Inboard Propeller sizing / theory ??

Discussion in 'Props' started by jav, Nov 8, 2004.

  1. jav
    Joined: Mar 2004
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    jav Junior Member

    Hi folks,

    I have a situation where my boats propellers were sized by a well known propeller manufacturer for a repower project. The predictions were a little off (overloaded the engines and had to be resized slightly). After resizing, I am now able to hit WOT lightly loaded. I've since read Dave Gerrs Propeller Handbook and understand the Crouch slip methodology and formulas pretty well (I am admittedly lost with BP delta). After running some calculations, I'm finding some factors such as slip, cavitation and blade loading are less than ideal. After talking to a few experts, all agree that blade loading is a problem and that "improvements" can be made. But, I seem to be getting different views on what should be done and what I can expect from the changes. I know its very difficult to predict the dynamic interaction of hull, sea and props but I would expect that with some real world data, minor "fine tuning" with reasonable expectations should not be completely unpredictable.

    I'm getting opinions ranging from new transmissions gearing / shafts / struts / props to, fixing the slip and cavitation with 4 blade props with no increase in speed, to living with some cavitation with larger 3 blade props to improve speed by 2 knots at WOT. All the opinions seem to be supported by "facts or empirical data", yet they differ approach and are inconclusive with rewards. Oddly, even though my blade loading is way too high, performance is not awful and I'm still achieving a top speed thats within 1-2 knots of published HP/weight curves using a 150c hull constant

    My problem now is that all the options cost a significant amount, but the return on the investment is not clear or guaranteed by any. I need help understanding what would provide the best return on investment or if that last 2-5% performance improvement I'm looking for is not cost effective to pursue.

    Many refer to "rules of thumb" based on my current numbers but they are not consistent. Can anyone enlighten me as to why some folks suggest that 1 inch of diameter is worth 2-3" of pitch while others argue thats its only worth 1" of pitch? Or how does one begin to determine if the lower blade loading and lower cavitation of a 4 blade prop is more or less beneficial than the higher loading, lower drag and higher cavitation of a 3 blade? Or why, despite the fact that my props are too small, is my boat performing so close to predicted expectations or the long term negatives of running this way?

    Any help would be appreciated?
     
  2. FAST FRED
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    FAST FRED Senior Member

    With some time on your current prop is there signs of cavitation eating away at the blade? Or do you only hear it ?

    Diameter is what is the power absorber , pitch just makes it more efficent , so I would be in the 1 dia is 3 pitch camp.

    FAST FRED



    FAST FRED
     
  3. jav
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    jav Junior Member

    Fast Fred,

    The blades look fine but there are very low hours on them (16 h). I can't hear or feel cavitation either. There are two reasons I feel cavitation is present and problematic. The first is that blade loading is over 16 PSI which clearly places the blades will into cavitation. Also, one prop shop has a Hydro Dynamics CAD package and based upon my sea trial data, they also suspect that I am well above an acceptable cavitation range.

    Your in the majority with 1" dia = 3" pitch. However, will be buying new wheels and the prop manufacturer is telling me that in my size range, 1"dia =1" pitch. I hate to drop over a grand on props only to find that they need to be adjusted and scanned yet again.
     
  4. jehardiman
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    jehardiman Senior Member

    Have you actually weighed the boat to be sure that it is built to the proper weight? Have you measured the running angle? The thrust angle? Do you know if there was ever an actual wake survey? Or are they just predicting from the hull form? Are you actually getting the power you think you are? Have you had the engines dynoed? Do you have real time rpm-torque-fuel flow data? What is your exaust gas analysis showing? What is the inlet air temperature? This is the type of stuff you will need to address if you want to wring every last knot out of the plant.

    Always remember that the last 5% of performance costs 10 times the first 90%. Unless your livelyhood is tied up in absolute speed of the boat (i.e. hydroplane racer or "small package trade"), is there really the need to get that last 5%? If it's a hobby or for bragging rights, remember you will never recoup a tangable investment for an intangable prize so blowing 10k trying to get that last knot has entertainment value in it's own right.
     
    Last edited: Nov 9, 2004
  5. jav
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    jav Junior Member

    No wake survey performed, predictions are based upon hull form data, weight and known speed vs RPM data. As far as the engines go, no dyno or fuel/power analysis has been done. I beleive that the engines are performing as rated only becuase the vessel performs so close to the hp/weight curve expectations. Could the HP be low? Sure. Could the weight be slightly higher? Sure. It would be very diffiicult and costly to proove either case.

