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Old 04-11-2018, 19:42   #31
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Re: Am I wrong to shy away from ply sandwhich?

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Derek, was that Airx foam or some other PVC?

Jim



Jim,


the slipway foam was Airex (linear pvc) but most of the boats are Divinicell or similar cross linked PVC. Linear is more heat sensitive so avoid using on decks.

Derek.
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Old 04-11-2018, 21:26   #32
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Re: Am I wrong to shy away from ply sandwhich?

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Jim,


the slipway foam was Airex (linear pvc) but most of the boats are Divinicell or similar cross linked PVC. Linear is more heat sensitive so avoid using on decks.

Derek.
thanks for that...

Our previous boat, a Palmer Johnson Satndfast 36, had Airex foam coring in the hull. When we sold her, she was 31 years of age and nearing 100k miles through the water. The hull coring was still virgin... no water intrusion detected by a hard nosed surveyor, so I thought highly of the construction. (Only fair to add that the glass layup was far thicker than modern practice) I'm always somewhat amused by the knee jerk anti-cored hull vents here on CF, for it can be truly long lived and strong.

And BTW, thanks for your many contributions here on CF. It is always good when a truly knowledgeable poster chimes in on tech subjects.

Jim
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Old 04-11-2018, 23:38   #33
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Re: Am I wrong to shy away from ply sandwhich?

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Hasn't the attitude of people changed over the last 20-30 years? Thirty years ago there was a special area right on the Swan River which was put aside for people to build their boats and there were over 100 boats on the go!! Everyone help each other and the place was a hive of activity on week-ends.

A few months ago I was reminiscing and spoke to the boat building yard manager who said there was just one boat under construction today. I think one of the reasons is that you can buy a good cheap boat cheaper than you can build one.

Clive
As far as I know it's still there. In 2013 I had arranged to take my steely there to do a hull refit but had the cancel after not making it any further south than Exmouth.

It was a good and bad place to build a boat, good because there were lots of other boat builders to get information from and socialize with and bad because there were lots of other boat builders to get information from and socialize with.

When I was there during 1986-87 there were quiet a few folks on the dole building boats. some of them even got finished and sailed off around the world. There were quiet a lot of wild and feral domestic ducks which hung about there on the river. You could make quiet a good estimation of how diligently the dole-police were working by the increases and decreases in the resident duck population.
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Old 05-11-2018, 00:47   #34
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Re: Am I wrong to shy away from ply sandwhich?

There's at least one production builder using plywood: Home - RM Yachts
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Old 05-11-2018, 01:21   #35
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Re: Am I wrong to shy away from ply sandwhich?

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As far as I know it's still there. In 2013 I had arranged to take my steely there to do a hull refit but had the cancel after not making it any further south than Exmouth.
It is still there but according to the Manager it is used by boat owners to store their boats.

I'm very much aware there were a lot of dole bludgers there building their boats which I didn't approve of. I always enjoyed talking to people building their dream boats. One guy had two pontoons supporting a VW Beetle. He had taken a prop shaft from the motor and attached a propeller for propulsion. Of course when he got to the destination he had ready-made transport.

I would have loved to have been there when (if) that was launched!!

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Old 05-11-2018, 04:52   #36
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Re: Am I wrong to shy away from ply sandwhich?

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I pioneered the use of foam a long time ago. A sandwich is two thin dense skins with a light weight core. I chose PVC foam, uni-directional glass (mostly) and polyester resin. All I know of are still sailing with surface maintenance only. For panel stiffness which is needed for most parts of a cat, a sandwich is a fraction on the weight. The theoretical weak link is the danger of damage from a sharp point impact. The incidents of impact damage are rare in my experience. PVC foam is monocellular and does not take up water. It is also elastic and able to resist impact damage better than say Polyurethane Foam. Foam has the added advantage of insulation and good buoyancy. Avoid balsa if interested in longevity. Avoid epoxy for health reasons if working on your boat or building a boat. I am a great believer in using one material for all of the structure. Yes it would take a few chapters to cover all the options. Happy boating, Derek.

