I'm trying to understand propellers
 

On Sun, 26 Feb 2006 23:03:22 GMT, Shortwave Sportfishing
wrote:

Yeah, but will those 671s even get to 6000 rpm?

With a little more boost pressure and nitro assist they might make it
for a few seconds. After that it would start getting messy.

I'm trying to understand propellers
 

But you would not over rev?

"Harry Krause" wrote in message
. ..
Injam wrote:
If you are testing your new boat and find that it does not achieve
recommended maximum RPM at WOT. That would be bad for the motor right?
Too much load. Then you would change to one that is a lower gear so to
speak. And if you over compensated and the motor went over the
recommended maximum RPM at WOT. That too would be bad for the motor,
right? Over revving. When testing should you take it to the recommended
maximum RPM and see if you have any throttle left?


Ideally, with your boat properly loaded and trimmed out, your engine RPMs
should be between the middle and upper end of the max RPM range suggested
by the manufacturer. As an example, one outboard I know of has a
recommended range of 5500 to 6000 rpm at WOT. If I got 5300, I'd probably
look for a slightly different pitched prop. If I got 5800, I'd be pleased.

I'm trying to understand propellers
 

"Wayne.B" wrote in message
...
On Sun, 26 Feb 2006 23:03:22 GMT, Shortwave Sportfishing
wrote:

Yeah, but will those 671s even get to 6000 rpm?

With a little more boost pressure and nitro assist they might make it
for a few seconds. After that it would start getting messy.


One thing I've always feared about diesels is the very rare potential of a
"runaway". My wife's uncle was a vocational school diesel instructor and
told me of this condition whereby something goes wrong (I forget what) and
the diesel continues to increase revs until it basically self destructs. If
it happens, you have to quickly find a way to cut off the air supply or the
fuel before it goes "boom".

RCE

I'm trying to understand propellers
 

This is one of those situations that most certainly has created a lively and
interesting discussion. It reminds me of a long time airline captain that
asked, "You have two identical airliners at the same altitude, and same
airspeed, but one plane weighed 50,000 lbs more than the other (easy with
the 747s he used to fly). Now, both airplanes lost all power at the same
time...which would glide farther?"

The answer seems obvious...or is it? :) Same with the prop discussion here.

--Mike

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 17:00:28 -0500, "RCE" wrote:


"Shortwave Sportfishing" wrote in message
. ..
But, just as a discussion exercise...

Everybody is forgetting how we got to 6,000 RPM. All else being
equal, more horsepower will get you to 6000 RPM faster with the net
result being more end speed at 6000 RPM. Once the boat is planing and
the boat is approaching 6000 RPM horsepower beings to show up
resulting in a higher end speed.

Yes/No?

IMO, yes .... and no.

The bigger engine will get you to 6000 RPM and the speed that 6000 RPM
produces - faster.
The smaller engine will take longer to get to 6000 RPM, but when it does,
the boat will be going the same speed.

So .... if it were a drag race, the big engine wins.

But for obtaining and maintaining a speed that 6000 RPM produces, both are
the same in this particular example.

You are discounting inertia and momentum as a factor. More horsepower
will overcome base inertia and add momentum meaning that the larger
engine will have to work less to maintain, or even increase, speed
given the same RPM.

I'm trying to understand propellers
 

"Injam" wrote in message
k.net...
Why? If the propellers are turning at the same RPM and they have the same
pitch, why would the 250 hp be faster?


"Harry Krause" wrote in message
. ..
Injam wrote:
Hello,

I'm trying to understand propellers.

If you have two identical boats, but the outboard horsepower ratings
are different. One boat has a 200 hp and the other a 250 hp. Both motors
have the same Gear Ratio. If the propellers are identical and the RPM of
the outboard is the same on each boat. Say 6000 RPM. Because of the
weight advantage will the 200-hp boat be faster than the 250 hp? I'm
thinking less weight less slippage otherwise they would be going the
same speed.



Sincerely,

Injam




If the 200 hp engine is properly propped, the 250 hp engine will most
likely be underpropped with the same prop. The weight diff between a 200
hp and a 250 hp is not usually significant, certainly not enough t0 canel
out a 25% increase in hp.

Now, to your question...the 250 still might be faster, even if it is
underpropped.



