Engine blower motor

[CarinaPDX]

If the OP is going to replace just the motor I suggest that he get the MTBF of any candidates. Most electric motors use bushings, not bearings, and have MTBFs often as low as 1000-2000 hours. The typical $100 bilge blowers are in this range, as are most cabin fans. A DC motor with bearings more typically has 35,000-50,000 hours MTBF. There are many bushing designs that have features that improve on the simple bushing, at a modest cost, but for this application I would want real bearings.

I bought the Delta-T 4" blower (spec sheet: https://deltatsystems.com/wp-content...cSheet2017.pdf ) to match an existing mushroom vent, but the 3-inch would have been a better match (https://deltatsystems.com/wp-content...Sheet-2017.pdf ). I put together a PWM speed controller so I could cut down the flow to a more appropriate level. Delta-T does make a speed controller for their 4-inch blower but the price is a bit steep. I am thinking about putting a thermostatic switch on it as well, as in the PNW engine room temp is not usually a problem. BTW the spec sheets do show the flow vs. pressure performance.

My Volvo-Penta installation manual has a section on calculating the required ventilation air flow for the engine. Before installing a blower it would be worthwhile to do the math.

Greg

[Beau.Vrolyk]

Originally Posted by Dockhead

Indeed the alternator is the device which most people worry about, temperature-wise, but not indeed HIGH output alternators, but low output ones. High output alternators, especially large frame ones, work just fine in high ambient temperature. It's small frame car type alternators, not designed for producing bulk power, which have thermal problems.




As to "positive displacement pumps" -- this has been discussed, but you can't say that one pump "moves vastly more air" than another, just because it is one type, rather than another. It depends on the relative size of the pumps and the resistance of the outlet. A specific example was named -- the standard Jabsco ER blower which moves 7 m3 per minute nominally. Even if it loses HALF of that volume due to a badly designed outlet duct, it's still moving more air than a 2 liter diesel engine running at redline.

Dockead, with all due respect. The large frame Balmar alternators are extremely sensitive to overheating. Having burned up many of the Balmar and Motorola small and large frame alternators, I can personally report that they are all highly sensitive to heat. The only exception I have direct experience with are the oil cooled alternators built for commercial applications which are designed to run at approximately 260 deg. F.

The Jabsco blower can be brought to zero output by simply putting your hand over the duct. The blades in the Jabsco blower stall and no air moves at all. In stark contrast, if you put you hand over the exhaust of your diesel it will simply blow your hand out of the way and keep going. Many boats have truly terrible routing of the ducting, and that duct is made of flexible corrugated tube with extremely high surface friction. It is quite easy to loose 50-70% of the output to back pressure in only 10 feet of this sort of tubing.

To make matters worse, many boats are built with flaps over the exhaust vent. As described in a few posts above, to keep water out. This further reduces the actual flow from the Jabsco 420 CFM blower. It's a simple matter to put a anemometer on the output and measure it, you'll find it is a small fraction of what the rated velocity is for the Jabsco, and no where near 420 CFM which is it's spec. Again, I have used these for years and measured the actually air flow on a number of boats. The absolute best case was a 60% loss of airflow at the actual exhaust.

[Dockhead]

Originally Posted by Beau.Vrolyk

Dockead, with all due respect. The large frame Balmar alternators are extremely sensitive to overheating. Having burned up many of the Balmar and Motorola small and large frame alternators, I can personally report that they are all highly sensitive to heat. The only exception I have direct experience with are the oil cooled alternators built for commercial applications which are designed to run at approximately 260 deg. F.

The Jabsco blower can be brought to zero output by simply putting your hand over the duct. The blades in the Jabsco blower stall and no air moves at all. In stark contrast, if you put you hand over the exhaust of your diesel it will simply blow your hand out of the way and keep going. Many boats have truly terrible routing of the ducting, and that duct is made of flexible corrugated tube with extremely high surface friction. It is quite easy to loose 50-70% of the output to back pressure in only 10 feet of this sort of tubing.

To make matters worse, many boats are built with flaps over the exhaust vent. As described in a few posts above, to keep water out. This further reduces the actual flow from the Jabsco 420 CFM blower. It's a simple matter to put a anemometer on the output and measure it, you'll find it is a small fraction of what the rated velocity is for the Jabsco, and no where near 420 CFM which is it's spec. Again, I have used these for years and measured the actually air flow on a number of boats. The absolute best case was a 60% loss of airflow at the actual exhaust.

