New Bow Thruster Tube bonding Into Old Thruster Tube

Trex450

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Hello Everyone,

I have an old bow thruster I’m replacing with a new one. Where the original tube is glassed to the hull is inaccessible from the inside without considerable work. And as such removing the old tube is not a practical option. Although not desirable, its common to glass a new tube into the original tube in such a challenging application.

The new modern more efficient Vetus thruster I’m installing has a smaller diameter tube and will fit within the old tube. The old tube ID is 8.375” and the new tube OD is 7.703”. The difference is 0.672” so there will be a 0.34” gap between the two tubes.

The typical process is to cut some holes in the top of the old tube, properly space the new tube within the old, glass the outside then fill the gap between the two tubes through the top holes cut into the old tube. There are more little details in the process but this is a high level summary of the main steps.

Filling the gap will prevent potential water ingress between the tubes. Everyone knows what will happen in the Northeast should water get trapped between the two tubes.

The question is what is the best material for filling the gap between the two tubes?

I was thinking of using epoxy with some sort of filler but I’m really concerned about the Exothermic reaction getting too hot with a large volume of epoxy. The gap volume is about 212.14in^3 which will take about 0.92 gallon of epoxy to fill. I have read to mix small batches and use slow cure hardener. I was also considering making a test rig with some PVC having a similar gap and running a test batch then monitoring the temp of the tube while it cures.

Does anyone have any suggestions of the best materials / fillers to use for such an application? Is epoxy the right approach?

Thanks for all your help.



Mike

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RobBaker

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Couldn't you just put a drain hole in the bottom of the old tube and not fill the void between the two tubes.
 

harpoon83

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Knowing little to nothing about bow thrusters, it sounds like it would be a whole lot easier to just replace the mechanical parts with the identical model that you allready have.

Is this not an option, is your existing model not made anymore?
 

Trex450

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Couldn't you just put a drain hole in the bottom of the old tube and not fill the void between the two tubes.
My only concern with this approach is the new tube would only be glassed to the outside of the hull. Normally its glassed on the outside and inside. Maybe if I were to use epoxy with some filler to make it nice and thick I could inject it into the gap about 1-2" deep around the full diameter from the outside of the boat and then glass over it. This would add extra bonding strength. Put in the drain hole as you suggested and not fill the rest of the void. That might work just fine. I do like filling the whole void though, it would be a really solid / strong installation.
 
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Trex450

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Knowing little to nothing about bow thrusters, it sounds like it would be a whole lot easier to just replace the mechanical parts with the identical model that you allready have.

Is this not an option, is your existing model not made anymore?
You guessed it. Its a discontinued Wesmar and the bevel gears in the drive are destroyed. No parts available. New Wesmar models will also need some glass work for the mounting of the new cradle. But I prefere the newer variable speed brushless motors and that requires a new, slightly smaller dia tube.
 

MAArcher

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What about making a fiberglass tube with a .34” wall thickness and use it as a spacer? Lube it up with adhesive before sliding it in.

I know nothing of bow thrusters other than I’d like one.
 

RIBen

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You could laminate additional glass around the new tube before installing it to reduce the volume of the casting. Will the motor accommodate being spaced off the tube by the additional thickness of the old tube and void.
 

MAArcher

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You could laminate additional glass around the new tube before installing it to reduce the volume of the casting. Will the motor accommodate being spaced off the tube by the additional thickness of the old tube and void.
After looking at the drawings more this is what I was thinking.
 

Trex450

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You could laminate additional glass around the new tube before installing it to reduce the volume of the casting. Will the motor accommodate being spaced off the tube by the additional thickness of the old tube and void.
Unfortunately the cradle is designed for the OD of the specified tube.
Originally I had the job quoted by one of the popular thruster installation companies. After the price shock I decided to do it myself. They mix up some sort of filler and poor it between the two tubes to fill the void. They didn’t get into specifics of what the mixture is they use.
 

RIBen

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Just cut away enough of the top of old tube that saddle has full access to inner tube and seal together in cutout before casting..
 

c1steve

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You could use West System Slow or Tropical hardeners to reduce the rate of cure, that is what I would do.
 
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