Advantages of sleeve valves for H-24 engines?

[Shortround6]

Also, as far as I know the R-2600 with 1800-1900 BHP for TO was not a production engine?

It was production engine.
There was no 1800hp production engine. 2 experimental though.
They jumped from the 1700hp engine to the 1900hp engine. This was the "BB" engine. It had the sheet metal cooling fins.
Again it was a new Wright engine that kept the bore and stroke of the "BA" engine and changed everything else.
They built just about 1000 of them in 1943 and over 8,700 of them in 1944.
It was only used in Avengers, Helldivers and some Martin Mariners (with fan cooling for trivia fans)
However weight was around 2045lbs.
it was allowed 2800rpm for take-off but only 2600rpm military?
At 15,000ft it gave 100hp more than the 1700hp version so the extra take-off power doesn't quite transfer over to flight performance.

I would note that the R-3350 was making 2400-2500hp (wet) with a two speed supercharger in 1944-45 (and later) on transport airplanes and went to at least 2700hp in later versions.

 

[AMCKen]

The sleeve valve engine was used in cars before aircraft.

Sleeve valve - Wikipedia

en.wikipedia.org

Knight engine - Wikipedia

en.wikipedia.org

 

[ThomasP]

Hey hobbes154, tomo pauk & Shortround6,

Thanks for the info on the R-2600-20, I missed that one for some reason. As I mentioned in my last post, my list is for the ratings at altitude (not TO). However, the -20 had an output at altitude of 1750 BHP instead of 1700 BHP. Corrected my lists upthread.

re the R-3350 running wet

As mentioned in my last post, all ratings in my list are dry (as far as I know) with 100/130 grade fuel. The ratings would be higher for most of the engines in my list if we use their ADI values.

 

[wuzak]

All the time RR was playing politics and some in the Air Ministry were dancing to their tune, which resulted in unfair decisions against Napier and Bristol.

Unfair decisions against Napier and Bristol? What were they?

The RR Eagle development was aided greatly after the Air Ministry removed an engine and all documentation from Napiers and sent it to RR.

Really?

Napier went belly up in 1942, and was to be sold to Rolls-Royce. But that was blocked by the Napier board, so it ended up being sold to English Electric.

One would think that the Sabre would have progressed more under Rolls-Royce, particularly with regard to altitude performance.

The Centaurus was also held back by the AM, and also part of the reason for the Centaurus numbers being so low during the war was they were built in an underground factory, which took time to make and staff. The staff were not exactly excited about working underground.

Bristol were also struggling somewhat with the Hercules, trying to get it sorted, make it reliable and make more power from it.

Any resources that were directed toward the Centaurus slowed development of the Hercules, which was a very important engine, particularly early war.

Note also that the Beaufighter was re-engined with Merlins in 1941 because production of the Hercules was initially slow, and the Short Stirling had priority. In the end not many were built.

Rolls-Royce had a number of programs cut in 1941 so they could concentrate on improving the Merlin and getting the Griffon into production.

In late 1942 the Air Ministry forced Bristol to provide Napier the manufacturing technology to make sleeves with the appropriate tolerances.

Which saved the Sabre, Typhoon and Tempest.

The Sabre program was at crisis point. the Air Ministry did what it could to put it back on track. That included procuring grinding machines from the USA that were originally destined for Prat and Whitney.

 

[wuzak]

RR drew up an H-24 using four Merlin cylinder blocks but it didn't go any further (?).

The H-24 Merlin was a double engine, like the DB606. Each half could be shut down in flight.

This is unlike H-24s such as the Sabre.

 

[z42]

The best thing Napier could have done for the war effort would have been to throw the Sabre on the scrap heap, and focus on developing the Culverin into a series of kick-ass two stroke diesels for tanks, MTB, and such.

Like a Cromwell or Comet powered by a mini-Deltic?

 

[Simon Thomas]

Unfair decisions against Napier and Bristol? What were they?

Bristol: "Fedden" Bill Gunston, It is worth reading the whole book to get the big picture. Two examples of many on p192 and p230.
Napier: I will go with my engine theft example.

Really?

As I have already stated, refer "By Precision into Power", Vessey, p143. First paragraph, second line.

 

[Shortround6]

Unfortunately a lot of what appears in web sites and common articles about the sleeve valve saga is codswallop, or at best, creative marketing.
we get stuff like this.