You've made a great contribution to this discussion by quoting some actual measured figures, instead of just theories. Good stuff


But still -- 60% loss of air flow, or even 80% loss of air flow, is still just not "vastly less air" than what the engine moves, in the typical case. A 2000cc engine moves 1 m3 per minute at 1000 RPM -- if we assume 100% efficiency, which it will not be. The typical Jabsco with even 80% loss of air flow will still be moving 1.4 m3 per minute; with 60% loss of air flow will be moving 2.8 m3 per minute, probably what the diesel moves around redline.



So can we agree that this: "you’ll find the diesel move vastly more air than your blower because it is a positive displacement pump while blowers are not" is not a correct statement?




As to Balmar alternators -- I will defer to your knowledge. I have no experience with them. Hearing this, though, I don't like them. My cheap Leece Neville school bus alternator is perfectly happy up to ridiculous temperatures, having been made specifically to thrive under the hoods of school buses in Texas. This is the typical high output alternator found around here -- often installed as part of the original build on Oysters, Moodys, and other larger cruising boats in the UK.

[transmitterdan]

Originally Posted by Jimmyhenry

Without a bilge blower the engine room will have negative pressure. Probably about the same as running a diesel at altitude. They hate that.
And without a blower any fumes in the engine room will be gone quickly out the exhaust.
So having a blower drawing air out of the ER and making the engine suffer even more, beggars belief.

More important is keeping the alternator cool. And here a small blower directed at the alternator “suck” side is best.

The tiny negative pressure created by an exhaust fan is negligible to a modern diesel. The benefit of cooler fresh air offsets the tiny reduction due to the fraction of an inch of negative pressure. And anyway, negative pressure is easy to avoid by providing an adequate size air inlet.

What beggars belief is that people still think its a good idea to force air into an engine space. That's a recipe for crew death when there is the inevitable exhaust system leak. And no one wants their cabin smelling like a diesel engine.

You are right that the alternator cooling is most important. But not important enough to risk my life.

[stnick]

Was your blower motor GOLD ? I just replaced my 2 blowers to the faster blowers. One fresh air in the other hot air out ! When the engine is on the blowers are always on !
If there is a any smoke due to oil burning or ant other heat problem the engine air out vent will thell you the story ! The high speed in line blowers Defender or Worst marine !

[james247]

Originally Posted by Jim Cate

Seems to me that the engine itself will keep the engine compartment at negative pressure relative to the cabin. It moves more air than most blowers that are fitted to small yacht engines. Additional blowers can mean a larger pressure differential, but dunno how much difference that makes in a practical sense.

Jim

Correct. To burn diesel you need oxygen
The main purpose of a blower / sucker is to remove heat and fumes from the engine room. The engine sucks what it needs from a vent and the fan sucks out additional air to improve the environment. Additional cool air in the engine room provided by suction helps cool alternator and refrig compressor if installed

[CarinaPDX]

I have occasionally thought that a delayed-off relay in the fan circuit would be a good idea, in order to keep removing heat while the engine cooled down. Up in the PNW it wouldn't be needed - the extra heat would probably be welcomed. But in the tropics that extra heat is miserable. Any thoughts?


Greg

[a64pilot]

Originally Posted by CarinaPDX

I have occasionally thought that a delayed-off relay in the fan circuit would be a good idea, in order to keep removing heat while the engine cooled down. Up in the PNW it wouldn't be needed - the extra heat would probably be welcomed. But in the tropics that extra heat is miserable. Any thoughts?


Greg

I keep playing with a simple adjustable thermostat, principle is the same though.
But yes, there is measurable heat that flows into the living space of my boat after shutdown, I mean just the other side of a lightly insulated wall of 3/4” plywood is a lump of several hundred lbs of 165 to 180 f lump of metal, that stays hot for quite awhile

[stnick]

i use in line blowers 220 CMF with Alu Dryer tubes. One 4 inch fresh air in and the other hot air out. My 384 the cowl vents right behind the wheel close to outboard on deck. I can place my hand there and tell if engine temp is correct. So the gage says. If the engine starts to smoke i can smell it minute one . ! If my engine is on so are my blowers. !

[Dockhead]

Originally Posted by CarinaPDX

I have occasionally thought that a delayed-off relay in the fan circuit would be a good idea, in order to keep removing heat while the engine cooled down. Up in the PNW it wouldn't be needed - the extra heat would probably be welcomed. But in the tropics that extra heat is miserable. Any thoughts?


Greg

My boat was built with a delayed-off relay on the blower.



I'm not sure it's a big deal -- ER temp otherwise will go up at shutdown but will go down thereafter. Maybe in a really hot climate.