Merc Driveline/Alpha 1 leg

[Big Zeke]

This might directly fit a fishing thread but I found a weird issue with my boat. It's a 21' Campion with a merc 4.3 on an Alpha 1 leg. Whenever I take off when trimmed up (more than halfway up the trim gauge) the boat takes forever to get out of the hole and tends to have a hard time getting on plane; the engine also cannot get above 3500 rpm with a top speed of 25 mph. It actually sounds like the prop is cavitating. If I bury the leg (trimmed all the way down) the boat jumps on plane in 30', the engine hits 4500 rpm in 30 seconds and hits a top speed of 40+ mph in less than a minute.

Is there something wrong with my driveline or is the boat supposed to behave like this. Outboards I've used on smaller boats weren't very sensitive to trim position when coming out of the hole (or at least I never noticed it a much).

Comments against Campions, Mercs or Ukrainians from Alberta are not necessary, I really like the boat.

Thanks in advance for your replies.
Zeke

 

[TenMile]

Sounds like it could be an issue with your trim gauge or maybe the trim system itself? From your description it sounds as though the leg is trimmed up really high, particularly if the prop is cavitating. If the leg is up high, the bow is going to stay high and it will take a long time to get up on plane.

When the leg is down (or close to down) is your top speed better? I'm not sure what the WOT rating of your engine is supposed to be, but I'd guess something around 5000RPMs. Can you achieve the top end of the RPM range at all? Have you ever been able to?

Other thing to check is your prop. Might be bent, dirty or cupped? With the low RPMs you may also have the wrong prop size -- either wrong pitch (too high) or wrong diameter (again, too big). Running with too much pitch or too large diameter makes your engine work too hard and is really hard on the engine. I'd take a look at iBoats and check your engine with a boat that has a similar weight as yours and see what they say about prop size and pitch, RPMs, top speed etc...

I just cured this exact problem on my boat and it was the prop. Came with a 24 pitch prop and I went down to a 20 pitch and picked up 1000 RPMs and my boat just jumps on plane now and gets much better fuel efficiency.

 

[The Fish Assassin]

Maybe with the way you have your boat loaded or something, you should try a different pitch on your prop. Also I usually trim down when I take off then trim up when I'm up on a plane. My best freind had the exact same problem with his boat, but he is a Ukrainian also!

Take only what you need.

 

[daddystoy]

You answered your own question. Trim the leg all the way down when coming out of the hole. Your boat is performing exactly as it should according to how you are operating the trim of your leg. After getting on plane adjust your trim as necessary or use trim tabs if you have them. You need all the power you can get when coming out of the hole and the only way to achieve this is to have the leg trimmed all the way down. If you say the prop sounds like it is cavitating then it defineately sounds like it is trimmed way too high.

 

[Degree of Freedom]

Sounds like its behaving properly to me. Trim it down. That being said, the Merc system has a trim limiter...when you're running, if you trim the leg with the trim switch (not the trailer switch) it should max out at considerably less than 1/2 way up the trim gauge.
Doesn't sound like anything's wrong with your drive, but there may be something wrong with your trim limiter. Not worth fixing until you have the leg off for a more major repair...in the meantime, use discretion when trimming.

You can do serious damage running with the leg trimmed up too high!