By 1927 Fedden had built a working two cylinder V as a testbed, with the idea of developing it into a V-12. However several problems cropped up on the design, notably that the sleeves tended to burst during the power stroke and strip their driving gears. This led to a long series of tests and materials changes and upgrades that required six years and an estimated 2 million pounds, but by 1933 the problems had been worked out.

The result was a Jupiter-sized engine adapted to the sleeve system, the Perseus, and its smaller cousin, the Bristol Aquila. The first production versions of the Perseus were rated at 580 horsepower 433 kW), the same as the same-year model Mercury, which shows that the sleeve system was being underutilized. However this was quickly uprated as improvements were introduced, and by 1936 the Perseus was delivering 810 hp (604 kW), eventually topping out at 930 hp (690 kW) in 1939.
Except that the problems had not been worked out and would require more years and more money.

Fedden's 1938 article pretty much compares the Bristol Sleeve engine with the Bristol poppet valve engine and NOT poppet valve engines from upstarts like the Americans.
How much engineering time/effort was going into the Bristol poppet valve engines is certainly subject to question. They were still lubricating the valve gear in the heads with grease gun fittings.
The Sleeve valve engines despite all the claims were barely beating the Bristol poppet valves in power. The Perseus XII in the Blackburn aircraft certainly didn't demonstrate any excess capability over the Poppet Valve Mercury and in fact, the Mercury responded fairly well to 100 octane fuel. The Perseus didn't.

Edit> Oh yeah, that 930hp rating for the Perseus in 1939? It was by using a cropped impeller supercharger and peak power was at around 6,500ft. <edit

The Perseus went to large scale production in 1938 (?) although we have to be careful about what large scale means. They had put 18 of them into the Vildebeest IV
"In this version, the Perseus had overheating problems and was deemed unsuitable for tropical service" I am not sure if 12 of them got Pegasus engines and were sold to New Zealand as Vincent's. At any rate 197 Vincent's were during/after the experiment with the Perseus engines.
The Skua may have been the 1st large scale user of the Perseus (see what I mean about large scale?) and while the Roc followed the Skua both the Roc and Botha first prototypes first flew in Dec of 1938. And Skua production saw over 95% completed in 1939. Only other large scale user was the Lysander.

Then we have the Taurus saga. Another sleeve valve engine that never lived up to the promises made for it. Main claim to fame is providing some partially finished sleeves to Napier to helps sort out the Sabre. This was done just short of bayonet point so complaints about RR have to be put in perspective. It seems a number of companies in England were out for No 1 and not the country as a whole.

And then we have the Hercules. Early production was slow and troubled for an engine that had all it's sleeve problems worked out in 1933 (over 6 years earlier). We are back to excessive oil consumption and short (sometimes very short) overhaul lives. We also have 220 Wellington IVs with P & W R-1830s to help make up for the lack of Hercules engines.
Please that while the Short Stirling bomber was supposed to get priority for Hercules production, both Short's factories had been bombed in Aug 1940 setting the program back by months.
Also note that the First Wellington III with Hercules engines flew on May 19th 1939. This was about 18 months before the First MK IIs with Merlin showed at No 12 squadron.
Also note the Beaufighter II with Merlins to help with the shortage of Hercules engines.
Except in late 1940 and early 1941 where were the Hercules engines going?????
The Stirling Factories had been bombed and it would take all of 1941 to complete 200 Stirling's.

Something smells in sleeve valve land with the all the problems had been solve in 1938 or before.

 

[Deleted member 68059]

The practical advantage of the sleeve valve vs poppet valve might be demonstrated by the relative BHP vs displacement. The list below shows the ratings at the end of the war (or just post-war) for the major Allied larger engines.

