Balsa-Cored Bottoms

[Hiracer]

Cruisers in the Pacific Northwest, including the inside passage and Alaska, regularly hit submerged and partially submerged logs. In my first season in Puget Sound I spoke with three boat owners who went up to Alaska and back. All three hit something. Two hit logs, and one hit a rock.

The two who hit logs got holed. Those were fiberglass boats. Both had to interrupt their trip to repair their hulls. One boat lost its entire summer season to the fix, and the other took several weeks for the fix.

The boat that hit a rock was steel and took a dent.

If you have a cored hull hull and hole it, I would imagine that the skill level needed to fix it properly would have be be more sophisticated. You might not find that level of skill depending on where you hole your boat. Something to think about. If you are just sailing local waters, no big deal. If not, then maybe, yes, a big deal.

Having run over in broad daylight a submerged log that was about three feet in diameter, I can attest that **** can happen especially if you sail where big trees grow. In my example, I am pretty darn sure that the log monster would have badly bent my prop shaft had I not had a prop encased in the skeg. In fact, think it could have snapped a spade rudder or bent that shaft. I never saw it until after we hit it (two very big thumps, one for the keel/hull and the other for the skeg) and it popped up many yards behind us, surfacing in slow motion like a whale.

I think cored hulls are fine, in some waters for some purposes. They do a lot of things well. I question their impact resistance and the difficulty of repairs. Certainly, it you hit something you need to pull the boat out or dive for inspection even if you are not leaking water into the interior because you need to find out if you are leaking water into the core. The outerskin typically is not very thick because the whole point of cored hulls is to save weight.

[Event_Horizon]

Originally Posted by bewitched

You definately want compressive strength in a hull, but it's not necessary in the core. Because the core doesn't take compressive loads.

Take these examples:

Foam core = Compressive strength 1.3 MPa

Balsa core = Compressive strength 6.3MPa

Now slap a skin of glass fibre each side for a composite construction and the strength is 293.8MPa.

The core isn't where the compressive strength comes from. As long as it can take the shear, that's all it needs to do.

Build it out cardboard if you want - just make sure you don't get it wet

All true unless you are expecting localized loading in that area. Around area's where you are going to have large point loads you'd better pick a strong core material or it might mean bad news bears for you.

Remember, core materials for your deck, and core materials below the waterline are subjected to different types of forces (typically) and this plays a role in core material selection.

Everyone is right

[GordMay]

Originally Posted by Event_Horizon

... Everyone is right

Yea us (er: you, them, & they) !!!

[bewitched]

Originally Posted by Event_Horizon

All true unless you are expecting localized loading in that area. Around area's where you are going to have large point loads you'd better pick a strong core material or it might mean bad news bears for you.

Remember, core materials for your deck, and core materials below the waterline are subjected to different types of forces (typically) and this plays a role in core material selection.

Everyone is right

Very true, and the answer most builders come up with is to locally put ply in the core. It's a cheap, easy and effective solution. But you wouldn't want to do that with a hull (if you want to keep the weight down).

For the hull, I think that Dockhead's boat builder has employed one of the neater solutions - apply an extremely high strength material, with extremely good abrasion and puncture resistant properties in the plane where it will have the greatest benefit.

There's a reason we don't see glass-fibre, ply or balsa bullet proof vests.

I've enjoyed batting ideas back and forth - my understanding of the subject has grown since the start of the thread. If I've appeared confrontational, I apologise, I don't mean to be.

[44'cruisingcat]

Originally Posted by bewitched

You definately want compressive strength in a hull, but it's not necessary in the core. Because the core doesn't take compressive loads.

Yes it does. Whenever you try to bend a cored panel, the skins will try to pinch closer together, like the way when you bend a hose, it flattens out. The core holds them apart, putting the inner skin into compression, and the outer skin into tension. (Like the vertical component of an "I" beam) The greater the compressive strength of the core, the stiffer the panel will be.

Try making a cored panel using foam rubber as the core - it will have so little stiffness as to be useless.

[bewitched]

The greater the compressive strength of the core, the stiffer the panel will be.

Take this data sheet of a foam cored composite construction. It shows that a 20mm foam core with a 0.53mm laminate of glass fibre each side is stiffer (about half the deflection for a given loading) of; 13mm of 5-ply plywood, 4mm steel plate or 6mm aluminium plate. The core material (foam) has a very low compressive strength in comparison to the other materials, but when used in a composite structure that addresses the compression and tension forces in the outer laminates, a very stiff structure results - this is not down to the compressive strength of the core, but its thickness and its ability to deal with shear forces.

The easiest way to visualise a shear force, is to imagine bending a paperback or a magazine parallel to the spine. It can easily bent (deflected) because the pages slip past one an other (there is no shear strength). Glue the pages together and you increase the shear strength significantly and significantly more force is required to bend the book.

This is why delamination of the core from the outer laminates is such a problem - it's a massive loss of shear strength.

[GordMay]

Originally Posted by bewitched

... The easiest way to visualise a shear force, is to imagine bending a paperback or a magazine parallel to the spine. It can easily bent (deflected) because the pages slip past one an other (there is no shear strength). Glue the pages together and you increase the shear strength significantly and significantly more force is required to bend the book.
This is why delamination of the core from the outer laminates is such a problem - it's a massive loss of shear strength.

Very apt analogy!

[matauwhi]

G'day, mates, Just updated our old Nilsson 3000 windlass to a Maxwell 3500 VMC. Here's a photo of the cored deck plug that I had to enlarge. 30 mm thick, excellent quality construction and still strong as. No worries with this cored deck. P.S. PAE beefed up the deck and surrounding areas to 140 mm for the windlass pad. We sleep well on the hook. Cheers.

[Event_Horizon]

Originally Posted by matauwhi

G'day, mates, Just updated our old Nilsson 3000 windlass to a Maxwell 3500 VMC. Here's a photo of the cored deck plug that I had to enlarge. 30 mm thick, excellent quality construction and still strong as. No worries with this cored deck. P.S. PAE beefed up the deck and surrounding areas to 140 mm for the windlass pad. We sleep well on the hook. Cheers.

That is one sweet sandwich!