Black art?

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

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    I reckon an accurate fix on what the effective pitch of the two props in tandem is, is key information. If you have a 2.2 to one reduction ( correct ?) and a governed RPM of 3600, that gives you the prop shaft rotating at a maximum of 1636 RPM, regardless of what props are fitted. Given that there is plenty of prop slip when a heavy boat is transitioning on to plane, it doesn't give you a lot of room for that slip (that is where the thrust comes from) if the pitch is not that great. From memory of driving heavy outboard powered boats with a 1.85 reduction box, they would be climbing well past 3000 rpm with a 17" pitch prop, before planing. But you have a 2.2 box, and the prop spinning slower. If going higher in pitch hasn't worked, possibly asking too much of a small prop disc with high pitch, on a heavy vessel, then going to a lower reduction box has to be a possible solution. I can recall testing some props on a boat, to get some more speed, and the 23" pitch would falter in getting the boat on plane, but worked when a hole was drilled in the prop hub, allowing some exhaust gases to lower the effective pitch of it. Don't recommend you do that, though. But if you can get a figure for what the effective pitch of your prop set-up is, maybe talk to the prop vendor, you can calculate what slip you have on hand, with that engine RPM and drive ratio and it would need to be around 35-40% to be getting enough thrust to plane your boat. I'd see if I could find what ratio others are running with the Bravo III, with engines of similar RPM on similar boats. Maybe the duoprop diameter increase at lower pitch, which is what happens in single prop series, and lower pitch will need more shaft RPM. Bigger diameter is better for thrust.
     
    Last edited: Jan 3, 2020
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    The give-away here is that no motor will red-line getting a boat on plane, unless there is cavitation/ventilation problems, or the prop and/or gear ratio is completely out of whack with what would be required to suit the boat.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Just looking at some figures, it isn't unusual for prop "slip" to be 50% with heavy boats coming on plane, so if your duoprop has 24" pitch and only advancing 12 inches per revolution at full revs, that is going to give you 15 knots, and that is the end of it. If 15 knots isn't going to give you a clean plane, then you won't get a reduction in that slippage, down to 15-20% at cruise, planing cleanly. Plenty of boats won't plane at 3000 rpm, but can be backed off to 3000 after planing, and still plane. In the absence of cavitation or ventilation being a factor, you probably have to change the ratio or try again with a 26" pitch, but you say that has not worked. Maybe that was cavitation related.
     
  4. gonzo
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    gonzo Senior Member

    The diesel engine only weights about 150 lb more than the gas engine. That is negligible on a heavy boat.
     
  5. scotch&water
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    scotch&water Junior Member

    Good morning, by the numbers, the fella is I and there # 12 shoes, as for nose up differential have not noticed most of the time tabs are down, with old Eng. and new Eng. going to make tabs wider, on the project list, list is longer then what gets done. I have run every prop pitch available from this company, with the old Eng. she would get to 25 MPH with lighter load. Old scenario departing Ketchikan AK. early in the morning, full gas 150 gal. live on board stuff ( spent 5 weeks) 2 big coolers with fish, one cooler down below, generator, boat has 15 HP kicker on the back 150 Lbs. 2 persons on board. Boat would not com on plain till 1/2 way to Prince Rupert and gas down to 80 or so Gal then I could push the throttle up and get on plain. So yes load has a lot to due with this and partly that prompted the Eng. and Drive swap. Was hoping for more power all around, that is where I am now have more power supposed better Drive BUT I am Stuck?
     
  6. scotch&water
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    scotch&water Junior Member

    Gonzo I think the difference is closer to 200 Lbs.
     
  7. gonzo
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    gonzo Senior Member

    Still, that is a small percentage of the total weight of the boat. It should not be preventing it from getting up on plane. What power do they rate the engine to?
     
  8. scotch&water
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    scotch&water Junior Member

    Peninsular rates the Eng. at 310HP. with 400Lbs in torque, more than the old 350 at 260HP. In addition a diesel will have more torque, but at a lower RPM.
     
  9. gonzo
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    gonzo Senior Member

    That is about 68% more than the maximum offered by GM, and that was for military vehicles. They really hotrod those engines. What is the life expectancy?
    The diesel may have more torque at lower RPM, but the gearing will determine the torque and RPMs at the propeller shaft. The power output is the important value.
     
  10. scotch&water
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    scotch&water Junior Member

    Gonzo, GM discontinued Manf. of the 6.5 in 2000 all rights where sold to AMG = Hummer fame, Eng. was redesigned an cast by IHC lots of integral changes marketed as the Optima 6.5 Marinized by Peninsular so actually HP is down to what some folks are running in trucks. So Gearing -- pitch and that kettle of FISH is What all this discussion is attempting to solve.
     
