Hull bottom materials?

[fallguy1000]

I see a thread going on alternant materials but I did not want to walk all over it.
My question is about using foam core in the hull bottom. I read where it is both good and bad to use a core for a glass boat and am wondering the thoughts are from here.

San polymer foams do not take water.

Most hull bottoms are more than a single layer of glass. Mine is two layers of about 21 oz glass or roughly 42 oz or 6 layers of tows.

If the bottom were damaged by impact; the impact area would need to be removed and replaced with a thinner core to allow for heavier glass on return. Heaven forbid.

The front section of my boat is 5 layers of 1700. Always seems a bit thin to me, but specs were for 3- 4 layers. It would be damned difficult to repair this area, but is most likely impact zone for a log or underwater immovable. The issue for the repair is there is no access to the back side, so you cut away damage and then have to fashion some sort of backer...most likely core.

[TomW1]

When I started this thread I was thinking of larger boats, say ones above 30 foot. Boats 20 foot and under to me there is no question that plywood is really tough to beat. The 20-30 foot area is where things get muddy for me. I was just sort of wondering how some of you guys felt. I have commercial fished plank, plywood, aluminium and glass hulls. they have all had their strong and week points. Nothing is perfect for boats or they would all be made out of it.

Fuzz if your talking about a 30' boat I would go with foam as I said above once you reach the 23-25' range especially above 25' foam has the advantage. The weight of the foam makes up for the extra fiberglass needed. If your building one of Jacques boats he will give you the lay ups for foam.

Tom
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[fallguy1000]

When I started this thread I was thinking of larger boats, say ones above 30 foot. Boats 20 foot and under to me there is no question that plywood is really tough to beat. The 20-30 foot area is where things get muddy for me. I was just sort of wondering how some of you guys felt. I have commercial fished plank, plywood, aluminium and glass hulls. they have all had their strong and week points. Nothing is perfect for boats or they would all be made out of it.

Fuzz if your talking about a 30' boat I would go with foam as I said above once you reach the 23-25' range especially above 25' foam has the advantage. The weight of the foam makes up for the extra fiberglass needed. If your building one of Jacques boats he will give you the lay ups for foam.

Tom
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Tom, I disagree with you on the foam/length issue.

Here is why.

A few years ago, I watched a video of a Woods multihull builder doing a foam weight reduction project on her boat. She went to great lengths to reduce her vessel weight by 10kg. I was rather amused by the effort and was laughing at her.

Then at the end, she showed a picture of her reward. It was a wine locker that allowed her 8 bottles of wine onboard with no weight penalty. It was at that point in time, I realized I was wrong and laughed at myself and a small part of me really thought she was a hell of a good woman.

The TD3 dinghy can be built in foam at 8 feet long and weigh a mere 40 pounds. Please don't think that there is no small boat tradeoff for foam.

The thing you need to realize is foam is harder to build for the weight tradeoffs. Lose the focus on length and adjust the focus to time. If you want the boat to be ultralight, build in foam, plan more time. If you are less concerned with weight, and have less time, build in ply. Ply is much easier.

[TomW1]

Fallguy Jacques has stated over and over that foam is at a disadvantage under 23-25 feet. 25 feet is really the break even point and where foam is at the best. It just takes to much fiberglass and epoxy to equal the the penetration strength of stitch and glue plywood. Sure you can do it and some have the cost is more and sales price may be better for a foam boat if one wants to sell there boat. Jacques is willing to spec almost any boat in foam as long as he feels it is feasible.

Tom

[fallguy1000]

Fallguy Jacques has stated over and over that foam is at a disadvantage under 23-25 feet. 25 feet is really the break even point and where foam is at the best. It just takes to much fiberglass and epoxy to equal the the penetration strength of stitch and glue plywood. Sure you can do it and some have the cost is more and sales price may be better for a foam boat if one wants to sell there boat. Jacques is willing to spec almost any boat in foam as long as he feels it is feasible.

Tom

Pretty sure he is specifying all his boats in foam. The issue is not length, but time to build.

[Lakesurfer]

regarding foam core benefits only after a certain length. What benefits are we looking for? weight always seems to be a pretty big one and I believe one would choose a foam core among many other reasons to save weight, corrent?

