I'm trying to understand propellers
 

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

I'm trying to understand propellers
 

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.

I'm trying to understand propellers
 

Q. What's a propeller?
A. A machine for propelling an aircraft or boat, consisting of a
power-driven shaft with radiating blades that are placed so as to thrust air
or water in a desired direction when spinning.

Now why would the 250 hp be faster? It would seen to me that in order to go
faster the propeller would have turn faster, but that are not.


"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 19:40:19 GMT, "Injam"
wrote:

Hello,

I'm trying to understand propellers.

What's a propeller?

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.

Identical boats, identical props, identical RPMs, it going to be the
more horsepower that wins the day.

I'm trying to understand propellers
 

It would seem to me that in order to go faster the propeller would have to
spin faster.

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

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.

Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually significant,
certainly not enough to canel out a 25% increase in hp.

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

Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

I'm trying to understand propellers
 

Think of it this way.

If I'm screwing a wood screw into a piece of wood and my friend Charley is
doing the same thing. The wood is the same and the pitch on the screws are
the same. Screws are identical, but Charley is a lot stronger than I am. We
turn the screws at the same RPM. Will Charley screw his screw in before me?

"Injam" wrote in message
ink.net...
It would seem to me that in order to go faster the propeller would have to
spin faster.

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

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.

Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually significant,
certainly not enough to canel out a 25% increase in hp.

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

Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...

Identical boats, identical props, identical RPMs, it going to be the
more horsepower that wins the day.


He's asked a good question. Assuming equal prop slippage, (or for the sake
of exercise, assume none), I think both engines would theoretically run the
boats at the same speed at a given RPM. The fact that the 250 hp engine
being a bit heavier might contribute to a bit more slippage in the real
world and may, indeed, be slower than the 200 hp.

Not to be confusing here... what I am saying is that if you treat the
engine and prop as a lossless, slipless gear train, then both engines should
produce the same speed on the same boat with the same props.

I think.

RCE

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

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.

Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually significant,
certainly not enough to canel out a 25% increase in hp.

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

Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

I hear what he's saying .... he's treating it like a fixed gear train and
both engines produce enough power to turn the prop at 6000 RPM. The speed
would likely be the same, with a possibly a slight edge to the smaller
engine.

I agree with Harry's point. It might be possible to increase the pitch of
the prop on the larger engine to take advantage of the extra 50 horses.
That boat would now be faster at 6000 RPM due to a lower gear ratio.

RCE

I'm trying to understand propellers
 

"Injam" wrote in message
ink.net...

It would seem to me that in order to go faster the propeller would have to
spin faster.


Or the same speed with a higher pitched prop. In your example, 6000 RPM is
about max for most outboards I know of. If the engine has enough extra
power to swing a higher pitched prop at 6000 RPM, it will be faster, for
sure.

RCE

I'm trying to understand propellers
 

wrote in message
oups.com...

Injam wrote:
Think of it this way.

If I'm screwing a wood screw into a piece of wood and my friend Charley
is
doing the same thing. The wood is the same and the pitch on the screws
are
the same. Screws are identical, but Charley is a lot stronger than I am.
We
turn the screws at the same RPM. Will Charley screw his screw in before
me?


Yes, because Charley is able to apply greater downward pressure on the
screw as he turns it and thereby increase the efficiency, even if the
torque and speed is identical. But that doesn't apply to your boat
example.


I'd have to chew on that one for a while, Chuck. There's not much efficiency
loss (i.e. slippage) when turning a screw into wood. Charley just won't be
as tired at the end of the day.



The higher HP engine could produce more torque, but the amount of work
being done to turn the prop 6000 rpm is not a variable in your example.
If the prop is turning 6000 rpm and all the other factors are the same
the two boats would travel the same distance per rotation of the prop.


Yep.

RCE

I'm trying to understand propellers
 

So you are saying that the heavier motor would not slip as much as the
lighter one?


wrote in message
oups.com...

