Where to trim motor at?

[camarostripes96]

ok well this is my first pontoon boat, and i have a 18ft sun tracker party barge with 40 2 stroke mercury, and i was curious about where to trim the motor (all the way down or what) for the best performance with 5 people or so? also how fast or slow do u think it will go?

 

[tashasdaddy]

Re: Where to trim motor at?

i have a heavy 20 ft, with a 2004 50 Johnson, i'm getting around 15 mhp, i also have TnT, but do very little adjusting. if i trim up much i ventilate.

 

[camarostripes96]

Re: Where to trim motor at?

oh ok well i was just curious , so i probably need to keep it trimmed all the way down? but it is fast enought to pull a tube and a decent speed , like for my daughters who are 6 and 12 years old?

 

[steelespike]

Re: Where to trim motor at?

Don't just arbitrarily adjust the trim.
Adjust the trim for the best performance with the least ventilation.Also
place the weight carefully to keep the boat balanced with a slight stern biased weight.Be aware if too much weight forward and a big wave comes on board it may submarine.It will continue to dive until you cut the throttle.
This is unlikely to happen with careful operation and good weight placement.

 

[camarostripes96]

Re: Where to trim motor at?

so what do u mean by ventilation? like keeping the prop in the water or what?

 

[Silvertip]

Re: Where to trim motor at?

The best way to learn what ventilation is would be to put an average load on the boat and head out for a ride. Get wide open and just keep bumping the trim up a little at a time. When the engine begins to rev up and you begin to lose speed, the prop has lost it's bite on the water and is now ventilating. Trim down until ventilation stops. You may actually have to chop the throttle to regain bite.

 

[Bigbobcat]

Re: Where to trim motor at?

I am also a newbie to boating !
I did almost the same way as Sivertip suggested. But my problem is the speedometer did not work and it was challenging for me to "feel" the speed ! So I had to run back and forth to check the prop to make sure it was not exposed to air or partially on air.
Any other experience do you have in this situation ? It seemed in my case the rpm did not change much or did I miss something ?

 

[Silvertip]

Re: Where to trim motor at?

If you have a tachometer, the engine revs increase suddenly when ventilation occurs. Speed may not decrease dramatically when ventilation occurs but you certainly can detect a change in engine revs with or without a tach. If engine revs do not increase markedly you are not experiencing ventilation.

 

[mla2ofus]

Re: Where to trim motor at?

OK, being the noob, I have to ask the dumb***** question. What is the difference between "ventilation" and "cavitation" ? I know pumps cavitate when they suck air along w/ liquid.
Mike

 

[Silvertip]

Re: Where to trim motor at?

There is really very little difference between ventilation and cavitation but the effect is very different. Ventilation is a situation where surface air or exhaust gasses are drawn into the propeller blades. When this situation occurs, boat speed is lost and engine RPM climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine. Cavitation can actually erode the blades of a prop due to the bubbles that form on the blades. Hear is a U-tube example of cavitation. Cavitation (or lack of it) is a function of propeller design. The Navy spends lots of money trying to design propellers with low cavitation for ships and submarines since cavitation also produces noise that can be used to detect ship or submarine location.

http://www.youtube.com/watch?v=mQvbispmUF4&feature=related

 

[steelespike]

Re: Where to trim motor at?

I am also a newbie to boating !
I did almost the same way as Sivertip suggested. But my problem is the speedometer did not work and it was challenging for me to "feel" the speed ! So I had to run back and forth to check the prop to make sure it was not exposed to air or partially on air.
Any other experience do you have in this situation ? It seemed in my case the rpm did not change much or did I miss something ?

You should really start your own thread.It gets confusing for all trying to
keep track of who is who.
That said If your motor ventilates you will know it without a tach or speedo.
Revs go way up speed usually decreases.If you can trim all the way up with no venting motor is probably set up too low.Not as big a problem on a pontoon but dragging extra motor through the water is inefficient.