F28 Exhaust Noise/fume

[Bigearl]

Last year I posted a question on the subject of exhaust noise and nuisance fumes we experienced on our F28. As pointed out in the replies my mufflers were probably shot and would need replacing to reduce the noise but that would not cure the fume issue on the rear deck. Oddly the noise was only an issue running at about 1200 to1500 rpm, once up on plane at 2600 plus rpm the noise was not an issue.
I decided to try extensions to the exhausts that would terminate underwater as some previous posters had done. Attached is a photo just prior to launching this spring. The experiment has worked well, on start up there is very little noise, mainly the bubbling sound of the exhaust gasses, at 7to 8 kts about 1500 rpm there is very little noise and no resonating sound when the motors are in sync. I must qualify this, in that the trim tabs must be up. As I go on plane the exhausts come out of the water but there is a lot of spray from the trim tabs that engulfs the end of the exhaust extension. The result is fume on the rear deck is much reduced and noise is no longer a problem without any negative impact on performance.
I was also concerned about the ability to verify that the cooling water was flowing, so I molded a copper pipe into the extension,at a location to catch the cooling water, and bent the copper tube to exit from the side of the extension where i was able to hook up a neoprene tube and clamp it where it could easily see it. On start up the pressure forces the water out of the tube and that way it's easy to confirm cooling water is flowing.

[larryeddington]

I had not noticed the fumes on my F28. The exhausts have a hooded turndown from Trojan. Not into the water though. Did your have the original fiberglass molded tips?

[aaronbocknek]

clever. would love to see close up shots of your invention. this is similar to the davis transom mount underwater exhaust system. carver used it on their 28 mariners i believe. here is a link to the original patent for the 'davis system'. click on the drawing to see it better...... http://www.google.com/patents/US3291252

there is a chris craft roamer flush deck at my mechanics marina that is undergoing a refit. he has a similar 'davis system' mounted to his vessel.

[mikeandanne]

clever. would love to see close up shots of your invention. this is similar to the davis transom mount underwater exhaust system. carver used it on their 28 mariners i believe. here is a link to the original patent for the 'davis system'. click on the drawing to see it better...... http://www.google.com/patents/US3291252

there is a chris craft roamer flush deck at my mechanics marina that is undergoing a refit. he has a similar 'davis system' mounted to his vessel.

I really like that idea Bigearl--- thanks for the link Aaron---- we had a Carver with that exhaust on it and it worked great--- I was thinking just yesterday of fitting the boat with something after tuning engines at the dock ( AFB's with stuck chokes. Lol )and dealing with the fumes---- a few close- ups would be good----did you have to retune at idle ---- Thanks Mike

[Bigearl]

Interesting that someone patented this years ago. thanks for the info Aaron. Others posted similar exhaust extensions on this forum a while back I just copied the idea. Attached are some close ups on the boat. The bottom of the extension is about 4" below the waterline with the boat at rest but you will see it is above the trim tab particularly when the trim tab is in the planing position. I have the original turned down exhaust tips, Larry, and left them in place, I attached the extensions with 3 of the 6 screws that are used for the exhaust tips and sealed the stainless flange I made with silicone between the tips and the flange. As the boat was stored a couple of hundred miles from my home the final fitting had to be done in the field and I molded the extensions to fit around the swim platform struts and then installed clamps to the struts, so this needs to be made neater before the boat goes back in the water.
I have also shown the copper pipe I used for checking water flow out the exhausts. I attached neoprene tubing to the copper and clamped it where I could see it through the slats of the swim platform. This worked well.
Hope this is of use.

[Bigearl]

Mike, I did not have to re-tune at idle and there is no difference in performance that I can notice.

[captainmaniac]

Underwater exhausts have been around for a loooonnnggg time - a lot of 60's vintage cruisers (including Trojans, Chris Crafts, Owens, and Sheppard) had solutions similar to the Carver example shown. Normally this kind of exhaust has some kind of 'tell tale' built into it, to confirm that water is being pumped (like the tube you added) - but in older ones it was just a 1/4" or 1/2" hole in the exhaust that would show some kind of spray if water was being pumped. But there is a reason that the 70's onwards vintages don't typically have these kinds of mufflers - they have the potential of being problematic.

The bigger problem is exhaust back pressure. At low speeds when things are submerged, is the exhaust going to be able to flow freely or will there be enough resistance there (because of having to push the water out of the way) to restrict flow, which will affect how the engines run. The tell tales help reduce back pressure; the bigger the tell tale diameter the better it reduces pressure (but also the more exhaust noise it carries through it).

A lot of cruisers have used underwater exhaust since then, but not using that design. They typically exhaust through the hull sides or bottom below the water line, but all have some kind of above the waterline tell tale to prove there is coolant flow and reduce exhaust back pressure at low speeds.

I am glad that you have come up with a custom noise /fume solution that works for you - just keep an eye out for anything that might indicate it is causing problems in other ways.

[Bigearl]

Thanks for your thoughts on possible adverse consequences of submerging the exhausts, this has been on my mind. As for back pressure I made these so I could easily cut the extensions down as they are just epoxy reinforced with glass fibre over aluminum flex tubing. My thought was to cut it back an inch at a time based on experience on the water. What I found was that at 1500-1600rpm about 7-8 kts the exhausts are close to coming out the water as a void starts to form behind the transom. If I drop the trim tabs slightly then the exhausts are totally exposed and I am back to noise and some fume, as a result I never did cut them back.
At idle I am getting maximum back pressure and I have not had to adjust the idling speed, it seems to be the same as before fitting these extensions, but it is possible that the motors have a little less power at idle when docking or maneuvering at idle using the transmissions only.

[mikeandanne]

This is quite interesting----- wonder what the difference is between what you have done and the same engine thru an outdrive---- exhaust velocity at the outlet or manifold design maybe-----they must be doing something to overcome this backpressure issue running the exhaust out the prop at idle and those outdrive legs are down a ways--the one on the Carver we owned had flappers at the top which would relieve gasses from the internal chamber and there were no mufflers----but again why if there is nothing on the outdrive like that----Mike

[larryeddington]

Not necessarily related but I noticed the direction of the side air intakes on your F28. One of my f28s still has the original plastic louvers on it and they are arranged down the side in following order intake then exhaust then intake then exhaust. Which make sense that way the middle bay (fuel) is vented and the rear (engine compartment is vented. I noticed it as my f28 which I will use is in the water and has the stainless aftermarket vents and they are different.

I looked on website for Dreamer F28 and the factory picture looks different.

Even though one f28 has original plastic louvers I am confused.

Anyone can clear up?

[Bigearl]

You make a valid point Larry, I looked up the factory photo of a new F28 and you are right the vents do alternate but the photo shows from bow to stern: exhaust,intake,exhaust,intake. I think the most logical is the point "prowlerfish" makes, the exhaust vents should be attached to vent pipes going to the lower part of the bilge to scavenge heavier than air gas fumes, while the intake vents supply air to the motors and exhaust vents so they do not have pipes into the lower part of the bilge. I will check mine for this configuration when I launch again. Thanks for your observation.

[larryeddington]

Here is a picture of my 77 f28 which still has the plastic vents, my 78 has the chromies like yours but have to check them.

[larryeddington]

FYI the vents on my functioning f28 are the same chrome type as big earl's and guess what mine are installed wrong as well, mine are bow to stern exhaust-intake-exhaust-intake. The first two vents are to exhaust center compartment (fuel and other things there) the rear two are to exhaust engine room. I am going to reposition them so they vent as originally intended as seen in the old f28 picture..