I'm trying to understand propellers
 

On Fri, 03 Mar 2006 06:19:04 GMT, "MGG"
wrote:

OMG people...this is so simple, I can't believe it's still going on. Two
boats, identical in every way *except* one has 50 more horses than the
other, turning a prop at 6000 rpm. At 6000 rpm, they will be traveling at
the exact same speed.

Correct, but one boat will either be under-propped, or over-propped.

Forget about the fact that the stronger engine may get
to speed faster...

That would be torque, not horse power.

__

"It's just about going fast...that's all..."

http://home.columbus.rr.com/ckg/

I'm trying to understand propellers
 

How often does it need to be repeated?

The OP stated that WEIGHT of the boats is different, so top speed is
different too.

Matt

I'm trying to understand propellers
 

On Fri, 03 Mar 2006 16:25:51 GMT, Shortwave Sportfishing
wrote:

Which is faster?

How hard is this to understand?

Faster acceleration, no question. That was not the original issue
however. Two cars, one with a more powerful engine, same RPM, same
drivetrain, same tires = same speed under steady state conditions.

The same is true for two boats with the same props and identical RPMs.

That's the difference, steady state conditions, as posited in the
original propellor post.

I'm trying to understand propellers
 

On Fri, 03 Mar 2006 17:12:17 GMT, Shortwave Sportfishing
wrote:

Correct, but one boat will either be under-propped, or over-propped.

Not necessarily. According to a Mercury prop site which I posted
earlier, for a prop designed for hole shot, mid-range performance with
high top end, there is no difference between a 200 and 250 Optimax.

There are two ways that engine horsepower can be increased. One is by
increasing torque (twisting force), the other is by increasing RPMs.
Horsepower is the net result of both torque and RPM, and how you get
there influences prop selection the same way that it influences gear
ratios in a car.

I'm trying to understand propellers
 

On 3 Mar 2006 09:28:48 -0800, "M" wrote:

The OP stated that WEIGHT of the boats is different, so top speed is
different too.

I don't believe that is true. The assumption was, as I remember it,
that the weight difference between the 200 and 250 hp engines was not
that great, if any.

I'm trying to understand propellers
 

"Wayne.B" wrote in message
...
On 3 Mar 2006 09:28:48 -0800, "M" wrote:

The OP stated that WEIGHT of the boats is different, so top speed is
different too.

I don't believe that is true. The assumption was, as I remember it,
that the weight difference between the 200 and 250 hp engines was not
that great, if any.



This whole thread has become confused and out of control. :-)

RCE

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...

Look at it this way - which car has more momentum? Obviously the car
with the lower ET. If you shut off the engines exactly at 6000 rpm,
which car will continue to accelerate?

It's the same issue with the boat question and it's even more relevant
because of momentum. The fact that one boat with more horsepower
imparts more momentum means that at 6000 RPM it will be faster when it
reaches it's limits.


Two words, Tom.

Steady state.

No acceleration. Momentum and inertia of each boat is the same, other than
the slight weight difference of the engines.

That was the original, stipulated condition.

RCE

I'm trying to understand propellers
 

Shortwave Sportfishing wrote:
On 3 Mar 2006 09:28:48 -0800, "M" wrote:

How often does it need to be repeated?

The OP stated that WEIGHT of the boats is different, so top speed is
different too.

That was an assumption, not a constant. His assumption was wrong. The
only true difference was horsepower.

I dont think the OP explained clearly what he really wanted to compare.
Thus the big discussion about the issue...

I understand he wanted to show a weight difference with the different
motors rather than a power difference. Thus the 6000 RPM limit...


The question should be rephrased before this gets out of hand .. .

Matt

I'm trying to understand propellers
 

"M" wrote in message
oups.com...


I understand he wanted to show a weight difference with the different
motors rather than a power difference. Thus the 6000 RPM limit...


The question should be rephrased before this gets out of hand .. .

Matt



It's already out of hand.

The OP posed the following question (paraphrasing)

Two identical boats, identical props, identical motor shaft gears, both
running at 6000 RPM.
One has 200 hp. The other 250 hp. Which boat is moving faster? The slight
weight difference between the engines was stipulated as a non-issue.

Answer: Side by side, "steady state" (not accelerating) both are going
the same speed.

Other inputs that may or may not be valid include the fact that the bigger
engine will accelerate to 6000 RPM faster, prop slip may be different, and
other variables where not part of the original conditions set for sake of
the original question and discussion.

That's the last of this that I want to think about.

RCE

I'm trying to understand propellers
 

agree with you ...