New Prop, now Exhaust Louder

[BuzzM]

I am running a mercruiser 1983 M260/350 GM with a pre-alpha outdrive in a 1983 Sunrunner 245 cruiser. I replaced a slipping 14 1/2 inch 19 pitch aluminum prop with a Solas Amita 3blade 15.5"x17 pitch aluminum from Iboats.
This boat went from a top end of 23 mph @ 4200 rpm ( GPS ) to 36 mph @ 4300 rpm. I love it.
However, as soon as I exceed 2100 - 2300 rpm., the exhaust is much louder now, sounds like a John Deer tractor. At first, I figured I just had the trim out too far and the exhaust ports were exposed. It still sounds that way with the trim tucked down at those r.p.m.s.
I also looked for any exhaust leaks around the manifolds, elbows, and as far down the " Y" I could get. The risers & manifolds were replaced about 6 months & 15 engine hours ago. I could not feel any air leaks using a strip of paper or see any water leaks, either.
In the slip at as high as 3000 r.p.m. ( obviously in neutral ), the engine & exhaust sounds fine and quiet. The same at low speeds, even under power
leaving the marina for example. There are no overheating issues.
The sound seems to definitely be coming from the aft side of the transom.
Could the increased diameter of the prop have an influence on how the water flows over the exhaust ports?
I did not change the flappers when I had the exhaust risers & manifolds replaced.
I am considering taking this boat offshore out of Texas for the first time, so I may just be getting paranoid.
I'd appreciate anyone's thoughts.
Jim

 

[WillyBWright]

Re: New Prop, now Exhaust Louder

It's the speed. Faster speed is sucking the ehxaust out quicker and spreading the exhaust bubbles containing the ehxaust noise out further. _

Okay, that sounds farfetched (and it is). It really probably has more to do with how high the boat rides in the water at the faster speed than anything else. The exhaust is closer to the surface. The deeper, the quieter.

Does your transom bracket have a bellows for exhaust, or an open tube that the exhaust port of the drive trims down into?

 

[BuzzM]

Re: New Prop, now Exhaust Louder

I am not real sure, but I think it has a " accordian " type bellows that is
very visible when the lower unit is raised to the " trailering " position. I think it is held on by clamps at each end.
I recently replaced the anodes, changed lower unit lube & greased all the zirts I could find.
While doing that, I looked for any cracks or rips in the rubber and found no problems.
I had the bellows & reseal job done in July 05, so I figured they should be o.k.
Thanks again for any suggestions.

 

[blues]

Re: New Prop, now Exhaust Louder

I have the same outdrive, merc 260 and would like to change to a different prop, as the one you have,
right now, a SS 14x20 top speed around 36 mph and rpm is below 4000
( 23 foot sleekcraft daycruiser)
I was thinkin of a SS prop 14.5x17 as clearance does not appear a 15.5 would clear???
did you change the tab, to get clearance?

 

[Haut Medoc]

Re: New Prop, now Exhaust Louder

Buzz, it sounds to me like you are good to go......
Blues you need to get your WOT up....
14x20 should get you over 4000 rpms.....
When was the last time a tuneup was done?
The lower pitch will help, but may be masking other problems.....
Your WOT should be 42-4600.......​

 

[blues]

Re: New Prop, now Exhaust Louder

just had complete tune,, but, changed engine from 350 chev to ford 302,.
keeping same 260 drive,

 

[Haut Medoc]

Re: New Prop, now Exhaust Louder

You might have mentioned that......
I would go 14x17......
You've lost 70 HP or so......​

 

[SuperNova]

Re: New Prop, now Exhaust Louder

You can't blame him, haut, it was a small detail.

 

[Coors]

Re: New Prop, now Exhaust Louder

Sure, small details don't matter.

 

[180shabah]

Re: New Prop, now Exhaust Louder

Yeah, but if he puts on some chrome valve covers and some of those trick braided fuel hoses he should be able to get back 40-50 hp

 

[Coors]

Re: New Prop, now Exhaust Louder

Hey, my boat come with painted valve covers; I cleaned them, and they are chrome- didn't see a hp increase; I feel cheated.

 

[180shabah]

Re: New Prop, now Exhaust Louder

Mine are plastic - no power at all

 

[zeezer]

Re: New Prop, now Exhaust Louder

I have the same outdrive, merc 260 and would like to change to a different prop, as the one you have,
right now, a SS 14x20 top speed around 36 mph and rpm is below 4000
( 23 foot sleekcraft daycruiser)
I was thinkin of a SS prop 14.5x17 as clearance does not appear a 15.5 would clear???
did you change the tab, to get clearance?

After a bunch of trial & error I finally ended up with a 14.5 x 19 p Ballistic Stainless prop. This did the trick. I have a 23 foot wellcraft Nova with the Merc 260 & Alpha 1. This is a heavy boat but we have her going 47 mph recorded on a GPS at 4400 rpm. I have sold the boat but did not give up that prop. I may sell it.