Adjusting Outdrive Trim.

[jon_bailey]

When trimming my Mercruiser 4.3 LX outdrive then to get up on the plane quickly the outdrive needs to be fully down. When on the plane if I trim up by more than a few degrees then the prop starts cavitating. Whilst everything is fine when trimmed down I wonder if I'm not missing something, e.g. better cruise, by not being able to trim down any further.

How can the outdrive be adjusted for optimum trim angle, or is is just trial and error?

Thanks in advance.

Jon Bailey

 

[BarryH]

Wieght distribution has a lot to do with it. How much gear you got stowed in the stern, also what boat is it? Like a lot of things, its all a bit of a compromise. When the boat is a rest is it stern down.

Wha'dya mean "I'm always playing with this engine" its the only way to get it to run!

 

[jon_bailey]

Boat is a Maxum 2100SC.

Nothing significant is stored in the back of the boat apart from a couple of fenders. More weight in the front (cuddy) if anything. Even when just the family (two slightly overweight adults and three small children), still get the cavitation and need to trim fully down.

 

[Guest]

We had a similar thing with a Maxum 2300 - there was very little range over which we could trim before ventilation set in. My own preference was to change the prop, but we sold the boat instead. I would look to fit an Attwood Ballistic stainless prop. I had one of these on a Fletcher a few years back, and it was almost impossible to induce ventilation.

OTOH, you may find the boat porpoises if you trim up further. In that case, you would at least still have the speed and efficiency benefits of stainless...

 

[wakeup]

I have the same boat but with the 5.0 V8 engine and I get the same.

It seems as though the trim range before cavitation is only half to 2/3 of the total trim movement.

I have found with some practice that the best way to trim it is to move the trim in very small increments and you'll find the optimum setting by listening to the enigine revs and looking at the speedo, best to do in open stretch where you can set the boat in a straight line and just play with the trim slightly and watch thte effect on the speed and the nose.

I find that with a full load of fuel and people on board the trim range is reduced before cavitation. With a light load and just two on board there is more to play with.

Now I've got the hang of it I find I am able to change the trim settings at the same time as I am adjusting the throttle to get the best out of the boat e.g. at the same time it comes on to the plane I start to reduce the throttle slightly and trim up alittle at the same time, and I do the opposite coming off the plane.




yada yada..

 

[terryw]

I have a 25ft boat with a 5.0 Volvo, and had the same problems when I first got her. After seeking some advice (many thanks to Col) I find the following works best for me.
I give her 3000rpm with the leg fully down and she pops up. I then adjust the leg, without touching the throttle, and watch the revs and speed increase (I use the GPS for speed as it is more accurate and easier to read than the one on the boat). At about 3700rpm the speed stops rising, and then begins to fall, but the revs keep rising, this is where cavitation is starting. I then lower the leg very slightly, and reduce the throttle to my prefered cruising speed which is normally back at 3000rpm and 24-25 knots.
Hope this helps.