Engine RPM and Propeller inquiry

[wildswan]

Hey everyone. I have searched the forum and could not exactly find what I'm looking for. I am just curious if I'm getting the same output as everyone else. I still have the original Yanmar 3QM30F on my boat. My propeller is 22 x16. I find that I can only run my boat at 1800 RPM for about 6kts. I can push it to 2000 RPM but it gets angry and I get a temp alarm after 10 minutes or so. I have read that the best RPM for this engine is more around 2600 or so. Just curious what speeds and RPM everyone else can get? Has anyone tried a different propeller and gotten better results? Anyone able to get close to hull speed? I was thinking a 20 x 16 propeller would allow to to get closer to that optimal RPM. With the current set up, I feel the boat is too under powered and there isn't enough reserve to get me out of a situation if any wind or current is involved. Thanks for everyone's help.

[chrisray12]

If you flat out gun the throttle, what is the rpm tach at?

[wildswan]

In gear, 2100. Not in gear, up to 3000 or so.

[chrisray12]

I am sure there are more knowledgeable minds than me on this forum regarding the 3QM, but it sounds like this is something other than the prop size. 22X14 is the 33t stock blade. You have a 22X16 and although moves more water and yes, you probably may loose a little RPM for max running, you would not be loosing 900rpm under load and certainly not overheating. Has this always been the case since you purchased? Under load and powering up, does the engine hit 3000 rpm then bog down to 2100? I would guess in my untrained opinion...

1- You have an obstruction on this shaft, monofilament, bad cutlass bearing with buildup, heavily fouled prop, tranny needs rebuilding. Can you put the Trans in neutral and turn the shaft freely by hand? Have you dove the prop to see if fouled with growth? If the shaft moves freely, is there any grinding in the tranny? On the next trans fluid change, check for any metal shavings. If all that looks good then you have...
2- an exhaust obstruction. Check the mixing elbow and waterlift for buildup or obstruction. Also check your exhaust line for buildup. This would be my $100 bet but if all this looks good then the next guess is...
3- Plugged Fuel Filter, failing Fuel Lift Pump or bad injectors. This would be more apparent if under load you reach 3000 rpm and then loose power to 2100. The engine is having a hard time keeping up with the fuel demand under power. It still would not explain the overheating but certainly a option.
3- Bad Diesel. This would be my least guess as you should should notice this with a fairly poor running engine, bad smoke out exhaust, missed timing.

[wildswan]

Thanks for the information, all very insightful. It isn't the prop, shaft or cutlass. It's all new except for the prop which I had rebalanced. All the fuel is quite new and came from reputable places and the fuel filters are new. The mixing elbow and hose is new as well, but I'm not sure how it all looks after the muffler. I may need to check all of that. The fuel pump and injectors I have not checked either. It hadn't crossed my mind, but you make a good point. Next opportunity I will check those items. I see that a lot of the HC33 boats came with a 22x16, but you said you have a 22x14. Do you have the 3qm30f as well? If so what rpm and speed are you getting? I was told by the prop shop that the 22x16 is a very strange size, I'd be willing to buy another one to try it. Thanks for the help

[chrisray12]

I have a 4jh and a 22x14 on a 43 mkii. I only know that the 22x14 is stock on the 33t because of the spec stock brochure. The problem we have on our heavy boats is the prop needs to be oversized with a flatter rake to maximize torque and not speed. For example, my 4jh is rated for a 3 blade prop size of 17x16 but we have a 3 blade 22x14. I just looked up the 3qm and it is rated for a 17x11. But on the 4jh I can still get to around 3500 rpm with the larger blade size (down about 150 rpm max) with a cruising speed of 6.8 knots at 2800 rpms.

The low fuel flow at higher RPMS also would not make the engine overheat, so I would throw that low on the list of possibilities or maybe a separate issue all together. This is why I think it had to be an exhaust issue. You should be able to pull up the water flow specs and run the engine at a certain rpm speed at the dock, drop a empty 5 gallon bucket below your exhaust thru hull and fill with exhaust water for a period of 30 seconds or 1 min and check the amount. This would be the quickest way to check for a raw water blockage. On my 4jh manual it shows raw water flow at 1000, 2000, and 3000 rpm.

I have very little 3qm experience but from what it sounds like to me, this is more a engine issue and not a over sized prop issue.

Keep us posted on any new developments.

