Fuel Consumption

Ranchi, no fair! It's an ASD, and everyone knows they don't plane - they use some super-secret method of skating over the water
Actually, I have heard of others, but they are very few and far between.
Steve (PS - nice boat!)

 

the old wise sad "even a potatoe can get in planing condition if you have power...."

 

Poida,
your boat actually has at least 3 different drags which have to be minimised to reduce fuel consumption. One is friction drag. To minimise it you have to minimise the surface area and you have to make it long and narrow instead of short and wide. That is because the aft bits are sort of in the wake of the bits in front. Think of bike riders all cycling one behind the other.
The second drag is wave drag. The less waves your boat makes the better because waves are energy and the energy comes from your fuel. To minimise wave drag your boat again has to be long and narrow.
The third drag is pressure drag. This is were the "pushing through the water" comes into action. An oar for example has high pressure on one side and low pressure on the other side. For an oar this is what you want but for a hull pressure drag is bad. V-hulls have lots of pressure drag at displacement speeds because of the turbulence at the stern. Basically the turbulence reduces the water pressure at the stern which otherwise would push the boat forwards. A well designed hull has very little pressure drag and friction and wave drag are much greater.
A higher displacement will generally require a greater wetted area and also produce more wave drag. For the same drag a high displacement boat has to be longer and narrower to produce the same drag as a lower displacement boat. So the combination of displacement and hull shape determine the drag and therefore fuel consumption.

 

Well written Karsten!
Have a look at hull forms from the last 60-70 years or so and we can clearly see that boats of all kinds have become fatter.
Decide how wide your boat's interior practically has to be, probably less than current trend, and use the rest on making it a bit longer

Mikey

 

Mikey -

"Decide how wide your boat's interior practically has to be, probably less than current trend, and use the rest on making it a bit longer."

Then all we have left to do is get marinas to charge by the square foot.

 

"Then all we have left to do is get marinas to charge by the square foot."

The DREAM of every Trimiran owner ever born!!

Although in California many marinas DO charge by the Sq Ft., that the only place I have ever seen that method of charging.

FAST FRED

 

Thanks Karsten.
You should write a book so people like me, (duh brains) can understand.

 

Many years ago there was a steel boatbuilding firm in the Louisiana bayou country. Their primary product was work boats for the petroleum industry. They decided to build some pleasure boats "because they could". My father had two of them. They were named Safti-Craft. They were heavier than comparable wood or FRP boats. The 34 footer had two hefty Gray Marine v8 engines. The boats were nice looking and worked satisfactorily. They had some serious faults. Principal among the faults was the enormous appetite for fuel. Some 600 horsepower would plane the boat at about 22 knots. Cruising speed of about 18 knots would use enough fuel to make an oil sheik wealthy.

These were steel boats and it was a full time battle to keep them from rusting out. The boats were said to be made of Cor-ten, supposedly a material that was not badly affected by oxidation. The skin was on the order of 3mm thick and it had a lot of flat bar on edge in the bilge. Over a fairly short time the bottom came to resemble a corrugated metal roof. It had a lot of ripples in the skin. The skin should have been 6mm thick at least. If so the skin would have been 4 times as stiff and twice as heavy. Twice as heavy means that it would have used even more fuel. Another disadvantage of the small steel boat is that it was unpleasantly noisy.

To the credit of the steel skin we did not rip the bottom out when we ran over a submerged wreck at about 20 knots. It did put a hellish gouge in the bottom but the boat never leaked afterwards.

I'll take wood or some other material that is NOT steel. A person who is skilled at fabricating steel can almost surely transfer those skills to a more sensible material like wood. If you simply must weld up a boat, then use aluminum and be happier in the long run.

 

Long and narrower

To make the hull longer and narrower will not only make it more fuel efficient, it will also make the boat more seaworthy and seakindly. It would be more resistant to capsize and track better, and the same wave action would generate higher accelerations on a wide boat compared to a narrower one. Seaworthiness and seakindliness, 2 forgotten factors today?

Mikey

 

Maintenance costs

Forgot maintenance costs, many places charge by the foot so a longer but narrower boat would then cost the same even though actual work done is less


Mikey

 

I would have thought getting a steel boat to plane, out of the question because of the weight and resultant fuel cost.

The one I was looking at making cruised at about 9 - 10 knots.

Rusting would not be a problem for me as I will probably die before it rusts out. In fact I would like my ashes scattered in the sea so if I die at the same time as it rusts out and sinks it would be quite convenient.

The problem at the moment is a place to build it.

 

Rust aint all that slow.

Poida,
if you are in fact looking out a window from which the setting sun is that close to the horizon (so to speak),
then maybe you might want to consider buying a boat rather than building one...

I am sure that many a home made steel boat has rusted out quicker than it might have taken one person to build it (especially someone who has yet to find somewhere to build it).

Maybe you can buy a wooden boat, and then when you feel just about done (ie the sun is about to set), flick a match with your last strength into the petrol tank, and boom, you have both your ashes, and your burial at sea...

Seriously though, Best of luck.

 

Thanks hansp77 for your touch of brilliance.
I already have a wooden boat and the sun could have gone into an eclipse before it set sending me to an early ashen departure.

Just recently a valve spring broke so I decided to do a complete overhaul of the head.

I didn't have the machinery to grind the valve seats so I sent it away.

While that was happening I did some modifications to the engine cover. I couldn't understand why there were small vents in the top of the engine cover as I didn't consider that they would be large enough to cool the engine and in fact I thought to myself that the person who puy them there must have been a nutter. I removed the vents.


After reading posts on this forum I realised they were vents to let any fammable gases out and not to cool the engine.

This forum may have saved my life.

Now I've got to put them back.

If you ever visit Western Australia buddy I'll take you out in her, it'll be a blast.