Cored Hulls ?

[BillD]

Most here know I'm researching and considering a move into a DE boat.
I've ruled out the 28 footers, too small for my boating needs.

OK, I've spoken with some builders and boat owners about "hull coring".
I'd like to understand the process, materials used, weight saved and the benefits and downsides of coring a hull.

OK, 1st question.

When a builder offers "cored hull" as an option for $XX thousands.
Typically what part of the hull is cored?
The whole hull keel to washboards?
Solid glass keel then coring up?
Solid glass from waterline "down"
Solid glass from up to the waterline then coring?

All all options?

OK, 2nd question.

What are the materials used these days. I know balsa was or is an option.
I understand about water absorption. Glassing sealing thru hulls etc.

OK, 3rd question.

Weight savings on coring. Let's compare two lengths.

30 footer, full cored hull, "keel to wash boards"? (if that's how it's done) Weight savings vs solid glass? What material?
36 footer, same questions.

OK, 4th and final question.

What would be the benefits and downsides of hull coring? Weight savings? Quietness/Insulation?
The cost is many $thousands$
Is it worth considering??

Appreciated,

Bill D
Is it worth the extra $thousands$

Thanks for your help! :-?

Bill D

 

[Powderpro]

Bill- I would save the money and go with a solid glass hull. I understand why some opt for a cored hull, but for my commercial fishing use, a solid glass hull is the only way to go. I think a cored hull would be fine if money was not an issue AND the builder was VERY competent at coring hulls. As far as weight savings go, I'm not sure there is enough weight savings to make a performance difference in a 30'-40' boat. There are other ways to save weight and gain performance if that is a big concern of yours. More HP and a big prop is the best sure fire way to gain performance.

 

[jerseysportfisher]

Bill,

The purpose of coring is to create strength. If you look at a cut away, the way the laminate bonds to the core material creates a structure similar to a truss. The benefit is it is nominally lighter then solid glass, and increase panel stiffness exponentially. The problem, not everyone does it right, not everyone uses the best materials for that solution. This can result in failures of the bond or failures of the core material itself. With that being said, way to many variables, i would get solid glass, atleast i know i can fix it myself.

 

[jerseysportfisher]

on a side note, hows a 28 to small, you have 3 pots and that dingy davit

 

[steveinak]

Use the money to buy 50 more horsepower and screw the core. If your going to try and save weight use coring for the cabinets and interior walls and bulkhead.

 

[BillD]

Thanks guys. I agree with a solid glass hull.

I want to understand the process, materials used and where the transitions are between core and solid areas if coring is involved.

Jersey, I have a big family and lots of friends who enjoy boating.
I know in for sure now 28 feet is to small.

Now on to the next sizes!!

I also deal in facts and you have you your facts wrong!.

I had "12" traps, not "3" @ the end of this season. LOL

 

[CEShawn]

Take away the doing it right part, because I agree...

The reasons why I was going to go with a cored boat was...

They were stiffer, doesnt that also equate out to possibility of more speed?

They are quieter inside the hull aren't they?

Also on some boats I saw when they weren't cored how they would sweat inside, wasnt sure if this was the case but that is what I was told.

 

[sevenjohn]

Bill D,
If you get a chance give me a call. I'm in the book in groveland.

 

[jerseysportfisher]

I had "12" traps, not "3" @ the end of this season. LOL

is that 3 12inch traps ?


Midcoast Yacht & Ship Brokerage (Tenants Harbor, ME)

 

[captainlarry84]

I concurr

Coring is stronger that a solid glass hull. They are stiffer, quieter and of course much light.

As far as the strength goes in heavy sea and dropping off waves the cord boats are better because the entire hull is bearing the load. If you hit a dead head log at 20 knots where all of the impact is in a small area that is when solid glass out performs coring in the strength department. On impact once the outer skin of glass is defeated the coring is naked to the elements giving zero strength with only the inner skin to stop additional penetration.

When I purchased my 31 JC in 1984 Jack Cadario gave me a very hard time as he insisted on coring & I insisted on solid glass. Needless to say the customer won. Jack complained because in doing the solid glass he had to add two additional full length stringers to make the boat as stiff and as strong as his corded JCs. With those extra stringers and the solid glass hull she is extremely ridged. When the boatyard hauls and blocks my boat they always comment on the fact that nothing moves or flexes when she is placed on blocks and stands.

The boat is now 28 years old and the hull is like the day she came out of the builder’s yard.

Coring on decks and top side are a must. The last place you want extra weight in about the shear line. Over the years I did a few very minor core repairs top side. Just cut the outer skin with a circular saw set a 3/8”, pull the skin off, replace the core and re-glass.
I would hate to try doing that on a vertical or bottom hull surface. Plus if the hull gets wet the water can travel down. You fix the area of damage and years later the trap water freezes and the coring gets pulled away from the fiberglass skins.

If I buy another boat she will be solid glass case closed.

 

[captainlarry84]

The core samples photographed below are all from my 1984 JC. The 3 by 1 thick photos are from the foredeck that was removed during the windless install. The core in those photos is 25 years old! And in excellent condition. The original bow bite was on these samples as noted by the black marks on the core sample. With a bolt running thru that was properly sealed there was zero core damage.