So I'm looking at a boat that is built that way with Polyester resin. Will that need regular coating of Epoxy every time you do a bottom job, or is one coating enough?
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Old 05-11-2018, 05:12   #37
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Re: Am I wrong to shy away from ply sandwhich?

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So I'm looking at a boat that is built that way with Polyester resin. Will that need regular coating of Epoxy every time you do a bottom job, or is one coating enough?
I assume you mean an epoxy barrier coat?

I've just finished putting a several barrier coats on my yacht (under construction). I most certainly don't expect to have to reapply a barrier coat every year but I would expect to have to apply anti-fouling maybe every year. Is that what you mean by "a bottom job"

Here are a few links





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Old 05-11-2018, 09:06   #38
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Re: Am I wrong to shy away from ply sandwhich?

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So I'm looking at a boat that is built that way with Polyester resin. Will that need regular coating of Epoxy every time you do a bottom job, or is one coating enough?

We always recommend a good quality top coat for either resin. Polyester is slightly less resistant to moisture and epoxy is less resistant to UV light. I know of one owner who also built, and sails three months of the year who has repainted one and a half times in 42 years. Incidentally similar with rigging replacement. Happy boating.. Derek.
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Old 09-11-2018, 07:46   #39
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Re: Am I wrong to shy away from ply sandwhich?

Foam core can be either closed cell or open cell. Open cell is less desirable, both are susceptible to deterioration from solvents or fuel, open cell can absorb water

All construction methods are trade offs between strength and weight - no method is absolutely bad and no method is perfect. Cored or cold molded hulls generally have more interior room due to the reduced need for stringers and ribbing

I would avoid plywood unless I could be absolutely certain about its origin, type and construction method. I would avoid any wood below the water line

One of the important considerations is how hull penetrations are done. If the hull is solid fiberglass and a penetration is made and properly bedded it is less likely to ever be a problem. if the hull is cored with anything (especially balsa wood) the method of doing penetrations is very important. You cant just drill a hole and bed the fitting - you risk water damage to the core. Very good builders carefully lay out the hull penetrations and use solid fiberglass in those areas to allow penetrations to not affect the hull integrity if they leak
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Old 09-11-2018, 08:21   #40
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Re: Am I wrong to shy away from ply sandwhich?

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I could put a hammer head through any conventional boat hull, with metal hulls taking several swings just as readily as I can still sink a set 16d nail in one swing.
Really? A couple of swings with a hammer you could breach my 6mm mild steel hull? When I cut a hole in the deck (which is only 5mm) with a hole saw to install a deckwash pump it took 20 minutes. I'm not that strong, and the hole saw was probably a bit blunt but even so .. i doubt you can put a hole in 6mm steel plate without a power tool. Indeed if you hit it in the middle of a plate (away from the structural supports) the hammer would just bounce off. Your only hope would be a weld near a support where the metal was both more rigid and more brittle.

I'm doubtful because I just misrolled a 5mm steel plate at school into a cylinder and we tried to make it circular with a hammer and couldn't even move the metal a half millimetre.

The fact here is that steel is stronger and lighter than GRP or wood. The main reason people don't use it on smaller boats is that it is a cored construction material. The steel is the core, PAINT is the material keeping the boat afloat. One scratch, and thin steel starts to corrode and then your screwed.

Most commercial and some recreational cats are made from aluminium. It doesn't corrode as easily, and is very strong. The main disadvantage over other materials is that it is very hard to repair without the right equipment and skills.
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Old 09-11-2018, 08:45   #41
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Re: Am I wrong to shy away from ply sandwhich?

There are several key weak spots on catamarans. The design and construction of these places is critical.

1. The bow struts which hold the forestay. Huge pressure is exerted on this point, in between the two hulls. The load must be distributed to strong points on the hulls. The forestay itself on a performance cat needs to be made of special wire with square strands otherwise it is either too heavy or not strong enough.

