Adjusting prop pitch .. Doesit make a difference?

[Nostrodamus]

Thank you for all your replies which helps me understand the problem a little more. There appar to b no definative answer but it seems as though sometimes we either just have to experiment or compromise.
My engine as was suggested seems to rev to about 3600 which when matched to the old Volvo 120 leg was fine. It is the new leg and gear ratios that is causing me real problems and now the 120 leg is out of production others may well have the same problem.

old leg gear ratio was 2.24:1
new leg ratio is 2.19:1

I know I need more bite when in marinas etc. In no winds or strong tides I can take things easy but if I really wanted some grip when manouvering forward there is nothing there.
Pitching the prop up is going to loose me 200 RPM for each notch. This would be good for manouverability but may strain the engine or loose me the ability to be able to punch through a sea and may leave me feeling I am in 5th gear all the time.
I try, like everyone not to be in really rough weather but at some point I will be. I usually cruise at 2,600 revs.
I am still in a dilema as to how best to cure the problem and have even concidered changing from the Bruntons variprofile i have to a fixed prop.
I suppose the only thing I can do is try changing the pitch and see but I don't want to spend a fortune adjusting something if then I have to adjust it back.

 

[Momac]

old leg gear ratio was 2.24:1
new leg ratio is 2.19:1

QUOTE]

That just 2.2% difference in gear ratio.
So the prop is turning 2.2% faster than with the old leg.
With the old leg was everthing okay?
What was the prop diameter and pitch with the old leg and what is it now?

 

[cmedsailor]

If you are saying that your engine's recommendation is 3,500-4,000 and you want to operate it at 3,800RPM then you don't have a problem. .

What is the meaning of this "range of maximum rpm"? Does it mean that any rpm within that range are OK?
For instance the manual of my MD2030D says maximum rpm 3200-3600 rpm. I can achieve (easily) 3500 rpm. Does the range "allows" me to use a stronger pitch propeller and achieve only 3300rpm with no problem?

 

[theoldsalt]

Cmedsailor,

If your boat already achieves "max hull speed" at max throttle then increasing the pitch of the prop may achieve little at high revs but probably increase fuel consumption. At low revs it might give the prop a little more "bite".

What are you trying to achieve by adding prop pitch and reducing max rpm?

 

[cmedsailor]

What are you trying to achieve by adding prop pitch and reducing max rpm?

Higher speed at cruising rpm (2400-2500rpm) or probably same speed at slightly less "cruising rpm" (thus lower consumption) as long as I am not oveloading the engine by been over-prop and also achieve the accepted, by the manufacturer, maximum rpm.

 

[Nostrodamus]

Cmedsailor,

If your boat already achieves "max hull speed" at max throttle then increasing the pitch of the prop may achieve little at high revs but probably increase fuel consumption. At low revs it might give the prop a little more "bite".

What are you trying to achieve by adding prop pitch and reducing max rpm?

If I had a choice I would not be trying to reduce RPM but I do need more bite.
A consequence of increasing the pitch will be a lowering of the available RPM.

The prop details I have are here
http://variprofile.de/en/index.html

 

[cmedsailor]

If I had a choice I would not be trying to reduce RPM but I do need more bite.
A consequence of increasing the pitch will be a lowering of the available RPM.
http://variprofile.de/en/index.html

It wasn't really my intention but this range of max rpm, which never really paid any attention before reading this thread and the idea of having a better speed at same cruising without overloading the engine (unless I am thinking wrong of course) made me thinking. I am planning on the next lift out to change my 16X11 2-blade with a 16X10 3-blade and I always though that the current max of 3500rpm is a must.

 

[theoldsalt]

As I have stated before the important engine characteristic is the power curve.

The amount of power that can be extracted from an engine depends on the load applied. Power can be extracted up to the max determined by the power curve at any given rpm. Load is applied to the engine via the prop, but a fixed prop is a compromise as it cannot be right at all revs. Variable pitch props is an attempt to overcome this. So it is important with a fixed prop to decide under what condition you need it to be best suited.

I have attached a typical power curve.

With a diesel engine the "throttle" is really a rev selector. It is the governor/injector pump that is really the "throttle". So imagine an engine running at constant rpm and the load (prop pitch) gradually increased. The governor would maintain the rpm roughly constant while the power produced by the engine would increase due to the governor instructing the injector pump to inject more fuel. This could continue until the power curve is reached at that rpm, after which the engine rpm would start to fall. This is when the engine is overloaded. The engine fails to burn all the fuel injected and produces a dirty exhaust.

So it should now be easier to understand the effect of changing prop pitch/diameter/blade number and area, all of which affect load.

An engine can be operated anywhere below the power curve depending on "throttle" and load.

 

[Pye_End]

If I had a choice I would not be trying to reduce RPM but I do need more bite.
A consequence of increasing the pitch will be a lowering of the available RPM.

Do you want more bite when the boat is moving at a reduced speed (eg punching seas or manoevering in a marina), or at cruising speed? If the former then a reduced pitch could work.

 

[Nostrodamus]

Do you want more bite when the boat is moving at a reduced speed (eg punching seas or manoevering in a marina), or at cruising speed? If the former then a reduced pitch could work.

Do you mean increased pitch rather than reduced pitch?

In marinas or at slower speed is where I need the bite but in an ideal world it would be more bite and more speed

 

[Pye_End]

Do you mean increased pitch rather than reduced pitch?

