Gentlemen-
I am hoping to remove the shell type vents on the side decks. Instead I plan to install vents in the hull sides to each engine compartment and one in the center of the transom. Like these with dorade boxes inside. I have read the regs and fumbled through the logarhymthic tables, swore and then realized it wasn't a big deal to look at the volume of the current 4 shell vents and increase that...
Four 4" holes yield 50.2 square inches. If I just beat that number am I ok??
Thank you,
Stephan
Why the vent in the center of the transom? My 31 has one, and it's the only one I've ever seen, and I've never understood it's purpose. Anyone got the answer??
LJMauricio,
The vent in the middle of the transom is for natural air flow, there is a FRP chute glassed into the inside of the transom behind the cockpit covering panel. It is an open funnel effect from just below the cockpit sole ledger board out the transom. These were only on the early 31's. When the vessel is underway the natural suction at the transom draws out air up from the aft bilge compartment.
Stephan, if your going to use those vents make sure you use atleast 2500CFM blowers attached to the aft hose, The way the old clam shells were set up, forward was natural flow in from front clamshell, blower was attached to aft clam shell to suck out fumes in engine compartment. I found that running the blowers all the time when running at high speed helps keep the engine box temperatures down. I run 4" hoses with 1 Attwood 3000CFM blower in each engine box.
Pete Fallon
Peete-
Thanks. My blowers are flange mounted to their own exhausts on the hull in each engine space. The shell vents were plumbed to the center of the transom spot you described.
I reckon a 3000 CFM blower would lower engine temps.... I've got a 105 CFM Jabsco blowers in each engine space...
Thanks Pete, just confirms that this is the place to go for B 31 info. You, Patrick, and the guys are amazing. I stay pretty quiet in the discussion, but have used much of the info I read here. Thanks again. We are getting ready to paint the hull this spring, and I was considering glassing over that vent.
another question about the vents. My 31 has 4 low profile clamshells on each side. 2 face fwd and 2 aft. Was this ever standard, or did some previous owner add 2 more? 1965 hull #314-413
LJ-
My boat is a 1972 FBC. I believe the 2 forward 2 aft shell vents were standard from the factory for my boat. The aft vents were plumbed to the spot Pete refered to in the center of the transom.
Last edited by Stephan on Feb 28th, '13, 13:01, edited 1 time in total.
I know this doesn't apply to diesels but I have 454s. I removed all the clam shells last year. I do have blowers on the sides of each engine compartment which I run before starting the motors, there is a dryer style like you pictured that runs down, blower motor is mounted to inside of hull with a small 6" cover on the outside. The theory is to remove fumes that potentially may have settled in the bilge. I turn my blowers off after the motors are running.
As far as getting air to the motors and airflow in general, if you have the old cockpit panels and the edges of the cockpit are not glassed to the hull there seems to be plenty of air movement that I haven't noticed a difference. I have a 4" gap the width and the length of the cockpit along with a hole in the panel to store lines that probably 2'x2'.
I was concerned that I'd need more ventilation, my plan was if I needed it to cut a 2'x2' hole in the side panels as far forward as possible and cover with a grill vent, kind of like the poor mans version of what Phoenix and Chimera did.
I am by no means an authority on anything but I wonder if adding those large vents is neccesary.
My other piece of advice is consider where you put the switch for the blowers, generally when I get on the boat the first thing I do is open the engine boxes and give a good sniff, then I go into the cabin and flip the breaker, then up to the bridge to turn on the blower. Then back down to do other stuff.
PeterPalmieri wrote:I know this doesn't apply to diesels but I have 454s. I removed all the clam shells last year. I do have blowers on the sides of each engine compartment which I run before starting the motors, ......... I turn my blowers off after the motors are running.
Without those clam shells I would keep those fans running, but that's just me........
Well I can tell you this. A running electric fan will just about run forever if not switched off. A bilge fan in a salt water environment that sees sporadic use will likely last longer if it's used more than less.
As far as the safety issue, perhaps Pete or Pat will have some insight.
Are the fans as necessary on diesel boats for cooling or ventilation ? I didn't consider cooling, just flammable vapors. I had a friend in Panama whose gas boat expolded at the Amador Marina. Should I be using those blowers while running with the Cummins ?
I was beginning to think I was the only one that had a B31 with the upper center transom vent. I have never seen it before either on B31s I have seen in person or in the many published photos of B31s. This 1963 boat #230 also has twin airscoops on each side facing forward alongside the cabin about even with the front edge of the motorboxes (have seen them on many B31s) and a single clam shell on each side even with the rear of the motorboxes facing to the stern. The blowers vent to the clamshells. This is a gas powered boat.
Is the transom vent necessary? I have been considering removing/glassing over it simply to create a more appealing look to the transom.
I do not wish to confuse gas rules with diesels---:
For gassers I thought I was required to have a passive ventilation plan that presented some flow and not just a static vent - so forward vents in the engine space and an exhaust at the transom. Can I instead just have larger side vents in the engine spaces? Also, I thought the ventilation was required to be to directly outside the boat and not to the cockpit area.
Thanks,
Stephan
As long as you have exhaust fans to draw potential gas fumes overboard. Otherwise all other vents are for supplying the minimum air flow to the engines at WOT and can be passive. The engines will draw in their needed air as long as the vent sizing is adequite...
PeterPalmieri wrote:Mike I was concerned that if leave them on all day I'd blow them.
Peter-
On my gasser 454, I had 4 clam shell vents on the gunnels, with the forward vents facing forward with the hoses going to the bilge in the engine compartments. My aft vents faced aft and the hoses ran to the transom and into the bilge like in the picture above. I also had the small metal grate vents in the hull sides with blowers with hoses that ran into the engine compartments. My drill was to open the boxes, go up to the FB and hit the switch for the blowers, start the engines, take a look and then shut the engine boxes--and leave the blowers running until I shut the boat down. The blowers were in there when I got her, and I ran the blowers like that for the 9 years I had her. They never missed a beat. I'm with Mike--keep 'em going.
I remember looking through the USCG requirements on this subject a couple years ago. I have not read through again but have attached a link.
Seemed like our clam shells would just barely qualify for the passive ventilation that the coast guard requires for gasoline powered vessels. I think they want an intake as far forward as possible and an exhaust near the transom. This way you have air flow at the bottom of the bilge where the heavier gas fumes might linger.
If I remember, compliance to the word of the USCG is almost impossible without putting a louver in front of the windshield. I have clam shells in both directions with several feet of winding duct work that suggests it is trying to do what the USCG describes.
Meanwhile... I am sure my blowers have more hours than the motors. I usually don't turn them off until I tie back up, even on the overnight trips.
with dorade boxes inside. I have read the regs and fumbled through the logarhymthic tables, swore and then realized it wasn't a big deal to look at the volume of the current 4 shell vents and increase that...
