Motorsailing tactics

[Daydream believer]

If you frequently motor sail, it is worth considering fitting a Bruntons Autoprop. As the name suggests, the pitch of the blades is automatically adjusted to suit the conditions. Say that under power 1000rpm gives you 2knots. You are sailing in light conditions, and only doing 2knots. With an Autoprop, if you switch on the engine as well, at 1000 rpm, you will do 4knots, because the propeller has automatically gone to a coarse pitch, thus putting some load on the engine. I have no connection with the company, other than being a satisfied customer.

I do not want to get into the debate about low engine revs for long periods as I am not qualified to do so.
However, I would fully concur with the recommendation of the Brunton Autoprop.
If one is sailing at 3 knts with a fixed prop surely it stands to reason that one has to apply enough revs to motor as if one was doing more than 3 kts otherwise the prop is just spinning in nothing.
I am not a great lover of the Autoprop for normal sailing. However, as a motor sailing prop, it is brilliant. One can just have the engine in fast tickover & it pitches up & one can feel the push. With 15-1700 revs the engine is very economical & the increase in motor sailing speed is significant.

 

[Babylon]

(When I was in the motor trade, we used to get steriotypical old ladies shopping cars in, "it's just not running very well". A good blast though the gears at high revs a few times usually had it running much better)

Italian tune-up then. When I was at uni in the 80s I'd borrow my mum's car at the start and end of each year to haul my stuff, and being young I'd give it full welly: the journey out was typically sluggish, but the journey back was always wizard!

 

[differentroads]

The choice is then: run the engine at low revs (typically 1800 or so), sip fuel and do normal motoring speed...or run the engine at normal revs (2200), enjoy a little more speed but burn diesel at the normal cruising rate.
Clearly if we're in a hurry it's option 2. But if we want to save fuel...option 1. In that event, should I be worried about running at prolonged spells at low revs? Appreciate any thoughts, thanks

I've done a good five hundred hours of motor sailing in the last two years (Ipswich, cross Biscay, Gibraltar Straits, Balearics with a much higher proportion of light and head winds). Having a heavily laden and slow 10m sailing cruiser, I had a lot of time on my hands and a strong motivation to get best speed v fuel efficiency.

My first insight was that my Yanmar 3GM30 has a remarkably flat fuel consumption curve. The effect is that over any given distance I use the same amount of fuel at 2000 or 3000rpm; burning more fuel per hour at higher revs but getting there sooner offsetting each other exactly. I wish id known that earlier. It would have saved scores of hours dawdling around motorsailing at less than optimal revs. Other boat and engine combinations will work out differently, but it's worth testing the assumption that low revs will mean less fuel used.

The second bit of received wisdom that I had punctured this year was that motor sailing in light winds is worthwhile. At wind speeds under about 8 kn, my Yankee Jib and mainsail turned out to be slowing me down by up to half a knot. I guess it's extra drag they create rather than drive, but after a few experiments at different wind angles and engine rev combinations on my admittedly underpowered and overladen cruiser, only with a wind in high single figures on the beam AND waves from behind, could I get a vmg better than motoring alone.