Use pressurized water as manuevering thruster?

SeaJay
The basic relationship is:

Force = Pi/4 * Water Density * (pipe ID)^2 * (Flow velocity)^2

So taking a 40mm ID pipe you will need a flow rate of 27.8m/s to get 1000N (1000/9.8kgf say 100kgf or 220lbf).

The required static head to achieve this flow is 400kPa (40m) (58psi). The pump power is 9.3kW without any losses. I would think 15HP would go close allowing for losses. The volumetric flow rate is 35lps (555GPM). This is the sort of flow required for a fire hydrant.

I looked at possible pumps and it will need to be at least 3". The attached link shows what a 4" pump could do:
http://www.godwinpumps.com/images/pdfs/cd100m.pdf
This has more than enough grunt. The 3" pump is marginal.

There are many pump manufacturers. I just found this one as it had a good range of simple pumps. You can get some nice compact electric powered pump of similar size but I did not find one with performance curves available on the www.

The length and size of delivery pipe is also important. I would be really concerned with water flow above 8m/s in a pipe so you need to have a delivery pipe of at least 75mm (3") and keep it short to reduce losses. If the intention is to run a good distance then you should determine the pressure loss and you might find it makes sense to make it bigger. It is normal to go larger than the pump outlet by 20% or so.

So you can see it is possible and not all that unrealistic. There are some other points to consider though.

Ideally you will jet above the waterline as having the jet below waterline will create back pressure and reduce performance.

Water from the nozzle at 27m/s could do serious damage to any nearby small craft if directed at them. Remember if something is placed in front of the nozzle it will experience a steady impact force of 1000N. Would easily capsize a small dinghy for example. 555GPM going into a small boat for a brief period could swamp it. So streams should be directed downward and just above the waterline

Trying to keep the pipe sizes down results in high velocity. You probably want nice butterfly valves or similar to give good control. Solenoid valves will cause hammer.

The nozzle has to be secured in such a way to resist the thrust. It is effectively a point load on the hull so may need localised support. Probably no more than any high integrity through-hull fitting.

This shows the sort of engineering required. If you have access to good hydraulics (as in water) designer then it may be worth doing more detail and comparing with more conventional thruster. The bigger the area the less power for the same thrust.

A force of 1000N is not insignificant and you might find that going down a bit in required force would make for a more economic solution.

The forces and flows you are requiring are in the vicinity of a fire hose. These are usually readily available in industrial sites if you want to get a feel for what you will be doing. You may have already had experience with such equipment. I would never man a fire hose single handed. They can generate large forces and need to be treated with care.

One thing about this solution is that you are dealing with readily available industrial components. It is not one-off type stuff for the luxury yacht market. You do need to have it engineered reliably though.

Rick W.

 

Rick,

Thanks for the detailed response. I'll spend some time with the numbers. However, the sizing you have arrived at would appear to push the system into the non-feasible range. (too heavy, too large of a pipe). But as you say, perhaps a smaller system would be suitable. Looking back at the Willdo system, I think they suggest the same thing. I see that they are recommending a system with only 60kgf and a 45mm pipe, and imply that their thruster works a little slower than conventional thrusters. With your information in mind, and reading their literature more carefully, I see where they disucss their "special thru-hull fittings". These likely are very important to their system performance.

However, you hit on my key concept that these components are off the shelf industrial units. Not to dismiss Willdo's work, but I'm betting I can come close. Anyway, thanks again for the input. I think I have what I need to start massaging the numbers.

Best Regards,

SeaJay

 

I had a look through the Willdo literature. The numbers stack up with what I have given above. They look compact, well engineered and neat. The price is reasonable - EUR1360 for a 30kgf thruster would be hard to better. I also think there is merit in minimising the length of pipe as they have shown.

I feel a bow thruster would provide just about all you need. I could not think of many situations where you would need both bow and stern.

Rick W.

 

Rick,

I’ve spent some time with your formulas and have a much better sense of the relative forces involved in using pressurized water as a thruster. However I am now a bit uncertain as to how Willdo is achieving their published thrust performance numbers.

