propeller calculations for a gillnetter

Discussion in 'Props' started by MLC, Feb 5, 2013.

  1. MLC
    Joined: Dec 2011
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    MLC Junior Member

    I have a 32' Bristol Bay gillnetter and I am trying to improve its performance. I have several Ideas that I plan to try sometime, but I think that the propeller is the place to start. I am also a naval architecture student, although I am not very far along in my course work, I have been learning about propulsion on my own. Unfortunately my knowledge is limited so I was hoping that someone could help me reduce the guesswork.

    There is cavitation erosion in the tunnel and at higher revs there is a lot of vibration in the aft end of the vessel. I was thinking that a new prop with more blade area would improve the performance, but I can’t figure out the pitch with any reasonable accuracy.

    BUILDER: Roberts Co.
    DISPLACEMENT: Not sure but here is and educated guess, 18,000lbs(with full fuel tanks or 350gal) give or take 1,000lbs
    LENGTH: 32'(max), 30'(WL)
    BEAM: 12'4"(max), 10'8"(max WL), 10'4"(max chine), 9'8"(chine at transom)
    LCF: Approximately 12.5ft (aft of transom)
    DAFT: 33"(light), 38"(full fuel tanks)
    TOP SPEED: 15kts(50% fuel)
    CRUISING SPEED: 9kts(@2,000rpm)
    POWER: 430hp at 2600rpm(rated), In practice it was under wheeled and was turning about 2775rpm.
    GEAR RATIO: 1.72 to 1
    PROPELLER: D24"xP19", Four Blade, The E.A.R is 0.69
    TIP CLEARANCE: 2in

    I have been trying to figure this out but I keep running in to complications that I am not comfortable guessing at.
     

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  2. MLC
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    MLC Junior Member

    I don't want to discourage anyone form replying to this thread. Any information is appreciated anyone who will offer their 2-cents. The solution to this project could come from an unlikely place. If anyone has any info on this hull or a 37' Roberts hull (this one is was built from a 37' Roberts mold), that would be a huge help.

    If you need anymore information or pictures then let me know.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    MLC

    Looking at your boat the length displacement ratio is rather low. Which means it’s a bit heavy for its length. You’re in the 4.5 area. Looking at this typical resistance curve of LD ratios v speed, you can see you’re not in the ideal spot.

    L-D ratio-1.jpg

    What this means is that minor changes can have a big effect, but usually never beneficial ones…it always conspirers against you at such low LD ratios.

    Looking at what your boat should do on a Lwl of 30 feet or 9.14m, and circa 8.1 tonne, and with 430HP, assuming the engine still pumps out that amount of horses, you should double check… then you should be getting around 18knots may be 18.5knots max, not the 15knots you cite.

    With your slightly higher rpm of 2775, with all things being equal (which they never are) you should have something like a 23”x 16” prop. The 24x19 is “on the money” for the design condition, but you’re getting more rpm’s, which doesn’t help…also that tunnel isn’t helping the prop much other than reducing the draft. The tip clearance is too tight and the skeg looks too wide, both of which affects the props performance, which is probably why you’re not getting the performance you expect.

    As for vibration..it could be anything. Without any more information, hard to say at this stage. Could be engine related or prop related, shaft out of balance etc etc…hard to say.
     
  4. MLC
    Joined: Dec 2011
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    MLC Junior Member

    Thanks for you input

    There are some places where I could shed some weight without much expense, but it won't change a great amount. More than a few bristol bay gillnetters have a fixed bottom extension. that is allowed extend 18in aft of the transom. They can cause some minor inconveniences but it would be interesting to try.

    If 18kts is possible that would be a nice top speed. I forgot to mention before that the top speed is about 12-13kts when the fuel tanks are full. I believe that this has a lot to do with trim, since an equal weight in fish did not have the same effect. I would try to keep the boat light on fuel to keep the speed up.

    The engine was installed before last year's fishing season. I mounted the new machine further aft in order to better handle the weather. I suspected that would cost me a knot or so but it was a worthwhile trade.

    I could try to modify the skeg in the not so near future, but I have plenty of other projects this coming season and cutting into the skeg looks time consuming. Having said that, I am glad you pointed that out.

    About the tip clearance, I might just need to find a suitable trade between it and the diameter unless I get the time and motivation to start that nasty project.

    Do you know if a higher blade area ratio might help?
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    You’ve hit the nail on the head there. When hulls have a low LD ratio, as noted above, small changes make a huge effect on performance. Getting the correct LCG for the speed is important. You can do some simple test to ascertain the effects. Trim the boat by 1foot (300mm) by the head, at a known displacement, by reading of the drafts. (Keep this constant). Run the boat and do some times speed runs over a known distance in both direction several times. Then again at 2 foot by the head and then 1foot by the stern and 2foot and finally 3 foot by the stern..all at same dipacment. Just by moving weights around to get the desired trim.

