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Old 02-11-2018, 10:10   #1
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Am I wrong to shy away from ply sandwhich?

I'm looking to buy a cat, and despite hours of research I'm still completely baffled by the relative merits of different construction techniques.



Is there a difference between fibreglass and grp?


Foam core is presumably lighter than solid grp hulls, but is it weaker?



Is ply core something to avoid like the plague?


Foam core is often described as unsinkable. Is it a closed cell foam they use?



My budget means I'm looking at boats around 20 years old. I really want something that feels solid and is going to not stress me out if I get caught in bigger seas on passage.



In my research, it seems like every boat I come across has different form of construction, and I can't find anywhere that is a good reference to their relative merits. Any help to clear the fog much appreciated?
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Old 02-11-2018, 10:24   #2
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Re: Am I wrong to shy away from ply sandwhich?

From the thead title, I was thinking I'd prefer multigrain sandwich.
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Old 02-11-2018, 10:25   #3
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Re: Am I wrong to shy away from ply sandwhich?

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.
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Old 02-11-2018, 10:46   #4
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Re: Am I wrong to shy away from ply sandwhich?

Quote:
Originally Posted by andypag View Post
Is there a difference between fibreglass and grp?
"Fiberglass" is American for "grp".

Quote:
Originally Posted by andypag View Post
Foam core is presumably lighter than solid grp hulls, but is it weaker?
Yes.. and no. Foam core is much stiffer for the same weight as solid grp, and so can be made significantly lighter. That is good. On the other hand, the thin outer skin is much more susceptible to a sharp pointed impact than solid grp. That is bad.

Quote:
Originally Posted by andypag View Post
Is ply core something to avoid like the plague?
All core materials will suffer serious problems over the long run if water gets into the layers. Plywood shows serious problems faster.


Quote:
Originally Posted by andypag View Post
Foam core is often described as unsinkable. Is it a closed cell foam they use?
All foams used are closed cell. If you want to be technical about it, balsa is a "closed cell foam." They have nothing to do with making a hull "unsinkable" other than they make it possible to make the boat light enough someone can claim it is "unsinkable". Just like the Titanic.

Quote:
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In my research, it seems like every boat I come across has different form of construction, and I can't find anywhere that is a good reference to their relative merits. Any help to clear the fog much appreciated?
Every decision made during a boat's design and construction is a compromise. Almost none of them are "all good" or "all bad."

May I suggest Dave Gerr's excellent book, The Nature of Boats as a fun and easy read about boat design. He takes the questions you ask here and spends many chapters discussing them. It's a bit more work than having a random stranger give you simplistic answers, but worth the effort.
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Old 02-11-2018, 11:05   #5
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Re: Am I wrong to shy away from ply sandwhich?

There are lots of different construction materials and just as many way of putting each of them together. A question like this will get 48 different opinions in 35 different replies. There are plank-on-frame (caravel) boats more than 100 years old, like "Curlew", still sailing. There are boats made out of reinforced concrete, too. From your posting it appears that you are looking at catamarans, where light weight is generally considered a good thing. For light weight coupled with strength, most builders seem to go to cored fiberglass. What the core is - foam, balsa, wood.... - and how the fiberglass is applied varies. Each ad you look at will say why theirs is the best. It is no wonder people get confused. Modern one-offs are often cold-molded of wood and then given epoxy/glass skins inside and out, making them essentially plywood boats that will probably last 50 years if they're well made and taken care of properly. The same goes for the other methods. So... read up on all the options and then YOU get to decide if it makes any difference and then what makes the most sense for you.
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Old 02-11-2018, 20:44   #6
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Re: Am I wrong to shy away from ply sandwhich?

