Exhaust/ silencer

[MtB]

On 03/04/2017 at 09:52, WotEver said:

Surely only if it's a 4 blade prop? If a three blade then it's only 3/8" off, no?

 

Have I misunderstood something?

If as proposed one takes 1/4" off the tip of each blade, no matter how many blades there are one has reduced the radius of the propeller by 1/4". This equals 1/2" off the diameter.

[WotEver]

1 minute ago, Mike the Boilerman said:

 

Have I misunderstood something?

If as proposed one takes 1/4" off the tip of each blade, no matter how many blades there are one has reduced the radius of the propeller by 1/4". This equals 1/2" off the diameter.

Oh yeah

[MtB]

On 03/04/2017 at 09:56, WotEver said:

Oh yeah

 

Lol this could rival the debate and confusion about how much less water a lock uses when you put a boat in it!

Even Daslandia who designs propellers got it wrong

[WotEver]

Just now, Mike the Boilerman said:

 

Lol this could rival the debate and confusion about how much less water a lock uses when you put a boat in it!

Obviously a big boat uses less water...

:::coat:::

[MtB]

On 03/04/2017 at 09:59, WotEver said:

Obviously a big boat uses less water...

:::coat:::

Going up? Or down? 

It will be different...

[WotEver]

2 minutes ago, Mike the Boilerman said:

Going up? Or down? 

It will be different...

It'll use less going down because of gravity. 

Have I covered all the misconceptions yet?

Oh no, I haven't... it'll empty quicker with a big heavy boat pushing the water out when going down too

[MtB]

Don't even begin to try to explain how a side pond works

[Dalslandia]

It would have been a April first joke, if I wrote it on the right day...

 

I suggest to start with a half inch on the diameter, that because it is what my thought and calculation say, and it is easier to take off material then add on.

 

Then for exhaust noise, using a pipe that goes straight down in to the water some inch, (with a 90 deg bend at the outlet from the transome) and drill a serie of small hole 1/8" on the rear side of the tube as a picola fluit. and form the end of the pipe as a oval, this sort of silencer is sometimes used on airplane. the holes will prevent it from suck water in when Engine is shut off.

 

The muffler Crow have is small, but there isn't much space for a bigger one under the "bonnet"

when lower one noise to the level of other noises, or less, the other noises need to to be reduced to lower the overall noise.

Edited April 3, 2017 by Dalslandia

[MtB]

I'm surprised you say tiny reductions in the diameter have such a big effect. I thought my suggestion of 1/2" off the tips was conservative. 

Agree with your point about it being far easier to grind it off though, than put it back! 

[Dalslandia]

 

5 minutes ago, Mike the Boilerman said:

I'm surprised you say tiny reductions in the diameter have such a big effect. I thought my suggestion of 1/2" off the tips was conservative. 

Agree with your point about it being far easier to grind it off though, than put it back! 

well it is a small prop to start with, at the same rpm the half inch diameter will reduce the Power demand to 82%

but it will only reduce the efficiency a Little. (2.5%)

the 1000/900 reduction in tickover rpm will reduce thrust by 20% also

Edited April 3, 2017 by Dalslandia

[MtB]

On 03/04/2017 at 10:28, Dalslandia said:

 

well it is a small prop to start with, at the same rpm the half inch diameter will reduce the Power demand to 82%

but it will only reduce the efficiency a Little. (2.5%)

Thanks. 

I wssn't disputing your calculations, just expressing surprise at the magnitude of the change. The diameter is 13" now IIRC, so a 3.9% reduction in diameter to 12.5" reduces the power demand by 18%. 

I commented earlier about how most of the work is done at the tips of a blade but I hadn't appreciated the degree to which this is true. You mentioned 5th power previously so I can see this is about right.

[Dalslandia]

Yes, if Everything els stay the same, the Power change with 5th Power on the diameter. (now the area will be reduced slightly too, on the other hand the aspect ratio go down, wider blade relative the blade length.)

If it stop smoking black the prop load will be right.

 

As Crow say, he don't want the efficiency to be as bad as with the outboard, so just enough is best. or Lagom, as the Swedish Word say.

Edited April 3, 2017 by Dalslandia

[MtB]

How does the power demand change with the pitch, if all else remains the same?

My guess is that's approximately linear other than at the extremes, (i.e. an almost flat blade or an over square blade).

Going back a step though, I seem to remember deciding long ago that reducing the diameter of a blade as we are discussing, also causes a change in the pitch, for a given propeller one is altering. Can't quite remember why/how I concluded this though. 

Edited April 3, 2017 by Mike the Boilerman

[Dalslandia]

There is no formula for pitch VS Power, just thumb of rule, but the thumb of rule vary with spokes person, and I Think the speed range of the boat. or different if over Square or under. seen both one inch diameter is same as 2 inch pitch, and the other way around.

