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Author Topic:   KING StarBoard® Transducer Mount
1985supersport15 posted 06-10-2008 08:43 AM ET (US)   Profile for 1985supersport15   Send Email to 1985supersport15  
I have decided to go with a KING StarBoard® plate to mount my tranducer to aid in adjusment and so it will be easy to add a spray blocking bracket at a later date. From what I understand 0.5-inch thickness of KING StarBoard® is more than adequate.

Will two screws be enough to hold the KING StarBoard® to the transom?

And if so will I need to get screws 0.5-inch longer than the manufacturer provided in order to make up for the purchase lost into the hull from the KING StarBoard®?

Countersink screws into starboard for cleaner look?

Do I want the screws to thread into the starboard at all or just the hull?

And would I still countersink the holes into the fiberglass?

Will marine 3M-4200 or 3M-5200 provide a good seal where the KING StarBoard® meets the fiberglass? I have heard these adhesives don't adhere to KING StarBoard®.

Lastly to attach the transducer to the KING StarBoard® just drill pilot holes shallow of the fiberglass? Is sealant needed?

What kind of screws to mount transducer to KING StarBoard®

Square block of KING StarBoard® or fabricate to match hull curve?

Apologize for all the specific questions just want to do it right.

If anyone has experience and pictures of their installation I would greatly appreciate it.

Newtauk1 posted 06-10-2008 09:26 AM ET (US)     Profile for Newtauk1  Send Email to Newtauk1     
I would suggest using something thicker than 1/2".I used 1/2" and now would have used something thicker. I used 2 screws to hold the Starboard to the hull. Counter sunk. I trimmed and beveled the edges to match the contour of the hull. 3M makes an adhesive for Starboard. I did not use it. I used 3M5200.
I will email you a photo.
jimh posted 06-10-2008 09:30 AM ET (US)     Profile for jimh  Send Email to jimh     
The concept of using an intermediate plate to mount a transducer on a transom is to allow all the fasteners for the transducer mounting bracket to be located only in the plate, not in the hull. A plate of 0.5-inch is probably the minimum, as that will provide just enough thickness of material for the transom mounting bracket screws to keep them out of the hull itself.

If you are working with a fresh transom that is not already full of holes, I would be tempted to forego the plate and just mount the transducer bracket directly to the transom. If you follow the advice of the transducer manufacturer, you will probably get it right on the first try; you won't need a plate.

Marlin posted 06-10-2008 12:52 PM ET (US)     Profile for Marlin  Send Email to Marlin     
I mounted my transducer directly to the transom, but I can really see the logic of an intermediate pad. You put on a brand X sonar, then 5 years later you put on a new brand Y sonar -- oops, the mounting hole spacing is a little different, time to drill a new hole. Given the typical long lifetime of Whalers, you could really end up with a bunch of holes back there.

If you use a pad, you only have 2 holes in the transom. If you replace the pad because it has too many mounting holes in it, you can mount the new one in the same 2 transom holes.


1985supersport15 posted 06-10-2008 02:18 PM ET (US)     Profile for 1985supersport15  Send Email to 1985supersport15     
I like the starboard idea in case I do get a new set up in a few years and it will also make it easier to fabricate a spray blocker and add it on without having to drill more holes in the transum.

So 3/4 in starboard would be better to give more room for mounting tranducer bracket?

I would use stainless machine screws bedded in pilot holes to mount on the statboard?

What size screws does anyone recommend to mount starboard to transum?

highanddry posted 06-10-2008 08:48 PM ET (US)     Profile for highanddry  Send Email to highanddry     
I used 3/4 inch and I attached it with two screws and 5200.
Henry posted 06-12-2008 07:03 PM ET (US)     Profile for Henry  Send Email to Henry     
When I went from a Humminbird to a Garmin device I used a 3/4" thick starboard pad screwed into the original Humminbird holes using counter sunk SS screws and 5200.

You might want to avoid making the mistake I made though. The mistake may be more or less easier to make depending on where the original holes are on the transom and how they relate to the holes that must be made for the new transducer bracket.

On my Montauk 17, the original Humminbird holes are not well placed in relation to the edge of the transom at the bottom of the hull and are so close to the Garmin holes that when I installed the transducer bracket below the screws holding the pad to the transom I could not adjust the bracket up enough to avoid (what I consider to be) excessive spray from the transducer. I am talking about 1/8 of an inch.

