Bow thruster install

[json]

I am going to be installing a bow thruster at some time in the near future, and was reviewing the images in Grady300's build thread of the cad drawings that Brian did for his install. I put my crash chamber bulkhead too far forward to use the measurements in the drawings, so, I have a couple questions around it.

Is there a hangup putting the tube further toward the bow? I believe there is a minimum width of the bottom of the tube, so as long as that requirement is met, and the minimum depth of the tube is also met under the waterline, is there any other reason that the tube needs to be located further aft as in the drawings?

If the tube is far enough forward such that the back of the tube becomes a perpendicular face toward the direction of the boat traveling, is there any reason to reenforce that back edge and round it off so that the water would flow off toward the outside of the tube?

I am attaching the drawings from Grady's build thread for reference.

[Brian.Dixon]

The bow thruster is placed where it is 'cuz that's where it works and fits the best, not because of where the crash chamber is.  Can you make it work as shown in the CAD drawings?  Also, you do want the tube-to-hull seam to be rounded nicely and well-glassed, and you should build up the forward edge to it deflects water over and past the hull (impossible, but it's recommended by the thruster company).

[json]

Thanks Brian. I could put the tube where the drawing shows, but I would probably be losing most of, if not all of, the floor space in the cuddy. If I could move it maybe 12-18" toward the bow it would make that a lot less. I could also move it aft about a foot but then I start running into the stringers so I don't think that's going to work. I would like to keep at least enough floor space in there to keep the head in there (a 5 gallon bucket with a seat). So since moving into the dead space inside the crash chamber isn't really feasible, maybe making another space just aft of the crash chamber could work. Definitely curious to hear your thoughts.

[Brian.Dixon]

 ... If you move the thruster tube further forward, then the tube gets too short.  Check the thruster docs ... (I'm going by fading memory here), but I think the thruster requires a certain minimum tube length.  And as you point out, moving the tube further aft conflicts with the stringers - basically cutting them off short.  Since this is not a deep-V style hull, it's a tight fit getting a thruster into it ... not even possible until Lewmar offered this one.  I believe Chuck Mazzola enclosed his thruster tube in a 'box' ... the result looked like a low cuddy deck with a stair step f'w'd.  It costs much more, but there's another style thruster that may give you more foot space in the cuddy ... kind of a dual jet, a hose coming down each side of the hull, each with a turn outward.  Can't remember the brand but you'll find it if you search these forums or look through photos on our Facebook page.

[json]

The docs state that the length of the tube should be 2x the tube diameter (which is 5.5" or close to). And actually, let me just include the relevant part here for the sake of discussion and future builders possibly doing research:

The actual position of the Thruster will depend on the internal & external construction of the Motor Boat or Sailing Yacht. For optimal performance the Thruster should be mounted within the following:

‣ As far forward as possible (Fig 3.1.1 / lever effect).
‣ 1 x Ø below the waterline to prevent air being sucked into the tunnel. (Fig. 3.1.2 / 0.75 x Ø minimum.).
‣ Minimum suggested tunnel length 2 x Ø.

NOTE: Ensure there is sufficient space for the Thruster assembly complete with motor and controls in the boat.

Ø = Tunnel Diameter

https://www.lewmar.com/sites/default/files/110TT-300TT%20Gen%202%20Issue%205.pdf

I had posted about the jet thruster not too long ago, it looks like a really cool option but was also going to take some work to get to fit well. If my memory serves the power consumption requirements were really heavy for that as well (it needed a battery capable of something like 500 cold cranking amps, so would have most likely required me to install a dedicated battery and all that entails). I contacted Lewmar support for clarification and additional feedback about their suggestions. If/when they get back to me I will update this with anything relevant.

[json]

I spoke with Lewmar's customer service rep, and gained a bit of additional information that was good. He didn't answer my questions though really, but searching around I feel like I have a somewhat ok grasp on what the trade-offs are. The relevant part of his response (where he wasn't telling me that I need to have a professional shop install it) is this:

"The further away from the barycenter the thruster is the more leverage it will have to push the bow (or stern).  The tube needs to be far enough below the water line so that the exit water does not interact with the surface or thrust will be lost.
At the same time you have to keep the tube far enough away from the hull bottom so there is enough strength to prevent it from breaking."

