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Old 06-12-2011, 08:53   #1
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Hull Extensions for Catamaran

I am considering adding hull extensions to my 1994 Solaris Sunstream 40 in order to compensate for the additional weight that is now being carried aft (eg., davits, dingy, outboard, solar panels, wind generator, radar). Various manufacturers have done this for the same reason - eg, the PDQ 34 became the 36, the PDQ 42 became the Antares 44, the Manta 40 became the 42, the Dolphin 43 the 46, the Gemini 30 became the 32 and then the 104, etc.); I have also read of various owners who have done the same to their own boats.

Adding a couple of feet aft will not only increase buoyancy aft, but also increase the LWL , reduce pitching, provide a swim platform and should raise the transoms back above the waterline and improve the bridgedeck clearance aft.

I am proposing to do the work myself and would be interested in hearing from anyone who has done the same. My current thoughts are as follows:

1. Make plugs from the existing transoms forward to about 6 inches past than the length of the proprosed extensions.

2. Cut out a section from the middle of the plug, measured so that the forward end of the plug will line up over the existing topsides 6" past the transoms.

3. Reconnect the two halves and fair the interior joint. Wax the inside of the mould.

4. Grind off the gelcoat from the corners of the transoms where the extensions will be attached.

5. Secure the overlap of the plug to the existing topsides with screws.

6. Lay up the glass/core/glass sandwich inside of the mould, over lapping the corners of the existing transoms.

7. Remove the moulds and then grind down, glass-in and fair the seams to the hull and the screw holes that were used to secure the plug.

8. Make the 'lids' and glass them to the hull extensions and exisitng transom (leaving, of course, an opeing for a hatch so that one can also glass from inside and install swim ladders and handholds.

9. Refinish with gelcoat to match the existing topsides.

Does anyone have any thoughts on this? I would, of course, especially appreciate hearing from anyone with actual experience in adding hull extensions, but am open to comments and suggestionis from anyone.

Thanks,

Brad
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Old 06-12-2011, 22:53   #2
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Re: Hull Extensions for Catamaran

I've done lots of hull extensions. Sounds like you're proposed extension is fairly short, but we're missing a lot of key info like that here. You're plan will work and is similar to methods I have used for solid laminate hulls, however I wouldn't do it quite like that on a cored hull. Basically, the problem is that your external skin and internal skin will both be folded onto the transoms with the core sandwiched and only a tape on the external joint. This means there will be a lot of grinding and fairing on the external joint, particularly if you grind it deep enough to put on a good strong tape. It also means that the new part is really only transfering loads onto the original transoms, unless you really grind back deep and far on the external tape seam, which kind of defeats the purpose of having a molded part. I would probably find a way to get around the mold or plug here, having done that on small cored extensions before, but even in a mold as you suggest I would pre grind the hull before attaching the mold and just lay the core and laminate the inside skin to the transoms, leaving the outside skin for after the mold is pulled. That means overhead laminating a substantial layup for the external skin, but it also means the external skin can be lapped onto the external skin of the original hulls with no tape seam. This means a much stronger joint and less fairing, if you're a good laminator. If it's not that big an extension sometimes you can forgo the plug completely and build a "plug" out of 5lb ridgid urethane core, laminate the outside from scaffolding, and then laminate the inside as described. Would love to see pics and talk more about it...
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Old 06-12-2011, 23:05   #3
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Re: Hull Extensions for Catamaran

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Ive done them like this before where I glue (hot melt) timber battens to the hull fore and aft with one screw through and into hull where the hull ends.
Lots of packing tape on transom where glass tape will go
This gives you a mould to lay foam into .
Once foam is in, glass it, tape it to hull, add additional steps and cove and tape as you go remembering that packing tape will release.

Once all cured, break off, fair foam, glass outside, bog fair and prime..

Grind up transom where tapes go and glue it in place, timber battens still there to support step.

Take battens off when cured and do glass tape, 2 staggered layers of 600db is more than enough .

Fair in and paint.

