Tornado 6.5 - Huge Fuel Consumption

CUSAC · 21 July 2020, 12:49

Hi all

I've got a 6.5m tornado with twin 90hp Merc 2 strokes (1997). It's a dive boat so always pretty heavily loaded but over several trips now it seems to be averaging 4 litres per nm and thats keeping the revs to 3.5k and cruising at around 18-20knots. Seems huge to me. Any thoughts?

I have had a good look through all the previous threads on the site and seems like with normal loading it should run somewhere in the region of 1.5-2L/nm at a slow cruise. Both engines seem to use a similar amount of fuel when topped up. I suspect its just due to the sheer weight of people and dive kit and the console being quite far forward so it runs quite flat (more hull in the water).

I noted a response from Willk previously saying one of his 6.5m with twin 90s ran 1.44L/nm so a massive difference.

Not really sure what the question is but the wallet is taking a big hit each time at the moment! Any experience of a similar set up appreciated.

Nasher · 21 July 2020, 13:02

I think that 1.44L/nm from Willk is likely to be per engine.

Twin old school 90s on an already heavily built boat loaded full of divers and kit.....

I'd have guessed 3.5L/nm anyway

Nasher.

willk · 21 July 2020, 14:18

Quote:

Originally Posted by Nasher

I think that 1.44L/nm from Willk is likely to be per engine..

God Lord no - that was the combined. I ran a fairly heavily loaded boat too - lot of fuel and extra kit. No lardy divers onboard tho...

Nasher · 21 July 2020, 14:29

Were they old school carbie 2-strokes though Willk?

willk · 21 July 2020, 14:49

Quote:

Originally Posted by Nasher

Were they old school carbie 2-strokes though Willk?

Good Lord, no. They wuz Ermintrude Etecs 90s M'Lud.

CUSAC · 21 July 2020, 14:57

Quote:

Originally Posted by willk

God Lord no - that was the combined. I ran a fairly heavily loaded boat too - lot of fuel and extra kit. No lardy divers onboard tho...

Very impressive if that's combined then.

Having done the numbers I estimate the boat at 1200kg (need to get on a weighbridge). Boat 550 + Engines 200 + Fuel 250 + A-frame/console/seat etc 200.

Weight of 6 divers plus kit (assume 80kg a diver plus 60kg of kit each) at 840kg.

So it is quite significant extra load.

Pikey Dave · 21 July 2020, 15:16

I used to run a Ribcraft 6.4 with twin 115 etecs. I used to do around 1.8l/nm combined. Cruising at 25kn with 4 techies on board.

CUSAC · 21 July 2020, 15:30

Quote:

Originally Posted by Pikey Dave

I used to run a Ribcraft 6.4 with twin 115 etecs. I used to do around 1.8l/nm combined. Cruising at 25kn with 4 techies on board.

Hmmm so I'm getting twice as much for a carb'd 2S vs DI 2S even running at a slower speed (have been trying to be frugal and keep the revs down). Nothing much I can do about it anyway though really and the cost of a more modern engine buys a lot of fuel.

Maybe time to repair the little imperfections in the hull and play about with weight distribution for marginal gains!

Pikey Dave · 21 July 2020, 15:37

Quote:

Originally Posted by CUSAC

Hmmm so I'm getting twice as much for a carb'd 2S vs DI 2S even running at a slower speed (have been trying to be frugal and keep the revs down). Nothing much I can do about it anyway though really and the cost of a more modern engine buys a lot of fuel.

Maybe time to repair the little imperfections in the hull and play about with weight distribution for marginal gains!

Have you checked the hull for trapped water?

CUSAC · 21 July 2020, 15:45

Quote:

Originally Posted by Pikey Dave

Have you checked the hull for trapped water?

Small dribble out of the drain plug each trip even on a really steep slip. Interesting thought about trapped water. Seems to sit nice and high when unloaded (tubes clear of water) so probably nothing significant. Under deck 200L tanks would take up most of the space!

Guess the only way is to get it on a weighbridge and the trailer and see if anything is suspicious.

beamishken · 21 July 2020, 18:13

Have you tried running faster and checking consumption? we find with our cruiser that if we run 27kts rather than 22kts that because the boat is planeing cleaner, more boat out of the water that we actually do get better mpg. That said we still burn 3L diesel per nm which is still significantly better than you & we are 8.5 tonnes in weight.
Just a thought, running slower isn't always the best option

davej · 21 July 2020, 18:26

I would have thought an old school 2S that 4 l/mm was not far off correct, my dive club uses a 5.5 tornado with a 115 2S yam, wonderful motor but it certainly has a drink problem when it’s loaded up.

If you divide the fuel use by the number of divers it’s not to bad a cost, well relative to what the commercial dive boats charge.

Typically with our boat one dive burns through 60 lt of fuel out of the 90 lt tank.

Use it and enjoy it.

Actually cruising at 18 to 20 kt seems fast have you tried backing off a bit ie 14 kits.

As someone else said it’s very heavily loaded.

Dave

Nos4r2 · 21 July 2020, 19:16

I'd be surprised if it even stays on the plane at 14kts, at a guess it'll be coming off the plane with hitting even the smallest waves.