FAQ – Getting a hull smooth – Torture Boards for strip planking and decks

“Potterage” on the woodwork forum was trying to fair his boat using a torture board – a big piece of ply with sandpaper attached.

You can never ever buy a torture board that is the right stiffness for your boat. Inevitably you must make one.

Happily they are very simple to make but the stiffness of the base is crucial to get right.

Torture boards are extremely useful for creating visual fairness. That is getting rid of waviness and unevenness. More than getting rid of little bumps, this is aimed at making the whole hull or deck smooth.

Some examples are

  • Fairing a strip planked hull.
  • Fairing a join between adjacent plywood sheets in a hull or deck
  • Fairing a composite structure
  • Fairing deck substructure. Deckframes and deck stringers faired ready to take plywood.

Here is a torture board being used to fair sub deck assembly on our SUP. Taal Stand Up Paddleboard design.

lightweight simple plywood SUP - stand up Paddleboard - storerboatplans.com

A torture board normally has handles scewed on with the drywall/plasterboard screws. And the sandpaper is floor sanding paper glued with contact cement, double sided tape or clamped under ply pieces screwed to the handles.

Anyway, our friend was having problems making the  torture board actually work.

Here I give advice on the thickness of the board.

And mention RSI – Repetitive Strain Injury.

Torture Boards. Not too thick and not too thin

Originally Posted by Potterage

The problem I have is marking out the high spots. I have tried rubbing a batten marked with builders chalk to pick out the high spots before using the Torture Boards but doesnt seem to work.
Cheers, Paul

Howdy Paul, Don’t expect to get too much visual feedback from a powerboard or torture board.

It generally works because the plywood thickness of the board base is the right thickness to match the curvature of the hull.  How thick is the ply that you used to make your torture board.

What you are relying on is the tool itself to make the decisions for you – because it is just not possible for us to SEE the small differences that will become obvious later.

Originally Posted by Potterage

Hi Boatmik
The answer to your questions is the torture boards 800 lg x 115 w x 3mm thick ply. I have attached a photo showing the handles. The boat is a 5.5m David Payne Trailer sailer.
Cheers, Paul

Howdy Paul,

First of all some sanding humour – relates closely to gallows humour!
torture board = tortured/bored.

There’s your problem Potterage – you board is way too thin. It is too flexible so can get into the hollows. It is so flexible that it conforms to the surface under the weight of the handles.

6mm (which is 8 times stiffer) or 9mm (which is 27 times stiffer would both be more around the ball park. Try 6mm.

Basically with your sanding weight (ie you leaning on the handles) on the board most of its surface should be flat on the surface apart from the very ends of the board – maybe the last half inch – it needs to be stiff enough to just hold that last bit off the surface.

What is happening now is that the board is so flexible you can get the ends down into the hollows without much effort – that’s why it is sanding them as well.

Sometimes you need both a stiff TB and a more flexible one – 9mm for sanding longitudinally and 6mm for sanding diagonally across the bilge.

3mm thick torture boards are for sanding round the bilges of canoes – but would probably still go for a 4mm (three times stiffer) for that use.

Laboursaving. Notched trowel to fill the area with lines of filler.

When filling large hollows

When screeding filler with a notched trowel don’t use one that has equal sizes of notched and un-notched areas either – you want the unnotched areas (say 1/4″) to be smaller than the distances between notches (3/4 or 1″) – it cuts the amount of materail that has to be sanded.

When you start getting back to the original flat hull surfaces in some places then it is ready to be filled. Maybe 50 to 70 percent back to original with an even spread of areas still filled. By the time you reach this point you start to have a good idea of the actual state of the hull.

When you fill between the screeding it is a good place to add some colour to the filler – so when you do further stages you know which parts indicate the end of the last fairing process. Just so you know where you are.

Where working with unscreeded surfaces I use pencil scribbled and sand until the pencil is gone from all areas – it can’t be blown away like chalk dust can – though I have used chalk too.

Multi person torture boards

One useful trick too – particularly for smaller boats (but can also be used on bigger boats for A grade work – if you make up a multi person torture board) is to make up the longitudinal sanding board around the same length as two of your temporary framing stations. This can get rid of the undulations they might cause.

But ALMOST NO-ONE goes to this much trouble because the torture board ends up so long that it is ergonomically dangerous to use. I once ended up with a six month case of RSI from sanding with a too long torture board so my arms were too spread out – it works the wrists through some really strange angles with every stroke – about 800mm is fairly safe I reckon.

(The RSI reoccurs now whenever I spend much time with a torture board – which might be why I like plywood boats!!!)