Measuring junk saling performance

<< First  < Prev   1   2   3   4   5   ...   Next >  Last >> 
  • 04 Sep 2018 19:54
    Reply # 6653308 on 4913961
    Anonymous member (Administrator)

    Sailing with Ingeborg’s Tohatsu swung up and down.

    Today I had another sailtrip in Ingeborg (no. 27 this summer). Apart from testing the Fan-Up Preventer, FUP, I also decided I would try to detect any speed difference (GPS) between when dragging or not dragging the Tohatsu’s propeller.

    This turned out to be less than easy, since I was alone on board, with no autopilot. I soon found that the easiest way to keep a constant drive was to head straight downwind and aim for some mountaintop in the distance. The problem was to keep the boat steady while I moved aft to lower the outboard. Several times Ingeborg rounded up 45° or more and it took time to turn back and pick up speed again  -  and the wind may then have changed. However, I think I hit quite well a couple of times. At one of those tests, the speed was about 3.0-3.1kts with motor up, and it dropped to some 2.5-2.7 with the leg down. On another go, the speed was around 3.5kts with leg up, and 3.0-3.2kts with the leg down.

    In other words, at this low speed, the propeller appeared to drag the speed down with about 10%. I noticed that as the speed increased, I had to push on the motor head to bring the leg down and lock it. Now, Ingeborg is 2150kg empty and with a SA=35sqm. Since Weaverbird is fitted with the same motor on a smaller and lighter boat (1200kg, 22sqm?), my guess is that the lowered propeller will have a bigger impact on that vessel.

    Btw, I tried to measure any difference between when the propeller was locked (reverse gear) and with it spinning freely. I can’t say I registered any real difference, although I felt a slight tendency to speed up a little with free propeller. I may be wrong on this. I hope to repeat this whole test with a crewmember keeping the boat on course while I handle the engine.

    Cheers, Arne

     


    Last modified: 05 Sep 2018 08:57 | Anonymous member (Administrator)
  • 03 Sep 2018 20:23
    Reply # 6651752 on 4913961
    Anonymous member (Administrator)

    Alan, the testing equipment that you have arranged sounds excellent. It's such an advantage to be measuring the conditions directly, and fascinating to think about what you might discover as this project goes on. I'm really looking forward to how this develops.

    David, absolutely, about only changing one variable at a time. I would be really interested to see that test include prop in the water/prop out of the water. Perhaps Weaverbird is not the simplest boat for testing that, but maybe somebody else with an outboard will be participating, and can run a comparison more easily. I have heard that it can be a full knot difference, with and without a prop in the water – and that was about a boat with an inboard and prop, from which the motor and prop were removed. It would be great to find what it is more definitively, with measurements to confirm near-identical wind conditions.

    Shemaya

  • 02 Sep 2018 14:21
    Reply # 6650206 on 4913961

    I don't know much about Spot GPS but the internet search tells me it costs $150 per unit, plus an annual fee of $200 per year or $20 per month. Plus another $200 for the weather station. So that's $500 per boat in the first year alone. I'm not sure our members or the JRA are willing to pay for this on "a number of boats". 

    I think it only sends your location (not speed) at the preset interval, so speed will have to be deduced from two successive locations and will effectively be an average over the period of 5 mins, or whatever period is chosen. I don't think it will transmit the data from the weather station, so some time after the event someone has to try and match the recorded weather station data (which will require another device to record it) with the GPS data. 

    Also, the GPS data will not give you boat speed and direction through the water, only over the ground, so including any current, leeway, etc.

    I think the data produced for boat speed through the water, wind speed, and direction, will not be very accurate, and therefore not very useful for comparing the performance of different boats and rigs.

    It could be that if lots of members were to fit the same Spot GPS device, and the same weather station, we could over time collect some averaged voyaging data. However, being independent minded folk, I suspect that they would likely fit different units from different makers, and then there would be questions about whether the calibrations of the weather units match, and so on.

    Meanwhile, we now have a set of instruments which can be transferred from boat to boat, (so the calibration is not an issue), which can measure quite accurately boat speed through the water, wind speed and direction. We have started measuring the performance of some boats with interesting rigs, and there are more being lined up as we speak. I plan to put an article in the Jan/Feb issue of the JRA mag to discuss the results already obtained.

    We are already up and running!

  • 01 Sep 2018 19:05
    Reply # 6649538 on 4913961

    would the "Spot" type gps that can upload a signal to the web through gps satellites coupled with a personal weather station that can be bought of amazon for under $200 provide useful data? 

    basically you'd have the ships position at the time of upload and weather conditions at time of upload and upload intervals can be set automatically form every 5 minutes and up to once per day.

    the Data wouldn't be terribly accurate, but if you had several boats contributing data you should be able to calculate a fairly meaningful average.


