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SingleProp versus DuoProp performance-gctid395440

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    SingleProp versus DuoProp performance-gctid395440

    I drew together some performance figures from my 3250 Conquest Single Prop to DuoProp conversion. Thought they may be of interest.

    In summary seems a ~ 22% improvement in efficiency, as measured by mileage at various speeds. Top end is effectively no different.

    Some specifics:

    * 3250 Conquest. Twin AQ260s, 1200 hours on one (a bit tired) perhaps 600 on the other? (not sure, PO did a swap). Quadrajet carbs, 280 legs. Dual flowscan. Same GPS speed tests, no tide impact, same amount of gear on the boat, same flowscan, same fuel lines so expectation is any errors are consistent - relative differences therefore are what I am looking at.

    * Old. 280 legs, 16x17 props in perfect shape.

    * New 280 legs, B3 props in 'ok' shape.

    Data points are a bit 'lumpy', probably due to reading guages whilst banging around the harbour. I think the overall trend is right. The single top-end datapoint on the old 280 legs might be suspect (maybe have been running down hill )

    Still early days so might be some more tuning up to do, but the results are going in the right direction!

    Attached files [img]/media/kunena/attachments/vb/699102=29178-temp.jpg[/img]

    #2
    Thanks, good info. How do the RPM's compare/differ?

    Comment


      #3
      It seems there is a bit over 1/2 knot. Off-plane you notice it as it is a bigger % eg. 1400 RPM is 5.9 to 6.5 knots - so 10%. Once on plane there is less of a % difference, at 3000 RPM it is 18.1 vs 17.5. Nice to have an improvement all the same.

      I'll be playing with the trim and will get some more data points.

      Comment


        #4
        I'm actually quite surprised to see your red line indicating a fall-off in fuel efficiency towards the top speed range. I'm also quite surprised to see that both graph lines end near 30 knots.

        I wish that I had taken some real numbers prior to adding my Duo Prop drives, because my experience with my own boat, and with the boats that I have fitted, have been quite different.

        Both a speed increase and ease of engine load seem to be the typical results.

        The difference that these made on my own boat was pretty impressive to me.

        Will your WOT RPM allow for trying B-4 props?
        Rick E. Gresham, Oregon
        2850 Bounty Sedan Flybridge model
        Twin 280 HP 5.7's w/ Closed Cooling
        Volvo Penta DuoProp Drives
        Kohler 4 CZ Gen Set

        Comment


          #5
          WOT right now is 4100, which is what I had before with the SPs. The engines are a bit tired so I think this is pretty good. Rated WOT specs are 4000-4200 according to the manual.

          I have B3s on it simply as those were the props the legs came with, so I thought to start there and then see.

          It does not seem close to being overpropped (ie it fairly leaps onto plane) so I am *thinking* it will run OK with B4s.

          I haven't found anyone around here (Vancouver BC) where I can trial some, so if I have to buy them then it will be a bit of an expensive test! Any ideas on this?

          Comment


            #6
            Just an FYI, I have been running B3 on my DP leg with an AQ260 w/ 600+ hours with no issue either. WOT is around 4100k as well. I do get a .1 NM/Gal increase in fuel efficiency though and I can cruise at roughly 100 RPM less than the B2 for the same speed. I came across a similar test done by an engineer in Oregon about a year ago. He posted his findings on the internet with videos as well. I seem to remember a 25% increase in efficiency as well but it's been a while.

            Gary.

            Comment


              #7
              nmuir wrote:
              WOT right now is 4100, which is what I had before with the SPs. The engines are a bit tired so I think this is pretty good. Rated WOT specs are 4000-4200 according to the manual.

              I have B3s on it simply as those were the props the legs came with, so I thought to start there and then see.

              It does not seem close to being overpropped (ie it fairly leaps onto plane) so I am *thinking* it will run OK with B4s.

              I haven't found anyone around here (Vancouver BC) where I can trial some, so if I have to buy them then it will be a bit of an expensive test! Any ideas on this?
              If you go to B-4's you'll lose 300 RPM or so which takes you to 3800 RPM WOT. Seems like the wrong direction for a tired engine??? If youÔÇÖre testing with the boat loaded as you normally do it looks like the B-3's are the ticket. If you usually load heavier B-2's might be better yet.

