Engine revs

[lw395]

No, I am not.

While the hp used at any revs is less than what is available except at WOT (assuming the boat is propped correctly) it is not less than 50% which is what your figure implies.

The real life average consumption of engines rated at 25-30hp is as quoted and reported by many. That is the only figure that has any meaning for planning passages or calculating ranges if using the engine.

The efficiency of diesels and the calorific energy of the fuel is well documented.
It makes little difference what the nominal power of the engine is, the power needed to drive the boat at e.g. 5 knots is the same, give or take propellor efficiency changing a bit.
The actual thrust needed to drive a boat at say 80% of hull speed in flat water is remarkably small.

Some years ago, I was on passage together with another yacht. There was no useful wind. It was remarkable how similar the fuel consumption was between a 25ft boat and a 35ft boat, at the same speed, despite us having a much bigger engine.

 

[vyv_cox]

I have often wondered about the fact that you should not let a diesel engine turn over without a load on it because of the risk of glazing.

You never hear of this being a problem when talking about diesel engines in eg HGVs. Or maybe it's just me who hasn't heard of such a problem.

Industrial engines run on tickover for weeks on end without suffering glazing. But they are running hot, when the problem does not occur. What causes glazing is running at no load from cold, which allows combustion products to condense on the bores. More of a problem on relatively new engines in which the honing scores are still sharp, giving the deposits something to adhere to. There is a page on this on my website.

 

[[2068]]

Must admit when I'm motoring I find it a bit boring so crack on a bit, the tank is 186 litres, according to the literature it is designed to give 36 hours motoring. At 2 ltrs/hr I could motor for nearly 4 days.... mind you I'd have died of boredom by then!

2 litres per hour?
Wow.
I think I’m using something like 1 litre per minute at 20kts.
I’ll look into consumption at slower speeds :ambivalence:

.

 

[Babylon]

2 litres per hour?
Wow.
I think I’m using something like 1 litre per minute at 20kts.
I’ll look into consumption at slower speeds :ambivalence:

.

Yes, you can go 3 times as fast as Jimi for 9 times the consumption.
For my smaller boat doing 5kts at 1.5l/hr, your figs are 4 times as fast and 10 times the consumption.

If I motored from the Needles to Alderney, it would take me 13 hours and cost me about £20 in diesel.
You'd get there in about 3 hours at a cost of £200 in diesel or £260 in petrol.

I don't know anything about motor-boat efficiencies, but assume that at lower revs you might be lower in the water and therefore some of your power would be dedicated to just pushing water out the way?

 

[Sybarite]

Industrial engines run on tickover for weeks on end without suffering glazing. But they are running hot, when the problem does not occur. What causes glazing is running at no load from cold, which allows combustion products to condense on the bores. More of a problem on relatively new engines in which the honing scores are still sharp, giving the deposits something to adhere to. There is a page on this on my website.

Thanks for that Vyv. One learns something everyday.