The thru hull for the engine exhaust hose on my H-36 (1982) is located such that when underway it is below the waterline, especially as the boat approaches hull speed. I am concerned that this increases the exhaust back pressure and results in a decrease in engine output. I have noticed that when underway under power, the bottom of the transum will be a good six inches in the water, while at rest it is a good two to three inches above the waterline. The thru hull for the exhaust hose is on the hull about six inches before the bottom of the transum, which places the exhaust exit in the water a good six to eight inches when underway. I am considering raising the location of the thru hull such that the exhaust will be above the waterline at all times, but not sure where to locate it, on the transum or on the hull side. I would appreciate hearing from anyone who has done this, or any discussion, pros or cons. Thanks, Ernie
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squat
I can't help about the exaust back pressure, that is still a bit of a mystery for me too. As for the boat sitting lower in the water, this is normal, its called squating. I believe all displacement boats will squat while moving, the closer they come to their hull speed the deeper they squat. This web site had a forum about diesel engine's and there was a LOT of information about back pressure. I think you can still look them up and post the question to him.
I can't help about the exaust back pressure, that is still a bit of a mystery for me too. As for the boat sitting lower in the water, this is normal, its called squating. I believe all displacement boats will squat while moving, the closer they come to their hull speed the deeper they squat. This web site had a forum about diesel engine's and there was a LOT of information about back pressure. I think you can still look them up and post the question to him.
Same is true of powerboat exhausts
The exhaust systems are are designed to allow the thru-hull be underwater at times. It won't affect engine performance, and should not be modified.
The exhaust systems are are designed to allow the thru-hull be underwater at times. It won't affect engine performance, and should not be modified.
exhaust location
My H33 has an exhaust location similar to what you describe for the H36. About the only disadvantage I have noticed to this exhaust location is that you have to lean out over the transom when first cranking to make sure that water is circulating out of the exhaust. Otherwise, this location is probably quieter since the outlet is further from the helmsman's ear, the transom is cleaner, and probably the folks in the cockpit get less exhaust in a following wind. Also, the exhaust is in a low pressure area of the hull anyway, the very reason that the transom squats in the first place so back pressure is probably helped, not hurt by that location.
My H33 has an exhaust location similar to what you describe for the H36. About the only disadvantage I have noticed to this exhaust location is that you have to lean out over the transom when first cranking to make sure that water is circulating out of the exhaust. Otherwise, this location is probably quieter since the outlet is further from the helmsman's ear, the transom is cleaner, and probably the folks in the cockpit get less exhaust in a following wind. Also, the exhaust is in a low pressure area of the hull anyway, the very reason that the transom squats in the first place so back pressure is probably helped, not hurt by that location.
Exhaust
Many of the Hunters of your vintage, and later, have the same configuration. If there was a problem with the location of the exhaust I am sure that Hunter would have modified its system a long time ago. In other words, unless you only want to see the water exiting from a more comfortable position I would leave it alone. See all the work this forum has saved you. Happy sailing
Many of the Hunters of your vintage, and later, have the same configuration. If there was a problem with the location of the exhaust I am sure that Hunter would have modified its system a long time ago. In other words, unless you only want to see the water exiting from a more comfortable position I would leave it alone. See all the work this forum has saved you. Happy sailing
Back pressure
I wouldn't worry about back pressure either-- I have actually seen old CCA boats (probably with gasoline auxiaries) with exhaust outlets BELOW the waterline. They were never known for powering prowess, but there it is. And yes, all displacement hulls 'squat' --my dad's in particularly, since they are so fast. [smile] The idea that the low-pressure area contributes positively to exhaust-line vacation is something I never considered before today, but it makes perfect sense and I'll concur with Peggie-- don't move the outlet. Then again, moving some weighty gear forward from the cockpit-seat lockers would not hurt boat trim under way. [wink] JC
I wouldn't worry about back pressure either-- I have actually seen old CCA boats (probably with gasoline auxiaries) with exhaust outlets BELOW the waterline. They were never known for powering prowess, but there it is. And yes, all displacement hulls 'squat' --my dad's in particularly, since they are so fast. [smile] The idea that the low-pressure area contributes positively to exhaust-line vacation is something I never considered before today, but it makes perfect sense and I'll concur with Peggie-- don't move the outlet. Then again, moving some weighty gear forward from the cockpit-seat lockers would not hurt boat trim under way. [wink] JC
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