              Around here the prop shops will let you trial out a set assuming if they work you will purchase.That said once you're sure, you can have your current B-3's re-pitched to B-2's or B-4's.

              Comment


                #8
                nmuir wrote:
                WOT right now is 4100, which is what I had before with the SPs. The engines are a bit tired so I think this is pretty good. Rated WOT specs are 4000-4200 according to the manual.

                I have B3s on it simply as those were the props the legs came with, so I thought to start there and then see.

                It does not seem close to being overpropped (ie it fairly leaps onto plane) so I am *thinking* it will run OK with B4s.

                I haven't found anyone around here (Vancouver BC) where I can trial some, so if I have to buy them then it will be a bit of an expensive test! Any ideas on this?
                As my grandfather used to say; "Sump'n aint right in Demark!"

                Either your drive ratio is incorrect (should be 1.95:1 for these engines), your engines are not in full tune, or your tachometers are not accurate.

                When all of the ducks are in a row..... fuel delivery systems, compression up to snuff, and so on, it typically boils down to Ignition timing.

                All too often we set BASE advance, and look no further.

                http://"http://www.baylinerownersclu...is thread.</b>

                This may not be your issue, but it can certainly become one item removed from the check-off list.

                NO...., I'd not try the B-4's just yet. Your WOT RPM numbers do not warrent this... not yet anyway.

                Do you know the final drive ratio? (this will stamped into the prop shaft bearing carrier, just ahead of the ring anode)

                Do you know what your ignition TA is at/near 3.2k RPM?

                Fuel delivery system up to snuff?

                .
                Rick E. Gresham, Oregon
                2850 Bounty Sedan Flybridge model
                Twin 280 HP 5.7's w/ Closed Cooling
                Volvo Penta DuoProp Drives
                Kohler 4 CZ Gen Set

                Comment


                  #9
                  First of all - thanks everyone for the comments. They are appreciated!

                  As far as answers to the questions:

                  * They are 1.95 ratios as confirmed by the stampings on the casing and good-ol' fashioned counting the turns .

                  * The best DP prop selection info I have spotted is in the table Ref http://volvopentaservice.dk/User_fil...e8f50357ba.pdf. It shows AQ260, 1.95:1, 4000-4400 rpm B3 as 26 to 29 knots, so it sounds about right so far. No reference to Denmark in the table though . What do you think I should be seeing?

                  * I haven't tested timing at speed, base/idle is good, and I can see it advancing with RPM increase but I didn't quantify how much. Both engines seemed to advance similar amounts with an eyeball check so I left it and went onto other matters. I'll mark the balancer and check properly if it is advancing enough - a job for later this week.

                  * I think the fuel system is ok. Filters all changed, Carb rebuilt on one engine, new carb on the other. The flowscans show that lots of fuel is flowing once the 4 barrels open up! With gas at $1.50/litre up here in Canada it can take your breath away!

                  * Good call on the ignition question. I have already chased a few poor connections through the ignition system and improved performance (including an old ignition switch that was leading to a voltage drop at the coil and misfiring under load). I think there might be one issue left on the stbd engine. There aren't too many things left to check so should be fast to sort.

                  Comment


                    #10
                    nmuir wrote:
                    * I haven't tested timing at speed, base/idle is good, and I can see it advancing with RPM increase but I didn't quantify how much. Both engines seemed to advance similar amounts with an eyeball check so I left it and went onto other matters. I'll mark the balancer and check properly if it is advancing enough - a job for later this week.
                    The only way to confirm this, will be a TA check. If you read the thread that I linked you to, it will explain how to do this.

                    I'm not a big fan of using the digitally advancing timing lights, but if you have a good expensive light such as Snap-On or ????, give it a try.

                    Plot the numbers out at the various RPM for each engine.

                    Post the results.

                    .
                    Rick E. Gresham, Oregon
                    2850 Bounty Sedan Flybridge model
                    Twin 280 HP 5.7's w/ Closed Cooling
                    Volvo Penta DuoProp Drives
                    Kohler 4 CZ Gen Set

                    Comment

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