Hunter 306 aft vent, blower and engine's air intake

Ralph Johnstone · Mar 20, 2020

Don't know the year of your boat, but from what you say it sounds like a previous owner (PO) did a little customizing which didn't go so well.

The combustion air intake is not a neccessity if its only intent is to supply cooler combustion air. Actually not too smart because its the alternator which really needs the cooling and with the comb. air not being taken from the engine space, the alternator runs hotter than it would otherwise. Below is an exhaust blower I installed which cools the engine space just for the comfort of the alternator:

The "aft vent" as you call it seems to be pulling air from the cooler region aft of the rear bulkhead. Hell, why not pull from the bilge which has the coolest air on board and requires less length of hose.

Perhaps you can take a few photos showing the hose routing from the engine casing to any aft vents. Sounds like you've got a few problems. If the PO was a little off the rails with this alteration, got to wonder what else is hidden away ?

BTW which marina are you in ? I'm over in Thunderbird.

Ralph Johnstone · Mar 20, 2020

take.yvr said:
I've attached a picture of the "aft vent" which is a louver on the port side transom.

Most interesting. I'll look forward to seeing the rest of the ducting.

Ralph Johnstone · Mar 21, 2020

Pretty much what you described. I think your PO was relying on a little basic knowledge but just didn't take it far enough.

Yes, sucking on cooler combustion air will supply slightly denser combustion air (see Charles Law). However, the diesel engine is a lean burn engine and would still have a small excess of oxygen at maximum power output providing the RPM is high enough. Get into large diesel engines and you will find a few variations on this theme but suffice to say, small engine are unaffected by air inlet temperature fluctuations once the engine is up to operating temperature.

I'd disconnect the hose if it bothers you by being there. The Yanmar 2GM could introduce as much as 29.1 CFM (@2600 RPM) of cooling air if the air intake were at the far end of the engine compartment which it is not.

All in all, much ado about very little.

Any other oddities you've found on the boat ?

jssailem · Mar 22, 2020

Welcome to the forum @take.yvr

Congrats on your new boat. The PacificNW is a great place to sail. Your in good hands with the insight @Ralph Johnstone has shared.

When you get a chance, share a picture of your boat.
Fair winds and fun sailing, good to have you here on board.

Ralph Johnstone · Mar 22, 2020

jssailem said:
Your in good hands with the insight @Ralph Johnstone has shared.

Thank you. Your cheque is in the mail

.

jssailem · Mar 22, 2020

Will that be USD? This time... The local coffee shop looked at a looney and kicked me out. I told them it was real money.....

Ralph Johnstone · Mar 22, 2020

jssailem said:
Will that be USD?

Surely you jest with the current exchange rate

?

Ron20324 · Mar 22, 2020

As long as the vent on the transom is not blocked, it allows air into the compartment on the inside of the vent. Then the hose sucks the air out of the compartment. I'd think the engine could suck more air initally/on demand than if it tried to suck when connected to that vent. My B323 has two such vents, and neither one has a hose on it- just a free-flow of air onto the engine compartment.

Rich Stidger · Mar 23, 2020

FWIW, I have a h40.5; a different boat. But I installed a blower and a 3" hose between a transom vent and ending directly at the alternator. My alternator is a 110A high output unit and can run very hot.
The blower is set to exhaust the hot air out of the engine compartment and take heat away from the alternator. I have yet to figure out the route of the replenishment air, but it works.

Ralph Johnstone · Mar 23, 2020

Rich Stidger said:
But I installed a blower and a 3" hose between a transom vent and ending directly at the alternator.

Great minds think alike (ya, skip the rest of that saying

). Installed mine for the same reasons you did. The make up air comes from the rear bilge area which is the coolest air on the boat. Lots of small air passages lead to the bilge area which supplies the make up air.

At one time I had posted (under owner mods) a .PDF on the fan installation but after a few format changes around here, it was sacrificed to the ether gods.

For what it's worth, here it is ..........................................

jaysweet · Apr 12, 2020

take.yvr said:
I'm a new owner of Hunter 306 and working on cleaning and fixes...

There is a 3-inch hose running from the engine room to the aft bulkhead and the bulkhead side of the hose is unattached. First, I thought the hose was for a blower and it came off from the aft vent. But the other end of the hose is connected to the engine's air intake, not the blower. It confused me because I couldn't find any information that suggested connecting the engine's air intake to vent. The other thing I don't understand was that there was a blower switch on the DC panel but I could not find the blower.
So I would like to ask 306 (or other Hunter) owners about the followings.
1) Is the aft vent expected to be unconnected?
2) Should the engine's air intake be connected somewhere?
3) Do any 306 owners have a blower?

Thank you

We have a 2005 Hunter 306, and I can confirm that this hose was originally a blower to exhaust hot engine room air to the aft vent. It originally terminated in the engine room right near the stuffing box, in a fan which was bolted to the starboard bulkhead between the stuffing box and the Seafire extinguisher. It's clear that someone just unhooked the hose at both ends to use it to supply "fresh" air to the engine air intake.

Jay

Rich Stidger · Apr 12, 2020

Ralph Johnstone said:
Great minds think alike (ya, skip the rest of that saying ). Installed mine for the same reasons you did. The make up air comes from the rear bilge area which is the coolest air on the boat. Lots of small air passages lead to the bilge area which supplies the make up air.

At one time I had posted (under owner mods) a .PDF on the fan installation but after a few format changes around here, it was sacrificed to the ether gods.

For what it's worth, here it is ..........................................

Excellent white paper on your blower installation. So simple even a caveman could do it!!

JamesG161 · Apr 12, 2020

I have blower on my Genset to remove heat, since its housing is insulated for noise.

Way down here in the South, where engine or genset heat, warms your cabin and both take a Air Conditioned Cabin Air for combustion...

I would use that to slowly remove the heat or provide outside air for combustion.

Jim...

Hunter 306 aft vent, blower and engine's air intake

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