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Post by Torsten on Feb 9, 2020 8:28:06 GMT
Hi folks,
since 2014 I have a speed log installed on my SO30i. I think I used the recommended position from Jeanneau, but I may be wrong. Here's where I placed it:
So it is placed approx. 20cm starboard lining up with the forward edge of the keel.
I can allign my NASA Clipper Duett to be similar to the SoG stated by GPS when going under motor (not heeled). When ever I am sailing the instrument tells me up to 0,5kn more speed than SoG if being heeled to starboard and up to 0,5kn less speed than SoG if being heeled to port. I am pretty annoyed by that.
SO30i owners: Where is you speed log sensor placed? Do you have similar issues?
Any ideas how to improve? I would accept if it is the same error for starboard and port (so up to +0,5kn or -0,5kn for both the same but not having +0,5kn on heeling to one side and -0,5kn when heeling to the other side)
Thanks for your attention
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Post by zaphod on Feb 9, 2020 9:13:32 GMT
That seems pretty far back, and perhaps too close to the keel. As your keel moves through the water, the water accelerates around the foil shape, which is what creates the lift. Your impeller could be in the "bow wave" of the keel.
Typically the knot meter should be on centerline and further forward, well away from anything that will disrupt the flow across the impeller, unless there is some peculiarity about the 30i that precludes it.
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Post by Torsten on Feb 9, 2020 9:38:50 GMT
Hi Zaphod, What you say makes pretty much sense. I need to check with the other SO30i owners about their placement and experiences. From what I can see for now is that the fresh water tank is placed in that area where it makes most sense to place the impeller. Thanks for your quick response! Best regards Torsten
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Post by rene460 on Feb 9, 2020 11:03:50 GMT
Hi Torsten,
The log paddle wheel in our SO30i was installed by Jeanneau, so was in place when the boat arrived in Australia.
It is pretty much in the same position as you have chosen, on the port side. We certainly often see that difference in indicated speed between tacks. However, I also find that even with the gps there is a difference.
As Zaphod says, the water flow at that point is not exactly fore and aft at that point, but I suspect that applies for any other point as well, especially when you are sailing heeled. The leeway inherent in sailing anything but direct downwind means that the hull is not travelling parallel to its centreline. Perhaps straight and level the flow on the centreline might be parallel to the centreline. I suppose there is also a point on each side at the maximum beam where the flow is also parallel to the centreline, but it might show even more difference between tacks.
I have experimented with angling the wheel slightly towards the centreline, so it is perhaps parallel to the flow I imagine at that point, and calibrating at that angle. It certainly does not make it worse, possibly slightly better, but I am not totally sure. Our lakes are almost not tidal, but I do notice, even when motoring in quiet conditions that we seem to have quite a bit of wind driven surface drift, which of course is the same as current to the instruments. I judge this by looking at the gps course over the ground compared with our heading when motoring in smooth water. Wind definitely makes a difference, but so does the minimal tidal flow.
Jeanneau installed the Raymarine ST 70 Instrument with the dst 800 combined log paddle wheel depth and water temperature, so only one fitting. The instrument failed after some years and was considered by the local technicians to be unfixable, so I replaced it with an i70. It was a real upgrade, so I did not regret throwing out the old instrument. The big advantage is that the log calibration is carried out at 6 different speeds, giving a calibration curve, which is more realistic. The calibration has held for some years over a wide range of speeds, it is still good when I compare it with the gps.
Overall I suspect the point you have chosen is as good as any. I tend to not put too much weight on the exact reading, but just concentrate on getting the best speed I can on the tack I am on, and use the leeway angle indicated by the difference between the heading and the COG to experiment with pointing up a bit or down a bit to see if I can minimise leeway.
Rene460
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Post by Torsten on Feb 9, 2020 12:11:21 GMT
Thanks Rene for your confirmation of the factory log paddle wheel position as well as for sharing your investigations and results. I really hoped also to get your feedback!
I also tried turning the orientation of the paddle wheel, hoping to compensate the effect a bit, but did not really succeeed doing this as well.
I also thought of having a second log paddle wheel on the port side, "adding" both pulses and calibrate the instrument for 50% correction. But I do not want to have more holes to the hull.
So you may be right, just use it as a relative indication in order to see if some trimming increases or decreases speed vs. heading.
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Post by sitara on Feb 9, 2020 21:23:33 GMT
The paddle wheel in my SO36i is on the center line in front of the keel. I notice changing errors in speed through the water when the boat is heeling significantly.
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Post by rene460 on Feb 10, 2020 8:40:38 GMT
Hi Torsten, you are welcome. Please feel free to send a PM or just mention my name in a post if you want me to respond any time. I follow the forum every day, but sometimes fall down actually writing my response.
