Man Do & Minaret for all this excellent advice. We are also doing a hull extension on our 45' Wauquiez cat, currently in Phuket, Thailand (we're in Seattle for a few weeks for holidays with the kids). The object is just to bring the sterns out of the water, extending the current underwater profile until they clear. This should result in an extension 2-3' long, & ~1' thick or so. We'd extend the outer wall to come to the end of the extension, but leave the inner wall as is, so the swim-platform would be open at the end & the inside.

Our contractor originally ground back the hulls a bit, then screwed 2 sheets of thin ply onto the outside of the hulls, wrapping it around. Then he waxed the inside & put on 6 layers of heavy biaxial. But when we removed the ply we found 2 problems:
  1. There was a bulging ridge where the 2 sheets of ply were joined. This could have been filled on the inside & then ground down on the outside, but then we'd lose hoop-strength (although all that biaxial is very strong).
  2. They didn't get the mold right up against the bottom at the very back, so there was a 1/4" step. This could have been ground & faired, but it meant that the angle was now shallower than our current underwater profile, so the extension had to go out 4' to clear the water - further than we wanted.
So we cut it off. Now a friend has sent us pics of a cat next to us also doing an extension. He's just put a big blob of pour-foam on his transoms, loosely held in place with some plastic. Presumably he'll remove the plastic, shape the foam as he wants, & then glass the foam in (& onto the existing transoms).


This seems both easier & more likely to produce matching extensions. No "taping" would be needed as the glass could be brought all the way fwd in overlapping sheets to where the transom gelcoat has been ground back.

I'm an engineer, but not a boat builder (obviously). Is this strong enough for a 2-3' extension? I would think so. (Note that our transoms are wider than the above pic)

This would obviously produce a solid cored transom, a sealed "crush-space" if you will. What sorts of problems will this method run into? How well will it work?
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Old 07-12-2011, 18:23   #34
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Re: Hull Extensions for Catamaran

How To Lengthen the Hulls on your Cruising Catamaran

I found this a year ago and thought it was cool. It has pictures of the steps they took and how they did it.

I'm not sure I would ever do it, but it seems like a good upgrade. Do you think it would be better to get a 30' catamaran for $30k less and add on this type of extension to make it longer? Could you shape these extensions to include stairs?
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Old 07-12-2011, 18:56   #35
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by Jon Hacking View Post

I'm an engineer, but not a boat builder (obviously). Is this strong enough for a 2-3' extension? I would think so. (Note that our transoms are wider than the above pic)

This would obviously produce a solid cored transom, a sealed "crush-space" if you will. What sorts of problems will this method run into? How well will it work?
I certainly would never consider doing an extension in this fashion as i am not familiar with the properties of of pouring foams, but I was under the impression that they are mostly non structural, so you will pretty much be relying on the sheer strength of the foam sticking to the hull to keep it in place plus I dont think they are closed cell, so it will suck water if skin is compromised.

It also appears to be not just an extension in step, but also a change in hullshape extending fwd several feet, so more than just a cosmetic change there.

Here is an article from a 2001 Multihull world and this is how I would undertake that sort of project.
Lets hope the scans work ok.

And I am not plugging ATL comp. or Duflex product, I dont use them myself and if doing this I would use any of the other breeds of foam, divinycel, herex, termanto etc along with resins and stitched cloth from your supplier of choice.
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Old 07-12-2011, 19:48   #36
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Re: Hull Extensions for Catamaran




Look at the pics in the manual. Those are some big tapes on those duflex panels!



From the Duflex manual-

Fibreglass and fibreglass taping

The KINETIX
R246TX high performance laminating
system is specified for laminating the fiberglass tapes in
DuFLEX kit construction, and for additional reinforcements
that may be required, such as curved areas of a kit that
need to be strip-planked and fibreglassed on each side,
or for extra reinforcing in specific areas. This system is
specifically designed for room-temperature fiberglass
lamination and provides extended working time and
excellent mechanical properties in the cured laminate.
Unless the design has been specified to have the core
rebated at panel joints, epoxy/fibreglass tapes are
applied on the inside and outside of where the DuFLEX
panels meet.
Prior to taping, it is important to prep the surface well.
Make sure the surfaces are free from contamination and
have been sanded well to key the surface for good
adhesion.
Use WEST SYSTEM resin/hardener with 403 Microfibre
Blend to create a neat cove in the join prior to applying
your taping. Ideally the coving and taping should be
done wet-on-wet to save work and time, and to give a

