too much heat

Discussion in 'Inboards' started by mudman, Oct 6, 2008.

  1. mudman
    Joined: Mar 2007
    Posts: 88
    Likes: 5, Points: 0, Legacy Rep: 72
    Location: Madisonville, LA

    mudman Junior Member

    I designed and built a 16 foot mudboat with a marinized subaru engine. The engine has an enclosed water cooling system and dry exhaust. It is pretty much set up like a car, except for the keel coolers. Well I have a heat problem, not the engine itself, but inside of the engine compartment.

    I took the boat out for a test run over the weekend and it was running fine. The tempreture gauge did not even move. The exhaust is inside of the engine cover, but exhausts out. Well, About 10 minutes into the run, My throttle cable melted, then some wiring melted, and a hose that I ran from the oil pan melted. (this hose was for easy change of oil). So I lost all oil pressure. I opened the engine cover, and it was like an oven in there.

    Will some insulation around the mufflers and the headers, solve this problem? Do I have to add more ventilation, fans, scoups, etc? I have some vents, but obviosly not enough. What else can I do? I don't want a fire or another meltdown.

    I've attached a few pics of when I was building the boat. Keem in mind that the back deck is now plated without air scoops. I figured that the expanded metal on the engine cover was enough.
     

    Attached Files:

  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    For every horsepower taken from the crankshaft, two HP of heat are generated.
    In a car, roughly half of this thermal energy is removed by the cooling system, the other half by the airstream. Your closed engine space cannot get rid of that last part, as you painfully found out.
    One option is to use a water jacketed exhaust, the alternative is to simulate the situation from a car by installing a VERY large fan or fans, controlled by thermostat(s) or electronically modulated depending on the engine rpm.
     
  3. BHOFM
    Joined: Jun 2008
    Posts: 457
    Likes: 14, Points: 0, Legacy Rep: 247
    Location: usa

    BHOFM Senior Member

    The mufflers are the problem, you need to build a shield
    around them and the pipes and force air through them.

    Like a outer skin spaced 3/4" with the blower at the
    point nearest the engine.

    Some vents on the rear deck won't hurt, but I don't think
    they will fix all the problem.
     
  4. mudman
    Joined: Mar 2007
    Posts: 88
    Likes: 5, Points: 0, Legacy Rep: 72
    Location: Madisonville, LA

    mudman Junior Member

    Raw water cooling is out of the question, since the boat will be run in very shallow water and soupy mud. Is there a way to pull water from the keel coolers to cool the exhaust?

    Are there any drawings or instructions to put either water cooling or the forced air on the exhaust?

    It was very hot in there. I mean like 400 degree oven hot. I can't believe how hot it was. The mufflers acted as a heating element and the aluminum reflected the heat back in like an oven.

    Can I just protect the wiring and controls from heat? It will get hot in there, but it's all aluminum around the engine. If the wires are protected from heat, would the heat in the engine compartment cause any other problems? How about painting on the inside of the engine compartment? This way the heat isn't reflected back in? I probably don't know what I'm talking about though.

    It's probably best to get rid of the heat in there though. Don't want to melt the bilge pump or worse yet the rubber boot on the packing box.

    Any other suggestions?
     
  5. BHOFM
    Joined: Jun 2008
    Posts: 457
    Likes: 14, Points: 0, Legacy Rep: 247
    Location: usa

    BHOFM Senior Member

    This is a rough idea of what I had in mind. It looks like
    you have plenty of room and it could be made of roof
    flashing and pop rivets! Some heat proof hose from
    the auto parts to connect the blower and shield. A bilge
    blower would do the job, as well as remove fumes. From
    the looks of your project, this would be a simple thing to
    make up.
     
    Last edited: Oct 9, 2008
  6. mudman
    Joined: Mar 2007
    Posts: 88
    Likes: 5, Points: 0, Legacy Rep: 72
    Location: Madisonville, LA

    mudman Junior Member

    Well I talked to some cajuns down here about what they do with their mudboats. First thing they asked is if I insulated the exhaust. Well when I told them no they all laughed. They said "Oh sha, you need to get dat K-wool and put it on dere at 2" thick. Afta dat, you can keep you drinks cold in dere." Crazy cajuns. They said that you can even touch the exhaust pipes, that they will only get warm to the touch.

    Well, I looked it up, and this K-wool (or kaowool) is ceremic fiber, good for heat up to 2000 F. I'll probably still put in some clam shell vents, but hopefully this solves the problem.
     
  7. Lt. Holden
    Joined: Sep 2007
    Posts: 137
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Western Massachusetts

    Lt. Holden Senior Member

    I think you are treading on dangerous ground here. I don't know what the rules are in LA. but in most states it is illegal to have a closed engine compartment with dry exhaust. I would suggest you think about getting marine manifolds and connecting them in series after the coolant goes through the engine block. That still leaves the problem with the muffflers and exhaust pipes. Perhaps the suggested enclosed duct air cooling would work, I don't know. How about adding a suitable car radiator with belt or electric fan(s) and thermostat?

    Is the engine carburetor or injected? Either way you need a USCG approved Spark Arrestor as well as suitable marine type starter, alternator and possibly fuel pump. Without these your boat is a bomb with a fuse of unknown length!
     
