5-Meter Power Cat with 6kW Outboard

This month I've been out twice on a small lake, experimenting.

The first change was to remove what I call the Flow Diverter. It's the part just ahead of the motor between the two hulls, intended to clean up the flow of water to the prop. It ran well without that part (weight: 10 pounds!), but seemed just a bit down on top-end performance. I had seen mid 20.x km/h speeds with just me aboard, and mid 19.x km/h speeds with two parsons aboard, but without the flow diverter, I lost that mid 0.x km/h on the top end (could only get to 20.0 km/h). Watching the spray at the back, it was clear that a lot of water is now hitting the upper part of the motor lower unit, basically the wakes off of the two hulls seems to be peaking there and causing a lot of drag when it hits the motor leg. Before, that wake was prevented from hitting the upper part of the lower unit by the Flow Diverter.

Today I tried removing the Anticavitation Plate. The thinking was that if that much of the lower unit is under water, removing that plate should cut drag down. The hope was that there would be enough water over the prop to not need the plate. On the first full-power run I got up to low 21.x km/h, about half a mile-per-hour faster than ever before. But then the ventilation started. Generally, I could not use more than 2/3 power before ventilation would kick in.

So it seems like I can't avoid the Anticavitation Plate (although I could try lowering the motor one inch). The easy next step is to just put the Flow Diverter back on, leaving me where I started. But I still feel like there's some performance improvements to be had by doing something there that's more suited to my situation. And wondering if anyone out there has experience in these matters. I'm sure I'll get lots of 'the details matter', which I totally understand. I haven't even posted a picture of the old Flow Diverter. I'll try to get to that soon.

A first thought I had was something like a long, thin trim tab, hinged to the underside of the deck, a few feet ahead of the motor. Then possibly I could adjust the angle between runs. I'm thinking like a one foot wide and three feet long thin aluminum plate (strengthened on the back side with something like hat channels or U channels). I could even imagine going a step further and being able to adjust the angle while underway, similar to how trim tabs on power monohulls work.

But a fair bit of water is coming into the problem area sideways, from the wakes of the two hulls. A flat trim tab would be open on the sides, and that water would not be stopped. So my Plan B was a long (3ft?) rectangular tube (12" x 3"?), cut on the diagonal, such that a thin 'box' wedge/fin would result, mounted just ahead of the lower unit (cut/open side up so the closed thin side pushes water down), ideally the bottom of which would stop where the Anticavitation Plate is mounted. Water coming towards the center from the hull wakes would hit the sides of that 'hollow rectangular fin' and get deflected mostly straight back instead of contributing to drag on the lower unit.

I completely understand if this is not something someone can help with just from pictures. I've searched online and this topic does come up here and there. But I've yet to find out the name of the part in question ('Flow Diverter'). If anyone knows what that thing is called, please post!

 

I'm surprised you did not mess with motor angles at all. Usually, a modest change up or down can impact, however, it sounds as though you have aerated water, or water becoming aerated at speeds. Dropping the engine down may offer cleaner water.

The forum has an expert on dynamics. He goes by @baeckmo . I'm not sure, but I may possibly annoy him, but when it comes to fluid dynamics; he knows a lot.

My personal opinion is that the motor belongs on the hull and not in the crashing wakes. This would require a proper transom and mount.

 

Thanks fallguy. I have the motor square to the deck currently. I've found that to work best, although earlier on, I found tucking the motor in five degree helped with ventilation, but probably at a cost performance-wise. Agree on the aerated water theory. I suppose dropping the motor down one inch is an easier experiment than building a new whatever-the-heck-its-called.

I've considered twins. It's a big change. Cost and weight goes up for the same propulsive thrust, but so does maneuverability and redundancy. I definitely concur that such an approach would eliminate the ventilation problem (Anticavitation Plates would be even with the bottom of each hull). In my case it would also move the motors back nearly two feet, so I'd have to work harder at keeping weight forward (and I'd likely need a longer Teleflex cable).

