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Post by manaia on Nov 19, 2018 18:30:53 GMT
FWIW, Jessabbe with the non turboed version of that engine would do RPM Speed (kn) Consumption (l/h) 1800 5.1 1.7 1900 5.7 2.0 2000 6.0 2.3 2100 6.4 2.6 2200 6.5 3.0 2300 6.5 3.4 2400 6.8 3.6 2500 7.1 3.9 2700 7.4 4.6 3000 7.9 6.0 Speed based on calm test and consumption from the manual Manaia has pretty much the same speeds / revs with her 56hp Yanmar - I did get the Maxprop pitch increased by an inch so that the max revs at WOT are around 3400-3500 without any black smoke. This is probably slightly over pitched but at 2700 revs, which is the 80% continuous running (?) recommendation, we are fast cruising at 7.5 knots.
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Post by sailbleu on Nov 20, 2018 4:32:57 GMT
Hi Sailbleu, no apologies needed. Now that I think about it a bit more, I think people use OTT (with two T’s) for over the top, so I certainly contributed by misreading it. Never mind, I don’t think of it as very far off topic either, as Alenka’s original question was about an appropriate speed for the engine to be running, though he was concerned about prop pitch settings and reasonable hull speed expectation. The question does seem aimed at optimising speed for quite low rpm, So, taking my usual approach, born of a career which always required an in depth grasp of equipment that had never run before, I tried to summarise some of the important factors in determining reasonable rpm. Then perhaps we can see a reasonable basis for our conclusions. I should have mentioned that my little 3YM20 has a rated maximum speed of 3600 rpm and a maximum continuous speed a bit lower. (I am still unclear about which one I should aim for 80% of, but 2900 rpm moves us along quite nicely, and is in the right range. Some of the larger engines seem to be rated for around 3000 rpm, so 80% of that. The other aspect of the question is how different engine rpm affects the load the propellor puts on the engine. This also introduces the topic of what the engine throttle lever actually does. What happens to the load on the engine as we reduce engine speed? That is also important to selecting reasonable running speed. As for saving on cold starts, again, I am in total agreement. Your practice develops incredible skill. I need a bit more courage to try it, and practice, out in open water where there is nothing to hit if I misjudge it. And nothing I have ever seen recommends running the engine at low load to charge batteries or heat water. It works if needs must, but I also feel it should be avoided where possible. I am with you 100% on the little separate water heater, and solar, and if really necessary a little portable generator for when the sun just will not cooperate. Also as one who has sailed trailer sailors for over thirty years, I don’t think having a little outboard on the back is silly at all. The brackets are readily available, so it is mostly about devising a suitable strong mounting on the transom. Room for lots of ingenuity there. You might construct a track or a linkage to lift it up to the rail when not being used, and preferably it could fill a dual purpose as the dingy engine, so not even be extra weight to carry. I often wonder if one of those little electric thrusters, like Vasko uses on his dingy, would be enough. If all else fails, the old fishermen used to move quite heavy boats quite nicely with one oar over the back. And in any case, having sails as reliable alternative power is ok in the text books, but is of limited use in confined situations, a little outboard, say 9 or 15 hp, would be very useful in most circumstances. But that is getting off topic. Perhaps we had better let Alenka tell us if we are getting close to the information he needs. We can always start a new thread on other interesting topics. Rene460 Remember the topic nearly 2 years ago Rene ? jeanneau.proboards.com/thread/5102/engineer-wanted-power-load-ratioWell both ( outboard and dinghy) winches are on and working for 2 summer seasons already. It's well worth the small investment gentlemen , especially getting the 6HP outboard in manually has been a real hassle in the past .Now it's just a turn of a switch be it up or down . Last summer I experimented with my electric outboard on the dinghy that was cross tied up with the bow to my stern . Man did it work , smooth see no wind mind you . But I'm reluctant to give in on the thought that I'm sold for the idea . Years ago I had a plan to use the swim platform-ladder a an outboard mount so a spare propulsion was there in case of main engine failure . As you can see the idea is still lingering & luring in my head . Would love to meet a boat/boater who has finalized this concept . But now I definitely get OT .Sorry for that. Regards
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Post by rene460 on Nov 20, 2018 10:55:38 GMT
Hi Sailbleu, yes off topic, but that doesn’t mean it is not interesting.