I also found this document that details lots of things around tunnel thruster installation:
http://www.marinewarehouse.net/images/sidepower/Tunnel_Installation_Guide.pdf

The question that seems like biggest shortcoming of installing the tube forward of the cad drawing is that the length of the bottom of the tube might be shorter, and might be closer to the surface and/or closer to the keel. The recommended length of the tube is 2x the diameter of the tube, diameter being 5.5". I was reviewing Chuck's install, and just ball-parking from the pictures it doesn't look like his tube is 11" long. I did speak with him about it though and he said the system works great.

So, couple more questions for you Brian. 1. Do you have the shortest length of the tube as designed in the cad drawing? 2. How close can the tube be installed to the keel to maintain structural integrity?

[Brian.Dixon]

Here's some more dimensions ... I think that Chuck and I came up with the best compromise installation that tries to maximize thruster performance without compromising boat structure, and also works well in getting the thruster tube above the on-plane waterline (boat rises, takes on a 2-degree bow-up trim at 21 mph).  Note below that moving the tube further forward also starts to cut into the interior fairbody filet and glass.  Personally, I'm surprised that we were able to squeeze this thing in and have it work well.  Deep-V boats have the advantage here ... the modest deadrise of the Great Alaskan or Kodiak makes bow thrusters rather challenging:

[json]

Ah, interesting. Ok, thanks for the info Brian, seems that the balancing act is a little tighter than I thought. Let me see what I can do to make this work as designed. I appreciate you taking the time to help me see the light.

[Brian.Dixon]

Ah, interesting. Ok, thanks for the info Brian, seems that the balancing act is a little tighter than I thought. Let me see what I can do to make this work as designed. I appreciate you taking the time to help me see the light.

Never a problem ... keep asking and I'll keep trying to answer.  What we need for these boats is a smaller tube diameter for a thruster.  I know they can do it, but will they?  The Great Alaskan / Kodiak has the shallowest deadrise that I've ever seen a bow thruster on ... fortunate that it can (now) be done.  But as you point out, there are trade-offs - mostly in foot space in the cuddy.  Fair enough, I guess.

[json]

They make a thruster with a 4 5/16" diameter tube, with 62 lbs of thrust - https://defender.com/en_us/lewmar-110tt-1-5-bow-thruster-complete-package-with-tube-joystick-cable-591102

Is that worth checking out?

[Brian.Dixon]

They make a thruster with a 4 5/16" diameter tube, with 62 lbs of thrust - https://defender.com/en_us/lewmar-110tt-1-5-bow-thruster-complete-package-with-tube-joystick-cable-591102

Is that worth checking out?

Chuck has the 140TT?  92 lbs thrust?  The 62# version might be fine since the Great Alaskan is not a deep-V and is easier to push around sideways.  I'd call their tech support and ask ... I'll bet the 110TT is a just-right answer and Chuck's probably has extra power?

[json]

I will reach out to their support and ask. A smaller tube allows the installation to move forward right? Doesn't have to be as long, doesn't have to have as much vertical space to work, also more torque generated if it's further from the pivot point...

[Brian.Dixon]

I will reach out to their support and ask. A smaller tube allows the installation to move forward right? Doesn't have to be as long, doesn't have to have as much vertical space to work, also more torque generated if it's further from the pivot point...

Possibly ... what you'll have to watch is the tube rising higher as you move forward due to the rising filet/glass as you go forward ... you have to maintain at-rest depth requirements to keep the top of the tube below the waterline.  I'd say that if you can keep the top of the tube no higher than the top of the 5.5" tube was, that you likely meet the depth requirements.  I'll stick it in CAD and see how much further you can go forward with the smaller tube, and will estimate the arm from center of tube to (typ) pivot point in the boat.  Tech support might want that info.

[Brian.Dixon]

I will reach out to their support and ask. A smaller tube allows the installation to move forward right? Doesn't have to be as long, doesn't have to have as much vertical space to work, also more torque generated if it's further from the pivot point...

According to my CAD estimates, it looks like the 4-5/16" tube can be moved forward up to 18" or so and still maintain the required depth versus the waterline and not cut into the fairbody fillet ... not sure how that lines up with the collision chamber bulkhead .. didn't look yet.

[json]

Thanks Brian! 18" is a good amount to play with, I think I should be able to find a good compromise in there. Can you include details on how to locate the initial tube in relation to the keel or other hight reference so I can use it to make sure I don't locate the tube too high? I think I should be able to eyeball not getting into the fillet or tape on the keel but I want to make sure I don't put the tube out of the water either. I can probably figure out how to translate the original measurement forward too, so if you have something there that I could use that would be super helpful!