Would you happen to know what that part is called? The pod/fairing just ahead of the lower unit on single-motor catamarans? It might help me to search for discussions related to it.

 

pivoting nacelle or pod, but these do not affect flow



anything in the flow will create drag

I would just put the engine on the transom. Offset the weight with the battery pack.

Add another smaller engine for maneuvering. It need not be same size. A cheap trolling motor would work. They could be attached with a bowden so one engine steers the same, but you could throttle steer them separately.

 

Alright, that's a thought. Agree that anything in the flow will create drag. But something more hydrodynamic ahead of the outboard leg can reduce total drag.



This is what I took off at the start of this series of experiments. 9.6 pounds. It's about 10" high and 4.5" across the back side. So wider than I need for my motor (most likely intended for an ICE outboard). I see that opening near the sharp end as one area of improvement. I could try temporarily taping that over at some point.

 

The fact that Rowing Solutions went to so much trouble and expense to fabricate and develop that flow diverter thingy tells you they probably identified an issue during testing, likely with bigger ICE motors with props spinning at higher RPM’s.

As Fallguy mentioned, hanging a motor out there in the open between the two hulls is not ideal.

I remember from your previous videos a lot of water disturbance behind your boat, and believe it is partly due to the stern shape and where the volume is placed along the hull length being that the hulls are designed for powered up performance sailing.

I have found a cavitation plate is a must to keep the prop from sucking air and am now running my props close to the water surface under cavitation plates. Too deep can hurt performance too.

Can you post a photo of your cavitation plate? Maybe I missed it but didn’t see one in the thread.

I still think your best option for improving performance is a more aggressive prop if you stick with the single Navy 6 hanging off the center.

Your motor was designed to be capable of pushing large displacement vessels and is “detuned” in order to protect the motor controller in high load applications.

It’s max RPM is 1500 and its power output is about 3.4kW.

With your craft being light displacement and a semi-planing catamaran, you should be able to get away with turning a more aggressive prop without overloading your motor controllers. You must of course monitor this closely when testing.

A more aggressive prop will not only get you a higher top speed, but will likely also improve cruising speeds and provide better response.

If it were my boat and I was seriously considering twin motors which was brought up, I would step down to a pair of 3kW motors which have a max RPM of 2300, a power output of 1.5kW, and are lighter smaller cased motors. They aren’t quite as efficient, but you’ll be running twins and you’ll have almost as much propulsive power as your single motor, but with be capable of higher RPM’s.

Then I would run a more aggressive prop on the smaller motors and make the cavitation plate work like an extension of the hull. Adding trim capability also makes a noticeable difference when the motors are hung off the back of each hull and really comes in handy with passengers and beaching.

My current setup is twin motors for 2kW of propulsive power turning 11.5” x 8.1” P 2-blade props at a max of 1350RPM.

Testing props is an easy affair with my boat. I just run one against the other, and since I use independent thrust to steer, the better prop will overtake the other and the boat will steer in the opposite direction.

My current set of props were the same pitch and diameter as my last, but made of a stiffer FRP material with more chord and slightly different twist at the tips, and the difference was night and day.

 

Thanks for the suggestions, SolGato.

A pair of Navy 3.0's has definitely crossed my mind. I just wish the price of a 3.0 was closer to half that of a 6.0, rather than nearly the same. For the price of two Navy 3.0's, I could have a GoldenMotor EZ-X20. Granted, I'd still have the current issue regarding drag reduction versus ventilation.

One thing I should clarify: there are no aftermarket props available for any ePropulsion motor (that I'm aware of). I would have definitely tried higher pitches by now if there were.

The 3.4kW of propulsive thrust is a result of the 57% efficiency (times 6kW input). I don't see it as a result of 'detuning'... if they could get higher efficiency, I suspect they would.

Adding trim capability seems like a big step. On pods like I've seen on your setup, hinged off the bottom/rear edge of the hulls, I can see it being fairly straightforward (linear actuators). I've considered adding in-flight height adjustment, but the smallest/lightest powered 'jacker' I've found weighs 23 pounds. I'd likely lose more from that extra weight than I'd gain.

