Vents For The Engine Compartment

Jackdaw · Jan 8, 2012

Your engine will run better and cooler for sure. Cooler because even with closed loop water cooling, ambient air temp effects operating temperature. Besides engine cooling, having fresh combustion air is a good deal. I would terminate one of the hoses right in from of your air cleaner. If you can, put a in-line hose fan on the exhaust hose. On our boat, the intake is in the cockpit and the exhaust is on the transom. I don't think it makes the noise worse, in fact i think it makes it quieter. But doing this you can better sound insulate your engine compartment. I assume now it was designed 'loose' to allow combustion air to leak in.

Steve Dion · Jan 8, 2012

Joe:

I don't know I would mess with this unless you consider adding a fan to induce fresh air into the engine box. These engines have been running just fine for the past 25 years.

If you think it is worth testing, why not just remove the cover over the stuffing box and see if you notice any difference. I have my doubts!

Allan12210 · Jan 8, 2012

There is an inspection port on the side of the engine box (at least on mine) where you can reach in and pull the oil dip stick. I leave the cap off and have a computer fan there wedged into the sound insulating foam that I just hooked up with some clip leads to the back side of the battery switch. I don't know if it helps, but I run it while the engine is on. Its your basic kluge that is part of a larger project to exhaust during and after the engine is shut down to keep smells out of the boat.

kloudie1 · Jan 8, 2012

I think there is plenty of intake opening area in the box, considering the bilge access around the propeller shaft and the holes in the grid members .. Exhausting hot air is a problem. On a long motor in 90F + ambient, the box sometimes gets hot enough to trip the thermal breaker in the air conditioning pump. (pump not running) My gut feel is that it gets around 160 to 180 F in there .. can't be all that good for the alternator.(it is 27 years old, but never has to work very hard). but worse, when ya shut down at night, all that heat comes into the cabin .. I have the in-line fan, I haven't installed it yet.

Ralph Johnstone · Jan 8, 2012

I Know Exactly ........................

............ what you mean. My Yanmar 2GM20F sits under the hat box shown in the attached photo, below the companionway steps. Dead quiet when the engine runs. The combustion air enters the cover from along the bottom of the hull with plenty of cross sectional area. However, the only air flow is the combustion air which amounts to about 30 CFM at 2,700 RPM. Not good as this doesn't cool the compartment and the combustion air is hot ......... about 128 deg. F. I initially looked at this problem as the alternator was getting cooked (170 deg. F) because it's been forced into hard labour with a newly installed Balmar ARS-5 alternator regulator.

But enough of why it's hot in there. A solution I'm presently looking at is with a 3" dia. Shurflo Yelowtail Blower. With all of the corrugated aluminum ducting hooked to it (clothes dryer ducting), I saw a differential pressure of about 0.9" across the fan
which would allow a flow of about 100 CFM according to their literature. Total flow with the engine running, about 130 CFM.

With the fan temporarily installed right near the engine compartment and discharge hose laying all over the cockpit floor, the interior temperature dropped to approx. 80 deg. F. Better for combustion and healthier for the alternator.

ONE BIG PROBLEM: I can't run the 3" dia. hose back to the transom and locate the 3" vent high enough so that I won't be
having a coronary each time I see a following wave ready to jump down the vent. And the vent must be on the transom as the
Shurflow blower is noisy, really noisy ................. it's a small fan which moves a lot of air at high RPM's. I've tried to insert sound
baffles in the aluminum ducting but this reduces the air flow and the engine compartment temperature climbs quickly.

A Shurflo fan may be the answer to your problem if you can come up with a safe discharge location.

Maine Sail · Jan 8, 2012

Joe Mullee said:
On my Hunter 34 there are no vents to allow air to pass in and out of the engine compartment. I was reading a blog by a Yanmar Distributor who suggested that adding vents would help the engine run more efficiently and provide an increase in power (noise too). I think he suggested two vents of 56 square inches for the 3GMF in my boat.

Has anyone done this or maybe the vents are already in place aboard your boat. For the engine experts out there is it a worthwhile thing to add?

