Loss of power - accelerating - Trim issue?

Question

Newbie, trying to get the hang of my boat...

I have a 17" with a 140 hp outboard. I am having trouble getting the hang of getting the boat up on plane and then trimming. I encountered a problem this weekend. Not sure if it is the motor or the operator.

The boat has a trim guage that tells you when the motor is trimmed "correctly". I have been trying to keep that "on the level" so to speak.

This weekend when I tried to take off, the boat would start to accelerate, hit about 4000 RPM and you could feel the propeller lose bite - the RPMs would shoot up but no power was transferred to the water.

I originally thought I broke something, but someone today was saying he thought it was more likely a trim issue - I was worried too much about setting trim and was actually creating a condition whereby the propeller was trying to grab water - but getting air - hence the revving. His suggestion - trim all the way down, get up to plane then trim. Make sense?

Answer 1

the idea is to get out of the hole quick as you can, then get as much of the boat and motor out of the water to get max top end.  Forget about the gauge, it can't tell you the optimum trim  position because it doesn't know the characteristics of your boat/motor combination or how it's loaded today, or todays water/weather conditions.  Start your hole shot with the trim fully down.  You'll have to experiment to find the best starting position, but start with it all the way in.  As the boat starts to climb out of the hole, the bow will start to fall; at that point start trimming out, a little at a time, giving the boat time to respond.  Keep trimming until you either start ventilating (drawing in surface air), or until RPM's go down instead of up.  

At WOT, your motor needs to run at the upper end of the recommended max RPM range - you don't say what kind it is, but if it's a johnnyrude that'll be 6000 RPM - any higher is overreving and will lead to running lean, which leads to piston failure;  too little is lugging which leads to unburned fuel, coked up cylinders, sticky rings and (you guessed it) piston failure.  Select a prop that lets you make the desired RPM's at WOT, and you'll be good at all throttle settings.  Decreasing prop pitch by one inch gives you approximately 200 more RPM, and vice versa (subject to differences in prop design).

If you can get to the outer trim limit without ventilating, you can gain more RPM's by raising the engine on the transom;  a bigger pitched prop at that point means more MPH.

One thing to watch out for in your quest for speed, is to make sure your getting plenty of water through the intakes - it's possible to run high enough to cause a fatal drop in water pressure - that's the gauge you want to watch.

Answer 2

i always have my trim all the way down in the water

Answer 3

Truenorth gave you excellnt advice.  The only thing I would add is if the cavitation (slipping) continues above 4000 rpm when the motor is trimmed all the way down,  the motor may have been installed too high on the transom.  Good luck!

Answer 4

OK, first ignore the gauge for the time being.

Down flat in the water, engine is all the way down.

Take off, let's try an experiment, leave the engine all the way down and get to cruising speed. Feel the boat, look back and see how the wake looks, and the water right behind the transom. Is the engine throwing out "wings of water"?

OK, now start moving the trim up, are the wings disappearing? Keep going, how is the boat moving? Did the bow come up? Did the bow start to slap? Has the wake become complete spray behind the prop?

OK, then that's too much. Put the trim back down until you have a nice smooth wake behind the prop. Somewhere between water wings and a rooster tail is the correct trim. If the bow is slapping trim down.



Play with it, experiment, and feel the boat. Pretty soon you will just know the right trim.

Once you know that, you can use the gauge to know where the engine should be without having to look back.

Good luck. Have fun.

Edit: without being in the boat I can only guess at what is causing the loss of power at the top end. First, if it is cavitation the cure is to trim down all the way and see if it still happens. Second, if you are still losing power above 4000 rpm and the prop has got a good bite then something else must be going on. Bad gear maybe?