Where I'm coming from is: that Flow Diverter (Pod, Fairing, Nacelle, ...) from RS was most likely designed for a much larger ICE motor (wider leg). I think I can do better. The Navy 6.0 leg is right at two inches across, where the part I took off is 4.5 inches across. 2.5 inches wide could work, if I can get it right up against the front of the lower unit.

Keep in mind that it works ventilation-wise with the old Flow Diverter in place (and motor antivent plate). So at worst I can reinstall it (them). The purpose of my recent experimentation was to expose how much the diverter was helping/hurting. It does seem marginally slower on the top end without it (but also more efficient at lower speeds). And it's clearly faster without the antivent plate, at all power levels (right up until it starts ventilating). That change was less about 'Is it faster?' than it was about 'Do I still need it to prevent ventilation now that the diverter is off?'. I got the answer, even if it wasn't the one I wanted.

It may sound like I'm shooting all of your suggestions down. More just pointing out that not all of them are within easy reach (for me). Certainly my other half will not want to see another $5k laid out, for what could amount to a one-mph gain at full power. The GoldenMotor EZ-X20 costs just over that, and would get me to about 17mph, admittedly with considerably less runtime, but I could run at current power levels if I needed to gain some runtime back (or add battery capacity, with the ever-present caveat of that extra weight dropping my speeds proportionately).

Here's my current plan, subject of course to modification, as more ideas come to the fore:

- drop the transom height one inch
- if that solves the ventilation, at least in calm water, and speeds are still better than baseline, call it good (but rough-water testing could bring me back to this list)
- if ventilation persists, raise it back up and reinstall the antivent plate, then
- consider options for an improved 'fairing'

Another recent idea I had was to add vertical 'fins' just ahead of and on each side of the motor, with the hope that they would effectively stop the inside wakes off each hull from causing massive drag on the lower unit.

Feel free to continue to make suggestions. That I might not find them all practical is my problem, not yours.

 

When I say “detuned”, I am referring to the prop and max RPM.

EPropulsion markets their motors for all types of applications and they have to perform reliably on dinghys to fully loaded bass boat and 30’ sailboats, and the Navy 6 is configured to move heavy displacement boats efficiently with its low RPM and mild propellor without drawing too many amps.

A comparable ICE motor’s prop would be spinning almost 2X the RPM which is why comparing it to a 9.9 is misleading as results in disappointment for some.

I may have mentioned it before, but one of the best references is Torqeedos propellor chart. Just like EPropulsion, when they first introduced their line of motors they were limited in the prop offerings, and then eventually they started developing props for better performance with light displacement planing vessels, later offering a 3 blade for their 2.0/4.0 for light planing application that’s 12.6” x 16.3P.

The 2.0 has a max RPM of 920, and the 4.0 1300 RPM. (They have new models now I think)

I’m guessing your prop is the 12.6” x 10.8P ?

If you compare that to Torqeedo, you’ll see those specs are what they recommend for heavy displacement applications on their comparably sized motors.

My point is, if Torqeedo can get away with swinging a much more aggressive pitched prop using a smaller 4kW motor, there’s a good chance you could too.

Yes, I understand you can’t just go out and buy a prop, however it shouldn’t be too hard to adapt one to the EPropulsion shaft since it is a straightforward shear pin design.

Reaming holes, making bushings and shims isn’t difficult, and you could always have a machinist do it for you.

You just need to try to identify a prop worth modifying/testing which will involve some homework.

You might even be able to use a standard style outboard hub prop with your motor housing being such a large diameter without the hub disrupting flow which might make adapting easy for a machinist in that an adaptor could be machined and pressed into the hub with rubber iso bushing.

I would give the Torqeedo 1923-00 prop a look and compare the shaft diameters, shear pin and hub diameters to see if it could be adapted. Diameter is same, but you get a lot more pitch.

Anyway, testing a prop would certainly be a lot less complicated/costly than reconfiguring the motor setup.

On the flow diverter thingy, my guess is they realized there was an issue when they ran bigger ICE motors thinking the boat would be faster.

