Could you give me more info about my boat

Discussion in 'Powerboats' started by Nidza, Dec 1, 2016.

  1. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    It is a no-brainer to be having an adjustable tab that is in front of, and below the cav plate. Asking for trouble. Whether getting rid of it solves your problem is not certain, but is has to go, next to useless anyway, even if it didn't interfere with in-flow to the prop. Something like a Permatrim would have been a better idea, but also offers minimal lift till you get moving faster than your current top speed. Incidentally, the other trim tabs visible look like a home made job, hard to imagine they hold their shape properly under load.
     
  2. Joakim
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    Joakim Senior Member

    The trim tab in front of the drive looks really bad for this boat and propeller. It seems to be permanent, not adjustable. Hard to see how the drive is installed relative to the hull.
     
  3. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    Central tab is permanent, not adjustable. I have already told earlier, anti ventilation plate is about 2 centimeters above the bottom of the hull in the middle of the boat (not keel bottom, hull bottom, that permanent tab is in line with the bottom of the hull and anti ventilation plate is 2 centimeters above that). Obviously, we all do agree now that central tab must be removed.

    At current top speed and even slower than that, the boat angle does react to the angle of two stainless steel adjustable trim tabs. Those tabs are about 3mm thick steel plates, so they will not loose their shape easy. Although at my top speed, using them to change angle is probably making more resistance than gaining speed due to changed angle.

    I got very useful answers from the owner of the boat in the video. Engine before reconstruction was diesel Mercedes 80HP/2500RPM, but it was not matched to the outdrive and prop, outdrive was just taken as it is from older original gasoline engine. That owner has been using it as a house on the water. Top speed was 5.5 knots at 2300RPM. He does not know why is that stern extension there, and as it was there he used it as it is, just made it stronger.

    Current engine is 265HP, about 600kg with outdrive and gear. He says that his new prop is 19 inches in diameter, but did not give me more data about it. He has left two original fuel tanks (he says 350 liters each) and 200 liters water tank in the middle. On the first trial day, he says the boat itself was 4000kg with 350 liters of fuel and no water. Speed tests were done with 250 liters of fuel, some water, 4 grown-ups and 5 children on board, therefore total around 4500kg. Do not know how did he measure weights though (except for the people). These are the results that he gave me:

    RPM Knots liters/hour
    1600 7,4 7,3
    1900 8,5 12
    2100 9,5 15
    2500 10,5 22
    2800 12 26
    3200 16,5 35
    3500 21 45

    I am a bit confused since I was expecting much less of total weight than he has described to me. If it would be less weight than he described, I would be very pessimistic of my results, since he has lighter engine and more HP, but I do not know what to think now. If we take fuel consumption as power reference, than it seems like he is achieving 16.5 knots at 206HP (35/45 * 265), that would be my very rough approximation for my boat, but not sure since described weights.

    One very important thing, he says that by comparing our boats, his keel is much deeper and that mine has a gradual diagonal finish/end and he does not have it that way and he is considering cutting it differently, to be smaller and has better fuel consumption he says. I was trying, but I really could not see that difference on the photos. I do not know now if his keel has original shape, but only my opinion is that I personally would never change original shape/design of hull, rather change the boat in that case. Hopefully, designers/engineers knew what they were doing (although with today short deadlines, one cannot be sure of that anymore).

    I am still optimistic about removing that tab, but not so optimistic about gaining the calculated speed (18-20 knots). Let's say if I take into consideration that he has maybe calculated/measured the weights wrong, I would expect that my maximum speed reach 15 knots if that tab removed and prop changed if needed. We shall see, one by one. At least I do have some comparable data now, in some way.
     
  4. Joakim
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    Joakim Senior Member

    The data doesn't quite add up. A 265 hp diesel should consume 55-60 l/h at full power, not only 45 l/h. Is this VW TDI 265-6, which is also used by Cummins Mercruiser? Just checked one measurement with that. 58.9 l/h at full power (4250 rpm in this case). 3500 rpm is also very low maximum rpm for this engine.

    45 l/h would suggest about 200 hp and 35 l/h maybe 160 hp. Maybe the maximum rpm is missing or the propeller is too big for the engine?
     
