Banging noise under sail, small(er) speed under power ?

Nov 12, 2016
9
O'Day 302 Sarasota
Just purchased an 1987 O'Day 302 3 weeks ago.
1st question:
First time I took it sailing, I had 20 knot winds. With only the jib I was doing 6.5 knots.
Exhilarating, but when boat speed surpassed 5 knots, a banging noise can be heard under the hull ....
I do not hear the noise while under power.... (shaft bearing or prop?)
2nd question:
Why is my power speed limited to 5.5 knots? Is it the prop? I assumed most boats do 6.5 knots under power. 1 knot is not a big difference, but it became an issue as I crossed FL on the canals.
3rd question:
My whole boat rattles when the engine is at idle (500 rpm). As soon as I give it gas, it starts running smoothly without any rattling. Bad shock absorbers would make it rattle all the time, not just at idle.
The bottom is in decent condition, and I don't want to haul it out unless necessary.
I listed all 3 problems because they may be linked...
Any suggestions are greatly appreciated.
 
Sep 2, 2011
1,041
Hunter 27 Cherubini Alum Creek State Park
#3. How many cylinders does your engine have? My single piston Yanmar calls for an idle speed of 850rpm and is much smoother there than at the 750 is was at when I bought it. I generally keep it at 1000 or so as it shifts better there and is smoother running.
 
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Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
Exhilarating, but when boat speed surpassed 5 knots, a banging noise can be heard under the hull ....
What engine? What gear were you in when you turned the engine off?

Why is my power speed limited to 5.5 knots? Is it the prop? I assumed most boats do 6.5 knots under power.
Max speed is based on hull length.

My whole boat rattles when the engine is at idle (500 rpm).
Idle is waaay too low. Again, what engine?
 
Nov 12, 2016
9
O'Day 302 Sarasota
Engine is Yanmar 2GM20F. 2 cylinders, fresh water cooled.
OK, I will increase the idle rpm to 850 (http://j30.us/files/yanmar-manual.pdf), as stated in the Yanmar manual.
Always put the engine in neutral before turning the engine off.
Hull length is 30 ft.. LWL is 24.17. Displacement is 7200 lbs. I had an Irwin Citation displacing 9100 lbs, with a Yanmar 2GM and I was motoring at above 6 knots ...
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
can you localize the banging sound? bow, stern? Also how often, once a second, once a minute...?
 
Nov 12, 2016
9
O'Day 302 Sarasota
The banging sound is under the cockpit... that's why I think about the shaft bearing or prop.
Keep in mind, there is no noise while under power - which is puzzling ... in talking to some boat owners, they suggested the prop.
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Certainly something in the power train outside the hull. Do you lock your prop when you sail. If so unlock it and see if you get the banging. I'd think the prop if the banging was rapid and stayed in sync with motor RPM other wise the shaft bearing or prop skeg coming loose. Could also be a rudder shaft bearing reacting to the turbulence from a powered prop but no to the smoother flow when sailing. Do you feel anything in the helm during the banging?
 

Joe

.
Jun 1, 2004
7,999
Catalina 27 Mission Bay, San Diego
Your boat speed, under power, will be limited by the water line length (not the boat length)
If you're using a gps, it measures speed over ground, and that can be affected by other elements... such as current and waves. If I were you.... I'd forget about it... it is what it is... BTW.... a one knot speed difference is huge in the sail boat world.
 

troy

.
Sep 30, 2006
7
Chaser 29 San Pedro
Do you have a folding prop? I wonder if it isn't fully closing under sail and the spinning unbalanced. My last boat had a folding prop that banged at idle. Once there was a load on it, centrifical force made it fully engage and the sound went away.

To exclude the prop you could shift it into gear while sailing.....with the motor off. Shifting it back and forth a couple time and listen to hear if the banging noise changes.
 
Jul 12, 2011
146
Oday 302 st pete
Gabriel - I too have a 302, and I also hear this noise when sailing with motor off and in neutral. If you shift into reverse it will stop, though Yanmar says to leave in neutral when sailing. I plan on replacing the cutlass bearing when I have her hauled soon.

