i have a 15/18 hp tohatsu that uses 7 L/hour at WOT. A good rough guide is half of the stated hp in L/hour at WOT for two stroke outboards.

your probably looking at 8-10 L/h and much less at 2/3 throttle.

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*I use my OB on a dinghy not a yacht but fuel burn / hour should be the same.

figures from a 2007 SW1000 - 2nd last 1000 before the XL.

the standard 9.9 HT Yamahas (350cc 2007 motors are very different to the 212cc newer ones) use 2.7L/h WOT. I found the best range was running one - WOT with no wind = 5.3kn at 2.7L - back it off to just under 5kn and the range increases significantly and I worked on 2nm/L to give a nice saftey margin.

The HT yamahas spin a big prop with a 2.92 reduction, the 20 tohatsu (Mercury) will be spinning a small diameter prop (with about a 2.0 reduction) that could be smillar pitch -The standard yamahas that SW fitted to the 1000 were actually quite over propped (no other prop option so too bad) so they were allways labouring. Mine would only do about 4400/4500rpm. The fact that they are overpropped will mean more fuel than they should be using in their sweet spot - which is why I am quoting WOT.

educated guess tells me that WOT for a 20 merc would be very close to 4-4.5L/h. Propeller pitch and blade shape will then determine how well the engine can push the boat (speed/RPM), so without these details, this is really a wide open question.

How fast does it go? WOT on one? WOT on two. As a guide the 9.9 are 5.3/7.2. The reason SW went back to the 9.9 was partly fuel usage and partly manouvability with the reduction and big prop.

The only good thing about the tank filling arangment in the SW1000 is the ability to refill at sea so take some Jerries if in doubt - I used to carry 4 or 5 20L gerries for coastal cruising. The other thing to mention is the tanks in my boat were definately not 60L. They had to be removed at one point and they definately did not take 60L to fill back up - more like 52 which is about 15% less range to start with...

scruzin said..
By way of comparison, my Lightwave 38 has twin 27hp Volvo Penta D1-30 engines. Running on a single engine the fuel consumption is approximately 3 litres/hour at a cruising RPM of 2,400 RPM. My fuel tank is 240 litres, so that's approx 80 hours or running. At 6.5 knots, which is typical at 2,400 RPM, that's a range of over 500 nautical miles.

yeah - the setup on the 1000 is really only good for day sailing on the bay. Lets not talk about the noise from little outboards in the tunnel. Your D1s are a hell of a lot quieter not to mention they actually charge the batteries, they only 1600hrs too.

I'll never have a outboard powered cat again. Been looking at boats in the 38-40' range, really don't get the new SW1160 Lite (I call it the 1160 Cheap) with outboards instead of diesels - well the accountant would but IMO it's not worth the saving. Given that we needed a generator for when it was cloudy for more than 4 days and the outboards only lasted 1600hrs, there's not much $ saving over the life of the boat....

Big fan of the Lightwave, 6.5kn on one engine at 2400 is nice.

The best way to determine fuel consumption is top up the tanks to a known/recorded level, then go motoring for 1 to 2 hrs (longer period better) then top up again and record what you have added. There is your consumption. Not the theoretical or other boat consumption just your boat fuel consumption.

Then go saling/motoring with confidence.

Happy and safe cruising.

Hi Guys

Thanks for the replies. Sorry I took a while to get back - been away; and a buggered back.

Harlie, the prop specs read like this: Propeller Mark = 9; 3 blade X 235 mm diameter X 229 pitch. I hope this is helpful. Where did you store your jerry cans of petrol?

Pekeri, I will be installing hour meters at some stage - hopefully as soon as I can. Or, initially, I'll do what your suggesting before I start the journey south.

Any other info would be greatly appreciated

Thanks

Relackson