Lower prop pitch to reduce fuel consumption theory..

[wipe_out]

I have a theory that I am interested in opinions on..

Early in the year I had a new engine fitted (5.7L with 4 barrel carb) and new anti-foul applied.. I found that with the new engine the old prop didn't have enough bite and was cavitating and over revving.. So I switched from a 17" pitch 3 blade to a 17" pitch 4 blade and it worked brilliantly..

Now there is some fouling on the bottom naturally the boat is taking more throttle to maintain it's 22-24kn cruising speed, usually at about 3200rpm.. With a clean bum and the new prop she would rev to about 4500-4600rpm and do 35kn.. Now she won't really go past 4200rpm flat out..

So my theory is that because there is more load on the engine now at cruising speed it's loading up the engine more and so opening the vacuum secondary more on the carb and so using more fuel..

If I was to fit say a 15" pitch 4 blade it would drop the load on the engine and therefore reduce the vacuum in the manifold so potentially not open the secondary as much.. Obviously to maintain the same speed I would be increasing the RPM to probably somewhere around 3500rpm but the WOT RPM would also increase from 4200rpm to something like 4600rpm (not that WOT is relevant because I hardly ever run flat out) meaning more available rev range..

So what would theoretically be more efficient.. Higher engine load with lower RPM and lower engine load with higher RPM at the same cruising speed??

 

[Canopy Locked]

I'm no expert at all, but have recently switched from an inboard shaft to an outboard, I've become interested in the science/art of propellers.

There is a definite school of thought that says that, "as the manufacturer states what the RPM range should be at WOT, if the engine cannot achieve that RPM, then the boat is probably over propped - Ie: prop is too big a pitch."

People say that they hardly ever run at WOT, but that's not the point... if the engine cannot reach the desired RPM at WOT, then it will be "lugging" throughout the engine range leading to increased fuel consumption / wear etc. I have heard it being likened to driving up a long hill on a motorway, in 5th gear towing a caravan.

With regards to changing from a 3 to a 4 blade prop - I think I'm correct in saying that, for an engine, swinging a 4 blade prop of x" pitch is equivalent to swinging a 3 blade prop of x"+2 pitch - so I'm surprised that you found it so much better - although the low speed handling in the marina would probably have been better with a 4 blade.

It would certainly be worth looking at the shape of the prop (the amount of cupping) and the material - SS is stiffer than Ali.

 

[wipe_out]

People say that they hardly ever run at WOT, but that's not the point... if the engine cannot reach the desired RPM at WOT, then it will be "lugging" throughout the engine range leading to increased fuel consumption / wear etc. I have heard it being likened to driving up a long hill on a motorway, in 5th gear towing a caravan.

This is my thinking and the reason for formulating the theory..

With regards to changing from a 3 to a 4 blade prop - I think I'm correct in saying that, for an engine, swinging a 4 blade prop of x" pitch is equivalent to swinging a 3 blade prop of x"+2 pitch - so I'm surprised that you found it so much better - although the low speed handling in the marina would probably have been better with a 4 blade.

The new engine is more powerful than the original which is why the original prop was over revving and the 4 blade of the same pitch was better..

It would certainly be worth looking at the shape of the prop (the amount of cupping) and the material - SS is stiffer than Ali.

Material is Ali.. It's a Solus Rubex prop 14.5"x17".. There is definitely more cupping than the original 3 blade but overall the blades look far more hydrodynamic than the original prop..

Props are definitely more art than science I think..

 

[CaptainMike1970]

We went through this with our last boat which at the sart of the season would hit he manufactures WOT max revs of 3,800rpm but mid season would struggle to get out of the hole and WOT was 3,400rpm at best. The boat was a bit under powered / marginally powered so mid season was always a bit of a struggle fully loaded.

We considered fitting hydofins to the anti-cavitation plate to give more stern lift, dropping a prop size to get back to 3,800rpm at WOT but in he end decided the best hing to do was just research anti- foul and find something more suitable to where we berth to reduce fouling and get a mid season lift and scrub. That way we had all 3,800 rpm to play with and the design top speed!

 

[ontheplane]

Clean the bum don't change the prop!!! All this is telling you is that the bum is too dirty, using more fuel and working the engine too hard.

 

[RobWales]

Clean the bum don't change the prop!!! All this is telling you is that the bum is too dirty, using more fuel and working the engine too hard.

+1

 

[wipe_out]

Ok.. Message received on the bum..

For interests sake let's forget the state of the bum, or even that it's a prop question..

If a specific engine application requires X hp to do what it needs to do (in this case move the boat constantly at 22kn)..

Would a petrol engine be more efficient at 3200rpm using ~85% throttle or 3500rpm using ~70% throttle?

