Hello,
I'm new on this forum and I have a very strange issue with my boat.
I have a small trawler with Penta MD21 and direct drive. Hull speed of the boat is 6 knots. The engine runs perfectly when not in gear with reasonable vibration on any RPM (700-2000). All 4 cylinders work, no misfire. Engine mounts are new and tight. Prop is balanced, engine and shaft are aligned to .0003"-.0005", shaft is straight.
When I engage the gear the magic begins.
1. Idle at 700 - 900 RPM - engine runs smooth and boat vibration is within limits.
2. 1000 RPM - boat vibrates a little bit rough but still ok
3. 1100 RPM - the boat starts shaking back and fore like something push her at stern with a frequency of 1-2 pushes per second.
4. 1200 RPM - shaking frequency increases, the whole boat begins pumping hard.
5. 1300-1600 RPM - Pumping/shaking amplitude increases, the whole boat is like in fever. The boat continue shaking until he reaches hull speed of 5.5-6 kts, then suddenly the shaking disappears and boat runs smoothly. Now I can reduce RPM back to 1300 and boat runs well - no pumping. Increase RPM back to 1600 and more - no issues: smooth running boat. Back to 1300 RPM - no issues.
6. Slowing down to 1100 - 1200 RPM - while the boat slows down, the shaking increases proportionally to the reducing of the speed and continue pumping until I increase RPM back to 1600-1800 RPM and boat gets hull speed.
I noticed, that when engine is cold, such effect is much lighter, may not appears at all. The shaking is less likely in cold May water than in August. Sailing against current reduces shaking RPM range.
The pumping occurs only in forward gear, reverse is fine and smooth in any RPM.
Please advice.
Thanks.
Alex.
I'm new on this forum and I have a very strange issue with my boat.
I have a small trawler with Penta MD21 and direct drive. Hull speed of the boat is 6 knots. The engine runs perfectly when not in gear with reasonable vibration on any RPM (700-2000). All 4 cylinders work, no misfire. Engine mounts are new and tight. Prop is balanced, engine and shaft are aligned to .0003"-.0005", shaft is straight.
When I engage the gear the magic begins.
1. Idle at 700 - 900 RPM - engine runs smooth and boat vibration is within limits.
2. 1000 RPM - boat vibrates a little bit rough but still ok
3. 1100 RPM - the boat starts shaking back and fore like something push her at stern with a frequency of 1-2 pushes per second.
4. 1200 RPM - shaking frequency increases, the whole boat begins pumping hard.
5. 1300-1600 RPM - Pumping/shaking amplitude increases, the whole boat is like in fever. The boat continue shaking until he reaches hull speed of 5.5-6 kts, then suddenly the shaking disappears and boat runs smoothly. Now I can reduce RPM back to 1300 and boat runs well - no pumping. Increase RPM back to 1600 and more - no issues: smooth running boat. Back to 1300 RPM - no issues.
6. Slowing down to 1100 - 1200 RPM - while the boat slows down, the shaking increases proportionally to the reducing of the speed and continue pumping until I increase RPM back to 1600-1800 RPM and boat gets hull speed.
I noticed, that when engine is cold, such effect is much lighter, may not appears at all. The shaking is less likely in cold May water than in August. Sailing against current reduces shaking RPM range.
The pumping occurs only in forward gear, reverse is fine and smooth in any RPM.
Please advice.
Thanks.
Alex.
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