You have a much better memory than I to find that one, but I am very glad that the winches are working well. I hope that my small contribution was some help.
On consideration of an outboard as an auxiliary, the typical bracket is a four bar chain that lifts the motor though generally leaving just the propellor in the water. It has springs to reduce the force necessary to lift the engine. The motor is then tilted to get the propellor clear of the water for sailing. However to lift the motor higher, or to avoid the need to tilt the motor, some people leave off the normal bracket and make a vertical track from main sheet traveller beams, and attach the motor mounting block using traveller cars. Then the motor can be lifted by pulleys or your winch. I suspect the normal bracket might not be so handy on your boat, and you may want to make the whole thing removable. Just thinking out loud. Not easy to describe, I can send you a sketch by email if it would help the explanation.
I hope Alenka now has the information he was seeking.
Rene460
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Post by alenka on Nov 22, 2018 12:52:18 GMT
Hi Everyone,
Just back from travels hence the delay in replying.
Following the comments here I finally tracked down the power curves for our engine. I have to say they came as quite a surprise to say the least.
The engine is rated to reach its max potential of 75hp at a very high 3,800 rpm. Personally I have never been on any sailing boat where the engine would rev so high and this coloured my expectations. In fact, on charter boats I was always instructed never ever to exceed 3,000 rpm and on the one occasion I did (to push through a pretty big swell) we soon got an over temp warning.
So.... for the Yanmar 4JH3-TE
3,800 rpm equates to 75 hp. 3,700 rpm is quoted as the max continuous power and develops 69hp
My original post questioned why the standard smaller engine (56hp) could reach 8 ish knots at around 2,000 rpm? 'Manaia' would seem to debunk this notion and reinforce my belief that a touch of optimism had crept into such claims.
My engine has to run at 3,400 rpm to develop 55hp which, again, seems to match what Manaia is saying for his smaller engine; At 2,700 rpm he is achieving 7.5 kts. At 2,700 rpm we are achieving 7 kts. Basically, by the time age related power loss and variations in prop pitch are taken into consideration, it is just too close to call. For me the power required to get us up to 8 kts probably equates to 3,000 + rpm. I say probably because believing this to be the max rpm the engine should be run at I never held this setting for too long, not only because I was under the impression that it was not good for the engine but also the noise levels are uncomfortable.
The big question is, if I increase the prop pitch to put more load on the engine will it get to closer to 8 kts at a lower rpm? Half of me says it would be worth it to lower noise levels... that making the engine work harder is no bad thing for a diesel... lower rpm may prolong engine life. The other half of me says the power/drag ratio will always remain the same and the outcome of fiddling might equate to just black smoke and wasted fuel. Akin to driving up a hill in the wrong gear.
It seems one thing is clear however. The turbo engine develops extra power because it is designed to run at a higher rpm. It does not develop more power at the same rev's of the smaller engine.
Comments invited.
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Post by reiner on Nov 22, 2018 20:27:26 GMT
Hi everyone, I compared both engines the 4jh3-e (55hp) and the 4jh3-te (75hp). The bigger engine has also more power with the turbo at a lower speed. Both engines need nearly the same rpm for the max power. Looking to for example 3000 rpm, you will get 20KW at the prop with the small and a bit over 30KW with the bigger engine. More power at the same rpm means a higher torque which can be used for a bigger prop. I have the 75hp engine on my 43 DS. With my fixed prop I reach 8,1knt at 3000 rpm but only if the hull and especially the prop are really clean. My idea is when changing to a folding prop to use a size where it is possible to reduce the rpm. My feeling is that the engine is strong enough for a reduction of round about 200 rpm.
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Post by optimystic on Nov 22, 2018 20:45:47 GMT
SO 379 and Flexifold three blade yields 6.5-6.8 knots at 2800 rpm. More rpms create little increase in boat speed. 3000 rpm is 7kts. I cruise at 2400 to 2800.
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Post by alenka on Nov 22, 2018 23:53:10 GMT
Hi Reiner,
It would seem your engine and rpm settings with the fixed pitch prop equate closely to what I am getting with the feathering prop.
I still have the original fixed pitch prop in the locker and it is quite a bit bigger in size, but obviously blade size is only part of the equation, pitch is the other.