nice, neat finish.
Take into consideration the number of layers of tape that
need to be applied and stagger the joins to reduce bulky
overlaps, and keep the tapes neat and straight. To
optimise the strength of these tapes, the fibreglass needs
to be oriented in the correct direction over the join. If in

doubt, ask your designer or materials supplier.
1. Unroll the reinforcement and pre-fit it over the joint,
cut it so that several excess inches extend beyond the
taping surface. After pre-fitting, roll up each segment of
reinforcement neatly and set it aside while you cove the
joint. Roll a neat coat of resin/hardener onto the surface
to be taped.
2. Unroll your tape and position it carefully over the wet
epoxy and cove, and in most cases the surface tension
will hold it in place. If the area is too vertical, you may
want to wait until the epoxy becomes tacky. Work out any
wrinkles by lifting the edge of the tape and smoothing
from the centre with your gloved hand or a squeegee.
3. Apply a second coat of epoxy with a foam roller to

thoroughly wet-out the fabric.
4. Squeegee away any excess epoxy before the first batch
begins to gel. Drag the squeegee over the fabric, using
even-pressured, overlapping strokes. The object is to
remove excess epoxy that could cause the fabric to float
off the surface, while avoiding the creation of dry spots
caused by squeegeeing too hard.
5. Finally, run a brush down the centre of the cove to
make sure you have good adhesion.
Repeat steps 2 thru 5 until you have applied the correct

number of tapes to the joint.
If tapes cannot be applied wet-on-wet, it is wise to apply
a layer of peel-ply tape to the last layer to avoid having
to prep and sand the surface prior to applying the next

layer of tape the following day.

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Old 07-12-2011, 20:30   #37
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by minaret View Post
Look at the pics in the manual. Those are some big tapes on those duflex panels!
Look again


You can clearly see on the "actual" panel where it has been joined with the z press, with no additional glass required, exactly like has been repeatedly pointed out yet still you maintain that this is incorrect.

Also the fact that you can see THROUGH the glass to the core demonstrates that the original layup is fairly light, not 6mm (1/4 inch) thick as you seem to maintain is necessary.

The wider glass tapes are joining components together to make one structure.
No where did I say that this would not be a requirement, and is in keeping with what I said here in an earlier post.
Quote:
Originally Posted by cat man do View Post
Take battens off when cured and do glass tape, 2 staggered layers of 600db is more than enough .
The fact that in the picture there appears to be more than 2x600gsm is because it is a bit more than a cosmetic 2ft extension.

Similar high load and transitional joins on my vessel, hull to bridgedeck panel for example, have 1 layer of 600db, 1 layer of 800 uni and 1 layer of 600db locking it all together at 100, 75 and 50 mm laps.

Where a joint is just a continuation of the existing structure, I have a single layer of the same weight as used in the panel it is joining to.
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Old 07-12-2011, 20:41   #38
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Re: Hull Extensions for Catamaran

It's amazing how on these forums, you'll find people who have clearly never used a product, or probably even SEEN the product, arguing with those who have....
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Old 07-12-2011, 20:47   #39
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Re: Hull Extensions for Catamaran

Great pic, looks like some big tapes to me, not no tape at all. I can see through my 1" thick solid laminate hull in spots where there is no gelcoat, transparency doesn't have much to do with thicknesss in a decent lam. Looks very similar to the duflex work I was doing, though we only used duflex panels for bulkheads, floors, interior structure, etc, not hulls. I'm not saying and haven't said that it's never done as you say, just that I've never seen it and would not be impressed by it. Cant imagine the few lbs per panel of weight difference that proper taping would add would make it a "slugomaran".
Also I'm pretty sure the OP's British cruising cat from '94 is not built this way and probably has skin thicknesses right around what I mentioned, so I'm not sure of the relevance. But maybe I'm wrong...
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Old 07-12-2011, 21:04   #40
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by minaret View Post
Great pic, looks like some big tapes to me, not no tape at all.
Now you are being deliberately silly.

No tapes is where the Z press was used, remember?

Quote:
Cant imagine the few lbs per panel of weight difference that proper taping would add would make it a "slugomaran".
Once again are you being deliberately silly?
There was no mention made of tapes, it was the entire panel adding many thousands of pounds to the overall build that made it a slugomaran.

I have quoted the passage below so you can read it again.
Quote:
Originally Posted by cat man do View Post
As an example, I worked on a 40ft "foamcored" production cat a while ago.
Its core was 10mm divinycell and probably 1/4 inch (6mm) of choppies and woven each side.
Needless to say the boat was an under-performing slugomaran.

