Jet bow thruster thoughts

Bobc

Bobc

Well-Known Member
Joined
20 Jan 2011
Messages
10,430
Visit site
I quite like the idea of the water jet bow thrusters, but not the price tag.

I was wondering whether you could "DIY" a solution using a swimming pool/hot tub 3hp pump powered from an inverter. Just thinking that they tend to only be running for a few seconds at a time, so wouldn't kill the batteries.

Thoughts please....
 
You don't say what size vessel you have. Personally, I feel that they are a complete waste of time for anything under 20 meters, I crew on a 30 meter vessel and rarely use it and have been known to sail off the pontoon, but I am sure the skipper was showing off.
 
Not thinking about a specific boat here, just wondering it's possible/do-able/sensible etc.
 
Bobc said:
I quite like the idea of the water jet bow thrusters, but not the price tag.

I was wondering whether you could "DIY" a solution using a swimming pool/hot tub 3hp pump powered from an inverter. Just thinking that they tend to only be running for a few seconds at a time, so wouldn't kill the batteries.

Thoughts please....
Click to expand...
Learn how you boat handles & save precious wiggly battery amps
 
I suspect that hot tub pumps are generally a) mains powered, & b) nowhere near powerful enough. Running a mains pump via an inverter is OK but you will need a big inverter to cope with the starting current. As a pump is directly coupled the motor starts under load, which can be up to six times running current for an induction motor.
 
Last edited:
Bobc said:
I quite like the idea of the water jet bow thrusters, but not the price tag.

I was wondering whether you could "DIY" a solution using a swimming pool/hot tub 3hp pump powered from an inverter. Just thinking that they tend to only be running for a few seconds at a time, so wouldn't kill the batteries.
Click to expand...

Not feasible really in the way you're suggesting. Water jets are less efficient than conventional bowthrusters. I have a Sidepower SE60 thruster, which has 60kgf thrust and uses 3.1kW. The less powerful 50kgf Jet Thruster has a 6kW pump. You can buy 6kW swimming pool pumps, but they're about £2500, then you'd need all the other kit. And of course you'd need a big inverter - rather bigger than 6kW to allow for the motor's starting load. Before you know it, you'll be up to the £7K cost of the Jet Thruster, with no guarantee that it'll work until you actually try it.
 
Why not drive it directly from the engine. Mind you I had some fun in a survival vessel for an oil rig that had a system like this to assist departure from rig in all weathers and it was certainly a lot more than 3 HP. It took a noticeable amount from the propulsion when activated but certainly was effective when broadside to the wind.
 
pvb said:
Not feasible really in the way you're suggesting. Water jets are less efficient than conventional bowthrusters. I have a Sidepower SE60 thruster, which has 60kgf thrust and uses 3.1kW. The less powerful 50kgf Jet Thruster has a 6kW pump. You can buy 6kW swimming pool pumps, but they're about £2500, then you'd need all the other kit. And of course you'd need a big inverter - rather bigger than 6kW to allow for the motor's starting load. Before you know it, you'll be up to the £7K cost of the Jet Thruster, with no guarantee that it'll work until you actually try it.
Click to expand...
So what magic motors are these guys using then?
 
Bobc said:
I quite like the idea of the water jet bow thrusters, but not the price tag.

I was wondering whether you could "DIY" a solution using a swimming pool/hot tub 3hp pump powered from an inverter. Just thinking that they tend to only be running for a few seconds at a time, so wouldn't kill the batteries.

Thoughts please....
Click to expand...
I thought that many years ago Vetus produced one for sub 40 foot boats but it disappeared after a few years. I think that it was hydraulic which didn't help on a small boat.
 
Yes, I have considered using a DIY water jet pump as a bow thruster. There are essentially two options: i.e a reversible pump or to direct the jet via the valves. My brother, who is a mechanical engineer, and I went through the possibilities and found there would be considerable savings over an off-the-shelf solution.

The main reason for considering the jet option was to avoid a 6" hole through right through our deep forefoot, adversely affecting what modest windward ability we have.

Since then, I have cleaned up and refined our rudder and deadwood. This has dramatically improved maneuvering and made the need for a truster unnecessary.
 
Sandy said:
I crew on a 30 meter vessel and rarely use it
Click to expand...
The thing about 30m vessels is that they tend to get massive, easy to use berths, so I'm unsurprised you rarely use them. Smaller vessels get tight and difficult berths in fairways smaller than the boat length and only slightly wider than the boat plus fenders. They also get blown off faster and generally damage easier than larger vessels. As such I'd argue that, to a point, smaller vessels need thrusters more than larger ones. That's not even approaching the fact that most boaters don't have the time or inclination to practice such manoevres when a perfectly suitable alternative is widely and cheaply available.
 
Bobc said:
A 2hp pump can shift 13litres per second. What's the force of that through say a 1" nozzle?
Click to expand...
The effect of pressure is negligible. Power is pressure difference times volumetric flow rate, so if you have 150W moving 0.013 cubic metres per second that's a pressure difference of 150/0.013 = 11.5 kPa, which across a 1" nozzle is 0.596 N or, if you prefer, 0.6 kgf.

What really matters is thrust from the momentum, which is density x volumetric flow rate ^2 / area = 333 N = 34 kgf. For comparison, the smallest standard Vetus bow thruster is 35 kgf and is suggested for boats from 7.5 - 11.5 m.
 
Last edited:
You must log in or register to reply here.

Share this page

Top