    Bill F

  • 01 Sep 2018 08:39
    Reply # 6649137 on 4913961

    The condition that I'm sailing in is exactly the same as that for a boat with an inboard engine and a fixed prop. That is, the majority of cruising boats. And as long as I don't introduce a variable by taking the outboard in and out, then we can get valid figures before and after  making a change to the rig. This is the major purpose of the testing. In the case of Calisto, to evaluate any gains and losses due to coverting to JR. In the case of Weaverbird, to evaluate any gains and losses due to changing from hinged battens plus a little camber, to wing sail. In a case of a boat upgrading from a flat sail to cambered panels or hinges, what is gained and what is lost? And then we have the case of, say, comparing Weaverbird and Miranda. Two boats the same length, with different rigs. To get a valid comparison, either both boats, or neither boat should be dragging a prop through the water. 

    And then of course there's Weaverbird, with the same rig, and with and without the prop drag. A valid comparitive test, to see whether Arne is right about how much speed I'm losing. In all these test procedures, it's important to change only one variable at a time. Then we will get a clear picture of what happens when we make that change. It really doesn't matter to me all that much, most of the time, that I'm sailing a little slower when I'm dragging the prop, for cruising. On the other hand, it would be nice to know how much speed I'm losing, if I were to be considering making a major structural change so as to lift the prop while sailing. 

    This testing programme is all about recording data that is in some way useful. 

  • 01 Sep 2018 00:13
    Reply # 6648836 on 4913961

    I don't know the circumstances but maybe it's time to think outside the box. Why not take the motor off Weaverbird until your trials are done? Perhaps rent a sailable mooring for overnight storage and hire a tender to get you there and back. Charge the NRA. No, wait..., the JRA. It's what our money is for.

    At my club we have a spar-dock and spar-crane. And, they will do tender service as well. Quick and easy.

    It seems pointless to do all that research and allow a confounding variable skew your results.

    As I said, I don't know your circumstances.

  • 31 Aug 2018 18:27
    Reply # 6648314 on 4913961
    Anonymous member (Administrator)

    Too bad, David  -  my sympathy.

    As you know, my Ingeborg has a similar engine to yours. Its propeller is on the large side, so I can understand that the original through-hull hole is on the small side. Does it mean that the well itself is so narrow that you cannot widen the hole in the hull without ‘going Titanic’?

    In that case, if Weaverbird were mine, I would fit a sliding outboard bracket to the starboard transom, with a 3:1 purchase on it. Dragging an engine in the water is simply no-no in my head. The ob. well still pays for itself as very good cockpit drain.

    Arne


    Last modified: 01 Sep 2018 14:50 | Anonymous member (Administrator)
  • 31 Aug 2018 17:46
    Reply # 6648270 on 4913961

    Quite impossible, Arne. Do you think I haven't thought about ways to do it? The well is too narrow for the prop to come through on a straight lift. If I could lift the motor out,  in a lumpy sea, without damage to myself, boat or motor, there is nowhere to store it. The only solution is the SibLim arrangement, and it's not worth the big rebuild.

    The later model, the Horizon 23, had a well cum locker offset to one side, and a four stroke could be tilted normally. Again, not worth the big rebuild.

  • 31 Aug 2018 16:54
    Reply # 6648177 on 4913961
    Anonymous member (Administrator)

    David,
    I can see your reluctance against lifting  out and storing that heavy Tohatsu each time you go sailing. Still, I struggle with understanding how you can put so much effort in R&D work on the rig and then let 0.5-1 knot go by dragging the engine like that.

    There are other ways: I suggest that you make an engine bracket which can be raised vertically. This will leave the engine in almost-hot stby. It will, of course involve a tiller which passes in an arc over the lifted engine, but that is no big deal. Come on, you can make anything you want  -  this is no problem for you to fix!

    In my town we are not always that speed-keen sailors. It is quite common to see boats romping along under the roller headsail alone, and even dragging a fender or two is regarded as a forgivable senior moment. However, sail about with the outboard engine still lowered, and you will be regarded as a hopeless, incompetent case, who even disregard the wellbeing of that poor outboard...

    Arne

    Here is what happened to Gary King's Ashiki when he modified the engine bracket to raise the engine.


    Last modified: 31 Aug 2018 17:00 | Anonymous member (Administrator)
  • 31 Aug 2018 15:26
    Reply # 6647989 on 4913961

    There is no raised position, Arne! This boat was designed in the days of two strokes that didn't mind being laid down. Four strokes do mind, and are much larger in size. I have to consider it as a saildrive.It's only feasible to take it out for winter layup. Anyway, bear in mind that Weaverbird is now a cruising boat, not a weekend leisure sailing boat, and my priority is convenience, not ultimate speed (but still, she goes pretty well).

<< First  < Prev   1   2   3   4   5   ...   Next >  Last >> 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software