Next time I am onboard I will look and see what the specs are and then ask variprop what settings they recommend to match. It seems from looking at 43DS's around the yard many, if not the majority, seem to have the same size variprop when they move away from basic fixed pitch so someone somewhere must have decided this was a good option.
If I can achieve a 200 rpm reduction and get to 8 kts at 2,800 rpm I will be much happier.
PS. Does anyone know what the specs are for the standard fixed pitch prop fitted by Jeanneau?
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Post by sitara on Nov 23, 2018 4:37:00 GMT
The standard factory prop fitted to my SO36i was oversized as it would not let the engine rev out. I could only get 3200 rpm. My local Yanmar agent recommended reducing the pitch so I fitted a feathering prop. I have adjusted the pitch on this to give me 3450 rpm maximum. So I don't know what governs the choice of propeller by the factory.
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Post by so36idavid on Nov 23, 2018 6:16:46 GMT
Sitara, I've never even tried to take my 36i to WOT. I'm surprised that it's rated to go almost up to 3500. I should probably give it a try just to see what happens. I get hull speed at about 2800 RPM.
I find that sea state makes a huge difference. If the water is dead flat I get around 7.1 kts at 2800 RPM. Small chop, even from astern reduces this. Anything from forward of the beam will immediately drop the speed down to around 6.5-6.75. There's not much point in burning more fuel to eke out another quarter knot beyond that. I almost always have one or more sails up when motoring. If nothing else, inducing a some heel will increase water length a bit and stabilize the boat.
I keep the bottom and prop clean. I have a 3 blade Flexofold.
David
P.S. If you need to motor to weather, try leading a preventer line forward from the boom over the midships cleat and to the primaries. You can pull the boom to weather a little, keeping the main full and giving 10 or 15 degrees more point. Some boats don't like this but mine does.
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Post by alenka on Nov 23, 2018 12:38:23 GMT
David,
It seems you have a pretty efficient set up. I calculate the theoretical hull speed for the 36i at around 7.6kts so to achieve 7.1kts using 700rpm less than max rated (75%) for the engine is good going.
I agree all of these figures are only achieved in flat calm conditions. Hence I disagreeumption that the up-sized engine on the 43DS was to help push it through any swell without breaking into a sweat rather than to get an overall higher speed.
Of course there will always be HP loss with age and engine hours. Equally, speed readout, inc GPS, will have slight variations so no two boats will get the same figures.
My next step is to try and get a suggested setting for pitch from Variprop and then see what is currently set.
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Post by alenka on Nov 23, 2018 12:46:07 GMT
'disagreeumption' is not the word I used but it cannot be changed?!?
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Post by so36idavid on Nov 24, 2018 6:47:07 GMT
David, It seems you have a pretty efficient set up. I calculate the theoretical hull speed for the 36i at around 7.6kts so to achieve 7.1kts using 700rpm less than max rated (75%) for the engine is good going. I don't know that I'm doing anything particularly smart. I try to keep the boat light and the bottom clean. I think it's also possible that it's overpropped, but if so, I don't really care. It works for me. Frankly I'm quite surprised that the theoretical max is 3450, I always thought it was 3300. I don't know that I've ever taken it over 3000.
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Post by rene460 on Nov 24, 2018 9:46:11 GMT
Hi Alenka, this thread has prompted interesting responses, as the questions apply in some form to every model, so to all of us, even if we got a bit off topic while you were away.
You should be able to edit that word in your post when you are signed in, you will see an edit button on the top right hand corner of the post. This opens the post in edit mode. When you are ready, click on the “save changes” button at the bottom of the edit screen. But it’s an interesting word in the tradition of the best poets and writers.
Your big question should be called big questions, (plural), as it opens so many issues. So just a few comments in response.
First, there is no physical connection between propellor pitch/rpm and boat speed, unlike a car where there is a direct mechanical connection between engine revolutions and wheel revolutions and distance travelled. (Unless the transmission is slipping of course.) The propellor produces thrust, which does vary with rpm, but the boat speed is the result of the balance between the thrust produced by the propellor and the resistance of the hull to movement through the water. The same propellor at the same rpm will produce a different speed on a different boat, and will even vary on the same boat if you have very different weight of crew, luggage, water and fuel, or a different degree of fouling, towing a dingy, head winds, etc.