Injam wrote:
Think of it this way.

If I'm screwing a wood screw into a piece of wood and my friend Charley
is
doing the same thing. The wood is the same and the pitch on the screws
are
the same. Screws are identical, but Charley is a lot stronger than I am.
We
turn the screws at the same RPM. Will Charley screw his screw in before
me?


Yes, because Charley is able to apply greater downward pressure on the
screw as he turns it and thereby increase the efficiency, even if the
torque and speed is identical. But that doesn't apply to your boat
example.

The higher HP engine could produce more torque, but the amount of work
being done to turn the prop 6000 rpm is not a variable in your example.
If the prop is turning 6000 rpm and all the other factors are the same
the two boats would travel the same distance per rotation of the prop.

I'm trying to understand propellers
 

On Sun, 26 Feb 2006 20:43:43 GMT, "Injam"
wrote:

It would seem to me that in order to go faster the propeller would have to
spin faster.

Given your conditions of identical boats, props and gear ratios, that
is correct. The only other variable is "slip" and that should be
about the same unless the bigger engine is significantly heavier.

If you want to go faster with the larger engine you will either need
to run at higher RPMs or switch to a prop with more pitch, more blade
cuppage, or both.

There is an excellent book available on propellers if you want to
learn mo

The Propeller Handbook: The Complete Reference for Choosing,
Installing, and Understanding Boat Propellers by Dave Gerr.

Available at www.amazon.com

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 16:04:43 -0500, "RCE" wrote:



I don't disagree with anything that's been said so far - everybody has
made great points. But I'm firmly in the camp of horsepower vs rpm.

With less horsepower, all else being equal, it makes sense to me that
the engine will transmit less direct power to the prop, thus more
slippage, thus less speed.

Well, this is fine how-do-you-do! We disagree.

The magic part of this exercise is the 6000 RPM. Again, all else equal,
including prop pitch, both boats are going to move the same distance in the
same time.

The bigger engine will get to 6000 RPM faster ... and *it* will probably
produce more prop slip in the process. Not unlike burning rubber in your
Vette. Get fatter or sticky-er tires and your slippage will decrease.

RCE

I'm trying to understand propellers
 

"Wayne.B" wrote in message
...

There is an excellent book available on propellers if you want to
learn mo

The Propeller Handbook: The Complete Reference for Choosing,
Installing, and Understanding Boat Propellers by Dave Gerr.

Available at www.amazon.com



Aw,,, Wayne had to spoil it. We were having fun confusing the banannas out
of him and ourselves.

RCE

I'm trying to understand propellers
 

Say you were going to buy a Triton 220LTS. You have a choice of Yamaha
F250hp V6 Four Stroke or F225hp V6 Four Stroke. If you went with the 250 hp
what diameter and pitch propeller would you buy? Also what diameter and
pitch propeller would you buy for the 225 hp?

"Injam" wrote in message
ink.net...
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

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 20:57:58 GMT, "Injam"
wrote:

If I'm screwing a wood screw into a piece of wood and my friend Charley is
doing the same thing. The wood is the same and the pitch on the screws are
the same. Screws are identical, but Charley is a lot stronger than I am.
We
turn the screws at the same RPM. Will Charley screw his screw in before
me?

If he's bigger and stronger, I'd bet on Charley. Even with the same
revolutions per minute, the depth of the stroke would be longer
because of Charley's available strength.

But, the total number of turns to screw it in will be the same.

RCE

I'm trying to understand propellers
 

Injam wrote:
So you are saying that the heavier motor would not slip as much as the
lighter one?


No. I'm saying that the greater amount of downward pressure applied
when turning a screw into wood, the more efficiently it drives. Much
different than you boat example.

We're not talking about the "engine" slipping at all. We're talking
about the prop slipping. All props slip. Two props of idential diameter
and pitch will slip the same amount at the same RPM in the same medium.