The second photo is from a bottom area during a transducer install. It measures in thickness. On the sides of the solid hull if memory serves me correctly the glass is

The one small hull sample of solid glass, weighs more that the two 3 foredeck samples. That gives you an idea of just who much lighter a core boat is.

They are lighter, faster, drier, quieter, and stronger in heavy seas.. however once compromised it means nothing. If you are in for the long haul and want to give up the qualities of coring then solid is the way to go.

 

[BillD]

Excellent information!

OK, thanks for all the info, private messages and phonecalls to me.

If I have a boat built it will be a solid hull, "double up" stringers and a non-balso cored top.

Bill D

 

[captainlarry84]

Thanks just a litle more info

Bill, Thanks I am glad that you fine the information useful. Sharing is a wonderful way to build friendships.

Back to coring:

Lets not write off balsa completely. Top side coring is important for nice stiff & light gunnels, cockpit solos, foredecks, hardtops, bulkheads & cabin sides. The key is to protect the coring from harm ways. If done correctly coring will last a long time as shown by those 25 year foredeck samples posted earlier.

On cored houses when windows are cut in, each window opening edge with exposed core must be sealed. The process is simple; you rack out about of coring on each opening then fill the void with quality dense fiberglass filler. Same should be done with rodholders and other deck items & thru hulls. The hardest to protect are external screws & bolts. For screws and bolts flat washers are important when practical. In all cases a good sealant should be used on each bolt & screw. Once the hole is drill, dry fit the bolt or screw. If it is a bolt push the bolt out about 1 and apply the sealer then push back in and tight by spinning the inside nut and not the bolt. On screws drive them home but not tight then back out half way, apply sealer and then re-screw into the hole with a hand driver or a screw gun turning very slowly.

With all of this above you may say then why not a cord hull.well we covered that pretty good, it is just risky for the long haul. As far as ride and sound, nothing drops off a wave nicer that a cored boat.

 

[F/V First Team]

Northern Bay 28 layed up with green gelcoat, one layer of kevlar, airex balsa core, one layer of kevlar, resin used was 50# lighter a barrel than regular resin.

Northern Bay 28 layed up with black gel coat, 2 layers of 1.5 oz mat, 2 layers 18-08, one layer of 1.5 oz mat, 2 additional layers of 18-08, resin used was regular GP resin.

Total weight difference between the two hulls: 120 lbs.

Boats were tied up beside each other, another vessel left a wake as it passed, green boat ended up with hole in it, black boat had zero damage.

Oh yeah, cored hulls are totally worth it

 

[F/V First Team]

Additionally, I took a direct hit in the bow this summer by a boat (might have been that wily black NB28, but who's naming names) as the operator backed down incorrectly. Area that was doing the hitting was the "porch" extended over the back of the boat. Had my boat been cored there would have been a wonderful big gash jammed through the laminate. As it was though, the solid glass just laughed as the 3/4" plywood covered with multiple layers of fiberglass crumpled under the force of the impact, the metal bracing and edging being destroyed on the offending member.

I would never suggest or recommend coring a hull. It's just bad math.

 

[petrel]

I agree solid is better for hulls, and why use balsa in decks and cabins when new materials are available for the same purpose where you hardly need to worry about a screw hole?

 

[Sailorgp]

I own a 1985 Sisu 26 with a partially cored hull. The coring starts 18" above the keel. I've heard and seen horror stories about cored hulls so even though I got a healthy survey report, I redid the hull penetrations the first year I owned the boat. I never found any wet core. The hull is incredibly stiff, rock solid from a vibration standpoint and generally built like a tank. Shaft alignment remains the same in or out of the water. Engine vibrations are very minimal. When I installed a bow thruster I really learned how robust the hull is. 3/16"-1/4" Glass skins on both sides of a 1/2" dry balsa core up at the bow. The builder seems to have done a great job laying up the hull. However, knowing that cored hulls DO have an achillies hill, I would still prefer a solid glass hull given the choice. But how much stiffness would be lost in heavy seas or dampening engine vibrations?

 

[tunamojo]

OK somebody explain the coring technique where the shell is all glass and it is backed by coring ????

seems you could use a thinner body of glass all the way around, then back it with the strength of coring ???

Anybody seen this ???

 

[DBM]

When we got the RP hull we ordered Down here, we determined that it needed to be stiffer. So in addition to additional stringers, we also laminated Balsa coring to the inside of the hull above the water line. Not only did it make it stiffer, but because i'm not tough like you Maine guys, it actually insulates the hull, and with the airex cored cabin and top, is fairly easy to heat.

DBM

 

[jerseysportfisher]

and why use balsa in decks and cabins when new materials are available for the same purpose where you hardly need to worry about a screw hole?

There have been quite a few tests in the composites world, the results of these test show over multiple panels, PVC foam holds a better peel strength then balsa, yet balsa wins in all other tests with panel integrity, compressive strength, torsional strength and shear strength. Peel strength is useless when your core material disintegrates under stress