Building a composite hull for a rowing boat

[Nick2]

I have been looking into different ways of making the hull of rowing boat lighter. It seems that the best way is to use Nomex as a core and then sticking pre preg to it, vacuuming it and putting it into an oven. Does anyone have any ideas about this or any pointers? Also are there any better materials for reducing the weight while not comprising the stiffness and strength of the hull?

[Herman]

If price is not an object, that would be the way to go for lightweight construction. You would need a mould to do this, and you would need to figure out a laminating scheme.

But what kind of rowing boat are we discussing? Do you have a picture, by any chance?

[Nick2]

The plan is to do this for a racing single scull. I already have a mould for one. I was thinking of using kevlar, carbon or a hybrid of both as the two layers sandwiching the nomex. The two layers would both be pre preg. Will the pre preg resin affect the nomex much? Also what is the best temperature for the mould to be cured at?

[Boatbuilder BC]

pre preg is will not affect the nomex but you will need to prime the core with a pre preg glue film. You will also need to debulk and spike your laminate to get all the air out. using a light laminate you will also suffer from print through. There are some low temp pre pregs, ie 60 degrees but they are more expensive. the norm is around 84 degrees. not sure what your mold is made of but you would be wise to post cure it to at least the temp you will cure at before you start to laminate.

[Herman]

Indeed, the mould should be capable of reaching the cure temperature of the prepreg. (60-180 degrees, depending on the prepreg. Try and settle for a 80 degrees version, less problems with open time, etc)

Using prepreg and nomex requires some skill, try a couple of test pieces first.

Things to be aware of (not a full list, other contributors can surely add to this)

-use of glue film
-debulking (if needed with a thin laminate)
-perforated film, which perforation
-get the hang of closing vacuum bags really good (and test before cure...)
-heat resistant vacuum materials (the usual stuff is OK at 80 degrees, but vacuum hoses are more susceptible to heat (collapsing)
-being able to follow the temperature ramp rates, THROUGHOUT the oven / part. This is harder than most people seem to realise.

As for laminate:
If the aim is a full-competition racing scull, throw away the kevlar. No use for it. Factors to design for are torsion (the deck helps to prevent that) and bending stiffness. Stiffness of a single hull panel is not critical. It might feel hairy to be able to dent the boat by hand, and a core can prevent that, but invest your weight in carbon UD as much as possible. Use some nomex at the seat area, to stiffen up things, but basicly that is it.

[Landlubber]

Hi,

I would suggest forget the prepreg idea and just go resin infusion. It is a simple technique really to do, we used it in the mid 80’s to make small dinghys, and it is currently being used to make full sized yachts now.

Simple as doing vacuum bagging really, the only difference is that the resin is added last to the project via a tube from the resin pot. If you cut 3mm square surface slots into the surface of the foam, the resin flows very fast and does not add much weight to the whole deal. There are many different ways to get the air out and the resin in, but basically it is done with standard laminates, foan core and a plastic sheet to seal the whole lot.

Private email me if you wish to get into it in detail.

[Herman]

I must admit resin infusion indeed is simple, but imagine a racing rowing scull:

Total weight of a scull should not be over 15 kgs or so. My suggestion is that even nomex core is too heavy. (and creates print through, at these thin laminates).

3x3mm slots in foam can be done, but the foam for a scull will probably be only 3mm, or even less...

I also admit that prepreg is a more complicated way of building, or at least, there is a learning curve to it, at a cost. You are not only supposed to do a good layup, which is hard at times, but also know about vacuum, and have control over your oven, to do exactly what you want it to do. (experience gained on test pieces).

Resin infusion in that respect is more simple: No pressure from resin going of, dry layup, applying vacuum, checking and rechecking the bag, and still no resin is mixed. Only after there is total confidence that everything is alright, the resin is mixed and infused.

If I should have to build a scull like that, I would probably opt for wet-bagging. (thin laminates, little core material, small object)

(this all might sound strange from someone having links to resin infusion sites in his signature...)

[Nick2]

I have experience in vacuum bagging and will have an oven soon. I am in the process of going down the pre preg route. Would anyone have in mind a substitute for the nomex core, which is similar in price and lighter? Also is the idea of possibly using just a single skin worth pursuing?

[Herman]

Basicly, nomex core is the lightest option available (there are several densities available, 48 kg/m3 is a populair one)

But indeed, as I said before, panel stiffness is of little importance. (using a sandwich gives you panel stiffness, but no lengthwise stiffness). How about skipping the nomex (saves you the weight of nomex and 2x bonding layer, which in fact is quite heavy) and use a monolithic (single skin) laminate. That way you can invest your weight savings in adding more carbon UD lengthwise, making for a lighter, but stiffer boat.

Perhaps in the seat area you will need some foam or nomex material to stiffen things locally, but that's it. Also try and get hold of some heavy glass prepreg, to put in areas that need thickness (where the outriggers attach).

[Nick2]

Would anyone have advice for making an oven. It would not need to be very big. I would be looking to be able to place a meter squared test pieces in it, for the time being.

[Herman]

There is that metal sheet-sandwich around, that they make large refrigerator units, more or less temporary buildings, and even complete buildings from. These panels usually are 1.2 meter wide, and available in different lengths. That material is great for ovens.

Forget about EPS foam, as that cannot stand the heat (90 degrees or more). In small scale you could try and use a paint stripper, in larger scale try your luck with a sauna heater.

Control of the temperature is critical for good results, so make sure you can influence the temperature. Having some fans in the (larger) oven sure helps to distribute heat.

[juiceclark]

I have enjoyed rowing my Maas 24 for 7 years now.
http://www.maasboats.com/maas24.htm

You don't use any core for shells...only glass or kevlar with a single spine running up the middle. It always seemed to me the biggest weight issue on lightweight shells was in the gelcoat. They need that to pop it from the mold but here's my thought:

what if you cold molded a shell from a series of jigs like they make big sportfishing boats? Lay 2 layers, sand it down thin, put a spine down the middle and give a thin coat of Imron when finished. That seems a fairly easy way to build a great shell. Hell, make it airtight and fill with helium...then you can fly it too...