  5. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    You could be wright, those are the data that he gave me and I was too excited, therefore not checking full power consumption for the given power and engine specs. Truth, if we take rough approximation that diesel engine uses around 0.2 liters of fuel per HP, then 265HP would really be closer to 53 liters/h of fuel consumption or 58.9 l/h according to specs as you have said. Anyway, I do believe that the owner was easily able to measure speed and RPM, so that is probably what he has. I will try to find out more details from him.

    I believe that it is VW TDI with displacement of 3.0 liters. Truth, all those VWs are spinning at more than 4000 RPM.

    If that is truth, better for my case, but I will wait for his response about that issue.
     
  6. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    The crane has measured that weight.

    At maximum throttle engine reaches 4000RPM, no data of speed yet, fuel consumption is 53 liters/hour at that RPM. He is considering greater prop diameter (20'') for gaining higher speed at lower RPMs.
     
  7. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    One more info, just to see the angle of the boat at different speeds. At the beginning of earlier video, boat is at full throttle taking over the boat with camera. At the end of the video, boat is riding at 10 knots near the boat with the camera.
     
  8. Nidza
    Joined: Nov 2016
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    Nidza Junior Member

    Hello,

    Due to nice weather it is time to wake up this thread. I am not yet in the water, but I have been busy removing the central tab and here are a couple of photos with better view, but unfortunately photos are of low quality due to to old phone. Anyway, I would appreciate if I could have a couple of questions answered according to our earlier conversation.

    In the first group of photos is represented bad situation with central tab mounted, and better situation with removed central tab. So, according to our earlier conversation, here are the first two questions:
    1. Do you consider that this drive is mounted too high? The anti ventilation plate is almost exactly in line with hull bottom (but not keel bottom, please, consider that I do not know all exact terms and names of hull structures, but it should be clear with included photos)
    2. Do you consider that the drive is far enough behind the keel?
    Photo0586.jpg Photo0587.jpg Photo0589.jpg Photo0590.jpg Photo0593.jpg Photo0608.jpg Photo0609.jpg

    The second group of photos represents protecting anodes. The first two photos are anodes from two side trim tabs that have been spent due to galvanic corrosion compared to original size. These tabs are whole made from stainless steel and not painted. Since I am currently in fresh/river water, these are Mg anodes. I must add to this that the river channel, where I keep the boat, is very polluted and I am sure that this increases galvanic corrosion. Anyway, tabs are also very big stainless steel plates in the water which is not at all optimal situation. So, here is the third question:
    3. Should I only replace the protecting anodes or you would also advise me to paint the tabs for additional protection? (just one more time, these are trim tabs anode protectors, not of outdrive)
    Photo0604.jpg Photo0606.jpg

    In the thrid group of photos, there is shown one of two protecting anodes on the drive, compared to original one and there can be seen that they are almost completely preserved. Now, I must add that these are Zn anodes, not Mg, but if I have been reading all the books well, main issue is that Zn anodes tend to oxidize on the surface area sooner in fresh water which makes them dysfunctional, so from time to time I sand them a bit to remove oxidation from the surface. By close inspection of the drive, I haven't seen any galvanic corrosion issues on the drive itself (after a year spent in the water). So here is the fourth question:
    4. Should I replace these with Mg anodes, or I can continue using Zn anodes by removing oxidation from time to time as explained?
    Photo0700.jpg

    Thank you in advance for your answers and opinions.
     
    Last edited: May 20, 2017
  9. Nidza
    Joined: Nov 2016
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    Location: Belgrade, Serbia

    Nidza Junior Member

    Joakim, thank you very much for the advice about the central tab!!! Actually, you have nailed it (as someone has already used the expression in the thread, but the conclusion was half right at that moment).

    Removing central tab gave a huge difference!!!

    Today, I have went from displacement to planning for the first time!! It would be impossible without trim tabs, but that is why they are there (compensating heavy stern). The peak maximum speed was 34km/h (18 knots) down the river stream. I do not have speed data upstream the river since battery of phone with GPS went dead. I must add, although speed was probably slower upstream, actually I made to planning faster, but I do not think it has something to do with stream, rather that in that run, my wife went to the bow instead of staying beside or behind me, is it possible that that gave better momentum/trim position/weight distribution (although she is not heavy at all :), but according to lever system... :))? Then again, I had a better experience of how to reach planing sooner since it was a third run.

