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Old 04-01-2009, 15:27   #16
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Not too bad...

Your reported maximum speed of 7.3 knots with average speeds of 6.4 to 6.8 knots looks very good to me.

With my longer waterline and much more power we top out at just over 8 knots under "perfect" conditions. A bit of headwind and current brings us down to just under 8 knots, even using full power.

I find I can cruise at 1350rpm and 6kts in what feels like a sweet spot (low noise and vibration) with modest fuel consumption.
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Old 04-01-2009, 20:00   #17
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We were happy with the results from the 3 days motoring . Even into the chop and headwind it only took a knot off the speed.

Got some vibrations to overcome. I did another alignment a couple of weeks ago and now that we have done a good trip, I will do it again. It is quite hard to get that tolerance below 3 thou of an inch. Vertical adjustments are easy (adjustable mounts) but the lateral adjustments are quite a challenge. I have to do the adjustments with the machined shaft saver insitu.
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Old 04-01-2009, 20:17   #18
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Quote:
It is quite hard to get that tolerance below 3 thou of an inch.
Yes, it is but it's still a good goal. You can hear the difference when you get it that tight.
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Old 12-01-2009, 16:30   #19
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On our run up to Sydney for the NYE fireworks we used teh GPS in a variety of conditions to record the vessel speed, the maximum wa 7.3knots with a tail wind with the engine running at 3600 rpm.

But, the average speed produced at 3600 was 6.4 to 6.8 knots.

The engine seems to run OK at max rpms, but seems happier at 3200-3400 rpms, little vibration, not too noisy and the boat speed is still good.

Diving around the boat we cleaned off most of the slime that was on the hull, but it was thin and there were no barnicles or weeds on the hull. I checked the prop and it was clean.

I still have to do a propper still water test and the weather looks windy for most of the week.

The engine reaches its max rpms without difficulty and we would gather that an increase in pitch may help with boat speed. Not expecting miracles though !
Ribbony,

It's not a good idea to power around at max RPM. That's kinda like driving on the freeway, in your car,with your foot to the floor. If you are under-propped, it's kinda like driving around town in low gear with your foot on the floor.

Your "Cruising" RPM should be somewhere between 1700 & 2500RPM. Cruising speed on your boat should be around 6 kts.

It will take some time of careful record keeping of fuel consumption at different RPMs to find out what is the most efficient for your vessel.

My Ford Lehman would burn nearly 5gal per hr at 2300RPM (max RPM is 2500) @7.9kts. I would get .75 gal per hr running @ 1600RPM @ 7kts.

Pushing your vessel up to hull speed can be expensive. Pushing it past hull speed just burns fuel while trying to get your boat to power up hill, trying to climb it's bow wake to get on plain (which, of course, is impossible).

You should only use max throttle under extreme circumstances and only for short periods of time.
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Old 12-01-2009, 18:28   #20
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OK, so it sound like we should run the engine at 1/2 to 2/3 of rated max engine RPM's for our extended "iron" sailing legs. I had thought that running diesels too low on the rpm's was not good for them either.
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Old 12-01-2009, 18:49   #21
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OK, so it sound like we should run the engine at 1/2 to 2/3 of rated max engine RPM's for our extended "iron" sailing legs. I had thought that running diesels too low on the rpm's was not good for them either.
I'm not sure what you're referring to here. However, a diesel engine can run at cruising RPM 24/7 for months without shutting down. It is better to have a load of some sort rather than to no-load idle for hours. Letting a diesel "Free-wheel" (run at high RPM with no load) is not good. That is one of the reasons not to under-prop the engine.

What is also not good is to "Lug" any engine. That is, why over-propping isn't good. The engine has to work too hard to maintain speed because the prop is too big and the engine cannot reach it's optimum power range.

BTW.....there is a difference between "Peak RPM" and "Peak HP RPM". You engine's HP is rated on a curve. The top of the curve (Max HP) is reached somewhere before the engine's Max rated RPM.
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Old 12-01-2009, 19:14   #22
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That is clearer. I will have to see if I can find a graph for our engines power and RPM's. Being an older model of Nanni, it has been hard to get any information at all on that motor. No service manual has been able to be obtained, there are summarised versions around but not the detailed editions.
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Old 12-01-2009, 20:02   #23
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Here is a typical high RPM diesel configuration:
http://www.yanmarmarine.com/uploads/...0_TechData.pdf

You will notice that the "Continuous RPM rating" is 3489. Max RPM is 3600.

