Engine Room Ventilation, blowers, sizing, gasoline vs diesel and other thoughts

[hughespat57]

So on a recent thread the questions was asked about "running blowers" when and why.
http://www.clubsearay.com/index.php?threads/blower-or-no-blower.95634/page-5
I am not going to rehash here "should you or shouldn't" run them on startup, yes you should.

But some side questions as to continuous running, ER heat, trying to get best performance etc. came up. With some valid and some not so valid assumptions. One of which is that the boat manufacturer designs to meet the requirements.

I'm not so confident, I'm a former USCG MK1, marine mech, ASE Master HD truck mech. And these question did bring back situations I have experienced where no the boat manufactures were idiots or barely competent.

So first thing is there are two distinctly different situations gasoline & diesel power. Both physically and legally they are quite different for ventilation.

Around 1990 on a 1984 45 ft Hatteras, original engines twin Detroit 6-71NA, owner re-powered twin Detroit 6V-92TI.
Customer comes to our shop, smoking & poor performance. On the test run you could not open the door to the engine room at WOT. It was physically impossible. With the door open you could feel the air sucked in around you. Turns out the boat did not even have enough vents to meet the requirements for the 6-71 naturals, now with 6V-92 Turbos the engines starved for air.

Saw the same with gassers. I believe it was about a 1994 Cruisers yacht 35 foot with twin 454 Crusaders. 454/7.4 liter engines need 680+ CFM of air flow per engine. Again at WOT you could barely lift an engine hatch. And could feel it give finally as you broke the suction. 3" vents just did not cut it.

Typical recreational boats have 3" or 4" Ducts and blowers. Mine has 4". I haven't looked at a newer boat closely in years is anyone running 6" ducts? Or larger? I don't see 6 inch blowers anywhere.

Mercury states the maximum vacuum in the engine compartment can be 1" H2O. Diesel engine manufactures typically state 0.25" is maximum and preference is 0" (no restriction) or slight positive with forced ventilation. Most diesels will specify how much CFM of air or square inches of venting is required else you void the warranty.

And there lies the big difference between diesel and gasoline.
Gasoline blowers MUST be exhaust blowers and the blower ducting MUST go to low in the engine compartment. This is part of the CFR. Gasoline is heavier than air and you want to suck it out of the bilge and send it outside.

Diesel boat should really have the blowers pushing air into the boat same ducting, low in the engine room. This pushes hot air out the higher "exhaust" vents. And all ducts are providing air for combustion if needed (WOT). But positive pressure can lead to engine odors in the cabins if not sealed well.

I have spent two days review airflow calculations and I have my doubts that most of the older boats breathe well, And really wonder about newer boats.

So what do you think? What have you experienced?
In "part 2" - shortly I will provide specific calculations and I have an Excel spreadsheet that does any number of what if for vents, HP, single/twin, and standard manufacturer requirements.

 

[hughespat57]

Attached is an Excel sheet that does what ifs
Enter your information in the Yellow boxes and see what you get....

 

[hughespat57]

So part 2 of the question. Focusing on gas engines.
My "little" 5.7/350cid twins and a 3kw generator need 1124 CFM of air.
Typical port and starboard vents two 4" Intake and two 4" Exhaust with blowers.
So if they are all free to flowing the total can flow only 780 CFM at 1"H2O of restriction.
So not good at WOT, where I would have just over 2" H20 restriction.

Now consider if the blowers are running they are trying to use two ducts to pull OUT 150 CFM of air each.
While the engine is trying to draw 1124 CFM air IN.
This makes me change my mind about running the blowers continuously...

 

[mobocracy]

I'm pretty sure Power and Motoryacht had an article in this vein within the last year, about insufficient engine room air flow.

 

[hughespat57]

I'm pretty sure Power and Motoryacht had an article in this vein within the last year, about insufficient engine room air flow.

You are correct good sir...
https://www.powerandmotoryacht.com/maintenance/how-to-properly-ventilate-your-boats-engine-room

Their example is larger boats but validates some of the formulas I had.

And that search lead to another larger diesel story
https://www.passagemaker.com/technical/the-holy-place-let-it-breathe-engine-compartment-ventilation

But I was surprised that even a smaller boat and engine setup like my '94 300 is marginal at best.

 

[Strecker25]

this is really interesting. I didn't plug my info in yet but it looks like CAT recommends some pretty serious flow even for our little 3126's and the stock bilge vents are not very large

 

[hughespat57]

A single engine boat with 4" vents is "good" up to an 8.1L @ around 370HP this is just under the 780CFM capcity of those vents.
As soon as you get into twins, anything larger than a pair of 4.3 / 190HP and the vents are maxed out.

 

[hughespat57]

this is really interesting. I didn't plug my info in yet but it looks like CAT recommends some pretty serious flow even for our little 3126's and the stock bilge vents are not very large

Yes, diesels are air breathers they need full scavenging and as much pure air as possible ingested. Because then you inject the fuel directly into the cylinder (not the intake like gas MPI) so the system is calibrated to "expect" a proper air charge to get the correct air/fuel ratio.

And this is where I also don't trust the manufacturers. The boats are built for gasoline, diesel is optional.
Which way do your blowers run? Intake or Exhaust? How big are your ducts? 4"?
Any extra vents you see on your boat? That may not be on gassers?

 

[b_arrington]

My boat was designed for diesel engines only. Happily it has a pair of massive air intakes on either side of the coach top. They are not directly open on the sides, but have a baffle so air goes upward in the grate, then down into the engine space. Being high up, it also helps prevent any water from getting in. The blower also discharges air to one of the intake grates. I've opened the engine hatch when running and and had no issues - although it's not a great test because the hatch has electric lifters.

