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.