prop design- newbie
 

Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning
about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My
goal is higher speed with lower fuel consumption without causing damage to
the drives. I know that the current props are 4 blade and generate 10 mph
at 12-1400 RPM depending on current and wind. My current fuel consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

Thanks for your help,

Marshall

On Sat, 28 May 2005 20:47:10 GMT, "MarshallE"
wrote:

Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning
about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My
goal is higher speed with lower fuel consumption without causing damage to
the drives. I know that the current props are 4 blade and generate 10 mph
at 12-1400 RPM depending on current and wind. My current fuel consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

Thanks for your help,

Marshall


Pitch:
a fixed pitch propellor has its pitch set to achieve an optimal angle
of attack at the desired shaft speed, and hull forward speed, both of
which contrl the value.

Diameter:
In general, a larger diameter leads to higher propuldsive efficiency,
until physical effects (like cavitation, or blade weakness) are
controling.

Projected area:
In general projected area is adjusted to absorb the available shaft
power at the desired shaft speed and target hull speed.

Can you say, "over-simplified?"

Hope this helps, all the same

Brian Whatcott

MarshallE wrote:
Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning
about prop design. My engines are 250 HP Cummings with Twin Disc Gears. My
goal is higher speed with lower fuel consumption without causing damage to
the drives. I know that the current props are 4 blade and generate 10 mph
at 12-1400 RPM depending on current and wind. My current fuel consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

You fuel consumption looks about right for an 80 foot boat at 10 knots
You are pushing the displacement hull a bit hard at 10 knots, do some
trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG
You should find a sweet spot somewhere between 8 and 10 knots.

The speed of a displacement hull in knots is determined by the square
root of the waterline lenth in feet, multiplied be a constant k

k = 1.3 is the max that you can hope for, fuel burn will be BAD
For best cruising MPG k will usually be between 1.0 and 1.1 but it
depends on the hull design. In some cases best MPG may come at lower
values of k, plot your graphs and see how they come out.

For example, for a waterline lenth of 64 feet sqrt(len) = 8
8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern hull.
8 * 1.2 = 9.6 knots -- Maybe the max for older hulls
8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel
8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around
here.

There is probably little to be gained by messing with the props.
If you want to go faster get a new boat with a semi-displacement hull
but be prepared to spend big $$$$ on fuel.

thanks for both responses...my son gave me a book by Dave Gerr about
propellers today...fascinating topic
thanks again
Marshall
"Chris Newport" wrote in message
...
MarshallE wrote:
Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning
about prop design. My engines are 250 HP Cummings with Twin Disc Gears.
My
goal is higher speed with lower fuel consumption without causing damage
to
the drives. I know that the current props are 4 blade and generate 10
mph
at 12-1400 RPM depending on current and wind. My current fuel
consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

You fuel consumption looks about right for an 80 foot boat at 10 knots
You are pushing the displacement hull a bit hard at 10 knots, do some
trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG
You should find a sweet spot somewhere between 8 and 10 knots.

The speed of a displacement hull in knots is determined by the square
root of the waterline lenth in feet, multiplied be a constant k

k = 1.3 is the max that you can hope for, fuel burn will be BAD
For best cruising MPG k will usually be between 1.0 and 1.1 but it
depends on the hull design. In some cases best MPG may come at lower
values of k, plot your graphs and see how they come out.

For example, for a waterline lenth of 64 feet sqrt(len) = 8
8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern hull.
8 * 1.2 = 9.6 knots -- Maybe the max for older hulls
8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel
8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around
here.

There is probably little to be gained by messing with the props.
If you want to go faster get a new boat with a semi-displacement hull
but be prepared to spend big $$$$ on fuel.

Try

www.fernandocarvalho.cjb.net




"MarshallE" escreveu na mensagem
m...
Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning
about prop design. My engines are 250 HP Cummings with Twin Disc Gears.
My
goal is higher speed with lower fuel consumption without causing damage to
the drives. I know that the current props are 4 blade and generate 10 mph
at 12-1400 RPM depending on current and wind. My current fuel consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

Thanks for your help,

Marshall

I read that book as well and found it extremely interesting although I have
not put that information to practical use. Dave Gerr also has a book about
boats that I find myself reading again and again, All about Boats

--
R. Scott Carroll
"MarshallE" wrote in message
m...
thanks for both responses...my son gave me a book by Dave Gerr about
propellers today...fascinating topic
thanks again
Marshall
"Chris Newport" wrote in message
...
MarshallE wrote:
Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in
learning
about prop design. My engines are 250 HP Cummings with Twin Disc
Gears.
My
goal is higher speed with lower fuel consumption without causing
damage
to
the drives. I know that the current props are 4 blade and generate 10
mph
at 12-1400 RPM depending on current and wind. My current fuel
consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

You fuel consumption looks about right for an 80 foot boat at 10 knots
You are pushing the displacement hull a bit hard at 10 knots, do some
trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG
You should find a sweet spot somewhere between 8 and 10 knots.

The speed of a displacement hull in knots is determined by the square
root of the waterline lenth in feet, multiplied be a constant k

k = 1.3 is the max that you can hope for, fuel burn will be BAD
For best cruising MPG k will usually be between 1.0 and 1.1 but it
depends on the hull design. In some cases best MPG may come at lower
values of k, plot your graphs and see how they come out.

For example, for a waterline lenth of 64 feet sqrt(len) = 8
8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern
hull.
8 * 1.2 = 9.6 knots -- Maybe the max for older hulls
8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel
8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around
here.

There is probably little to be gained by messing with the props.
If you want to go faster get a new boat with a semi-displacement hull
but be prepared to spend big $$$$ on fuel.