Engine room vents on 1974 GB42 problem

[Nick F]

This post is probably only relevant to those with older GBs powered with John Deere engines. For a couple of years the engine room vents on the fibreglass boats were located on the outside of the hull (actually on the bulwark, below the gunwale and above the side deck level).

During very heavy rain I noticed a slow rain leak into the ER. I final traced this to the stbd aft ER vent. I then removed the vent grill and was amazed to discover that the air passage which it covers is tiny. The passage is about ¼” x 15”. The ¼” dimension is simply the small gap between the inner and outer walls of the bulwark.

Due to this thin passageway, I suspect that the ventilation capability of the vent is minimal and I am thinking of blocking it off completely to stop future rain entry. I am in the PNW, my ER does not have vent fan(s), and the ER temperature is not a problem here.

Comments?

 

[Comodave]

Sometimes things look good on paper but don’t work out in real life. If the cross section is that small it can’t be doing much good. If it is leaking then IMO that would be a good reason to close it up. Can you add a vent somewhere that actually might get some air flow?

 

[Diep]

You also need to make sure enough air can get in for combustion/exhaust. I would be hesitant to completely seal things without providing another vent somewhere.

You could test it by sealing the openings with tape and seeing what happens when you run wide open.

 

[Delta Riverat]

Don't seal up the vents. Yes, the engines need those to breath. They need lots of air. If I open the ER access with the engines running I can feel the breeze passing by me. The 2 foot by 3 foot door has a lot more area than the vents do so, more flow with lower resistance. That's with (2) 5.5 liter turbo diesels at idle.

 

[C lectric]

Pay attention to the warnings above or you may create other problems.

Can you come up with a vent protector , a large piece of S.S. fastened just above the vents that are tilted outward, and are large enough to cover your existing vents from above, forward and behind.? That way the rain will fall away from the hull and vent.

There are usually SS fabricators near boatyards that could do a good job.

 

[Comodave]

Well he said the current opening is 1/4” x 15”. So that is about 4 square inches. Not really much at all. If it is leaking why not close it up and put a new vent in that will actually pass some air. A 4” vent will have about 12.5 square inches so it will flow much more air.

 

[Nick F]

Well he said the current opening is 1/4” x 15”. So that is about 4 square inches. Not really much at all. If it is leaking why not close it up and put a new vent in that will actually pass some air. A 4” vent will have about 12.5 square inches so it will flow much more air.

Thanks Dave - finally someone who sensed my dilemma! Not only is the current vent only some 4 sq in, it is wide and thin and hence much more restrictive than a 2" x 2" opening.

Since the engines currently appear to perform OK, and there is no apparent vacuum in the ER (which would pull the door open), I can only assume that there are plenty of other leakage paths which are effectively feeding the engines.

Thinking further on this, I think I will temporarily blank off all 4 vents and test for vacuum.

 

[Comodave]

No problem, glad to help.

 

[GB42-424]

I have the same boat and engines. It came with no blowers (other than those engine mounted fans). To cool the ER after shutdown I added an exhaust blower just aft of the port door. There was space behind my cabinets for more than one 4" dia hose. There also was an unused grill at that location. If you have the same cabinet configuration pull the draw and take a look.

 

[SteveK]

Well he said the current opening is 1/4” x 15”. So that is about 4 square inches. Not really much at all. If it is leaking why not close it up and put a new vent in that will actually pass some air. A 4” vent will have about 12.5 square inches so it will flow much more air.

I had a GB36 with the same vents. I am sure they vented considerably more than 1/4"x15". At least the visible vent grill openings. By memory air went in from side and then up and over the visible box seen behind grill when removed.

 

[Nick F]

I had a GB36 with the same vents. I am sure they vented considerably more than 1/4"x15". At least the visible vent grill openings. By memory air went in from side and then up and over the visible box seen behind grill when removed.

