5-Meter Power Cat with 6kW Outboard

I understand that this is a Boat Design / Electric Propulsion forum, and I didn't design this hull, but I did package it with battery/motor. Also, most of you are discussing inboard motor designs, and this is an outboard motor. But you may be able to use it as a data point:

LWL: 15.5ft
hull style: semi-displacement
displacement: 1,500#
power: 6kW
max prop RPM: 1,500
prop pitch: 10.8"
top speed: 13mph

 

Welcome to the Forum Bob.
That is an impressive wee cat! And she is stomping along very nicely in the video.
Can you post some more details or closer up photos / video of the hull design please?
What make of electric O/B motor are you using?

 

Thanks for the Welcome!

I'm using the Rowing Solutions RS CAT16 model (no motor, so not the eCAT model):

https://www.rowingsolutions.com/

They have some decent pictures and videos there.

Then the motor is the ePropulsion Navy 6.0 Evo (long shaft):

https://www.epropulsion.com/navy/

I took this yesterday evening after installing the rear seats (which are just the front seats that I replaced):



Feel free to ask more about 'Electra' (she's still a work in progress).

 

Nice job!

I think a reporting of power consumption versus speed would be helpful to others.

For example, at a cruising speed of 5MPH, what is the motor system reporting as the watts being consumed?

Most reference data provided by electric propulsion manufactures is based on a small aluminum fishing boat hull that cannot reach plane, and not a catamaran.

I would expect the motor to perform better when pushing an efficient pair of catamaran hulls as that has been my experience.

Also what battery system is Electra using?

Do you plan to add solar?

One really nice feature of EPropulsions smaller Spirit motors with integrated battery is that they can take an external charge from a MPPT charge controller while under power which can drastically increase range and allow for more autonomy as the battery doesn’t have to be removed for charging.

My next solar electric catamaran build will likely use a pair of Navy 3.0 or 6.0. Still undecided. It’s not that much more to step up to the 6.0, but they would require a bigger battery due to their max amp draw when running a pair.

 

I built my own battery out of 16x 200Ah LiFePO4 prismatic cells. On paper, I can go for 90 minutes at full power and still have 10% capacity left. That's close to twenty miles of range. More realistic is an average of half throttle ('cruising speed'), giving me three hours of runtime and close to thirty miles of range.

I do want to add solar panels at some point. Four 300Wp panels would be perfect, but I'm having trouble finding flexible panels that powerful. The reason is that I can use these very affordable/small/waterproof MPPT controllers between each panel and one battery 'module' (four cells in series):

https://powerwerx.com/mppt-300w-solar-charge-controller

With 1Kw of solar input I can extend my three hours to 4.5 (or go faster for some of the trip). The main reason for avoiding non-flexible panels is weight. I don't need the flexibility.

I have no BMS, just a 200A fuse on the output. Each four-cell 'module' is well-balanced, as they were bought at the same time and have the same amount of cycling on them. So I have no trouble topping the individual modules up with a 14.6V current-limited power supply. Between modules there's a bit of a mismatch due to different amounts of cycling. I observed this recently when testing if a 56V current-limited power supply (Meanwell RSP-1000-48) would get them charged evenly.

I understand that there are those who will insist that a BMS is necessary. I've made it work without one so far. Overdischarge is protected against by the motor, as it reduces power draw at 42V. At some point I want to add coloumb counting, so I can have an accurate picture of the battery SOC.

On a power-to-speed table, I definitely want that soon, but still have some kinks to work out (I'm adding an anti-ventilation plate soon). My understanding from the hull manufacturer is that power is related to speed by a 'squared' relationship. I've seen others state that it should be 'cubed'. Possibly the difference relates to semi-displacement versus full displacement. My two fairly-solid data points are when I had a 2kW motor (~7.5mph) on it, and this 6kW motor. Doing the math did show that the constants ('C') derived when assuming p = Cv^2 for the two data points were close. Admittedly, there was low precision on the inputs to that calculation.

Assuming p = Cv^2, and using 6kW (in watts) and 13mph to derive a C of 35.5:

C = 35.5
p = 6000, v = 13
p = 5000, v = 11.9
p = 4000, v = 10.6
p = 3000, v = 9.2
p = 2000, v = 7.5
p = 1000, v = 5.3

I've seen similar numbers to that last data point, as the speed limit on the river near where we live is 5mph. The posted video is only the third time out with this motor, and on the previous two outings, either rough water or river currents interfered with trying to get repeatable power-to-speed readings.

