Black art?

Discussion in 'Sterndrives' started by scotch&water, Dec 30, 2019.

  1. gonzo
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    gonzo Senior Member

    A quick calculation using HP = (Torque x RPM ) / 5252
    At idle = (320 x 650) / 5252 = 39.6 not 110HP as the chart shows
    Further, the torque curve shows an almost linear increase to 460 ft. lb at 3500 RPM. However, the specifications show a maximum of 469 ft. lb. at 2800 RPM. The curve in the graph should increase, plateau and then drop; like a normal torque curve. I don't think it can be trusted.
    I don't know why you wouldn't use a Volvo Penta. They have all the technical information to match gears and propellers. You can't expect to get good results by using old propeller theory though.
     
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  2. baeckmo
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    baeckmo Hydrodynamics

    Yup Gonzo, I saw you had tried to find performance info as well. If these engines are sold with this kind of documentation, it is fraud, plain and simple.
    I would like to know what kind of sales info S&W got from the seller regarding the performance.

    Re the Volvo issue: the TAMD 63 was just the final drip. After a long period installing about 30 Volvo engines without problems, there were all sorts of kinks popping up on brand new engines. If you look at the power curve of the 310 hp TAMD 63 from that era, it shows a lack of power at low rpms. On the "last" one we had the injection pump calibrated at our own cost; the individual settings were scattered all over the place, rendering the power curve even worse. Based on that, the engine was bought back by Volvo, but we never got any compensation, not even an excuse. And Gonzo, the propeller calc was done by qualified people, using best info available, mind you!

    Sorry for hi-jacking the thread S&W, enough with complaining for now!
     
  3. scotch&water
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    scotch&water Junior Member

    Good morning gents. It looks like answers are coming thank you for your efforts. I will start don the line to answer your posts, gonzo yes Peninsular has closed its doors sold most of its stuff to an outfit in the plains of Texas. Baeckmo great explanation that even on old Fiber Glasser and boat patchr can understand. The fact that you are pointing to the 181:1 ratio is good, on the Bravo III the 181:1 2.0:1 220:1 are linked together= I only need to change upper gears, with 165:1 one needs to change upper and lower gear sets, so more $$$. Mr Efficincy when I indecat hitting red line it is 3600 RPM and I pull back I have, with the low pitch props bumped to 4000 short bump and it did not help. Over proped was the 28 and the 26 pich props, as baeckmo indecated smoking like a steemer( over fueling) we did pull the rubber 90 going to the turbo off ( more air ) and could just get to 3600, not over the hump, as indecated going to 4 inch intake and elimenating the muffler, to get a fue more horses. as to the Cumins Eng. great power plant, it is way too high to fit in to my Eng. bay so I am stuck. might look at turning the turbo up a bit, the is a delecet issue. Thanks all for now. Fritz
     
  4. scotch&water
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    scotch&water Junior Member

    As to information recieved fro Peninsular the sheet in the post is what I got. the reason I picked this Eng. !. $ over half of a Volvo others a not sufficient information and like to stay from electronic fuel conteols, last have 2 Trucks and a Suburban with that Eng. have worked on them for 30 years. But like the Cmmins 12 valve have 2.
     
  5. gonzo
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    gonzo Senior Member

    Do you still have the Bravo II?
     
  6. scotch&water
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    scotch&water Junior Member

    Yes and I think the ratio was 165:1.
     
  7. gonzo
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    gonzo Senior Member

    Why don't you try the engine with the old drive and see what it does? It only takes 20 minutes to swap.
     