Using information from their site and your formulas, the 30 kgf model would have a velocity of 15.3 m/s (30+ mi/hr) and a flow rate of 300+ gpm. The 60 kgf model, which would be recommend for my boat, would have rates of 21.6 m/s (48 mi/hr) and 430 gpm. As you have noted, and other sources have confirmed, velocities of greater than around 5 m/s start to have an erosive effect on pipes. I can readily see the potential harm that could arise if these streams were misdirected.

Estimating a complete Willdo set up at around 4,000 Euros (6,300 USD), it does seem unlikely I’d better the price and actually not a bad deal. Especially considering that the price includes the hydraulic pump, which could then be used for other applications. (However, I’m open to suggestions if any of you mad scientists out there have one!)

Thanks again for all of your help. This has been a very informative exercise.

Best Regards

SeaJa

 

A pipe operating above 5m/s is typically not recommended for continuous use or long length. With a system that is going to do a few minutes a month at most it is not going to do much harm to operate at much higher velocity. The main limit will be extra drag with length. This increases power requirement. The nominated power may be with short hoses. At some point the allowable pressure in the hoses will limit thrust.

You could put the question of pipe length to Willdo. What is the maximum allowable length. What is the power requirement for a particular system with a given hose length. What is the maximum allowable pressure.

Note the size of the inlet. This is the size necessary to avoid high suction loss as there is only around 15psi to play with in a small craft. I expect if you do the sums for this section of piping you will get a velocity under 5m/s.

Rick W.

 

Hello everyone,
New to this forum. I am in the process of building a 25 foot pilot type bass boat (displacement 3500 pounds) and just about to close the floor.
Been exploring this idea of a water jet thruster for a while and at this stage of building, have to decide if it's a go or no go with the whole thing.

I also believe that I can (maybe) better Willdo's price but also customize the controls to have some kind of directional control on the stern thrusters to move me away from the dock where depth is very shallow and could damage the prop on my outboard engine.

Rick,
Thanks for the formulae, I ran the numbers and come up with 215 GPM with hose ID 40mm and thrust of 15 KgF (according to Willdo). Maybe I missed something but can you tell me how to derive the required static head and pump power ? I would run on battery power alone and if the draw is too much, I will scrap the idea unfortunately.

Sea Jay,
I am curious to know if you went ahead with Willdo and how are their services after the sale ?

Thanks
Daniel

 

Daniel,

I am not to the stage of construction where I need to purchase the unit however, I am providing the necessary access for future installation. With my uderwater profile I just don't have the room for a 10" tunnel so the smaller piping of the jet system is what is driving my decsion. Otherwise I'd just plan on a standard propeller type thruster with likely larger aftermarket support. Please keep me posted of your developments.

Best Regards,

Doug

 

You can apply Bernoulli equation. This assumes perfect conditions for the piping. The pressure ends up at 62kPa for your conditions. Resulting pump power is 0.85kW. Realistically more like 1.5kW accounting for losses. You need about a 6" pump doing 1500rpm. You might be able to find some pump curves. If your pipe is long and only 40mm diameter it will chew up a lot of power.

Rick W

 

Rick and Sea Jay,
Just got a reply today from Willdo. They talk about a pump and dc motor assembly with 40mm ID hose for my boat.
I have to tell you that I find 215 GPM is a lot of water for a little dc driven pump. My pump dc motor search so far is less than good. The biggest I found dc driven was in the 10GPM range. I am wondering if you have any better sources than I in this field.
I will probably end up looking at this challenge the other way around. Find the biggest pump dc motor available and use Rick's formulae to find how much thrust it can give with a 40mm ID hose. This will probably be my go no go answer.
By the way, Willdo tells me that their unit draws 300 amps with the dc motor they have. I wonder where this motor pump assembly comes from.
Any suggestions are welcome. I will have to make my decision soon and would be dissapointed to close the floor without the unit only to find out it could have been done later.
Daniel

 

for reference:

http://duckworksmagazine.com/03/r/articles/jet/drive2.htm

 

Daniel
There are some very nice 240V pumps available like Davey but they do not come with 12V DC motor.

