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Old 10-01-2012, 16:11   #76
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Re: Hull Extensions for Catamaran

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Originally Posted by 44'cruisingcat View Post
It will also improve your LWL/BWL ratio, and thus reduce the effect of the theoretical hull speed limitation.

For me the biggest concern would be the fact your boat has 16 year old gelcoat. Matching the colour could be difficult. Getting it to STAY matching for a long time could be near impossible.
On our last Seawind we put a graphic where the new stern extension met the old hull. Couldn't tell the difference in color.
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Old 11-01-2012, 04:35   #77
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Re: Hull Extensions for Catamaran

Actually 44CC I have been pretty successful at mixing up batches og gelcoat to match for small repairs, although admittedly over an area of that size it could prove problematic. And SMJ, that's an excellent suggestion - although it must require a great deal of care when laminating in and fairing the joint for the extensions so as to keep a relatively straight line.

Brad
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Old 11-01-2012, 04:43   #78
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Re: Hull Extensions for Catamaran

PS I should thank all of those members with experience in adding hull extensions/building cats who, rather than attacking the idea (or how I have equipped my boat) have provided invaluable advice both in this thread and, for those who didn't wish to be attacked, in PM's.

Cheers!

Brad
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Old 11-01-2012, 09:47   #79
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Re: Hull Extensions for Catamaran

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Actually 44CC I have been pretty successful at mixing up batches og gelcoat to match for small repairs, although admittedly over an area of that size it could prove problematic. And SMJ, that's an excellent suggestion - although it must require a great deal of care when laminating in and fairing the joint for the extensions so as to keep a relatively straight line.

Brad
Pay attention to 44CC's last sentence - it is dead easy to match gel coat, but it will be a near certainty that the colors will age differently. Most people don't know that their boats are changing colors right before their eyes because all of the gelcoat started out the same. But you can walk on any boat with a couple year old gel coat repair and notice it immediately.

I think the idea of using a graphic around the joins is brilliant. Doesn't even have to be a straight line join or be completely covered with the graphic. If the graphic has sufficient breaks and non-linear areas, the color difference will not be noticed through or around it.

Mark
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Old 12-01-2012, 05:47   #80
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Re: Hull Extensions for Catamaran

No doubt sound advice and yes, I am aware that as gelcoat ages it changes color.

Brad
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Old 12-01-2012, 06:16   #81
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Re: Hull Extensions for Catamaran

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I think the idea of using a graphic around the joins is brilliant. Doesn't even have to be a straight line join or be completely covered with the graphic. If the graphic has sufficient breaks and non-linear areas, the color difference will not be noticed through or around it.

Mark
+ 1. The eye is easily distracted.

One approach I use is, where I can't hide something to think about whether to make a feature of it.....i.e. in this case either mould in a raised rim or fix on some bling trim (s/s?) over the join.......or even paint the extentions a different colour entirely! (matching any other stripes / graphics?).
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Old 31-01-2012, 13:22   #82
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Re: Hull Extensions for Catamaran

Dunno if you can get it in the US, but Australian Multihull World has an article about a guy who extended the transoms on a Seawind 850.

Doesn't go into a lot of detail, but he did a very nice job of it.
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Old 28-01-2014, 21:33   #83
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Re: Hull Extensions for Catamaran

Cruising Cat - can you give me some details on the construction of your composite arch? My PO had a massive aluminum structure made up for dingy davits and while incredibly strong, I am sure it weighs far too much. I would like to take a lot of weight off my sterns.
thanks..Ed
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Old 28-01-2014, 23:42   #84
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Re: Hull Extensions for Catamaran

There are four legs, box section, made of Duflex and plywood, glassed with a heavy uni tape over the outside. Each leg weighs maybe 4kg, and I could hang from each one seperately, bouncing up and down a bit. (I'm about 115kg)
The legs are taped onto the inner sheer panels with heavy uni tape.

The top "wing" is just made from plywood over plywood ribs, with curved Duflex spars running along the length of them. The leading edge is pvc pipe, the trailing edge conduit. The whole lot was glassed over with 440 gsm double bias.

It's pretty strong, I once used it to try and lift a snagged stern anchor. Broke a 40 mm Ronstan block that was hanging from the dinghy lifting eye.
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Old 29-01-2014, 00:20   #85
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Re: Hull Extensions for Catamaran

It's been interesting reading through this thread again. I wonder if Southern Star ever did his hull extensions, or did he get so much "good advice" he abandoned the idea?
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Old 29-01-2014, 08:34   #86
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Re: Hull Extensions for Catamaran

I forgot about this thread and about the need to consult naval architects, and about how adding extensions will actually be a negative buoyancy and sink in the stern, as well as somehow also pushing the bows underwater because of too much buoyancy, and how I should be buying a new boat instead.