  8. scotch&water
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    scotch&water Junior Member

    Gonzo I have considered that, the negative that set up had to be at 4000 RPM or greater to plain, so since I am limited to 3600 RPM it translates to purchasing a new prop, for maybe a one time application? I have been digging in to the power from the newer 6.5 from Peninsular and it seams the had a good reputation for dyno testing to 320 HP. Still digging.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't get why you would be thinking in terms of screwing the turbo up, etc, those tactics are only relevant to getting higher engine RPM at a given load, you are already at the red line so it is meaningless. Whether getting to the red line is occurring without excessive cavitation or ventilation, needs to be established. I would get a very accurate boat speed reading for the boat at the 3600 rpm, with the duoprop you have fitted, then go looking for boat performance tests with the same drive and prop, and see what the rpm vs speed figures are, especially at the "hump". Regardless of the engine used, that is irrelevant, it is the propeller shaft speed that matters, and if the drive ratio is the same as yours, that will be a direct comparison. If the test boat is comparable to yours, but the boat speed figures are completely out of whack, that might suggest the prop bite is compromised. But if they are similar, it is telling you, that you don't have enough prop shaft RPM. And you don't help that at all by fiddling with the engine. it is already turning as fast as permissible.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Exactly, for now there is more power available, than the props can handle. As to the old drive: The Bravo II was never offered to the 350 engines with 1.65:1 ratio. On the other hand, these engines were offered together with the Bravo I drive with 1.65:1 ratio.

    To my knowledge the B II, 1.65:1 ratio was only provided together with the D 7.3 liter diesel. That ratio would not allow the gas engines to reach normal rpms (4600-5000), except if heavily cavitating. So there are three alternatives:
    • Either your old drive was a Bravo II, but with another ratio; if so, what ratio and what propeller was used?
    • Or, it was in fact a non-standard rig with severe cavitation issues (mounted too high?), explaining the 4000+ revs you mentioned.
    • Or you have mixed it up, in fact having had a 350 engine with a 1.65:1 Bravo I.
    Note that the installation instructions for Bravo drives say that in a heavy application, these drives should be installed with the antiventilating plate 25 mm BELOW bottom line! This is the first thing to be checked (after you made clear what drive and propeller was used before) when the fat lady sits on the dry.

    Edit: ...and there may be no obstructions, like keel, deadwood, water intakes, sounders etc 1.5 m forward of transom and 0.3 m each side from shaft cl.
     
  11. baeckmo
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    baeckmo Hydrodynamics

    It may be easier to comprehend the interaction between prop and engine when seen in a graph as attached below. The propeller data for the D 7.3 l case are based on "real life" measurements, which gave real "as built" values for the torque coefficient. For the graph I have extracted data for two speeds, corresponding to two loading conditions. The 12 knot case is a full throttle towing condition.

    These Kq values were then recalculated to the shaft rpms in the S&W case, showing the low loading when using the 2.2:1 ratio, and the hump condition with the 1.81:1 ratio. Considering the degree of uncertainty here, I should have used a much wider pencil ("Peninsular style"), but you should be able to understand the differences.

    Now if you look at the z=2.2:1 curve for the Peninsular, at 4000 rpm ("burst speeds" according to S&W) it would need some 260 to 280 kW, which just are not available. This tells me that the propeller is cavitating at this speed.
     

    Attached Files:

  12. gonzo
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    gonzo Senior Member

  13. scotch&water
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    scotch&water Junior Member

    Mr. Efficiency, the comment regarding the turbo was a search for more power, boat speed with last fitted prop @ 3600 RPM 15 MPH and just plowing. as to related boats most are gas with 7.4 power plants so totally different, so the ratios will be different, a build 7.4 can twist to 5000 RPM. The comparing to the IHC 7.3 diesel is closest to my 6.5 but none that I have seen, please not the hull on my boat is a 1993 so information is sketchy. Placement of the Bravo II and Bravo III are in OEM location with no obstructions on the hull for 6Ft. drive ratio on the Bravo II leg, I could be of, will check that out.
    Baeckmo the compering 6.5Eng. to the 7.3 is good, the IHC has more power, the ventilation cavitation problem, is that not style of prop related? There is no more testing till spring the Ice is getting thick.
    Thank you all Gonzo you are right on the installation's of the drives.
     
  14. scotch&water
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    scotch&water Junior Member

    Thank you all for the great help in solving my black art question, you have enlightened me to a point I see shade of gray, so I will change gears to 181:1 keep my fingers crossed and start searching for the proper prop set, so thank you once more. Fritz from cold Montana where water turns hard.
     

  15. gonzo
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    gonzo Senior Member

    Glad to get you from darkness to barely twilight. :rolleyes:
     
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