Steve - the "visible box" is the inside wall of the bulwark. Also, the size of the vent grill openings is totally misleading based on seeing the resticted passage which they cover.

 

[Nick F]

I have the same boat and engines. It came with no blowers (other than those engine mounted fans). To cool the ER after shutdown I added an exhaust blower just aft of the port door. There was space behind my cabinets for more than one 4" dia hose. There also was an unused grill at that location. If you have the same cabinet configuration pull the draw and take a look.

Which cabinet, please?

 

[GB42-424]

The two drawers my cabinet just aft of the port door are shorter by 6" than the depth of the entire cabinet. Also had an unused teak grill to the outside. Maybe you have a freezer in this space. My freezer is top loading below the seating.

 

[Nick F]

The two drawers my cabinet just aft of the port door are shorter by 6" than the depth of the entire cabinet. Also had an unused teak grill to the outside. Maybe you have a freezer in this space. My freezer is top loading below the seating.

Thanks! I have a freezer there. It would be nice to find a suitable location like you did.

 

[Comodave]

I had a GB36 with the same vents. I am sure they vented considerably more than 1/4"x15". At least the visible vent grill openings. By memory air went in from side and then up and over the visible box seen behind grill when removed.

Did you look at the photos he posted. Not much air will flow through that small opening.

 

[Endeavor GB 32]

What is on the engine room end of the airflow? Not sure if this is applicable to your boat, on our later 32, there is a baffle, the incoming air flows down, then changes direction, up and over the baffle, similar to a dorade vent. There is a drain hose from the low point to the bilge. When we got the boat, the drain hose went into a gallon plastic bottle. It had a small amount of water in it, I never emptied it, never noticed the level going up. Our air intake was on the side of the cabin. FWIW, I would not seal the air opening. Good luck!
Regards,
Scott

 

[GB42-424]

here the space behind my liquor cabinet and past icemaker. Enough to install a buss heater. Also note access panel to see dorade box.

 

[Nick F]

here the space behind my liquor cabinet and past icemaker. Enough to install a buss heater. Also note access panel to see dorade box.

Here is a view of the hole I cut while trying to better see the connection to the vent. The second photo is a close up of the box around the back of the vent (dorade?)

 

[GB42-424]

Similar to mine. Not sure if this is a proper dorade. Never looked in from below to see if there are baffles or drainage. I'm wondering if they are wide enough to capture both external air vents. The aft-most vent might just be ventilating this dead space.

 

[Nick F]

Similar to mine. Not sure if this is a proper dorade. Never looked in from below to see if there are baffles or drainage. I'm wondering if they are wide enough to capture both external air vents. The aft-most vent might just be ventilating this dead space.

This box in the photo only covers the aft vent. I will look to see what is opposite the forward vent.

 

[Nick F]

This box in the photo only covers the aft vent. I will look to see what is opposite the forward vent.

Ok - Today I looked to see what is attached to the forward vent and the answer is . . . . nothing! This presumably means that the forward vent is just open to the "dead space" under the side deck. I will confirm this better with a coat hanger wire.

 

[Nick F]

Today I performed a little experiment and measured the engine room vacuum.
I rigged up a piece of nylon tube as a manometer and inserted one end of the tube into the ER. The other end was open to the inside of the aft cabin.
I ran the two engines at 1500rpm and checked the manometer.
I then blanked off all four ER vents on the boat's bulwarks and repeated the manometer measurement.
Check the photos of the manometer during the two conditions.
Basically, there was no measurable vacuum with the 4 vents blanked off!
Out of curiosity, I checked the small grill (lazarette vent) located on the side of the boat close to the stern and could sense a "draft" with my fingers.

 

[Delta Riverat]

Looks like a 1 mm difference. I would think you have a lot of "other leakage" into the engine room. Perhaps from the aft cabin?

I can feel the breeze when I open the ER access door at idle, about 2' x 3' because that's bigger than the ER vents.