Thanks,
Joe Mullee

Joe,

If you can borrow a "draft gauge", which measures negative or positive draft pressures, this is the best way to test the engine bay. They are used in the heating industry for setting up boilers & furnaces and are quite sensitive to either negative or positive pressure. Run the engine for 30 minutes or more at cruise and see if you can measure a negative in the engine bay. If no negative is noted then no need for vents.. If you normally run the bilge blower than you'll want to test with that running. Just because you add vents it does not mean you'll create a dramatically cooler environment you'll often just break any negative pressure.

Dan Johnson · Jan 8, 2012

Joe...

I can only echo what Allan, Claude, and MaineSail have said. I see no need for added air INTO the engine box, BUT, getting heat out after a long run is of a large concern.

I've never had a problem running 6-7 hours in the 2,600 - 2,800 RPM range and never felt the engine was "starving" for air.

I do have a digital indoor/outdoor thermometer at the companionway, with the outdoor sensor hanging above the engine near the heat exchanger. On the hottest day I've never seen it get above 145* in the engine compartment after a long run, but it does exhude heat into the cabin once it's shut down. Even with engine box insulation, the surrounding paneling is quite warm to the touch as a consequence.

I have a bilge blower, hoses and some fittings aboard in a v-berth cubby to install a "heat remover" like Claude plans to do. I just haven't gotten to it as yet.

Allan did give me the idea to rig a temporary test set-up using the Beckson port by the dip stick to run a hose to the blower in the cockpit on a lighter plug-in to see if the benefit is worth the engineering to make it a decent, clean installation.

Rick D · Jan 8, 2012

Maine Sail said:
If you can borrow a "draft gauge", which measures negative or positive draft pressures, this is the best way to test the engine bay. They are used in the heating industry for setting up boilers & furnaces and are quite sensitive to either negative or positive pressure. Run the engine for 30 minutes or more at cruise and see if you can measure a negative in the engine bay. If no negative is noted then no need for vents.. If you normally run the bilge blower than you'll want to test with that running. Just because you add vents it does not mean you'll create a dramatically cooler environment you'll often just break any negative pressure.

Huh. I would have guessed there would be some negative pressure with the engine at cruise loaded RPM. No? So, maybe you can elaborate a bit more for those of us trying to follow the concept.

Maine Sail · Jan 8, 2012

Rick D said:
Huh. I would have guessed there would be some negative pressure with the engine at cruise loaded RPM. No? So, maybe you can elaborate a bit more for those of us trying to follow the concept.

Not been able to measure any negative pressures on any of the boats I've tested. I thought my C-310 would be negative but it was not. If you have negative pressure you are beginning to starve the engine.

You can also use a manometer but I find the draft gauge to me a lot more sensitive...

The Bacharach Draft Gauge I use looks like this:

Rick D · Jan 8, 2012

Maine Sail said:
Not been able to measure any negative pressures on any of the boats I've tested. I thought my C-310 would be negative but it was not. If you have negative pressure you are beginning to starve the engine.
You can also use a manometer but I find the draft gauge to me a lot more sensitive...

Thanks. That's very helpful. I often wonder about mine and this provides a tool for objective measurement vs. seat-of-the-pants. I appreciate it.

Benny17441 · Jan 8, 2012

These engines are small and are getting all the air they need. To add ventilation ports in order to increase the flow of air is pointless and will increase the noise level. A gasoline engine benefits from a source of cool air intake as it provides a denser fuel /air mixture which results in added power. That effect is mostly lost in diesel engines which by design heat the fuel/air mixture through compression to cause the detonation. There is no electric ignition in Diesel engines. The other aspect is engine running temperatures and that is efficiently regulated by the constant replenishment of raw cooling water which efficiently draws heat from the engine or exchanger. The air temperature inside the engine cover does not affect the engine in a significant manner as the fuel/air mixture will heated by compression. This final comment just reflects on my way of thinking and has no pro or con consequences. These engines are considered auxiliary engines to supplement the power of the wind on the sails when strength or direction are not favorable. There is not much to be gained by trying to tweak them. If I wanted to increase performance I would put my money on the sails and the rig and not on the engine.

Terry411 · Jan 9, 2012

Hot air equals less power more smoke from the exhaust. Alot of time money engineering goes into coldair intakes and intercoolers. Getting heat out is good for wires hoses belts and other components mentioned.