The biggest 12V stuff is for bilge pumps, solar circulating pumps and low voltage pond pumps. I doubt that you will find any in the size you need. I found a bilge pump that will produce about 2kgf for a fellow wanting to power a canoe in shallow water. Would not be much use for you.

You can certainly get low voltage motors that give you the required power off 12V but they are not coupled to small pumps. They are not sealed though so are not well suited.

The photos for the makeshift jet boat linked in the previous post gives you an idea of the sort of flow you will need. It is a lot of water. The jet will reach about 20ft high if directed upwards. Not quite a fire hose but still significant.

Rick W.

 

Rick,
Something bigger has to exist somewhere. Here is some info (attachment) from Willdo about a 20KGF 12 vdc driven pump. Looks nice and pretty compact too.
Does that give you any hints as of what kind of motor or pump type is used ?
Unless I am wrong, the 20KGF unit shows 12v, 2KW motor. In a previous email from Willdo, the rep talked about 300 amp draw. Some thing I don't understand is that 2000 watts / 12 volts = 166 amps , not 300 amps. Are they taking into account for losses or is my calculation wrong ?

Also, I seem to notice like a bypass tube on top of the unit.
If I can work this out, I would prefer having the pump going only on demand.
What I mean is I would not want the motor driving the pump all the time in a recirculating way until there would be a demand (opening a passage valve). Instead, I would like the motor to come on only when I would send a signal to the unit, then the motor would come on and the appropriate passage would open also. This would restrain the power demand I believe. Is that feasible ?
Thanks again for your insight and sorry for this long reply.
Seems to me the more I know, the more I find I don't know !
Daniel

 

When we went through this previously we concluded Willdo represented fair value for the system. The nominated power of 2kW for a 20kgf system is certainly close. The electric motor is likely to be around 80% efficient so you have to allow for this. Electric motors will easily operate above their rated power for short periods. The 300A might be related to starting or the 14V you get from the battery when being charged. If the motor is directly connected then rpm will be linear with voltage and power will be cube relationship. Hence current will rise with the square of voltage.

Personally I do not like the idea because it requires through hull fittings below the waterline. The security of your boat is reliant on the security of all this plumbing. Think how quickly you can sink it if a line fails and it allows 200gpm into the hull. So with this consideration you want very reliable plumbing. The electric drive is not too difficult to do on the cheap. Model plane motors can get close to the required power on a 12V system:
http://www.hobbycity.com/hobbycity/...80-100-B_130Kv_Brushless_Outrunner_(eq:_70-55)
These are not rated continuously but you only need burst power anyhow. You need an electronic controller because they a 3-phase motors. But the controllers are also quite low cost:
http://www.hobbycity.com/hobbycity/...hexTronik_PRO_120A_BESC_w/_PC_Programmability
I have a couple of smaller versions of these and they are outstanding value.

The issue with this stuff is that you need to separate the motor from any wet bits. Pump sealing is important. You can get magnetically driven chemical pumps but they start getting expensive.

Ultimately you get back to looking at Willdo as being fair price. There are a lot of considerations that they have already worked through. It is reasonable that they get something back for their development effort.

Rick W.

 

bow thruster

Hi i have looked at the wildo range who recommended a 25kgf, el25 pump and controls which can be used in a force 6 . I have a 25ft motor boat and decided to build my own. I used skin fittings just below the water line with 3/4 fittings at the bow, ajustable flow 12v dc solinoids and a single cyclone pump in a y connector from the head pick-up (19mm pipe id). The flow was rated at 100lmin at 0,1m head. wildo sujested a 500lmin pump at 2-3bar.

My system works in calm conditions at a very slow speed (ideal start) I am currently looking for a small pump that is close to the wildo spec. There pump is reversed engineered from bigger units used in the stena line oil rig support craft.

I am covinced that a good pressure pump may be better than high flow?

I used a wash down pump a hose and a rod to try this before starting work. since we only need a gentle nudge from time to time a small pump looks logical.

 

Click !

Hi !

post a picture of your pump if you have !
Thanks !