Double Skin hulls / yachts really better?

[mrk123]

To be honest I was not considering a double skin / closed cell foam hull, but I am reading they are advantageous and stop condensation, is condensation really a big issue with none of the above?

[ItDepends]

The answer is... it depends.

You specifically asked about a cored HULL, not a cored DECK, so that is what I will talk about.

There is NO question that a typical cored hull will have a lower thermal conductivity than a typical one of solid fiberglass. Because of this there absolutely will be conditions where a solid hull will sweat and a cored hull will not, but the claim that a cored hull will "stop condensation" is bogus.

The real question should be is "Does the difference matter in the real world?" The answer to this is, no, not really. The range of air and water temperatures and humidities where a solid hull will sweat and a cored hull will not is not wide enough to make it a meaningful selection criteria.

If your boat is in a place where the water is cold enough, and the interior is warm and humid enough, you will get condensation on bare hull surfaces cored, or not.

The reason to use a cored hull is you can make a lighter boat cheaper than you can with solid glass. In the extreme case, you can make a boat lighter than is possible with solid glass. There are no other significant benefits.

[goboatingnow]

Originally Posted by mrk123

To be honest I was not considering a double skin / closed cell foam hull, but I am reading they are advantageous and stop condensation, is condensation really a big issue with none of the above?

Unsinkable sounds good

[valhalla360]

I will agree with "it depends".

After that a lot of design considerations.
- The proper term is "cored", not double skin.
- It works very similar to an I-Beam. When you flex an I-beam, the top and bottom flanges are in compression and tension. The web experiences relatively little tension or compression but it does experience shear forces but they tend to be comparatively small, hence why a narrow web can handle them. With cored construction, the foam takes the place of the web and resists shear forces. Fiberglass is heavy but pretty good resisting tension and compression.
- Almost all fiberglass decks use coring. Sometimes balsa and sometimes foam. The hulls are where you see more variability. It's typically done to keep weight down.
- Just like an I-beam, coring is used to get more bending strength out of less material. For otherwise same skin strength, the further you can keep them apart the stronger it is against flexing.
- You still need a minimum thickness on the skins, particularly the exterior to resist punctures if you hit something solid.
- They are more particular about good construction techniques. In order to transfer the shear to the foam, requires good adhesion from the skin to the foam. Delamination can cause substantial weakening.

A properly built cored boat can be very good quality. A poorly built cored boat can be horrible. You also want to be very careful about adding hardware in a cored section. Even with foam, introducing water is a bad thing (really bad with balsa coring).

Now to the insulating question...Yes, it really does help with condensation if you are somewhere cold. In extreme conditions it may not eliminate it 100% but even 80-90% reduction helps a lot when you are trying to keep the boat dry. We spent a few winters in Georgia/Carolinas and fighting condensation is a really issue. Cored boats suffered far less.

[mrk123]

Originally Posted by valhalla360

I will agree with "it depends".

After that a lot of design considerations.
- The proper term is "cored", not double skin.
- It works very similar to an I-Beam. When you flex an I-beam, the top and bottom flanges are in compression and tension. The web experiences relatively little tension or compression but it does experience shear forces but they tend to be comparatively small, hence why a narrow web can handle them. With cored construction, the foam takes the place of the web and resists shear forces. Fiberglass is heavy but pretty good resisting tension and compression.
- Almost all fiberglass decks use coring. Sometimes balsa and sometimes foam. The hulls are where you see more variability. It's typically done to keep weight down.
- Just like an I-beam, coring is used to get more bending strength out of less material. For otherwise same skin strength, the further you can keep them apart the stronger it is against flexing.
- You still need a minimum thickness on the skins, particularly the exterior to resist punctures if you hit something solid.
- They are more particular about good construction techniques. In order to transfer the shear to the foam, requires good adhesion from the skin to the foam. Delamination can cause substantial weakening.

A properly built cored boat can be very good quality. A poorly built cored boat can be horrible. You also want to be very careful about adding hardware in a cored section. Even with foam, introducing water is a bad thing (really bad with balsa coring).

Now to the insulating question...Yes, it really does help with condensation if you are somewhere cold. In extreme conditions it may not eliminate it 100% but even 80-90% reduction helps a lot when you are trying to keep the boat dry. We spent a few winters in Georgia/Carolinas and fighting condensation is a really issue. Cored boats suffered far less.

Insightful info - from all posters thanks...

I was looking at ETAP boats, and heard they used cored (or what I wrongly called double skin). Would these be good cored boats? So it's basically just 2 relatively skinned fibreglass sheets with foam between? Sounds simple enough?

