Firstly consider what it is you are trying to achieve. Are you looking to make emergency repairs at sea or are you located somewhere where spares are not easy to come by? If its the latter then carry a full service kit including filters, belts, anodes, etc and re-order after use.
If you are simply looking to cover yourself for breakdowns at sea, it depends on what sort of mechanical set up you have. For both diesel and petrol engines I would carry a set of belts, spare raw water impeller + necessary gaskets O rings etc., spare water separator filters (2). I would be less worried about oil filters as really, changing the oil is a dockside activity and even if you had a catastrophic loss of oil, if you could fix the leak at sea, it would do no harm to simply top up with the existing filter in situe and change when back in port. Obviously you should carry oil, both engine and gearbox. A spare air filter is also good in case for some reason the existing one gets choked. Hoses in sizes to facilitate emergency repairs plus stainless jubilee clips are good. Another essential is a box of assorted nuts, bolts, screws, washers, clips, electrical fuses, electrical wire, connectors plus various sizes of plastic ties, rubber O rings, silicon sealer, tube of instant gasket.
For diesel engines a spare injector and some injector piping is prudent.
For petrol engines, carry a set of plugs and points (if your engine has them). Outdrive boats can usefully carry a spare prop.
Another thing to remember is that trying to do things at sea when, potentially, the boat is rolling about and the engines are hot is not easy. Make sure you have the correct tools e.g. something to pull an old impeller, to make the job as easy as possible.
Finally, if talking emergency repairs, it is prudent to carry assorted wooden wedges and cones to use to wedge packing (sheets, towels, pretty well anything which reduces the water flow) into holes should you be unlucky enough to hole your boat.
The above should be good enough to ensure you draw about 3" more once you've got it all aboard!
Definately emergency only stuff. Have read horror stories of RNLI rescues due to idiot skipper on board not being able to do basics. Have read a few diesel books and am relying on car engine skills from teenage years to get me through a crisis.
Engines are twin Diesel 7.3L D-tronics with Bravo 3 drives.
Spose it depends where you are estuary, river, coastal etc. How about
Decent transportable light 12v for prodding about
Reasonable qty of cable ties, and union clips.
But after a certain point in amassing spares as per Nick R etc above/below , but slightly before spare crank, then safety gear becomes more important cos you ain't goona be able to fix everything. In addition to usual flares, liferaft grabbag, lifejkackets, praps consider:
Inflatable and outboard which could be lashed to side as get you in/home
Really long, strong lines for a tow.
Enough chain to reach the bottom and stop the boat moving while any problem diagnosed/fixed.
Decent goggles for groping about under boat, spare goggles for finding the other goggles, and dangerous breadknives for slashing rope off props (even if you have ropecutters).
Trip line - small buoy and long line, plus and boltcutters for dumping and marking fouled anchor.
CO2 (i think) the gas one with a horn thing - not just for fires but to cool down that hot thing you need to fix but obviously careful not to breathe the stuff much. Probably a better thing for this.
Decent gloves and overalls for protection for the twiddling in hot engineroom.
mobile phone 12v charger, and portable vhf incl charger and battery handheld gps inc spare batteries permanently on the boat for the long night out waiting for tide or spares or seastart.
Wetsuit if you're in cold waters. Or a drysuit! Don't laugh, I've got one although never used it.
5 or 10 gall diesel jerrycan for when you run out since boat will use lots more with all this lot. Have we sunk it yet ?
Yes, you have to draw the line somewhere. Years ago we used to go fellwaking in the lake district. Lots of bods turned up with big rusacs even thos they weren't camping out, just out for a day. We used to take fruit pastels and no bags at all, just run up and down: the other guys lugging all water and sarnies about ...neded to stop and rest cos of the weight they were carrying.
Seastart is quite good. Weight and cost saving of all above is of course to get wife/crew to think v positive thoughts and/or quiet prayer for boat prior to departure. Then it's their fault if it breaks down for not praying enough!
Twin engines is a big safety factor in itself. Conventional wisdom says they should feed from separate tanks in case of fuel contamination although this is a bit of a mute point as, you don't fill up one tank at a time and you'll put the crap in both.
My advice (from the school of hard knocks) is if one engine goes, limp back on the other engine and fix it once tied up. Repairs at sea are difficult, injury prone (especially burns/scalds) and if you are going to be seasick, you will be so in the engine room with hot engines, oily smells and no view of the Horizon.
The point about a spare prop for outdrives is valid - if you have driven over something which wrecks your props, the chances are it's both props and one new one will get you home. However, do not try changing at sea in anything other than more or less flat calm, using the dinghy and not the bathing platform.
If you have a problem never be afraid or embarrassed to call for help. Depending on the issue, it does not need to be the emergency services - most boaters (even yachtsmen) will help out with a tow home if necessary. If you lose an engine, tell the coastguard that you are limping and they will give especially vigilant cover.
You have certainly had some advice on this one! All of it is good advice but it's a bit like 'before driving your car check oil, water, tyres, tyre pressures, etc' - more important with boats of course as we can't sit on the verge waiting for rescue and to call the coastguard because we have been a prat is embarrassing but necessary if you are in danger!
Preventative maintenence is the order of the day with boats! Regularly set aside a time [say? every 10 hours on water] to check things over ... I sometimes do it at anchor when the missus is asleep? Check water hoses, oil level and smell the oil on the dipstick because that can give you a good indication of its condition [not rocket science, fresh oil smells different to burnt or contaminated oil - compare them next time to change the engine oil!] and change the engine oil and filter according to makers recs and at the end of the season prior to layup [do this as the oil is fresh and has fresh corrosion inhibitors to protect the engine over the winter]. Carry the spares that you can't see like water pump impellers although unless you run the pump dry a lot these rarely fail but still carry a spare [if the last one you replaced - makers recs! is still OK then this is a good 'get you home' spare but don't forget a new gasket! Fuel can be a problem but more with diesels and carrying spare, clean fuel is good but also a way of feeding it to the negines other than pouring it in the tank would be a good idea?
I/we could go on for ages - common sense prevails and as someone else said 'with twin engines it's unlikely [other than fuel probelms] for you to lose both!?