    You’ll get a ncie plot of trim/LCG v speed…and will help you decide what is possible for future work.

    Minimal tip clearance is fine when it is a proper true tunnel, like Servogear do…but a small partial, just makes it problematic.

    Only if you have cavitation issues. It reduces the load on the blades.
     
  6. johneck
    Joined: Nov 2011
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    johneck Senior Member

    Resistance reduction is the key to going faster. The earlier posters are right on in their comments.
    But your prop is also way overloaded and the tip speed is quite high. Combined with low ( 8%) tip clearance that means heavy vibration. The DAR probably should be close to 1.0 and I would think about using a 5 bladed propeller.
    The tunnel shape does not look the best and there appears to be a full keel in front of the prop. That is leading to very low axial velocities at the top centerline and thus cavitation right up against the hull. A variable pitch prop would also go along way towards reducing the hammering at high speed and RPM.
     
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  7. JSL
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    JSL Senior Member

    You can compare your boat to this 32 x 14. Yes, more horsepower but similar displacement (about 20,000 lb at Depart Port Condition) so you would need similar propeller blade area.
    The prop is in a partial tunnel (pocket - approx 10% tip clearance) and it has the usual skeg, shoe, rudder arrangement. Yes, the D/L ratio is high but these fellows like to squeeze a quart into a pint and go fast
     

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  8. MLC
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    MLC Junior Member

    Reducing the blade loading is what I was thinking, but I could Imagine some conditions that complicate the calculations. Here are some thoughts about some of the possibilities.

    (1)I figure that a reduction in blade loading would reduce the slip at a given thrust, so less power would be required to produce that amount of thrust. This should mean that the required torque should be less.

    (2)Increased area should increase the frictional resistance, thereby increasing the required torque.

    (3)A higher vessel speed should increase the advance speed so less torque is required.

    A good place to start might be to figure out the resistance, but here are some things that prevent me from puzzling this out.

    I don't know how overloading the prop will effect the thrust produced, or the torque required.

    Figuring out the torque would be simple if I knew how much the engine puts out at rpm's above the rated rpm, but I don't.

    Another way to figure out the torque would be to use a propeller series chart, but I don,t know the wake fraction.

    Another thing I should mention is that I have some Wageningen B-series charts, but I don't know how closely they apply to Props with ogival sections.

    I looked through "The Propeller Handbook" but I am concerned that the calculations might be to generalized to be accurate for a "Weird" Bristol Bay Gillnetter. Maybe I am making this more complicated than it needs to be, but I am not an expert.
     
  9. johneck
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    johneck Senior Member

    You are correct on all three points, but 1 and 2 are pretty small compared to 3. The way to get to a higher speed of advance is to increase speed by reducing resistance.

    The best way to figure out what is going on is to use the existing propeller as a dynomometer. If you know diameter, pitch and RPM; you can calculate thrust and torque with a fair degree of accuracy. The Wag-B series is probably within 5% of a segmental prop, so for engineering purposes may be close enough. The segmental series is also available, but much of the data is for 0 skew. The wake fraction is a secondary factor, so unless you way off it will also be OK. I would estimate something like 0.85 for (1-w).
    My earlier comments were aimed towards reducing vibration, not increasing speed. it will be very difficult to get more speed simply by changing the prop. The efficiency gain is not sufficient to overcome the increased resistance.
    If you can reduce displacement or find a better LCG you might be able to reduce the resistance and get a noticible speed gain. Perhaps you can see sensitivity to LCG by moving fuel and water around and see effect of displacement by running with a light fuel and water load.
     
  10. MLC
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    MLC Junior Member

    JSL,
    That is an impressive performance for a Bristol Bay boat. My dad had a 32'x13' Plier boat with 600hp and a 28" diameter prop, and it could make about 17kts when it was light on fuel.

    Is it about 13' wide at the chine. It definitely less bottom loading than my boat. My water plane area is about 280 square feet and the bottom area is a little less.

    johneck,
    I guess that cavitation doesn't affect the thrust and torque as much as I thought. This makes it less complicated, but on the other hand it doesn't leave much room for improvement. The only thing going for that dream is that it is only about 125rpm between 13.5 and 15kts so it might be nearing the top of the hump.

    There is likely a few hundred pounds that go, but I will likely get it all back when I add RSW. The RSW unit will need to go in the stern, but I will move the water forwards about 11ft.

    I just need to be careful not to move the LCG to much. The improvement in seakeeping since shifting the LCG aft is tremendous

    Speaking of the trim, one of my crazy ideas was to install an intercepter trim tab behind the tunnel. The purpose for this placement is to hopefully create lift at little or no speed in order to counteract the suction created by the tunnel when in shallow water. I came up with the idea last fall when talking to an ex-bristol bay fisherman, who was looking at buying a new boat. We were talking about what makes a boat work well in shallow water. It turns out that it is not just the draft. My boat could scrape over the bottom best at an idle. Increasing the power only sucked the keel against the bottom harder. The intercepter should counteract this effect, but how much is the big question.

