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#11 waikiore

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Posted 19 January 2017 - 04:29 PM

yes I would be looking for 6 knots ish at 3000 and being able to get near 3600 as she sinks into her own hole going a little quicker (gives you reserve for weather conditions)


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#12 wheels

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Posted 19 January 2017 - 06:11 PM

OK...here is the story. A diesel engine develops Hp in relation to RPM. The only main difference between the older designs and the more modern ones, that are computer controlled injection, is that the Torque curve is different. So lets say we have a 20Hp @ 3600RPM engine. At 1500RPM, it would be closer to just 10Hp(not exactly, but close enough for the discussion). So lets say the prop has too much pitch and all the engine can rev to is 2600. That means the engine is going to develop just 14Hp. That may or may not be enough to move you about as snappily as you need to. The next issue is that the engine is still getting a command to deliver all the fuel it can into the cylinder. But not all the fuel is being burned, so you can get Black smoke issues, but even worse, you can be getting Diesel Fuel washing the sides of the bore and causing damage. Plus the Big ends are getting a hammering.
        The way to work out if the prop is correct is as follows.

Firstly, while out of gear, open up the throttle and see what RPM the engine will make. If it is a 3600RPM engine and the Tacho reads 3600RPM, then that is great. If it only reads 3500RPM, that is not a major concern. It could be that the throttle is not set to open fully, or it could be the Tacho is not adjusted correctly. Not a major. Although it would certainly be helpful to get a hand held tester and set the engine to correct RPM with throttle open fully.
Now pull the revs back and put the engine in gear and open the throttle fully again. If the engine goes to 3600RPM or exactly the same as it did out of gear, then you may well be under pitched. There is always effort spinning a prop. Even with no pitch at all, there is friction caused by the surface of the disc spinning in the water. As the pitch increase, there is more effort required to move the water. The sum of Surface Friction and movement of water equals about 10% of the effort of the engine at it's full Hp/RPM. Or if I explain that a little differently, with engine in gear and at full RPM, the engine should rev to max RPM less 10%. Or 3600, less 360 = 3240RPM. NO LOWER THAN THAT. If it is lower than 10%, then the prop is over pitched and both engine and box is going to be overloaded.
You can be anywhere within the space of 3240 to say 3590. (because 3600 would be hard to tell if the engine is working or not working). I would certainly be suspicious if it makes 3600 and be looking at more pitch on the prop. 
This max RPM has no relationship to the RPM you want to cruise at. If you want to cruise at 1400RPM, that is no problem at all. The load is in proportion to the RPM and the engine will be happy to work at any RPM you select. But you have the power and the best prop efficiency when you need it when you open the throttle.




 


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#13 Chrisc

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Posted 19 January 2017 - 07:08 PM

Thanks Wheels. Very succinct.☺
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#14 Muzza

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Posted 19 January 2017 - 07:47 PM

Fish. With the boat being that far short of max revs on the engine I would look at a reduction gear change. I have done this on several boats first case a 23ft waterline yacht with single cylinder yanmar 1GM 10.  The engine supplier suggested a 13 x 7 Briski two bladed folding prop but engine was 600 rpm short of  max revs and max speed was under 6 knots. Engine was showing signs of over loading from 2800rpm. Henleys suggest that data from the boat indicated a 12 x 6 would be more appropriate for that shaft speed. I knew this was the wrong way to go so decided to changed reduction from 2.2:1 to 3.2:1 engine was now under propped so added another three inches of pitch 13 x 9 end result was max speed now 6.6 knots at WOT 3600 rpm being easily reached. With  displacement boats you ideally want to keep prop shaft revolutions down to around 1000 rpm. On displacement boats a slower turning prop is always more efficient than a higher speed one. In this case propeller slip decreased from about 32% to about 22%.  Henley's took the later figures and then worked out the ideal would have been a 14 x 8 however 14 inch diameter was cutting down propeller tip clearance to the hull to under 10% so the decision was made to stay with 13 inch diameter.


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#15 Fish

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Posted 19 January 2017 - 08:44 PM

That's interesting Muzza,
Our gearbox is a bit difficult to fiddle with. It's a 54 kg hydraulically actuated PRM 160 box, a fair bit different to the modern 12 kg mechanical PRM boxes.
Understand the logic of having a shaft speed of about 1,000 rpm. We cruise at that, or a bit more if we open her up.
How do you change the reduction ratio, get a new box?
Our prop is a Kiwiprops, so it is very easy to reduce the pitch slightly. I'm not convinced I need to just yet, but I am keeping an eye on my speeds and revs to make sure. I need to check my notes on when we get the black smoke, but think it is actually fairly close to 10% of max revs.
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#16 wheels

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Posted 20 January 2017 - 06:15 AM

Yep a new box. So it is not fr the faint hearted.
I have never heard anyone say the 1000RPM thing before. Interesting. I will keep that in my mind. It does make perfect sense. The faster you want to spin a prop, the more resistance there becomes due to water resistance on it's surface. Hence why you don't see 4 bladed props on high speed race boats.


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#17 native

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Posted 20 January 2017 - 08:23 AM

1000 is on the low side, 1200 is fine but the whole idea is to get the largest slowest turning prop possible. So the process is working backward from the prop, but in the case of an existing install you have to work with what you've got. The consensus is that you should be able to achieve max revs at WOT with the boat clean and in flat water- just. Max cruising revs will be 200-600 rpm lower depending on the engine. 

 

Overpropping is not a good idea, it leads to premature engine wear although some prefer it for motor sailing. Noise and vibration are solvable problems, noise by good well fitted insulation and both noise and vibration by careful engine alignment, ensuring mounts etc are in good nick. A balanced prop, straight shaft and aligned motor is good for several hundred RPM s well.

Fish, sounds like a slight de-pitching is all you need.


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#18 Fish

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Posted 20 January 2017 - 08:48 AM

 

Fish, sounds like a slight de-pitching is all you need.

I'm going to have another look at my numbers. De-pitching is as easy as adjusting three allen screws for us.

Strangely we haven't actually used the boat that much since putting the bigger prop on. Something about it still being spring, and wanting to wait until summer before taking the kids out a lot....

 

We have done the full job on sound proofing, and replaced and upgraded the engine mounts to good cushy-floats - that made a massive difference. We also put a silencer on the air intake - that made a massive difference too. Took out the deep base-note boom boom. For a 30 year old long stroke engine that is twice as heavy as the modern equivalents, it is going really well.

I'm not convinced we've got the alignment right on the drive train yet, but that is getting toward the too hard basket and its much easier to just go sailing.


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#19 Fish

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Posted 20 January 2017 - 08:51 AM

Chris, out of interest, what size props are you going from and to?

The various calculators suggest the hull speed for the H28 is 6.4 knots, and you should need 22.5 Hp to do it, so it does sound like your original prop is under done, and your new prop is over-done.

I find this stuff fascinating (maybe I need to get out more). Will be interested to hear how you go with the re-pitched prop.


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#20 Muzza

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Posted 20 January 2017 - 10:53 AM

Changing the reduction gear often doesn't mean a new box it just means changing components in the existing gearbox. As rotation speed of the prop goes up so does propeller slip. Take rotation speed x pitch in inches and you can work out a speed with no slip then find your actual speed through the water and that will tell you how much the percentage of  slip there is. If diameter gets compromised a solution might be to move from two blade to three blade feathering to increase blade area and load engine up more.


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