See the intakes on the representative model below. They are like 5 feet long and the air intake is about 4-5 inches along the length, and serve a single Cummins QSC 8.3 600.

 

[Strecker25]

Yes, diesels are air breathers they need full scavenging and as much pure air as possible ingested. Because then you inject the fuel directly into the cylinder (not the intake like gas MPI) so the system is calibrated to "expect" a proper air charge to get the correct air/fuel ratio.

And this is where I also don't trust the manufacturers. The boats are built for gasoline, diesel is optional.
Which way do your blowers run? Intake or Exhaust? How big are your ducts? 4"?
Any extra vents you see on your boat? That may not be on gassers?

almost certain 2 blow out and 2 blow in but now I can't wait to check

 

[spikedaddy99]

almost certain 2 blow out and 2 blow in but now I can't wait to check

I had the same thought!

 

[b_arrington]

I had the same thought!

My 270 AJ had a single passive intake routed low and a single powered vent. But the vents were larger than the 4" ducting.

 

[hughespat57]

almost certain 2 blow out and 2 blow in but now I can't wait to check

I had the same thought!

See the 410DA was offered in both Gas or Diesel so I wonder what it was "designed for"
Now the 500DB was only Diesel, like the Back Cove 37, so more likely Sea Ray took the diesels into the design

 

[ttmott]

I think you will find that Sea Ray exhausts to the outside all air in their engine room / bilge powered ventilation systems. They do this, in part, to keep odors out of the cabin spaces. Unlike large yachts the bare minimum is done to seal and isolate the engine rooms in SR boats from the living spaces. I think to make manufacturing simple, SR in their boats that were offered in both Diesel and Gasoline power the exhaust systems had ductwork that drew air and vapors from the lowest part of the bilge as required for gasoline engine boats; my diesel 400DA was like that conversely my 52DB (only offered in diesel) extracts air rather high up in the engine room. SR really did a decent design job in their engine room/bilge ventilation design aside from using POS exhaust fans. So don't confuse the exhaust fans with the hull air supply for the engines; they are different in purpose. I can assure you that the entry hatch to the lazarette on my 52 is no different to lift open when the boat is running full cruise as it is in the slip which confirms there is little or no differential pressure present. That hatch is about 16 square feet; it wouldn't take much of a differential pressure to make it impossible to lift open. 2 In H2O differential would be about 166 pounds of force on that hatch. As a calibration 2 inches of water column is 0.072 Pounds per square inch.

 

[hughespat57]

And being exclusively designed for diesel I would hope Sea Ray did a good job. Exhaust high wither pulled out by blowers mounted there or by pushing air in low is the preferred flow for diesel engine rooms. (In low/Out high)

The parts books only shows the two blowers 4" and their vent hoses. But doesn't clearly show what other intake ducting is built in or how big that is I would be curious to know that if it's obvious to see.
As you pointed out for your previous 400DA, that's what is more of a concern. Designed for the least common denominator gasoline to meet the legal requirements if a gas engine. Again the parts book only shows a 4" exhaust system, but no mention of how big the intakes might be.

If the intakes are large enough then no issues, but my intakes are only 4".

 

[b_arrington]

I think you will find that Sea Ray exhausts to the outside all air in their engine room / bilge powered ventilation systems. They do this, in part, to keep odors out of the cabin spaces. Unlike large yachts the bare minimum is done to seal and isolate the engine rooms in SR boats from the living spaces. I think to make manufacturing simple, SR in their boats that were offered in both Diesel and Gasoline power the exhaust systems had ductwork that drew air and vapors from the lowest part of the bilge as required for gasoline engine boats; my diesel 400DA was like that conversely my 52DB (only offered in diesel) extracts air rather high up in the engine room. SR really did a decent design job in their engine room/bilge ventilation design aside from using POS exhaust fans. So don't confuse the exhaust fans with the hull air supply for the engines; they are different in purpose. I can assure you that the entry hatch to the lazarette on my 52 is no different to lift open when the boat is running full cruise as it is in the slip which confirms there is little or no differential pressure present. That hatch is about 16 square feet; it wouldn't take much of a differential pressure to make it impossible to lift open. 2 In H2O differential would be about 166 pounds of force on that hatch. As a calibration 2 inches of water column is 0.072 Pounds per square inch.

This is a good point about the hull intake area vs. blower hull area.

Re: engine compartment sealing, I found my 270 AJ's engine compartment to be well sealed. There was copious silicone applied to the forward bulkhead areas. The aft bilge was separated from the forward bilge, so nothing travelled forward. It's a sample size of one, but I thought SR did a good job.

 

[Henry Boyd]

This is a good point about the hull intake area vs. blower hull area.

Re: engine compartment sealing, I found my 270 AJ's engine compartment to be well sealed. There was copious silicone applied to the forward bulkhead areas. The aft bilge was separated from the forward bilge, so nothing travelled forward. It's a sample size of one, but I thought SR did a good job.

Ditto on the engine compartment & living space on our 280. They probably used a carton of caulking tubes to seal the wo spaces. On our Sabre there is basically an open air space under the sole & deck from bow to transom. From what I have seen of other models and sizes of Sabres this feature appears to be uniform for the brand.

 

[b_arrington]

Ditto on the engine compartment & living space on our 280. They probably used a carton of caulking tubes to seal the wo spaces. On our Sabre there is basically an open air space under the sole & deck from bow to transom. From what I have seen of other models and sizes of Sabres this feature appears to be uniform for the brand.

Same on both my Back Coves. It's a single bilge that extends from the bow to stern.

 

[Henry Boyd]

Same on both my Back Coves. It's a single bilge that extends from the bow to stern.

Any idea of what they do on the outboard models?

 

[The Bill Collector]

I only run my blowers at the end of the trip to exhaust heat out of the E.R. at dead idle speed. (diesel engines)