Am I wrong to shy away from ply sandwhich?

[DerekKelsall]

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.

[coopec43]

Originally Posted by DerekKelsall

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.

Derek Kelsall WOW!! What a blast from the past?

I remember your name well. Two other designers/constructors of Trimaran fame back in the 60-80s that I recall were Arthur Piver and Hedley Nicol

Clive

[DerekKelsall]

Originally Posted by coopec43

Derek Kelsall WOW!! What a blast from the past?

I remember your name well. Two other designers/constructors of Trimaran fame back in the 60-80s that I recall were Arthur Piver and Hedley Nicol

Clive

Hi, I never went away, A few hundred different foam sandwich catamarans have gone afloat since "Toria" but it is some time since we designed a tri.
Happy boating, Derek

[captlloyd]

Originally Posted by fatherchronica

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.

I may have misunderstood you but I believe you said you could put a hammer through even a metal hull. Not on the hull I built. 3/16 sides and 1/4 bottom. Not happening.

[Jim Cate]

Originally Posted by captlloyd

I may have misunderstood you but I believe you said you could put a hammer through even a metal hull. Not on the hull I built. 3/16 sides and 1/4 bottom. Not happening.

Be careful... FatherChronica is Thor's screen name!

Jim

[Djarraluda]

Originally Posted by Cadence

A great reply.

Except I would have to argue on the first point. Sorry to be pedantic but “Fiberglass” is actually the American for “Fibreglass”. The fact that it is a GRP is secondary in this case. I have a sneaking suspicion that there was some sort of trademark or registered name involved here back when it all began.
Roger

[valhalla360]

A rough breakdown:
- Fiberglass/GRP: This is mesh of glass fibers (sometimes carbon fiber or other fibers are used for specialty purposes) encased in a plastic resin. Poly-ester and vinyl-ester are the most common and epoxy is sometimes used.
- You can build with straight up wood with plywood being a variation. Not very common anymore. Plywood in particular can vary wildly in it's suitability. Cheap home depot luan, is not a great choice. As soon as any water gets in, it will delaminate and lose it's strength. Top of the line marine plywood (with top of the line prices) holds up quite well. We had 25yr old marine plywood boards on our cat that were fine after years exposed to water with just some bottom paint on them.
- One solution for wood is cold molding. It uses strips of solid wood encased in epoxy. If done right and maintained, it keeps the wood dry so no rotting occurs. The wood provides the structural strength.
- Another solution for wood is sheathing. This is used with plywood. Since keeping it dry is important and most places where the sealing epoxy is likely to be damaged is on the exterior, a couple layers of fiberglass are laid down to resist abrasion. The interior is less likely to be abraded. Again the wood provides the structural strength.
- Sandwich construction comes in multiple flavors. The basic idea is similar to a steel I-beam. I-beams need lots of strength in the top & bottom flanges and not much in the middle. Sandwich does this but replaces the center with foam or wood.
- End core balsa is probably the oldest version. The key advantages are it's light and because the strands don't go along the panel, if water gets in, it takes more time for the rot to travel thru the core.
- Plywood core has issues because the grain follows along the panel and will wick any moisture across a larger area increasing the area where rot occurs. Also, it can delaminate which significantly weakens the panel.
- Foam core is newer (but really not that new). The biggest issue is it usually takes more care to get good adhesion between the fiberglass skin and the foam. If the adhesion is poor, you will have a weak panel.

The reason you see cheap wood boats for sale is very few are built on a production basis. If you do it right and keep them sealed and dry, it is an excellent material but lots have been built by armatures and are total junk. As a result they all suffer lower resale value because it's not always easy to know (particularly for a newbie) which are the good ones.

[catsketcher]

One reason for custom boats have a supposedly lower resale may be the modern buying public rather than any problems with any boat type. The modern condocat buyer is not the same as the buyer from 20 years ago.

Back then most buyers knew a lot about the boats and chose them because of the sailing performance of the design, the background of the designer and a relationship with the builder. Now most people buy boats like they buy a car - interior furnishings, interior room, appointments.

So the custom boats are a reflection of a different boating approach. It is the approach I prefer. We used to have very few people who boat cats depending on the galley furnishings instead of designer background. Many buyers are different now - it doesn't means the boats are better, they just reflect the bulk of modern buyers.

[coopec43]

Originally Posted by catsketcher

One reason for custom boats have a supposedly lower resale may be the modern buying public rather than any problems with any boat type. The modern condocat buyer is not the same as the buyer from 20 years ago.

