RPM-Related Issues

[Katsumodo]

Hi everyone, I'm a new boater and I've face a few issues on my motor yacht. My boat weights 18.1 tonnes, I have a twin yanmar engine 400 horse each. The yacht is a stern drive and does not have rudder. Here is my problem. When I'm at 2500 rpm the speed is only at 10 plus 11 knots. When I increase the rpm to 3000, the speed actually drops to 9 plus 10 knots. Before I did a full engine servicing I could reach up to 16 knots at 3300 rpm. What happened? The boat's manual say I can reach up to 24 knots but not I'm struggling to hit 12. Do give me your valuable suggestions. Thank you.

[boasun]

When was the last time you had the bottom cleaned??
Also a boat is a hole that gets filled with all sorts of weighty items. You may need to do spring cleaning and redistribute the weight.

[perchance]

Originally Posted by Katsumodo

Hi everyone, I'm a new boater and I've face a few issues on my motor yacht. My boat weights 18.1 tonnes, I have a twin yanmar engine 400 horse each. The yacht is a stern drive and does not have rudder. Here is my problem. When I'm at 2500 rpm the speed is only at 10 plus 11 knots. When I increase the rpm to 3000, the speed actually drops to 9 plus 10 knots. Before I did a full engine servicing I could reach up to 16 knots at 3300 rpm. What happened? The boat's manual say I can reach up to 24 knots but not I'm struggling to hit 12. Do give me your valuable suggestions. Thank you.

Check the trim adjustment of your stern drives.

[Tellie]

Are you blowing a lot of black smoke?

[Katsumodo]

boasun - well I've cleaned the hull and props less then 2 weeks ago. As for weight I don't really have alot in the yacht.
Perchance - I'll look into tat cos at the moment the trim tabs r down. I more thing, my diesel tank, my water tank">fresh water tank and mine waste tank and generator are all at the bow area, is it still necessary to use the trim tab?
Tellie - no jus a steady streak of smoke, but I must say it really has pretty low sea water discharge. The engine don't seem to use much sea water.

[Katsumodo]

Any body got any idea how I can solve my problems pls help thank you....

[barnakiel]

Fouled props perhaps?

b.

[never monday]

what did you do in the engine servicing?

what sterndrive are you using?

[ATC]

Some Yanmars have mixing elbows on the exhaust that over time will become blocks with carbon. This restriction reduces the flow of the exhaust and you will not be able to reach the RPM's as before.

[Mark Johnson]

Originally Posted by Absolute

Some Yanmars have mixing elbows on the exhaust that over time will become blocks with carbon. This restriction reduces the flow of the exhaust and you will not be able to reach the RPM's as before.

This is very true. I was told "by Yanmar", that it is the first thing to change out on Yanmars, and I changed my SS version out @ 1,000 hrs. When I got the old one off, It was about 1/3rd carboned up. I suppose I could've just cleaned it out, but since I had already bought a new one, I installed it instead.

An every 1,000 hr removal to inspect and clean, (at least), is a good idea. If they rust out internally or carbon up, you have a really bad day!

Mark

[delmarrey]

Here is a couple youtubes that deals with trim. If you can not get the boat leveled out it will push too much water. And yeah I would check the mixing elbows too.
If you getting black smoke out at the higher RPM's then the motors are working too hard, and that would tend to carbon up the mixing elbows.

http://www.youtube.com/watch?v=C9_FcrI_mwc

http://www.youtube.com/watch?v=6v8gbT9XskY

[never monday]

Originally Posted by Absolute

Some Yanmars have mixing elbows on the exhaust that over time will become blocks with carbon. This restriction reduces the flow of the exhaust and you will not be able to reach the RPM's as before.

on the auxiliary diesels this is a big problem. While still possible, it's just not seen as often on the big engines