[C320-list] Bilge blower

[Jeff Hare]

Hi Joe, 

You've gotten a lot of info already on this.  This is a bit long, but hopefully it'll get you thinking more critically about reaching your goal...

First, I'm sorry to say, your dealer wasn't correct about running it continuously so you can safely just forget he ever said that.  The blower wasn't intended for that purpose and really isn't doing what you would think it's doing, read on. :)

Let's analyze this a bit using some info from Boat Mechanical Systems Handbook. I'm rounding up heavily in the math below.

- The Yanmar engines in our boats are just under 30hp (27-29hp). The minimum intake airflow formula I believe Caterpillar Diesel uses is ~2.5 * HP for an engine at full throttle, but let's use 2.7 for giggles and extra margin. This says they would pull less than 85 CFM of air at full throttle. 

- So, this means the engine area needs about 9 sq inches of unobstructed ventilation area to meet its minimum air intake requirements into the engine area with passive airflow (no blower). (Formula for amount of sq/inches needed is roughly Engine HP / 3.3).  

- Catalina provided a pair of 4" holes cut into the shaft tunnel shared with the engine. When unobstructed, those 2 should easily supply 15-20sq inches of air intake area. There are other places where air will enter the engine area as well since it isn't a sealed compartment by any means. So, /without/ the 4" holes being obstructed, there's plenty of cool air for the engine.  REMEMBER: The engine is going to be both drawing cool air in and due to the position of the air intake port on the engine, it will also draw excess heat out of the upper part of the engine compartment, through the engine and out the exhaust.  You don't need an exhaust blower fighting that process when the engine is running. The engine will have far more power to draw air in than the blower has trying to draw hot air out.

- The temp of the air being drawn from the bilge/below inner liner and aft locker is typically the coolest air around since it's being drawn across the area of the hull floor that's in the water.

Now let's look at the blower system.

- Problem 1: I don't really know what it's CFM rating is, but there is virtually zero chance that this will protect you against a propane leak. 
Let's assume that you had a leak in the propane locker AND ALSO the vent hose was broken and open into the aft locker instead of running out the drain. That would fill the aft locker and likely work its way into the bilge and under the floorboards in the main cabin and some of it would undoubtedly end up in the shaft tunnel where the engine is but only some of it. The Blower is pulling limited amounts of air to begin with AND only from the shaft tunnel area for the most part. That would remove only a tiny amount of propane that happened to find its way into that shaft tunnel squeezing around the hoses. You'd know you had a problem long before you went to start the engine because you'd be smelling it in the main cabin.  A leak in the hose leading to the stove would likely not even get into the engine tunnel until it backed up high enough to leak into that compartment.  The blower was designed to remove gasoline fumes due to a leak at the carburetor or fuel line at the engine. That simply isn't a thing on our diesel boats.
To protect yourself from propane leaks, shut off the propane at the tank and install a propane alarm in the bilge. 

- Problem 2: The blower vent hoses are very heavily corrugated dryer vent hoses with loads of air resistance. They are practically useless for moving air by vacuum. They collapse if you just look at them and make lots of sharp bends further restricting airflow. The one connected to the blower draws very little air out in practice. The other one travels a crazy route winding all the way across the boat and is supposed to provide fresh air in. Total fail. LOL!  The result of all this is that they only serve to block the natural unobstructed airflow into the engine compartment through that pair of 4" vent holes.  When you RUN the blower when the engine is running, you're actually fighting the engine trying to draw air in and the engine will win every time.  So the blower running with the engine is doing nothing good whatsoever. Guaranteed.  Even when you run this blower with the engine off, it's unlikely to be removing enough heat to notice. You could prove this to yourself if you wanted to do some measurements and run an experiment.  Note that the hose in and the hose out are quite close to each other in the shaft tunnel. What do you think the airflow pattern really is going to be there?

In Theory the STB side vent hose would draw air in for the engine when the engine is running and the PORT vent hose connected to the blower would draw gasoline fumes out when run before starting the engine. It doesn't work in practice. Neither hose moves sufficient air to serve the intended purpose so the engine happily pulls air from other sources and gets on with business.

Solution:
==========
- Remove every scrap of that white blower hose.
- Add mesh/screen over the aft intake vents and let air get drawn in, or flow out naturally as needed by the engine, or when the engine isn't running, excess heat from the fridge will exit those vents and be replaced by air from the bilge and keep the fridge running a little more efficiently.
- The engine compartment will cool down just fine after a long run with no help, but you can get nearly silent IP65 rated "computer" fans that will move more than 100cfm of air and run on 12vdc and just wire one up right at the port aft vent without being routed to the engine tunnel. That would make it cool the aft locker, keep the fridge from running so often and gradually draw air out of the bilge area. You can accomplish the same thing by cracking open the port aft locker over the fridge and letting the heat rise up out there. 

Best bet is to insulate the engine compartment like many have done using the heavy Sound Down dense foam and let a lot of the heat dissipate through the hull over time. It'll also be as quiet as it can be while running the engine.

Crack a beer, don't over-think it and just have fun sailing.

-Jeff Hare
Formerly C320 #809 (Sailed 2001-2021)
Currently C355 #17
  
-----Original Message-----
From: C320-list <c320-list-bounces at lists.catalina320.com> On Behalf Of Joe M
Sent: Wednesday, December 15, 2021 2:23 PM
To: C320-List at catalina320.com
Subject: Re: [C320-list] Bilge blower

I want to hot air out, fresh air being pulled in for engine as dealer told me to leave it on running when I bought it 3 years ago. My first diesel, first inboard, first sailboat. Previous boats of 30 years were all outboards.

Joe 2002 C320 hull #902

On Wed, Dec 15, 2021 at 11:09 AM Troy Dunn <troutwarrior at gmail.com> wrote:

> Joe
>
> What is your goal for continuous operation?   Is it reduced engine room
> temps, or something else?
>
> I have been studying this problem solely from the perspective of 
> getting cooler air to the alternator and subsequently removing the 
> heat from said alternator.
>
> I don’t think the original bilge blower was designed for continuous 
> operation nor does it seem to move much air and it’s noisy as all get 
> out…as you’ve stated.
>
> I think the solution is higher cfm 12V fans designed for quietly moving
> more air.  Probably a squirrel cage centrifugal blower.    I haven’t got
> past trying to design a duct system to get the air directed at the 
> alternator so I haven’t focused too much on the actual blower yet.
>
> Cheers
>
> Troy Dunn
> Hull#514
>