No idea of what to expect

[captain647]

I'm thinking of upgrading my day boat to a modern cruiser (3/4 berth) for use on rivers and inland tidal waterways, but so many seem equipped with 4L V6s or 5L V8s which seem too powerful for river use. What sort of consumption can I expect from these engines at river speeds of 4-5 mph - or are they not designed to potter about at these low speeds and revs ?

 

[rafiki_]

I'm thinking of upgrading my day boat to a modern cruiser (3/4 berth) for use on rivers and inland tidal waterways, but so many seem equipped with 4L V6s or 5L V8s which seem too powerful for river use. What sort of consumption can I expect from these engines at river speeds of 4-5 mph - or are they not designed to potter about at these low speeds and revs ?

I had a 25' sports cruiser on the Norfolk Broads. 5.7 V8. Sipped 1 gallon per hour. Seemed happy to do so.

 

[jct1981]

I've got a Sealine S24, with a single 4.3 V6 on Duoprops. I get about 4mpg, or 1 litre per mile, if I keep it easy to about 4.5 / 5 knots, with the engine at about 1,800 rpm. Much faster and the planing hull is working inefficiently and it drops back to 2mpg (or below), until I start planing.

Diesels have a different consumption rate; normally much lower once you start putting your foot down. However, petrols, I believe, can run happily at lower revs, whereas some diesels, especially the more modern KADs and later, like to be run harder from time to time to engage the super & turbo chargers.
Someone with a bit more knowledge will no doubt be along soon to add more / correct me.

james

 

[Mr Googler]

A large Diesel turbo engine will not be happy at low load all the time and give you issues. A normally aspirated small diesel as found in canal boats will be more than happy but you won't find them in a sports cruiser so all that's left is a big fat petrol engine which won't care what you do!!

Given petrol boats are cheaper to buy in the first place, I would imagine you will never spend the difference over a diesel boat in fuel. Happy cruising!