Hull bottom materials?

Ask questions before buying our plans or request a new design. Anybody can post here
Fuzz
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8921
Joined: Tue Aug 02, 2011 4:23 am
Location: Kasilof, Alaska

Hull bottom materials?

Post by Fuzz »

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.

Lakesurfer
Frequent Poster
Frequent Poster
Posts: 21
Joined: Fri Nov 15, 2019 5:38 pm
Location: Tampa fl

Re: Hull bottom materials?

Post by Lakesurfer »

Fuzz wrote: Mon Nov 02, 2020 4:58 pm 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.
Im an expert on nothing especially when it comes to boats and i have the same questions. being in aviation with a majority of my experience in aviation composites it depends on application, for airplanes at least. the Boing 777x has a composite wing that has no core but is skin on frame. I believe wings need to flex slightly for some aerodynamic or structural reason. the 747 from the wingtip can flex up to six feet and again no core. in the fuselage they use a core as well as flight control surfaces, raydomes, landing gear doors and so much more. so from what i've researched and been trained a core not only get its strength from the core itself but from the skins as well. think of it like a platform made out of composites. one platform is made from only 5 layers of fiberglass and no core and the second has 3 layers of glass a core and 2 more layers of glass. the non cored platform will bend and be all kinds of flimsy where the cored platform with be a lot more stable and able to support more weight. the stress of the cored material is spread out a greater distance. and to make the non cored platform just as strong ass the cored, youd have to use several more layers of glass but it would also be a ton heavier. I've seen structures without cores but have used hollow stringers much like a boat but coreless. these are effective because the skin has more surface area to distribute the stress. so for boat stringers you could technically use cheap styrofoam and put several layers of glass over them and the strength would be mainly in the glass. where a wooden stringer relays largely on the wood and less on the fiberglass. so your core in a way is tricking the structure into thinking it has more layers of fiberglass than it actually does. but the sacrifice with a core vs a solid laminate is that the cored hull skin will be far more likely to suffer from puncture type damage. where i live in florida we have all kinds of oyster beds that are super easy to run into on lower tides. if I ran into an oyster bed with a cored hull that was 1/2 thick, the actual fiberglass would be less than 1/8 and would puncture much easier than a solid hull that was 1/4 thick.

That being said, i cant answer your question with any real data or numbers but I can say that the hull needs to be able to withstand abuse from slapping on the water and probably shouldn't have much flex if any at all. so if the design calls for a core and you don't want to build it that way then you'll have to compensate someplace else. most certainly several more layers of glass on the hull and wider, stronger or more glass on the stringer and frames. removing the core will require adding more weight and more weight will change the characteristics of the boat.

cored structures main benefits are to save weight and add strength but there will always be a cost, as nothings free.

im looking at the PH15 and found PCV foam similar to divinycell but the density is 1 lbs less but $70 less per sheet. so im trying to determine if I can use this as a core and maybe add another layer of glass to the hull to compensate for less density.

either way good luck

-Mark

Fuzz
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8921
Joined: Tue Aug 02, 2011 4:23 am
Location: Kasilof, Alaska

Re: Hull bottom materials?

Post by Fuzz »

I have built a skiff with foam core and it worked out well. The reason I am asking is I see that on larger boats many folks do not want a core below the water line. I think it is for both water intrusion and ease of repair if needed.

Lakesurfer
Frequent Poster
Frequent Poster
Posts: 21
Joined: Fri Nov 15, 2019 5:38 pm
Location: Tampa fl

Re: Hull bottom materials?

Post by Lakesurfer »

Fuzz wrote: Tue Nov 03, 2020 4:20 pm I have built a skiff with foam core and it worked out well. The reason I am asking is I see that on larger boats many folks do not want a core below the water line. I think it is for both water intrusion and ease of repair if needed.
Hmm. I’ve repaired foam cored wing components on airplanes and I think it’s pretty easy to repair. Unless you get a huge rip or hole but If you boat is solid glass with major damage, the repair will be just as complicated. Cut out the damage and replace with new. For every layer of glass on your damaged part your repair laminates should extend 1 inch from the outside of the damage. So if you have a 1 inch circular hole in a panel that has 4 layers of glass you would at least 4 repair patches, the first being a 2 in circle the second 3 in, 3rd would be 4 in and so on. If you can get to the inside of the panel you would want to put a couple layers there as well.

So a cored hull repair in my experiences usually are pretty easy just a couple more steps.

User avatar
Netpackrat
Very Active Poster
Very Active Poster
Posts: 1017
Joined: Thu Sep 14, 2017 1:35 am
Location: Anchorage, AK

Re: Hull bottom materials?

Post by Netpackrat »

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.

narfi
* Bateau Builder *
* Bateau Builder *
Posts: 2433
Joined: Tue Jun 07, 2016 6:55 pm
Location: Bush Alaska
Location: Bush Alaska

Re: Hull bottom materials?

