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Old 29-01-2014, 13:57   #91
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Re: Hull Extensions for Catamaran

When you keep it hinged like in the 1st pic, you can store a jet-ski in each stern! Plenty buoyancy for that and high James Bond factor
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Old 29-01-2014, 13:58   #92
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Re: Hull Extensions for Catamaran

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Originally Posted by colemj View Post
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
Wow! Even though you've completely ruined the boat, it looks great! Excellent work.
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Old 29-01-2014, 14:31   #93
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Re: Hull Extensions for Catamaran

I ruined mine too.

Sail Delmarva: Extended Transoms: The Process

My primary interest was to make boarding from the dingy and kayaks easier for elderly and arthritic, and in that I was completely successful. As for handling and speed, small but measurable improvements.

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Old 29-01-2014, 15:47   #94
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Re: Hull Extensions for Catamaran

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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.
I have done a lot of work in epoxy, not as much in poly and some in vinyl. I wouldn't have any problem at all doing the whole thing in poly or vinyl. I did tab the bottom hull and box beam with epoxy, but that was more of an over-engineering moment of over-think than a necessity.

There was an extra piece of polyester tabbing I didn't need, so I took a chisel and hammer and whanged away at it today. Couldn't get it to debond from the old hull so I had to resort to grinding it off.

Vinyl is a bit harder to work with because the environment plays a larger role (humidity, etc). Epoxy is much harder to work with than poly - especially if someone hasn't done a lot of work with it. It is less health friendly also and requires more careful attention to fumes and contact.

I think epoxy works best when you need to lay unidirectional fiber to control forces or loads and you can bag or infuse it to control the resin ratio. Or for areas where you have questionable bonding ability. And for ultralight builds - which is again directional fibers and resin control. Other than that, it is often just expensive overkill.

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Old 29-01-2014, 15:50   #95
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Re: Hull Extensions for Catamaran

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Originally Posted by s/v Jedi View Post
When you keep it hinged like in the 1st pic, you can store a jet-ski in each stern! Plenty buoyancy for that and high James Bond factor
We already considered that! Also considered cutting a hatch in the step and using it as a live-well - carry around our lobster and conch!

I had the idea of installing torpedo tubes there, but Michele veto'd it.

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Old 29-01-2014, 16:15   #96
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Quote:
Originally Posted by colemj View Post

I have done a lot of work in epoxy, not as much in poly and some in vinyl. I wouldn't have any problem at all doing the whole thing in poly or vinyl. I did tab the bottom hull and box beam with epoxy, but that was more of an over-engineering moment of over-think than a necessity.

There was an extra piece of polyester tabbing I didn't need, so I took a chisel and hammer and whanged away at it today. Couldn't get it to debond from the old hull so I had to resort to grinding it off.

Vinyl is a bit harder to work with because the environment plays a larger role (humidity, etc). Epoxy is much harder to work with than poly - especially if someone hasn't done a lot of work with it. It is less health friendly also and requires more careful attention to fumes and contact.

I think epoxy works best when you need to lay unidirectional fiber to control forces or loads and you can bag or infuse it to control the resin ratio. Or for areas where you have questionable bonding ability. And for ultralight builds - which is again directional fibers and resin control. Other than that, it is often just expensive overkill.

Mark
I guess that would be where our thoughts would differ, i consider any polyester thats not green as being an area of questionable bonding. Using epoxy for the tabbing of the bottom of the hull and the box beam to me would be a necessity rather than over-engineering. For probably $20 more in resin you have a structure which in my mind is much stronger and less likely to pull apart. I understand the need to tab the rest of the structure in with polyester as you want good adhesion with your gelcoat, and to me you've made this possible by using epoxy to tab in what are probably the highest stressed areas. I do agree that a fully epoxied boat would be an overkill unless going for an ultralight build. I think the way you've built it is well thought out.
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Old 29-01-2014, 18:01   #97
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Re: Hull Extensions for Catamaran

colemj,

Nice work, but why would you extended the stern 27"?

Hull Extensions for Catamaran
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Old 29-01-2014, 20:20   #98
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Re: Hull Extensions for Catamaran

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colemj,

Nice work, but why would you extended the stern 27"?

Hull Extensions for Catamaran
I don't understand the link you provided - it sends me right back to this page of this thread.

The main goal of extending the sterns was simply to get the sterns out of the water. We are heavier than the design was made for since everything we own is on board and we cruise full time. We just can't seem to make the hard choices to go without things like our SUV dinghy with large engine, spare parts for remote areas, food stocks to spend 6 months away from any stores, large watermaker, etc.

This is easier!

The secondary goals were to change the staircase area to be more usable and to change the shape and height of the transom.

27" was just where the lines naturally pulled out enough to reach our goals. It brings the bottom of the transom to just out of the water fully loaded (I think), increases the transom height from 1" above waterline to 7" above while changing the shape to a more usable one, and allowed us to get rid of a step to make two longer steps and even out the step falls to all the same height.

Also of consideration was that I was not interested in changing the position of the rudder, so did not want to effect steering by having too much stern aft of the rudder. And effecting trim/ride by having too much buoyancy in the stern.

And we wanted it to look natural. Because of the complicated curves and dimensions of the step rails on this boat, going much further would have made us completely change the look of the design and the transom would have looked like a bolt-on. Manta did extend the 40 into the 42 in a similar fashion, so our extended boat still looks like a natural member of the family.

Our design, however has more buoyancy than Manta's extension - and for us, a more usable step and transom arrangement. But to a first approximation, one would not know that it was an extended Manta 40 and not a 42.