I'm trying to understand propellers
 

What if we ran these boats for 100 miles?
Who would get there first?


"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 16:35:18 -0500, "RCE" wrote:


"Shortwave Sportfishing" wrote in message
. ..
On Sun, 26 Feb 2006 16:04:43 -0500, "RCE" wrote:



I don't disagree with anything that's been said so far - everybody has
made great points. But I'm firmly in the camp of horsepower vs rpm.

With less horsepower, all else being equal, it makes sense to me that
the engine will transmit less direct power to the prop, thus more
slippage, thus less speed.

Well, this is fine how-do-you-do! We disagree.

The magic part of this exercise is the 6000 RPM. Again, all else equal,
including prop pitch, both boats are going to move the same distance in
the
same time.

The bigger engine will get to 6000 RPM faster ... and *it* will probably
produce more prop slip in the process. Not unlike burning rubber in your
Vette. Get fatter or sticky-er tires and your slippage will decrease.

Again, I'm not disagreeing with anything that's been said as it all
makes sense to me.

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?

I'm trying to understand propellers
 

"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.

RCE

I'm trying to understand propellers
 

"Injam" wrote in message
ink.net...
What if we ran these boats for 100 miles?
Who would get there first?



The smaller engine because the big one will run out of gas.

RCE

I'm trying to understand propellers
 

But it does not.

"Harry Krause" wrote in message
. ..
RCE wrote:
"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

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.
Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually
significant,
certainly not enough to canel out a 25% increase in hp.
Now, to your question...the 250 still might be faster, even if it is
underpropped.
Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

I hear what he's saying .... he's treating it like a fixed gear train and
both engines produce enough power to turn the prop at 6000 RPM. The
speed would likely be the same, with a possibly a slight edge to the
smaller engine.

I agree with Harry's point. It might be possible to increase the pitch
of the prop on the larger engine to take advantage of the extra 50
horses. That boat would now be faster at 6000 RPM due to a lower gear
ratio.

RCE

If the 200 is properly propped to hit 6000 rpm at WOT, then the 250 is
going to over-rev 6000 with the same prop.

I'm trying to understand propellers
 

"Harry Krause" wrote in message
. ..

If the 200 is properly propped to hit 6000 rpm at WOT, then the 250 is
going to over-rev 6000 with the same prop.

Yup.

RCE

I'm trying to understand propellers
 

On Sun, 26 Feb 2006 21:45:35 GMT, Shortwave Sportfishing
wrote:

Ah what does he know - he owns a Grand Banks. :)

With a really big (and expensive) pair of propellers.

I'm trying to understand propellers
 

On Sun, 26 Feb 2006 16:35:18 -0500, "RCE" wrote:

The bigger engine will get to 6000 RPM faster ... and *it* will probably
produce more prop slip in the process. Not unlike burning rubber in your
Vette.

Yes, because the bigger engine will presumably have more torque.

I'm trying to understand propellers
 

On Sun, 26 Feb 2006 21:48:29 GMT, "Injam"
wrote:

What if we ran these boats for 100 miles?
Who would get there first?

100 miles at the same speed, uhhhh, that's a tough one.

Got it.

Who ever started first. :-)

I'm trying to understand propellers
 

On Sun, 26 Feb 2006 21:39:48 GMT, "Injam"
wrote:

Say you were going to buy a Triton 220LTS. You have a choice of Yamaha
F250hp V6 Four Stroke or F225hp V6 Four Stroke. If you went with the 250 hp
what diameter and pitch propeller would you buy? Also what diameter and
pitch propeller would you buy for the 225 hp?

Picking the right propeller for a given boat and engine is as much art
as science. It even gets down to how much fuel, and how many
passengers you expect to carry, and what conditions do you want to
optimize. I would go with dealer or manufacturer recommendations for
a starter.

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...

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.

