Twin engines for cruising cat

Richard - put two 200 watt panels on your targa bar and you can have the Dc juice coming out like a river. We run the DC fridge and everything else and our battery never goes below a float of 12.8 volts. I sometimes just sit there watching the charging regulator. I was not kidding about running a 600 watt breadmaker EVERYDAY if needed (Queensland in Oct - Dec)- we don't eat that much bread. Alternators are for monos with no dinghy davits - get the cheap panels on and put two fridges in if needed. Our fridge is a Waeco BD 50 out in a custom box with heaps of insulation. Run time about 40%. Batteries only 200 AH.

To be serious the availablity of juice was huge change for our last cruise in 2014. I never worried about power which was a big change. My son is making me an autopilot (based on drone technology - I hope it works) and I am going to power it with a big and powerful geared 10cm diameter motor - we make more juice than we can use so I am going to get the steering to really drive down that line.

cheers

Phil

 

here the real weights:
http://www.yanmarmarine.com/theme/y...-datasheet/English/Yanmar-BrochureSD25-HR.pdf

29hp=157kg
21hp=144kg
cruisingcats>40': don't even think about outboards!
<40': 4stroke & ideally in wells
(btw: the outboard is not exactly weightless either: Yamaha 15HP electricstart=55,7kg
25hp=67kg)

 

Pivoting outboards on a successful 44 footer - a good sailing boat though. A sistership can do about 9-10 knots under power with a similar arrangement.

http://yachthub.com/list/yachts-for-sale/used/sail-catamarans/oram-44c/184184

 

Direct therefore angled diesel drives lose 10% efficiency of power delivery, and you will need folding or feathering props or endure that drag which could be valued at around 1 knot per prop.

Diesel sail drives appear to be the yard built favourite option as they are compact self aligned solutions, but drives are vulnerable without protective skegs which reduces depth. Stern drives are still horribly expensive and most examples suffer corrosion difficulties, high repair costs, and they dont seem designed for low output engines.

At the end of the day a lot can be said of outboards and some come with higher capacity alternators, long shafts (extensions can be provided too). The ideal a 3:1 ratio isnt available but ratios approaching 2.5:1 are around as are 4 blade props. Outboards not in use can be lifted to avoid drag or corrosion alleviating the expense of folding feathering props.

On outboard location, they are best situated in line with the keel of the hull, the more inboard to centreline located engines will encounter cavitation at some point, the higher the power demand (yet still fairly modest power levels) the more likely cavitation will be encountered. Cavitation will destroy shaft and gearbox bearings, damage props through pitting and reduce drive and therefore fuel efficiency markedly. At issue is the aerated flow of water caused by wave interaction between the hulls. Centreline bridgedeck located single engine outboards can further suffer poor depth control due to sea conditions with the possibly of over revving, therefore damaging the engine.

While premix should be out of the question, injected 2 strokes (Evinrude is one I know of) can be supplied with large capacity oil tanks with alarms for low oil that can endure 300 hrs use.

As diesel outboards are 'in the real world' unavailable and unserviceable, I think the outstanding issue to do with outboards is the volatility of the fuel particularly in large hull tanks. I havent heard of anyone using fuel bladders or products like explosafe. Id be interested in whoever can comment on that and other fire prevention issues.

A database resource of suitable outboards is really needed including reference to shaft length, alternator outputs and drive ratios. Verified fuel consumption figures would also be very useful.

 

A large proportion of powerboats are still gasoline powered in the US and while I have personally witnessed one explode at the gas dock it is very rare. A cat offers much better opportunities to store the fuel in a safe manner in tanks on the bridgedeck in lockers that vent overboard, a much safer arrangement than inboard engines and tanks in the hulls. most boats that have diesels still carry gasoline for their outboard on the tender, where do they store it? it appears that you have a better selection of suitable outboard in OZ than we do in the US, we cant get the Yamaha 25 high thrust with a 25" shaft here while I believe you can there.

 

Don't worry about petrol

I'm with Steve re petrol. I store mine in vented lockers in the sterns or if I have too much (and trim is an issue) I put the rest in the anchor locker. Everything drains overboard and there are no electrics anywhere near.

A mono with a small outboard is a far worse fire risk than any cat with bridgedeck or other vented lockers. I would lean against large installed petrol tanks. A good sailing cat can slip along with a 20 liter tank and get it topped up from a jerry can when required. Been doing that for a while now.

 

a 'sailing cat' yes
not sure about topping up the tank at sea from gerry cans

The possibility exists for a displacement cat designed to run on one of the outboards at 8 knots or so. Such a vessel can transit long sea journeys exceeding 1000 nm but it does of course require large tankage in the area of 50-70 Imp gallons per engine.

That said I like the idea of vented lockers as its a genuine solution for more low end requirements, and its also a good point that monos would be in a more critical situation.

 

Another option: On the 18m C60 harryproa, we have a 7.5m/25' tender which sits between the beams and hinges down to act as a giant sled. The outboard is a 27 hp Yanmar diesel (94 kgs), which should push the mothership at 8-10 knots and the tender at 20. The cost/weight savings of a single engine, no sled plus the advantages of a large, fast, powerful tender are significant.

With the large fore and aft rudders, single engine manoeuvrability should be acceptable. If not, an electric outboard will be mounted near the helm on a rotating mount so it can drive the boat in any direction. When not in use, it will lift up flush with the bottom of the bridge deck.

http://harryproa.com/?page_id=1177
http://harryproa.com/?page_id=1187
http://www.balticmachinery.com/products/yanmar-d27

 

Topping up tanks is fine in most weathers if you use a jiggler syphon. I don't have to tip tanks. I don't really like the idea of built in tanks and although I have put three different steering systems on my boat, three different cabin/dodger tops, re-arranged the cockpit, added aft deck I haven't ever wanted to build in the tanks. I need jerry cans anyway (as I don't go into marinas so take them to the fuel wharf in the dinghy) and I have to fill the 20 litre fuel tank every fortnight or so depending on motoring.

Think like a sailor - the motor is just there for when the wind doesn't blow. You don't need much fuel. You can keep lighter and sail better and carry even less. 80 litres is more than enough.

 

Those jiggler siphons are an amazing things, there is no way I'm tipping a full 5 gallon jug at sea, again its a lot easier to hold a jerry can in a secure position while siphoning into a tank on a cat than a mono. I have a pair of 12 us gal built in poly tanks with spin on filters in bridgedeck lockers. I like having just two fuel sources rather (gasoline and propane) rather than three as I need gasoline for the dinghy anyway. We will be driving to florida in a couple of days to measure up for outboard brackets for my son and his girlfriends Simpson cat and will be looking for bridgedeck locations for fuel tanks among many other things.

 

Funny to hear the comments against petrol. Petrol and outboards on larger boats are becoming the norm, we aren't moving toward diesel we are moving away from it. My little 22' flats boat has a deck mount 24 gallon fuel tank. Most smaller boats place that under the seat. It would be simple to have 10 of those mounted in the cockpit under a large bench seat. I have yet to smell any fumes from my tank. Im surprised nobody has mentioned electric power, that's the only way I would go. If I'm going to have a sail boat then I am going to sail, not motor. I would only motor when I need to maneuver in tight spaces.

 

I fitted one this spring and really love it. Have had other brands made in Europe and this cheap "eco temp" works far better. It runs on its own 1lb refillable cylinder so the leakage potential is somewhat limited, but the cylinder is in the head compartment just behind the bulkhead from the shower. Located in the opposite hull from the galley and other ignition sources.

 

this is a compressor type unit and so needs to vent heat from the compressor coils? How do you balance protection against ventilation?