Offshore style exhaust

[Roflhat]

Hi,

can someone talk me through the pros and cons of the offshore style exhaust?
I've got a Yamaha 70CES, so the midsection would have to be converted to offshore style.
Is there a certain way of going about doing it?

Cheers

[Roflhat]

. ?

[Markus]

Pros: The exhaust may keep your cowling dryer, and water exits the mid faster

Cons: The mid is weakened and the engine becomes much noisier

If you want to drill, drill holes in the side of the gear case instead.

But better keep it stock.

[Roflhat]

Thanks Markus,

I've drilled holes in my previous engines gearbox, was just wondering if there was any gain/loss from the exhaust pressure inside the leg.
I quite like the sound of the open exhaust myself. the main reason for looking into it was it I got an alternate gearbox, where the exhaust would have to exit the mid rather than through the gearcase.

[mackeral5]

Several years ago I punched a couple of holes in the rear wall of the mid. The wall I'm referring to is the one that you can see when looking up into the mid with the gearcase off. Keep in mind this is a 25" mid, on a 20" mid you will have to cut similar holes in the exhaust can as well. This applies to 90 degree v4 and v6's. I am not sure about inlines and 76 degree v6's.

The result is exhaust relief that exits through the slots on the sides of the gearcase above the cav plate. No external visible indications of exhaust relief. In my opinion very little if any weakening of the mid or lower.

I've run this setup for many years. I don't believe there was any performance gain, other than eliminating some prop slippage on take off when running props with PVS type holes. You definitely get a distinctive change in sound when trimmed up and on plane. At speeds where the gearcase is buried you don't hear any of the exhause relief.

With all of that said, future plans are to replace my mid with a non-relieved unit as my current preferences are to make my motor less noisy.


It's all about personal preference.