    The intercepter tab should also be useful for reducing resistance. Hopefully its placement behind the prop will not cause any unwelcome surprises.

    Here are some pictures showing the difference in trim before and after the repower
     

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  11. MLC
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    MLC Junior Member

    I don't know if I should open up a different thread since the prop is no longer the main topic, but all of you have been helpful and informative so I would like to ask a few more questions.

    What is the difference between a true tunnel and a partial tunnel? I looked at the Servogear website and I could see some major differences in the setup as a whole, but as far as the tunnel itself there were only two meaningful differences that I could see. The first is the lack of a keel. The second is the reduction in tunnel area aft of the prop, which they seemed to hint that this is so the tunnel geometry matches the narrowing of the flow of water due to the increase in velocity induced by the prop.

    Any major modification to the tunnel is likely impractical due to my narrow time window and outdoor workspace, but I am very interested in the design of tunnels.

    Out of curiosity, How much does overloading the prop effect the thrust and torque? JSL suggested that in my case the effect will be relatively small. Are there any graphs or equations that could be used to estimate this.

    It sounds like weight and trim are the big factors here. The weight will not likely change much for the better, but I could extend the spray rails all the way aft and fill out some highly curved shape back aft. A "swim step like" bottom extension could also be added. These should reduce bottom loading, and reduce the squat.

    As for the LCG, I could play around with this more than I thought. I could likely induce 1 to 2 inches of trim by shifting around some stores.

    There are some other places that might reduce the resistance that would be simple to do.
    1) My bow thruster is not faired in to the hull in any way. From the pictures, It looks like the thruster tube is still submerged even at the top speed. I don't know how much difference it would make. By the way it is an 8 inch thruster.

    2) There are two 12 foot coolers, (one on each side). I have no plans to get rid of them, but maybe one of you knows a good way to reduce their drag.

    3) I wonder if rounding the leading edge of the vee-struts would gain anything. I don't expect anything noticeable but it should take about 10 minutes to do.

    4) Would a foil rudder be less resistance than a plate rudder. If not then hopefully it will improve the steering (which is not bad, but more is better).

    Thanks for your replies everyone.
    MLC
     
  12. johneck
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    johneck Senior Member

    A wake adapted propeller can easily be designed to account for the mean inflow that is present at any radius. As long as the wake is relatively constant around the rotation, this works well and will result in an efficient smooth running propeller. Problems occur when there is circumferential variation in the inflow. In that case the propeller can only be designed for the mean condition and the variations will cause unsteady forces and cavitation.
    The servogear website does seem to indicate that the propellers are wake adapted to make best use of the inflow, but in a partial tunnel there will still be a large untseady component that the blades must go thru. I would chalk this up to marketing, though with a properly designed tunnel, appendages and propeller, they can probably provide a very good system.
    You will not get there by picking a constant pitch propeller out of a catalog, but the gains once again will mostly be in cavitation and vibration, not vessel speed. The other challenge you face is the skeg at the CL of the tunnel. This makes the wake deficit far worse at the TDC position and will make it very difficult to get a smooth running system.

    As to the losses due to cavitation, many of the Wageningen props were tested for cavitation and the data is available. Keep in mind that propellers can be fairly heavily cavitating and not experience any measurabe thrust breakdown. This indicates that the loss in efficiency is very small, probably only a few percent. When (if) you begin to get thrust breakdown, then the losses get large quickly.
     
  13. MLC
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    MLC Junior Member

    JSL,

    It sounds as if the difference between a full-tunnel and a partial-tunnel is not clearly defined. To my understanding the difference is that a full tunnel is designed to minimize the difference in velocity, at different points of rotation, of the flow in front of the prop. I would think that for most tunnels the goal would be the same, except that the design process for a full-tunnel would involve detailed optimization of an entire system consisting of the tunnel, hull-geometry, propeller, and any relevant appendages.

    If any of that convolution that I just wrote is comprehensible, could you tell me if I am on the right page?

    Very interesting so far, thank you.
     

  14. JSL
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    JSL Senior Member

    Just a quick reply
    I looked over the Servogear site and don't see anything that is significantly new over that of the past 30 years. Regardless of installation and tunnel depth, shape, etc.(if fitted) , the prop needs the right environment to work properly (area, water flow, support, etc).
    Tunnels (or pockets) if necessary but not necessarily tunnels.

    Draft reduction is one reason - Alaska has a twice daily tide with a range up to 24' so reducing the draft a bit can be helpful.... and a tunnel may permit a reduced shaft angle.
    The tunnels I have used in boats are partial (pockets) up to 50%D(max), but I have used full tunnels on smaller special service craft of 7' to 16'.
     
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