Performance foam cored boats are as I described earlier
THICK core, lighter skins
Boats like this
and
are made from 20mm divinycel, 400gsm kevlar in and out and a 400gsm BIAX on the outside only for impact/abrasion resistance.
This would be a skin thickness of 1/16th or less
Quote:
Also I'm pretty sure the OP's British cruising cat from '94 is not built this way and probably has skin thicknesses right around what I mentioned, so I'm not sure of the relevance.
The relevance would be that the OP obviously has issues with the vessel squatting in the arse like many older vessels do due to excessive weight in the ends.
To compensate for this, extensions are often employed.

To make the extension as light and as buoyant as possible will help the problem
To make the extension heavy, like the original build and what you propose will not.

All that does is add even MORE unnecessary weight at the ends of the vessel exasperating the original problem.
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Old 07-12-2011, 21:11   #41
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by 44'cruisingcat View Post
It's amazing how on these forums, you'll find people who have clearly never used a product, or probably even SEEN the product, arguing with those who have....
+1, and its getting quite boring as well
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Old 08-12-2011, 06:28   #42
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Re: Hull Extensions for Catamaran

Thanks so much for the advice everyone - even if it led to a spirited debate, it provided me with so much more information than I had before and has encouraged me to undertake the project (although in this climate, it will have to wait for spring). While my original plan - taking plugs off the back of the existing hulls, would minimize fairing, the batten method makes much more sense to me: it will be less expensive and will enable me to curve the aft end of the extensions up slightly in order to ensure that the transoms are well clear of the water.

The only real debate seems to be on the thickness of the laminate and the size of the area that should be ground back for taping on the extensions. I anticipate doing more than I had originally planned, albeit less than the 3 feet suggested my minaret.

On extensions of this size I see no need for bilge pumps or limber holes: I will have a water tight hatch opening on the deck (necessary in order to access bolts for the hand holds and folding swim ladder) and then foam in the bottom portion of the extensions. If they take on water, it won't be much and it will be quite easy to remove with a hand held bilge pump through the hatch opening.

The only remaining question I have is whether anyone has experience spraying gelcoat over epoxy resin? In the past I have heard that it cannot be done, although I have since heard that it is possible. I will use epoxy resin as, of course, it bonds better. However, I would rather attempt to use gelcoat and then fair that in with the existing gelcoat - paint will always show overspray and, with two part polyurethanes it will be very difficult for a non-professional to get a colour match.

Brad
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Old 08-12-2011, 07:48   #43
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Re: Hull Extensions for Catamaran

Is the original layup done in epoxy or poly? I would match the original laminates resin as well as matching the laminate schedule, ie if the boat was built in poly build the extension in poly. If you really want to use epoxy for the lam and finish with gel you can tie coat with 545.
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Old 08-12-2011, 07:54   #44
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Re: Hull Extensions for Catamaran

Are you going to do the mold method with both internal and external skins going on to the transom and only a tape for the external seam, or are you going to laminate the external skin on to the external skin of the original hull as I suggested? Just curious, as it seems to me this is the biggest difference from your original concept. It will make a huge difference in the strength of the joint, although the amateur hour crowd here didn't think to bring it up.
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Old 08-12-2011, 08:23   #45
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by cat man do View Post
Now you are being deliberately silly.

No tapes is where the Z press was used, remember?

Once again are you being deliberately silly?
There was no mention made of tapes, it was the entire panel adding many thousands of pounds to the overall build that made it a slugomaran.

I have quoted the passage below so you can read it again.



The relevance would be that the OP obviously has issues with the vessel squatting in the arse like many older vessels do due to excessive weight in the ends.
To compensate for this, extensions are often employed.

To make the extension as light and as buoyant as possible will help the problem
To make the extension heavy, like the original build and what you propose will not.

All that does is add even MORE unnecessary weight at the ends of the vessel exasperating the original problem.



Hilarious, we're talking about adding a couple of thousands of pounds of buoyancy volume to the hull, and you think it matters whether the part weighs ten pounds or a hundred. Either way it will produce way more buoyancy than it weighs, to the point that it's weight makes little difference as long as it's not heavier than the original hull. I'd be much more concerned about building a part that won't delaminate from the hull, which you obviously arent since you think both skins should be laminated to the transom only and not the hull. How is suggesting he use 2" thick core and 1/16" thick laminate on a boat built with 1" core and a normal laminate helpful? This is information which only applies to your very rare build method. I see you want to impress everyone with the perceived superiority of your build method, but I for one am not impressed, and I don't see how it helps the OP with his question.
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