  8. mudman
    Joined: Mar 2007
    Posts: 88
    Likes: 5, Points: 0, Legacy Rep: 72
    Location: Madisonville, LA

    mudman Junior Member

    I have the spark arrester, and there is venting on the engine compartment, so it isn't totally enclosed. They have boats like this all over louisiana. Wildlife and fisheries doesn't call for fuel pump or alternator as far as I know, but I'll check into that. USCG may have different rules and regulations.

    The drive train is equipped with a radiator with dual electric fans.

    The problem was that the mufflers were producing too much heat and acting as a heating element. Even with the fans, and the expanded metal on the coverings, the heat just could not escape quickly enough. Well, I talked with some people that build heater vessels and boilers about this K-wool stuff. The manufacturer claims that a 1" thick wrap will cool 1000 degrees to 160. So I just bought 50 sq ft for 62 bucks. If I'm at 1000 degrees, I think that I would have melted the aluminum.

    I think that I'll put 2" to be safe, and install more venting. I'll also look into the explosion proof alternators and fuel pumps. I would not want to put myself into harms way, especially in the swamp out here. Not very fun to spend the night in a swamp, I've done it.....once.

    Any other advice on how to make this boat safe and reliable? I thought that it was safe the first time, but I was obviously mistaken. Really wouldn't want a repeat of any sort again.
     
  9. srimes
    Joined: Sep 2008
    Posts: 283
    Likes: 30, Points: 28, Legacy Rep: 214
    Location: Oregon

    srimes Senior Member

    I'd consider using a mechanical fan instead of the electrics and have it pump air through the compartment as well as the radiator. Electric fans are great on cars because there's plenty of airflow without them so most of the time they aren't needed. When a fan is needed electric is less efficient as well as less reliable. In your case you may need the fan more than not so I'd go mechanical. Try a clutch fan first and see if it cools enough, and if not take off the clutch.
     
  10. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    By insulating the mufflers in a confined space you create a potentially dangerous situation.
    The metal will quickly reach the 1000+ degrees that your exhaust gas has. That will reduce the life expectancy to almost nothing, the metal will be red-hot under the thick insulation layer and the whole construction will expand and contract much more than designed for.
    In a nightmare scenario a wall ruptures and the hot gases freely enter the engine bay, melting wires and fuel lines....
     
  11. Lt. Holden
    Joined: Sep 2007
    Posts: 137
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Western Massachusetts

    Lt. Holden Senior Member

    I agree CDK. The real problem is not to isolate the heat but to get rid of it! I think in the end you have to to water cool the entire exhaust system. Since it is a 'mud motor' and you can't use raw water to dump out the tail pipes; you need a closed circuit freshwater (or coolant) exhaust system. What about using a 'waterlift' muffler and keep the exhaust pipes after that as short as possible before they dump out the boat. You could use the K-wool on that very short section where it exits the boat. Your use of automotive type mufflers is only making the situation worse as they are trapping even more heat.
     
  12. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Take the mufflers out of the engine space , and mount them up like on a big truck.

    Insulate the portion of the exhaust in the engine space and the exhaust manifold.

    Put screening or something to keep the outside exhaust from being grabbed by anyone.

    FF
     

  13. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 490, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You have a number of issues going on there. The metal box and structure around the enclosed spaces are heat sinks. Your exhaust gases at the port are in the 1,200 to 1,400 range with 1,000 degrees pretty much to the mufflers. Not much will survive for long in that type of environment. I'm surprised you haven't distorted some of the enclosure.

    Yes, "header wrap" will cut down on a lot of the heat, but as the locals said, you'll need lots of it, which isn't cheap and this will cause other issues.

    The simple solution, as has been mentioned, is to move the restriction point (the mufflers) outside of the box. You appear to have room for this.

    Next I would strongly consider forced ventilation of the engine compartment. I've found the best way for this is to create a negative pressure inside the box, with a fan mounted to pull the compartment air out. If you mount a fan to blow into the compartment then and out vents, then you also blow air into the bilge and any, non-air tight openings in the enclosure. This is usually dangerous as the fumes can enter unventilated or poorly vented areas of the boat, dramatically raising the potential for things to go boom. Also, a blowing fans sends fumes into the "people" spaces, which isn't any more desirable either.

    Put some clam shells or louvered vents on the forward top of the box and a good size fan (or two) to suck the air from the bottom rear of the compartment and overboard. With this setup, all the air will enter through the clam shells and exit through the rear vents. To remove lots of heat, you needs lots of fan volume, so don't be shy about fans.

    Also, use some good insulation inside the box. This too isn't cheap, but it works well. You have to remove the heat. Reflecting it will just cause you to cook everything inside the compartment, including gaskets, electrical components inside the starter, alternator, etc. These pieces aren't designed for this level of heat. You have to physically pump the engine compartment contents out, the heat will come with it.

    You may have to use one of those pancake style of fans they use on cars. These will move a bunch of air, but you have to allow for the volume. Size the vents to at least equal the fan blade area (fan diameter times 3). So, if you have a 10" fan, that's a little over 30" in area. A home made clam shell that's 3" tall and 10" wide, would be minimum (for a 10" fan).

    Typical marine fans are the 3" or 4", inline style of blower. If you used a 4" fan like this, you'd probably need 3 or 4 of them, mounted at the back of the compartment. A 4" inline blower has around 12" of area, so size the intake vents accordingly for the total volume of air you're moving. You could even hook them up to a thermostat, so they'd come on when you're compartment temperatures reached, say 275 degrees and off if it wasn't.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.