Simple.
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Old 07-12-2011, 04:28   #4
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Re: Hull Extensions for Catamaran

Thanks Minaret and Cat man do - great to hear voices of experience. Minaret, you are not doubt correct about the need for additional laminates at the connection to the topsides - although I was hoping to get by as described because the extensions are only two feet long and run up only to the first step (probably 12 inches). Cat man do, what time of foam are suggesting to spray (pour?) into the mold?

Brad
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Old 07-12-2011, 05:09   #5
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Re: Hull Extensions for Catamaran

On a 40ft cat I have done 20mm H80 divinycell or equivalent (sheet)
around 1000gsm of triax should be more than enough (800 would probably do) , I have done 200gsm kevlar and 400gsm biax over the top for abrasion before with no problems on a now 41 ft cat with a 4 ft extension.

On a 2 ft extension I have done 15mm H80 and 600gsm biax in and out with an extra layer of 200gsm db (total 800) where you walk.

The timber battens attached to the hull I speak of is so you end up with a frame like this allowing you to attach the sheet foam

could be some info here for you
http://www.f-boat.com/pages/construction/index.html
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Old 07-12-2011, 05:33   #6
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Re: Hull Extensions for Catamaran

Thanks Cat man do - that was hugely helpful!

Brad
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Old 07-12-2011, 07:01   #7
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Re: Hull Extensions for Catamaran

If you use just the battens it is very difficult to end up with exactly identical transoms. Make up a transom mold for each hull and fasten the battens to it at the correct angle and it will help you to acheive identical shape on both sides. I would never use a two layer tape on a structural member like this, even if it is only two feet long and the inside skin laps on to the transom. The external tape grind should tie in to the whole external laminate if you do it that way. This means you should be almost down to the core at the seam after grinding. The tape in the pics that were linked to is ridiculously inadequate.They look about 6" wide. I would grind onto the hull and part a good foot to foot and a half depending on thickness of external lam. That means a glass seam 3' wide and 1/4" - 3/8" deep. Hard to fair on a part only 2' long. Thats why I suggested no mold method. It's silly to build a fair part and then grind half of it off for the taper on the external seam. Much faster and easier to glass the external skin in one piece lapping onto the hull with no tape seam. Stronger too.
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Old 07-12-2011, 13:33   #8
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Re: Hull Extensions for Catamaran

Thanks Minaret - obviously wider taping is better than narrower, although I certainly wasn't expecting I'd need 3 feet of overlap (i.e., 1 1/2 feet on the hull and the same on the extension) if it was properly attached to the transom. One concern I have is that by attaching the battens to the existing hull and then installing the core directly to the battens, once I have layed up the exterior laminates over the core material I will end up with extensions that are wider than the hull to which they are attached. Or am I once again missing something?

Brad
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Old 07-12-2011, 14:13   #9
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Re: Hull Extensions for Catamaran

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Thanks Minaret - obviously wider taping is better than narrower, although I certainly wasn't expecting I'd need 3 feet of overlap (i.e., 1 1/2 feet on the hull and the same on the extension) if it was properly attached to the transom. One concern I have is that by attaching the battens to the existing hull and then installing the core directly to the battens, once I have layed up the exterior laminates over the core material I will end up with extensions that are wider than the hull to which they are attached. Or am I once again missing something?