It will be faster to the same speed. But, the 2 engines should not have the
same pitch prop. The 250 hp will overspeed with the correct pitch for the
200 hp. Same pitch and same RPM, other than a weight difference, both boats
would be the same speed.

I'm trying to understand propellers
 

"Calif Bill" wrote in message
ink.net...


It will be faster to the same speed. But, the 2 engines should not have
the same pitch prop. The 250 hp will overspeed with the correct pitch for
the 200 hp. Same pitch and same RPM, other than a weight difference, both
boats would be the same speed.


Thank you very much. I agree.

RCE

I'm trying to understand propellers
 

On Mon, 27 Feb 2006 00:49:23 -0500, "RCE" wrote:
One thing I've always feared about diesels is the very rare potential of a
"runaway". My wife's uncle was a vocational school diesel instructor and
told me of this condition whereby something goes wrong (I forget what) and
the diesel continues to increase revs until it basically self destructs. If
it happens, you have to quickly find a way to cut off the air supply or the
fuel before it goes "boom".

As I understand it, that can happen if the rings get badly worn to the
point where the engine is able to start running on residual crankcase
oil. Most of the 2 cycle Detroits are equipped with an emergency
shutdown device which is basically a spring loaded metal door which
closes off the air intake when activated. When we bought the boat,
the 671s in my Grand Banks had the shutdowns installed on the engines
but no remote cables for activating them. That was one of the many
upgrades that we added last year before heading north since the idea
of being up close and personal with a 3,000 lb runaway engine was not
too attractive.

I'm trying to understand propellers
 

"Wayne.B" wrote in message
...
On Mon, 27 Feb 2006 00:49:23 -0500, "RCE" wrote:
One thing I've always feared about diesels is the very rare potential of a
"runaway". My wife's uncle was a vocational school diesel instructor and
told me of this condition whereby something goes wrong (I forget what) and
the diesel continues to increase revs until it basically self destructs.
If
it happens, you have to quickly find a way to cut off the air supply or
the
fuel before it goes "boom".

As I understand it, that can happen if the rings get badly worn to the
point where the engine is able to start running on residual crankcase
oil. Most of the 2 cycle Detroits are equipped with an emergency
shutdown device which is basically a spring loaded metal door which
closes off the air intake when activated. When we bought the boat,
the 671s in my Grand Banks had the shutdowns installed on the engines
but no remote cables for activating them. That was one of the many
upgrades that we added last year before heading north since the idea
of being up close and personal with a 3,000 lb runaway engine was not
too attractive.


That was it. Running on it's own oil supply.

I had one educating experience with one of the Volvos on the Navigator the
first year I had it. We pulled into Indian River Marina in Delaware to
escape a heavy thunderstorm and I could not shut down one of the engines. It
sat idling for an hour while a Volvo tech in PA tried to walk me through it.
The problem was that every time I went into the engine room to do whatever
he was telling me, I couldn't hear him on the cell phone anymore. Finally
found the mechanical lever to manually shut off the fuel supply at the
engine and noted it's location, however the real problem was a tripped
"reset-able" fuse on the control box. Reset it and the solenoid that shuts
the fuel off now worked again with the keyswitch.

RCE

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...


I just went to the Mercury site and spec'd out a prop for an Sal****er
EF. I set up for good overall performance between a 200 and 250.

http://www.mercurymarine.com/prop_selector

Props were identical for a 20' bass boat at roughly 3000 pounds for
hole shot, mid-range performance and top speed.

There was a one degree pitch difference in a different type of prop.

So.......horsepower rules.



I think we are on two different wavelengths, and your's is short ....
Heheheh....

Are you saying that two identical boats .....
with identical weights .....
with identical shaft gearing in the engines ....
and identical props .....

one boat has 200 hp ....
the other, 250 hp ....

both are run side by side with the engines at a steady 6000 rpm ....

That the 250 hp boat is going to be moving faster?

No Way, Hosie ..

RCE

I'm trying to understand propellers
 

On Mon, 27 Feb 2006 08:56:22 -0500, "RCE" wrote:

the real problem was a tripped
"reset-able" fuse on the control box. Reset it and the solenoid that shuts
the fuel off now worked again with the keyswitch.

Yes, that's a different issue. It's good to know where that solenoid
is so that you can activate it manually if needed.