twin motor question

[ewenm]

hi

i have a 32 foot boat with twin 4.3 efi mercruisers, this being my first twin engined boat
i have to ask, and here may not be the best place but here goes,

i get a noticeable vibration a thrum thruming noise, and wondered if this is normal for twins, i have 3 blade aluminium props and clean or dirty its the same

the only method of synchronising the motors is to balance the rpms with the tachos
and sometimes by slightly mismatching them i can reduce the vibration

the motors are solid mounted and there is no timber in the mounting points, its a fiberglass moulding that the mounts are through bolted to

it could be just twin motor harmonics, but i thought i would ask anyway

and do you think 4 blade props would help, i dont need anymore get up and go as this thing flies for its size and with full tanks (500 litres fuel and 280 litres of water) and 6 adults it easily planes and will run out to max rpm

so not really looking to change the props, however if it was thought that i could reduce the vib this way i would make the change

so what do yah reckon

Ewen

 

[rodbolt]

Re: twin motor question

are the inboards or sterndrives.
32ft with twin 4.3s is a vastly underpowered hull.

 

[zbnutcase]

Re: twin motor question

4.3s, even with the balance shaft, do vibrate quite a bit.

 

[jtmarten]

Re: twin motor question

Is it an actual vibration, or harmonics? I have a sychronizer gauge, and when sync'd, the tachs are slightly off. I can tell by listening and feel when they're sync'd.
I'd agree that twin 4.3's makes a very underpowered 32 footer.
If it is simple harmonics, new props won't help. You'll just have to play around with the throttles and see if you can eliminate or reduce the vibration. Don't pay attention to the tachs, adjust throttle by ear/feel.
Run with just one side and see if the vibration is in either engine. I had to send a prop/hub back to Turning Point about a month ago to be checked. Had a very slight vibration that moved to the other engine when I swapped the props. (Turning Point has awesome cust service - sent me new prop/hub; vibration gone)

 

[ewenm]

Re: twin motor question

i think its a vibration, my previous boat had a VP and they are rubber mounted maybe im just being over sensitive

as for under powered i cruise at 27 mph at 3700 rpm (GPS) and can run to 5000 rpm

ive seen over 40mph, plenty fast enough for 32 feet

oh its a stern drive (b2,s) Australian built "Mustang 3200 sports cruiser"

other than the vibration / harmonics ( and i will try varying the rpms on each engine) i couldn't be happier with this boat, in my humble opinion its the perfect balance between size, performance and fuel economy.

my personal test for any boat is to load it up with fuel and people and see how easy it plains

and i suspect that this boat is slightly under propped, in that it can pull 5000rpm where the specs call for 4800 as wot rpm, however this thing jumps on plain easily

it was also available as a single petrol engine, a diesel, or twin petrol up to 300hp each

i wondered if anyone with twins had experimented with aftermarket 4 blade props??

Ewen