Greetings to all
I am repairing a large hole in an eight. So have a 2 part question for the brain trust.
1. looking for advice in using honeycomb without prepreg
2. What are the (probably many) reasons to use honeycomb in boat construction in the first place? It is lightweight obviously but that is insufficient as a reason.

Tony
Sherbrooke rowing

On 24/07/2023 22:03, tony tarlo wrote:
> Greetings to all
> I am repairing a large hole in an eight. So have a 2 part question for the brain trust.
> 1. looking for advice in using honeycomb without prepreg
> 2. What are the (probably many) reasons to use honeycomb in boat construction in the first place? It is lightweight obviously but that is insufficient as a reason.
>
> Tony
> Sherbrooke rowing
>
Hi Tony -

The usual honeycomb material is Nomex, from Kevlar-composite paper
material. Of course it is largely air. There's also Aluminium
honeycomb, & expanded foam sheets.

Honeycomb & expanded foam serve the same function as the web in a
structural I-beam (the vertical part in the capital letter I). Bonded
between 2 layers of resin-laminated carbon, Kevlar or glass-fibre, the
core maintains the physical separation of those layers & prevents them
from moving relative to each other - the laminate layers act as the top
and bottom of the I-beam to carry in tension or compression the applied
forces that would otherwise deform the laminate.

The cores contribute next to nothing to strength & stiffness along the
plane of the surface laminations but, by thickening the laminate, they
increase its panel stiffness (its resistance to bending or buckling).

The resulting laminate is vulnerable to puncturing as there is minimal
"give" to absorb the incoming energy under perpendicular or oblique
impact. You also need to ensure that the skins, despite being quite
thin, are not porous as you don't want water to enter the cells of the
honeycomb. And there must be sufficient excess resin for the laminate
layers to completely bond to the honeycomb edges.

Pre-preg is the favoured form of fabric for industrial lamination (with
heated moulds to fuse and cure the laminate) as it gives good control of
resin content & easy handling. You can of course pre-impregnate the
cloth layers with freshly-mixed (wet) resin, maybe laid onto very thin
polythene film to improve stability and handling. In any case, it is
good to use vacuum to help to press the fabric onto the Nomex edges
while the resin cures.

There are, of course, other ways to create stiff, light & more resilient
shells, but that wasn't your question ;-)

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: tinyurl.com/2tqujf
Email: carl@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

On Monday, July 24, 2023 at 6:29:06 PM UTC-4, carl wrote:
> On 24/07/2023 22:03, tony tarlo wrote:
> > Greetings to all
> > I am repairing a large hole in an eight. So have a 2 part question for the brain trust.
> > 1. looking for advice in using honeycomb without prepreg
> > 2. What are the (probably many) reasons to use honeycomb in boat construction in the first place? It is lightweight obviously but that is insufficient as a reason.
> >
> > Tony
> > Sherbrooke rowing
> >
> Hi Tony -
>
> The usual honeycomb material is Nomex, from Kevlar-composite paper
> material. Of course it is largely air. There's also Aluminium
> honeycomb, & expanded foam sheets.
>
> Honeycomb & expanded foam serve the same function as the web in a
> structural I-beam (the vertical part in the capital letter I). Bonded
> between 2 layers of resin-laminated carbon, Kevlar or glass-fibre, the
> core maintains the physical separation of those layers & prevents them
> from moving relative to each other - the laminate layers act as the top
> and bottom of the I-beam to carry in tension or compression the applied
> forces that would otherwise deform the laminate.
>
> The cores contribute next to nothing to strength & stiffness along the
> plane of the surface laminations but, by thickening the laminate, they
> increase its panel stiffness (its resistance to bending or buckling).
>
> The resulting laminate is vulnerable to puncturing as there is minimal
> "give" to absorb the incoming energy under perpendicular or oblique
> impact. You also need to ensure that the skins, despite being quite
> thin, are not porous as you don't want water to enter the cells of the
> honeycomb. And there must be sufficient excess resin for the laminate
> layers to completely bond to the honeycomb edges.
>
> Pre-preg is the favoured form of fabric for industrial lamination (with
> heated moulds to fuse and cure the laminate) as it gives good control of
> resin content & easy handling. You can of course pre-impregnate the
> cloth layers with freshly-mixed (wet) resin, maybe laid onto very thin
> polythene film to improve stability and handling. In any case, it is
> good to use vacuum to help to press the fabric onto the Nomex edges
> while the resin cures.
>
> There are, of course, other ways to create stiff, light & more resilient
> shells, but that wasn't your question ;-)
>
> Cheers -
> Carl
>
> --
> Carl Douglas Racing Shells -
> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
> Find: tinyurl.com/2tqujf
> Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
> URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

It's like tossing a bottle into the sea and hoping for the best.
Thank you for this Carl.
Tony