Straight from the plans for the PH 15:

wood core:
LOA: 14' 6"
Draft: 4"
Weight: 600 lbs

Foam core:
LOA: 14' 6"
Draft: 3.25"
Weight: 375 lbs

If time and money are not a big factor than foam core for the win?

-Mark

[Lakesurfer]

With aircraft, the repair itself may not be difficult, but depending on the tech data requirements, creating the requisite conditions may be a challenge. There's likely to be a cleanliness requirement, a temperature requirement, as well as possibly a vacuum requirement which may be difficult or impossible to meet in a drafty, dusty hangar, let alone out on a ramp. Particularly in my part of the world, for half of the year. On a smaller airframe or component which you can take into a proper composite shop/clean room to make a repair, that's one thing (provided you HAVE such a facility), but it's usually difficult to do that with a whole airframe. It may be possible to set up those conditions in a hangar for an on-wing repair, but as soon as somebody comes along and opens your hangar door, you'll probably have to start the cure time all over.

I am not totally sure what my point is there other than to say that calling aircraft composite repairs easy isn't really a one size fits all statement. I'd rather repair a cored boat any day than a composite aircraft structure. Most small to medium sized operations are going to send the damaged part out to a dedicated facility for repair.

I think the thing with not wanting a core below the waterline, is if you are going to hit something, that's the most likely impact zone, and single skin/solid fiberglass will have an advantage there. Some of it's probably also just concern about getting water into the core in general. On an aircraft you aren't really worried as much about those things. Impact damage is still a possibility but not really to the same level of concern as with a boat hull, and while weight savings is important to a boat, strength to weight is everything to an aircraft. The aircraft is going to be much more closely engineered in that regard, and so on the whole you are going to see more restrictive repair requirements as well. The boat can afford to put more weight into durability, although as has been repeated here many times, there is a cost to that since resin and glass are the expensive part of the hull structure.

For discussions sake neither the FAA nor the manufactures of just about any composites on any AC would allow a repair to be facilitated on any ramp regardless of the outside environment. Clean rooms, calibrated vacuum, calibrated heat lamps or heat blankets with specific cure times and temps and so many other requirements are needed. repairs on airplanes are never a choose your own adventure, they are all outlined in a structures manual and if the damage you have to repair is not in the manual then you must got to the manufacturer or authorized aviation engineers. so in that sense the repairs on airplanes are easy because everything you need to do is outlined in the manual including tooling and environment. So I say they are easy to repair because of my previous work in an off wing composites repair facility. any of the tools or environmental factors can be adapted, adjusted or altered to meet your requirements of diy garage boat/composites repairs.

Tools and having the correct ones make seemingly difficult jobs much more easy. I consider knowledge and ingenuity to be the most important tools with any project. Although I do have most of the equipment to vacuum bag parts in my garage i typically cannot be bothered to set it all up and choose the fastest route. Currently I'm building interior trim pieces for my car in "forged Carbon" fiber. for vacuum bagging im using those vacuum bags that people use to put cloths and blankets in to compress them to fit into tighter storage. and I need to do larger pieces then all I really need are the vacuum fittings that are on the bags and some construction grade plastic sheathing. get some butyl tape from the hardware store to seal the bag and the vacuum fitting up and pick up some nylon fabric and polyester batting from the fabric store and you have a completely adequate (for our purposes) vacuum bagging system. I can completely understand that building a foam core boat without any understanding of vacuum bagging or composites could be a complete disaster though.

In a lot of composites foam cores are only to give a structure for laminating material to be laid on. so you would be relying on the strength of the cloth vs the combination. so for example the PH 15 has a sandwich of 2 layers of glass 5 lbs density foam and 2 more layers of glass. if one wanted a "tougher" hull skin you could use some combination of (for simplicity) 3 layers of glass 4 lbs foam and 3 more layers of glass. I've repaired Airplane parts that did something similar to this where the outside skin was several layers thick and on the inside had several thin width soft foam core stringers running the length of the part and then several layers of glass over the foam. so the inside of the panels fiberglass had far more surface area to cover because it followed the contours of the foam. this made for a pretty strong panel.

I not sure what my point is but if you figure it out let me know

-Mark