Balsa core advice

[NickRobinson]

Hi All-

I'm looking at a Mirage this weekend which is the first 'non-solid' boat Iv'e looked at.

A previous survey refers to two areas, coach roof and cockpit sole, 4' long of suspect delamination/deterioration but describes the structure as sound.

Recommendations then were to investigate by drilling cores. The vendor is looking through his receipts from then to see if rectification was indeed carried out if necessary.

I realise the old survey carries no warranty to me.

So- what should I look for/do/ask about please?

Any other Mirage 28 advice gladly received-

Nick

 

[maby]

Was there a suspicion of water ingress? If so, steer clear - expensive job and very difficult to leave it looking good.

 

[exfinnsailor]

Had a FINN sailing dinghy a number of years ago with a balsa fill to the floor. The floor cracked where you moved about and this had let some water in. I decided to repair it. I used a router to remove the top layer of floor where the damage was and found the balsa core soaked with water. All you can do is make the hole bigger until you find dry balsa. Once you have found that you then have to dry everything and I then epoxied in place new core material ( foam typre ) and rebuilt the floor with epoxy and mat. The problems you have are getting it to look right. After alot of sanding and filling I painted the floor with none slip and that was it. It would be imposible to do a job like this from the underneath.

If you know of two bits that are delaminated how much more is there that you don't know about. It could cost a considerable amount of money to repair.

Someone suggested to me that I could just have drilled a load of holes and used an allen key to make a void and then fill with West Epoxy. All this does is create hard spots in the floor which will then crack round as you stand on other parts of the floor. This happens because the balsa core will give slightly but the West does not.

 

[PCUK]

All depends on the final price whether it is worth buying and doing the job. Not a difficult job apart from the finish. How about covering with Dek-king fake teak or similar?

 

[William_H]

Balsa core failure

As said the core can fail due to water ingress or from high point loads as in cockpit floor or deck.
I would not consider it a terrible problem however it will require work to fix. When it is fixed you will have a smooth or worse rough floor of the cockpit which will have to be painted. This could be covered up with wooden grating but not so easy for the cabin top.
A full paint job will be necessary after repair and you will need to apply non skid granules (Intergrip is one)
and the original non skid pattern and gell coat will be lost forever. However once done you can easily keep it up to scratch with repainting and repairs will make it no worse. So for an old boat resorting to paint and filler is not so bad. So you decide. Is the boat the right price. Are you up for f/g repairs? good luck olewill

 

[oldsaltoz]

The best way to repair a balsa deck or floor is from below, this ensures the original deck finish is preserved and repairs are all invisible.

Get a price on under repairs and take this off the asking price.

To do the work yourself, find the soft spot and mark below deck, get a 6 mm drill with a depth stopper on it and drill holes on a 50 mm grid paten, cutting through the bottom layer of glass only.

Check the shavings off the drill for any sign of moisture, keep widening the grid till you get no moisture and mark the dry hole/s with a felt pen.

When all the outer holes have a dry hole mark on them you have an outline or the area to be removed.

Cut this out in one piece with a small router bit and save it for re-fitting later.

Now remove all the soggy balsa then put a an Allen key in the chuck of a drill and remove the layer of dry balsa around the edge to 15 or 25 mm leaving a lip of glass.

Next clean the whole area and sand the underside of the deck then wipe down with acetone.

Make a template of the area in two halves because you will have to fit the new filling over the ledge of the under deck laminate.

Cut your closed cell foam to shape, you may have stick a couple of layers together to get the right thickness, it should be 10 mm thinner than the gap between both layers and the edges all round.

If the area is curved, saw cuts close together and 5 to 10 mm from all the way through will allow the foam to bend to any shape.

Now use the 6 mm drill to drill holes right through yjr foam on a 25 mm grid paten.

Mix a blend of epoxy resin and hardener, then add equal quantities (by volume) of closed cell balloons and micro-fibres.

Now paint a coat of epoxy resin over the area to be repaired including the exposed underside of the deck and the bals around the edges.

Now plaster the mix of balloons, migro-fibres and rein all around the edge of the area and on the top side of the foam.

