Difference between outboards and car egines

[timddo]

OK. My commondore uses 12 litres and hour doing 100km's and the boat use hmmm at best 1.5km/l. Thats 66litres for 100km.

Why so much different. Wouldn't it be good if we can be that efficient 12 litres for 100km traveling?

[Noelm]

the biggest problem is in Water you are forever in the wrong gear and always going "uphill" with a slipping clutch, so as you can see, you will never achieve car economy in a Boat.

[mini696]

A prop pushing water is more inefficient than tyres. And there is more drag induced by the water. Also since a car has gears it can stay in the "sweet zone" for fuel efficiency easier that a boat can.

It basically comes down to the fact that a boat has a less efficient propulsion mode and more drag to overcome than a car does.

Driving a boat through swell/chop is like driving a car up a steep sand dune where the surface goes from hard to soft constantly causing your motor to surge.

To get better mileage... Drive in flat water only, have the boat trimmed down properly, drive on the plane, use a flat bottomed boat, dont have any extra weight in the boat. etc etc.

Its not that the boat motors are less efficient, its that they work a LOT harder to do the job they do.

Remember a 200HP car might get to 250-300kmph. But a 200HP boat will only reach 100kmph at best.

[Roughasguts]

Blame the Prop at best a half descent 6 seater Air craft will do 1 minute per litre of fuel as well, but it would have travelled 3 miles in that time.
But then an Aircraft can glide and save heaps on fuel for miles a boat can't they almost stop dead un less you have a huge ship.

On the other hand we had a 19 foot glass displacement hull with a 4 cyl ford prefect motor in it. You could put 10 bucks in that and drive it all weekend, yeah okay at 12 knots max, but still have a half tank the next weekend.

[FNQCairns]

Dunno just how much harder an outboard engine is working at cruise compared to that same engine fitted into a car and traveling in 5th at 100kph. Suspect it could be assumed if a person measured the throttle position of each in real time.

I am waiting for an infinity (probably not but dunno the the correct name) torque sensing adjustable type of ratio arrangement fitted somewhere below the powerhead, adjust with a breaker bar or the likeand dial in the exact setting for the prop and weight.

cheers fnq

[Roo]

drive a 5spd manual commodore in 2nd gear for 100ks @ 100kmh and watch the fuel gauge tumble. that should just about triple you fuel consumption.

[Outsider1]

drive a 5spd manual commodore in 2nd gear for 100ks @ 100kmh and watch the fuel gauge tumble. that should just about triple you fuel consumption.


Whilst towing a loaded trailer (weighing the same as your boat) with the trailer brakes seized on full!

Cheers

Dave

[Stuart]

Speed boats are in fact very efficient as the boat literally skims across the surface of the water. The same can be seen in skipping a rock across a flat lake. Off course rough weather is totally different. It comes down to gearing in an outboard, a small wheel being your prop as apposed to a much larger prop. Perhaps if we could buy an outboard with better gearing and a much larger prop you have far superior economy. Most cars only rev out to around 2800 at 100kmh and getting 18mpg while the standard boat is doing 5800rpm wot to achieve 30 to 40knts and sucking 1.2 litres per mile. I think outboard manufacturers need to start looking at the drive system instead of just looking at the power head all the time. I really think we could come ahead in leaps and bounds in terms of outboard efficiency if they invested a fair bit of time in the gearbox set up of outboards. Seems to me that most if not all outboard companies look at how fuel is best being burnt rather than looking at the engine as a whole.

Stu

[Noelm]

the problem is not just the gear system, it is more the fact that we only have 1 gear, so it must be of a size/ratio that will enable a Boat to take off and go full speed with the same gear, unless we have a two speed setup, to be of use as we Outboarders know it, we must have a smallish high speed prop, a big slow turning one (as in a ship) does not give us what we want, if we moved to slow speed then we have a whole new set of rules to play by and a large diameter slow turning prop becomes our friend, a ship Motor may only do 90 to 100 RPM flat out, that would be useless to us speed freaks, be kind of spooky though to have a 2 metre diametre prop churning away on the back! would certainly dig up the sand, and maybe chew up a couple of Kite Surfers as well coming across the bar!

[Noelm]

OH and of course we also need to take into account that our "road" is not a stable surface, it changes all the time, so outright speed is not always the King, we need compromise, we get it, but it costs us Fuel!

[Flex]

All comes down to inertia. What happens when you take your foot off the pedal on a flat road in a car? you roll for ages before you come to a stop.

What happens in a boat? your off the plane within 20-30meters and slow to a halt pretty soon afterwards. Basically your boat is having to always overcome the opposing force of water drag against the hull to make it plane.
But in a car you dont need to overcome this at all+ you have the inertia of the vehicle+spinning tyres(which have a very low friction co-efficient because of bearings)

Where to plane a boat you need to physically drive uphill 100% of the time+ a very inefficient form of propulsion.

[Kevaclone]

All comes down to inertia. What happens when you take your foot off the pedal on a flat road in a car? you roll for ages before you come to a stop.

What happens in a boat? your off the plane within 20-30meters and slow to a halt pretty soon afterwards. Basically your boat is having to always overcome the opposing force of water drag against the hull to make it plane.
But in a car you dont need to overcome this at all+ you have the inertia of the vehicle+spinning tyres(which have a very low friction co-efficient because of bearings)

Where to plane a boat you need to physically drive uphill 100% of the time+ a very inefficient form of propulsion.

not to mention you have 18 inches of metal dragging through the water at the rear

Kev

[Stuart]

Well thats it, Im using wind up rubber bands.::)

Stu

[Roughasguts]

Sails would have to be your best option then, no more petrol costs.

Now imagine a car on sails, only you wouldn't get far would you, be lucky to get out the drive way, and towing forget it.

[Hornet Rider]

What about jet boats? Are they any more efficient than an outboard?

[Dicko]

I was just thinking about this the other day, my bush maths reckon they're about 1/2 as efficient as a car.

My ski boat with a carbied 350 chev at around 300 to 350 hp burns about 12 to 14l per hour for general skiing varying around 20 to 35mph, so roughly that equates to traveling approx 50km over water.

The same engine in a car traveling at 100 kmh would use the same 12 to 14 litres, but cover 100km.

Load & Drag play a huge role.

My 90TLDi on the tinny only sips fuel during normal fishing use, but throw a couple of skiers on behind for the afternoon & the consumption climbs dramatically to 10 to 12 l per hr. This brings it pretty close to running the chev which has the benefit of approx 3 times the power & bucketloads more torque.

The outboard is sitting on 4500 to 5500 rpm most of the day & the chev is sitting on 2500 to 3500 rpm.

Comparing grunt out of the hole with skiers on is no contest. ;D

Other mates running big 2 strokes 175 & up for skiing are using double what my chev does.

[Noelm]

What about jet boats? Are they any more efficient than an outboard?
NO! they are even worse than a prop driven Boat for eficiency, that is unless the water is very shallow.

[KGW3]

A Boat is constantly displacing water, so needs to continually use the energy to travel forward. Some boats do use "gears" in the form of a variable pitch prop.