2. The centreboard slots, if you have them. If you don't have them you lose a lot of performance into the wind. If you do, they're a weak point and almost impossible to repair if damaged.

3. The undercarriage. This is the bit that holds the hulls apart, from at least the mast position back. It is subject to massive torques as the hulls twist due to uneven passage through rough water, the greater the beam, the more the stresses. It is also regularly pounded by waves, and indirectly holds the mast up. Cats are far more rigid in the water than monos, so the rig is subjected to much greater loads. Luckily, the stays are seated further apart reducing the load on the wire. But the vertical forces on the mast seat are much higher so the foot must be very securely connected to the structure.

In my view .. if you don't know a lot about sailing .. don't think about taking a cat blue water. In bad weather there's a key difference to a mono: you can just let the mono look after itself. In bad weather, you have to SAIL a cat to keep it safe. For coastal sailing, cats are better. But marina berths are a lot more expensive.
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Old 09-11-2018, 11:47   #42
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Re: Am I wrong to shy away from ply sandwhich?

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Cored hulls are more rigid, They also have the possibility of delamination and water intrusion. They may be lighter vs strength, but I'm not convinced they are just plain lighter weight.... at least if they have an adequate outer shell thickness. However, they are how cats are built mostly and seem fine in most cases.
I would definitely avoid plywood, it has not proven to be a long term construction solution in most cases.
I have seen cored cats with huge holes in the hull, so no, not unsinkable. i'm quite confident I can put a hammer thru a cored hull easier than a solid hull.
Fiberglass and GRP are the same thing.

******** about plywood. Well maintained ply boats are still around after their glass cousins have gone to the shredder.
I have a ply epoxy boat myself, over 30 years old. People ask me if it is new!
Don't let uninformed opinion on the internet put you off plywood.
And. Two big advantages. Ply resists cyclical loading better than any plastic, and deterioration is easy to spot and fix.



Solid glass is way to heavy for catamaran construction, and doesn't resist the hammer test as well as a cored panel the same weight.
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Old 09-11-2018, 14:38   #43
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Re: Am I wrong to shy away from ply sandwhich?

My experience tells me to avoid plywood in any shape or form unless YOU built the vessel yourself and know exactly how much treatment you gave it--because YOU are not making a profit and YOUR life depends on how well you saturated the plywood, and how well you bonded the glass to it, how well you saturated the glass, and of course you used only the finest of materials and took the utmost pains in saturating cut edges, returning to them time and again until they refused to absorb any more expensive epoxy or vinylester resin.

Plywood vessels usually die from the inside outwards--so I always dumped hands full of swimming pool salt into the bilges wherever water or any damp surfaces could exist.

Even so--rot in the tropics is unrelenting. My advice is never to buy plywood. ANYTHING else is better as long as it was competently made and currently seaworthy.

Osmosis is bad--but fixable. Plywood rot is VERY expensive to fix--and often just not worth it.

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Old 09-11-2018, 16:32   #44
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Re: Am I wrong to shy away from ply sandwhich?

Exactly. the condition & upkeep is the point here , Iv sailed 50 year old ply boats which wasn't even glassed but had good owners over the time & still sailed the pants of modern go fast machines . Spencer yachts in NZ was a typical example, Hard chine, car box ply quick sailing craft.
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Old 09-11-2018, 19:37   #45
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Re: Am I wrong to shy away from ply sandwhich?

We surveyed four 15 to 20-year old cats and all of them had water in the core - most below the waterline. Two had de-lamination to the point it was structurally significant and cost prohibitive to repair. We surveyed a TPI Lagoon 42, two Privilege cats and a Maxim 38. It was a very expensive, time consuming and disappointing year of boat shopping. We learned A LOT! We concluded never to consider any boat with coring below the waterline - period. We concluded it is just a matter of time before water gets into the coring - for many, many reasons. As I recall, in the mass produced price range only Dean is solid below - but my memory may be wrong. Talk to a quality surveyor who specializes only in cats!



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