Agreed. But in this exercise all we need to deal with is sufficient power
to overcome base inertia to achieve 6000 RPM and we have stipulated that the
200 hp engine can, for purposes of discussion. The rest is gearing.

RCE

I'm trying to understand propellers
 

Thanks to all who responded. It helped---------kinda.

Sincerely,
Injam

"Wayne.B" wrote in message
...
On Sun, 26 Feb 2006 21:39:48 GMT, "Injam"
wrote:

Say you were going to buy a Triton 220LTS. You have a choice of Yamaha
F250hp V6 Four Stroke or F225hp V6 Four Stroke. If you went with the 250
hp
what diameter and pitch propeller would you buy? Also what diameter and
pitch propeller would you buy for the 225 hp?

Picking the right propeller for a given boat and engine is as much art
as science. It even gets down to how much fuel, and how many
passengers you expect to carry, and what conditions do you want to
optimize. I would go with dealer or manufacturer recommendations for
a starter.

I'm trying to understand propellers
 

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 17:46:11 -0500, Wayne.B
wrote:

On Sun, 26 Feb 2006 21:45:35 GMT, Shortwave Sportfishing
wrote:

Ah what does he know - he owns a Grand Banks. :)

With a really big (and expensive) pair of propellers.

On a slow boat to China. :)

Did I mention to you that my lovely and long suffering wife was so
impressed with your boat that she's been beating on me to look into
obtaining one?

I've been telling here that it's way to expensive and besides, I can't
tow it.

~~ mutter ~~

Bring her over to Kingman this spring or summer. Mrs. E. would love to give
her a tour of a smaller cousin to Wayne's boat. Maybe we'll take it out and
"open 'er up". All the way to 8.5 kts.

RCE

I'm trying to understand propellers
 

"Injam" wrote in message
ink.net...
Thanks to all who responded. It helped---------kinda.

Sincerely,
Injam


Injam,

Like Wayne said, the right prop for your boat is going to be a bit of an
experiment exercise. If you are buying a new boat, confirm with the dealer
that he will assist you in obtaining the optimum pitch after you take
delivery and have some experience with it. A 25 hp difference in engines is
not going to be a huge factor. Things like how you load the boat, how many
people typically ride with you, etc. will probably have more of an affect.
The simple method is to determine what pitch allows your engine to achieve
it's recommended maximum RPM at WOT. I think you will find that a
compromise of sorts will be required though because the ideal pitch will
change, again based upon how you load the boat.

I had an old Century I/O years ago and had two props. I used the higher
pitch when I was just cruising by myself or with one other person. If four
or more people showed up for a ride, I changed the prop to a lower pitch.

RCE

I'm trying to understand propellers
 

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?

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 18:12:42 -0500, "RCE" wrote:


"Injam" wrote in message
hlink.net...
Thanks to all who responded. It helped---------kinda.

Sincerely,
Injam


Injam,

Like Wayne said, the right prop for your boat is going to be a bit of an
experiment exercise. If you are buying a new boat, confirm with the dealer
that he will assist you in obtaining the optimum pitch after you take
delivery and have some experience with it. A 25 hp difference in engines
is
not going to be a huge factor. Things like how you load the boat, how
many
people typically ride with you, etc. will probably have more of an affect.
The simple method is to determine what pitch allows your engine to achieve
it's recommended maximum RPM at WOT. I think you will find that a
compromise of sorts will be required though because the ideal pitch will
change, again based upon how you load the boat.

I had an old Century I/O years ago and had two props. I used the higher
pitch when I was just cruising by myself or with one other person. If
four
or more people showed up for a ride, I changed the prop to a lower pitch.

Very true and well stated.

The thing I would add is that most props are compromises anyway for
the exact reasons RCE, Wayne B and other have mentioned.

One of the things I did with the Ranger when I bought it was pretty
simple with regard to props. I asked what was the best prop for speed
and the best for load and split the difference.

That's what your dealer will probably do anyway.

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.