_____________________________________super___________valve______BHP
_______________in3______Type________ charger__________type_______dry
Merlin_________1650_____V12_____2-stage/2-speed_____poppet_____2030
V-1710________1710_____V12_____2-stage/2-speed_____poppet_____1800
Griffon________ 2240_____V12_____2-stage/2-speed_____poppet_____2200
Griffon________ 2240_____V12_____2-stage/3-speed_____poppet_____2400
Sabre_________2240_____H24_____1-stage/2-speed_____sleeve_____ 2600
Hercules______ 2360_____R14_____1-stage/2-speed_____sleeve_____ 1800
R-2600_______ 2600_____R14_____1-stage/2-speed_____poppet_____1750
R-2800_______ 2800_____R18_____2-stage/2-speed_____poppet_____2300
Eagle 22______ 2817_____H24_____2-stage/2-speed_____sleeve_____ 3200
Centaurus_____3272_____R18_____1-stage/2-speed_____sleeve______2500
R-3350_______ 3350_____R18_____ turbo-supercharged_ poppet_____2200 (2500 for A-bomb missions?)
R-4360_______ 4360_____R28_____ turbo-supercharged_ poppet_____3250

Take note of the BHP per displacement of the H24 engines (in particular) vs the equivalent size poppet valve engines. While this list does not give anywhere near all the various pluses and minuses, I think it indicates a very good reason why Napier and Rolls Royce pursued the sleeve valve in their liquid-cooled engines.

The sleeve topped out at about +17 lbs boost because of thermal conductivity problems and sleeve distortion issues, whilst the poppet continued to +30 lbs boost in wartime.

Having an H-24 for a given displacement (Sabre = Griffon in size approximately) enabled short stroke high crankspeed design which helps increase power, but it is not enough to offset the limitation in boost caused by the nature of the sleeve.

Hence the Centaurus never passed 56 hp/litre and the Sabre-VII 83 hp /litre (with water injection) whilst the plain old Merlin-100 was type tested at 85 in wartime despite a considerably lower crankspeed and no water injection.

I`m actually being very generous to the Sabre here too as the Sabre VII was never mass produced, whereas the Merlin-100 flew in war operationally in the Hornet.

There is no example of a sleeve valve ever surpassing a poppet valve of the same era in bhp/litre.

 

[hobbes154]

Something smells in sleeve valve land with the all the problems had been solve in 1938 or before.

If we are being generous, there was one set of problems getting prototypes to work, and another set of problems getting them into mass production? So plausibly the claims you quote only apply to the first?

The Wiki on Hercules says:

These smooth-running engines were largely hand-built, which was incompatible with the needs of wartime production. At that time, the tolerances were simply not sufficiently accurate to ensure the mass production of reliable engines. Fedden drove his teams mercilessly, at both Bristol and its suppliers, and thousands of combinations of alloys and methods were tried before a process was discovered which used centrifugal casting to make the sleeves perfectly round. This final success arrived just before the start of the Second World War.[3]

Which may still be optimistic, or perhaps there were other problems with the production engines?

 

[hobbes154]

There is no example of a sleeve valve ever surpassing a poppet valve of the same era in bhp/litre.

If we are comparing like with like the Hercules and Centaurus stack up OK against the R-2600 and R-3350? I guess because the air cooled engines are already constrained in bhp/litre by heat limits so the boost limit doesn't matter as much (unless you are racing at Reno)?

 

[Simon Thomas]

There is no example of a sleeve valve ever surpassing a poppet valve of the same era in bhp/litre.

The Hercules XI had more power and less fuel consumption than the Wright R-2600-A5B, at maximum continuous weak power settings at all altitudes.

Short Stirling Engines - Hercules XI & Wright GR2600A5B

Does anyone have a copy of the performance curve for the Wright GR2600A5B fitted to the Short Stirling? I am hoping to compare the performance curves of that and the Hercules XI. In the book "Short Stirling, The First of the RAF Heavy Bombers", Pino Lombardi writes However, Hercules engine...

ww2aircraft.net

When tested in the Stirling, "three aircraft - N3657, N3711, and R9188 - were fitted with Wright Cyclone engines at Swindon, performance and fuel consumption trials to assess performance and fuel consumption produced disappointing results."

So in the real world, the sleeve outperformed the poppet. You will know that the Hercules capacity was 2300 in³, rather smaller than the R-2600.

Comparing numbers in tables doesn't always show the situation. TO/combat/WEP power is a transient, and is only a small part of the entire performance envelope.

 

[Simon Thomas]

If we are being generous, there was one set of problems getting prototypes to work, and another set of problems getting them into mass production? So plausibly the claims you quote only apply to the first?

There were plenty of issues of both categories. There were also difficulties once in service, eg the RN had missed a step in the procedure (opening an oil drain), which resulted in quite a few failures.
I may come across as a bit of a sleeve valve junkie, but that isn't really the case. I am impressed by their very clever engineering. I just (foolishly) want them to get a fair trial.

