Solar power catamaran prop size

[caymantri]

I am in the design phase of a 42’ power cat WITH 40’ of water line and 24” width at the water line loaded
Very easily driven hull shape powered by twin 10 kw electric motors rated at 3700 RPM
I have heard that the most efficient propeller is a larger size turning at a low speed.
I would like the simplicity of a direct drive but not sure if that would strain the motor even if I used a low pitch prop
I’m only looking to cruise at hull speed 8-9 kts with the top speed of 12-14 when needed
I have seen two to one belt drive reduction systems but would like to avoid them if possible
Anyone have any experience with electric motors

 

[montero]

Hi Caymantri.
What engines do you have in mind specifically?
For such a large and, I assume, heavy boat, they're a bit underpowered.

 

[SolGato]

How will you drive the propellors? Prop shaft thru hull?

Keep in mind not all electric motors are designed to handle the thrust load from the shaft and prop.

If you don’t like the idea of a belt/chain driven reduction there are inline gear reduction boxes available.

 

[caymantri]

Hi Caymantri.
What engines do you have in mind specifically?
For such a large and, I assume, heavy boat, they're a bit underpowered.

I plan on using a pair of golden motors BLDC 48 volt with their controller 10kw continuous and 14kw peak.
I don’t think they are underpowered there are 2 converted sailing catamarans using pairs of Epropution 6kw outboards one is an old Catalac 9mtr and the other is a 37’ prout and typically cruise at 3 to 4 kts using less than 2 kw per side. They are capable of more speed but have only about 4kw of solar and limit their speed to their intake. I will have close to 10kw of solar
There is another utube channel called “fantasia sailing” that uses the exact same motor as I want to use on his 50’ custom cat. He installed the electric on one side and left the 30 HP diesel on the other and has done extensive comparison. At full power he has got 7.5 kts from the single 10kw motor and he still has the sailing rigging he absolutely loves the electric but I think he is powering through a sail drive. I am trying to find out if he has gear reduction

 

[caymantri]

How will you drive the propellors? Prop shaft thru hull?

Keep in mind not all electric motors are designed to handle the thrust load from the shaft and prop.

If you don’t like the idea of a belt/chain driven reduction there are inline gear reduction boxes available.

Hi SolGato
I’m looking at 2 ways to drive. One is the conventional prop shaft thru hull if I use this I will need to have a shoe under the prop running back to the rudder so I can ground the boat safely and possibly bring it up on the beach on rollers

The other way is a little out of the box idea of using Thai long tail drives but mount the motors forward just inside of the hulls so the propellers are near a normal position using kick up rudders this would allow for a potentially shallower draft and complete safety for beaching to keep the shaft angle from being too great pods could be attached to the inside of the hulls closer to the waterline and probably access the motors from inside the hull

These motors are typically used with an extra thrust bearing to handle the load
I did find some gear boxes for reduction but if I find the the motors can efficiently the direct drive that would be my preferred option

 

[caymantri]

I plan on using a pair of golden motors BLDC 48 volt with their controller 10kw continuous and 14kw peak.
I don’t think they are underpowered there are 2 converted sailing catamarans using pairs of Epropution 6kw outboards one is an old Catalac 9mtr and the other is a 37’ prout and typically cruise at 3 to 4 kts using less than 2 kw per side. They are capable of more speed but have only about 4kw of solar and limit their speed to their intake. I will have close to 10kw of solar
There is another utube channel called sailing fantasia that uses the exact same motor as I want to use on a 50’ custom cat. He installed the electric on one side and left the 30 HP diesel on the other and has done extensive comparison. At full power he has got 7.5 kts from the single 10kw motor and he still has the sailing rigging he absolutely loves the electric but I think he is powering through a sail drive. I am trying to find out if he has gear reduction

Actually the utube channel is fantasia sailing

 

[montero]

The other way is a little out of the box idea of using Thai long tail drives but mount the motors forward just inside of the hulls so the propellers are near a normal position using kick up rudders this would allow for a potentially shallower draft and complete safety for beaching to keep the shaft angle from being too great pods could be attached to the inside of the hulls closer to the waterline and probably access the motors from inside the hull

I had similar ideas with combustion engines.
Maybe think about a solution like surface drive but with only vertical movement. It won't be very difficult.

 

[SolGato]

Whether or not to use reduction is also partly dependent on if you will have the power to spare with regard to range and runtime, displacement, sea states, windage, etc..

You see a lot of conversions with reduction because the vessels are high displacement, are hull speed limited, and people want lower voltage systems with a minimal battery bank size that provides good range and efficiency while not overworking the motor and electronics (reliability).

Maybe the best approach would be to design your drive system with the ability to easily incorporate reduction down the line if you find you need it.

One of the advantages of the types of systems sailboat conversions use is that you can play with the reduction ratio easily.