I need to make one correction to my earlier post, the original instrument was an ST 70 (I found it in a box today, another “ one day” project to explore). perhaps the error was due to 10 years of ST 60 instruments on the previous boat. I have edited yesterday’s post to minimise future confusion.
I also wanted to add an extra observation. I have made a practice of resetting the trip log on the i70 and one of the trip logs available on the gps unit display each time it gets up to 100 nm or so. I have noticed that despite the different speed indicated on each tack, the total accumulated by each of the logs differs by only a small amount, often only two or three percent. It shows that the errors are indeed compensating, as the i70 log relies entirely on the paddle wheel for distance as well as speed. The first time I noticed it, I was so astounded that I have been a keen observer of the issue ever since. Not sufficiently organised to have recorded the results unfortunately.
This means that your idea of using two transducers and combining the readings mathematically some how might well work. It would mean two hull openings and I agree with your reluctance in that area. And it would also involve inserting some extra hardware to do the combining in real time. I don’t know if the Raymarine equipment I am using has the necessary hardware for simple installation, and it is certainly beyond me to make up something myself that could be inserted into the system. I have read of the big guys in off shore racing using two transducers and switching between them or perhaps combining the output in some other way, but I suspect they use a more expensive system than I could afford. Certainly at my level, I think it is better to keep it simple. But it is interesting to try and work out what is going on.
Hi Sitara, it is interesting that your transducer is actually on the centre line, and to learn that it does not eliminate the problem. That seems to confirm my feeling that the flow around the hull includes a cross ways component, and is further affected by heeling perhaps in addition to waves. Obviously not worth making the extra hole in the hull to find out the difference. Thank you.
rene460
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Post by ForGrinsToo on Feb 10, 2020 18:49:36 GMT
First, like Sitara, the speed/log on our 36i is centerline and perhaps 20-25 cm forward of the keel. It is also proud of the hull by maybe 4 - 5 mm, which is certainly going to affect water flow past the paddlewheel. About the only time it's reasonably accurate is when motoring in flat water, and then the Raymarine ST60 has only a single point calibration (really, just an offset) so it's only close at 5.5 - 6 kts and deviates significantly below or above. Also, like Rene observes, the log function seems to agree remarkably well with the GPS - and I don't really see how that can be. Anyway, because it supplies readout to 0.01 kt, I tend to use the ST60 mostly just for sail trim. I use a GPS for SOG.
But what I really wanted to point out is that there are now on the market at least two ultrasonic speed/log units which mount inside the hull, and do not require a hull penetration, from AirMar and NKE, though I've just read B&G has one, as well. That would be my choice to replace the ST60 when it goes.
Geoff
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Post by Torsten on Feb 15, 2020 14:38:42 GMT
Thanks to all for your good feedback! Cheers Torsten
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Post by NZL50505 on Feb 16, 2020 0:57:13 GMT
I agree that worst case you can just use WSP log for sail trim and GPS for SOG. But I still like the keep my paddle wheel in good condition because there is a second benefit - namely wind & tide. Like many others here my Raymarine instruments are all networked to the plotter (Axiom Pro in my case) which gives some useful overlays on the screen - including true wind and tide. And it calculates both from the WSP paddle wheel not GPS SOG. I hope that one day Raymarine do a firmware upgrade that changes this but for now I believe it’s worth a regular paddle wheel scrub to help preserve decent overlays for both true wind and tide.
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Post by rene460 on Feb 16, 2020 7:41:43 GMT
Hi NZL, I did not mean to imply that I have abandoned my paddle wheel. I remove replace the transducer with the plug every time we leave the boat and put it back clean and spinning freely when we return. And whenever we are motoring I check the reading against the gps reading to see that the calibration is still ok. It has held better than I ever imagined it would.
So when motoring, I do use the difference between COG and heading to get an idea of currents. They are more than I would ever expect on a nearly closed lake system, so I try and make sense of it. I think the currents I see are mostly wind driven surface currents. And of course it gives an idea of leeway, though affected by current as well.
But I don’t get to up tight about the different readings on each tack.
rene460
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Post by alenka on Feb 17, 2020 11:42:01 GMT
Paddle wheels are notoriously fickle. Take it out, clean it, and put it back in just a fraction off the optimum angle to the water flow and the reading changes. At least it does in mine as there is not lug or slot to keep it in a set position.
Maybe if you experiment with the paddle wheel angle to the flow of water to try and get a consistent error on both tacks and then put an electric correction in that might be a potential solution.
I would tend to think that jeanneau must have an awful lot of experience in fitting paddle wheels and any errors in location would be sorted in pre-production models.
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