Slide each half of the foam into place and push it up so that any trapped air will be squeezed down through the drill holes; now place the bit of glass you cut out earlier under the repair and put a seet of plastic under it to hold in the required position with blocks of foam and tape or props.

Let this cure overnight, then remove the plastic and grind away any unwanted material and glass over the gap between the old patch and the rest of the under deck laminate.

Sand and clean after curing and coat with a layer of flow coat. Job done.

Good luck and fair wings.

 

[NickRobinson]

Quite complicated -

Isn't quite like osmosis on a old heavy low-ish hull then- (benign neglect..)

I'll be keen to see evidence of remedial work. Also will walk all topsides feeling for give.
Nick

 

[orangemikey]

I would much rather have a solid GRP boat with osmosis than a boat with a soggy balsa core!

 

[maby]

I would much rather have a solid GRP boat with osmosis than a boat with a soggy balsa core!

Here, here! osmisis is a cosmetic nuisence, soggy balsa core can be a real pain!

 

[nimbusgb]

Here, here! osmisis is a cosmetic nuisence, soggy balsa core can be a real pain!

Cored construction of ANY type relies on the bond between skins and core. Without it you are into failure territory. Whilst a soggy balsa core may not have become debonded from the skins the chances that it will stay bonded for long are minimal!

A cored construction in pristine condition will support many times the load that a de-laminated one will, probably 50 times as much.

 

[NickRobinson]

Cored construction of ANY type relies on the bond between skins and core. Without it you are into failure territory. Whilst a soggy balsa core may not have become debonded from the skins the chances that it will stay bonded for long are minimal!

A cored construction in pristine condition will support many times the load that a de-laminated one will, probably 50 times as much.

So is this method still used? When I first heard about it years ago, it sounded as daft as those house doors with bits of toilet roll holders inside! (But they work...)

 

[maby]

So is this method still used? When I first heard about it years ago, it sounded as daft as those house doors with bits of toilet roll holders inside! (But they work...)

It's very hard to find any new boat that isn't cored at least in the deck areas - believe me - I tried! They'll tell you it's for performance - I think it's to save money!

 

[Tranona]

It's very hard to find any new boat that isn't cored at least in the deck areas - believe me - I tried! They'll tell you it's for performance - I think it's to save money!

It is used because it has superior properties to solid GRP. The boat that is the subjectt of this thread is over 30 years old and built at a time when the process was not well understood. Things have moved on and the manufacturing processes now produce a much more reliable composite than in the past. Builders no longer bolt fittings through cored areas which removes one issue and the adhesives used to ensure the skins and core stay together are much better.

 

[nimbusgb]

Cored construction is used extensively. Balsa has taken a back seat to foam cores and even aluminium or other thinks like nomex.

Weight for weight a sandwich is much stiffer than an equivalent solid piece of frp. I had some pieces of cored layup about the size of a mug coaster ( taken out of a damaged aircraft wing skin) . Approximately 6mm thick with no more than 2 layers of 200 g/m2 epoxy layup on either side of a pvc foam sandwich material. ( skins about the thickness of business card either side ). They weighed nothing. I battled to bend one of them in my hands!

 

[Ludd]

Isn't quite like osmosis on a old heavy low-ish hull then- (benign neglect..)

I'll be keen to see evidence of remedial work. Also will walk all topsides feeling for give.
Nick

Don't walk----RUN LIKE HELL and don't look back!

 

[penfold]

Cored construction is used extensively. Balsa has taken a back seat to foam cores and even aluminium or other thinks like nomex.

Weight for weight a sandwich is much stiffer than an equivalent solid piece of frp. I had some pieces of cored layup about the size of a mug coaster ( taken out of a damaged aircraft wing skin) . Approximately 6mm thick with no more than 2 layers of 200 g/m2 epoxy layup on either side of a pvc foam sandwich material. ( skins about the thickness of business card either side ). They weighed nothing. I battled to bend one of them in my hands!

I forget the exact figure, but sandwiches offer something like 80% of the stiffness of solid layup of equal overall thickness. The biggest problem is avoiding point load damage; with wing skins spanners and other tools dropped by AvTechs is as big a problem as the aircraft encountering hail while aloft. On boats the risk of point load damage can be mitigated by increasing the skin thickness, but it's not an exact science.