    Mathematically, it has been relatively easy to show that the existing props are not ideal from a slip, cavitation and loading standpoint. That point has been consistent in my calclualtions and everyone elses. I would like to improve those specific parameters just for the sake of doing things "right" and not necessarily with the expectations of an increase in performance. However, I am trying to understand the best approaches towards a fix, and why correcting these generally accepted "problems" wouldn't result in an improvement in speed, efficiency or both. After all, if a boat performs acceptably with known propping problems, and the solutions to those problems make it perform the same or worst, then the solutions become suspect in my view. Thats what I'm trying to understand.
     
  6. jehardiman
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    jehardiman Senior Member

    That is my point, just attacking the propeller may not solve your problem and any solution may be expensive. From your previous posts, you also pointed to a large wake and large slip, these are consistant with overweight/incorrect planeing angle/incorrect thrust angle. Especially if, as you state, "the trim tabs are still effective and I haven't changed the LCG such that I can't trim her out". This points to the trim tab angle being excessive which could easily be 10-15% of total drag at speed.

    As I understand right now you have WOT and ~35% slip at ~3600 rpm (with twin 200hp@3800 Volvo diesels replacing 225@4000 318's which did 25 knots with the same 1.5 gearing ) and 21 knots in a Trojan F-32 ( modified vee hull with relativley low deadrise) turning 16x16 3 bladed wheels.

    Looking at powering, power required is proportional to velocity squared, so the maximum expect speed with 50 less hp is 23 knots. Current turns per knot is 1.9. Red line speed would then be 22 knots. Therefore the engines are presently overloaded. Either by the prop being too aggressive in pitch or the hull got heavier/more drag.

    Now look at the props. J is ~0.65 and cavitation number is ~ 0.78. You are well into the back bubble and back sheet cavitation, but are still getting high face side thrust compared to prop rpm and are on the correct side of the efficency hump. This means that your wheel is too small or you are pitched too low or your blade area is too low. This also means that the props are absorbing the entire engine output, so they should not be the problem.

    If you have no wake survey, then the estimated slip may be incorrect, especally with excessive trim tabs. This would also effect you cavitation number. In a larger sense though, cavitation is not a bad thing depending on your geometry. Your cavitation number is so low that you might consider a super cavitating blade (as shown by the recommendation to more aggressively cup the blade if you keep the same diameter) as you may be in the region where this will benefit you.
     
  7. jav
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    jav Junior Member

    Wow, I am thoroughly impressed and appreciative of the obvious effort it took to assimulate this and my old data! Thanks so much! Now I feel bad that I didn't update my older posts with new info.

    Just a quick update: Tach issues were resolved and they weren't reading max RPM's. Loaded boat weight was adjusted up 500# to 16500# (found a higher factory dry weight rating and re- itmized all additions). A new sea trial was conducted (in 2 directions) on a very calm day with no wind or current. The vessel was not at full load, it was 1400# shy of max (15100# estimated) The data was 2 way averaged. New results show engines exceeding WOT (1 hits 3860, other hits 3900 ). Top speed = 23 knots. Apparent slip started around 45% just above idle, peaked @ 52% at the planning threshhold (10-12 knots) and sloped back down to 31% @ max RPM. Other than this not being a fully loaded vessel, I thought the results were OK. I submitted the new data for evaluation and ran my own check numbers, which is where the cavitation and blade loading concernes started.

    Your point about the props not being the problem is well taken and logical given the numbers. Other than cavitation, blade loading and my concerns over full load changes, I wasn't overly concerned with the results. But, most seem to agree that the blades are overloaded and that improvements can be made.. so the question is; where do I go from here?

    I don't think I can remove any weight and I'm pretty sure I don't have any unusual or correctable drag issues. (I don't think the tabs are accutely applied at any speed , but thats just my perception... I could be dead worng and don't know how to check for that other than to say that I can over tab and have to raise them to run well). What is involved with a wake survey? I had resolved myself make the best decision possible and just spend the money on new props, even if just to avoid cavitation prop damage or throwing a blade from overloading. Unfortunalty, the more I learn, the harder the right decision becomes.

    Any thoughts on how to proceed?
     
  8. jehardiman
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    jehardiman Senior Member

    Ok, your new data shows that the present props are about right (setting aside the cavitation and loading issues). You want to go faster... you have 3 options. 1) increase horsepower and rpm, 2) decrease drag, 3) increase prop efficency.

