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Cabo 43 FB Performance with 28x40 props

Discussion in 'Cabo Yacht' started by CSkipR, Jan 11, 2009.

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  1. Sherwy

    Sherwy New Member

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    Gents,

    firstly, thank you for your swift reply's, all very greatly appreciated.

    Now to address a few questions raised. No, I don't mean leaning to one side or another. I have found this is inherent to most fast boats that carry a deep v all the way aft. This customer's last boat was a 38 Express with CAT C12's and it had the same problem. You just have to play with the tabs a lot. I have several customers with Palm Beach 32's, 50's and a 55. These hulls are more like a New England lobster boat style, like the Hinckleys etc, and have an extremely deep V forward but it flattens out dramatically as you run aft. These boats have no tendency to lean at all.

    I am talking about a 2002 Cabo 43 FB. And those are the consumption figures we measured pretty accurately on a recent 8 hour run. Engine and gearbox specs in the OP.

    I fail to see how a 1/2 inch increase in diameter can cause ventilation. The gap between the hull and the blades has only decreased by 1/4 of an inch, that gap now is about 3 and 1/2 inches or more. I gave Veem the hull gap information in the ordering process and they didn't seem concerned by this at all.

    I believe the problem I have is ventilation not cavitation. My understanding is cavitation is generally (but not always) caused by poor blade design, creating tiny bubbles of air which can erode the props surface in extreme cases.

    Ventilation is caused when air is "introduced" to the propeller from an external source causing a much more aggressive loss of "bight" or "grip", and the revs rise sharply and the vessel turns towards the side that is slipping. Please correct me if I'm wrong on these definitions.

    My problem happens mostly on the port side, and is most prevalent when you try to accelerate beyond 2100 RPM not matter how gently you accelerate. Once or twice I have managed to get it to "stick" up to 2250 for several seconds, the speed rose to an encouraging 32.9 in that short space of time. Several times it has ventilated whilst accelerating around 1500 - 1600 RPM just as the boat is "climbing out of the hole". I put this down to being too aggressive with the throttle. It seems to make no difference where the tabs are, full up, partially down or full down.

    So generally I feel that Veem have nailed the sizing of the props spot on, we seem to be right in the ball park of all the sea trail data I have seen for this engine/gearbox/hull combo. The lack of top end is disappointing but not the end of the world. It is wonderful just to have a boat that is finally performing as it should at cruising speeds. But I would like to cure this ventilation if possible and was wondering if anyone has come across this problem before and if so how they went about fixing it.

    It is important to note that we have installed a new Furuno 1 kW Chirp sounder with a fairing block on the port side. It is slightly inboard of the prop shaft, I'd say at least a foot. It's about a foot in font of the main engine so at least 15 - 20 feet forward of the props. All of the other sounders on board are not "through hull", they are the internal "oil bath" style that attach to the inner skin of the hull so their signals must penetrate the hull laminate. I hope to hell that this is not the problem but am starting to suspect it as the port side is definitely worse that starboard.

    I look forward to hearing your thoughts. Thanks again.

    Sherwy.
  2. PacBlue

    PacBlue Senior Member

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    I think you have identified the issue with the port side 1kW transducer fairing block. At this stage you can either move it to centerline within the transducer's specs or use one of the new style that transmit through the hull.

    Cabo used to have mold inserts for factory installed transducer blocks that would then be flush mounted and would not through turbulence into the propeller as you are experiencing.
  3. Sherwy

    Sherwy New Member

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    I was told that the new CHIRP sounders don't perform well unless they are mounted through the hull. (externally) The internal "oil bath" style was not an option.

    I spoke today with a friend who is a naval architect. He feels that the sounder is most likely the issue. The vertically mounted fairing blocks end up being asymmetric in shape because of the vee in the hull. This causes a directional swirl which disturbs the flow in front of the prop. He has seen this style of transducer cause issues even when they were mounted a "long way" forward of the props.

    I guess I'll have to move the bugger!
  4. Capt J

    Capt J Senior Member

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    Talk to the engineers at Cabo as they will tell you the perfect location to mount the transducer and where they recommend. Keep in mind the fairing block for the 1 kw is quite large. I don't remember ever seeing a transducer mounted that far foward on a Cabo from the sounds of it. But I cannot remember. Center bilge under the generator seems to ring a bell for locations of transducers on some, in the center bilge at the front of the engine room on others. In the Lazzarette was where the oil bath ones were always mounted.
  5. Sherwy

    Sherwy New Member

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    All the transducers on this boat are mounted just off centre at the front of the engine room so we have run out of room there.... I might have to go under the generator at the back. And you are right, the fairing block for a 1kW CHIRP is bloody enormous! I guess I'll need another one.

