dear all,
thank you for the plurality of the arguments/ideas as well as the interesting points raised.
A few general comments and some more questions / individual requests, wont try to reply to every post as it will take too much time and there'll be some repetition.
OK, first, craft is 70s designed, originally with DD fitted, just like Alf's Navigator.
For sure NO silencer ever fitted, doubt the concept existing back then, just straight throughs, with the twist of half gas/water path inside the e/r and the other half outside. When re-engined 20yrs ago they did a fair job overall with stock risers that within 500h ruined the port turbo. Not impressed with their job tbh...
Finally the option of rerouting the exhaust via the lazarette is a no go, as there are two 250lt water tanks on the bulkhead sides leaving no space for such mods.
The noise inside the saloon, (yes, engines are just under the saloon) is different, dumpened by insulation and carpet, and will eventually install some rubber underlay, but that's NOT my worry. If in the middle of the summer going someplace someone is in the saloon is probably taking a nap, engine noise is probably helping him/her to it...
Concern is the bass bumping noise from the transom exits even at 7kn, ok it's better at 4kn but unlikely to go anywhere at that speed!
I can understand that noise reduction is proportional to money thrown at it as a problem to an extend. I also imagine that design and shaping silencers according to the rules following the speaker enclosure designs avoiding standing waves et al, can help (and is not easy and can help shape and fit a custom box within the available space...).
From the ones that have followed the MiToS rebuilt, you are aware by now that a bit of diy is not something I'm worried about, on the contrary I'd clearly say I'd be looking forward to now that other serious jobs are more or less over...
What I'm currently missing is clear understanding of the principles and some starting points.
Also not sure I would like to go to diy underwater exhausts just like that, unless someone can help and support the design process.
As far as of the shelf bits of kit that would work there are three types:
dustbin size (not exaggerating for my 5inch pipes!) risers that I could with a bit of careful arrangements fit in there but make the access to the outside of the engines a slight nightmare. Cost at around 1.2K euro both sides. http://www.asap-supplies.com/eu/bra...k-vernalift-grp-wet-exhaust-waterlock-1500116
inline silencers, shortest I found is approx 800mm long which is pushing it A LOT on the port engine where the turbo is on the port side and there's a tightish curve and straight out. Stbrd side is doable. Cheaper probably 700euro both sides, and probably will need some serious mod to introduce a 45deg elbow at least for the port inlet. http://www.asap-supplies.com/eu/bra.../centek-vernatone-minimax-grp-muffler-1050500
inline SS mufflers some fancy SS circular U-shaped sections full of I guess 8-10mm holes in the perimeter that claim to reduce noise without hampering performance. I guess soundwise they wont be anything special but are even cheaper at approx 500euro a pair. Just an idea: http://www.ebay.co.uk/itm/221790836272?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT
All these are simple e/r mods leaving the long side grp exits untouched. Unfortunatelly none of these would work on the outside channels, except maybe for the last one but wouldn't be able to get it all the way to the front where the exhaust exits the hull.
Summing it all up it looks like I'd be tempted to look more carefully and proceed along the following routes (with your help of course!):
Going the JFM route with diy dustbin watertraps/mufflers, John I'd appreciate some of your nicely detailed and dimensioned sketches
Introducing baffles and noise reduction design features on the side "channels" As I already said, they are U shaped (approx 220X160mm in section) bolted to a double skin plywood hull side with half a zillion countersunks, will be a happy evenings work to remove. Would happily close them to a full section after introducing baffles if that's going to help. Cannot imaging being difficult, cannot imagine introducing massive amounts of backpressure considering the massive volume involved.
Modifying the ending of the channel as Alf says, Alf I did loose you at the end there, any pics or can you elaborate a bit more how the exhaust gasses and water is exiting?
That would well be and add-on to the baffling of the channels themselves following removal.
Finally consider underwater exits within the e/r (with great care!)
Or any combo of the above!
So, as always looking forward to your suggestions/sketches/ideas!
cheers
V.
. On board at any speed there is virtually no exhaust noise. Also the genset exhaust is exiting under water and in a light breeze you cannot hear it running on the aft deck. On th Fairline the genset exhaust was merged to the port main exhaust which made it quite noisy. I was considering converting the Fairline to have underwater exits but the Halyard bins would have had to be removed to reduce the back pressure. 
Sounds like a problem (Design? Size? Can't tell for sure) of THOSE dustbins, rather than of the system in general.
Well, if it is, makes me wonder why no boat builder have ever used that system, to start with.
The best and cheapest way of getting rid of exhaust noise is to convert the outlets under water with sc goose necks on the exits. One of the reasons for me to trade in the Phantom 38 for the Ferretti was to get properly designed underwater exhaust. The Phantom was reasonable silent at displacement speeds but at planning speed the exhaust noise was too much on long runs, even on the flybridge for me. The Phantom had large Halyard dust bins but at planning speeds they were blown empy and the system worked like straight pipes. The Ferretti has an arrangement like JMF described above with small by pass outlets above the water, still giving a nice muted rumble when standing on the dock behind the boat
So watertraps might reduce your low speed noise issues however at higher speeds you still have the noise and back pressure might become an issue. Underwater exhausts need to be carefully designed also to avoid too much back pressure. It can be done quite easy however.
Porto,
looks like excellent material thanks for the pointer, but I struggle to see how I could use it. Only use I could think of, is wrap it around the 127mm exhaust hose that Alf recommends I stuff within the side channels. However, doing so, it will be:
A. a tight squeeze and I doubt it's going to be very efficient under the circumstances
B. susceptible to seawater contamination. From the literature, it doesn't seem to be an option, so whole thing should be carefully sealed in order to perform.
Let's see how the whole set of ideas develop and I'll keep it in mind as a channel wrap.
I'm also worried that it's not only dB we are talking about it's also low frequencies as Montemar points on post #19 and they are more difficult to tame unless you play with baffles. At least that's my understanding from the time I was into speaker enclosure designs a couple of decades ago (OK, make them 3...)
tbh I'm more concerned on D speed noise suppression, so the bins sound like a plan. Problem is I've no idea how they look inside, saw a cutout with a horizontal baffle and a tube going bottom to top in the middle, but engineeringwise (and with my limited knowledge on the topic) I wasn't really convinced.
Let's see what sketch JFM comes up with
cheers
V.