If the propeller have constant pitch along the blade the diameter change don't change pitch, but if pitch is changing from the original 60% radii and in toward the hub,

Edited April 3, 2017 by Dalslandia

[MtB]

On 03/04/2017 at 10:56, Dalslandia said:

There is no formula for pitch VS Power, just thumb of rule, but the thumb of rule vary with spokes person, and I Think the speed range of the boat. or different if over Square or under. seen both one inch diameter is same as 2 inch pitch, and the other way around.

If the propeller have constant pitch along the blade the diameter change don't change pitch, but if pitch is changing from the original 60% radii and in toward the hub,

 

Hmmm yes I can see that would be the case if the blade angle increases towards to hub accurately to whatever is necessary to keep the pitch is the same, but on a typical NB propeller I suspect they don't. My suspicion is all blades use the same casting and during manufacture the outer three or four inches of blade get twisted to accurately to calibrate the prop and give it a particular pitch. Hence my suspicion that lopping bits off the tips might affect the pitch.

This seems especially likely given 80% of the work is done by the outer 20% of the blades. Or worse!

[Dalslandia]

The nominal pitch is said to be at 70% radii on boat props, (75% on airprops) what I can see in my air prop software, the center is Close to 65% on a typical 100-200 mph plane.)

those props with reduced pitch into the hub, from 50-60% radii, have some % better efficiency. the pitch is normally 60% of nominal at the hub.

I don't know how they make the typical small boat props, you might be right.

 

This is probably a ship propeller, (ship as in bigger boat)

 

Edited April 3, 2017 by Dalslandia

[Neil2]

1 hour ago, Dalslandia said:

Yes, if Everything els stay the same, the Power change with 5th Power on the diameter. (now the area will be reduced slightly too, on the other hand the aspect ratio go down, wider blade relative the blade length.)

If it stop smoking black the prop load will be right.

 

As Crow say, he don't want the efficiency to be as bad as with the outboard, so just enough is best. or Lagom, as the Swedish Word say.

I don't understand that Jan - I may be missing something so can you explain the calculation how you get 18% reduction in power from a 3.9% reduction in diameter?

[Dalslandia]

Just now, Neil2 said:

I don't understand that Jan - I may be missing something so can you explain the calculation how you get 18% reduction in power from a 3.9% reduction in diameter?

At the same RPM, for starter, if we let the rpm go up the pwer goes up naturaly, but say the governor is limiting the rpm to 3000 or so.

taking of blade area, diameter or pitch will naturally reduce the need of Power to get 3000 rpm.

 

12,5/13= 0,9615

0,9615^5 =  0,8217

[WotEver]

2 hours ago, Mike the Boilerman said:

Don't even begin to try to explain how a side pond works

That's easy. The heavy boat squirts the water from the lock into the pond above it. 

[Dalslandia]

How the water come back up in the system is the most common question we get from the Tourists.

Sometimes they ask if there not must be a different way back to the start Point, because you can't go down in the locks, can you?

Edited April 3, 2017 by Dalslandia

[Neil2]

24 minutes ago, Dalslandia said:

At the same RPM, for starter, if we let the rpm go up the pwer goes up naturaly, but say the governor is limiting the rpm to 3000 or so.

taking of blade area, diameter or pitch will naturally reduce the need of Power to get 3000 rpm.

 

12,5/13= 0,9615

0,9615^5 =  0,8217

Ok forgive my ignorance I have it now.

I contacted Crowthers last year with my boat/engine details and they suggested I should go from 18" dia to 16".  If I do the calculation above it gives a massive 45% reduction in power suggesting my boat is way overpropped.  I didn't have the nerve to make such a big change, it sounds like just grinding a fraction off the existing prop would make quite a difference.    

[Dalslandia]

I guess it boild down to how much off you are from start. then what you want to achive.

say you are down on rpm,

(but keep the pitch)

16/18= 0,8888^5=0,555

or 18/16=1,125^5 = 1,8

giving 21-22% more rpm if not rpm limited

But when changing a prop to a smaller prop and keeping the BAR ratio the blade area goes down.

Edited April 3, 2017 by Dalslandia

[WotEver]

17 minutes ago, Dalslandia said:

How the water come back up in the system is the most common question we get from the Tourists.

Sometimes they ask if there not must be a different way back to the start Point, because you can't go down in the locks, can you?

You have to wait for the heavier boats to squirt it back up

[MtB]

On 03/04/2017 at 12:59, WotEver said:

You have to wait for the heavier boats to squirt it back up

 

I think you'll find the water automatically flows back up to the opposite end of the canal each night, once it gets really dark.

[WotEver]

3 minutes ago, Mike the Boilerman said:

I think you'll find the water automatically flows back up to the opposite end of the canal each night, once it gets really dark.

Ahh yes, over the culverts and back to the top, I remember now.