 

[Shortround6]

If we are comparing like with like the Hercules and Centaurus stack up OK against the R-2600 and R-3350? I guess because the air cooled engines are already constrained in bhp/litre by heat limits so the boost limit doesn't matter as much (unless you are racing at Reno)?

An awful lot depends on when. The R-2600 is a bit of puzzle in itself. There are three totally different engines that share the same bore and stroke. The 3rd engine (the BB) started production in the middle of 1943 and is the 1900hp for take-off version but that doesn't line up with the Military power even at low level. This engine was only used by the Navy with few, if any, commercial sales after the war. Wright may (guessing) have seen the hand writing on the wall and concentrated on the R-3350 and the R-1820 and not tried to split their effort 3 ways. They had enough problems with R-3350 in 1944/45 anyway.
So this makes it rather difficult to compare to the Hercules engine/s.

Engine..........................take-off...................................Military high gear..............................Max continuous high gear
R-2600 BB....................1900hp/2800rpm..............1450hp/2600rpm/15,000ft.................1350hp/2400rpm/14,800ft
Hercules XVI................1615hp/2900rpm.............1545hp/2900rpm/15,500ft.................1300hp/2400rpm/13,500ft
Hercules XVIII.............1725hp/2900rpm.............1600hp/2900rpm/15,500ft.................1300hp/2400rpm/ 9,500ft
Hercules 100...............1675hp/2800rpm.............1625hp/2800rpm/19,500ft.................1415hp/2400rpm/16,500ft

Most any Hercules after the 100 really blows the R-2600 into the weeds, but the R-2600 was pretty much frozen in time from the Spring of 1943 onwards.
Now if you want to go backwards a 1940 R-2600 with 1600hp (the A ) for take-off edges out the 1940 Hercules III
They built about 1900 R-2600 A's in 1940.

P & W tried to build a post war R-2180 14 cylinder engine of 1800hp take-off (wet) and lost their shirt. (1/2 an R-3360) Only one company (SAAB) bought any.

There is a lot of the Hercules saga that is not easy to find.
One book goes through the cylinder head something like this.

Development stage, single piece sand casting, got to 540 sq.in cooling area.
First production engine, die cast single piece, 581 sq in cooling area.
early war time (MKs not given) two piece with die cast bottom shrunk into the flange, 728 sq in.
later heads with better casting got 777 sq in with increase fin pitch. decrease in CHT of 15 degrees C and cut 3 oz of the weight.

A number of experimental heads followed with a machined copper based head being the head used after the war. It was at least the 3rd head incorporating copper for better heat transfer.

 

[wuzak]

Bristol: "Fedden" Bill Gunston, It is worth reading the whole book to get the big picture. Two examples of many on p192 and p230.

What are the examples?

Or do you expect me to buy a book to read two pages?

Napier: I will go with my engine theft example.

As I have already stated, refer "By Precision into Power", Vessey, p143. First paragraph, second line.

Is there proof? Or just suspicion?

At the time it would seem that the Air Ministry did not trust Napier management.

They did go broke around that time and had to be sold. The Air Ministry wanted Rolls-Royce to buy Napier, but that was blocked.

So if the Air Ministry did take documents and give them to Rolls-Royce then it may be that they thought that was the best way to get a useful engine for the future.

Also, the Air Ministry were so unfair towards Napier that they compelled Bristol to help Napier with sleeve production and redirected machine tools.

 

[wuzak]

I may come across as a bit of a sleeve valve junkie, but that isn't really the case. I am impressed by their very clever engineering. I just (foolishly) want them to get a fair trial.

You don't think sleeve valve engines had a fair trial in WW2?

What would have been a fair trial?

 

[Shortround6]

The Hercules XI had more power and less fuel consumption than the Wright R-2600-A5B, at maximum continuous weak power settings at all altitudes.

When tested in the Stirling, "three aircraft - N3657, N3711, and R9188 - were fitted with Wright Cyclone engines at Swindon, performance and fuel consumption trials to assess performance and fuel consumption produced disappointing results."

So in the real world, the sleeve outperformed the poppet. You will know that the Hercules capacity was 2300 in³, rather smaller than the R-2600.

Comparing numbers in tables doesn't always show the situation. TO/combat/WEP power is a transient, and is only a small part of the entire performance envelope.