I suggest you study each construction, and each vessel. Much can be learned online or in books and magazines, but real knowledge is obtained in person. Go out to boatyards, talk to owners and repairmen. Learn what to look for in each case. My boat has a wooden hull and glass/ply decks. She's fifty years old and very sound. But I've spent my life using and studying wood boats. Glass on ply can be very durable, and it's stiffer than glass alone. But if you look at a ply boat, bring someone who knows what to look for. Foam or balsa core gets soggy too. Start looking at boats, go sailing if possible. When you think you've found your boat, pay for a survey. Good luck, keep us posted.
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Old 02-11-2018, 21:33   #7
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Re: Am I wrong to shy away from ply sandwhich?

Quote:
Originally Posted by Cheechako View Post


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.
Putting a hole in a boat is not the same as sinking it.

To the OP.

There are some excellent boats built from plywood. And some rubbish. The same applies to every material.

The suitability of a material will depend somewhat on what you want from a catamaran.

If you'd like to be able to rely mostly on sails for propulsion, then solid fibreglass is unlikely to work for you. Cored construction will produce a much lighter, better sailing boat.

You'll likely find plywood cored boats cheaper to buy. They're generally cheaper to build. Foam cored would be the most expensive.

Plywood can rot, foam can delaminate, metal can corrode, everything needs maintenance.

I wouldn't let the fact that a boat has any type of core or construction rule it out. Just get a survey to make sure there aren't any serious problems.
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Old 02-11-2018, 22:20   #8
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Re: Am I wrong to shy away from ply sandwhich?

Quote:
Modern one-offs are often cold-molded of wood and then given epoxy/glass skins inside and out, making them essentially plywood boats that will probably last 50 years if they're well made and taken care of properly.
There's a big difference between cold-molding (or strip planking) in epoxy and then glass, and glass over a plywood hull. In a cold molded hull, compound curves are do-able, all the bonds are in epoxy, and the glass is only there to keep worms out of the outside and dry rot from freshwater intrusion from the inside.

Glass over ply most often involves hard chine designs, and the ply can be of dubious type and origin. Many such boats were 'home built' andthe skills of the builder unknown (and his ideas on how to save money during construction as well!).

Of course, there are some excellent examples of glass over ply floating about. There are some of lesser quality, too.

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Old 03-11-2018, 04:32   #9
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Re: Am I wrong to shy away from ply sandwhich?

Andypag

What about looking at these links.(You might have to copy and paste each one)

Catamaran build with different core materials

https://www.boatdesign.net/threads/c...terials.55302/

https://www.boatdesign.net/threads/c...s.55302/page-2

https://www.boatdesign.net/threads/c...s.55302/page-3






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Old 03-11-2018, 09:48   #10
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Re: Am I wrong to shy away from ply sandwhich?

First of all, it is not really "ply sandwich", but glass over ply. Usually the glass is on the outside only and most of the strength comes from the plywood (which is an amazing engineering material but not really suited to production builds). You are not wrong to shy away from them but there are many well made examples afloat and a lot of them outperform their mass produced, fiberglass cousins, and the possibility exists to obtain one for much less than the cost of a comparable production model catamaran, or if you are considering a trimaran or a Wharram type cat, most are built in ply and there are many, many alive and well to this day and have somewhat of a "cult" following. There are several reasons to consider a glass over ply multihull:
1, You like wooden boats. The sound and feel of them is more pleasing, they can be constructed, modified, and repaired with simple carpentry techniques, are more traditional and material can be sourced anywhere.
2, Performance. Lighter=faster and glass over ply multihulls can be built much lighter than their fiberglass cousins (assuming that you can't afford carbon fiber).
3, Cost. You can't afford a production catamaran the size you want at the price that you can afford.
4. Design options. You want something different. Or unique. Or a boat that can be easily modified.

Anybody feel free to add to or expound on the merits of plywood. Also there are wooden cold mold and strip plank multihulls which are very good. I will leave it to others to explain why not to buy one although I am sure that very few of them have any practical experience at all on the subject. Mine is for sale and is floating happily near some similar sized production cats with similar capabilities but they are asking two or three times the price!
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Old 03-11-2018, 10:00   #11
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Re: Am I wrong to shy away from ply sandwhich?

