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Mercruiser Alpha 1 gen 2
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If by "efficient" you mean (hydraulic output power)/(mechanical input power), then this statement is wrong, since the pump efficiency is varying with rotating speed (and flow and head), also for the flexible impeller type pumps, since they are somewhat "semidynamic". Only strict displacement pumps (piston, gear aso) are nearly independent of speed up to a limiting value.
If by efficient you mean "pumping adeqate amount of cooling water", then the answer would perhaps rather be "enough at all speeds".......
That said, with reference to similarly sized pumps as the one in the Alpha rig, I'd guess the speed range 2500-3000 rpm is about the optimum for this pump size. The maximum efficiency of flexible impellerpumps at optimum speed and flow is just below 40%, but at off-design flow it is considerably lower.
That statement pops up every now and then; let's look at some numbers and see if it is true. The Alpha rubber impeller has a diameter of about 50 mm and we check the 3000 rpm condition. The tip speed of the impeller is then 7,8 m/s (probably less when squeezed into the housing). That corresponds to a dynamic pressure of 30420 Pa in cold fresh water, or roughly 3,1 m of water column, which is the maximum "centrifugal pumping pressure" you could get at zero flow. This is roughly about 10% of the possible working pressure of this impeller. Note that this alleged "centrifugal action" is directly acting to push water out of the inlet port as well as out of the outlet port. The net effect on the pressure increase is thus close to zero.With increasing flow, the dynamic part of the pressure increase is reduced. Ergo, the essence of the statement is a myth.
What really happens when speed is increased is that the vanes cannot expand ( or spring back) quick enough after passing the cam in the housing. Add the piping counter-pressure and you find the vanes "waterplaning" inside the housing, with leakage increasing with increasing speed and pressure. It is very clear once you see the actual pump curve. So what we have is a displacement machine with varying internal recirculation due to speed and pressure, which has very little to do with the action of a centrifugal pump.
There is a reason to understand the difference, because the operating point (and with that the efficiency) is determined by the piping system, and centrifugal and displacement pumps behave differently.
What really happens when speed is increased is that the vanes cannot expand ( or spring back) quick enough after passing the cam in the housing. Add the piping counter-pressure and you find the vanes "waterplaning" inside the housing, with leakage increasing with increasing speed and pressure. It is very clear once you see the actual pump curve. So what we have is a displacement machine with varying internal recirculation due to speed and pressure, which has very little to do with the action of a centrifugal pump.
There is a reason to understand the difference, because the operating point (and with that the efficiency) is determined by the piping system, and centrifugal and displacement pumps behave differently.
tpenfield
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I'd say it is efficient enough for its intended purpose, based on its overall success. Is there a specific reason for the question?
Generally, the Mercruiser Alpha pump is not quite strong enough for closed cooling applications, and the pulley driven pumps are used in its place.
Specific question
Thx for Your reply .I've read about that early Alpha used half closed cooling using raw water pump for wet exhaust collectors .
I trying to design experimental drive for 33' weird catamaran .
Thx for Your reply .I've read about that early Alpha used half closed cooling using raw water pump for wet exhaust collectors .
I trying to design experimental drive for 33' weird catamaran .
tpenfield
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I just checked through Mercruiser's parts website. It looks like only the small engines (3.0L) relied on the Alpha pump with closed cooling. The larger engines (V6 and small V8) had a pulley driven raw water pump if there was closed cooling. Big V8's were Bravo only (which has no pump in the outdrive).
So, I think the Alpha drive has its limits as to the amount of flow that it can support. Probably depends on the sizing of your engine(s).
So, I think the Alpha drive has its limits as to the amount of flow that it can support. Probably depends on the sizing of your engine(s).
So the biggest V-? outboards use bigger capacity pumps or outboard setup is more heat efficient ?
tpenfield
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Not a specifications expert, just going off my 20+ years of experience with Mercruiser power packages . . .
It looks like anything 4.3 V6 and bigger with closed cooling called for the pulley-driven pump. The key thing is closed cooling as compared to open (raw water) cooling. Outboards can generally be considered open cooling, same as raw water cooled I/O engines. Alpha drives generally can handle open cooling into the V6 and small V8 range, as indicated by their product offerings.
Closed cooling requires a bit more pressure & flow than open cooling, since the heat exchangers used are typically '4-pass' type.
IMO - If you are doing a custom application, then you will probably need to test the cooling system if you are going with the Alpha drives and be ready to add an engine driven raw water pump.
It looks like anything 4.3 V6 and bigger with closed cooling called for the pulley-driven pump. The key thing is closed cooling as compared to open (raw water) cooling. Outboards can generally be considered open cooling, same as raw water cooled I/O engines. Alpha drives generally can handle open cooling into the V6 and small V8 range, as indicated by their product offerings.
Closed cooling requires a bit more pressure & flow than open cooling, since the heat exchangers used are typically '4-pass' type.
IMO - If you are doing a custom application, then you will probably need to test the cooling system if you are going with the Alpha drives and be ready to add an engine driven raw water pump.
Ok. Thx again for usefull infos . I have couple of questions about Alpha lower unit :
Are OEM gears and bearings best or strongest ?
Are the counter lower units the same reliability as clockwise ?
Are there any improvments or tips good for Alpha lower unit ?
I'm don't won't make wave jumping at high rev . And rev will be around 3000-3500 at the lower unit shaft.
Are OEM gears and bearings best or strongest ?
Are the counter lower units the same reliability as clockwise ?
Are there any improvments or tips good for Alpha lower unit ?
I'm don't won't make wave jumping at high rev . And rev will be around 3000-3500 at the lower unit shaft.
tpenfield
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My experience is with Bravo outdrives. . . but . . . There is nothing 'strongest' about an Alpha drive. Their HP rating is 300 for gasoline engines (which I think is a stretch) and 150 HP for diesel engines (I assume a torque limitation).
In my times taking apart/rebuilding Bravo drives - the bearings were Timken brand, so decent enough . . . not sure about the Alphas. The Bravos are basically big gears and big bearings.
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Here is a video that talks about the differences in the Alpha 1 Gen 2 drives since they were released. You may notice that the drive shaft input gear is a bit smaller than the Bravo gear in my picture.
As I've said before, the engine is probably what matters the most in terms of the fitness of the Alpha drive for your application.
In my times taking apart/rebuilding Bravo drives - the bearings were Timken brand, so decent enough . . . not sure about the Alphas. The Bravos are basically big gears and big bearings.
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Here is a video that talks about the differences in the Alpha 1 Gen 2 drives since they were released. You may notice that the drive shaft input gear is a bit smaller than the Bravo gear in my picture.
As I've said before, the engine is probably what matters the most in terms of the fitness of the Alpha drive for your application.
I don't like those bulky lower units , all of them , sucks . I'm sure that I don't need reverse at sea or underwater exhaust . But there's are very limited options available .
Same like the cars , most of them sucks .
Same like the cars , most of them sucks .
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