All Electric Jet Drive Discussion

There is nothing revolutionary in your concept. We all know its possible. However what remains to be demonstrated is this application of a "speed boat" recreational type craft, is it feasible? That is, the cost of all the highest tech gear you will need in order to create a system of high enough power density is prohibitively expensive. If money is no object and your happy to blow a small fortune on this, then let's keep talking, otherwise it still looks unfeasible. There are batteries with much higher power densities than the common electric car lithium batteries, however there are safety concerns with other battery chemistries and also cost barriers on yet others...

Have you a budget for this or is it all just hypothetical?

 

I know it's not going to be cheep, but I'm still interested.
Now, not being all that familiar with Naval Architecture, my biggest concern is, and always has been, to obtain the power requirements of the pump to push the approximately 8,000 pound gross weight welded aliminum boat (4K boat + 3K propulsion + 1K occupants) at a constant speed of 50 to 75 MPH so that I can choose a proper motor. It would also be very beneficial to have the complete velocity vs power curve for the boat. Until we get a good handle on this propulsion combination, we are still hypothesizing.
Maybe the propulsion system exceeds the estimated 3K weight and the requirements to maintain the speed goal diverge. If so, then I will have to cut back on my speed goals.
Maybe, after its all said and done, I will just have to go with gasoline, but I don't want to have to plow $25,000 or $50,000 into a supercharged V-10 or whatever.

 

cant see electric being cheaper

 

I totally agree, but electric is much simpler, more compact and a whole lot quieter. Plus, one can get multiple props off of one ICE. I'm expecting to spend something on the order of $75,000 to $100,000 for an all electric system.

 

So you want a 3600 kg boat to reach 43-65 knots? Just to give you a ballpark I made a calculation with my Savitsky program for a 28' boat at that weight. With optimal weight distribution and 65% propulsion efficiency 43 knots needed about 300 kW and 65 knots about 600 kW. Those figures won't be easy to reach at those speeds, especially with a jet propulsion.

With gasoline installation you can save more than 20% of that weight and thus also need more than 20% less power. The biggest standard sterndrives at 425 HP would probably get you close to 50 knots. Those would be around $25 000.

Yes you can get 65 knots with that, but then you need a much lighter boat. Maybe around 1500 kg total.

An electric system with more weight would need to be about 370 kW for that same 50 knots. How much would it cost? How long can you "cruise" at 200-300 kW with your batteries? Say 1000 kg of 200 Wh/kg is 200 kWh and thus less than an hour. The cost of the batteries alone is maybe $200/kWh thus $40 000.

 

I like what you're saying. 300 KW for 43 knots doesn't sound bad, but I'm hoping to keep the total power down to 250 KW, so I may have to sacrifice speed and or length (weight).
I don't expect to "cruise" at anywhere near full power - more like quarter to half full, with most of my cruise time being spent at 1/8 full, so I can be on the water for about 2 hours between chargings, or add an onboard 25 KW generator.
I know the batteries are the limiting components with both their weight and cost. A few years back I priced an 85 KWH battery pack at $60,000. I recently found some 30 KWH batteries at 723 pounds apiece, which would total less than 1,500 pounds for a pair of them @ 60 KWH or 2,250 pounds for a 90 KWH 3-pack.

 

Earlier you wrote " My goal is to have a system that can cruise at 45 to 50 knots and have a top speed of approximately 75 knots. "

A 3600 kg boat cabable of 65 knots or even 40 knots, is not going to be very efficient at low power. You will likely need more than 100 kW to keep her planing (say 20 knots), so about half the maximum power and your 60 kWh is only going to last for half an hour.

At 30 kW power you will do about 7 knots. Much better then to slow down to 6 knots and use only 10-15 kW or even to 5 knots and use less than 10 kW.

 

Frank, what your saying is so far from feasible it's not funny, many reasons have already been given above, and the are still more reasons not to do it. I don't think you grasp the fundamentals well enough to understand why this proposal sounds absurd. I'll try to summarise it,

A heavy boat needs lots of power to go fast, a lighter boat can do it with less power;
Electric propulsion is heavier than gasoline systems, thus you will need more power as the same boat is heavier with electric power;
Electric power systems are more expensive than gas, mostly in the batteries.
Electric propulsion is therefore suited to efficient low powered vehicles and boats, not high specific energy speed machines and racer type craft as the energy density is not there- power to weight ratio.

Your barking up the wrong tree with this, pie in the sky... 70mph via a jet drive is also silly, you would be better off with a surface drive if you wanted a boat to do those sorts of speeds....