In marinas or at slower speed is where I need the bite but in an ideal world it would be more bite and more speed

As theoldsalt said, the propellor is a compromise. What is better for full RPM at full hull speed is not best for when the boat is hardly moving, which is one of the reasons why the Bruntons Autoprop is so popular - it adjusts the pitch according to the different conditions.

If you think about the way that pitch is calulated, it looks at the hull speed; RPM and the slippage. Therefore at slower hull speeds you are better off with less pitch. For best slow speed grunt you want plenty of surface area, and low pitch, but this is not what you want at hull speed, or when you are sailing.

 

[bedouin]

Do you mean increased pitch rather than reduced pitch?

In marinas or at slower speed is where I need the bite but in an ideal world it would be more bite and more speed

I am very confused as to why you think you want more pitch - in my opinion a finer pitch is almost always better.

Rather like gears on a car, you can't have more torque and more speed. Finer pitch = lower gear - and would probably give better handling at slow speeds. Coarser pitch = higher gear, better top speed but likely to give poorer handling and slow speed and possibly more prop walk and the like

 

[ianj99]

I am very confused as to why you think you want more pitch - in my opinion a finer pitch is almost always better.

Rather like gears on a car, you can't have more torque and more speed. Finer pitch = lower gear - and would probably give better handling at slow speeds. Coarser pitch = higher gear, better top speed but likely to give poorer handling and slow speed and possibly more prop walk and the like

WRONG!

Fine pitch = more revs, more noise, less responsivness, less 'bite' from the prop.

SInce fitting a Kiwi prop, the handling of my 10t long keel stell ketch has been transformed:
1) Manoeuvering much better - just needs a very brief burst to push the stern out without picking up speed.
2) Going astern to slow down actually works.
3) Opening the throttle in either direction can be felt to be doing something. With the previous finer pitched fixed prop, I could have sworn the clutch plates in the gearbox were slipping such was the barely detectable change in velocity when opening the throttle.
Downside - there is more pronounced prop walk going astern but if you know what to expect, you can allow for it!
Ian

 

[ianj99]

Boat speed and prop pitch calculators

You can play aorund with these calculators to see how much power is needed for your boat's hull speed and then find the prop pitch. You need the engine power curve as well and the 4108's is attached.

http://www.psychosnail.com/boatspeedcalculator.aspx

http://www.castlemarine.co.uk/pitch.htm

Don't take the results as gospel, but you can compare various options for prop size and pitch versus engine rpm.

As an example using my boat: Hull speed of about 7kts needs 33bhp. From the power chart, the 4108 makes 33bhp at 2200rpm. So putting these and the reduction ratio of 1:1.792 into the prop size program, produces a prop of about 17 x 14. My Kiwi prop is a 17" x 22degrees - equivalent to about a 13-14" pitch so the result is very close. The engine revs quickly to 2000rpm and then maxes out at about 2150 for a speed of 6.8kts (gps), whilst cruising at 6kts at 1800, 6.5kts is at 2000rpm.

The previous fixed bladed prop was a 17" x10" and gave poor performance compared to the Kiwi.

 

[NormanS]

Phew!, I'm glad I've got a Brunton's Autoprop to do all that for me.

 

[Nostrodamus]

Now I am confused.
One says I need finer pitch for more bite and another says more. It has to be more.. dosen't it?

 

[bedouin]

Now I am confused.
One says I need finer pitch for more bite and another says more. It has to be more.. dosen't it?

The problem is you haven't defined what you mean by "bite" and it is not a generally understood term. It may be that in fact what you need is not a change in pitch but more DAR

Basically

Fine pitch = max thrust
Coarse pitch = max speed, lower rpm, better economy

 

[Nostrodamus]

The problem is you haven't defined what you mean by "bite" and it is not a generally understood term. It may be that in fact what you need is not a change in pitch but more DAR

Basically

Fine pitch = max thrust
Coarse pitch = max speed, lower rpm, better economy

So where did DAR suddenly slip in and who is he.
My problem is that if going astern and I give it a burst of power forward very little happens for quiet a while. The best description was given by the posterwho said it was like the feeling of a clutch slipping on a car.

 

[bedouin]

So where did DAR suddenly slip in and who is he.
My problem is that if going astern and I give it a burst of power forward very little happens for quiet a while. The best description was given by the posterwho said it was like the feeling of a clutch slipping on a car.

Well IMHO that particular problem is better fixed with lower (finer) pitch - rather like using a lower gear on a car.

 

[CelebrityScandel]

Basically

Fine pitch = max thrust
Coarse pitch = max speed, lower rpm...

As a generalisation that is correct. It cannot be refuted, but I suspect some will try .
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The original poster's own situation depends on the boat hydrodynamics, displacement, aperture size, engine horsepower, etc and the propeller's own characteristics at low and higher advance speeds (it is said by the prop builder to be "optimized" so who knows what those characteristics are) and RPM's, and may be best improved by pitch change, diameter change or a combination of both.

I assume that out of all that the prop builder has recommended a pitch increase but has said that max engine RPM will drop by 200? If he has not said that then the OP is just guessing.

As I mentioned before I am surprised that the prop builder cannot solve this for the OP (assuming they recommended the specific prop in the first place, if they did not then the OP is on his own). They are far more likely to be able to than any amount of discussion in a forum unless someone has the same boat, engine and prop and can speak objectively from experience of that combination.