Fuel Consumption

[Poida]

Hi
I was looking at building myself a steel boat, mainly because I have all the equipment required to build it.

I realise that it would be heavier than using other materials but does anyone know how that would effect the fuel consumption.

For example is most of the fuel consumption used in pushing the boat through the water, with the weight of the boat being of minimal difference?

[antonfourie]

What kind of boat are you referring too, power, sail, monohull, cat etc ?

[Poida]

Monohull Power

[antonfourie]

Is it a planing or displacement hull

[SailDesign]

Just a thought, anton, but have you heard of many planing steel boats?
Not saying it hasn't or can't be done, but....
Steve

[antonfourie]

Good point SailDesign, in that case as most commercial boats are made of steel it would not be a issue to build in steel, Poida

[FAST FRED]

A steel boat is usually of a size where the hull material is less of a handycap, 40ft or so plus.

Be sure to only use flat bar as reinforcement , as the T bar or L bar is far too hard to sandblast under so the vessel life will be compromised.

FAST FRED

[Poida]

Thanks for everyones response but nobody's answered the question.
Obviously building with steel is not an issue as it is done all the time. The question was, what does the weight of a boat effect the fuel consumption.

My guess is that most of the power is used to push the water out of the way and the weight of the boat is of little importance and probably why shipping is still the most cheapest mode for moving goods.

I don't kneed to know the answer, I was wondering if anyone knew.

Cheers
Poida

[antonfourie]

Easy answer then, heavier = more fuel

[fcfc]

For a boat that goes displacement speed = low.
displacement speed in knots = slower than 1.3 * square root (waterline length in feet).

for the same length of waterline, for the same speed, by calm weather, the power needed so the comsuption is about linear with the weigth. A boat 10% heavier, same length, same speed will burn about 10% more the fuel.

A boat 20% longer, but with the same weigth will go 10% faster with the same power (same fuel burn per hour).

This is very broad because you do not give too much parameters.

[fcfc]

No.

You do not "push" the water.

When you move a boat, you make waves and you have friction with the water.

At very low speed, resistance is mainly the friction. you do not make big waves. The friction is directly dependant of the wetted area of the hull. Typically, heavier hull, bigger wetted area.

At higher speed, wave resistance is main. It is most dependant of the weigth of the boat.

So in all cases, a heavier boat will always burn more fuel than a ligther boat, same length, same speed. How much is all the question.

[riggertroy]

Just a thought - a wooden sailing ship (incl aux engine and fittings), displacing about 300 tonnes, LOA 145foot, Steel sailing ship LOA 150foot, displacing 220tonnes. Actual ships. They were built for different purposes so that would be the reason for the difference in displacement but maybe the weight difference is not that great once you get up to larger vessels. My 22' yacht is timber built but including weight of keel is around 4.5tonne, still restoring her, was 6 when we salvaged her, all that mud inside I guess

[Poida]

Good point Riggertroy

When I was thinking of building a steel boat I immediately thought, "what about the extra weight and fuel consumption."

However steel probably has a good strength to weight ratio. The sides of the boat were 3mm (1/8") where a comparitive fibreglass construction would have to be (at a guess) around 12mm (1/2").

Both fibreglass and timber absorb water making it even heavier, so a steel constructed boat as indicated by the figures you quoted is probably not much if at all heavier.

I was suprised to hear that boats do not push water away and find it hard to grasp. Of course there would be friction between the water and the boat but if there was no force required to push the water away, you wouldn't need the bow to be wedge shaped.

Anyway this is getting all too much for me I'm going to bed.
Goodnight all.

[tom28571]

Actually a boat does push water out of its way. That is what makes waves.

Maybe the reason that you did not get the answer you wanted is that you biased the answers by asking a bad question. It's not one or the other. More weight equals more water to be pushed ouf the way. Then there is hull shape, hull length to beam ratio, speed you intend to run at and a lot of other stuff to be considered.

It's possible for a large and heavy hull to have less resistance at a particular speed than a small and light hull.

Read Dave Gerr's "Nature of Boats" or Ted Brewer's book on design to get an inkling of what this is about.

[RANCHI OTTO]

Hi SailDesign,

Just have a look on my web, "RED SUGAR ASD", steel and 60 knots with a length of 12 m.....

Perhaps is the only one in the world...