  11. gonzo
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    gonzo Senior Member

    I tried to find Peninsular online, and they seem to be out of business. Were these old stock? I am wondering about the HP rating.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    Ok guys, let’s have a second go at this problem.There are two matchings to perform:

    · matching propeller rpms to engine characteristics

    · matching propeller to hull

    We have to get the first right before anything can be said about the second. Since there is a number of original Mercruiser engines matched to the Bravo III drive and all share the same set of propellers, we can use this info. The key to this matching is the Torque coefficient:

    Kq= Torque/[(water density)*(shaft revs per second)^2*(Prop dia)^5];

    For this comparison, the combined prop set can be treated as a unit, using the maximum available propeller diameter (16”) as the unit diameter. This comparison is relevant for all tandem, contrarotating drives, but is not compatible to the values for single props.

    Taking data from Mercruiser sales and propeller selection documentation, we get values for Kq between 0,045 and 0,060 for various gas engines with propshaft powers between 180 and 415 hp. The D7.3 liter diesel is an outlier with its 0,067 (ratio 1,65:1) and 0,1 (ratio 1,81:1). It was offered in both ratios, but we only saw the 1,65.1 version installed here. The “normal” values correspond fairly well to the values seen in connection with the Volvo 290 Duoprop drives, for which I have some background info.

    Now to the GM 6.5 liter engine. As installed it is supposed to produce 310 shaft hp’s at 3600 rpm (found data at the "Simplicity homepage", not altogether convincing though). Using the same mechanical efficiency (88%) as used by Mercruiser, and the 2,2:1 ratio, we get a value for Kq of 0,154. This is way too high! With ratio 1,81:1 we get Kq = 0,086, which might be tolerable, but it is loading the blades heavily in terms of required lift coefficient. The question is if the 1,65:1 ratio (Kq = 0,065) is to be used with this engine.

    To settle that, it is necessary to look carefully to the engine torque curve; a comparison between the Mercruiser D7.3 and the GM 6.5 reveals significant differences. The D7.3 has a much higher low- to medium speed torque than the GM. This difference tells us that the Peninsular has to “move lightly” in the hump speed region, where maximum torque is needed. Otherwise it will “bog down” and just produce smoke and no power. As a consequence, the Peninsular should not be combined with the 1,65:1 ratio in a boat with high hump resistance!! Remember it could not even pull the 28” pitch propeller with the 2,2:1 ratio! So the conclusion is to use the 1,81:1 ratio.

    Engine D7,3: torque@1900 rpm: 763 Nm, torque@2800 rpm: 679 Nm
    Engine GM 6.5: torque@1900 rpm: 590 Nm, torque@2800 rpm: 636 Nm

    Normally, when designing propulsion systems, there are complete data sets available for the propeller types in question, but when dealing with tandem prop units like the Bravo III and Volvo AQ Duoprop, the propeller info is “inhouse secret property”. This is why we have to use generic info to find reasonable compromize. I think the Kq method as above is as close we can come for the engine/propeller matching in this case. Unfortunately, it still leaves the hull/propeller matching open for problems to be tackled next.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    "in-house secret property" ? How helpful for solving problems ! The duoprop is the wild card, I was under the impression they reduced cavitation and ventilation problems, because the load was spread over more blades, so I don't know you would be expecting those two as a cause. I can't get away from the maximum prop shaft speed of 1636 RPM as being the killer, you have a little 16" inch prop disc being asked to plane a 5 ton boat, and at a speed less than sufficient. They can only do so much work per rev. The fact is the boat now runs a higher gear ratio, with a lower engine RPM, which is a compounding effect of reducing prop shaft rotations.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Did it fail to hit the 3600 RPM limit ? He doesn't specify, just says it was over-propped, which would imply it did not hit that rev limit. That needs to be clarified. But if it did reach 3600 rpm, that doesn't prove it was "biting" and not cavitating. That's what a rev-limited engine can do, mask cavitation and ventilation issues, and especially if it hits the limit easily when the props are biting properly.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    Peninsular 400ta.jpg
    Yes, I fully agree with your conclusions here. Also that the low shaft speed (due to the 2,2:1 ratio) is a "killer", which is obvious from the Kq analysis. When the diesel is running along the fuel limiting speed limit, you can not say anything about the power produced, other than it is lower than maximum.

    I'm diving into this, because I have seen the problem before. Back in the early 1990-ies we built a couple of diving support / fifi boats. The first had a Volvo TAMD 63 (310 hp) driving through a ZF leg with 19" dia screw. The prop was calculated with all numbers correct, but the engine could'nt get over 1600 rpms, smoking like a steamer and the ducks were swimming faster. When tilted to draw some air, it could be lured to the point where the turbo kicked in. From there we had good acceleratin up to top speed a tad below 30 knots. That engine was swapped for a Cummins 6liter BTA-something with a beautiful, fat power curve.

    The second similar boat was equipped with the Mercruiser D 7.3 with a Bravo III ratio 1,65:1, which operated perfectly from day one.

    That experience promped a research into the tandem prop territory; the numbers in my previous post come from that excercise, as well as a decision never to use a Volvo in any of my boats again.

    Edit: The performance curves found at the "Simplicity-Marine" homepage for the Peninsular engines are not to be thrusted; the power and torque numbers don't add up. Peninsular 400ta 310 hp sheet is attached.
     
    Last edited: Jan 5, 2020
    philSweet likes this.
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