Externally Mounted Bow Thruster vs. Traditional Tunnel Type

[CarlF]

If drag was anywhere close to 10%, most tunnel thrusters on sailboats would have shutters.

In the early days of bow thrusters a cruising sailboat builder raced two essentially identical new boats (one with tunnel thruster, one without). They were trying to figure out what kind of shutters to put on the tube. They found the two boats had identical speed and forgot about shutters.

The theory is that as long as the tunnel is completely submerged the un-compressible water in the tube becomes a solid and smooth surface to the flowing water across the tube outlets and the laminar flow is unaffected.

Planing powerboats are a different situation and fairing at the tube outlets is important.

[CarlF]

I assume part of the external thruster sales pitch was that the housing didn't snag lines? Seems pretty easy.

My understanding from people who claim to know fluid dynamics, is that drag at the bow doesn't work as you'd expect. It's all about wave shaping. That's why large ships have those huge bulbs out front.

Maybe if you stuck the thruster on a bulb ahead of the bow you'd go faster

[Tonyjay]

CampbellsSloop while I can't comment on the differences in performance I believe there is a maintenance advantage to internally mounted. Our Lewmar internally mounted thruster has the drive shear pin mounted inside so access to change is easy and does not require haulout. We had ours replaced last summer - quick four bolt removal of the motor to access the pin on the drive shaft.

[boatpoker]

A little fairing in front of the tube makes sense, so that's the way I installed it (no fluid dynamics study).
I'd also suggest doing it yourself, it's a weekend job with a little knowledge and planning. Of that 10g's the yards quote, five and a half are labour

[Lancerbye]

I have a motorsailer with twin screws and a bow thruster, I seldom use the bow thruster, but still find it handy at times. I single hand mostly so docking has to be easy since I am too old to do dock jumping. The boat came equipped that way when I bought it so I have learned to enjoy it.
In the marina where I keep my boat, one guy has come up with a very efficient and cheap way of having bow control. The winds in this area are very consistently across the slips. So if you have a boat moored lee to the wind you have a problem holding your boat against the slip dock finger. The individual put an electric outboard like a Mincota mounted on a square long piece of aluminum tubing and made a clamping bracket on the bow. This is a 38 ft. sailboat with a single engine. The rig is normally left on the deck and is attached to the bracket just prior to entering the marina. He has the controls remoted to the cockpit. That setup can move the bow sideways better than most bow thrusters I have seen. It probably cost him around $300 total for the rig.

[delmarrey]

Originally Posted by Lancerbye

The individual put an electric outboard like a Mincota mounted on a square long piece of aluminum tubing and made a clamping bracket on the bow. This is a 38 ft. sailboat with a single engine. The rig is normally left on the deck and is attached to the bracket just prior to entering the marina. He has the controls remoted to the cockpit. That setup can move the bow sideways better than most bow thrusters I have seen. It probably cost him around $300 total for the rig.

Sounds like a two man operation. How does he start the motor and how does he control the throttle. And it must be fun to watch as green water comes over the bow when out to sea.

[CampbellsSloop]

Boatpoker - Some advice I can use. Thanks so much. Too bad you do not know the brand of the thrusters that developed corrosion. Supposedly, Yacht Thruster uses a totally sealed motor, but I will ask about a corrosion warranty. And, HopCar, I will let you know if I go with the Yacht Thruster external mount.

[DeepFrz]

del, I would like to see him looking for the starter cord on that electric outboard. Standing there scratching his head...

[delmarrey]

Originally Posted by DeepFrz

del, I would like to see him looking for the starter cord on that electric outboard. Standing there scratching his head...

Yeah, wasn't reading too good. still got sleep in my eyes.

[valhalla360]

Originally Posted by delmarrey

Sounds like a two man operation. How does he start the motor and how does he control the throttle. And it must be fun to watch as green water comes over the bow when out to sea.

Shouldn't be any worse than when they mount it on a fishing boat that takes green water. Now if you buy the $99 fresh water cheepo version, you are probalby correct.

We were considering this exact option for our Gemini (plenty of deck space to mount it) when we switched from a twin inboard power boat. We were going from a pair of V8's turning big props to a single 25hp outboard. But after getting the hang of it, we decided it wasn't worth the trouble and we take the Gemini into places, we would never consider with the old boat.

I think it's like insurance. If you get into an accident, it's good to have insurance but that doesn't mean you do things likely to cause an accident. It should aid good boat handling skills once in a difficult situation not encourage someone with bad boat handling skills to ignore learning how to handle the boat and get into difficult situations.

[boatpoker]

If you do decide to go with a tunnel thruster, consider going with a 24volt unit run off two batteries at the bow with a series/parallel switch. Much more power with much lower amperage draw.

[markpierce]

Originally Posted by boatpoker

If you do decide to go with a tunnel thruster, consider going with a 24volt unit run off two batteries at the bow with a series/parallel switch. Much more power with much lower amperage draw.

My 24-volt thruster works quite well using the house batteries (Dual 24 volt 200AH) located 10 or so feet away from the thruster. (The boat's electrical system is 24-volt, with 12 volt DC and 110 volt AC peripherals.)

[Pegleg Sands]

Originally Posted by DeepFrz

del, I would like to see him looking for the starter cord on that electric outboard. Standing there scratching his head...

I much prefer the electric start electric motors

[cappy208]

@CampbellsSloop: I have never had the opportunity to use a thruster on a yacht. Sure, seen it done many times. But your base question is answerable. Having worked on a 525' tug and barge with a wheelhouse controlled thruster they certainly can allow you to go into many tight holes that you otherwise would need a little tugboat to get you out of trouble. BUT, as others have mentioned, the usefulness of an external thruster will change the way you boat. I am surprised no one has mentioned it. Don't you ever beach your boat on a sand bar so the grandkids can jump off and go to the beach? That would kill an external thruster immediately.

If money is no object, I would go with an internally mounted, with internal access to the motor, via a waterproof tube and hatch in the bottom of the forepeak or under the Vee berth. They design them so the top of the hatch is ABOVE the water line to prevent haulouts for motor replacement and maintenance. The down side, to replace the prop still requires a haul out or a diver (who knows how to do it)

I don't believe any of the hype of the salesman. Internal, external, if .2 of a knot is going to kill you then you probably need bigger engines and fuel tanks!

[botanybay]

Originally Posted by robert sailor

Is there any support for that 10% loss of speed with a tunnel bow thruster??

That was for a very specific boat / configuration. Also all of their studies were from pulling so equiv to dead down wind. I wonder about the external appendage when going to weather. Drag forward of the keel is significant.

I spent some time trying to quantify the impact of a tunnel thruster and was not able to get a consistent answer.