 

With regard to the need for a BMS, I think you’ll find that eventually with use (especially as you discharge down to 10%) you will start to see a voltage deviation between cells within each module.

The problem as you probably know will continue and repeat with every charge and discharge cycle as some cells will reach their target V and hit full capacity, while others wont quite get there, causing some to discharge to a lower capacity than others because they started at a lower capacity.

Eventually that 10% “reserve” capacity will start to diminish as you begin to reach the low voltage threshold of the motor more quickly.

Having a shunt with a BMS really is the way to go as I would not expect the EPropulsion predictions to be accurate unless you were using their display with their battery system.

Do you have any over charge protection?

This would be extremely important if you don’t have any cellular balancing.

There is a 4 in one isolated balancer available (HA02) that you might consider using to handle the balancing of the 4 12V modules with one another.

Solar panel wise, SunPower has just introduced a new 300W flexible panel that might be perfect for your multi module independent MPPT plan.

 

Hi SolGato, I understand the concern you express. In my experience, fully charging each 12.8V-nominal module results in virtually-equal open-cell voltages on the four cells in each module (+/- 0.01V/cell). Maybe it's because the four cells in each module were closely capacity-matched from the manufacturer (and they've been cycled equally), or my other theory is that, as charging approaches the target voltage, any cell reaching that 3.65V/cell slightly before the others can just absorb excess charge until the others catch up.

On overcharge protection, my go-to charger is a current-limited 50A supply with adjustable target voltage (Meanwell RSP-750-15). I set that target voltage to 14.6V. When I see that the charge current is under 1A I feel like I can disconnect the charger, but I've also come along after leaving the charging unattended for short periods and disconnected the charger with the charge current well under 1A. I've seen no ill effect to leaving the charger connected for short periods of time (under an hour) once charging is effectively complete.

Another thing to keep in mind when concerned about how top-balanced a battery pack might be, is that the per-cell voltage difference between 100% SOC and 99% SOC is more than a quarter of a volt, and I'm talking about deltaV's on the order of one hundredth of a volt.

'BMS or Not' can easily fill its own thread, so I'm hoping to leave that discussion there. It does sound like you agree that IF I can achieve a balanced full charge, the discharge half of the cycle is okay with no BMS, given the two protection mechanisms I mentioned: 200A fuse on the battery pack output and low-voltage-power-reduction on the motor. I agree that if I rarely-or-never had the chance to bring the battery pack back up to 100% SOC, I'd likely experience the issue you raise. Given that I fully recharge after each outing, in my experience, that allows any cell-to-cell SOC drift to be effectively reset.

I'm looking into that HA02 isolated module balancer and the SunPower 300Wp flexible panels now. Thanks for those suggestions.

 

Before I got the 6kW motor I tried a 2kW motor. It was a tiller model so I first had to convert it to remote control. I saw close to 8mph top speed. The final run was eighteen miles in three hours, around all of Lake Sammamish, on Halloween weekend.

 

Had another testing session on Lake Ballinger on Sunday.



For this session, the 'Anticavitation Plate' is new. That allowed me to return the trim angle to zero. It wanted to ventilate at zero trim with no plate.

But the transom is too low now, due to having been lowered to counteract ventilation (one inch). Today I raised it back up the the original height, so the next outing will be a test of that new configuration.

Speeds in the above video were virtually identical to those of the previous test session (12.x mph), so I'm hoping to pick up a bit more with less of the motor in the water. Turns out the mounting collar on the anti-ventilation plate blocks a fair bit of water, so if I can get the bottom of the plate right in line with the water surface at speed, I should see less drag (mounting collar is above the plate surface).

 

Way too much wake for that little boat. Probably what you said, but even the hulls seemed like making too much. I hate to be critical because it is so cool, but perhaps the stern is too narrow?

 

It's hard for me to have an expectation on wake size for this style and configuration of hulls, as most of my experience involves planing monohulls. There are a few videos from the manufacturer here: https://www.rowingsolutions.com/. The wakes seen in those video seem comparable to what I see in my videos.