 

[MapisM]

I'm afraid there isn't a theoretically correct answer to this question, because the only thing you can be sure of is that your hull requires - for any given prop, load, fouling, and sea conditions - the same power to cruise at 22 knots (or whatever), regardless of the rpm.
So, whether your specific engine burns a bit more/less of fuel at 3200/3500 rpm with a higher/lower load respectively, that's anybody's guess.
But don't expect the difference to be relevant, anyway, since the engine is still producing X HPs, at both the lower and the higher rpm.
Btw, what's the ratio of your outdrive?
By heart, 35kts @ 4600rpm with a 17" prop makes me think that either you've got an unusual ratio outdrive, or the prop slip is too high.

 

[Spi D]

Lots of good and correct answers.

Your engine will burn fuel in relation to the work carried out, as stated. For a calculating excercise: Use 200 grams of fuel per kW per hour and factor in the weight of petrol to get gallons/liters.

What you use the power for doesn't matter - the engine could be running any device. To move your boat you need to overcome a number of factors like displacing water, wind, current etc. The power needed depends on the hull shape, condition (preferably smooth like a baby's bum) and not least the gross weight of the vessel.

If you aim for optimal performance, all of these factors must be considered. The easiest (and free (!)) gain is from removing dead weight. The second is to keep the hull clean. Third is to cruise at economical speeds (ie, when fuel is used to move the boat, not to keep bow high or dragging a huge wash). Outboards have a benefit here as they can be run higher on the boat, hence leaving less in the water to create drag. Even to an extent where the prop breaks the surface and works in a blend of water and air - designed for exactly that.

The engine is designed to deliver max. power and cope with work for a long time, provided conditions are kept within specific limits. Load on the thing is a key point and that is defined by 'when the engine can reach xx rpm at WOT, the load is so that things will work optimally'. Now, the only way in a boat is compromise, since we have only one gear and can't shift while under way like in a car, we can't combine engine rpm and torque to match various loads. The best blend for the compromise is built into the max. rpm range at WOT, too.

To ensure proper rpm range is achieved, the prop size (dia & pitch) is matched. To ensure the prop rpm respects the physics about prop blade speed, the driveline gear ratio comes into play. Logically, at boat that cover a wide speed range, eg. 0-35 knots, need an engine that revs. over a wider rpm range that a boat doing maybe 0-10 knots.

When on outboard or sterndrive, you can alter the tilt angle while underway. By tuning the angle you optimize prop efficiency (or spoil it!).

Prop's properties are known to be 90% related to design, blade number and area, and 10% related to material, so careful selection is crucial. More than often repeated tests are the only way if you are after absolute fine tuning.

If you like to know more, Quicksilver's free book 'Everything you need to know about propellers' can be read or downloaded (20 mb pdf) here:
http://prop_.spidybot.com/Quicksilver_Propellers.pdf

 

[tinkicker0]

Too many variables. However a petrol engine is a device that ultimately turns chemical energy into motive power. It does not matter what is soaking up your power, may it be tide, wind, load or a dirty bum, to achieve a set amount of work requires a set amount of energy stored in your fuel.

The biggest variable in a petrol engine, assuming that everything else stays the same is the throttle plate position and pumping losses caused by the vacuum in the manifold. At open throttle a modern fuel injected petrol is pretty much as fuel efficient as a diesel with only the calorific value of the fuel differing with diesel fuel having a higher specific impulse, or stored energy per unit of mass than petrol has.

 

[wipe_out]

Btw, what's the ratio of your outdrive?
By heart, 35kts @ 4600rpm with a 17" prop makes me think that either you've got an unusual ratio outdrive, or the prop slip is too high.

I don't know the drive ratio.. When it was all cleaned up the engineer painted the leg so the plate hs been painted too.. I should probably lift it and try and remove the pait to see what the ratio is..

How much prop slip is the 'right' amount of slip?

 

[wipe_out]

Your engine will burn fuel in relation to the work carried out, as stated. For a calculating excercise: Use 200 grams of fuel per kW per hour and factor in the weight of petrol to get gallons/liters.

That was the other side of the theory I was thinking.. If X hp is required to do the work it needs Y amount of fuel and the engine efficiency difference between 3200rpm and 3500rpm is probably negligible..

 

[wipe_out]

The biggest variable in a petrol engine, assuming that everything else stays the same is the throttle plate position and pumping losses caused by the vacuum in the manifold. At open throttle a modern fuel injected petrol is pretty much as fuel efficient as a diesel with only the calorific value of the fuel differing with diesel fuel having a higher specific impulse, or stored energy per unit of mass than petrol has.

Interesting.. So what you are saying is running a more loaded engine to deliver the required power with a more open throttle would reduce the vacuum in the manifold and so be more efficient because the pumping losses would be less vs a less loaded engine with a more closed throttle having higher pumping losses?

 

[tinkicker0]

Indeed, a smaller engine at high throttle is more fuel efficient than a bigger engine at low throttle for the same work produced.