When you increase the propellor pitch, I would expect that you would increase the torque required, hence power required to drive the propellor at any given speed, but the more important issue is whether it produces more thrust. The speed which results from that extra thrust is dependent on the hull drag-speed curve. I suggest that if you carry this too far, the propellor might just churn the water in a spiral with minimal extra thrust. It is only the axial component of the velocity of the water leaving the propellor that produces thrust, the radial and spiral components in the plane of the propellor absorb energy but do not produce thrust. I would expect there is a more or less optimum pitch and engine for each propellor and engine combination, and at some point, more pitch might not help. We need to remember that blade area is an important factor in determining the propellor ability to produce thrust, as well as the blade outline and the thickness profile of the blade, so different propellors are not directly comparable. Remember it is the propellor speed, not the engine speed which is important in comparing different engine/gear box/propellor combinations. It is all quite complex, and information on how it all works is not easy to find.
It is fairly easy to determine a drag curve for your hull. But the propellor is much more difficult. The most practical solution is probably trial and error. But even that is not so simple, involving either diving or lifting out for propellors with adjustable pitch. And not many of us have access to a bunch of different pitch fixed propellors to swap and compare. So comparing performance achieved by the members of the forum is probably the next best thing. The main requirement is to identify the critical variables, hull, engine, gear ratio, propellor diameter, pitch and manufacturer, while eliminating other factors such as waves, wind, hull and propellor fouling. Engine revs need to be compared against rated speed, as some engines seem to be rated at 3000 rpm while others are rated at 3600, possibly related to using the basically same engine on 50 Hz and 60 Hz alternators. You start to see why there is not a simple yes or no answer to your question.
I am certainly with you on the desirability and benefit of reducing noise, especially on a longer passage, but 20% of the rated speed is a significant drop. And it is worth knowing what the manual specifies as the maximum rated speed and for maximum continuous speed for your engine, and also the gear box reduction ratio, the manufacturer does know the rpm the engine is rated for. I would not advocate running at maximum continuous speed on the basis of noise production, and also on fuel consumption, it rises rapidly towards that wave making limit of the hull, when fuel consumption rises disproportionately for minor speed increases. Basically, the propellor keeps absorbing more power to provide more thrust, but the hull drag curve is steep in that area, so the increase in speed is minimal from that extra thrust.
Probably exceeded my allocation of a few comments, but there are so many issues involved in your big questions.
Rene460
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Post by alenka on Nov 24, 2018 10:29:49 GMT
Rene,
Thanks for your in-depth insight.
The easy bit. Editing the aforementioned word is easy but every time I save the edit it just reverts back to the word I am trying to get rid of... Very Strange!
Cannot fault any of your logic regarding power, props, speed, etc. As you rightly point out altering the pitch after launch is very messy and lifting just to make a minor adjustment expensive. I will see what Variprop say and then compare it to the current settings. I think it might stand a small tweak without creating too many problems.
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Post by Don Reaves on Nov 24, 2018 12:57:46 GMT
There is yet another issue that hasn't come up in this thread. That is the accuracy of the tachometer. I would hope that if your boat has a digital tachometer, you wouldn't have to worry much about accuracy, since the only questionable component of the measurement is time, and quartz clocks are adequately accurate. I'm assuming the input is a simple revolution indicator, of course.
On my 2004 SO 35, I have an analog tachometer, and after repeated calibration measurements, I have determined that it reads about 6% low over much of the useful cruising range. So a 0.5 knot difference in two boats for a given RPM might be due in large part by tach errors.
Don
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Post by pagoda54 on Nov 24, 2018 21:28:04 GMT
There is yet another issue that hasn't come up in this thread. That is the accuracy of the tachometer. I would hope that if your boat has a digital tachometer, you wouldn't have to worry much about accuracy, since the only questionable component of the measurement is time, and quartz clocks are adequately accurate. I'm assuming the input is a simple revolution indicator, of course.
On my 2004 SO 35, I have an analog tachometer, and after repeated calibration measurements, I have determined that it reads about 6% low over much of the useful cruising range. So a 0.5 knot difference in two boats for a given RPM might be due in large part by tach errors.