    Now of the 3, increasing prop efficency is what you most likely want to look at right now. This will also address the cavitation and loading issues. The problem here is that in order to increase efficency (which is just the ratio of thrust to torque), the prop must get larger, slower, and have less BAR (i.e. a slip decrease with lesser wetted area). Most likely you will not be able to increase diameter too much and still maintain proper tip clearence and wake and cavitation effects will increase with a larger diameter. You could increase the reduction and pitch as a diesel's power curve is flatter than a gas engine's (the exhaust gas analysis mentioned earlier would help on that) but this may make it harder to get out of the hole. Unless you really increase the diameter and slow the shaft down, decreasing BAR is not really an option unless you want to try supercavatating. What this does is to decrease the wetted surface while greatly increasing the loading on the pressure face and the form drag of the section. The trick is to increase the blade loading faster than you increase the section drag. Supercavitating blades tend to be poor performers at off speeds.

    I would tend to groom the boat for a year or so while working the data before deciding on a course. I have done analysis to build some really efficient props for a given operating condition, but they would have been useless in real life because of acceleration and structural considerations. Given the varancies of real operating conditions, see how these wheels work overeall before changing them.

    As for a wake survey, normaly what is done is during the design phase a set of dynamic pressure measurements are made just ahead of the props with a pitot "rake" to determine the differences in flow velocities into the blades from the hull outward. This data is used to determine the "average" velocity into the wheel and to avoid such problems as blade oscliation, torque oscliation, and intermittent tip cavitation. This is very important with highly raked shafts.
     
    Last edited: Nov 10, 2004
  9. jav
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    jav Junior Member

    jehardiman,

    thank you. I've learned alot and I'm still learning. With what I understand now, I probably should have gone to 2:1 gears but that would have meant many more changes. Even then, with my current weight and horsepower, I'm not sure the improvements of doing so would have been significant speed wise.

    Looking at prop options, your opinion is consistent with one prop expert that suggested sticking with the current props and repitching them to 16x15 with a medium cup. He felt the cup would help (not eliminate) the cavitation and that it may create some lift aft which might help slip as well.

    Another prop expert that has many cad tools and supporting data suggests that my best option is going to a 17x13 4 blade (.69 DAR) with a medium cup. This approach would bring cavitation to an acceeptable level and slip down to 13% at WOT. This approach was the most expensive of the prop options but its do-able. My problem is that the numbers don't appear to support such a change where I operate the boat most (85-90% of max RPM). The pitch ratio (.764) and slip predictions place that blade near 60% efficiency according to Gerrs book (which is even lower than the current blades). Also, I agree the much higher blade area would help with loading and cavitation but I was concerned that the increased drag would hurt speed more than help. He thought that this combination would yeild 25 knots fully loaded and I fear thats too optomistic.

    The last option was recomended by MI Wheel which was 17x14 DJX 3 blades (.61 DAR). This also had me interested between the better efficiency of a 3 blade, the newer skewed blade design and the higher DAR ... however, I am scared its too much wheel where my 16x16 uncupped blades are absorbing full engine power now (this is the 1" of diameter = to 2-3" of pitch question). If I go down to 17x13, the pitch ratio again becomes inefficient and returns become questionable. They made no speed prediction but did say that these blades would be in partial cavitation mode much of the time and that this was OK.

    Also, most everyone tells me I should be using Nibral... any thoughts on that?

    While I could run this setup for another year, I thought with the boat out of the water, this would be a good time to "try" to make improvements. I just don't trust my knowledge enough to make that decision quite yet.

    Thanks again for all your great insight.
     
  10. jehardiman
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    jehardiman Senior Member

    Sorry this took so long, but life interviened.

    Without having all the data and several days of work, I'm about at the end of what I can say. As I pointed out, the last little bit is the hard part.

    Two last points. First, remember what cupping does, it increases the apparent pitch and stablizes the back (suction side) cavitation. Second, what skew does is to increase the resistance to back and face cavitation, not tip cavitation. See http://www.hydrocompinc.com/knowledge/publications/PropellerCup.pdf for a paper on the details and whys of cupping. As for skew your wheels are now well into the tip and back cavitation region, and as tip cavitation often starts back cavitation, excessive skew may be required (and still not solve the problem).

    As for props, I assume you mean "nibral" as a generic material (nickel-aluminium bronze) as opposed to a tradename. I would use MONEL if you can afford it (and the shafts can take it), Ni-Al-Bronze is next best. Ni-Al Bronze almost as good as MONEL but is attacked by some marine growth (amonia in concentration will attack it) and it may also require hardened shafts to prevent cutting the taper. Normal propeller bronze is next on the list. I myself would stay away from "stainless" props for a boat that stays in the water all the time as it can be iffy in corrosion due to many variables concerning activation and passivation of the surface.
     
