Outboard pros and cons...

[porth]

My Volvo Penta MD1B has now been pronounced dead. New engine required! No money! Not a lot at least. So I am thinking of putting an outboard on the back of my ancient 29 foot 3 ton cruiser. I sail 99% of the time inside the skerries of western Norway, so waves not a prblem. Anyonewith any experience of outboards on this size of boat? I see Ken Endean has one on his Sabre 27, and he is a lot more ambitious than we are. How small engine can we get away with? PS we have almost no electrics and sail mostly weekends: Thinking of buying an extra battery and charging in the berth.
Thanks for help folks.
PS anyone in Norway want a mooring weight?!

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[snowleopard]

pro: lightweight, cheap, can be turned for manoevering

con: less reliable, easily stolen, less thrust, cavitation, fuel consumption

for your boat i would say a 9.9 would be ideal. some manufacturers do a yacht auxiliary special of that size with dc charging output included. you can retro-fit electrics to any engine at a price.

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[claymore]

Agree with that - we had one on a 25ft boat and I used to become rather concerned about the gallons of fuel I was carrying in tanks in the cockpit. I felt I had to on longer passages but perhaps this is not such a consideration for you?

Charging batteries is fairly straightforward - Mercury saildrive outboards were a step in the right direction.
Fuel consumption averages at around 1 gall per hour per 1HP. Windless summers are the drawback when it all gets a bit costly! 1995 & 96 cost an arm and a leg!!

<hr width=100% size=1>regards
Claymore
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[extravert]

That sounds a bit expensive - 10 gallons an hour for 10hp?

I'm on my second outboard powered boat now, and although neither is particularly similar to porth's boat, I don't use anything like that amount of fuel. Here are my consumption figures...

Extravert 22 mono, 7m LOA, 1200kg with a 7.5hp Honda 4-stroke
1 gallon in 4 hours at 5 knots (20 miles per gallon).

Dragonfly 920 tri, 9.2m LOA, 1800kg with a 10hp Yamaha 4-stroke
1 gallon in 2.2 hours at 6 knots (13 miles per gallon)

The above figures are for flat calm conditions, but that's the only time I motor any significant distance.

I would expect porth to get 5-10 miles per gallon in flat calm conditions with a 4-stroke engine of 10-15hp.


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[snowleopard]

fuel price difference

in the uk we have a strong preference for diesel on cost grounds as we can buy duty-free diesel but not petrol, meaning that an outboard costs 5-10 times as much to run as an inboard diesel. that applies in very few other countries.

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[Jacket]

Have a look at how readily available petrol is in the harbours around you. I've no idea what Norways like, but I spent a few weeks in Sweden cruising in company with a couple on an outboard powered trimaran, and they had a real struggle at times to get hold of petrol in the harbour. They often had to resort to carrying it from the nearest petrol station- often a couple of miles away.

I think across much of europe petrol is getting harder to find for boats, due to the implementation of new, harsher regulations making it uneconmic for harbours to store, given most boats use diesel.

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[extravert]

Re: fuel price difference

> outboard costs 5-10 times as much to run as an inboard diesel

For a small sailing boat like porth's that is not true. What you mean is that the fuel costs are 5 to 10 times greater, but fuel costs are only part of the total costs of motoring for a small sailing boat.

Look at the engine replacement costs for a boat like porth's, say 10 to 15hp...

Diesel inboard £5000, 15 year life, £333 per year
Petrol outboard £1200, 10 year life, £120 per year

That's a £213 per year difference. Even with our cheap diesel porth would have to motor about 500-800 miles a year before petrol running costs became more expensive. Where marine diesel is not low tax, porth would have to do 5 times those distances per year for diesel to be cheaper.

The major cost of running a diesel engine in a small sailing boat like porth's, for the use he makes of it, is replacement when it dies, as he has just found out.




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[claymore]

Re: fuel price difference

Correction - with yet more apologies - of course you are correct I meant to write
1 gall per hour per 10 hp - which my calculations verified as it took me around 10 hours to get to the Isle of Man in flat calm windless conditions and we used around 7 gallons with an 8 hp engine.
Sorry for the confusion

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Claymore
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[qsiv]

Thats useful info. The upcoming boat (40', 4000kg mono, very easily driven) is powered by a 15HP OB motor - I had been wondering about fuel consumption. It looks as though 2 GPH would cover it reasonably comfortably, which implies the 12 gallon tank should give me about 6 hrs motoring - which is two hours short of that required in ORC special regs. Seems like a bigger tank will be needed ...

That only leaves the issue of getting a permanently installed OB, mounted in an inboard well classified as an inboard engine ....

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[duncan]

Re: thats true for....

non direct injection 2 strokes at WOT - ie the maximum consumption that the engine could be forced to consume!
On a displacement yacht hull a 10 hp outboard running at 50% revs should be down at 30% of that - or 1 gallon every 3 hours. If that gave you 5 knots you would get 15miles to the gallon. I don't know if you would get the 5 knots at that setting though!

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[qsiv]

Re: thats true for....

So ... would a 15HP motor push a 4000 kg 40footer at a sensible speed (I amagine it must - the builders chsoen to do it this way before).

I need to obtain sqrt(LWL) as a cruising speed (ORC regs again), which translates to a shade under 6 knots. Cavitation and ventillation due to waves will be of minimal concern, and the speed only has to be reached in flat water (if there's any wind the motor is irrelevant as the boat is fully powered up at 8 knots true wind)

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[Boathook]

Re: thats true for....

9m Catalac with a 30 hp Honda cruising between 4 and 5 knots gets between 6 and 7 hours from a 5 gallon tank. Flat sea or being able to motor sail can increase time up to 9 hours from a 5 gallon tank. Engine produces 10 amps to charge batteries so they are normally ok.

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[porth]

Thanks to everyone for the help!
Any makes better or worse than others?

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[snowleopard]

worth knowing the following: both mariner and mercury are re-badged tohatsu. you'll probably get a better price on a tohatsu because the name is less well known.

four-strokes are better on consumption. you won't need to worry about the extra weight as you won't be lugging it around every time you want to sail.

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