 

[boatmike]

I agree with all that Spi D says. However he might have added that propeller design is always, as it should be, calculated to give maximum performance (knots) in optimum conditions, at the RPM that produces maximum shaft horse power. Once the optimum has been achieved then reducing the pitch or diameter of the prop will make it less efficient and you will consume more power for any given speed than before. Don't mess with it. Clean the bottom as others have said and spend your money on booze rather than new props. You need to transmit the highest percentage of power the engine produces to pushing your boat along rather than having to rev higher to achieve the same speed with a less efficient prop.

 

[MapisM]

How much prop slip is the 'right' amount of slip?

Well, the lower the better of course.
But with an alu prop you can hardly get a slip lower than 10-12%, while a s/s prop nicely matched with the boat definitely can give much better results - possibly achieving a slip as low as 5-6%. Of course, this is clearly a case where YMMV, as they say...

Anyhow, since I didn't want to have thrown in a worrying comment with no reason, I just made a quick check, though I had to make assumptions.
Knowing that you have a 5.7 engine with a single prop outdrive, chances are that you have the Merc Alpha outdrive.
And this drive came mostly with a 1.5 ratio for the first models, changed to 1.47 in the early 90s (Gen 2).
With the latter, your prop slip would be exactly 20%, while with the first you would have 18.4%. Either ways, definitely too high, also for an alu prop.
Otoh, also a 1.61 ratio was available (though I've never seen one, so I would think it's pretty rare). If that's what you've got, the slip is 12.4%, which would be ok with an alu prop.

Then again, you might as well have a VP, an OMC, or whatever...

 

[wipe_out]

Well, the lower the better of course.
But with an alu prop you can hardly get a slip lower than 10-12%, while a s/s prop nicely matched with the boat definitely can give much better results - possibly achieving a slip as low as 5-6%. Of course, this is clearly a case where YMMV, as they say...

Anyhow, since I didn't want to have thrown in a worrying comment with no reason, I just made a quick check, though I had to make assumptions.
Knowing that you have a 5.7 engine with a single prop outdrive, chances are that you have the Merc Alpha outdrive.
And this drive came mostly with a 1.5 ratio for the first models, changed to 1.47 in the early 90s (Gen 2).
With the latter, your prop slip would be exactly 20%, while with the first you would have 18.4%. Either ways, definitely too high, also for an alu prop.
Otoh, also a 1.61 ratio was available (though I've never seen one, so I would think it's pretty rare). If that's what you've got, the slip is 12.4%, which would be ok with an alu prop.

Then again, you might as well have a VP, an OMC, or whatever...

Sorry, probably would have been worth mentioning it's a VP SX drive.. I really should find out what the ratio is even just for interest sake..

I have just read an interesting article on Brake Specific Fuel Consumption which for the most part answers the question about low rpm high throttle vs high rpm low throttle.. Also suggests that the difference I was talking about between 3200rpm and 3500rpm woudl probably make very little difference at all..

Here is the article for anyone interested..
http://www.autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_112611/article.html

 

[MapisM]

Sorry, probably would have been worth mentioning it's a VP SX drive.. I really should find out what the ratio is even just for interest sake..

Aha. I'm afraid I can't help there, 'cause I have zero experience with VP SX.
But maybe volvopaul or some others can help...
...You might well be lucky and discover that the SX mated with the 5.7 is available only with one ratio.
Btw I suppose the vintage can also help any VP expert to give you a hint.

 

[wipe_out]

Btw I suppose the vintage can also help any VP expert to give you a hint.

Boat is 2002 so I guess the drive is somewhere between 2000 and 2002.. When I looked before it seems the SX had a number of ratios but as you say maybe there are one or two that were mated to the 5.7..

 

[ulyden]

To find out this you need tree maps. Hull resistance map (curve)engine efficiency map and propeller diagram. For a certain speed you need at certain power. When the power demand is fixed different combinations of pitch and speed is considered.In some cases diameter is considered but normally a limitation due to space and torque. When you then have the highest propulsion efficiency you consider the engine map. Target is often easy. Utilize maximum engine power and at economic hull speed have the best combination of propulsion efficiency and engine efficiency. On larger boats this is a big focus on this.

I have been study a lot of propeller maps and engine maps. On a big boat running below maximum speed (like most container and tank ships are doing these days) there is a lot of potential saving.

But normally at above 60% of rated speed you get the best propeller and engine efficiency by lower the speed/increase pitch.

On a otto engine cooled by petrol you might find operation in max torque area having a better efficiency than lambda one area.

You will never find specific fuel consumption at 200g/kwh on a lambda one petrol engine. I think 238g/kwh is the lowest i have ever seen. AUDI 1.8TSI 170hp. Most engines are drinking more. At full speed and load 300-320g/kwh is more the rule.

300g/kwh is close to 0.3l/hph.