 

[caymantri]

I had similar ideas with combustion engines.
Maybe think about a solution like surface drive but with only vertical movement. It won't be very difficult.

Thanks Montel. Those Thai drives are almost surface drives or certainly can be by just raising them which I would be able to do

Whether or not to use reduction is also partly dependent on if you will have the power to spare with regard to range and runtime, displacement, sea states, windage, etc..

You see a lot of conversions with reduction because the vessels are high displacement, are hull speed limited, and people want lower voltage systems with a minimal battery bank size that provides good range and efficiency while not overworking the motor and electronics (reliability).

Maybe the best approach would be to design your drive system with the ability to easily incorporate reduction down the line if you find you need it.

One of the advantages of the types of systems sailboat conversions use is that you can play with the reduction ratio easily.

Whether or not to use reduction is also partly dependent on if you will have the power to spare with regard to range and runtime, displacement, sea states, windage, etc..

You see a lot of conversions with reduction because the vessels are high displacement, are hull speed limited, and people want lower voltage systems with a minimal battery bank size that provides good range and efficiency while not overworking the motor and electronics (reliability).

Maybe the best approach would be to design your drive system with the ability to easily incorporate reduction down the line if you find you need it.

One of the advantages of the types of systems sailboat conversions use is that you can play with the reduction ratio easily.

Thanks SolGato
I will design it so the reduction could be added later. I will first try direct drive with a low pitch prop and if it doesn’t work then I’ll add the reduction and go to a higher pitch. It will be interesting to see how it effects the amp draw on the motors related to speed

 

[montero]

I am not referring to the specific characteristics of the surface-piercing propeller itself, but rather to the transom pass-through assembly featuring a universal joint—specifically for the purpose of raising and lowering the propeller.

 

[SolGato]

As a point of reference, I have a pair of Torqeedo 10kW POD drive motors.

They use a planetary gear reduction coupled inline with the motor.

Gear reduction specs aren’t published, so I pulled one out and checked the ratio.

The ratio is approximately 5:1.

The motors turn at around 1100 Max RPM at the prop.

The specs for the props (diameter and pitch) are published, and I believe they offer a few styles dependent on if you are pushing a large displacement hull or light performance.

Compare that to an ICE equivalent spinning the prop at 2X the speed and you can see why people are often disappointed with the top speed performance of electric propulsion.

 

[rnlock]

It's my understanding that brushless motors aren't all THAT complicated and that it's possible to make them with low kv values.That is, relatively low rpm per volt. Such motors might be short but with large diameters. In model aviation, we often used gears with brushed motors, but now we usually use brushless ones that have higher torque at lower rpm and don't need the gears. Check out this old spec sheet for some motors:Joby motor specs If the rightmost motor can put out over 20kw at only 2,500 rpm, no doubt it can put out 10 kw at a lower rpm. In aviation, propellers are much larger because of air's low density. This puts a premium on low rpm, to keep the prop tips well below Mach 1. Otherwise, they rend to be noisy and inefficient. You might look up the notorious Thunderscreech aircraft to see what happens when this is ignored. I don't know what mods are required to keep a motor cool when it's on a boat, though. There are limits to just how large a prop is optimum with slow boats, though I imagine there are usually other constraints on diameter that are reached first.

Warning: One of my follow up checks indicated that the following was in the approximate ballpark. The other didn't. I'm now too tired to figure out which was right. I may have egg on my face. Sigh. In any case, even if my calculations are correct, they're only meant to be rough estimates

The Light Eagle human powered airplane had an 11.3 foot propeller, and something like one fortieth the power of one of your motors.,The flight speed was comparable to your boat's speed. Water is around 800 times as dense as air. So, an equivalent prop for you might be (sqrt 1/800)*(sqrt 40)*11.3 = about 2.5 feet in diameter.
I suppose another approach might be to find an outboard motor with a dead engine and an appropriate prop size. Don't they usually have reduction gears? You might have to do some significant mods to allow a large enough prop. I suppose you don't need quite as much efficiency as the Light Eagle, so maybe you could go a bit smaller. There are probably more suitable programs, but if one called Javaprop has a provision to change the density to that of water, it might be a good way to look at the tradeoffs involved.

 

[rnlock]

Hmm... If that 2.5 foot prop has a p:d of 1:1, and the pitch speed is 15 knots, you're looking at about 600 rpm.

 

[gonzo]

One of the issues to consider with large diameter propellers is the tip velocity. Too fast will cause cavitation.

 

[HelmutSheina]

I am not referring to the specific characteristics of the surface-piercing propeller itself, but rather to the transom pass-through assembly featuring a universal joint—specifically for the purpose of raising and lowering the propeller.

Also consider a CV joint if you can keep it dry. Volvo used them on earlier stern drives.