  11. jav
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    jav Junior Member

    Many thanks for your thoughtful help.
     
  12. woodboat
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    woodboat Senior Member

    I have been a little reluctant to jump in as you are dealing with theories and numbers versus real world data. I would base my recommendation on how the boat currently performs and how you expect to use it. What you haven't given is how it "feels" I will use my last two boats as examples. 28 ft Marinette, single 360 gas, 1:1 swinging a 14x11. WOT light 4400, WOT loaded 4200. When light would plane @ 2600. When loaded it wallowed. Needed to be pushed up to 3300 Rpm to "feel" right. 98% of the time boat was lightly loaded, at least bearable, with my wife and five children. If I had a lot of guests a lot of the time I would have sacrificed top speed for a lower cruising RPM with this loaded condition. I would have gone to a larger diameter and pulled some pitch. Interestingly the twin model used the same size and pitch propeller but had about an 8 MPH advantage on top speed. This tells me I had lots O' slip. Current boat is a 50 Burns Craft with twin 350 Chevys. Don't know the gear reduction off hand which is NOT like me at all being a "gear head". Would guess is a 2:1. Props are 19X16. Boat seems to run the same whether I am alone or if loaded with 20+ people. So even carrying 3000 Lbs of people there is no perceptible reduction in performance, not that it is a ball of fire or anything. Soo....... If you are generally lightly loaded I would add a little cup and be done. If you plan to take 20 guys out fishing I would seriously consider adding diameter and sacrificing speed.
     
  13. jav
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    jav Junior Member

    Woodboat,

    you make a very good point about "feel". I'm a gear head too and I fear my "need for speed" might taint my perception. My sense is the boat doesn't feel right until it hits 22-23 knots (which is the current max). Under that, it feels like its in a hole and pushing too much water aside, even at the 20 knot cruise. The seatrial numbers show otherwise though. My planning hump is between 10-12 knots and thats where slip is highest (above 50%) . from 16 knots to 23 knots, the slip only drops from 36% to 31 %, not a huge difference in the number but it just doesn't feel "right" at 16 knots, and only slightly better at 20 knots.

    80% of the time, I run the boat with a medium load but thats still some 2500 pounds above what the manufacturer claims the wieght is and is likely why the 16" 3 blade props are so overloaded. I usually run the fuel tank between 3/4 and 1/4 becuase I don't need the range and hate lugging the extra weight around. I can feel the difference between a full tank and a 1/4 tank but I've never felt any difference with the quatity of people on board. The boat normally weighs around 15000# so a 1000# either way has little effect.

    All this is what makes the prop decision so hard. I'm convinced I have way too much weight for the size props I'm running. I'm convinced a 4 blade prop would be best to address that problem. However, I'm concerned that I will loose speed with the 4 blade and I don't know how that will make the boat "feel". If at 20 knots, it made the boat "feel" like it does at 23 knots with the 3 blade, I think I would be happy with that, even if I could no loger get to 23 knots.
     
  14. woodboat
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    woodboat Senior Member

    I would guess that your hull is very much like my marinette in that it has little or no lifting strakes. The marinette felt like it was in a hole when cruising at 2900 RPM but had a lot of lift when WOT. I couldn't really argue with mileage though, was about 5 GPH. Given that you currently exceed MAX rpm I still think I would add cup. Your max speed would go up, WOT would go down and there is a likely chance that it would help cavitation. As far as being in that hole, you would need to decrease weight or add lift. I do not know if they work but there are add on lifting systems available. I had considered http://www.thesmartrail.com/ for the marinette but bought the burns instead
     

  15. jav
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    jav Junior Member

    Woodboat,

    My mileage is good too and alot of guys at my marina would kill for a 20 knot cruise at my consumption. But as you said, the "feel" is just off.


    I do have lifting strakes, 3 per side if memory serves but they are not especially large. Removing weight would be tough. As far as exceeding WOT, your correct but keep in mind that the seatrial was not at full load. I was told that in order to keep from overloading my diesels, I should be aiming for 3900 RPM fully loaded. I can't quite get there with a medium load so I think my propping is pretty close if not a little on the over propped side.

    I was really intrigued/hopeful by the lifting properties of cupping and thought that between that and the higher blade area of a 4 blade, I might have a shot at improvement... with all the variables already discussed, that may have been wishful thinking. I guess the only real way to know dymanics of propping is to try it.
     
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