 

Three things:
1. I am confident that surface drives will work because when you take the heat off at speed you still rumble along at a decent speed. I probably will go dual surface drives with a deep vee hull before it's all said and done. I like the jet for its more reduced draft and fully protected impeller, but I'm concerned with its being a closed system.
2. Thrust is what drives any vehicle forward -- whether on wheels or on the water. Linear thrust is proportional to the torque generated by ICE's and electric motors. The torque output of an electric motor (BLDC) remains constant as the RPM's drop off to zero. Not so with ICE's. . . .
3. I imagine that your calculations are relative to ICE's, in which case they cannot be compared directly to electric motor power.
TO OVERCOME THESE SHORTCOMINGS:
A. Let's relate boat speed to thrust.
B. Then. compare that with jet thrust and torque vs RPM
C. Finally, correlate these curves to motor torque vs RPM for motor selection.
Note that I am not steadfast in my all electric jet, but I am determined to go all electric and fast, with minimal draft.

 

Also the torque reguired by a propeller or a jet pump impeller drops to zero when RPM drops to zero. The required torque drops at a rate of about RPM^2 thus there is no need for high torque at low rpm.

Most ICE have rather flat torque curve from just above idle speed to close to full power and the charged ones even can give much more torque at a rather low RPM compared to higher RPM.

The two points when high torque is needed are getting up to plane and full speed. For a 28' boat having maximum speed of 40 knots the first will be about 50-70% of full rpm and the latter at full rpm. For a jet drive the first one is likely to be even higher rpm.

50-70% of full power rpm is just the spot where a typical modern ICE has its maximum torque.

With a much faster (e.g. 60 knots) boat the RPM at getting up to plane may be rather low, say 30% of the full rpm, and then it may be a problem. But not for a jet drive, which typically has a much higher RPM at that stage. Here the problem is not so much that the engine has lower torque at that point compared to full power, but the torque reguired may be higher than at the full speed. The reason for this is that the boat and the propeller (or a jet drive) optimized for 60 knots is very inefficient below 25 knots.

 

As a rough guide, this was output from a savitski planing prediction spreadsheet using your input values of 28ft LOA, 22ft LWL and 8000lb displacement and a bunch of other assumed variables of a typical planing hull of this dimension. You can see that getting to 40kts might be realistic, but 60kts is basically a different ballgame... I also think that you would struggle to build this and have a fully ready to run boat @ 8000lbs.

Another thing you have to consider is the C rate at which your discharging your batteries. If you have a small battery pack to save cost and weight, and yet wish to pull a huge current from it for only a short blasts of speed between charges, the battery will get very hot and will not last many cycles. So you will end up damaging your very expensive battery very quickly if the pack is not sized appropriately for the currents your expecting from it.

speed Peffective
[kn] [ehp] [ekW]
15 52 39
16 56 42
18 64 48
20 71 53
22 79 59
24 88 65
26 98 73
28 109 81
30 122 91
32 136 102
34 153 114
36 172 128
40 216 161
50 373 278
60 607 453

And thrust or resistance as follows;
[lbf] [kN]
1,127 5.0
1,145 5.1
1,162 5.2
1,167 5.2
1,176 5.2
1,195 5.3
1,225 5.5
1,268 5.6
1,323 5.9
1,390 6.2
1,468 6.5
1,556 6.9
1,762 7.8
2,430 10.8
3,300 14.7

 

You could optimize the effective power need at 60 knots close to 300 kW (10 kN thrust, about 500 kW engine), but then you would need much more power at 20 knots due to hull form and weight distribution being far from optimal at lower speeds.

 

Which is not a problem at all (for ICEs). At low rpm the prop is not going to need much torque to spin. The motors ABILITY to provide more torque is unnecessary and provides nothing.

This is silly argument and gets brought up way too frequently - high torque from zero rpm is of no meaning on boat propulsion. In the end kw=kw (hp=hp) no matter if its created by steam, gas, diesel, chipmunks pedaling or electric motor.

 

Four things prompted me to go for electric:
1. Full torque all the way down to below 1 RPM for extremely low speed operation.
2. Because of item 1 above, electric motor has fatter power curve than ICE for getting up on plane.
3. Ability to rev well past normal.
4. Silent operation.
Special thanks to Groper and Joakim for their input to this discussion. I am now implementing ICE only into my design. The thrust and power chart provide by Groper is most helpful.
Item #1 above is not an issue because it is solved by Hamilton Jet.
Item #2 is not all that big an issue with properly selected and tuned ICE.
Item #3 is not really an issue.
Item #4 losing silent operation is the price to pay for speed.
Now, my only concern is being able to obtain a jet drive with enough speed capability. Hamilton Jet ads typically support power up to 50 MPH, and I'm shooting for 60 MPH -- does Hamilton have a model that can exceed 50 MPH? The chart provided by groper indicates a huge power step to get from 50 to 60 so I may have to "settle" for 50 to 55 mph.
Then, we get into single or multi-stage pumps and the pros and cons of each. Two- or three-stage make a lot of sense to me, but the water velocity at the intake concerns me.
I suppose we need to go to a different forum to discuss this.

 

I'll put a thread with what I've been doing. Shortly.
Hoping to be in the water Halloween.