The R-2600-A5B was a 1939-40 engine but that is probably what the US was willing to export at the time.
The R-2600-BA engine was the 1941 version (443 built in 1941) and this was the 1700hp take-off power version. Used a steel crankcase instead of the Aluminum of the A series engines.
Might very well have used more fuel than the A.
But the BA could run 100rpm faster and had about 3000ft more altitude at max cruise.
BTW the R-2600 A5B was supposed to be rated on 90-91 octane fuel, the R-2600 A5A was supposed to run on 100 octane (US 100 octane)
Hercules XI was supposed to run on 100 octane (British)?

I am all for comparing engines, as long as the comparison is fair.
The British ordered the A5B engines, they got A5B engines, that may be all they could get. They got the results they got.
However that doesn't mean that the test shows the results of a best practice poppet valve engine vs a best practice sleeve valve engine.
Things changed by the year if not sometimes a bit quicker.

 

[Shortround6]

You don't think sleeve valve engines had a fair trial in WW2?

What would have been a fair trial?

Comparing a 1941 sleeve valve to a 1940 poppet engine or a 1946-47 sleeve valve to a 1943 Poppet?

 

[marathag]

I wouldn't expect so as it's really two flat 12 cylinder arrangements and flat (horizontally opposed) designs
ran valves.

More development of the Fairey P24 Monarch, going from 2 valve per to 4?

Fairey P.24 Monarch Aircraft Engine

Designed by Richard Forsyth, the P.24 Monarch was Fairey Aviation’s final attempt to enter the aircraft piston engine business. The 24-cylinder Monarch was essentially two engines in a common crank…

oldmachinepress.com

has this patent drawing

so in theory could have been built

 

[ThomasP]

Where to begin . . . where to begin

Unfortunately there are just too many variables involved in the sleeve vs poppet argument to give a compact coherent comprehensive statement/answer. Just for the power curves alone we would need everyone to post whatever they have - in order to compare them. Then we would have to sort out the ones using 130 vs 150 grade fuel, ADI or no ADI, type/efficiency of supercharger, whether they should be considered similarly mature/developed or not, along with what other factors (ie other than sleeve vs poppet) contributed to the engines performance.

The following items are not necessarily meant to counter what others have said above. But they are details that I believe need to be taken into account as they contribute significantly to the pros and cons of the sleeve vs poppet argument.


1. re Using maximum boost as a measure of efficiency. What was the cylinder CR (Compression Ratio)?

This is important because, if everything else is equal, the maximum usable boost will be:

CR1 x "Hg1 /CR2 (approximately)

An example would be if we compare the Merlin 25 to the Sabre. Both engines were liquid-cooled with 1-stage/2-speed superchargers. The Merlin 25 had a CR of 6 and a maximum boost using 130 grade of +18 lbs/66.5"Hg. The Sabre had a CR of 7. This means the best we should expect in terms of boost from the Sabre, everything else being equal, would be:

6 x 66.5 / 7 = 57"Hg or +13.3 lbs when using 130 grade fuel

What was the highest realistic boost used by the Sabre in service - when using 130 grade fuel? IIRC the boosts were:
Sabre IIB_____2400 BHP at 3700 rpm at +11 lbs (1944)
Sabre VA_____2600 BHP at 3850 rpm at +15 lbs (1946)


What if everything else is not equal?

Again, using the Merlin 25 as the engine baseline for comparison.

2. Is the supercharger more or less efficient (in terms of heat added to the charge)? As far as I know the Merlin 25 supercharger was considered more efficient than the Sabre supercharger. Greater heat rise due to the supercharger would result in a lower altitude rating and/or a lower boost rating and/or lower BHP.

The Griffon engines in my lists are the 65 and 85, both well developed and using the 'Hookerized' supercharger and inter-/after-cooler. As far as I know none of the Sabre series used an inter-/after-cooler. Yet the Sabre still had the advantage in output. If the Sabre had a supercharger and inter-/after-cooler similar to the Griffons we can assume significantly improved performance from the Sabre.


3. Is the engine air-cooled or liquid-cooled?

As shown by my lists upthread, in terms of comparable displacement and type of cylinder cooling, none of the poppet-valve engines matched the BHP/in3 of the sleeve-valve engines. This is (I think) unarguable?

