Car engine vs. outboard

[F.H.B.]

Hi,

I don't get it. A 20 HP outboard uses 3-4gph. My big Lincoln uses 4gph at 65mph. What gives? Would I be better off with a cheap inboard car engine in cases where it can fit?

Thanks!

[Frosty]

Thats apples and oranges, a boat is not a car.

[Obsession]

If you turn off the engine in both the car and boat going a similar speed, compare how far each goes before coming to a stop. The boat has a lot more resistance and requires constant horsepower; the car spends most of its time at low rpm and putting out only a fraction of it's wot horsepower as the car rolls or coasts along.

People find it difficult to believe that their car only needs about twenty horsepower to cruise along at a steady 65 mph.

http://zfacts.com/p/821.html

[F.H.B.]

Thanks. The friction part and how a car usues little HP to keep going makes a lot of sense.

[tunnels]

How many mpg will you car use climbing a steep hill with you foot flat down for a couple of hours ?? Because that the differance between a car and a boat .
The boat motor is using 80% or more hp all the time it is moving . A car uses 10 % to 15% of its hp to move along the road . sometime a little more . get the idea ??

[CDK]

In the British TV series Topgear there was a contest converting a car into an amphibian. Jeremy Clarkson converted a Nissan truck by sealing the cabin and adding a very large Honda outboard: he could barely reach 4 kn.

[hoytedow]

Originally Posted by CDK

In the British TV series Topgear there was a contest converting a car into an amphibian. Jeremy Clarkson converted a Nissan truck by sealing the cabin and adding a very large Honda outboard: he could barely reach 4 kn.

I loved that episode.

http://www.youtube.com/watch?v=KzxuEOxYSLE

[FAST FRED]

4gph at 65mph

So your car is aerodynamic enough to only need 40hp or so at road speed.

FF

[powerabout]

what was that old Porsche 928 add from 20 years ago...
15hp for 55mph?

[FAST FRED]

Aerodynamics are everything.

Back in the day I took a 1500 normal out of a 54 Porsche Coupe that usually got 32 or so on the highway.

Replaced it with a Corvair , 4 carb kit , slow fan pulley and Abarth muffler.

Mileage only dropped to 29 on the highway,

Sleek is sleek!

[Marco1]

That's right. Fuel consumption is a factor of power delivered not potential power. You could fit a V8 Ford 5 litres in a Toyota Corolla and test fuel usage with another standard Toyota and the difference will not be much. If you compensate for the extra weight putting two passengers in the standard Toyota, you may even get some savings with the V8, depending at what speed you will be doing your test.

Having said that, and from the amount of fuel the outboard in question is using, the original post compared a probably old 2 stroke outboard with a car engine. Clearly the old 2 stroke are really bad when it comes to fuel.
Perhaps upgrading to a new 4 stroke outboard may be the go?

[Marco1]

Originally Posted by Obsession

If you turn off the engine in both the car and boat going a similar speed, compare how far each goes before coming to a stop. The boat has a lot more resistance and requires constant horsepower; the car spends most of its time at low rpm and putting out only a fraction of it's wot horsepower as the car rolls or coasts along. http://zfacts.com/p/821.HTML

Not very accurate on both accounts. When it is true that power requirements are widely different for boats and cars, to say that you only need 20HP or 40HP in a car is not true or we would have them on the market.

A car on the road battles mainly with two factors, gravity and inertia. Road friction and air resistance are the factors that require lesser power and the one that must be overcome when cruising along and can be done away with lesser power.
Gravity going up hill and inertia when in need to accelerate be it from stand still or on the go are the two factors that require 100 or 200 hp to give a reasonable performance even with small loads. To tow a caravan or a boat, we are talking even more.
A boat on the other hand, goes at constant speed and only occasionally must overcome inertia and when it may go up hill on a choppy day, that does not compare with a real hill. May be compares with a bad road.

And the example of comparing friction on the road and in the water is of course not correct on a planing boat and can only be applied to a displacement hull or one doing displacement speed.

All in all, to compare the power requirements of a boat vs a car, one should put both vehicles in similar conditions, and so a car does not cruise along all day and a boat does not do stop and start and uphill ever.

[CDK]

Originally Posted by powerabout

what was that old Porsche 928 add from 20 years ago...
15hp for 55mph?

There is some truth in that. I have a 928, now collecting dust in the garage. Driving at steady speed the 4.9 ltr V8 needs less gas than my wife's Ford Ka. But getting there from 0 is another matter: it is the only V8 I know of that has 9 fuel injectors!

[powerabout]

Originally Posted by Marco1

That's right. Fuel consumption is a factor of power delivered not potential power. YClearly the old 2 stroke are really bad when it comes to fuel.
Perhaps upgrading to a new 4 stroke outboard may be the go?

Nothing wrong with a new 2 stroke

[Frosty]

Originally Posted by CDK

Driving at steady speed the 4.9 ltr V8 needs less gas than my wife's Ford Ka.it is the only V8 I know of that has 9 fuel injectors!

Experimental engineer you may be but are you not also experimenting with our tolerance of ********.

There are examples of bigger engines in some models of cars being slightly more economical than its smaller model but lets not go over the top.