Prop selection question for 6m foil assisted power cat

Discussion in 'Props' started by cmaas, Dec 15, 2023.

  1. cmaas
    Joined: Sep 2007
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    cmaas Junior Member

    I have built a 6m electric foil assisted, stepped hull cat. Early tests were very successful. You can see in the photo that very little of the hull is in the water. 20 knots was achieved with a 18Kw motor.

    However, the finished boat is now heavy enough that performance has suffered drastically. Barely reaching 13 knots.

    What seems to happen now is that the prop ventilates, I think, before the boat can generate enough speed and lift to get up on the foils with the hulls planing freely.

    The outboard is set quite low. Especially at low speeds before the foils lift the stern. The ventilation is happening well before the stern has lifted to where you see it in the photo.

    It seems like the prop is slipping to the point were it's simply chewing a hole in the water. At 13knots prop rpm is about 1850.

    Adjusting foil AOA has not helped the basic problem.

    I'm using a 9 7/8 x 13 prop now and thinking of trying less pitch. Maybe add some cup?

    Before I do anything drastic, Like fitting a bigger motor, or switching to an ICE outboard (which would save 190kg in battery weight) I'm hoping someone here can offer some thoughts on prop choice.

    Thanks for any thoughts. IMG_3360.jpg
    IMG_7606 (1).jpg
     
    SolGato likes this.
  2. SolGato
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    SolGato Senior Member

    Are you using a converted outboard?

    What size leg and what style prop blade-wise?
     
  3. cmaas
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    cmaas Junior Member

    Yes, a Yamaha 25. 20" leg. Three blade prop.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    Please provide picture of what you have in front of the OB leg and down to the AV plate. I've built a number of single engine cats, and there are a few do's and don'ts to keep in mind to prevent ventilation.

    Btw, what is the all-up weight?
     
  5. cmaas
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    cmaas Junior Member

    This is all I have for a fairing. It is clear of solid water when the stern is lifted by the foils. Though it is in a lot of spray.

    The weight is approximately 500kg.

    View attachment 192276


    IMG_3383.JPG

    This is a previous fairing from when I was using a single foil attached to the outboard. The foils are on either side now.
    IMG_2665.JPG
     
    Last edited: Dec 15, 2023
  6. kapnD
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    kapnD Senior Member

    A longtail motor would work well here, the angle/depth of the propeller being easily adjusted as the hull climbs up onto its foils.
     
  7. cmaas
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    cmaas Junior Member

    That's an interesting idea. It could work well with direct drive from an electric motor. Not super practical in tight docking spaces like I often deal with though.
     
  8. kapnD
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    kapnD Senior Member

    Two things I found curious in the last photo above.
    The foil is in front of and in line with the propeller, and that the foil turns with the motor.
    The outboard leg is a very convenient projection to mount foils on.
    I’d try wings off of the motors cavitation plate, they could be angled down.
    I’m assuming there’s another foil further forward?
     
  9. SolGato
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    SolGato Senior Member

    Is your current max rpm the same as it was before when you were able to reach higher speeds when the boat was lighter?

    It’s unfortunate modern outboards legs all have reduction gearing built in.

    With the torque capabilities of an electric motor, there really isn’t a need for torque multiplication gearing like there with an ICE.

    Imagine the speed/efficiency you could achieve with 1:1 AND foil assist to reduce wetted surface area.

    Anyway, regarding your theory about the prop slipping/ventilating, can you see the prop doing this?

    I found performance slightly improved with the props running closer to the surface on my boat versus having the motors deep.

    To me it sounds more like you are not reaching a high enough RPM and speed to get the added weight you have now up on plane with the foils.

    Since you should have plenty of torque and are running a gear reduction, I would experiment with a more aggressive prop to try to get your speed up, stern lifted and on the foils.

    How close are you to pulling max Amperage under load when trying to get on plane? Do you have some headroom to work with?
     
    fallguy likes this.
  10. baeckmo
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    baeckmo Hydrodynamics

    The pod you show has a bad shape from two perspectives.