Back then most buyers knew a lot about the boats and chose them because of the sailing performance of the design, the background of the designer and a relationship with the builder. Now most people buy boats like they buy a car - interior furnishings, interior room, appointments.

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

[valhalla360]

Originally Posted by catsketcher

One reason for custom boats have a supposedly lower resale may be the modern buying public rather than any problems with any boat type. The modern condocat buyer is not the same as the buyer from 20 years ago.

Back then most buyers knew a lot about the boats and chose them because of the sailing performance of the design, the background of the designer and a relationship with the builder. Now most people buy boats like they buy a car - interior furnishings, interior room, appointments.

So the custom boats are a reflection of a different boating approach. It is the approach I prefer. We used to have very few people who boat cats depending on the galley furnishings instead of designer background. Many buyers are different now - it doesn't means the boats are better, they just reflect the bulk of modern buyers.

I think it's a lot longer than 20yrs ago that most cruising boats were custom built. I'm thinking at least 50-60yrs ago to get to the point where it was even 25% of boats were custom. I know back in the mid 90's (25yrs ago) when we bought our first cruising boat, custom boats were already oddballs with unknown provenience unless you had specialty knowledge.

It's very similar to restaurants:
- If you go to a chain restaurant, you may not get the worlds best meal but you probably won't get a bad meal.
- Go to a one off restaurant...if it's the right one, it may be the best meal you ever had for a bargain price...or it could be total crap. You won't know until you try.

Spend $40 on a meal and it turns out bad, it's no big deal. People are willing to take the risk. Spend six figures on a boat and it turns out to be a cluster with bad resale...that's too big of a risk for most.

[JOHNMARDALL]

Whether it's cold moulded (a plywood boat covered with a layer of fiberglass), or a cored laminate (fiberglass inner and outer skins separated by a plastic foam, balsa or plywood core), the carefulness and expertise of the original builder and subsequent modifiers and installers of equipment in making penetrations of the hull and deck will determine if a particular boat is a good buy for you. Every hole or cutout for a through-hull/skin fitting, stay or shroud base, stanchion base, windlass, hatch, etc. is a place where, over long periods of time, water can get into the core or plywood, and aided by freeze-thaw expansion and contraction in cold climates, delaminate and damage the structure of the boat. This is more common on decks than hulls, as they have more holes. Every hole in plywood must be soaked in resin, preferably epoxy, every hole in cored laminate must be edge-reamed and filled with resin putty so that no hole has untreated edges or sides that can absorb water.
So, on an existing boat, you have to employ a surveyor who will sound the hull and deck with a mallet, to locate rot and delamination. On an older boat there will always be some, and the surveyor will help you to decide if it's too extensive and under critical load points (stays and shrouds, winches, cleats). Except for very small areas, water damage cannot be repaired by drilling holes and pouring in epoxy, but will require cutting out the damaged area and rebuilding it.
Your decision making should also be informed by how and where you're going to use the boat - rounding Antarctica at one extreme and mid-summer cruising on the Chesapeake Bay at the other. As a beginning sailor, with an adequate supply of good judgement about the weather, you're unlikely to break the hull-deck joint but may sail into conditions where the stays and shrouds are heavily loaded and could pull out of a weakened hull or deck.


I hope this helps and wish you good luck
John Mardall
Vetus Maxwell Boat Equipment

[andypag]

Thank you all so much for these excellent replies.



The mist has definitely cleared. And I'm also grateful for the explanation of the different types of GRP. That has made a lot of boat maintenance research suddenly make much more sense.


I really appreciate the time and effort so many if you have put into your considered replies.



Brilliant community.

[Cadence]

Clive, you nailed it with "it is easier to buy a good used boat."

[DerekKelsall]

Comments -
We still have misleading comment on foam, which apply to balsa for example but not to all foam. I used offcuts of PVC foam as spacers on a slipway I built. When last seen, 40 years later, washed twice a day by the tide, the foam was still standing proud of the concrete. PVC (about 70 kg density) is the right stuff. There are two simple ways to ensure a bond between the skin and the foam - laminate directly onto the foam or use vacuum. A shop vac will do the job. Resin infusion is standard with my clients. I can take you to 40-50 yo sandwich tris still sailing and looking good for a few more decades yet.

It is a pity that owner build has gone out of fashion. It was how I started. There are few things more satisfying than being able to say - all my own work. A big advantage while sailing is that you know the boat and its systems.
Happy boating, Derek.

[Jim Cate]

I used offcuts of PVC foam as spacers

Derek, was that Airx foam or some other PVC?

Jim