Mounting HDPE...through bolt with sealant

Tin Man · Tuesday at 10:06 AM

I am preparing to install my new Lowrance HDS PRO 10 with 3 in 1 HD Active Imaging transducer.

I have mounted HDPE blocks on the transoms of several boats. All were epoxies using flame treatment and West System G-Flex. The were all still attached after 2-3 years. However, this new transducer is larger, expensive, and heavier so I am considering through bolting the HDPE black using sealant between block an hull.

Questions:

1. Are there are any drawbacks to through bolting? I was planning on two (one upper, one lower) #10 SS bolts with flat and lock washers and nuts.

2. What adhesive is preferred between HDPE and painted alum hull? I am using a 5"x 7" x 1" HDPE block.

3. Ond of the attached pics show my inner transom access. The orange pencil (bottom) shows where I placed a bolt through existing transom hole (from previously mounted transducer that was screwed) and the second hole will be drilled 4" directly above it . As you can see, there is a 3"x 3" alum tube (part of transom reinforcement)....I would use the current hole (bottom) and one additional hole above the tube. Bolt holes would be 4" apart (as show on blue tape placed on HDPE block.

NOTE: There is not a lot of room to work with inside transom and the bolt on bottom would require me to use "feel" only when attaching the washer., sealant, washers, and nut.

Thoughts?

Thanks!

airshot · Tuesday at 5:02 PM

If it were me.........I would add a bigfer, longer block just in case any future add-ons might be needed. One never knows what the future has in store. This way once you have two good bolts holding it in place, you won't have to do this again !! If it were me.......bigger block and a couple bigger bolts then add-on whatever the future brings !

Tin Man · Tuesday at 6:24 PM

airshot said:
If it were me.........I would add a bigfer, longer block just in case any future add-ons might be needed. One never knows what the future has in store. This way once you have two good bolts holding it in place, you won't have to do this again !! If it were me.......bigger block and a couple bigger bolts then add-on whatever the future brings !

when you say, "bigger," what does bigger mean....taller, wider, ? It is 1" thick which is plenty for this application. I do not have much width....to the right is the hull's chine, to the left is the live well intake. I was considering a taller block....to encompass the upper hole seen in pic (this upper hole is used for a cable clamp).....it would make new block about 9" tall. This would mean I would be reusing current existing holes for this project....NO new holes would need to be drilled. However, I do not believe I would ever need to mount transducers "higher" as most, if not all, transducers mount within 1"-2" of variance of suggested height.

airshot · Tuesday at 8:26 PM

Just expressing an opinion, if it were me....I would put as large a piece as I could make fit as a just in case....been there, done that....never have I been sorry for going bigger, but many times have I been sorry for not making it bigger! Your call, your boat....with all the work and difficulty of getting a piece mounted, I would use that effort to go as large as I could. If you never need it, so what...but if you do ??

guyg · Tuesday at 8:45 PM

I wouldn’t trust glue on hpde at all. In industrial applications, it’s always heat welded. Through bolting should work well, but seal the holes well with 3m 5200 or similar. Good luck!

thill · Wednesday at 8:49 AM

I have done this on every fishing boat I've ever owned. I think it's a very good idea.

I use either (2)1/4" bolts or four, depending on the size.
I always countersink the heads flush, with just a tiny bit of wiggle-room in the holes
Loctite "Stick'n Seal" is my favorite for doing this. "SealAll" is good too.
The trick is to sandwich the sealant between the block and the transom.
I typically run a bead around the perimeter, then beads across the back of the block, with emphasis of full, thick circles of sealant around each bolt/hole.
When you press the block in place and tighten the bolts, the sealant should squeeze out around the full perimeter, and also all around the bolts. You should generally see it squeeze out around the washer on the inside in an aluminum boat Clean it mostly up, making the bead of sealant around the perimeter of the block very small.
I like to torque 90% initially, then tighten the nut on the inside a little bit more the next day to physically compress the "gasket" a little. Do NOT twist the bolt head after the sealant dries, or you can break the outer seal, which is the most important.
For the transducer, I oversize the screw diameter as much as possible and pre-drill the holes for the screws. Also, I use screws longer than the block is thick, then cut them off just short of full thickness. This gives me full bolt strength and prevents the bolt from pushing the transom block away from the hull.

This methodology has been 100% successful for me. No corrosion, no wet transoms, no issues, regardless of how many holes or 'ducers I put in the block.

I agree with Airshot, about making it as big as possible. Most of my boats have 2-3 transducers back there, although my current tinnie only has one. Strangely, not sure why, but when you have 3 transducers side-by-side, they all read BETTER at high speeds than when alone. Alone, you have to get them set pretty perfectly.

A trick I learned is to set the BACK of the 'ducer slightly lower than the front, if you are having issues at speed, and the transducer is at the proper height. This keeps the water running across the bottom clean and pressurized. It works much better than burying the 'ducer and causing a roostertail.

Hopefully this is helpful.

Tin Man · Wednesday at 1:16 PM

What are the thoughts on the size I have shown based on the cardboard template shown in pics? It is approx. 5" wide by 6"-9" tall by 1" thick. I angled the bottom to follow angle of hull.

As mentioned, the hull chine is to the right of my current proposed placement so I making it wider to the right would not be useful, as it is not recommended to mount transducers directly over chines.

Wider to the left puts me at the live well intake...not sure I would ever mount a transducer right next to this through hull.

Also, I plan on using (2) #10 SS bolts with nylok nuts. The #10 size because the current holes in hull at bottom are this size. I can't imagine 2 #10 bolts would not be strong enough.

For sealant....I'm considering one of the following. Loctite is a polyether, Boat Life Like Caulk is a polysulfide, and 3M 4200 is a polyurethane. I don not want a permanent adhesive such as 3M 5200. I prefer a high grade sealant more than an adhesive, as there will be bolts used for attachment.

https://www.loctiteproducts.com/pro...ite-pl-marine-fast-cure/SAP_0201XFL02WQG.html

https://www.boatlife.com/product/life-calk-sealant-tube/

https://www.3m.com/3M/en_US/p/d/b40066971/

thill · Thursday at 6:16 PM

What you have looks fine, as close to the livewell pickup as possible. Two screws should be fine for that one. I gave you my choices of sealant. Not so thrilled with those you chose, but I think others use them with not too many issues.

Mounting HDPE...through bolt with sealant

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