I'm terrified.. What should be the diameter gap for pod drive housing?

Hello, I'm a terrified amateur boat builder. I'm about to cut a hole through my boat..

I'm building a wooden boat. I have a pod drive that should go through the keel.
- The diameter of the housing is 120mm (4.72 inch)
- I will use a sealing compound 3m 5200 or tikalflex contact 12 to fill the gap between housing and the hole.

I have no clue how much the diameter of the hole should be?
Too tight and the metal housing may touch the wood unless perfectly centered and water will seep in? Too large and maybe sealant won't work?

I have a base plate that came with the engine with diameter of 122mm (4.8 inch) but it looks super tight.

Thank you!

 

The problem is not the diameter of the hole, but how to make it watertight. Look for one of these watertight passages in the store.

 

I would definitely make it round with 0.5 gap and what is important impregnate the hole with deep penetrating epoxy. After the resin has set I would fit, if it gets too tight I would sand it down. Precision will be hard to achieve.

Second option is laminate sandblasted stainless steel sleeve with retention studs with epoxy. Then I would fit the hole tightly, without sikaflex. Silicone maybe. And it could be taken apart.

 

0.5 is millimeter I suppose? Do I fill the gap with a sealant like sikaflex?

 

My plan was filling the gap between shafthousing and the hole with sikaflex or 3m 5200 for water tightness.

 

3M 5200 is designed to be used at 1/8" (3.2mm) thickness according to their tech support. Less or more can affect the cure.
So in this case 120mm plus 3.2mm/side = 127mm or perhaps 128mm (maybe not 130mm) diameter hole (would error slightly larger than smaller).
The idea above to seal wood with epoxy is good, then wait for full cure of epoxy, wash off amine blush, sand, then 5200.
Is the pod your own design or someone else's? Seems like a manufacturer would offer guidance in this case.

 

Make it watertight and durable, no matter hole diameter (within an order).

 

Well, that was not really helpful. Maybe I misunderstood you. Of course I want to make it watertight and durable, that is the end goal.
But I suppose 200mm hole on a 120mm housing wouldn't work. There should be a guideline how to do it?
Too tight fit and the sealant may not work, too large of a hole and sealant wouldn't work? There should be a general rule to follow maybe? Like Housing diamater + x gap with a specific sealant?

 

Please see my post above for 5200.

 

Ah, thank you! That was very helpful.

 

I am very sorry but I don't think anyone would be so bold as to give a figure for the diameter. It will depend on the sealing element, its capacity to penetrate the resulting gap, the shrinkage it experiences when drying, etc. Consult an expert because here, in my opinion, few will be able to give you a definitive answer.

 

What do the mounting instructions say? How is torque handled in this application? This isn't a window where you only need to account for differential expansion and must have a minimum gap, the shaft experiences thrust.
Be advised, 5200 is a permanent solution.

 

I don't consider curing conditions of 5200 , as TOPS mentioned above. You can always make test on some pipes then cut it trough and check.

 

As well as everything some have mentioned (read Rumars' post carefully) you should consider how the device is anchored to the hull structure.

 

Don't panic, just do it to the best of your ability, with due research into your sealant / adhesive, re- preparation, gap / thickness needed, temperatures optimal, compression / torque needed at the joint, all that fine print on the container, or on the data sheet (look it up). Seal your hole in the wood with epoxy once it is made and the fit is tested good. If it doesn't work out, it's easy to plug the hole with wood and epoxy, and try again in a slightly different way, as long as you don't damage your drive unit. Wood can be re-glued with epoxy. Silicone sealant can shrink and crack, mastic can be messy but re-doable, ask a local boat repairer what they would use; even bluetack would probably work for a while, but I wouldn't recommend it. Securing the base plate to the hull should be done properly, because if/when your pod hits something at speed, and wrenches your woodwork, you don't want to sink straight away. It may pay to make a longer base plate to spread any shock loads from the pod hitting something.