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M25XP Quieting


Jul 17, 2020
Catalina 30 mkII Apostle Islands
Hello all,

I thought I'd share a project I recently completed on Bella Donna, our Catalina 30 mkII.

One of our ongoing annoyances with the boat has been how loud the Universal M25XP sounds, especially down in the cabin. It was to the point that carrying on a conversation in the cockpit while motoring was difficult and impossible when in the cabin.

I have lurked on a variety of older threads about how folks were able to quiet the motor. One thread I read thoroughly (found here How I made my M25XP quiet. ) involved purchasing a Kubota motor air filtration kit and using it to dampen the noise from the air intake.

However, as I looked into this I found that most of the Kubota parts are no longer available.

I did some research and came up with my own solution.

Based on a rough air flow requirement calculation (52 cubic inch engine at 3000 room) (calculator is here: Air Flow (CFM) | Widman International SRL ) it would appear that the motor needs around 41 CFM of air flow.

I found a filter unit, a Donaldson G042544, that exceeded the air flow need ( Donaldson G042544 ). I purchased this from Amazon. I purchased a 1.5" rubber elbow joint, 1.5" coupler,, and 1.5" PVC pipe from a local hardware store.

I angled the pipe and filter to hang underneath the galley drawers. They are currently supported with zip ties but I plan on ordering a mounting kit (I didn't want to do anything permanent before seeing how things worked).

The filter made a HUGE difference in the motor sound. It still sounds like a diesel--but a conversation in the cabin is now possible, and sitting in the cockpit sounds no different than sitting in the cabin of my diesel Dodge pickup.

We motored around in a variety of weather and water. I did not notice any power or performance difference with the motor. I usually run the motor at 2800 RPM, we saw 5.8-6 knots most of time (except when we motored upwind in 3-4 foot swells--then it was more like 5 to 5.5 knots, per the speed sensor).

I also purchased an oil separator to route the crankcase vent into the air intake, but I decided to go sailing instead of working on that project.

I'm not sure if others have made similar modifications, but I figured folks may be interested in the outcome of this project.

I'll post some pictures later (the app doesn't seem to like attaching files via my phone at the moment).
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