    The main problem is that the inflow velocity into the propeller has to be accelerated; in your case at 13 knots the advance velocity is about 6.35 m/s, but the velocity in the midposition in the disc area should be about 7.74 m/s. The dynamic pressure difference necessary for this speed increase is close to 1 m water column (or ~0.1e5 Pa). When there is a free surface in front of the prop, this pressure difference can't build up, and there will be a free vortex starting from the propeller and ending on the surface, and there you have your ventilation channel created.

    What makes the hump situation extra sensitive is the combination of increased thrust requirement and propblades working with increased incidence. This results in a pre-rotation of the incoming flow, making for an easy start of the vortex.

    The remedy is to create an "antiventilation plate" forward of the drive leg, where there normally is a hull surface doing the job. In addition, it has to shape the flow in front of the crude upper parts of the OB leg. There must be no upswept surfaces along the buttock lines of the pod; that may work in air, but not where fluid spray is involved. When your boat is on the dry, take your garden hose and direct the jet slightly upwards, say 3-4 degrees towards your present pod and see where the water spray goes!

    I went down to my workshop scrap bin and dug out the remnance of a pod we used for some propulsion experiments on a 28' workboat cat. In that case we run a Suzuki 150 hp outboard with varying load. The pod has a "main body" which is shaped to "shadow" the mounting bracket of the OB. The aspect ratio of that part is about 4:1 and its transverse dimension just covers the bracket. NOTE its bottom is flat! Any spray or fluid hitting it is deflected down/sideways, not upwards!

    The lower "pod body" is shaped with the required additional AV-plate, in this case 150 mm wide and ~600 mm long. It is carried by a "plug" that is wide and high enough to "shadow" the OB leg above its AV-plate. For this Suz engine, it had to be 110 mm wide and roughly 120 mm high in order to get the additional AV surface 10 mm above the engine dito. The aspect ratio of the lower body should not be less than 5:1 and there is nothing to win above ~6:1. Sides nearly vertical, plan shape a pointed parabola.

    I can not see where your foils are located, but you also have to consider the downwash effects on the propeller inflow, but I suggest you revise the pod arrangement first, then we can see what needs to be done to the propeller.
     

    Attached Files:

  11. cmaas
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    cmaas Junior Member

    SolGato - Previous max RPM was 4500 @ 270 amps. Now max RPM is about 3800 @ 190 amps.

    I have been alone in the boat the first two outings so haven't had a look. It sounds similar to when an outboard is tilted up too far. Then prop wake looks more foamy. The outboard also takes a little jog to port right when it happens.

    It would be interesting to try a direct drive from an electric motor. Maybe with a surface prop. Probably not on this boat though.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    The rpms you refer to above must be motor, not propshaft eh? Otherwise there's something weird going on...... What is the voltage then?
     
  13. SolGato
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    SolGato Senior Member

    So those figures are RPM’s at the motor?

    The RPM and amperage differences do seem to suggest the prop is slipping and or ventilating, otherwise I would expect the amperage to be higher if the prop had good bite and was loading the motor while trying to push the hull to planing speed.

    You can almost think about it like tires on a car and putting power to the pavement.

    I have seen examples where small trolling motors (very mild props) were used to push large Catanaran hulls, and at a certain speed the props would “dig a hole” trying to push the vessel faster, limiting the speed as the displacement and wetted surface area of the hulls were greater that the bite of the prop.

    Knowing what little I know about foils, there seems to be a particular speed one must reach before the boat can lift just like an airplane taking off, and I suspect the extra weight added might be limiting your ability to reach that speed (which may be higher now with the added weight) causing the prop slip because it doesn’t have enough bite to push the boat over that speed.

    If the RPM’s above are motor speed, and you have at least a 2:1 reduction, that means you are spinning that prop very slowly compared to an ICE. Imaging trying to get a boat on plane with a similar sized ICE outboard, but using only 1/2-2/3rds throttle.

    If you have headroom with your max discharge amperage, I would try a more aggressive prop with better bite to try to get you over the “hump”.

    This is of course just speculation. It’s hard to know all the variables at hand, but when I used to follow the electric foiling crowd during the early development of their technology, they constantly struggled with the balance between RPM and prop sizing with regard to weight of the rider and getting on plane.
     
  14. cmaas
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    cmaas Junior Member

     

  15. cmaas
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    cmaas Junior Member

    Yes, motor RPM. 72 volts.
     
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