Bow Thruster alternative

[amanphoto]

I was wondering if I put two through hole fittings the bow of my boat. One on each side. Then connect two a high flow water pumps to them. If I pump water out the fitting on the opposite side of the boat that I want the bow to move to. Will it shift the bow? I don't need a lot of power. Just enough to shift it over for docking. I have a 25' fiberglass boat. My thought is this will be a cheep and easy way to aid in docking. I've seen trolling motor type that are bolted to the bow, but they look ugly. Thoughts?

 

[Silvertip]

Whether this would work or not depends on lots of factors so here are a few things to consider:

1) You probably plan to use an electric motor to drive the pump. That would require a big motor/
2) What size plumbing are you thinking about for the thruster? I'm not a hydraulics engineer but I would guess you need something on the order of 2-1/2 inch diameter or larger.
3) The volume of water you need to push to move the boat requires a big pump (high volume) which again requires a big motor which requires something more than 12 volts and probably on the order of 36 volts which requires a boat-load of batteries.

Are you now thinking this may not be the best way to destroy the value of your boat?

 

[H20Rat]

No... Simply not feasible in the size boat you are talking about. The size of the tubing and required pump would be LARGE

 

[amanphoto]

http://www.hollandmarineparts.nl/jetthruster

Something like this. But smaller scale.

 

[Frank Acampora]

Newton's law: For every action there is an equal and opposite reaction. Thrust is a function of mass times velocity squared--that is a small amount of water at a very high speed will generate the same thrust as a large amount of water at a slow speed. That is why the jet system can replace the propeller bow thruster.

However, you can not go off half cocked! You need to calculate the amount of thrust needed to move the bow sideways at a decent speed and you need to calculate the amount of thrust delivered by your nozzles. Then you need to determine the size of the pump necessary to deliver the water at the head you require. Just throwing some components together into a thrust system may simply waste time effort, and have un-necessary holes in your boat.

Obviously, any amount of thrust will have some effect but you don't want to spend all day nestling up to the dock.

 

[Slip Away]

Waste of time, money and re-sale value of your boat. Just learn to deal with currents and wind and how your boat handles it.

 

[H20Rat]

That is why the jet system can replace the propeller bow thruster.

But... It doesn't replace the power requirement, in fact, it actually increases the requirements. Still need room and power for a sizable pump system.

 

[K-2]

Waste of time, money and re-sale value of your boat. Just learn to deal with currents and wind and how your boat handles it.

Exactly,
Practice Practice Practice

 

[smokeonthewater]

soooo to put this in perspective you would need AT LEAST a 10 hp pump...... The fine print, is that your boat really isn't big enough to be putting a bow thruster on.....

practice practice practice

 

[amanphoto]

No Title

Patrice is one thing. The fact is that I back my boat into a spot that has 18" on each side. The front doesn't pivot when I swing the back around. A small burst of a thruster would help. I have get close - jump off the back and pull the boat into it's spot.

 

[Illinoid]

a 12v trolling motor can put out over 50lbs of thrust but they are designed to be very efficient. It would be hard to find the right pump to efficiently move the water you need to move and size and curves of the plumbing all affect efficiency of the system. The pump could have a very short duty cycle like a starter on a car.
I think the pump should be high volume, low head, I can't see how a low volume, high pressure 12v spray pumps would do any good, but a 12v sump pump will move a lot of weight of water from one side of the boat to the other. I have considered doing this for my toon but hadn't figured out how to overcome all the problems and I always trailer so practice is the solution to my maneuvering challenges.
The simplest solution is a bow mounted trolling motor...IMHO.

 

[smokeonthewater]

there are other ways to get docked... such as hooking up a spring line as you start backing in so it pulls you tight to the dock...

IF you still feel that you need a thruster the only practical way to get it done is open the pocket book and have a real thruster installed...

My last boat was a 26' single engine carver... when I first got it I tried (like you) to figure out how I could get a thruster on the cheap.... after lots of practice I could slide it into any slip easily.... often I needed to go faster to be able to maneuver than what I had previously been comfortable with....

Eventually the only time I had any trouble at all was with strong cross wind and current and that is tough with any boat...

a boat hook goes a long way toward getting you in and out without issue

 

[Slip Away]

Boat hook and spring line, two things every slipped boat should have in use. Much more practical than jury rigged bow thruster.
18" on each side is plenty of room to back that small of a boat in to it's slip.

 

[muskyfins]

Like the OP, I try to find creative (home made) solutions to problems. I agree this is impractical, but I also feel like we've been a bit harsh on the OP. I hope he doesn't take it personally. (or maybe I'm just feeling overly sensitive today?)

Some of the posts have the fluid dynamics incorrect. We use a 10 HP pump to pump water up about 150' in a 2" pipe. This equates to about 66 PSI or about what a strong tap water supply is. You don't need anywhere near this level of power to move the bow of a 25' boat. I haven't done the calculations, but imagine what a 10 HP kicker would do? The problem with a sump pump is that it moves large quantities of water at low velocities. To make a pump powered system work, you need a high velocity pressure. (essentially the mass x velocity of the water at the exit point. As opposed to the static pressure which is what we usually think of when we say "pressure") So the interals of the pump are what makes it succeed or fail. Just like prop selection, we need to find the correct volume and velocity and impeller to create them and then determine how much horsepower it would take to make that happen. Plus any loss through the piping and fittings. I found a pump capable of moving 60 GPM at 60 feet TDH. Roughly 26 PSI. Probably good enough to move the bow for what the OP expects. Takes about 2 HP. The pump I found costs around $1500 in 220 VAC format. Probably close to $2000 as a DC system and not sure what voltage but at least 36 , maybe 48. Now you need 4 more batteries and probably an additional alternator on the engine, which would be a custom job. Pipe, fittings, hull fittings, wire, fuses, switches, valves. Suddenly, the costs do indeed exceed the bolt on trolling motor type or a regular hull mounted thruster.

 

[Silvertip]

Although the OP has indicated he intends to fabricate his own cheap thruster system, a few things in the linked brochure should raise some questions. Such as:
1) The pump must be located below the water line. Priming and continuous flow issue.
2) The smallest "50-size" example in the brochure indicates their pump requires a 6KWh 12 volt motor. Can you say expensive -- even used.
3) This unit draws 850 amps and they recommend a 165 AHr battery (which provides 4 minutes operation max). Obviously doubling up batteries would double that time but maybe 4 minutes once a day is enough time.
4) The installation drawings show that this type of unit occupies a good amount of space - space that a 25 ft boat may not have.
5) I used their calculator to price the "30" size unit and came up with over $4,000. To "cheap it" would likely run in the $1500 range unless most of the parts were free or a ludicrous lucky find occurred.
6) A household sump pump may or may not work but they designed for submersion so the inlet issue would need to be worked.

Without delving into this any further, it just seems to be a poor choice from an economic standpoint, impractical for the size of the boat, and that very system may scare away any potential buyers in the future. But -- as was mentioned, if this is something that needs to be done, go for it. We would all be riding horses and still wishing we could travel by air if some folks never took a shot.