Can anyone please help with advice? A large tree branch fell on my Merlin and has put a 3 foot long crack in the beautiful mahogany gunwale! The crack doesn't go right through; in fact it takes considerable effort from me to open the crack even to thumbnail thickness. This leads me to believe that the underlying cedar structure of the gunwale is still solid but that the overlying mahogany (well, utile actually!) has cracked as the structure has flexed under impact. My thoughts at the moment are that, rather than contemplate the replacement of the entire gunwale, I should glue it and cramp it back together. Pulling it all back together by hand leads me to suppose that this could produce a virtually invisible repair. The problem is what glue to use? My current inclination is to buy some syringes and inject epoxy into the very thin crack. I am, however, concerned that epoxy is fairly viscous and may not go into the crack. I thought of using slow hardener and heating the epoxy (over boiling water?) to make it more fluid. I am reluctant to use watered down PVA as it may not be up to the job long term. Any help would be much appreciated.

Epoxy varnish/resin should do it. You can thin it even more if you want, it's expensive but you will not need a lot, someone might be able to give you enough or a friendly boat builder also. Sadly you can't post the stuff or I would but I am miles away.
Glass resin may be a cheaper alternative it sticks alright and flows fairly freely.
As you say, as long as there is no water staining in the crack it should be invisible to all intents and purposes. Do make sure it is dry maybe with a hair dryer the warmth might help the resin of whichever type run more feely. Do check the crack doesn't go right through and if it does put some masking tape along to prevent the resin dropping out and forming a stalagtite of resin!
Good luck.

One other thing suitably fine syringes are hard for us ordinary folk to buy even boatbuilders, try the Vet or your GP they have loads of used ones, they are easy enough to clean, there may even do thatv for you, possibly insist, and they'll know you are not going to use them for the wrong purpose. Ask for the one they used for your flu jab when you have it?

The best place to buy syringes is from a farm supplies store.  They are cheap (ie about 30p) and you can get all sizes.  The needles are also a tad larger and better for pumping expoxy through than those used on humans.  I use a Countrywide Farmers store (http://www.countrywidefarmers.co.uk) which is just down the road from me.

Unless it was your own tree, claim on the insurance and get the full professional job by someone like Pete Vincent or one of the builders.

Warmth is the key.  Make sure that everything is warm - the boat, the air and the epoxy. Select your epoxy carefully; the multi-purpose glueing/ coating ones like SP320 are really viscous.  Remember that epoxy plus hardener creates an exothermic reaction, so don't warm it too much, make the smallest quantity that you can get away with and don't put it in anything that insulates it too much or you'll end up with a steaming mess that goes off before you have time to do anything with it.  I would guess that ~30C is about the top working temperature.

A different way would be to use Zap which is available from Axminster tools in a small bottle. It is a type of superglue designed to run deeply into cracks. it is very runny and is drawn in by capilary attraction. Unlike epoxy a little higher moisture content in the wood is fine. (for epoxy it must be very dry). It is strong and waterproof but perhaps not as strong as epoxy. The other advantage is that you dont need cramps to close the split up - just press hard for a few seconds. It will leave an invisible repair whereas epoxy you have the problem of getting into the crack first and then squeesing it (nearly) all out again or you will see a glue line. If the crack is not very structurally important I would consider this.

I wouldn't use a used syringe for epoxy.  I doubt any GP will hand them out rather than putting them in sharps bins and you would have to clean and dry it first, flu jab syringes are typically less than 1ml not terribly useful for epoxy.  Pharmacists have supplies of syringes for squirting things into babies' mouths that would be suitable.

If you warm the crack up just before squirting in the epoxy, the epoxy gets sucked into the wood as it cools.  Farm stores for syringes not pharmacists

Or order them online and have them delivered to your house.........