My 302 has a new 2YM15, so slightly less power, and I motor at 5 - 5.5 kts at about 2900 rpm.

Let me know if you have other 302 specific questions.
 
Feb 14, 2014
7,399
Hunter 430 Waveland, MS
Any suggestions are greatly appreciated.
A commonality of all your symptoms, if linked together ...

Engine Transmission

You can record and post a short movie for each noise with just the sound track. More help from others if you do.
Jim...
 
Nov 6, 2006
9,884
Hunter 34 Mandeville Louisiana
The other thing to check is if the prop shaft is centered in the log where it goes through the hull. I had a shaft that would hit the log as it spun under sail..
 
Sep 21, 2014
4
MacGregor 25 Seabrook
Out of curiosity, I ran your LWL through an online calculator and at 24.17 ft, it came back as 6.5878 KTS. But as was previously stated, it varies on conditions. If you are bucking a current or beating into swells, you'll find boat speed to be less. I've also seen boats exceeding hull speed for short spurts, while surfing down waves on a broad reach or downwind run.

Like Joe said, your speed under power is not that big a deal. If you are racing and need every ounce of speed you can muster under sail, you might want to think about things that might cause drag underwater. Hull obstructions, things you snag while sailing (nets, line, crab traps, etc.,) through hulls, even marine growth can effect boat speed.

Sail trim, amount of heal, weight distribution and a host of other factors go into getting boat speed up as well.

Ultimately, you are performing as expected under sail, so the only other variable is going to be power of your engine, and no, they aren't necessarily going to bring you up all the way to hull speed, even with a clean bottom and not dragging a bucket.

FYI, I was being lazy using a calculator. You can figure out boat speed using this formula: Hull Speed = 1.34 * √(LWL)
 
Jul 18, 2014
14
Hunter 34 Sydney
A sailboat beside my slip had the same noise. Turned out, following a diver's inspection, the propeller shaft zinc was loose and sliding back and forth depending on throttle and gear, banging as it went. This might be your problem - and a simple fix!
 
Aug 28, 2015
190
Oday 28 St Joseph, MI
Take someone sailing and have them steer while you crawl around and listen. Put your hand on the prop shaft and rudder shaft. The only two things that can move down there while under sail are the prop shaft and rudder. I once had an intermittent banging I couldn't find until I did just that. It was the cabin entry washboards sliding around on the quarter berth.
 
Sep 3, 2013
16
Columbia 8.7 ROCKLAND & ST. PETE
Just purchased an 1987 O'Day 302 3 weeks ago.
1st question:
First time I took it sailing, I had 20 knot winds. With only the jib I was doing 6.5 knots.
Exhilarating, but when boat speed surpassed 5 knots, a banging noise can be heard under the hull ....
I do not hear the noise while under power.... (shaft bearing or prop?)
2nd question:
Why is my power speed limited to 5.5 knots? Is it the prop? I assumed most boats do 6.5 knots under power. 1 knot is not a big difference, but it became an issue as I crossed FL on the canals.
3rd question:
My whole boat rattles when the engine is at idle (500 rpm). As soon as I give it gas, it starts running smoothly without any rattling. Bad shock absorbers would make it rattle all the time, not just at idle.
The bottom is in decent condition, and I don't want to haul it out unless necessary.
I listed all 3 problems because they may be linked...
Any suggestions are greatly appreciated.
 
Sep 3, 2013
16
Columbia 8.7 ROCKLAND & ST. PETE
Hull speed is a function of waterline length. Period...and can be diminished by hull fouling, dragging along a prop, unfair surfaces...ad nauseum. Speed under power is no different, but engine performance and prop selection are big factors. Expecting to routinely achieve hull speed either under sail or power is not that useful an objective. Under sail, just have a tuned rig, good sail that are well set and sail well. Under power approaching hull speed requires disproportionately more power and corresponding fuel consumption and engine stress and wear. A reasonable motor cruising speed is what you get running at about 75% of top power. If the speed doesn't satisfy, maybe another craft would suit better.

That noise may well be your centerboard, if you have one. If not....I'd be worried and haul ASAP to investigate. Fair Winds.