That "Continuous RPM rating" is what you can run the engine at for long periods if you need to. It is not the recommended cruising RPM.

The HP flattens out on this engine at 3250. Some engines drop off from there, this one continues.

Move to the torque and fuel consumption charts and you will see that the torque drops off dramatically after 2500RPM and the fuel consumption increases dramatically after 2500. At 2500 RPM this engine burns about .5L per hour. At 3600 RPM, it burns 2L per hr.

This is a pretty typical set up for those high RPM diesels. I wouldn't expect yours to be too much different. It looks like the optimum RPM range on this engine would be right around 2500.

IMHO......you would want the prop pitched so that this engine can turn no faster than 3600 at full throttle. This should give you optimum performance.
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Old 12-01-2009, 22:43   #24
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IMHO......you would want the prop pitched so that this engine can turn no faster than 3600 at full throttle. This should give you optimum performance.

I like that last quote, that is a good goal to aim for with RPM's and performance .
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Old 13-01-2009, 15:17   #25
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Just one other note here.

Often times the Max Continuous RPM rating is calculated on the oiling and cooling capacity of a particular engine. In other words.......(for the engine that I posted a link to) after 3489RPM, the engine may not have enough oil lubrication and/or cooling capacity to sustain that engine for more than just a very few minutes.

If you study those charts, you can tell that there is very little to be achieved out of that engine after 3250RPM, even in heavy seas and strong winds.

Powering into such conditions is usually done to enter a harbor under extreme conditions with the wind on the nose and a heavy chop. These are just the sort of conditions that are the most challenging for any engine. The harder that you run the engine in these conditions, the more likely an engine failure. In circumstances like that, engine failure could be catastrophic for the vessel and crew.

I would simply never run that engine past 3250, no matter what the conditions. The engine has achieved peak HP which should keep the prop running at it's max efficiency. You will also notice that the torque curve drops off after 3250. That means that it would be even harder to sustain a higher RPM in adverse conditions, which would put further stress on the engine.
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Old 17-01-2009, 12:28   #26
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Quote:
Originally Posted by Kanani View Post
Here is a typical high RPM diesel configuration:
http://www.yanmarmarine.com/uploads/...0_TechData.pdf

IMHO......you would want the prop pitched so that this engine can turn no faster than 3600 at full throttle. This should give you optimum performance.
The manufacturer of the engine referenced above has a slightly different recommendation:

"It is recommended that new vessels be propped so the engines can operate at 100 - 200 rpm above the Maximum Rated Power Output rpm (3700 - 3800) to allow for some added weight and hull resistance. The engine must be able to reach the Maximum Rated Power rpm (3600) under full load at all times." The engine RPM should be independently verified with an optical tach, since the instrument panel tach error can be 200 RPM or more.

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Originally Posted by ribbony View Post
I had thought that running diesels too low on the rpm's was not good for them either.
A diesel should be at least occasionally allowed to reach full rated rpm to avoid carbon build-up and glazing of the cylinders. Again, Yanmar recommends occasionally racing the engine by accelerating (in neutral) from low speed to high speed and repeating this process five times. This is supposed to clean out the carbon deposits from the cylinders and injector nozzles.

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Move to the torque and fuel consumption charts and you will see that the torque drops off dramatically after 2500RPM and the fuel consumption increases dramatically after 2500. At 2500 RPM this engine burns about .5L per hour. At 3600 RPM, it burns 2L per hr.
Generally, the "sweet spot" cruising RPM that will give you most horsepower for fuel consumed is at the maximum torque. So 2500 RPM is about right for the Yanmar 3YM30 referenced above (a minor nit is that the fuel consumption @2500 rpm is 0.6 Gallons per hour, not liters.)

Also, some diesels (e.g., Bukh) are rated for continuous operation at max RPM, and the governor is set so you cannot exceed this maximum.
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