My biggest concern is this "ply sandwich" terminology.

These are just plywood catamarans with a thin veil of glass over the plywood to protect it from abrasion. The extra glass on a plywood Catamaran is not a structural component.

Don't be confused by that.
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Old 03-11-2018, 10:30   #12
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Re: Am I wrong to shy away from ply sandwhich?

I believe, using my old memory, that Reuel Parker has a construction technique for constructing round bilge sharpies using layered strips of plywood that in my mind could be labeled plywood sandwich. I don't recall him using that description though. Also, having been around boats for 64 years so far, and having started my construction career nailing 1x6 diagonal sheeted floors on apartment buildings at about age 8, I think at my advanced age, with 2 right shoulder surgeries and no bursa on that side, that 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. I met a loudmouthed braggart a few years ago in Mexico who loudly and drunkenly boasted of the strength and superiority of his heavy full keel cruiser at every opportunity and when it sank like a rock after supposedly hit a whale he blamed the whale and not the construction.
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Old 03-11-2018, 10:35   #13
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Re: Am I wrong to shy away from ply sandwhich?

Quote: "... despite hours of research I'm still completely baffled..."

Were you really expecting to get a grip on something as varied and complex as yacht construction in a few hours of sculling around on HolyMotherNet?

Others have pointed you in very sound directions in regard to reading material that will give you the beginnings of insight into this enormous subject. Since you haven't bought a boat yet, I think there'd be wisdom in your dedicating the coming winter months to doing some serious reading about both boat construction and boat maintenance.

You imply, though you don't say so outright, that you have ocean cruising in mind. You might like to spend considerable time this winter to study also just what that entails.

We are happy to help, but you need to be specific in your stated objectives and realistic in your expectations otherwise the discussion will get totally amorphous and therefore trivial.

All the best

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Old 03-11-2018, 11:28   #14
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Re: Am I wrong to shy away from ply sandwhich?

GRP means glass reinforced plastic btw and that is fiberglass as others have said. The resin used in grp is really key. In ascending order of price, strength and waterproofness they are polyester, vinylester and epoxy. Poly is the cheapest and easiest and most boats are built with this. Bondo is polyester. Poly boats are not technically waterproof so that is why an epoxy barrier coat is applied to the bottom before antifouling. Epoxy is also widely used in all sorts of industrial tank linings because of this. Also it is pretty standard to use a really thin penetrating epoxy for the first layer before glassing. This saturates the core to some exent. I had damage to the cedar core on my tri and it was months before I could haul it out. I was sailing it regularly also but that penetrating epoxy really held off all but a few inches of water intrusion thru the core. The biggest problem with all cores is improper sealing around all the hardware and screws which lets thing get wet.

Customs and one offs can be great values but make sure you go with a known designer so at least you know the builder had proper plans to begin with.
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Old 03-11-2018, 12:15   #15
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Re: Am I wrong to shy away from ply sandwhich?

Quote:
Originally Posted by billknny View Post
"Fiberglass" is American for "grp".


Yes.. and no. Foam core is much stiffer for the same weight as solid grp, and so can be made significantly lighter. That is good. On the other hand, the thin outer skin is much more susceptible to a sharp pointed impact than solid grp. That is bad.


All core materials will suffer serious problems over the long run if water gets into the layers. Plywood shows serious problems faster.



All foams used are closed cell. If you want to be technical about it, balsa is a "closed cell foam." They have nothing to do with making a hull "unsinkable" other than they make it possible to make the boat light enough someone can claim it is "unsinkable". Just like the Titanic.



Every decision made during a boat's design and construction is a compromise. Almost none of them are "all good" or "all bad."

May I suggest Dave Gerr's excellent book, The Nature of Boats as a fun and easy read about boat design. He takes the questions you ask here and spends many chapters discussing them. It's a bit more work than having a random stranger give you simplistic answers, but worth the effort.
A great reply.
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