I understand you suspect it might be too wide, but your motor housing is also quite large on the Navy 6.

Certainly worth a modification or redesign to try to improve flow, but I think a few things may need to change to make a significant difference.

You could also try a Kort Nozzle type shroud setup. A friend of mine had some success with a kayak catamaran he built with a motor mounted in the center. He built it more as a prop guard for his kids, but it ended up helping with flow and reducing ventilation.

 

I'm considering propeller improvements as orthogonal to the current problem space. I get it: by increasing pitch, RPMs will go down a bit, reducing drag on the prop. As long as I'm not up against a torque limit of the motor/controller, that should deliver more propulsive thrust, 'all other things being equal'. I may come back to that front at some point, but if I were to do a motor upgrade that would be a lot of effort to have gone through, then abandon. I also consider it as something of a crap shoot, as there are so many parameters of a propeller (diameter, pitch, blade count, rake, hub diameter, EAR, trailing edge cupping, ...) that to find one that matches in most areas but has a bit more pitch seems like long odds. I agree that the Torqeedo model you point out would be a good start though.

"Try increasing the propeller pitch" is a great answer to this more general question than I'm asking: "How to increase overall performance, with changes not restricted to a specific set of parts?". Instead I'm just trying to get the dust to settle on an experiment I embarked on earlier this month: "Can it work better without the Flow Diverter, and if not, is there something better I can use in it's place?" Of course this touched on a couple of related areas, the antivent plate and transom height (and some would suggest trim angle). Note that I've already had the motor mounted one inch lower, during a similar initial round of experimentation I underwent before adding the antivent plate. So it's trivial to return to that transom height: remove motor, unbolt/rebolt transom to the lower set of holes, reinstall motor. I can get it done in an hour, far less time than finding a suitable 'donor' prop and modifying it to fit (only to find that it has enough other differences that it doesn't end up being an improvement).

I feel like if I get the lower unit 'fairing' arrangement improved for the current propeller situation, that result will also be an improvement for any pitch increases I may make there. And even a different electric outboard motor is likely to have a similarly-thin lower unit 'shaft'. You mention the much larger dimensions of the actual motor housing on the Navy 6, but the lower unit 'fairing' just needs to deflect water around the lower unit 'shaft'. I've considered a Kort Nozzle but feel that the overall drag would be higher that way compared the to current antivent plate. Plus the latter needs no fabrication as it already fits and works. It's just the drag it creates with no lower unit fairing that I'm trying to sidestep.

If I can clear up the ventilation problem by lowering the motor, I'll move on to a better-shaped fairing. Otherwise, the antivent plate goes back on, transom height goes back up, and I still move on to a better-shaped fairing. Any ideas for what kind of shape might work best there are welcome. I mentioned a few ideas I have earlier. If nothing I come up with improves the situation, so be it. At least I tried. Without trying, I'd be wondering if there are any gains to be had in that area. And of course, even after trying and failing, that's not conclusive. I just might not have hit on the better approach before giving up and reverting to the prior arrangement.

 

Maybe I’m misunderstanding the purpose of the flow diverter.

You mention its need to redirect flow around the leg of your motor, but I believe its intended design purpose is to act like a transom to provide good flow to the prop.

The motor leg is already an aerofoil and not all that different from any outboard motor leg, but a big difference is having the motor and prop hanging out there all by itself without a hull and transom feeding it.

My guess is the diverter is designed to act like a hull to better feed the prop and something that may have come about when lack luster performance was noted using bigger ICE motors which have expected performance benchmarks.

The reason I bring up the motor housing diameter is because having such a large housing in front of a small prop doesn’t do you any favors, it’s one of the downfalls of the design, but at slower trolling motor cruising speeds worth the benefits of keeping the motor cool. But when you are going for speed, it starts to work against you trying to push such a big housing through the water that’s just in front of the prop.

How does the width of the back of the diverter compare to the width of your motor housing and blade area?

If the diverter is wider, then perhaps matching it to the width of the motor housing would be worth a try.