So I foolishly went and started extending our sterns without all the above advice. Just zipped up the first one yesterday. Needs the seams ground and faired yet and then needs gelcoating. Non-skid also.

Extended the stern 27" and changed the entire staircase - fewer, but longer steps with a different step height than before. Added internal dorade structures for the blower vents so that there are no longer any clamshell vents intruding into the sterns like before.

It was a bit more complicated because of the 3D shape and complex curves of the step rails and hulls in the original. Not to mention that the sterns are mirror images of each other, so one hull can't be used for the other.

The new extension is made in polyester, nidacore (steps and hull sides - hull bottom is solid) and 1808 biax.

The main connection structure is a nidacore box beam that connects to the old steps and catilevers onto the new hull. The new steps are taped to it. The new extension butts onto the old hull and is taped inside and out.

The new extension was built similar to how Brad described wanting to do his - a mold was splashed off the old hull and rails, a wedge was cut out of this mold to fit the necessary dimensions for the extension and rejoined. The steps were made with a plywood plug and the rails and hull molds joined to it. Glass was added as needed to these pieces to form a complete stern mold, then that mold was cut horizontally so that I ended up with two molds - one containing the steps, rails and most of the hull sides, and the other containing the hull bottom and partial remaining hull sides.

The new stern was laid up inside these two molds.

Worked great! Once everything is faired, I will splash a hull and staircase mold off of it, splash rails off the other transom like before, connect those rails to the hull/step mold and make the other transom. This should be much quicker because there will be no making wood staircase plugs, fussing with getting the fitting of the wedge cutout and rails correct, etc. And much less post-mold fitting and fairing to exact shape.

The new build came in at 135lbs (that's 61kg for you cutting-edge measurement people). The added volume gives 650lbs (295kg) of buoyancy, with 500lbs (227kg) of that within 10" of the waterline. I could have saved weight by cutting off the covered part of the old transom, but I did not.

The first pic is dry-fitting the two molded parts of the extension - you can see the box beam cantilever already glassed to the old hull. The new hull and staircase will later be glassed to it also. The second picture is everything all glassed in. You can see the complicated step rail shape, although the compound curve in it and the hull side is not very evident in the pic. The holes are just access holes for glassing and will be glassed over.

Without this complex and mirror image shape on our boat, the stern extension done similar to how Brad proposed would be very easy and work well. In my opinion only, of course.

Please don't tell me how much I screwed up doing this! I read back through this thread and already know how much peril I am in…

Mark
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Old 29-01-2014, 08:48   #87
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Looks really good. She should feel like a new boat once finished! Are you using epoxy to tape the extensions to the hulls?
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Old 29-01-2014, 08:52   #88
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Re: Hull Extensions for Catamaran

Thanks. The box beam and hull bottom piece was taped with epoxy. The rest is taped with polyester.

BTW, anyone interested in more pictures and info - Michele keeps a facebook page on our boat, which also has links to blog entries on our website (don't worry - we don't have any paid ads, beg for money or any other revenue generation on either one). She has been documenting this.

https://www.facebook.com/CruisingOnReach

Mark
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Old 29-01-2014, 11:50   #89
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Re: Hull Extensions for Catamaran

Quote:
There are four legs, box section, made of Duflex and plywood, glassed with a heavy uni tape over the outside. Each leg weighs maybe 4kg,
CC44 - seems quite light. Can you tell me approx what the dimensions are?
And after this time using it, would you make any changes if doing over again?
Ed
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Old 29-01-2014, 12:32   #90
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Thanks. The box beam and hull bottom piece was taped with epoxy. The rest is taped with polyester.

BTW, anyone interested in more pictures and info - Michele keeps a facebook page on our boat, which also has links to blog entries on our website (don't worry - we don't have any paid ads, beg for money or any other revenue generation on either one). She has been documenting this.

https://www.facebook.com/CruisingOnReach

Mark
I was just reading your blog on the stern extensions. Impressive work. I really like the box beam, major strength there.
A friend of mine is trying to find someone to do extensions on his Seawind 1000. So far he is having a hard time finding a craftsman that will have anything to do,with epoxy! They all want to use polyester or vinyl ester saying it is just as or stronger than epoxy. I don't understand, is it the added $100 they're worried about for the extra cost of the epoxy resin, or are they just not comfortable working with epoxy. To me, using epoxy for the structural secondary bonds is a must.
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