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Old 29-01-2014, 20:22   #99
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Re: Hull Extensions for Catamaran

Oh yeah - it will be a Manta 42.25 and not just a bog-standard Manta 42.

My guitar amp also goes to 11…

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Old 29-01-2014, 21:30   #100
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Re: Hull Extensions for Catamaran

What was your laminate schedule for exterior and interior skins? Core thickness?
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Old 30-01-2014, 02:47   #101
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Re: Hull Extensions for Catamaran

Quote:
Originally Posted by colemj View Post
I don't understand the link you provided - it sends me right back to this page of this thread.

The main goal of extending the sterns was simply to get the sterns out of the water. We are heavier than the design was made for since everything we own is on board and we cruise full time. We just can't seem to make the hard choices to go without things like our SUV dinghy with large engine, spare parts for remote areas, food stocks to spend 6 months away from any stores, large watermaker, etc.

This is easier!

The secondary goals were to change the staircase area to be more usable and to change the shape and height of the transom.

27" was just where the lines naturally pulled out enough to reach our goals. It brings the bottom of the transom to just out of the water fully loaded (I think), increases the transom height from 1" above waterline to 7" above while changing the shape to a more usable one, and allowed us to get rid of a step to make two longer steps and even out the step falls to all the same height.

Also of consideration was that I was not interested in changing the position of the rudder, so did not want to effect steering by having too much stern aft of the rudder. And effecting trim/ride by having too much buoyancy in the stern.

And we wanted it to look natural. Because of the complicated curves and dimensions of the step rails on this boat, going much further would have made us completely change the look of the design and the transom would have looked like a bolt-on. Manta did extend the 40 into the 42 in a similar fashion, so our extended boat still looks like a natural member of the family.

Our design, however has more buoyancy than Manta's extension - and for us, a more usable step and transom arrangement. But to a first approximation, one would not know that it was an extended Manta 40 and not a 42.

Mark
it all looks logical and an excellent addition to me.
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Old 30-01-2014, 06:31   #102
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Re: Hull Extensions for Catamaran

colemj,

What does a hull extension upgrade like this cost? Looks very labor intensive.
Did you pick the marina your at, because they are known for doing good glass work?
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Old 30-01-2014, 07:18   #103
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Re: Hull Extensions for Catamaran

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What was your laminate schedule for exterior and interior skins? Core thickness?
For the cored portions:
1 layer of light CSM followed by 2 layers of 1808 biax, 1/2" Nidacore followed by 1 layer of 1808 biax.

The solid hull bottom was 4 layers of 1808 biax and the top side edges of the step rails were 3 layers of 1808.

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Old 30-01-2014, 07:41   #104
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Re: Hull Extensions for Catamaran

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colemj,

What does a hull extension upgrade like this cost? Looks very labor intensive.
Did you pick the marina your at, because they are known for doing good glass work?
Well, I haven't finished them yet!

But the estimate cost was about the same as a basic haulout and bottom paint job in Ft. Lauderdale. In other words, much less than we anticipated.

The first one was somewhat labor intensive because we were kind of making up the details and fit as we went along and the compound curves were a bit of a hassle. The second one will be much simpler because 80% of it will be a mold taken off the first. The remaining 20% is more custom, but I have already learned all the lessons from the first one.

I chose this marina because when we hauled out to just paint the bottom, I discovered that in the past, they had strangely ordered a bunch of Nidacore and biaxial glass to have in stock, but had never actually used the stuff before. They also had high quality resins (epoxy and polyester) and very good gelcoats.

So I have been teaching the workers here how to work with composite structures. They have generations of fiberglass knowledge, but it is all based on one simple rule - "pile in the CSM and heavy woven roving to an inch thick and pour resin into it until it is pooling all over the ground".

At first, they thought it insane to put plastic and air inside the fiberglass, let alone only using very thin layers of fiberglass, and none of it woven roving (they worship woven roving). So I built a couple of small panels of 1/2" core with thin layers of biax, put them between horses and jumped on them and whacked them with hammers, etc. I think they still think it all was a trick, but they are pretty enthusiastic about sticking core into things now and are starting to think stitched directional fabrics are impressive. Maybe they just think it is funny that I'm building a "toy boat"?

It is still difficult getting them to control resin ratios, though. They don't understand exactly how fiberglass/resin interact, and view the resin component as the more important strength part. So I spend a lot of time during layups running between glassing my parts and going to theirs and squeegeeing off half gallons of resin and saying "no mas!, no mas!"

But they are learning quickly and really starting to understand how strong light things can be when the correct materials and techniques are applied.

In one month, I have gone from no Spanish ability to being able to hold entire conversations now! The funny thing is that all of my Spanish is pretty useless outside of the boatyard because my entire vocabulary consists of Spanish words for "bulkhead", "angle grinder", "catalyst", "tape measure", parts of boats, etc. People stop by who speak good Spanish and have no idea what we are talking about! Outside the boatyard, I don't understand anything because I don't know any of the common vocabulary topics.

Mark
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Old 30-01-2014, 08:21   #105
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Re: Hull Extensions for Catamaran

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What does a hull extension upgrade like this cost? Looks very labor intensive...
Our 33" extensions cost about US$ 4,000 here in Phuket, Thailand - done in epoxy, foam, & biaxial glass. But ours weren't nearly as scientific as Mark's. Much of the original work was done in April 2012, but steps & swim ladders & stuff were added in later weeks (see nav-bar on left side of page). And speaking Thai is MUCH harder than Spanish
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