Hull Extensions for Catamaran

[Southern Star]

I had always heard that epoxy resin adheres better to polyester than polyester. However, the hull was constructed out of polyester resin and is solid glass below the waterline, but foam cored above (and I am unsure of the type of foam).

I was going to use core throughout (there will be no thru-hulls to worry about) and lay up the core and inside laminates while the stringers are attached to the hull; thereafter remove the stringers and laminate to the hull as suggested.

Brad

[minaret]

Originally Posted by Southern Star

I had always heard that epoxy resin adheres better to polyester than polyester. However, the hull was constructed out of polyester resin and is solid glass below the waterline, but foam cored above (and I am unsure of the type of foam).

I was going to use core throughout (there will be no thru-hulls to worry about) and lay up the core and inside laminates while the stringers are attached to the hull; thereafter remove the stringers and laminate to the hull as suggested.

Brad

Epoxy does adhere better to poly than poly does, but that's not the only thing to take in to account here. The reason you want to match the original hull construction in laminate schedule and resin makeup is because you want the extension to have the same physical properties as the hull. If the extension is a stiffer section of hull than the rest of the boat, it will stress the joint unnecessarily, and vice versa. You want the extension to flex and move at the same rate as the rest of the hull. Poly will adhere just fine too, I've done major hull extensions on vessels vastly bigger than this one in poly, never had a failure. You just have to do it right. Personally I wouldn't hesitate to do it in poly, in fact I'd prefer it for this job, for the reasons just stated, as well as for economy and speed of use. You'd be surprised how much time it saves not having to wait for epoxy cure times. And then you can gelcoat without tie coating.
If you are doing the batten method with offset battens to compensate for skin thickness and the core installed against the battens, remember that you can then glass the whole exterior skin lapping on to the original hull. This means there is no grinding on the part, only the original hull, and no tape seam. That means you can take the scarf ratio out to the ratios I tend to use without it being a big deal, just grind on to the hull for a foot and a half instead of a foot. No big deal, not much more glassing or fairing, but a much better structural joint.
I'd be hesitant to core below the waterline, obviously because I think a primary concern here is matching the original structure for reasons already mentioned. However, it's a small part and coring below the waterline will let you use the batten technique with no molding at any point. I think the labor savings involved are worthwhile and the risks minimal for such a small extension. I wouldn't build it that way myself, but that's because we warranty our work forever and my boss wouldn't let me do it like that. We have a reputation to protect in the industry, all that stuff...

[Southern Star]

Thanks Minaret. Frankly, if laminated properly I cannot imagine much flex in a 2 foot extension that is attached, both inside and out, to the existing transom. Furthermore, my hull scantlings are such that there are no signs of flex - even after 17 years there are zero stress cracks on the hull or bridgedeck, in spite of structural bulkheads that were tabbed to the hulls.

Brad

[minaret]

Originally Posted by Southern Star

Thanks Minaret. Frankly, if laminated properly I cannot imagine much flex in a 2 foot extension that is attached, both inside and out, to the existing transom. Furthermore, my hull scantlings are such that there are no signs of flex - even after 17 years there are zero stress cracks on the hull or bridgedeck, in spite of structural bulkheads that were tabbed to the hulls.

Brad

Very good, that means your hull is pretty stiff. If you build a lightweight extension with a laminate schedule that is not as stiff as the rest of your hull clearly is, you can see where it might want to flex more than the hull does, putting undue stress on the joint between the two. Just match the original hull's schedule and remember to bring any structural members such as stringers out to the new transom if thats how they were done before and you should be fine.

[Aussiesuede]

Originally Posted by cat man do

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.

Jon feel free to correct me if I'm in error, but the process described seems to be the "Lost Foam" method? If that's the case, the foam is sacrificial and used to create shape only. After the foam is sanded to shape, the exterior glass and gelcoat are added and AFTER the cosmetic external "shell" is completed, the foam is "melted" away with a solvent and then a ribbed structure added to the interior of the glass that's load bearing. Again Jon, correct me if my estimations are in error, but this "Lost Foam" method has been used for years in other applications.

[cat man do]

Originally Posted by minaret

Hilarious, <snip> 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..

