Engine Choice?

[Martman]

I am need of an outboard motor for my 16.5 open bow. I have found 2) a dealer fresh rebuilt 1979 Evinrude 100 with warranty. I have also found a 1987 Johnson 120 VRO, rebuilt a couple of years ago with 30 day warranty. The rebuilt 100 is cheaper than the 120. Is the 1979 engine obsolete or is it still a good engine to work with. It will not be used many hours a year.
I am replacing a shot 120 VRO, but will the 100 do the job for me I am more concerned with fuel economy than total performance. If the 100 will run me decent speed 30-35 mph I would be content.

 

[wilde1j]

Re: Engine Choice?

The 120 is the motor I would pick. Way more power. You didn't say how much for either, so I can't offer a value judgment. I owned an '85 140 (same block) for over twenty years. Outstanding motor.

 

[Martman]

Re: Engine Choice?

The 100 rude is $2600, the J120 is $3400. These are canadian prices no deals in this country on recreational products.

 

[wilde1j]

Re: Engine Choice?

The 120 is eight years newer, loop charged and a better design. The 100 is a cross flow, which is OK, but the looper is a better motor. For $800, I'd opt for the 120, which will give way better performance and probably won't cost much more to run unless you like to run wide open. The 120 is prop shaft rated for HP, and the 100 is crankshaft rated, about a 5 to 7% difference, so your looking at more like a 93HP vs 120HP. If your guessing the speed with the 100, you may be too optimistic because of the net HP difference between the two motors.

 

[jtexas]

Re: Engine Choice?

I had to make some assumptions regarding things like displacement, beam, deadrise, center of gravity, length at waterline, etc.; I assumed 5% gearbox loss for the 100hp and 7% slip for both, but didn't penalize the older motor for age on account of the recent overhaul. Here's what I get:

At WOT, the 120 will provide about 7.5% more speed while burning 20% more fuel. I suspect that at a cruising speed of say, 38-40 mph, fuel burn might be about the same, but I have no scientific evidence to support that.

 

[Martman]

Re: Engine Choice?

Jtexas
I am also looking at the fact that the 100 hp engine is just about 50 lbs lighter, and a full seasons warranty. Gear case was resealed, lift cylinders overhauled, as well as a complete power head over haul with 4 new oversize pistons.

 

[archcycle]

Re: Engine Choice?

You'd be surprised what upping the power can do for (rather than to) your fuel economy. I used to run a 90 on my 16' cape horn (max recommended) and it wasn't enough but I was happy at the time that it sipped fuel WOT. But that was sort of a fallacy. Yes it did, but my most efficient cruise speed was 22ish. Now I'm running a 140 and though it can swallow my gas tank if I tell it to, I now cruise 28-30mph at 3000rpm on smooth days. On top of fuel flow at lower RPM, remember higher MPH means less time spent burning fuel per hour! Since our efficiency has to be measured in gallons per hour, you spend less time running and you can be more efficient

Edit: I'm not necessarily insisting you would get better fuel economy, but it's almost an apples to oranges comparison between the two if the boat they are on stays the same.

 

[jtexas]

Re: Engine Choice?

In my model, the 50lbs didn't make a noticeable difference (-2.5% of the weight, +0.1 MPH)

Should have mentioned this before -- this an excel model from boatdesign.net, page 3 of this thread:
http://www.boatdesign.net/forums/design-software/savitsky-power-prediction-2187.html
It estimates the hydrodynamic drag of a hull given its size, weight, and about 25 other characteristics, and computes the required thrust at various speeds, and the effective horsepower given the speed and thrust. The tricky part is estimating "effective horsepower" from "rated" horsepower; crank- to prop-shaft losses could be anywhere from 5-15%, and prop efficiency could range from 90-95% at full throttle down to 65% or less at 2000 RPM.

So, don't expect 47 MPH from the 100, but it will certainly exceed your criteria of 30-35. At that speed, the 120 might well burn less fuel and still have power to spare.