Post by narfi »

I also know very little, but read a lot :)

For boats the 3 typical cores are plywood, balsa, and foam. They each have their own set of compromises, advantages and disadvantages.
For aircraft composites, there is a huge difference in worlds between people who work on Boeing type aircraft and people who work on small cessna/piper type aircraft and people who work on Rutan type aircraft and people who work on molded kit aircraft, all different worlds. Then you have the difference in locations and climates, someone building or working or repairing in Florida has a completely different set of compromises than someone working in Alaska in January.

Plywood is easy to use, balsa is light and cheap, foam is most water resistant.
As for cores under the waterline, each designer will design for the loads and resistance needed.
In aircraft where weight is the primary concern, the compromise is that by the time you make the skin thick enough to be puncture resistant, it is already stronger than needed to carry the loads, the designer will run out a spreadsheet of numbers to determine the lightest way to get the strength needed as well as the puncture resistance while keeping weight down. (I do not know the math)

Wood has a long history and well known properties over time in use. Foam has a relatively short history, and there are concerns and fears both founded and unfounded on the different types.
For example one type of foam may be more prone to crumbling over time and impact, while another foam may withstand impact and time well and maintain its primary purpose of holding the two skins apart. However, if someone has experienced failure with the former type of foam, they have a bad picture in their mind that ALL foam is bad. In the same way, someone who has experienced a failure in poorly bonded skins to a balsa core that ended in a waterlogged and rotten worthless hull will never be convinced that properly bonded balsa cores could ever be used below the waterline.

A properly engineered composite can be much lighter with a core vs. a solid laminate. This works the same way as a steel I beam is lighter than a solid steel bar of the same strength. In a cored composite, the core acts as the web of the 'I' and the glass or carbon or whatever acts as the caps. The core only has to be strong enough to hold the caps apart from each other and thus can be much lighter. In Aircraft the math gets even more interesting because you need to choose between cored skins and massive cored structures. Which is lighter, a wing made of a solid chunk of foam with glass skins around the outside (glass, foam, glass) or with cored skins(glass, foam, glass, air gap with ribs, glass, foam, glass)? What is the difference in labor? What is the difference in cost? What is the difference in weight? What is the difference in error free repeatability?

In the end, I think there is science and there is superstition that both drive the decision behind cores used as well as the decision to go solid without cores.
I choose to pick reputable designers with a history behind the design I choose, or at least a history of well proven designs and trust that they know what they are doing, because I do not have the knowledge or education to make those decisions myself.


Edit to simplify my ramble.
A solid non cored hull bottom will be more puncture resistant and heavier. A cored hull bottom will have varying properties depending on the core used and laminate schedule specified by the designer who ultimately has done all the math and decision making already.

Online
TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: Hull bottom materials?

Post by TomW1 »

Fuzz there is not much difference between plywood and foam until you get up above 23-25 feet. At that point foam takes over on a weight basis and boats will be lighter as the weight of the foam is less than the plywood needed. Below that the weight of the fiberglass and epoxy needed to support the foam weighs as much or more than a plywood boat. The smaller the boat the bigger advantage plywood has.
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

AlScal
New Poster
New Poster
Posts: 9
Joined: Wed Nov 11, 2020 2:39 am

Re: Hull bottom materials?

Post by AlScal »

Planning a new boat can be great fun. After thousands of miles on my last boat, I better understand what I want my next one to do. For bottom material I like aluminum, it’s tough, relatively light, and easy to integrate equipment beds and integral tanks.

Fuzz
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8921
Joined: Tue Aug 02, 2011 4:23 am
Location: Kasilof, Alaska

Re: Hull bottom materials?

Post by Fuzz »

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.

cracked_ribs
Very Active Poster
Very Active Poster
Posts: 511
Joined: Sat Apr 11, 2015 11:58 pm
Location: Western Canada
Location: Ladysmith, BC

Re: Hull bottom materials?

Post by cracked_ribs »

This is just my take but on a boat that size, the only way I'd have a cored bottom would be if either A) it was layered, like a cold moulded hull, or B) the inner skin would support the hull well on its own at least in a stationary setting.

That's simply because if I have to repair it, I want to either A) only be dealing with rot or damage in the outer layer, or B) because I want to be able to cut the whole core away without worrying that the hull will deform without the core in the area that needs repair.

I haven't looked at cored boats in that size range so I don't know what the laminate schedule looks like; maybe they have two layers of 17oz biax on the inside and are stiff as steel even when the core is saturated and wrecked, I have no idea.

But that's what I'd be looking at, design-wise. If the core gets wrecked in some section, I want to be able to cut it out and replace it without worrying that the hull is going to deform in that area while I have it apart. That's why I don't mind large cold moulds - there's epoxy between everything, and the whole structure seems stiff and the rot or damage, very localized. But a single layer of 12oz on the inside, over a single layer of core, say? No, that I would avoid.

Just my take.
I designed my own boat. This is the build thread:

viewtopic.php?f=12&t=65349

Post Reply

Who is online

Users browsing this forum: No registered users and 13 guests