1992 115 evinrude v4 more speed please

[kennyt]

" Hi,

I have the above moto

" Hi,

I have the above motor. I am currently only operating at 42 mph on my 1989 178 astrocraft bassboat. I am running a 13 x 18 laser prop. I was running a standard omc prop (13 x 19) and was only achieving 38 at wot. My question is: the motor is only operating at about 4900 rpm. Can I expect more speed with a smaller pitch prop. I realize I probably should have the max motor (150) for this boat but cannot afford it. WHat kind of rpms should I expect out of this motor at wot and do you think that I capable of achieving speeds of 45mph or greater with another prop or jackplate.

Which brings me to my next question? Will a jackplate help. I still have alot of boat in the water and they tell me jackplates increase leverage and will get more boat out of the water, hence reducing friction and increasing speed.

Thanks Ken "

 

[jon]

" Ken,

I'm not sure

" Ken,

I'm not sure what the operating range of that engine is; at full throttle & a light load, with whatever prop you are using, you want to be at the top of it. Outboards put out their best HP at the top of the operating range; so to get the best possible efficiency (speed etc..) you want that engine to hit the right RPM at WOT.

Going down in pitch will make the prop spin faster in the water, and will give it less grip.. if you are at the right RPM to get your max HP, then more slippage is only going to lessen efficiency, increase friction and drop your top speed, while increasing RPM. If your WOT RPM's are too low, you won't get that max HP out of the engine as it will never hit it's best operating range.. going down in pitch might help top speed, but it really depends how far you are off etc.. as dropping pitch can lead to less effieciency as well, although it will probably give you increased acceleration.

A stainless prop will give you better top speed and increased efficiency as the stainless flexes less than aluminum and has a bit more weight to it; which I believe gives you some extra torque/inertia as a flywheel would. The only downside to stainless is that it's expensive and also transfers the force from a blow more directly to the propshaft, which tends to destroy it - alumimum or composite tends to bend/break from a blow, saving your lower unit.

As for the jackplate; getting the engine back a bit from the transom puts the prop in "cleaner" water; less disturbance from the boat = greater efficiency. Also, if you go with a hydraulic jackplate you will be able to keep the angle of the engine constant and raise/lower it while underway - fine tuning everything for increased speed. For instance, with power trim, you trim the engine down a bit for acceleration, and once up to speed, you trim it up for a few extra MPH or better fuel efficiency - pulling the engine as far as possible out of the water to decrease friction, while not allowing it to loose traction. The only downside with power trim is that it's affecting the angle of the engine - a jack-plate is directly up and down - you will still have the power trim for angle adjustments as well.

They also do things to props (ie cupped props) that allow them to grip while still very close to the surface of the water; this is something else to consider w/ a jackplate.

In addition, they make low water pickup nose-cones for outboards; they help streamline the lower unit and give you greater efficiency at top speeds.

My .02 is, it's probably a waste of time, unless you are racing, to spend $$ on an outboard, for a few extra MPH. For extra speed, I'd trade up and go for the 150 HP or more, the biggest thing to worry about when upping HP is weight, not HP - just change the decals for legal issues . Another thing to consider as well though, how fast do you really want to go?.. Gets pretty scary chine walking @ 60 MPH. Good luck,

Jon "

 

[sparky]

Have the motor checked on a d

Have the motor checked on a dynamometer to see if it is putting out the rated horsepower. No amount of fine tuning will help if the motor is in bad shape mechanically.

 

[boogie]

Kennyt: that model boat is n

Kennyt: that model boat is notorious for "Dragging" water. I had one with a 140 rude on it and it never did perform like the motor should have by all spec's.