    The critical speed to overcome is from 20km/h up to around 23km/h, after that, boat almost immediately reaches 27km/h in planning mode (downstream), after that, up to 30-31km/h throttle is very responsive, reaching above this speed is a game play of tabs, trim and throttle. Boat starts to rise from about 2000RPM on engine and maximum rising maybe before 2500RPM, somwhere close above this value planing can be reached.

    Now one question. At those maximum speeds, according to engine RPM gauge, motor can again go above maximum RPM (3000RPM), that has confused me a bit. Is it really possible that he is still ventilating at those speeds when planing and that it should be different type of prop for those speeds (greater BAR as you said, more blades)? But I must say that I would not at all be tempted to go to higher pitch props since huge amount of torque is needed to reach planing and I would be afraid for outdrive gears (engine makes a bit of smoke at the highest transition process, after that it sings perfectly without smoke). What would be your suggestion on prop according to these measurements and big torque needed to reach planning, what would be wise? I must add, according to experience of people in the port managing the boat on the ground, they say that boat is probably heavier than 4000kg. Or is this the prop which should not be changed according to described situation?

    One more question, what is better for the engine and drive, to increase throttle close to maximum for the transition process to reach planing or to increase throttle gradually as I have done today?

    And one more, very important, thing is also achieved. Steering is so much better! Before, at lowest speeds, boat was wandering pretty much, it was turning almost 30-40 degrees from left to right and vice versa and it was art steering at that speed to go straight (although possible with practice). And at higher displacement speeds, there was not wandering, but I had to make corrections very often which was very tiring on longer rides. Now, the boat is going very straight and I need only small corrections from time to time (no matter at what speed).

    Everybody, thank you very much!!
     
  10. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Good to hear you had success. Seems like your prop does the job.
     

  11. Nidza
    Joined: Nov 2016
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    Location: Belgrade, Serbia

    Nidza Junior Member

    After one season of experience with this setup I have more hull, propeller and speed related questions and I would appreciate your knowledge and opinions according to new data that I will represent here. I have chosen to reopen same thread beacuse I will usesome of data already mentioned earlier.

    My last response here was a bit too rushed about actual speeds and engine RPMs because I was very excited that I made it to planning speed (but the river flow is not to be disregarded). Now, I have measured more different data so we can discuss it here, probably not enough yet, but I am a bit impatient so I will give what I have until now.

    1. Actual maximum speed of boat without river flow is varying between 27km/h and 28.5km/h, but this is accomplished at 3500RPM. I was confused and at first I did not believe to electronic RPM gauge, although it is very accurate since it is using pulses, not analogue voltage, so I have checked that with mechanical gauge and it really is set like that. It seems that PT fuel pump is probably set to 3300RPM so I presume that 3500RPM is governor limit (fuel pump has MVS governor). I will try to contact a friend from service to ask him for the actual setup. Anyway, I did this speed only a couple of times for the test purposes only, but I intend to keep the engine operating below 3000RPM and even 2800RPM since that is where the engine is designed to operate reliably. Again, I do dominantly drive it at displacement speed due to range and costs and reliability of equipment. This test is done with half full fuel tank (full tank in 220 liters), half full water tank (full tank is 260 litera) and two people on board.

    2. Boat can reach planing without using trim tabs, but the bow will be a bit in the air, but after putting the bow down, speed gain will be about 1-2 km/h, not much more. Outdrive is at -5 degree angle always, changing the trim angle a bit up was rather returning the bow up than gaining speed. Only once I felt gaining a bit of speed with trimming up, but no speedometer at that time and I presume that the weight distribution was a bit different since I had four people on the board and probably the trim tabs had different angle.

    3. Maximum planing speed has less than 1km/h variation due to big weight changes. I made to planning with 8 tall, grown up people on board (of course, they were evenly distributed) and reached 26km/h without river flow. Fuel and water tanks were also half full.