    Sherwy.
  6. Capt J

    Capt J Senior Member

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    Call Cabo and see what they have to offer for you. They are very knowledgable about their boats and where to place things and many times even have drawings and measurements.
  7. Sherwy

    Sherwy New Member

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    That's where the plot thickens, I have zero faith / confidence in the local Cabo Rep. Does anyone have any good contacts at Cabo?

    Sherwy.
  8. Capt J

    Capt J Senior Member

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    Just call or email the factory directly with the phone number on their website and ask to speak to one of the engineers there.......tell them what you're trying to do and they'll get you to the right person.
  9. CSkipR

    CSkipR Member

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    Cabo 43

    Good luck getting any assistance from Cabo. I've made many calls to them regarding questions and most of the time they have no knowledge. The majority of the employees who worked for Cabo in the Calif days are long gone.
  10. Capt Ralph

    Capt Ralph Senior Member

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    How many sonar transducers do you have? Near the same location?
    Even different freq 'ducers will interfere with each other.
  11. Sherwy

    Sherwy New Member

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    Hello Gents,

    I won't bore you with the sordid details of the saga that has played out since my last post in April but here it is in a nutshell. Moved transducer - still ventilating - port gearbox failed - remove, rebuild an re-install gearbox - yay happy days! 36.2 knot top speed @ 2350 RPM and 25 knots cruise @ 1750 RPM @ 150 LPM.

    So it was the gear box slipping all along, not ventilation from turbulance from the transducer.

    We have started developing some cavitation burns near the roots of the blades on both sides of each blade. The prop speed had almost completely eroded away after only 50 hours. Now at 100 hours the blades themselves have started to errode.

    I have given the props to Colin Lough at performance propellers here in Brisbane, he is confident he can tweak them to fix the issue, he has had good success in recent months on similar vessels. I'll keep you posted.

    FYI if you are interested in hearing my rant / saga over the last 7 months let me know and I'll post the full story. Get a couple of beers and big bucket of popcorn, you will laugh, you will cry, it is an epic tale of triumph despite some of the worst customer service and advice I have experienced in 20 years in this industry.
  12. Capt J

    Capt J Senior Member

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    I would talk to Veem on the prop burn first as they should warranty it and come up with a fix for the prop burn. Usually the Veem equipped Cabo's didn't get prop burn, just the Michigan wheeled ones.
  13. Bill106

    Bill106 Senior Member

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    A lot of the hub burn I've seen has been attributed to mismatch between hub and strut diameter or incorrect location of shaft zincs. If the wheel hub is larger than the strut it will likely cause this issue but Veem usually asks this question specifically to avoid it. If the wheel is larger, a chamfer on the leading edge of the barrel may help. A shaft zinc located too far aft will also cause turbulence that can lead to burn and can also restrict water flow to the strut cutlass bearing causing premature failure, I always locate them 1-2" behind the shaft tube on the forward end of the shaft and never at the aft end (like I see all too often!).
  14. ScrumpyVixen

    ScrumpyVixen Member

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    Do tell.........
  15. dubba

    dubba New Member

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    Please do tell.

    You just cannot leave an intro like that hanging...
  16. Sherwy

    Sherwy New Member

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    CAptn J,

    I've been in touch with Austal props and the Veem engineer who spec'ed them for us, sent them a whole heap of photos and had zero response. Very disappointing when you pay top dollar. I'll give the Teinbridge's a go next time!

    I'll post my rant about customer service later tonight! Sorry to keep you hanging, I've been flat out.
  17. CSkipR

    CSkipR Member

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    I know this has been addressed before but do you have shaft zincs on the shafts? Because Cabo has the brush shaft grounding system it is recommended not to put zincs on the shafts as it can cause prop burn.
  18. Sherwy

    Sherwy New Member

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    Really? Every boat I've ever worked on has had shaft brushes AND zincs on the shafts.
  19. CSkipR

    CSkipR Member

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    Well then you better call Cabo because they are not suppose to be installed on the shaft.
  20. CSkipR

    CSkipR Member

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    Here is a lengthy post about zincs on shafts. Note number 2.

