Ideal cruising revolutions for Bukh 36

[mashua8]

Ideal cruising revolutions for Bukh 36.   I have heard various figures given from 1700 to 2200 as the ideal cruising revoultions to obtailn the best economy and speed. My LM 30 is a bilge keeler so this may make a difference. I would be interested to hear what other LM Owners think?

[Sula]

I am not sure there really is any objective 'ideal':  you will have to decide for yourself the best compromise between fuel consumption and speed, factoring in noise, vibration, boat movement, etc.

That said, there's probably a comfortable range that would suit most people, but even that will vary boat to boat and depend on prop size, how clean the hull and prop are, wind/waves, etc.

I've only a few hours experience with my Bukh 36, and in a different hull, so can't give any practical advice, but will share my theoretical musings in case that's of any interest.

The current Bukh 36/48 manual has a graph showing fuel consumption per power output (grams per kWh) against engine revs.  As I understand it, this suggests that for the same amount of power the fuel consumption falls gently from 1,500 rpm to its 'most efficient' speed of 2,400 to 2,700 revs, rises gently again up to 3,000 revs and a little more steeply up to 3,600 revs.  The performance difference between the revs is very modest, with less than a 10% difference between the most efficient and least efficient revs. 

Those differences will be outweighed by the rise in power required the faster you go.  This rises gently, and gradually more steeply, as boat speed increases, then increases very markedly as you approach 'hull speed' (which would definitely not be an ideal cruising speed).  

In short, the faster you go the more it will cost; above 2,700 revs you've got two factors adding to the cost; and approaching hull speed you are spending good money/diesel to little effect (but circumstance may dictate). 

All of which gets you back to the first paragraph, and what works for you!  Hopefully someone will be along shortly with some objective experience of that engine in a similar boat, and what works for them in practice.  Happy motoring!




   

[Deleted]

I have an LM 30 with bilge keels. At 2000 rpm I get 5 knots, low noise and at least 60 hours from a tank full. At 2200 I get 6 knots and I'm not really aware of a big difference in fuel consumption, well, not enough to worry about even crossing the channel to the CI's where you can fill up at a fraction of the UK cost. I can get around 7 knots at 2700 but very very rarely need to go that fast.