Silent Thunder Question
 

I’m just curious as to the purpose of the black rubber flappers on the silent thunder exhaust tips? Anybody out there know what these are for? I was told you need to be careful slowing the boat down to fast as it may damage the engine?! Is this the reason the exhaust has these flappers?

Thanks in advance for any input.

Mike


:cool:

Yes, the flaps are part of the system that prevents water from backing up into the exhaust and then into the cyclinders. Due to the baffling system within the platform, you could ram too much water in there and create excessive back pressure or even force that "extra" water up into the engine.

There are "main" anti-reversion valves in the exhaust tips where the exhaust goes through the transom. However, with the age of our boats, those valves are most likely toasted by now.

Any dry running of the engine, even for short time periods will cook the exhaust tip flappers. I would bet that if you haven't had your platform off, those flappers are not only burnt, but probably totally gone. Mine were. I rebuilt my Silent Thunder platform totally, new wood, reglassed, new seals, the works. I found all four exhaust tip flappers to be non-operative and two totally blown away!

I decided to cut out the remnants of any pieces that were left and let the outer rubber flappers (installed new ones) do the work. Problem here is that you need to remember that just those external flappers are protecting you engine. Fast deceleration, fast reversing and backing into following seas or wakes are no-no's.

Adding to this, many guys here on the board cut out large relief holes in the bottom of the platform to increase exhaust flow (less back pressure). This practice by-passes the external flappers. This should ONLY be done if you know for a fact that your main flappers in the exhaust tips are in operating condition. The only way to know that is to remove the platform for inspection. You could remove the rubber exhaust hoses from the manifolds to the transom exhaust tips from inside the engine bay and look in that way, but the inspection is not as clear from that direction.

Bottom line: Keep your exterior flappers in place and in good condition. They may be the only thing protecting your engine from a huge gulp of sea water!!:eek:

WOW! Thanks for the tip! I have the boat at my local shop to have the swim step re-sealed and the outdrive serviced so I will call ASAP and make sure he checks these valves. He is the Donzi, and authorized Mercruiser dealer in Seattle and I has been so for 25 years so I'm hoping he will do a good job.

Mike

I just found a receipt from 3/9/2001 (less than 75 hrs use since that date) that was for removing the Silent Thunder to specifically check the flapper valves ... it indicated that they were in good working order. Exterior flaps are in excellent shape.

:D

Sounds good. Make sure he is familiar with these Silent Thunder platforms. I've seen and heard of people have exhaust leaks "fixed" by boat yards. The yard will simply seal the platform to the transom.

WRONG!!!!!!!!!!!!!!

There are "donut seals" inside the board that do the real sealing job. These "donut seals" seal the platfrom to the transom at the exhaust tip outlet. These are the seal that need to be replaced. In doing so, the wood and fiberglass in that area need to be inspected to assure its not rotted (it probably is, just getting you ready for the worst). Then there is another seal that goaes all the way around the periphery of the platfom. This seal doesn't do much, just keeps water out of the mating area of the platfrom to the transom. All of these seals can be had from Formula (and they are unique).

Soooooo, you need to make sure your repair guy knows all this chit, and knows it well. Otherwise, you're going to get a chit repair job done.

Do a search on this subject within this forum, you'll learn plenty from those of who have been where you are now;) .

The reason I took the boat in is the starboard upper nut on the inside of the transom started to weep along with a small weep in between the two lower outdrive bolts. I put a socket on these bolts to see if they were tight and found them to be snug but by no means tight. So I immediately pulled the boat out of the water and checked the exterior bolts on the transom and outdrive and found the same thing. (snug not tight) The swim platform was pulled off in 2001 and checked by the previous owners mechanic and found to be in good order.

The boat has been used exclusively in Lake Tahoe and dry stored until I purchased it this month and towed it back to Seattle. I had the boat in the water for 5 days and ran it for approximately 10-12 hours before I noticed the weeping condition … I did check the bilge every time I ran the boat and it was dry until the 4th day at my dock. Is it possible that it loosened up during the 800-mile bouncy trip home? Or is more likely that it has had this problem coming on for some time and it is now showing itself?

Thank you for your input.

What baffling system in the box? I sure don't have any. Mine's an 89', bolt on system. You look up inside, see the short 2 inch thru hulls, and it's just one big honkin' box. That's it. No signs of any baffling system ever was in place, no place anything looks cut out, except where I cut the partitions out.

I am very careful though about coming off plane and backing up. ESPECIALLY into waves.

Besides, mine's coming off.

Well completelymobile, I fear the worst:( . I'm quite certain your platform is slowly deteriorating, just like almost everyone elses.

Even if you did try to tighten those bolts up, you'd most likely break right through the fiberglass. I really think your platform needs to be rebuilt. New wood, reglassed, resealed, etc, etc. If and when you need to do this, I would suggest you do it yourself. It is not technically difficult. But, it needs careful attention to detail and good workmanship to assure longevity. This kind of work is outside the normal for a most boatyards. They'll just bone it all up on you!! It's more of a structural/fiberglass type repair. If you're handy at all, I'm sure you can do it. Done any fiberglass work?

Or you could buy mine......:eureka:

The partitions you cut out ARE the baffles I'm referring to. With the baffles in place and the outside flappers gone or caved in, you can "pile" excessive amounts of water in there during hard backing (for example). This can result in high back pressures and/or inversion problems, even when going in forward because the water can't drain fast enough.