Can an existing GRP hull be made too stiff retroactively?

Hi,

I'm rebuilding my 1980 GRP sailing monohull 43'.

I've removed the interior and would like to add some thermal insulation throughout the entire hull as well as create a simple, durable and easy to clean surface on the hull interior (to replace the original vertical battens and horizontal wooden strip design.)

I've become fairly proficient with vacuum bagging and vacuum infusion, and am considering applying what is essentially a structural core (PET 20 millimeters or 3/4 inch) and and inner fiberglass laminate (2 layers 600 gram/meter stitched biaxial) to the inside of the hull, which itself is simply a single GRP skin with some vertical wooden strips laminated very lightly every 50 centimeters/20 inches or so. I assume the laminate (hat shaped profile) provides some panel stiffness. The strips (formers for the hat shape profile) also double as the aforementioned battens for the interior strips. I would remove these battens.

The deck was a cored design to begin with, so that (i.e. the ceiling) is not part of this discussion.

The idea is to use the thermal insulation properties of the core, and the easy to maintain properties of the laminate to achieve my goals.

I realise that many may consider this idea prohibitive in terms of cost and/or effort and/or expected insulation value. But let's just leave that aside for now.

My question is this: could I be creating a structural issue by effectively, but unintentionally, stiffening the hull panels significantly? On the face of it, one would think: the stiffer the better. But I'm not an engineer, and have found that things aren't always as simple as the layman (me) thinks.

The hull originally only makes use of bulkheads (some partial), longitudinal stringers, some of the furniture and the light panel stiffeners to stiffen the hull. I would say a very typical design for its time.

The design is fairly heavy for its length, with a cut away long keel. Never meant to be light and nimble, with a corresponding laminate schedule I would imagine.

What do you think?

Thanks!

 

I think you will need to re-engineer the structure. Stiffer is not necessarily better. The increased stiffness is likely to create stress risers. That means that a force that was spread out over a large surface, is now applied to a small area due to the stiffness increase.

 

The vertical wooden strips are just there to provide a place for the screws holding the ceiling.
There's no reason to use 2x600g/sqm fiberglass over the core, unless you specifically wanted to stiffen the panels.
Not even infusion will give you a surface that's good enough as interior finish without further filling, sanding and painting. To achieve the best results for insulation and finish use rigid PU foam and a light woven cloth (plain or twill max. 200g/sqm).

 

Hi Rumars,

Thanks for the comments.

The vacuum bagging/infusion is not about the finish. It's about gluing/ laminating to a vertical curved surface.

I agree that a much lighter laminate would reduce the stiffening effect and would therefore also reduce any unintentional side effects. Although perhaps the thickness of the core contributes more to the stiffening than the thickness of the skin?

Thanks!

 

If I may…

You ask us to set aside the concern over the difficulty and only consider hull stiffness. But then suggest 1200gsm glass over a vac bagged core over ?a painted frp substrate? The entire exercise seems unreasonably difficult. Is it to avoid condensation? You ought to make this clearer.

You are creating a very heavy inside laminate, so it is hard to take you seriously and Rumars is much kinder than me here.

I’m not sure I’m qualified to answer the stiffness question, but I see a lot of problems as a builder. Bonding the new core to the existing hull cannot happen at the bulkheads or any other structural areas, as Gonzo suggests. So, you would have thinner hull at all the areas where hull continuity is really expected. For example, you never stop the hull for structural elements like wood strip ending on a bulkhead. In your case, the old hull will always be thinner at the most critical elements. Even something like a thru hull will be in a weaker section of hull than that around it unless redone.

Circling back to that which you set aside. If you simply wanted to insulate the hull better, I believe you could do so with spray foam. This would not affect the hull. You could increase the existing furring to the need and build hull liner from any number of products. The added stiffness of the insulation to the hull would be marginal. And the production speed would be ?tenfold?.

I’d say unless you have a structural reason, you don’t make a structural mod.

 

Hi Fallguy,

Where did you read that I was suggesting to laminate on a painted FRP substrate? I would find it hard to take myself seriously if I had suggested that.

And if you do find it difficult to take me seriously, then please simply refrain from replying to my question.

Your last comment is basically why I'm asking this question to the forum in the first place. Your position is clear.

Thanks!

 

If the existing hull is a solid layup (likely 1" or so thick given the age and size of the vessel), then adding a 3/4" core and an inner skin is just making yourself all sorts of sectional modulus issues. Yes, it would add a little stiffness; but most likely just cause core to inner skin shear issues. If you want to add insulation, add insulation...but removing air circulation (the reason for the space behind the ceiling) is just creating a petri dish no matter how smooth the interior is.

 

"vertical wooden strips " you mean frame ?

Take photos of the interior with some scale element, e.g. a beer can.

I'm pro of your idea . Core kind and inner shell surface preparation are the main things to reconsider . Inner cloth gramature also .

Or move to the Caribbean or something.

 

I like the idea, vacuum infusion makes a lot of sense in this case. Core and inside skin any other way would be a nightmare. My biggest concern is bond of the core to the existing shell.

 

Have you been watching Sailing Uma on youtube? They recently laid in a foam/glass new interior. Not everyone agrees with it, and im not qualified to judge.

 

Yes, I do watch Uma. And I love their 'let's just get on with it' attitude.

I've watched many, many boat rebuild channels, read a lot of books and articles, and hacked and sawn my way through quite a few areas of my current boat.

I've come to the conclusion that sailing yachts are generally overbuilt, in the sense that very few actually disintegrate while underway, regardless of how old or decrepit their hulls are. Problems usually have to do with other issues or damage which was not detected earlier (keels, rudders, rigging, prop shaft).

I've also concluded that naval architects are not very enthousiastic when it comes to making recommendations for a yacht for which no original plans or laminating schedules are available.

Uma laminated a whole new frame to the inside of their hull and stiffened their coach roof (and of course completely altered their bulkhead arrangement with a new interior). I guess time will tell if that has any unforeseen negative effects on the integrity of the hull in practice. Somehow I doubt it.

 

Not sure why bonding to the existing laminate would be an issue. Surface prep is everything of course. Any specific reason for bringing this up? Maybe I'm missing something regarding bonding?

 

Moving to the Caribbean would solve many issues (but create new ones too) ;-)