I could remove the transducer bracket drill two new holes into the pad above the Humminbird holes and reinstall the bracket farther up and hope that I can adjust the bracket down enough to achieve a good adjustment with good readings and minimal spray. I haven't done this because of the difficulty of drilling perfect holes into the pad already installed on the transom and because I consistently get excellent bottom readings and the spray isn't too bad.

Jefecinco posted 06-12-2008 07:09 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
I mounted the Starboard with a screw in each corner and liberal application of Boatlife caulk. I then used 1/2 inch screws to mount the transducer.

If I had it to do over again I'd use a thicker piece of Starboard. My excuse of using 1/2 inch the first time is that I had plenty of that thickness on hand. Also, I would probably try to mount the Starboard with adhesive only and forego the screws at each corner. That would leave the transom absolutely free of penetrations below the water line. A desireable condition IMO.


jimh posted 06-12-2008 10:22 PM ET (US)     Profile for jimh  Send Email to jimh     
Even if only 0.5-inch material is available, you could easily double it in thickness. The seal between two layers of intermediate plate material does not have to be perfect.
Jefecinco posted 06-13-2008 09:21 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

I agree that doubling of 1/2 inch would have been smart in my case. Next time I'll do a better job. If I layer two pieces I'll probably join them with screws at the corners and attach the assembly to the transom with 3M 5200 adhesive or similar using duct tape to hold the assembly to the transom while the adhesive cures.

Fewer penetrations below the waterline are better and fewer on the transom even better.


1985supersport15 posted 06-19-2008 03:07 PM ET (US)     Profile for 1985supersport15  Send Email to 1985supersport15     
I purchased a shee7 of 3/4-inch KING StarBoard® to fabricate a transducer plate. [Give me] advice as to how to create a good seal between the KING StarBoard® and the hull of my [fiberglass boat]. I plan to fasten it to the transom with two screws. However, I am not sure if simply applying marine 3M-4200 liberally in the screw holes and on the screws will create a sufficient seal between the KING StarBoard® and the fiberglass at the site of the screw holes. The reason for my concern is that I have read that it is very difficult to adhere any adhesives to KING StarBoard®. I had planned on gooping a lot of 3M-4200 at the site of the screw holes so that when I fasten down the KING StarBoard® it would create a seal that would at least adhere to the fiberglass and the screws. I figured that [the 3M-4200] would be sandwiched between the KING StarBoard® and the fiberglass near the site of the screws, and that it would create a good seal. But am not sure.

I also contemplated recessing a stainless washer on each screw between the KING StarBoard® and the transom so that the 3M-4200 would adhere better and create a more sound seal.

[Give me] advice as to how to get a good seal without covering the entire backside of the starboard with 3M-4200? I am trying to avoid making it messy, so if I do ever have to remove it it won't be a mess.



jimh posted 06-20-2008 09:22 AM ET (US)     Profile for jimh  Send Email to jimh     
My impression of KING StarBoard® and other high molecular weight plastics is that they are difficult to bond using the usual adhesives. I advise you to accomplish the installation using the screw fasteners as the primary source of the strength of attachment of the plate to the transom. Don't rely on the adhesive to provide the attachment strength.

As you describe, the holes where the screw fasteners pass into the fiberglass transom must be well sealed. Because it is not clear to me that any advantage will be gained by using 3M-4200 as an adhesive sealant, you may find that using just a normal polysulfide boat caulk will accomplish the same goal.

It is probably a good idea to maintain a water-tight seal between the plate and the transom, again not for strength, but just to keep water out. A bead of sealant around the perimeter of the plate should provide a good seal without making a "goopy mess."

aubv posted 06-20-2008 04:17 PM ET (US)     Profile for aubv  Send Email to aubv     

I have used 3/4" Starboard with 5200 and no screws to mount my transducer to the hull.

First, I would make the pad at least 1/2" larger than the width and height of the transducer. Then I beveled the 3/4" corners and the length and width outer edges. This leaves all exposed edges relieved, with no sharp corners. A small table saw with make very short work of this.

I then scored the surface of the starboard block, that will mate to the hull, with a utility knife. Score at an angle, in a crisscross pattern.

I then taped a square with blue painters tape around the area of the hull where the block was to be place. I then sanded the hull inside the taped area, lightly with 220 grit sand paper.