Then again, like I said maybe I’m misunderstanding the purpose of the diverter.

Regarding the other changes to improve performance, I have found that some should be sorted out before finding the “perfect” prop, but others had to be changed/adjusted during testing and after swapping, so anything you can make adjustable will be worthwhile like trim, diverter width, cavitation plate height, etc., as you may find different changes need to be made for different props.

One handy feature if you get to the point of testing a more aggressive prop (I would just focus on something sane diameter and blade count with more pitch) is the EPropulsion throttle limit programmability.

Ideally what you would like is a higher pitch prop that still slips enough when you throttle hard from a dead stop so it doesn’t overload the ESC, then at the top end you are still able to get close to the max rated RPM without exceeding current draw. It would be easier to work with a top end issue (use the throttle limit feature) than have to worry about initial throttle loading (unless there’s a way to program the motor for that as well?).

Does the display give you an Amp draw reading?

 

On second thought, after looking closely again at the photos and seeing the dimensions of the diverter, I’m thinking it might be designed to combat the converging flow coming off each hull meeting in the center of the boat as it is too narrow and shallow to mimic a transom, although it’s hard to tell exactly without seeing it all together in the water.

Maybe you could mount a camera and take some footage of what is going in front of the motor and behind the motor flow-wise when underway?

 

My first take on the Flow Diverter was a stand-in for a transom. Push the turbulent water down to close to the level of the antivent plate. After removing it and noting a slight decrease in performance, I'm more of the mind that it's for both that AND the second purpose you mention (combat converging flow). My thinking in removing it was that the water surface level should high enough without it to avoid ventilation, after all, "all it does" is push water down. Once I noted the slight performance decrease, I realized that the only way that could happen is if drag went up. Water that used to hit the diverter is now hitting the lower unit leg. I get that the lower unit leg is fairly sleek, but apparently not as slippery as the diverter. Then I took a look back at speed as a passenger (and got video, to be posted soon), and it was clear that water was hitting the leg very high up, and of course all along its length. That could only be due to the waves off each hull converging there (or near there). So the purpose of the diverter became more clear: the wide sides it has deflect that wave action back away from the centerline. AND there's some amount of acting like a transom, supplying the 'bottom edge' of an artificial hull. The drag from deflecting that water back away from the centerline is apparently less than allowing that water to hit the lower unit leg. OR the water is actually piling up so high that it's hitting the underside of the deck, which is not sleek at all without the diverter in place (and you can see that the upper/horizontal part of the diverter, which mounts to the underside of the deck, could be helping with that). A rearward-facing camera under the forward part of the deck (as you suggest) would help see if that's happening (and I had that on my list of things to consider trying). A view from behind the motor is more challenging, as I'd need to rig up some scaffolding, but not out of reach.

Width of diverter is 4.5", width of lower unit leg is 2", width of motor housing is 5". I feel like if an improved diverter could be right up against the front edge of the leg, it only needs to be as wide as the leg. The 4.5" is most likely for an ICE leg, which are generally wider, as they need to pass exhaust, cooling water, drive shaft, and gear shift linkage. The Navy 6.0 just needs a couple of high-current wires, and maybe some control signals, like for temperature sensors.

I have a Watts readout on the motor control. At full power it hovers right at 6,000, so it's clear to me that it's feeding back on that value and trying to limit it duty-cycle-wise to that (via PWM). At 48V, 6kW is 125A, but of course my voltage is a bit higher than 48V typically, so maybe 120A. But no direct amps readout. REALLY want a prop RPM readout, but they're only on the side-mount and dual-mount controls, and I have the top-mount control.

Agree that finding the perfect prop generally has to interact with other improvements. If drag were to go down, pitch would want to go up to take advantage of it (assuming pitch was perfect before the drag reduction). Mostly, for now, I'm after drag reduction, after exposing a source of drag I can control to some degree. If I get somewhere drag-wise that I'm happy with, that can be a jumping-off point for a better prop. Who knows, by then there may be aftermarket options.

I appreciate your input.