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Skiff way, way, way too LOUD!

Old 06-10-2021, 12:46 PM
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Default Skiff way, way, way too LOUD!

Hey everyone, this most likely has been discussed way too much but I want some fresh input if possible. I have a 25' Jefferson Fiberglass skiff (South Louisiana shrimp boat, Lafitte skiff) with a 1993 Isuzu 6BD1 diesel in it, straight 4" wet exhaust. Its crazy loud, very loud. I cant decide if its more engine noise or exhaust but its definitely both. The engine box is just glassed in wood. There isnt a lot of room to insulate so I want opinions on whether to put in rolled Soundown type deadening or the paint in stuff like SR 1000. Also, what muffler? At idle the exhaust is below water line but after that its not under water.

Thanks everyone.
Loring in Madisonville, LA
Old 06-10-2021, 02:27 PM
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Is there a rubber seal around your engine box? Make sure that is as airtight as possible, maybe upgrade it. Yes on quality sound insulation inside the box if possible. Throw a couple layers of Pig blanket in the bilge as well.

Glassing in a step or platform over the exhaust might do a lot for reducing the sound getting into the boat. Deep grooves in the bottom of such a step would tend to dissapate the air pulses.
Old 06-10-2021, 07:55 PM
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You asked "what muffler?" - Is there no muffler now?
A water-lift style muffler of sufficient size for the exhaust gas and water flow produced , and Sound-Down wrapped, may provide more sound attenuation than a horizontal type.
Does your engine by any chance run on the hotter than normal side gauge readings? That , with the loud exhaust, could indicate insufficient RW flow being injected into the wet exhaust stream.
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Old 06-11-2021, 05:01 AM
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Originally Posted by 30West View Post
Is there a rubber seal around your engine box? Make sure that is as airtight as possible, maybe upgrade it. Yes on quality sound insulation inside the box if possible. Throw a couple layers of Pig blanket in the bilge as well.

Glassing in a step or platform over the exhaust might do a lot for reducing the sound getting into the boat. Deep grooves in the bottom of such a step would tend to dissapate the air pulses.

Thanks, great pointers. What is Pig blanket though?
Old 06-11-2021, 05:06 AM
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Originally Posted by TTaxi View Post
You asked "what muffler?" - Is there no muffler now?
A water-lift style muffler of sufficient size for the exhaust gas and water flow produced , and Sound-Down wrapped, may provide more sound attenuation than a horizontal type.
Does your engine by any chance run on the hotter than normal side gauge readings? That , with the loud exhaust, could indicate insufficient RW flow being injected into the wet exhaust stream.
Correct, there is no muffler. Also I don’t have a lot of room for a muffler. The bilge is about 20” wide by 8” deep. I figure she flows about 5gpm this is because I ran her on the trailer with a hose in 5gal bucket. I’ve got well water and it flows about 5-7gpm and the bucket never overfilled. Runs about 190F out on the water. After I post a few more times I’ll post a picture so you guys/gals have a better idea

thanks for your reply’s they’re very helpful!
Old 06-11-2021, 08:21 AM
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OP, a very good thin insulation at a reasonable cost at Lowe's or HD and easy to apply is the aluminum foil with plastic bubble wrap packaged inside of two sides of aluminum foil. It comes in various widths and is very pliable for unusual shapes. It works well for cold insulation, and I can only guess it will work well for sound? If this will not work for your motor cover maybe something similar will.
Old 06-11-2021, 08:26 AM
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Do you have any pics of the boat? Googled it to see what it was and looked like some pretty cool boats
Old 06-11-2021, 09:12 AM
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I have a 25 with a 454 in it. Dual 4in straight wet exhaust. My engine box is just a basic fiberglass doghouse like most. Mine isnt overly loud unless im running higher rpms <2500ish. I know gas burner vs diesel is apples to oranges but i would expect a diesel to be quieter if anything. They are some badass boats. I can run 37 mph if i want to or pull a 35ft trawl at 1200 rpms sippin fuel.
Old 06-11-2021, 09:23 AM
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Originally Posted by 25Jefferson View Post
Thanks, great pointers. What is Pig blanket though?
Pig oil absorbent mats absorb oil but not water, good for bilges under and around the engines. They keep the bilge much cleaner, keep the Coast Guard and environmentalist happy. They aren't designed to absorb sound, but they help with absorbing engine noise that would reflect off the bottom of the bilge, or sound from props under the boat.

If you dig around there are thick, 5'x30" pads that are actually refills for outdoor ditch pad systems, that is what I use under my engines.

New Pig - Absorbents, Spill Control & Handling & Plant Safety Products
Old 06-13-2021, 07:42 AM
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Originally Posted by ScottBoy View Post
I have a 25 with a 454 in it. Dual 4in straight wet exhaust. My engine box is just a basic fiberglass doghouse like most. Mine isnt overly loud unless im running higher rpms <2500ish. I know gas burner vs diesel is apples to oranges but i would expect a diesel to be quieter if anything. They are some badass boats. I can run 37 mph if i want to or pull a 35ft trawl at 1200 rpms sippin fuel.
yeah man this thing is an old school diesel that’s just loud. Like first gen Dodge Cummins loud. I might have to do an engine swap. What gearbox are you running?
Old 06-13-2021, 08:06 AM
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Make a new box. Line it with dynamat on the inside then foam board then plywood. Wrap the whole thing in carpet.

second, you can get a muffler or look into a resonator. The resonator will get rid of the drone sound of the diesel. Mufflers come in all shapes some flow through will not be any bigger than the pipe that’s already there. Wrap your exhaust pipes and muffler in fiberglass, the old boats used to use asbestos.

put a flapper on the exhaust exit. It’s amazing the difference that alone can make.

we had an old ski boat that the risers corroded out on and decided to straight pipe. We found all these things helped.
Old 06-13-2021, 08:29 AM
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A 6BD1 spooled up to 3000 is LOUD. And on a 25' boat no way to get very far from it.

Turbo engine? That makes the exhaust quieter.

Smaller inboard diesel skiffs at planing speed usually have three primary sources of noise: Engine combustion/turbo noise, exhaust noise and prop noise. Need to know what is bothering you before suggesting a fix.

There have been several boats I ran that noise was so bad I wore either ear plugs or ear muffs. That is your cheapest and most effective fix.

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