 

[Dockhead]

It depends on the method used. Eventually builders figured out how to encapsulate blocks of end-grain balsa in resin. And besides that, to create solid areas where point loads are carried. This method is reliable, and any damage will be localized and repairable. The final structure is much better than a solid structure, as it can be thick enough to be really stiff without being prohibitively heavy.

Balsa core construction is is more expensive than solid GRP construction, but I don't think decks are made any other way, since at least the last 15 or 20 years. And hulls in more and more boats are made with encapsulated balsa block cores -- at least on high-end boats like HR, Najad, etc. (except Oyster). Cheaper production boats (Bavaria, Beneteau) have solid hulls.

Non-encapsulated balsa construction is a nightmare if it has been soaked, as others have said.

 

[aluijten]

And hulls in more and more boats are made with encapsulated balsa block cores -- at least on high-end boats like HR, Najad, etc. (except Oyster). Cheaper production boats (Bavaria, Beneteau) have solid hulls.

As said, most "better" builders use cored hulls above the waterline and in the decks. Not only for stiffness but also for thermal insulation.

But the real clever builders don't use balsa anymore but special closed-cell foam. Hallberg Rassy has done so for the past 20 years or so. The stuff is called "Divinycell" (http://www.diabgroup.com/europe/products/e_prods_2.html). It has strong water-rejecting properties.

 

[Deleted member 36384]

De-Bonding or De-Laminating Are Two Different Issues

.... A previous survey refers to two areas, coach roof and cockpit sole, 4' long of suspect delamination/deterioration but describes the structure as sound.

Recommendations then were to investigate by drilling cores. The vendor is looking through his receipts from then to see if rectification was indeed carried out if necessary. .....

Nick,

Fixing a de-bonding type of problem is not as hard as one thinks and easily within the capability of DIY if you can work thoroughly. It will be time consuming though.

As a rank amateur I always refer to the manual below for my GRP work and it has served me well over the years. In my opinion an amateur can do a good job, and in fact better than some yards at the fraction of the cost: -

http://www.westsystem.com/ss/use-guides/

Download 002-550 Fibreglass Boat Repair & Maintenance which has a section on core failures and how to repair them.

There are two distinct areas that you need to be concerned about: De-lamination of the GRP and de-bonding of the core. De-lamination of the GRP is serious as the actual GRP layers are separating from one another and in my opinion this is could be a serious issue which would be a reason not to buy, this is less likely. De-bonding of the core, more likely, can be dealt with and the areas repaired. The price should reflect this repair. Do your homework and get prices of similar boats on the market to establish a range.

I would estimate that 2 areas of 4' x 4' area of de-bonded core, that required the core to be replaced, would cost between £2-3k to repair in a yard (its a ball park figure for sure at £50 per hour x 8 hours x 5 days). Note that the £50 per hour labour charge is at the high end in my opinion but good enough for an all in spread rate calculation.

At the end of the day if you are serious about the boat discuss with the seller what the expectations are if you hire a surveyor and the faults are still evident. A surveyor should give you more accurate prices for de-bonding repair work. My experience of people is that they are reasonable and prepared to discuss a favourable outcome for both parties if there is a serious intent to do a deal.

Then again there are 3 Mirage 28's for sale here: http://www.ybw.com/forums/newreply.php?do=newreply&p=3171755

If the price is right and one of these does not have issues with de-bonding, then it could be worth a trip around the country for a look. Of course you need to take into account costs to get the boat to your cruising grounds.

Hope this helps and good luck,

BlowingOldBoots

 

[Ludd]

As said, most "better" builders use cored hulls above the waterline and in the decks. Not only for stiffness but also for thermal insulation.

But the real clever builders don't use balsa anymore but special closed-cell foam. Hallberg Rassy has done so for the past 20 years or so. The stuff is called "Divinycell" (http://www.diabgroup.com/europe/products/e_prods_2.html). It has strong water-rejecting properties.

My NAB/Rasmus HR has divinycell decks,and she was built in 1973!