If my wakes *are* somewhat larger than theirs, it may be because I have more weight towards the rear than they have. The battery pack on mine weighs 160 pounds, and is not very far forward. I believe their battery pack is smaller/lighter, and is under the front seat area. I'll definitely be trying different battery mounting locations over time.

And you may be on to something with the 'stern width' concern. The sailing cat from which this hull/configuration is derived has considerably more beam: RS CAT 16 – the most stylish, easy, exciting for clubs and families https://www.rssailing.com/project/rs-cat-16/#cat16specification. Nearly 8 feet compared to more like 6 feet. I think the smaller beam was chosen for better trailerability. I'm unsure if that difference would affect the wake size, but I think the hulls being closer together causes more 'wake interference' under the deck.

 

If your motor is pulling max power, I doubt you will achieve any higher top speeds until you start experimenting with props, and you might be able to improve efficiency by finding the best balance, but I think the hull shape is what is really causing the excessive wake.

Being a short non-planing design with most of their volume near the center with little in the stern, you are probably pushing the “fat” part of the hulls against the water causing a lot of the water disturbance. In the video it looks like it is building near the center and not from the sterns dragging, and that the hulls are riding bow up.

Small catamaran and newer beach cat hulls like those tend to be designed to float a lot of weight at slow speeds until they are powered up by the wind and lifted out of the water.

From my experience, keeping the weight toward the front (like you have done) to level the boat out and allow it to push through the water seems to work the best under motor power. Wind, you want to be as far back as possible.

Hopefully EPropulsion gets their act together and starts offering an optional speed propellor for light displacement applications like yours, similar to those Torqeedo offers for their larger motors.

I don’t remember what the prop specs are on the Navy 6 prop (a 3 blade I think), but remember the props on their other models are pretty mild in order to protect their controllers across a wide variety of applications.

Since your boat is a low displacement design, you could afford to give up a little on the bottom end to get a higher top speed, and you would probably gain some efficiency at cruising speeds.
.

 

It looks to me as though hull immersion is too high. No easy fix for that...but you ought to be able to see if I am wrong easily enough.

Have you charted rpms and speed? I'd be willing to bet not getting over any hump and your are operating more semi-d...kinda what SolG said.

Wake drag, while real, is probably less an overall issue than the hull shape. Do you have a picture of the stern and stern bottom out of water?

Did some checking and the length is 472 and the beam is 40cm. They offer displacement and hull weight as well, but falling asleep now. Really, these are in the realm of semi-displacement and not planing hulls. So speed will be limited a bit. I thought perhaps you figured it'd get up on plane.

Enjoy them for what they are, work on that engine issue.

(this from a guy building a semi-displacement power cat)

 

I appreciate everyone's feedback and suggestions.

ePropulsion has three different-pitch propellers available for the Navy 6.0 Evo, but the two lower-pitch props are nearly the same in pitch, differing more in material (aluminum versus composite). I'm using the highest-pitch prop they make (10.8"). With their maximum prop RPM of 1,500, that comes to 15.34mph 'theoretical' top speed (zero slip). My ~13mph observed (so far) comes to about 85% of that, so some combination of slip and just not achieving maximum prop RPM is involved. I do want to see prop RPM, but so far I've not found it on the top-mount control I have. I do see the RPM icon when the display goes into self-test mode, so it may be available. And the other two styles of control (side and dual) have the RPM readout. Once I can see prop RPMs, I can better see a way to increase speeds. Their low-pitch offering (8.5") has a theoretical top speed of 12.07mph, below what I get now, so that won't be an option unless I run with considerably more weight. That's of course assuming that 1,500RPM is really the maximum. If it's higher, then that moves the crossover point closer.

 

Here's a good shot of the hull shape along the bottom. Definitely semi-displacement. I opted for that over a full planing hull so lower power levels wouldn't suffer from poor efficiency due to being only partially on plane. I'd really need more like 15kW to get the size boat I'm after (2-4 passengers) on plane. This way I can get good performance with a fraction of that power, and still get what I think will be reasonable efficiency if/when I increase the power. This boat is rated for 15kW / 20hp, and should go right at 20mph at that power level. Granted some smaller planing hulls will do better there, but they wouldn't allow efficient cruising at, say, half that speed.