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Thread: Quick question

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    Fasttimez's Avatar
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    Quick question

    The more I get into fabbing up my own FF exhaust, the more I'm wondering how ocean water doesn't fill up your exhaust and find its way back into your engine. I mean it's a big open hole in your hull. I've googled it and searched around but still no deffinately answer. Remember guys I'm new to water craft so take it easy on me....lol


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    Carbonic's Avatar
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    Gravity and the waterbox would be my best guess..

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    Fasttimez's Avatar
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    I figured the water box had something to do with it, but then I saw a ski with no waterbox. Is there some kind of one way valve in front of the waterbox ?

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    SpeedDemon99's Avatar
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    I've been wondering the same thing because I have been considering a through hull exhaust. I thought the idea of drilling a big whole in the back of the ski was a sure way to get water into the exhaust and engine but now I see the stock setup has a big hole also. What is the function of the waterbox and how does is work? Is it to keep water from flowing back into the engine?

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    Fasttimez's Avatar
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    To my best understanding, the waterbox is used to muffle the exhaust noise and also to cool the exhaust gases before they exit the ski.

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    rosey's Avatar
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    Great questions. if you look at the location of the exhaust port it is at the bottom back of the boat. it then has a pipe that rises 1.5 feet or so over to the the water box (i think that is what it is). from there back down to the muffler (I think that is what it is). water does not flow up hill (up the pipe through the port in the hull)(especially with the engine on and pushing air out). on another note I took my exhaust mostly apart last weekend when i screwed up the impeller and was trying to figure out why it was not performing like it should (turns out it was the impeller). I found that the large metal can (muffler?) had water in it, as well as the waterbox. I may be wrong but i think the cooling system actually flows water around the internal exhaust pipe that goes to the muffler to cool it and then is blown out throught the system with the exhaust. I may be wrong though. that is just my first look and take on it.

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    64physhy's Avatar
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    Actually, the 2 plastic boxes are the baffles (mufflers) and the aluminum canister that sits down low on the left side is the waterbox.

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    rosey's Avatar
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    I want to take that plastic crap off then. make it loud

  9. #9
    Fasttimez's Avatar
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    Quote Originally Posted by rosey View Post
    Great questions. if you look at the location of the exhaust port it is at the bottom back of the boat. it then has a pipe that rises 1.5 feet or so over to the the water box (i think that is what it is). from there back down to the muffler (I think that is what it is). water does not flow up hill (up the pipe through the port in the hull)(especially with the engine on and pushing air out). on another note I took my exhaust mostly apart last weekend when i screwed up the impeller and was trying to figure out why it was not performing like it should (turns out it was the impeller). I found that the large metal can (muffler?) had water in it, as well as the waterbox. I may be wrong but i think the cooling system actually flows water around the internal exhaust pipe that goes to the muffler to cool it and then is blown out throught the system with the exhaust. I may be wrong though. that is just my first look and take on it.
    I asked a Marine Engineer today about this. He said the exhaust is designed with bends and such to discourage the flow of water back into the system, kinda like a P-Trap in a sink. He also said the weight of every watercraft and how low it sits in the water is taken into consideration when designing an exhaust because the exit should be just at the waterline. He said if the exhaust exit was sitting 2-3 feet underwater, it would back fill all the time no matter how many twist or kinks you tried to put in the line because the pressure of the water pushing in at that depth is great enough to overcome gravity trying to pull the water back down. Turns out it's all simple physics...lol

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    SpeedDemon99's Avatar
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    That makes sense about the upward curve of the exhaust pipe to stop water from flowing back.

    I still don't fully understand the waterbox though. After you flush the engine and shut the water off we rev the engine up and down for 10-15 seconds to clear the waterbox. Why is there water in there and where does it come from? I assume it's connected to the cooling system some how as there is water flowing out the back of the ski with the hose connected.

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