Speed Loss

[michaeel]

Recently replaced the mixing elbow on my Yanmar 3YM20. The engine runs great as normal with no signs of trouble. However, it seems since this replacement, I don't get the motoring speeds I used to. Before the replacement, I could easily hit 4 knots at 2700 RPM. Now I get around 3 knots and must increase RPMs to 3500 to hit 4 knots. Any suggestions on what I should check?

 

[kappykaplan]

When was the last time the bottom and prop/shaft were cleaned?

 

[michaeel]

The bottom was repainted with ablative in Sep 2012 and it gets cleaned monthly. However, the bottom cleaner has reported the prop no longer has any paint on it.

 

[tahomathundr]

Have you checked the paddle wheel on your knot meter as it may have a lot of growth on it giving you a false reading. I get 6.8 to 7.3 on my boat at 3500 rpm depending on headwinds. Bob C309 #20

 

[michaeel]

Not using the transducer...using GPS for speed. I used to get up to 6.9@3500, but lately, the boat is much slower under power...feels almost like I'm dragging something. Under sail, it moves very fast. So I think it must be something with either the marine gear, shaft or prop. BTW, I grew up in Kirkland and recently moved to FL from Newcastle, WA. I miss Puget Sound!

 

[tahomathundr]

Do you have the std 3 blade prop, folder, or feathering? If a 3 blade I doubt it would be the prop unless it is really fouled, or slipping on the shaft which would mean the key is missing and the nut is loose. The others may not be functioning properly resulting in loss power. The other possibility would be slippage in the transmission, check the fluid levels and look for burn type residue discoloration of the fluid. The engine seems to be working OK if you can get 3500rpm under load. Be happy you do not live in Newcastle next week as the DOT in shutting down I-90 west bound at the east channel bridge for a full week. I live in Issaquah and backups could will tie up the whole eastside. I hope you find your problem soon. Happy sailing Bob

 

[michaeel]

Thanks Bob. It is a std 3 blade prop. I will check the items you've mentioned.

 

[tahomathundr]

Another idea, You probably considered this but, have you hit anything that could have bent a blade on the prop? Bob

 

[RichH]

All good suggestions so far.
Here's another - next time you shut down the engine after a long run, immediately check the temperature of your propeller shaft 'stuffing box'. If its HOT it means friction from a too tight packing (can swell if you have flax installed), and such will/can 'bleed-off' a lot of engine HP. A too tight stuffing box will be acting as a BRAKE on the propshaft.

Ditto - check the transmission's lube oil level.

 

[jsalley]

All of the C309's have PSS shaft seals, unless an owner changed to a stuffing box. First thing I'd check for is a fouled prop - I found on my previous boat (2006 C309 with standard prop, Chesapeake Bay) that even a small amount of hard growth on the prop caused cavitation and a drastic effect on performance under power. Fouling is even more likely where you're located....

 

[michaeel]

How do you check for prop slippage? I'm going to be diving under the boat this weekend to check for fouled or bent prop and was wondering how to check for slippage, loose nut or missing key?

 

[tahomathundr]

The prop should not move on the shaft. It should not rotate or slide back and forth on the shaft. The only way the key would be missing would be if someone removed the prop and forgot to put it back in. The nut on the end of the shaft will control sliding back an forth on the shaft. You would probably hear a clunking noise or vibration when shifting from forward to reverse if this was the case. Last year I had a loose nut on the zinc on the end of my shaft that sounded like the prop was ready to fall off. Put the trans in gear so that it does not rotate when you check the prop. Bob

 

[michaeel]

After checking all the great suggestions from this forum, nothing came up as the problem. I didn't check the intuitively obvious thing until last, the tachometer. If was reading 800 rpm higher than the engine was running. I calibrated it and the boat is back to normal.

 

[plodder]

Hi - glad you solved the engine power/tach meter issue, however ...

I am interested in the mixing elbow on your Yanmar 3YM20. I too have a 309 which I need to replace but I think a previous owner has installed a different mixing elbow. The one on my boat is upside-down 'U' shaped and made of cast metal, the one on the diagram for Yanmar motors looks only slightly bent and made from tube steel.

The problem with the straight steel one is i can't see how the rubber exhaust piping would fit onto the 'slightly bent' original at the back of the engine before the wooden wall separating the aft bunk area at the rear of the engine.

I ask as I have the 'slightly bent' original looking one - sent from Aust to PNG ( I live in Port Moresby) but can't figure out how the exhaust pipe fits at the rear of the engine and then goes down under the floor to exit at the back. Are you or another Forum member able to describe or post an image of how the exhaust and elbow on your boat are fitted? It would help me enormously and save me sending what may be the correct thing back to Australia to exchange.

 

[michaeel]

The elbow I replaced was the U-shaped elbow. I've seen pictures of the elbow you are talking about, but there is no way that elbow would work on my 309. Attached are pictures of the old, uninstalled elbow and the new installed one. When I get home tonight, I will post the Yanmar Part numbers...

 

[michaeel]

I've attached diagram from Yanmar Service manual page 173. The part numbers are:

104214-13521 Mixing Elbow​

104214-13580 Exhaust Joint​

128370-13201 Gasket​

Note that the connector for the seawater coolant hose on the mixing elbow comes with a straight out connector, not the 90 degree on. I simply reused the old one. Not sure if there is a different mixing elbow part number to get the 90 degree connector.

 

[jsalley]

The 309 is designed to use the U shaped elbow to prevent back flow of seawater into the engine, as it is below the waterline. Yanmar also supplies the 90degree elbow for above waterline installations, but you should NOT use it in this boat!

 

[michaeel]

Not sure which 90 degree elbow you are speaking of jsalley. On my 309, the seawater output hose connects to a 90 degree hose connector to the u-type mixing elbow. However, unlike the diagram shows, my seawater output hose goes through an anti-siphon loop before connecting to the u-type mixing elbow.

 

[jsalley]

My mistake - you are absolutely correct. I was referring to the 90 degree mixing elbow that Yanmar also offers for the 3YM engine series, which likely wouldn't even fit in the 309's tight engine compartment.

 

[plodder]

thanks

Thank you both for the information and advice. I shall get underway to order those parts. I took off the old exhaust mixer and it is totally clagged up with carbon deposits. I put a temp one on and tested it and it makes all the difference with engine performance. It is terrific to have those images and part numbers - very much appreciated.