I'm trying to understand propellers
 

On Mon, 27 Feb 2006 16:49:29 GMT, Shortwave Sportfishing
wrote:

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

You're looking at it the wrong way. The boat with the 250 hp engine
has the POTENTIAL to go faster but you have to change to a more
agressive prop, or run at a faster RPM, to make it happen. Otherwise
it's like two cars with the same tire diameter, wheels spinning at the
same speed. Doesn't matter what the horsepower is.

I'm trying to understand propellers
 

"Wayne.B" wrote in message
...
On Mon, 27 Feb 2006 16:49:29 GMT, Shortwave Sportfishing
wrote:

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

You're looking at it the wrong way. The boat with the 250 hp engine
has the POTENTIAL to go faster but you have to change to a more
agressive prop, or run at a faster RPM, to make it happen. Otherwise
it's like two cars with the same tire diameter, wheels spinning at the
same speed. Doesn't matter what the horsepower is.


Exactly. I was about to do the car analogy, but you beat me to it.

RCE

I'm trying to understand propellers
 

Aint no way Jose.
The higher HP engine could overrev with the prop matched to a lesser HP
engine. But, since revs are limited to 6000 on both engines, gear rarios are
both the same, and prop diam. pitch and cup are the same on both engines,
boat speed will be the same.
JIMinFL.
"Shortwave Sportfishing" wrote in message
...
On Mon, 27 Feb 2006 11:02:43 -0500, "RCE" wrote:


"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....

ROTFL!!!

Oh wait...

HEY!!!!

Are you saying that two identical boats .....

Yep.

with identical weights .....

Yep.

with identical shaft gearing in the engines ....

Yep.

and identical props .....

Yep.

one boat has 200 hp ....

Yep.

the other, 250 hp ....

Yep.

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

Yep.

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

Yep.

No Way, Hosie ..

Way, Jose..

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

You also have to consider how these boats got to 6000 RPM and I'll say
it again - horsepower wins everytime.

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

In this case, it's nothing but a gearing issue, including the props.


You also have to consider how these boats got to 6000 RPM and I'll say
it again - horsepower wins everytime.

We're in agreement here - the big engine will get to 6000 RPM faster.

RCE

I'm trying to understand propellers
 

"JIMinFL" wrote in message
ink.net...

Aint no way Jose.
The higher HP engine could overrev with the prop matched to a lesser HP
engine. But, since revs are limited to 6000 on both engines, gear rarios
are both the same, and prop diam. pitch and cup are the same on both
engines, boat speed will be the same.
JIMinFL.

I've got a feeling that by the time we convert Tom to think gears instead
of horsepower in this exercise, the original poster will be on his third or
forth boat and laughing his ass off at us.

RCE

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

You also have to consider how these boats got to 6000 RPM and I'll say
it again - horsepower wins everytime.


Ok. Time to try another analogy. (Pun intended)

Two 60 Hz, synchronous clocks.
One is running on 120 volts AC, 60 Hz.
The other is running on 110 volts AC, 60 Hz.

Is the 120 volt clock gonna run faster?

Hmmmmmmm.......

RCE

I'm trying to understand propellers
 

"RCE" wrote in message
...

"Shortwave Sportfishing" wrote in message
...

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

You also have to consider how these boats got to 6000 RPM and I'll say
it again - horsepower wins everytime.


Ok. Time to try another analogy. (Pun intended)

Two 60 Hz, synchronous clocks.
One is running on 120 volts AC, 60 Hz.
The other is running on 110 volts AC, 60 Hz.

Is the 120 volt clock gonna run faster?

Hmmmmmmm.......

RCE


An aside. Friend of mine moved to England and to manage an office there.
None of the local people would tell him why when he complained his clock was
running slow. And he was also an EE.

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...
On Mon, 27 Feb 2006 06:45:19 GMT, "Calif Bill"
wrote:


"Injam" wrote in message
ink.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.

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.

That's not the point. The point is that both run at 6000 RPM. Over
rev isn't an issue.

The issues as I understand them would point to horsepower being the
determination.

As long as you can turn that same prop at 6000 RPM the boat will move at the
same speed. If I put your vette engine in my boat, at 3200 RPM I am still
at the same speed as my 330 mpi. It does not know that there is an extra
300 hp available.

I'm trying to understand propellers
 

On Mon, 27 Feb 2006 12:52:27 -0500, Harry Krause
wrote:


I pointed this out last week. Also, the boat with the larger HP engine
will accelerate faster to its top speed:

Not true...flat out wrong

more torque, winds up to WOT
faster.

Huh? What does that mean? "WOT" is a throttle position, ie; "wide
open".

What kind of boat do you have again?
__

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

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

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...
On Mon, 27 Feb 2006 13:16:25 -0500, "RCE" wrote:


"Shortwave Sportfishing" wrote in message
. ..

I look at it this way. There is 50 more available horsepower at 6000
RPM. Thus, it makes total sense to me that given the same factors -
meaning inertia and momentum - that 50 additional horsepower driving
the props will give better speed.

You also have to consider how these boats got to 6000 RPM and I'll say
it again - horsepower wins everytime.


Ok. Time to try another analogy. (Pun intended)

Two 60 Hz, synchronous clocks.
One is running on 120 volts AC, 60 Hz.
The other is running on 110 volts AC, 60 Hz.

Is the 120 volt clock gonna run faster?

Hmmmmmmm.......

No, because the clock is dependent on slicing the 60 cycles and is not
dependent on voltage - in a perfect world that is.

Ok. In the exercise we have been discussing, under the conditions
stipulated to, both engines are slicing 6000 RPM in the same manner as the
clock. The extra 20 volts or 50 hp does not matter.

Hmmmmmmm???????


RCE