Engine Room Exhaust Fans

RickB wrote:
Marin's note of how a conventional marine generator responds to a heavy load doesn't really apply. The typical marine generator operates at full rated speed and is capable of delivering full rated load as quickly as the governor can deliver fuel. The generator follows the load, it can't anticipate it and that fact creates the effects you see and hear.

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Right, but wouldn't if follow that if the main engine was also driving an AC generator sized to run AC appliances like hot water heaters, stoves, and whatnot, as these loads were applied the main engine would have to work that much harder to combine the load of the generator with the load of moving the boat?**If that reasoning is correct,*you'd either need a larger engine to maintain the same propulsion power while powering the loaded-up generator, or you'd need to accept less propulsion power with a smaller engine whenever a significant load was put on the generator.

The temperature in my engine room stays quite comfortable while we are under way as the engines draw in fresh (relatively) cool air from outside. When we park up and switch off, the temperature shoots up as the heat soaks out of the engines.

Wow you guys are amazing, I'm glad you're not billing me for the advise!

My inverter is a Victron "Quattro" 24v 5000w. I do not believe it will supply 240v, I believe I can pair it with a second and produce 240, but it would seem that the DC feed would be enormous.

I was talking today to a Krogen 52 owner who moves a lot of Krogen 55's and 58's and he says Krogens theory on boats this size is two gensets, with the smaller be essentially a disposable one. Assuming one is good for several thousand hours, I'm not so sure this s crazy.

I'm still going to work on getting power to my existing fans and house bank without the genset.


The other advantage I see to this is some day in the future if I can replenish house bank with wind and sun, we could stay at anchor for some time with little genset use.

-- Edited by Just Bob on Wednesday 21st of December 2011 05:15:35 PM

-- Edited by Just Bob on Wednesday 21st of December 2011 05:16:13 PM

Marin wrote:
Right, but wouldn't if follow that if the main engine was also driving an AC generator sized to run AC appliances like hot water heaters, stoves, and whatnot, as these loads were applied the main engine would have to work that much harder to combine the load of the generator with the load of moving the boat?**If that reasoning is correct,*you'd either need a larger engine to maintain the same propulsion power while powering the loaded-up generator, or you'd need to accept less propulsion power with a smaller engine whenever a significant load was put on the generator.

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*That is correct, you can't create energy, you can only convert it. The shaft generator can only convert horsepower that is not being used by the propeller and it does take fuel to do that.

Shaft generators are usually sized to produce X amount of power at 100 percent shaft rpm or power (if a CP wheel) and a more or less linearily decreasing amount of power down to about 50 percent of engine power. You can't just add a shaft generator to an existing powerplant that is already required to produce 100 percent power to move at X speed and expect the speed to remain the same. You can't add a shaft generator to an engine that burns X gallons per hour at Y knots and expect to burn the same fuel at the same speed and produce Z kW as well.

Where a shaft generator would work on a trawler yacht would be on one that is grossly overpowered but normally operates at low speed so that there is a large margin of "reserve power" available to convert to electricity.

Given that most trawlers don't cruise at constant rpm for extended periods the chances are that the shaft generator would frequently trip due to rapid rpm and load changes. To negate this problem, a shaft generator can be paralleled with an inverter or smaller generator to share the load. Either method means that load management is still as important as it is on shore power. If you only have a 30A shore power supply you know you won't be running the hot tub, the dryer, and the galley oven at the same time and having a shaft generator doesn't automatically eliminate that issue.

Now, if you have lots of space for batteries and still need a lot of AC power, it might make more sense to keep it simple and just install a very large alternator on the main (if it is big enough) and feed inverters through the batteries. You can parallel inverters to support large loads and use the batteries to provide "peaking" power when needed or to make up for short term main engine power reductions.

The bottom line in all this stuff is that you are trying to change the fuel in the tanks to heat and lights or cold or some other more useful form and the means to do that are as varied as the reasons to do it. It is all a compromise and no one method is better than another for all people all the time.

Just Bob wrote:
My inverter is a Victron "Quattro" 24v 5000w. I do not believe it will supply 240v, I believe I can pair it with a second and produce 240, but it would seem that the DC feed would be enormous.

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*Pretty sure you can't pair 2 - 120 volt inverters to make 240 volts. Inverter power consumption is based on 120 or 240 volt load, not the capacity of the inverter. I drive a 16,500 btu airconditioner with a 5,000 watt 12 volt inverter. The inverter could consume 420 amps at 13.2 volts to produce 5,000 watts. But running the AC unit, it only consumes about 120 amps at 13.2 volts.

It would be helpful if you could post the amp draw of the fans, so we could bloviate better.*

I'll be able to post that fan amperage draw next week when I get to the boat.

Just for knowledge, I checked Victron's website and this inverter can be paralleled with another to produce 240v. Not sure I want to (read can afford) that option, but I guess it's nice to know it's there. I think I would prefer to see if I can change the fan motors to 120.

"If that reasoning is correct, you'd either need a larger engine to maintain the same propulsion power while powering the loaded-up generator, or you'd need to accept less propulsion power with a smaller engine whenever a significant load was put on the generator."

This might be a hassle if the engine was properly sized and working hard and efficiently.
With most pleasure boats the engine is way oversized

A TT using 3gph at cruise is perhaps using 45 HP certainly under 60hp.

With frequently 120 or 135 hp farm engines , another 20 hp for 10KW would not require a vessel slow down.

Just Bob: curious as to where you've ended up with all this, having just seen this thread for first time. I am an OA owner (older, 52ft). I think the problem you face stems from the design decision to use Delta-T system at all...but that's another story and now you have it irrelevant anyway. But to 'design' for full-time genset use these days is crazy...your boat is just not that big ! Do you now have a solution to the engine room ventilation problem that doesn't require the genset to be run?