Stevens 1140 or 1180 fuel consumption @ 5 or 6 knots

efanton

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Simple question. Can someone tell me the typical fuel consumption for a Stevens 1140 or 1180 cruising at about 5 or 6 knots?
boat has 2X 100 HP Volvo Penta TMD31
Hoping someone has a similar boat and can answer from experience


I have seen the formulas for calculating fuel consumption, but I intend to use the boat for river and canal cruising in Europe. The boat will rarely (possibly never) be near full throttle, but instead generally at around 5 or 6 knots. Problem for me is that I don't know whether cruising at 5 or 6 knots would be half throttle or 1/4 throttle, or what revs would give a 5 knot speed.
I am also assuming the formula for fuel consumption is not linear, correct me if I am wrong, so the formula really doesn't help?
 
On our current and previous boats with larger engines (currently 265hp) at river speeds I reckon around a gallon an hour each engine isn’t too far out. Engjnes tickover around 700rpm which gives around 3-4 knots so 5-6 knots is around 1000rpm but will vary depending on current/tide. I know you are talking about smaller engines but they will be working harder and I don’t think they will be far off that - if memory serves we got similar with a pair of 106hp engines in a 32 foot boat.
 
We have a 11.6m semi displacement twin engined boat with two 145hp turbo diesels, our engines tick over about 450-500rpm for about 8km/h, each engine consumes about 3l/h at up to 12km/h which is about 700rpm.
A lot of time we are only using one engine to propel the boat as at the low speeds one engine is more than enough, it makes maybe as much as 1km/h difference between using one or two engines.

We are in Belgium, on the canals there speeds are quite low and counted in Km/h speed over ground, not through the water, our canal has a speed limit of 8km/h, the larger canal we are attached to has a speed limit of 10km/h and 12km/h, the Albert Kanaal near us has a limit of 15km/h and 18km/h in sections.

The big downside of having twin engines is that you cannot get too close to the sides of the canal so a lot of moorings are too shallow at the sides for us as the propeller could touch, we have seen that with boats similar to yours with single engines that they can get right alongside and we cannot.
 
SOG depends very much on the water flow on the canals, we can see up to 3km/h of flow. When we were in the Hasselt Marina this summer there was about .75m rise and fall of the canal every so often, that marina is attached to the Albert Kanaal which is a main water route between Antwerp and Maastricht, you can visibly see the water rise and fall over a few minutes as the locks cycle through
 
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