comparitive fuel efficiency of inboard vs outboard?

[Iliade]

GPH don't mean a lot when amount of miles covered is not in the equation

For my boats, the speed under power is/was about 4kt cruising or 5kt flat out, regardless of the engine type considered.

(Granted, before it expired, the 25hp Volvo i/b made the boat go faster, and burned more diesel to do so, but that was well up the hump of the hull speed graph)

 

[Boathook]

My experience is a little opposite. Having started off with 4stroke outboards on our catamaran i found them noisy, heavy on fuel and not reliable. By comparison the diesel we now have is super fuel efficient and quiet. Different boats but I cant see an outboard solution for a 19 ton yacht being viable......

My outboard is as reliable and compared with those I know who have inboards, it would seem more reliable. It does use more fuel which is a down side but I would put an inboard on a 19 ton boat unless it was on an inland waterway.

 

[peteK]

A diesel inboard returns unused fuel back too the tank.

 

[geem]

My outboard is as reliable and compared with those I know who have inboards, it would seem more reliable. It does use more fuel which is a down side but I would put an inboard on a 19 ton boat unless it was on an inland waterway.

My comments were somewhat tongue in cheek! People talking about the merits of outboard to inboard dont mention size of boat. Nobody in their right mind would consider an outboard on a large monohull if they are crossing oceans. On the other hand, if you have a trailer sailer or weekender I can see the attraction. Investment in a new inboard diesel compared to an outboard is considerable. I would always choose the inboard diesel personally. In my case the 4.4litre diesel inboard is the only way to go

 

[Topcat47]

I found this on another thread.

http://www.torqeedo.com/en/technology-and-environment/performance-and-efficiency.html

This may go some way to explaining things, but I"d rather expect Saildrives to suffer from the same inefficiencies so perhaps it's not the whole answer.

 

[lw395]

In calm weather at 5.5 knots my 30' trimaran at 2 tons uses 1.6 litres per hour which is about 16 nm per gallon. I don't recognise the 4 litres per hour figure at all. Maybe they are revving the nuts off the engine or got the wrong prop or something.

A tri is a bit of a special case.
My Impala used to use about a litre an hour or less of diesel.
Those with 2T outboards would be around 3 litres if you weren't in a hurry, easily 4 litres/hr against the tide.
4T perhaps 2/3 the fuel of 2T?

It's not the cost of the fuel, it's how much you want to be carrying, particulary when people used to race across channel and need to be back for Monday.
From choice, I'd never motor the thing much anyway.