I am contemplating finishing a boat in epoxy but have noticed that it darkens the mahogany considerably and wonder if I will be able to see a nice finish with the graining visible or will it just look a VERY dark brown compared to varnish. Kate was epoxied but I believe that she is pine planking rather than mahogany so may not experience the darkening to the same degree.   I have seen epoxied decks on other Merlins which look really quite light in colour so am I 
a) doing something wrong or
b) using the wrong epoxy (it is West system)

Any advice on what to use?

Garry,

It might be worth a phone call to Alan Jackson at Chipstow about this - I am fairly sure I remember him saying that priming with UCP solved this problem.

GGGGGGG

West system hardner has a yellow color to it which will darken the wood.
Varnish does too, the solids in the varnish is where the UV protection comes from I think.
You don't want to finish in just Epoxy, you'll need to varnish too otherwise your finish will degrade pretty quickly in the sun, sorry you guys only have liquid sunshine don't you!
There are some epoxies which are crystal/water clear which will reduce the color change, but I think you'll find you still need varnish for the UV protection on top of that. The varnish will darken the color of the wood some.
According to the West System book the epoxy is structural, the varnish is the bright finish!

should be clear with a little colour change, mainly more intense colour and grain same as varnish. if you wet the bare wood surface and it looks that dark, then that's the way the wood is. if not, then there may be something afoot. in any case if you don't finish with a good uv protective varnish, then it will go very yellow.

did you store the epoxy in a warm place over winter or did it fall below zero in the shed? just a thought

Ditto talk to Alan Jackson.
Basic facts all timbers will darken in some degree when epoxy or varnish is applied, Most builders will apply a UV stabilished primer first UCP or Interstain (my favourite) this takes out iregularities in the veneer then one or two coats of 'flow coat epoxy' talk to your local supplier or check out their websites. I used SP 116(I think?) normally used on surf boards, as stated you'll then need to overcoat this with 2/3 coats of a good UV proof varnish (Epifanes).
Like all things nautical various owners/builders have there favourites and it's well worth chating to a few to find the common thread & economic resoning, like mine being living 4 miles from SP Systems! with a couple of friends working there!! .
 Good luck, Barry  
ps don't forget you must remove all old varnish & get good working conditions for best results.

Correction SP 115, Barry

Interstain is a woodstain not just a primer - now that's guranteed to darken the wood!!

Avoid if you don't want to change the woods colour.

My personal favoite is SPs eposeal which is literally like water and the wood just drinks it.

Had a long and very useful conversation with Alan Jackson.  Bottom line seems to be that any epoxy will make mahogany very dark IF it has been in contact with the water over a long period of time.  New wood is not a problem - witness the decks and superb new-hull finish seen on the Chipstow website .  Sooooo - something obviously happens to mahogany when it is wet which reacts with the epoxy leaving a very dark brown.  For many reasons according to Alan it is probably best not to go with an epoxy finish on old mahogany - even with a UCP coat first as the plus of epoxy is that it should soak into the wood (for strength) and not just sit on top of a barrier layer.  So it looks as if it will be UCP and then a single pack poly varnish of some manufacture now that Blakes No 1 is off the market.  He recommended Venziani from Bear Marine so once Easter is over will see what they have to say.

Whats wrong with a dark finish anyway?

The problem is that it isn't just dark - it turns almost black and you don't see the grain of the wood (which seems a shame) - it really does go that dark and that's what I am trying to avoid.

Now you see what I mean about varying opinions/experiences, I believe that if your woodwork has been subject to long periods of water exposure to the extent that it has turned black then you will have to fully correct that prior to any good long life finish. The black you see if still there after striping & drying is a biproduct from the chemical reaction (mould/water/UV/heat,etc) 
To remove you have three main ways to go, 
1 Chemical (commercial available) 
2 light sanding(remenber most veneers only .5-1mm thick!) solid mahogany carefull planing.
3 Replacment. may seem daunting but often more economic in time & money

Left unchecked the dark staining will turn black leading to full on rot the break down decay of the fibres leading to decomposition of the fibres, the wood itself.
Now the carefull use of Interstain will not darken the good! wood to the extent that it will hide the grain if properly applied but will enhance the grain reversing the UV action. Obviously you cant use dark mahogany stain on old light gaboon or light veneered ply and expect a 'natural' finish, if you have old sound ply of that nature which has responded well to drying & sanding  then UCP is the way to go I personally would have no reservations on then using epoxy if the timber is sound(perhaps that is a reservation!) If it is'nt go straight to option 3
Choice of epoxies SP,West,Epiglass its up to personal choice all have there devotees even the companies have historical links, personel moving from one to another ,etc. The same with paints there are many good finishes available, I would advise to look for a good repeatable source but try & avoid 'mixing' products which I believe is where many of the problems arise. 
Anyway thats enough from the confessional for now have fun,  Barry.

ps, As soon as my server will let me I'll email a couple of shots of 'Inter'd' decks,etc.