Are AMEL boats for example just a huge mass of fibreglass wall?

[goboatingnow]

Originally Posted by mrk123

Insightful info - from all posters thanks...

I was looking at ETAP boats, and heard they used cored (or what I wrongly called double skin). Would these be good cored boats? So it's basically just 2 relatively skinned fibreglass sheets with foam between? Sounds simple enough?

Are AMEL boats for example just a huge mass of fibreglass wall?

Arnt HR hulls fully cored. Amel uses cored decks.

[mrk123]

Originally Posted by goboatingnow

Arnt HR hulls fully cored. Amel uses cored decks.

So only Amel decks are cored, and their hulls just single lined?

[goboatingnow]

Originally Posted by mrk123

So only Amel decks are cored, and their hulls just single lined?

That’s my understanding , just like my Bavaria

I remember in the HR hull fitout factory ( I didn’t visit the hull factory ) asking about the composite hull , they argued it was stronger for a given weight, even if repair was more complex. Not sure if all HRs are so built.

[Matt Johnson]

Originally Posted by mrk123

Insightful info - from all posters thanks...



I was looking at ETAP boats, and heard they used cored (or what I wrongly called double skin). Would these be good cored boats? So it's basically just 2 relatively skinned fibreglass sheets with foam between? Sounds simple enough?



Are AMEL boats for example just a huge mass of fibreglass wall?

Etap is the exception. The foam in these boats are used for flotation and not necessarily for stiffness. It's actually a solid hull laminate and a separate fiberglass interior liner, and then a two part liquid foam is poured into the cavity between these two pieces to add buoyancy. It's also thicker than a foam used as a structural coreing.... Something like 38mm 1-1/2" depending on boats. At this thickness, it does have a large effect on condensation.

[boatman61]

There are also Sadler 26 and 29ft boats that are unsinkable available as bilge keel and fin keel..

[deblen]

Cored hulls will eventually get water in between the inner and outer skins.
If you store a "wet" cored hull on the hard in freezing conditions.the water will freeze,expand & cause delamination. Other problems will follow.
Cored decks can have this same problem,but most production boats have cored decks,so we must live with with them.

Cheers/Len

[FPNC]

[QUOTE=Cored hulls will eventually get water in between the inner and outer skins. If you store a "wet" cored hull on the hard in freezing conditions. the water will freeze, expand & cause delamination.[/QUOTE]

Having built many cored hulls in my life, I can state that this is an absolutely true statement. Cored hulls must be built with the utmost care and planning and using the best quality materials and craftsmanship. Even at that, they are susceptible to water intrusion over the long term. I would never recommend purchasing one.

[Tortuga's Lie]

Originally Posted by deblen

Cored hulls will eventually get water in between the inner and outer skins.
If you store a "wet" cored hull on the hard in freezing conditions.the water will freeze,expand & cause delamination. Other problems will follow.

Cheers/Len

I don't believe this is an absolute, If the outer skin is thick enough, water penetration should be of little concern. I have a 24 year old Tartan 4100 and the core is sound. I had a 1977 Tartan 37 for 20 years, sold it in 2015 and no problems with the core. A friend of mine bought it and as far as I know, he hasn't had any issues with it either. Both boats have balsa cored hulls but they also have an outer skin that is at least 1/2 inch thick, thicker in the 4100 (5/8"). In fact, the total thickness of the hull (both skins & core) is close to one and a half inches.
The problems with coring as mentioned, is improper hull penetrations. The use of barrier coating can also help with water saturation.
As a side note on this topic, Tartan utilizes vacuum bagging to ensure proper resin/hardener ratios and penetration throughout the hull and deck layups

[SV-Viento]

Originally Posted by goboatingnow

Arnt HR hulls fully cored. Amel uses cored decks.

This is straight from the HR FAQs: “ Hallberg-Rassy use Divinycell closed cell PVC foam that does not absorb water but insulates well against cold, heat and noise. Hallberg-Rassy use Divinycell cored hulls on all Frers designs. No Enderlein designs except the 382 have cored hulls. Horizontal areas of the decks are always cored. The transom is never cored. Hallberg-Rassy has never used balsa, which may soak water and rotten.”

HR like other quality builders utilizes solid laminates in many high load and structural joint areas including a large for to aft structural joint throughout the bilge of the boat that varies in width and frequently the entire area below the waterline.

I think others up thread have more fully addressed the insulating/condensation question. But in general there are arguably many reasonable quality boats that use cored construction in appropriate areas primarily for structural/weight reasons and secondarily for insulating quality.