Richard Bryer · Jan 9, 2012

I have found the heat in the engine compartment to be a problem in the summer when after a run of a few hours it makes the aft cabin hot for sleeping. Our solution has been to leave the engine compartment hatch open ( put a companionway board in so no one steps into the opening).

seadaddler · Jan 9, 2012

Exhaust Blower

I think most new Hunters have exhaust blowers and that's what I use to help elimate some of the heat in the rear berth in the summer and most always when ever running the engine,I have a rear berth hatch that I leave open to help also in the summer months.
Nick

Jackdaw · Jan 9, 2012

Colder air is denser air and per cubic unit contains more o2. Diesel or gas, all engines run better supplied with cool, denser air.
And regardless of the existence of a water based cooling system, the correlation between high ambient air temps and higher operation temps is well documented.

The use of fans in ductwork to both both cool the engine compartment and supply combustion air is pretty much SOP in current sailboat design; Hunter's too I bet. Must be a good reason for it.

kloudie1 · Jan 11, 2012

Dunno, Joe.. I am still scratching my head over how to run the hose.. I think it may go thru the bulkhead above the batteries and back to a clamshell vent on the aft cockpit combing above the transom.. Going to try to get the hose up high to keep following sea water out.

dqb24 · Jan 12, 2012

Ralph Johnstone said:
............ what you mean. My Yanmar 2GM20F sits under the hat box shown in the attached photo, below the companionway steps. Dead quiet when the engine runs. The combustion air enters the cover from along the bottom of the hull with plenty of cross sectional area. However, the only air flow is the combustion air which amounts to about 30 CFM at 2,700 RPM. Not good as this doesn't cool the compartment and the combustion air is hot ......... about 128 deg. F. I initially looked at this problem as the alternator was getting cooked (170 deg. F) because it's been forced into hard labour with a newly installed Balmar ARS-5 alternator regulator.

But enough of why it's hot in there. A solution I'm presently looking at is with a 3" dia. Shurflo Yelowtail Blower. With all of the corrugated aluminum ducting hooked to it (clothes dryer ducting), I saw a differential pressure of about 0.9" across the fan
which would allow a flow of about 100 CFM according to their literature. Total flow with the engine running, about 130 CFM.

With the fan temporarily installed right near the engine compartment and discharge hose laying all over the cockpit floor, the interior temperature dropped to approx. 80 deg. F. Better for combustion and healthier for the alternator.

ONE BIG PROBLEM: I can't run the 3" dia. hose back to the transom and locate the 3" vent high enough so that I won't be
having a coronary each time I see a following wave ready to jump down the vent. And the vent must be on the transom as the
Shurflow blower is noisy, really noisy ................. it's a small fan which moves a lot of air at high RPM's. I've tried to insert sound
baffles in the aluminum ducting but this reduces the air flow and the engine compartment temperature climbs quickly.

A Shurflo fan may be the answer to your problem if you can come up with a safe discharge location.

Ralph-
Take a look at the Vetus SCIROCCO clamshell vent for 3" hose for your transom.

Angled to the side/pointing to the horizon and slightly down for rain, that should discopurage any following wave from entering. Also put a gooseneck loop inside the boat (I have one for my diesel tank vent that is coming in from the transom close to where I would place a vent - not sure where yours is). You may want to use the diesel tank vent placement as an indicator as to where the boat designer felt is was safest to locate such as item - as you can imagine they defintely do not want water from waves getting to the diesel tank.

As Claude said, placement on the aft cockpit coaming (on the H34) is a very good idea, and Allan's idea to do inital testing using the dipstick inspection port is brilliant.

Keep us posted all.

Dave

redhead78 · Jan 12, 2012

Hi all I had 2- 4" static vents that looks like original equipt.the hoses run from either side of the engine compartment and terminate on either side of my helm seat,and there are vents. The original plan I think was to let passive air in or out. When I repowered I chose a 100 amp alternator, and the manufacturer recomended rather insisted I install a bilge blower fan. I installed to exhaust on one hose, the other as intake, can not hear it when running, and it can't hurt to turn over the air in there.I installed it on a seperate switch, it does help to let it run after motoring back to the mooring, it cools the space otherwise its like having a big radiator that stays hot for quite a while. I can't really say whether or not it helps the engine run better, but my alternator may last longer, it puts off some heat...Red

Vents For The Engine Compartment

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