[jonneely]

when was the last time the raw water side of the exchanger was cleaned? We where having a similar issue with our 3qm, not being able to go above 2000rpm w/o the temp raising over 185 and not stopping until reducing rams to at least 2000. cleaned the raw water side from about 10 old zinc anodes and other sea creatures. With the 160degree thermostat installed for the warm water of the tropics we had a hard time running the engine long enough to get it to temperature. I for some reason was alway scared to hell to open the changer caps, but after doing it that once I am no longer afraid, and actually you should make it a general 500hrs inspection. I think it might actually say to do that in the manual plus you might be already doing such a thing already.

Cheers,

Jon

[wildswan]

The bucket idea is a good one, I'll definitely look that info up and try it when I get back to the boat . Raw water exchanger is a good thought too. I cleaned it out years ago when I bought it, but not since. Barnacle growth is a major problem in my area too. That would tie in with the whole exhaust idea too. Definitely next on my to do list. Thanks everyone.

[bramfam]

I'm following this discussion with interest because the 4JHE on my new HC33 only gets to 2800 rpm. Thanks for the troubleshooting ideas guys.

[lonesome boatman]

Hi All
Respite has a three blade prop stamped 21 x 16 which seems to be correctly pitched for my 3QM30F and 3.0 forward reduction KH18 box on paper. I cannot confirm that the stamping of 21 x 16 is the correct diameter and pitch as the prop may have been modified by previous owners. I initially had problems with black smoke at full throttle in forward gear trying to get to hull speed. The smoke was considerably less in reverse which is logical as their is more reduction in reverse according to the gearbox specs. After considerable engine diagnostics and head scratching on prop/gearbox specifications I concluded that the current prop is slightly to highly pitched to achieve hull speed unless all conditions are optimal. Firstly I checked the engine, clean filters, adjusted the valve clearances and fitted a new exhaust elbow/flexible hose. I then ran some trails using a cheap hand held tachometer I brought of ebay, the non contact type which uses a reflectors stuck on the engine crankshaft pulley and prop shaft. I discovered that the Yanmar tach fitted was way out of calibration and have since replaced it with a more accurate one. The Yanmar type I have are not adjustable, but I believe some have an adjustment screw. Note I did confirm that the hand held unit I brought was accurate by using if on our Subaru cars. The following are my findings and observations. I could only get hull speed of 7 plus knots with a clean hull, flat water and no wind at 2400 rpm. Any growth on the hull limited the speed to 6.5 knots and black smoke. No overheating at full revs 2300 - 2400 under load and smoke is now minimal if I can get up to near 7 knots just as the stern starts to squat down. Out of gear the engine will rev to 2800 no problems at all and would I suspect go slightly higher if I let it under continual throttle. Prop shaft speed and engine revolutions matched and were spot on to the calculated specified gearbox reductions at various rpm's. My conclusion is that a slightly less pitched prop would be more suitable and allow me to reach the optimum rpm of 2600 at around 6.5 knots. The current prop has some de-zincification so it needs to be replaced, I am considering my options at the moment.

[wildswan]

Thanks for all of the information. I thought I would write what I have been able to discover thus far. I disassembled the entire raw water cooling system to check impellers, hoses and the heat exchanger. The hoses were definitely past their prime and were replaced and all the impellers were in great shape. I turned my focus to the heat exchanger. When I shined a flash light through it, all the holes were clear, but upon removal, I could see a lot of hard growth. I decided to dip it in a Muriatic Acid of approximately 30% strength for 5 minutes then dipped in a Baking soda mix. This had a shocking result of lots of hard growth coming off. Upon inspection, I saw there was still more, so I dipped it again with better results. This will definitely help with the over heating situation I feel. I also checked out my Tachometer as mentioned above. Just by adjusting the throttle I quickly realized the Yanmar Tach is not reliable at all. Just by tapping it I got different readings. Next I plan to replace it with a more accurate one.
I took the boat out to anchor to clean the bottom and the prop. Upon my return, I pushed the engine a bit and was able to get about 6.4 knots on mostly calm waters. I wasn't able to run it long enough to see if it would overheat, but I feel that maybe with the cleaned exchanger, that problem may be better if not resolved. I still plan to clean the fuel system and check the exhaust and will report back. I still have the sneaking suspicion that my propeller is too big, but a new and accurate tach will help with that.

[singlehanded]

[singlehanded]

[chrisray12]