    #1 Zincs on a boat should more correctly be termed "Sacrificial Anodes". Zinc is used because it has a higher voltage in the water so the current will be more inclined to flow from it than from your propeller. To complete the electrical circuit, the zincs must be connected to the items they are intended to protect. Usually this is no problem because the zinc is electrically connected to the shaft via shaft brushes (do not forget jumper cable on shaft half coupling) the and bolted right though to underwater housings. Non metal boats should usually have a copper bonding wire inside that connects all the underwater metal items together so they all share the protection from zinc anodes. Since engines use the metal frame as the negative battery connection and the engine is connected to the prop shaft, the engine and the negative side of your 12 volt system are also part of this bonding connection.
    If other currents are allowed to get into this bonding circuit they can easily overpower the small voltage available from your zincs and defeat the protection you need. This is usually the most destructive form of electrolysis and you notice it because your zincs get eaten up very quickly trying to keep up. Under normal circumstances, zincs should last at least a year if they are working normally and much longer if you don't have any problems. If they are being "sacrificed" in a shorter period you need to find where the external current is getting in.
    The most common source of this external current is the shore power connection, especially the ground lead. Pontoons are notorious for bad wiring and often the ground lead is not connected to ground, is connected to the neutral, is being used for carrying current to a mis-wired boat, and all other sorts of problems. So the ground lead should never be directly connected to the ground bonding system we talked about earlier. The purpose of the shore power ground lead is to provide a return path for current if there is a short circuit or power leakage from an appliance or the wiring on the boat. You don't want it to connect all the underwater items on your boat to the underwater items on all the other boats and the dock because now your zinc is trying to protect everyone else too.
    Unfortunately it is not always possible to keep the circuits separate due to interconnections such as shore power chargers. There are a number of ways to separate the shore power ground from the boat's underwater bonding system. The preferable and safest way is to use a galvanic isolator to introduce a 1.2 volt insulator in the circuit. This is enough to isolate most galvanic voltages but it will still conduct electrical faults and keep the boat safe in the event of a ground fault in the wiring or in an appliance. The galvanic isolator must be rated for the size of your shore power circuit.
    #2 Shaft mountded sacrificial anodes are a complete nonsense on most pleasure vessels. Why do so many vessels have them, because the people who fit them know little about the fundamentals of marine engineering. If stern gear is protected as in note #1 at best shaft anode serves no purpose. However fitting a shaft anode can have many undesirable side effects.
    (A) Any propeller must have “clean” water ahead of it if it is to be efficient. A lump of scrap bolted to the shaft certainly does a good job of spoiling chances of getting a nice flow of clean undisturbed water for the prop to get its teeth into.
    (B) If shaft anode starts to deplete then the situation becomes even more unhappy, as the depletion is never equal all the way around, now not only creating even more “dirty” water for the propeller to eat. Now compounded by the imbalance of this ugly piece of scrap. Say 3,000 rpm engine 2:1 box, shaft spinning at 1,500 rpm. Builder elected to have four blade prop for optimum smoothness, defeated by lump of unbalanced metal on our nice sweet drive line.
    (C) Finally have you ever seen the effect of a shaft anode letting go, dependant on reduction ratio and rated engine speed, they can cause structural damage to the hull when they come adrift.
    #3 I did not want to get into a blade number discussion as not actually relevant to this post however;
    The most efficient propeller has one blade as it has no other blades ahead disturbing the water flow. However from a practical perspective as much use as one legged sailor as ass kicking party.
    Without rambling on forever on the subject, three bladed propellers are generally proven to have the best compromise between balance blade area and efficiency.
    Propellers with four or even more blades are useful for two reasons. First more blades create more total blade area for a given diameter. Nasty modern and relatively inefficient hull designs with tunnels need to absorb a lot of power for a given diameter. However you cannot ignore the fact that four or more blades scramble each other’s water flow, fact.
    The second persuasive reason to use four or more blades is to reduce vibration, every time the propeller passes the strut or under the hull they cause a change in pressure (or suction). Consider this; the blades of a three bladed propeller passing under the stern 3,000 times a minute equals 50 Hz. The four blade propeller still at 1,000 rpm would change this to 4,000 times a second or 66Hz the smoother the feel and less likely to cause hull resonance.
    Starting to ramble now as I feel blade loading, blade tip clearance etc etc coming on. But will call it a day here.
    As a final aside looking at the pictures of the effected propeller blades i would suggest prop started life as a good quality Nibral casting.