Don
I had some issues with the idle speed and loading of my Kiwiprop in reverse on our old Sunrise (3gm30). Ahead was fine but going to reverse was problematic due to the aggressive reverse pitch. The mechanical Rev counter looked OK, but i bought an inexpensive digital gauge from Ebay. It worked from reflective marks on the crankshaft pulley. That proved I was almost 200RPM low using the mechanical gauge. I adjusted the idle speed up to spec and lo and behold had no problem engaging reverse any longer. Where prop pitch has a strong influence on the available torque , unduly low idle RPM will make the problem much worse. You might not use the digital handheld gauge that often but once you have adjusted the engine to actual RPM spec, you will feel the difference.
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Post by rene460 on Nov 25, 2018 10:37:01 GMT
Hi Alenka, given that probably none of us have the necessary software and design detail to answer your question about achieving any given speed, we need to think about what the theory does tell us (unless there are some naval architects looking in silently). All the theory is not much use if it does not help guide a practical action.
I am totally with you in feeling that your engine and propellor can take a small tweak without having any really detrimental effect, so if you are keen to get a little more speed at lower rpm, give it a try. I do keep coming back to my mechanics insistence that I check the wide open throttle performance after I fitted the feathering propellor, not because he has any special expertise, but because it is also a totally consistent engineering basis for checking that a load is an appropriate match for the engine. When I specified engines, or more usually turbines or large electric motors in my working years, I normally specified the engine to be 10% more powerful than the the load required, however most of this was to allow for the tolerance on the predicted power requirement of a specifically designed, but not yet built compressor. We are running the actual engine with the actual load, so no margin is really required. (I can talk about engine power loss later if it is of interest.). It is a good basis to asses changes.
The first step is to know the maximum rated speed specified for the engine. We have already noted that the rated speed varies across different engine models. Then make sure you have a clean hull and propellor, and engine recently serviced so it is running well. It is also worth noting those comments about tacho calibration. Results need to be corrected to accurate rpm for repeatability.
Then in calm water with minimum wind, and preferably no current, run at wide open throttle for enough time to find the maximum speed reached. A word of caution, only go to about 10% over the maximum rated speed, even if you are not at wide open throttle. This could happen if the prop is not nearly enough load for the engine. You may have this data already.
The maximum rpm reached is a good measure of the propellor load. Now the thing about propellors is that the torque required is proportional to the rpm squared, but the power required is proportional to rpm cubed. A small difference to the rpm is a more significant difference in engine load. But that gives the starting point for comparison with the result of any adjustments.
If wide open throttle gives lower rpm than maximum rated speed, it would be considered “over-propped”, a not very precise technical word, but well understood. This is not necessarily a bad thing, as we all tend to prefer the lower noise levels of lower rpm. That rpm cubed law means that an engine is significantly under loaded by the prop that was “just right” at maximum rpm. The disadvantage is that you might not be able to use the maximum power your engine could provide if ever caught in a really bad situation. However, if the engine will not reach the specified maximum rpm, there is probably less incentive to increase the pitch further.
On the other hand if you reach that maximum rpm before you reach wide open throttle, the engine could clearly handle a bit more load. So when you next haul out make a small adjustment, and carefully record what you do, in case you want to go back next opportunity. It seems a long process, but each time we have these discussions, I remember that I have now had the boat nine years, time really does fly when you are having fun! Had I made a small tweak each lift out I would have all the required information by now. The propellor manufacturer might be prepared to give some guidance on how much to turn the pitch screw for a sensible increment, recognising that you will have to live with the result for the season.
If your propellor is like mine, it has a second screw to independently adjust the reverse pitch, and the manufacturer set them both a little differently. However, I have been very happy with the performance. Much less drag when sailing which of course varies with speed than the original fixed prop (which also performed exceptionally well, it was for sailing that I made the change). We don’t get quite the same maximum speed as the fixed prop, but not enough difference to motivate me to start the program, and also very good reverse performance for stopping power, very necessary as I motor forward into the pen with enough speed to have steerage with twin rudders and a single prop in between them, then need to stop in about half a boat length. No problem, even taking my time in changing from forward to reverse.
That seems a very long way to say, I agree, it won’t hurt to give it a little tweak to see what happens. It probably won’t be too bad to put up with for a whole season. And it probably the most practical way to determine the answer to your questions.
Rene460
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