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115 merc 4 stroke

S

Silverline

New member
Hello
I have a 5.9 m alloy pontoon boat which I use for open water sea fishing here in New Zealand.
It currently has a 2001 saltwater series 90 hp 2 stroke (Merc). While it has proved to be a very reliable motor I feel it is a bit under powered for some conditions. I am currently considering a 2001 115 merc 4 stroke with 115 hrs from new. My only concern is there seems be be a bit of chatter on these forums about excessive noise in neutral. Aparently comming from the lower leg? Has anyone heard if this issue has been resolved and if so what is the outcome.
Cheers
 
The 27% increase in power would be wiped out by the extra 40kg.I would go as far as saying you would lose more than the 27%.
I have tried to find Graham Lamb's formula for the increase in power/speed to increase in horsepower ratio,but no luck.It's in the archive somewhere.
If I was considering it,I would be looking at the gear ratio,along the lines of a bigfoot.More torque,probably less speed and a prop adjustment to keep the engine within the recommended RPM's.Sure wouldn't you miss the smell of the two stroke:D
 
I have run the numbers thru a calculator and you are correct. Only gain 2 - 3 mph. But the real advantage I see will be in more torque. At the moment the boat tends to fall off the plane very quickly at the bottom of a swell and tends to require full throttle to get her up planning again. Was hoping the 4 stroke will deliver more constant power, push thru the trough and not have the tendency to bog down as the 2 stroke is doing. Not really looking for more speed as in these conditions about 25 mph is any amount. So back to the question are these motors any good or is this anoying noise at low rpm still an issue.
 
I haventaclue about four stroke outboards,some would say I haventaclue about two strokes either,they could be right.
Four strokes are heavier,need a lot more servicing and attention and don't take as much abuse as a two stroke. On the other hand,they are quieter and more economical,well so I have been told.And that's as much as I know about four strokes.
As you're not looking for speed,a lower unit for your 90 merc with a lower gear ratio will give you more torque,change prop accordingly to keep within the min-max RPM's for the engine.And may be cheaper than going for a ten year old four stroke.Even if it has only 115 hours on it.
I have never noticed excessive noise from my own or any of my of my friends two stroke outboards.We could talk easily while at idle.That includes my beautiful 1969 Merc 800.Well maybe not WOT.:D
 
Thanks for your input. The noise I am refering to is specific to the first generation Mercury 4 strokes which had a Yamaha power head. The noise apparently is a grinding noise which comes from the lower leg when in neutral. Not applicable to the 2 strokes.
 
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