Brad
I notice you say these "extensions" are only going to be approx. 2 ft long and 1 ft. high. For a part that small you could probably get away with glassing either way and keeping the lap fairly tight, it's not like it's integral to the boat. I'm notorious for overbuilding stuff like this, but that's why they pay me the big bucks. Technically scarf ratios for a joint like this should be a minimum of 10:1, though most pro's use more like 20:1, ie for a 1/2" thick laminate you would back grind 10" either side of a seam for a 20" seam laminate. In practice you need to take the gelcoat back a little further to make room for fairing, and when laminating something like a hull extension I tend to take the ratio a little further than this even for a really strong joint that will let you sleep at night. I'd guess your external skin is about 1/4"-3/8" depending on where on the boat it's measured. So your external seam should be a minimum of 8" per side for a 16" seam, technically speaking. I would do more, obviously.
There are several standard methods for dealing with your concern with the batten setup. You can pre-grind the hull seam edge nice and fair before installing the battens, so when you install the core against the battens it will be flush with the original core, as I mentioned earlier. This technique only makes sense if your backgrind is quite long, or the sharp angle of the grind will affect the fairness of the battens. You also need a carefully devised transom pattern to assist with the fairness as well.
Another commonly used method is to use fairly substantial battens which you cut a notch out of for the length that will lie on the hull to the depth of the desired thickness of the external laminate. So when you put the battens on to the hull, without grinding it first, they actually are thicker for the part not on the hull by the desired thickness of the lam. Sorry hard to describe in text but I hope you're getting the idea. You cut the length of the batten that will lie against the hull down on a tablesaw by the desired thickness of the laminate. So when you have installed the core against the battens and glassed the inside, you remove the battens and the core is once again aligned with the original core in the original hull, ie the core will be low by the desired thickness of the external laminate. I'll try to find you some pics of an extension done this way....
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Old 07-12-2011, 14:17   #10
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Re: Hull Extensions for Catamaran

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Thanks Minaret - obviously wider taping is better than narrower,
Not necessarily
Look at duflex z joints for evidence of this

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Old 07-12-2011, 14:22   #11
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Re: Hull Extensions for Catamaran

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If you use just the battens it is very difficult to end up with exactly identical transoms.
But they will match each hull, believe it or not in the past I have worked on cats where there have been slight discrepancies in hull width and length, which is why the batten method is good as you end up with a step suited for that hull and no other.

Its also easy if you have an eye
If it looks right, invariably it is
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Old 07-12-2011, 14:23   #12
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Re: Hull Extensions for Catamaran

Obviously on a big extension as I would normally be doing proffessionally, which would generally be a big powerboat or monohull sailboat, for a proper pro extension job I would be cutting out the original transom and glassing the inside seam of the extension to the inside skin of the original hull on a similar scarf ratio as the outside seam, while building an entirely new transom. Obviously you would do this because part of the point of the extension is generally increased interior space which needs to be used in a particular desired fashion, and the running of gear which often goes through the transom like exhausts etc. is simplified if it doesn't go through "two" transoms. For your purposes though keeping this space seperate without limber holes could have advantages, ie an aft "crash" compartment equivalant. But it would probably need individual bilge pumps if you did it that way...
Cutting out the transom, however, opens up the option of installing the battens from the inside in some cases. I've done an integral swim step on a big powerboat like this. Oh, and be careful hot gluing battens, often you will end up off by the thickness of the hot glue, which can be a lot in terms of fairing. I prefer to use more screws and deal with the fallout later. Usually my seams are so wide that most screw holes are covered anyway, at worst there might be one or two rows of screw holes to repair outside the seam. You can gelcoat these at the same time as the rest of the exterior so it doesnt cost much time.
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Old 07-12-2011, 14:27   #13
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Re: Hull Extensions for Catamaran

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I'm notorious for overbuilding stuff like this, but that's why they pay me the big bucks.
A bad trait to have when building multihulls
Overbuilding is easy.
Building light with appropriate strength is more difficult.

One area you don't want excess weight is in the ends of a multi, especially if that weight is not related to the structural integrity of the vessel.
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Old 07-12-2011, 14:28   #14
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Re: Hull Extensions for Catamaran

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For your purposes though keeping this space seperate without limber holes could have advantages, ie an aft "crash" compartment equivalant. But it would probably need individual bilge pumps if you did it that way...
Why oh why would it need limber holes and bilge pumps?
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Old 07-12-2011, 14:37   #15
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Re: Hull Extensions for Catamaran

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Not necessarily
Look at duflex z joints for evidence of this

Look at the thickness of the laminate skin on that panel. It's about 1/16th, so thats easily a 20:1 scarf ratio. That is not intended to be a structural joint, it's just to join the panels together for positioning prior to glassing both sides of it. Would you actually go to sea with a boat made of panels with 1/16th skins on either side?
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