Practical Boat Owner — January 2018

Reinforcing

a dinghy hull

Glass fabrics are ideal for strengthening

laminates. David Parker describes using

woven cloth to sheathe the inside of a dinghy

M

any of us will probably be

familiar with using polyester

and epoxy systems for general

jobs on board. Typically with polyester

resins this may involve beefing up an

area or bonding in materials using

chopped strand mat and resin. Epoxies

are widely used for coating and when

used with glass tapes they are perfect

for reinforcing epoxy fillet joints and

strengthening edge bonds for example.

When it comes to using woven glass

fabrics on a larger scale however, we may

associate that more with sandwich

construction or when sheathing a

strip-planked hull. But these versatile

fabrics, which come in a variety of weaves

and weights, are ideal for smaller jobs too.

Once such job is reinforcing an area of

GRP laminate which has been subject to

heavy wear, as was the case with my

workhorse tender. This small open craft

remains on the water pretty well all year

round with the interior subject to all

weathers and constant UV exposure. The

tender is pretty old too and the inside floor

of the dinghy was starting to show

significant signs of wear.

The original interior sole appears to have

had a gelcoat finish painted directly over

glass fibre matting. The result had

produced an irregular surface which was

showing increasing pockets of damage

known as ‘stress cracking’. This damage

had the appearance

of star crazing, where

fine cracks radiate out

from a central point.

This was probably

caused by point

loading as opposed to something more

serious such as impact damage which

tends to show up as longitudinal cracks as

well. The nature of the cracks looked like

they were caused by someone’s weight on

a weak spot which had caused stressing of

the laminate underneath. The gelcoat

membrane had not been able to take the

pressure and so had cracked as a result.

This was because normally gelcoat is laid

up in a mould and its main function is just

to provide the cosmetic outer skin to the

glass fibre or cloth which is then bonded to

it in layers to form the hull structure of the

boat. The mat lay up provides the strength

while the gelcoat gives the smooth, shiny

visible exterior surface.

Stress cracking

Once bonded in, gelcoat offers reasonable

abrasion resistance but being essentially a

brittle membrane it can be vulnerable to

damage, particularly from impact. In the

case of my tender where gelcoat had been

used as an internal finishing coat, the

irregular surface it was bonded to had left it

prone to extensive cracking over time,

particularly because the lightweight internal

floor is subject to flexing.

On an external hull when you get gelcoat

star crazing or stress cracking a typical

Step

by

step

Woven cloth reinforcement

can be used to repair

vessels of all sizes

PRACTICAL