Interesting engine thrust calculations

[jlastofka]

I'm a mechanical engineer, so I find this kind of thing interesting. It occured to me to figure out how hard an outboard is pushing on the back of the boat. To use round numbers I assumed 90 HP and 30 MPH.

One HP is 550 foot pounds per second. 30 MPH is 44 feet per second. So the calculation is 90 times 550 divided by 44 and the result is 1125 pounds. That's what 90 horsepower represents at 30 miles per hour. 1125 pounds of thrust.

At the other end of the spectrum, a 9.9 HP outboard pushing the boat at 6 MPH is putting out 619 pounds of thrust.

The slow speed case is displacement mode, plowing more of the hull through the water, and the high speed case is planing mode, plowing much less of the hull through the water. Hence the thrust only changing by a factor of two, while the speed changes by a factor of five.

I'm supposed to get my CD 22 this weekend, so it won't be too long before I see if these speeds are reasonable guesses. I take it easy on equipment, so I'll be breaking it in a while before doing any speed tests. My understanding is that outboards are rated at propeller shaft output, and I think I've read elsewhere here that these boats run at speeds close to these, so I'm sure I'm in the ballpark anyway.

I'm also aware that 30 MPH would require SMOOTH water, careful trimming and a LIGHT load in the boat. I'm sure 20ish is a lot more reasonable cruising speed. Still, it's fun to get a rough idea of the forces involved on our little plastic boats.

Jeff Lastofka

[mikeporterinmd]

My 22 gets kinda funky and light feeling at about 28-29. The only way
to get it to go faster is to lift more out of the water, but then the flat
bottom doesn't seem to have a good grip and so doesn't track very well.

Runs fine at 25mph, but it takes a lot of effort to deal with wakes at that
speed. Runs at 20 pretty easily, and 15-17 is a nice easy cruise.

Mike

[ppriest]

O.K. Now how about this one - how to determine the optimal combination of motor tilt and trim tab position, first for maximum speed, then for best fuel economy.
The presumption is that use of trim tabs trades off more desirable attitude in pitch against added drag. The question is whether or not this is more readily solved empirically.

Paul
J.C. Lately

[Sea Wolf]

[jlastofka]

For $360 at West Marine and probably less elsewhere you can get a Floscan fuel flow meter and do just this sort of thing. You'd need to divide your speed in MPH by the displayed GPH to get MPG. For $660, there's a model that will take GPS input and display MPG directly.

You could then juggle the throttle, engine trim and trim tabs a bit to optimize the MPG over the ground, taking in the effects of current, wind, whatever. Maybe even shift weight around in the boat.

The three possible outcomes are:

1) Hey, this makes a real difference!

2) Well, this is a waste of time.

3) I'm an engineer and I like figuring this stuff out and it makes the whole experience a lot more fun and I don't care if it helps or not....

The sensor goes in the fuel line coming from the tank, so if the main and kicker both feed from the same source (mine will), one could use the meter for either engine. At real low flow rates (kicker, idling main, etc) it may not be very accurate. I'll look into this more after I get the boat and sort through some more important stuff. Next week, I hope.

Jeff Lastofka

[mikeporterinmd]

You can buy an Lowrance LMF-400 or LMF-200 kit with an EP-10
fuel flow probe. If you have GPS data available on a NMEA 2000
bus, then you can get MPG for about $150.

It is interesting to watch. Lots of tab definitely increases mileage.
I get roughly the same mileage anywhere from something like
13mph on up to 20. The other day, I was playing around and ran at
25mph for an extended period of tiime (5000 rpm), but forgot to
check the mileage. I have mpg set up on an analog style gauge on the
custom display on the LMS-337C DF. It didn't catch my eye as being
way out of normal.

Mike