Whats hilarious is that you are so desperate as to make things up and then believe it.

Show me where I said not to glass it onto 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?

More made up stuff.
Where did I suggest a 2'' thick core?

This is information which only applies to your very rare build method.

Very rare build method?
Thats friken hilarious

Every non production multi in Australia, NZ and probably the US uses these methods.

Tenant
Crowther
Kelsall
Simpson
Schionning
Pescott
Given
Shuttleworth
Chamberlain
Lidgard
Grainger
Bloomfield
Farrier
Arber
Oram, to name a few designers ALL recomend and use these methods to build boats.

Foam and or any sandwich construction is nothing new, though it apparently is for you.

I see you want to impress everyone with the perceived superiority of your build method,

Sorry, I thought that was you, after all, you seem to twist the truth at every opportunity an an attempt to push your method.

In reality all you seem to have shown is a lack of knowledge (in modern construction techniques) as has been repeatedly pointed out.

but I for one am not impressed,

Oh well

and I don't see how it helps the OP with his question

Well lets see what the OP thinks shall we.

[cat man do]

Originally Posted by minaret

Personally I wouldn't hesitate to do it in poly, in fact I'd prefer it for this job, for the reasons just stated, as well as for economy and speed of use. You'd be surprised how much time it saves not having to wait for epoxy cure times.

I did my own extension in Poly, steps and cabin



and then attached it with epoxy.

That was 15mm foam and 600gsm Biax inside and out
All edges cored out and filled with hairy bog (High density filler with glass fibres) for impact resistance
Never been a problem.

[44'cruisingcat]

I started doing the edges by de-coring and then filling with a rolled up uni tape. I found it to be quicker and easier than glue/bog, and gives a near indestructible edge.

[minaret]

Originally Posted by 44'cruisingcat

I started doing the edges by de-coring and then filling with a rolled up uni tape. I found it to be quicker and easier than glue/bog, and gives a near indestructible edge.

LOL, we do the same in a multimillion dollar temp and humidity controlled environment, using a laminate trimmer for decoring and pneumatic proset guns with mixing wands for fillers. I've done miles of panel edges this way. After we fill flush with proset we radius and tape each edge. The environmental control is to prevent off-gassing in the core causing bubbling in the edge tapes as the resin exotherms. Makes your method seem kinda primitive...

[Southern Star]

cat man do, for whatever it is worth I never took it that you wanted to laminate the extensions only to the transom - indeed, your original post in this thread makes it clear that you suggested putting layers over both the hull and the transom. I also don't recall you suggesting a 2 inch core.

Brad

[minaret]

I only assumed he wanted a 2" core because he recommended a 1/16" thick laminate for the part's skins, and I cant imagine how you would have any strength at all without also using the rest of his build method, which requires very thick core. 1/16" laminate on both sides of a regular core would be a terrible idea on this boat IMHO. And I suggested to you tieng the external skin to the external skin, he thought a two layer tape was "more than enough". If your external skin thickness is 3/8", how does a 1/16" two layer tape properly tie it in to the hull? It doesn't...

[44'cruisingcat]

Originally Posted by minaret

LOL, we do the same in a multimillion dollar temp and humidity controlled environment, using a laminate trimmer for decoring and pneumatic proset guns with mixing wands for fillers. I've done miles of panel edges this way. After we fill flush with proset we radius and tape each edge. The environmental control is to prevent off-gassing in the core causing bubbling in the edge tapes as the resin exotherms. Makes your method seem kinda primitive...

Whoopee doo for you! And yet as you say, you do the same thing.....

What do you build by the way? Monohulls?

[Southern Star]

Minaret, 44 C's methods may be primitive in comparison, but so will mine. Afterall, the question here concerned a DIY project and I am grateful for the contributions of others who have done the same.

Brad

[Southern Star]

And Minaret, don't get me wrong - I am also grateful for your contributions. There is a natural tendancy for people to have strong opinions in favour of what has worked for them in the past.

Brad

[minaret]

Agreed, I only have a problem with that when what has worked for them has little or no relation to the issue at hand. Your boat and his are two completely different animals, requiring totally different techniques, IMHO.