Just stick to a steady 2000 rpm on my my merc om636 which delivers about 6 knots and acceptable economy-for what its worth its this sort of rpm that delivers best economy on most diesel cars I have possessed from a 2.8 litre Nissan Terrano to my current fuel efficient 2 litre Citroen Berlingo-usually about 60mph on level road in car.
If motor sailing usually run at about 1500 rpm to maintain similar speed.
Constantly changing rpm/speed is one of the less efficient ways of fuel management particularly as most marine diesels have a very simple fuel injector pump.
Ultimately of course they will start putting together marine versions of the latest computer controled common rail diesels BUT as for example Citroen are the only people prepared to fix my HDI car engine!!!!-the thought of having to take my boat /engine to an "Arnold Clark Marine outlet"(our Citroen dealership up here) who couldnt service a bicycle properly!!!!
Couldn't agree more, this common-rail technology with all sorts of weird sensors scattered around the engine may be fine on the M4 but definitely not suitable for use at sea. Though if the emmissions freaks have their way I'm sure we will have it imposed on us one day.
I'm told by a friend who is a fully qualified expert that with the latest generation of Ford Mondeo diesels, only a Ford garage with a special Ford device can bleed the fuel system, and this needs to be done after every fuel filter change - if you don't get Ford to do it, you are in for a £2000 bill having the injector system replaced as it will be wrecked if not bled with their device!
I find that with my Volvo 2020, which is a fairly high reving diesel (3600 max) that between 2200 and 2600 rpm seems to give the best balance of economy v speed, depends on the sea state, wind and how clean the bottom is .
I reckon that even if one wants to go as fast as possible, the best way with a diesel, is to open it right up and then gradually back off on the power until the boat starts to slow down a bit, then if you settle for about 0.25 of a knot below max speed, you'll find on most boats that you are only at half throttle, which I reckon means you are only using between 0.25 and 0.3 of the amount of fuel you would use to get that extra 0.25 knot.
But of course I have a fairly easily driven racing hull and am deliberately a bit over-propped. I fitted a 16 x 11, in place of the recommended 15 x 10 so that I could be at max hull speed at the peak of the torque curve rather than peak revs/power. I haven't experienced any carboning up problems and the engine is nicely under load even at low revs, which helps keep its temperature up. I did this not primarily to economise but 3600 revs is just too damned noisey and busy! BTW I notice on the latest generation of this engine the Volvo D1, they have increased its stroke and reduced its revs to give more torgue and a flatter torque curve.
Richard J's figures for the Beta BZ 482 are absolutely fascinating, and for which many thanks. I have one in my Halcyon 27 with a 3 blade prop, and just from listening to the engine and watching the instruments , I concluded that in still water and towing the 2.6m Zodiac, 2000 revs and about 4.2 knots is most economical, which almost exactly agrees with Richard's figures. If I give the Halcyon full grunt with the Beta I do around 7.5kn (v. approx - only did it once on trials) and have to put the bung in to stop the cockpit floodling!
I was reading Tom Cunliffe's RYA Seamanship manual yesterday and in general terms he suggests (page 16) that the most economical rate of progress falls around the root waterline and that a good compromise is found around halfway between this and hull speed (considered to be the square root of the waterline in feet x 1.34).
The waterline length of my Yarmouth is 17.9 feet giving the square root as 4.2 and the hull speed as 5.7 kts. The halfway figure is 4.9 kts - bang on what I expect at 2000 RPM in flat water. However, these figures are 'depending upon engine size' - I'm impressed that you managed 7.5 kts flat out - I can't push the revs much above 3000 with my large 3 bladed prop and that gives me something over 6 kts.
At the end of the day, thinking about this has rammed home (again) the importance of tidal gates - I could stand 3.7 kts through the water if the SOG 5.7.
As regards the posts referring to the use of vegetable oil from the local supermarket, I do hope, David, that you have taken care to read the HMRC notice carefully.
What it says, at HMRC Notice 179E, para 2.4, is:
"If you produce 2,500 litres or more biofuels a year, or use them as motor fuel on which duty has not been paid [my emphasis], you will need to contact us so that we can make arrangements for you to account for the excise duty due on any products set aside, or delivered for use, as a motor fuel. "
This means that you might be right to say you can use converted chip fat to power a boat (if you are sure that is not "motor fuel"), or any other off-road use, but you very definitely cannot do so for road vehicles. This is explicitly stated in HMRC Notice 75 at para 2.1:
"What fuel can I legally use in a diesel-engined road vehicle?
The law requires that you must always use fully duty paid fuel. [My emphasis]"
Don't mess with the excise men - they do NOT muck about!
Sorry Paul you have it mixed up Biofuels does not mean vegetable oil-bio fuels are veg oil and the like that has been mixed with neat alcohol, cooked and titrated with sodium hydroxide-the result a form of diesel(plus a lot of lye soap)-as such you have to pay duty if used on a Public Highway..
There is however now an exemption for veg.oil bearing in mind you either need to blend it with diesel or install tank and inline heaters to keep it thin(all readily available on the internet).The exemption was given in my opinion simply because it was already happening and impossible to police.
I used 22 litres of diesel to motor 98 nm in slight sea, no wind (20 hours). The Vetus (22HP 3.10) manual said that I should expect to use 3 litres per hour - ie 60 litres!
My consumption was almost 300% better than expected! - maybe against sea and wind the story would be different.