In my list I have only the Griffon to use in comparison to the Sabre for liquid-cooling, and the Sabre clearly comes out ahead in BHP/in3. Possibly there are other developed engines that could be compared (maybe German or Soviet?) that might provide a significantly difference aspect to things.

In the air-cooled sleeve vs air-cooled poppet category, it seems to me that the difference is less, but still noticeable. As my lists show, the Hercules 100 had 240 in3 less displacement, but still matched/exceeded the R-2600-20 engine output (both engine models being best of their war-time series?). As Shortround6 pointed out, the Hercules 100 was a slightly later development than the R-2600-20. If the R-2600-20 had the same supercharger design as the Hercules 100 how much difference would it have made?


4. Scale effects, ie how much would the difference in engine size make?

There are no (I think) reasonably developed sleeve-valve liquid-cooled engines in the 1650-1710 in3 range. This makes (I think) any direct comparison to the Merlin or V-1710 impractical.

Unfortunately, there are (I think) also no reasonably developed poppet-valve liquid-cooled engines in the size of the Eagle 22 sleeve-valve engine. So again, there is (I think) no way to make conclusive comparisons. Possibly someone would like to try comparing the less well developed US large hyper engines? Unfortunately, as far as I know there is not a lot of confirmable data on most of the engines. The Allison V-3420 is the only large displacement poppet-valve engine comparable in size to the Eagle 22. But it is an earlier design, with significantly less efficient supercharger(s), and it was not really fully developed (I think) even though it was operated in various platforms over a significant period of time. What data I have shows it at very great disadvantage to the Eagle 22.

The only other well developed air-cooled poppet-valve engine of note was the Dagger VIII. It was an air-cooled H24-cylinder design with a displacement of 1037 in3 - very significantly smaller than the Merlin & V-1710 - with a CR of 7.5 and max boost of +5 lbs on 87 grade fuel. Its output was 1000 BHP at 4200 rpm at +5 lbs. In terms of output this is more in the range of the Peregrin V12 poppet-valve engine. The 1296 in3 Peregrin had a CR of 6 and put out about the same 1000 BHP when running at 3000 rpm and +9 lbs boost on 130 grade fuel. Even though the Dagger was air-cooled, on 87 octane it ~matched the 30% greater displacement liquid-cooled Peregrine engine at +9 lbs. When the Dagger VIII was rated on 130 grade it reached 1115 BHP at 4400 rpm at +7 lbs, which ~matched the Peregrin's 1100 BHP at 3000 rpm at +12 lbs.


4. Combinations of the above differences

In practice, it took the R-4360 to match the Eagle 22 in output, but it used 4360 in3 of air-cooled poppet-valve vs 2817 in3 of liquid-cooled sleeve-valve - or 55% more displacement for the same output. The R-4360 variant I used in my lists is the one used in the B-50, so it had turbosuperchargers with inter-/after-coolers. The Eagle 22 is the mark used in some of the post-war prototype aircraft, and it had an improved 'Hookerized' 2-stage/2-speed supercharger with inter-/after-cooler.

NOTE According to RR/Fedden the Eagle was 'overbuilt' partly in order to accommodate the stresses of the large contra-rotating props, and partly to accommodate future growth in output. It was thought that 4000 BHP was not unrealistic with further development using higher grade fuels and ADI.


5. In addition to the above, I think we need to add reliability in the actual applications.

How much of the difference in performance between the V12/R-xx poppet-valve engines and the H24/R-xx sleeve-valve engines was due to inherent issues with the concepts, and how much was due to more or less success in practice (inherent manufacturing problems vs quality control for example) and/or timing (ie X years earlier/later design, put on the back burner due to priorities, knowledge/technology not available at the time of comparison, etc).


In conclusion, I suspect that this argument is similar to the one of whether 2-, 3-, or 4-blade propellers are more efficient. In theory and reality, a well designed 2-blade propeller will be more efficient than a 3-blade prop, if the 2-blade prop can absorb the BHP (This is basic physics and hydrodynamics). If the 2-blade prop is already at maximum allowable diameter, and you want to increase the usable BHP, then when using similar blade design you will have to go to a 3 (or more) blade prop. While each individual blade of the 3-blade prop will be acting less efficiently than the individual blades of the 2-blade prop, when combined the blades or the 3-blade prop will be able to absorb more BHP than the 2-blade prop.

Bleh need more coffee.