    4. I have measured fuel consumption and speed at some specific RPMs , but I do intend to measure at more different RPMs. It is time consuming process, therefore not easy to reach all values very fast, well at least if you want it to be accurate enough. Since I have removed the central trim tab, I do have better fuel consumption. I do sail often at 1500RPM at displacement speed and before removing central tab my consumption was around 6.35 liters/hour at speed around 11km/h without river flow and the boat was harder to steer straight due to prop ventilation. And in the thread was earlier mentioned that it is a high consumption. So now, at same same RPM without central tab, my consumption is around 4.5 liters/hour and the boat is sailing very straight on his own (when no wind gusts of course). Anyway, beside showing other aspects of removing central tab, measuring fuel consumption gave me, maybe the most interesting data of all (well at least to me). I have gained data of 3 real examples, three different setups of this hull. The first one is original factory setup with twin older Volvo penta 150HP gasoline engines (data taken from manual/specifications book), the second one is from boat shown on the youtube, already mentioned in the thread, with single 265HP VW diesel engine (data given by the owner earlier in the thread) and the third one is my own setup with single heavy 195HP diesel engine (described in the thread earlier). So from given consumption, speed and RPM I have reconstructed hull speed vs engine power and put all three on same diagram. The diagram is in the attachment. Take into consideration that I did have only three points measured for my setup until now, but it seems that it has very similar behavior to other two boats, but for some reason which I would like to know it is "missing the action" at higher speeds/RPMs. I presume that at slower speeds, difference between two gasoline engines and one diesel engine is because of double the slip of two propellers and because slip is higher at displacement speeds (all adding up), but also since old gasoline engines were not as efficient as diesel engines. Anyway, at displacement speeds, both diesel setups act almost identical, but at higher speeds my setup is losing performance. All this would be very interesting to discuss. I must say that there is one approximation in diagram showing my setup and I will explain, I "thought" that would be the most accurate way for approximation (I am electronic engineer though). Measured fuel consumption at 3500RPM/28.5km/h was 41.68 liters/hour. According to diagram shown in this link:

    https://gce.cummins.com/gce_hist/curves/3800_389.9/C3802A.PDF

    consumption at maximum power (185HP) at 3000RPM should be 45.42 liters/hour (12 GPH) (from diagram 3 in link), so I took my own consumption at 3500RPM and found on diagram in the link that that fuel consumption should be consumed at 2925RPM (on diagram 5 in link) which corresponds to 175HP (on diagram 3 in link). So my approximation is that I am using 175HP at 3500RPM and in my diagram, my setup is related to that, tell me if this is OK or should I "fix" it.

    5. All those equipment on the engine is taking some power of the engine.How much power is being lost due to gears and bearings through the outdrive/sterndrive? Reduction 1.6 from engine to propeller is actually in the sterndrive combined in upper and lower gears and gearbox itself is 1:1.

    6. If we discuss changing of propeller take into consideration that maximum torque that outdrive can handle is 350 lb-ft and engine torque is 341 lb-ft at 3000RPM/195HP according to formula:

    T[lb-ft]=(5252*HP)/RPM,

    but the engine on dynamometer is capable of 375 lb-ft at 1900RPM. Anyway 341 is close to 350 and because of that and also specific and heavy hull, the manufacturer of outdrive was probably a bit reserved with selecting higher pitch of prop (that was really long time ago). Although, in the manual book of outdrive if you use their formulas, you will get this prop for my setup or very close to it. The oudrive manual can be found here:

    http://www.sternpowr.com/pdf/100serieshires.pdf

    7. I will give the table of speeds of my boat here and RPMs so ifanyone can tell me from Burril's chart what is happening with cavitation now, I have tried, but could not trace every possible data yet how to read that Burril's chart. Here are the data:
    %Hullspeed for my Bora 3 (single 200HP diesel engine)
    %RPM km/h liters/hour
    %1100 9
    %1200 9.5
    %1300 10
    %1400 11
    %1500 11 4.5+
    %1700 12
    %1800 12.5
    %1900 13
    %2000 13.5
    %2100 14 9.34
    %2200 14.5
    %2300 15
    %2400 15.5
    %2500 16
    %2700 17.5
    %3000 19.5
    %3500 28.5 41.68
    OK, here is also the diagram for nicer representation in the attachment.

    I have probably forgotten to give some of the info, but I think there is enough for start of discussion.

    Some of questions would be - Cavitating propeller, still needs grater bar? Pitch too small and engine underloaded? Boat too heavy? Still ventilating? Are trim tabs enough left and right from the center of outdrive to prevent additional ventilation (pictures can be seen earlier in the thread after the central tab is being removed)? All other questions and opinions desirable.

    Regards,
    Nidza
     

    Attached Files:

    Last edited: Oct 17, 2017 at 4:45 AM
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