I then buttered 5200 to the scored mating surface of Starboard block and pressed and wiggled the block to the hull. I removed the tape from around the block which removed any excess 5200 from wiggling the block as it was pressed against the transom. I then used painters tape and ran 4 long pieces across the block. They ran diagonally across the block corner to corner and from 6 o'clock to 12 o'clock and 3 o'clock to 9 O'clock. Do not run the tape along the hull to the block, up the side of the block and across the block. You want the tape to angle from the hull up to the block as this will keep the block where you placed it, there will be a tapered gap under the tape starting a couple inches away from the block.

After the 5200 cured for at least a couple of days, I mounted the transducer and it has been in place for 5+ years.


jimh posted 06-20-2008 05:43 PM ET (US)     Profile for jimh  Send Email to jimh     
Re StarBoard and 3M-5200:

See last comment by macfam

jb4146 posted 06-21-2008 12:13 AM ET (US)     Profile for jb4146    
Just a couple of months ago, I mounted the transom transducer for a Norcross Hawkeye digital depth finder on a 1/2" piece of KingStarboard on my Sport 15. I used a single stainless screw (countersunk) and 3M 4200 adhesive to attach the starboard to the transom, and 2 stainless screws to attach the transducer bracket to the Kingstarboard plate. I made sure the single screw into the transom was well sealed. I was prepared to go to up to 4 screws for the bracket if necessary but it wasn't. The key was getting stainless bracket screws that were a little long and then grinding the ends down a bit to get threads in the full 1/2" thickness without going into the fiberglass. Before mounting the plate, I used a router to put a 1/4" radius quarter round on the KingStarboard edges. I'm very happy with the cosmetic result, and I get uninterrupted depth readings at any speed. Ieknib kindly supplied me with a small piece of King StarBoard to do the project.
Marlin posted 06-21-2008 09:27 PM ET (US)     Profile for Marlin  Send Email to Marlin     
King plastics says:

Because King StarBoard sheet products are designed to resist water and grime, they cannot be painted and the use of adhesives is not recommended. If gluing is absolutely necessary, products such as 3M’s Scotch-Weld DP-8005 and Lord Corporation’s 7540-AB have been used with some success. Special welding rods, designed for use with polymer sheets, are also available.

See for King's "Working with King Starboard" product sheet.


Fishmore posted 06-22-2008 01:37 AM ET (US)     Profile for Fishmore    
I too used 3/4 starboard, 5200 and two ss screws to bond the starboard to the hull. But after reading this post it got me thinking that maybe a better idea would have been to use a 3/4 pice of hardwood that was coated in epoxy several times and then attached with 5200 and no screws. Even if the wood eventually went bad (after several years) it would not be too hard to remove and replace and you would not have any holes in the hull. Has anybody tried that instead of starboard?
alfred posted 06-22-2008 10:00 AM ET (US)     Profile for alfred  Send Email to alfred     
How big a piece would you use to mount the tranny? I mean how much allowance will you need on all sides of the mount plate?

1985supersport15 posted 06-25-2008 03:17 PM ET (US)     Profile for 1985supersport15  Send Email to 1985supersport15     
I like the idea of just using sealant and no screws but I do not like the idea of having to score up the fiberglass to get good adhesion between the starboard and the transum.

How difficult is it to remove marine 4200 or 5200 from fiberglass after it dries? I am undecided as to which sealant it better for tranducer mounting. Many have said taht 5200 is too rigid and very difficult if not impossible to remove. I heard 4200 will still work for underwater applications but is a little more pliable?????

I think I will end up doing two screws through the starboard to the fiberglass. In order to make a good seal between the starboard and the transum at the site of the screws I am still undecided.

I thought about scoring the starboard only around the screw holes.

I also thought about recessing a stainless washer on the back side of the starboard to give the sealant something to adhere to.

My final option would be to put a bead of sealant around the entire plate but I am not sure if the sealant will remove cleanly if I ever sold the boat.

Newtauk1 posted 06-26-2008 12:55 PM ET (US)     Profile for Newtauk1  Send Email to Newtauk1     
4200 or 5200 will come off fine.
1985supersport15 posted 07-01-2008 09:45 AM ET (US)     Profile for 1985supersport15  Send Email to 1985supersport15     
Is it recommended to fabricate the starboard to be exactly flush with the bottom of the hull or a little raised. I am not sure if either situation would disturb regular waterflow off the transum.

To seal the plate roughing the starboard and fiberglass with heavy grit sandpaper will help the sealant adhere? I plan to put a bead around the perimeter of the plate, on the edges and obviously in the holes and on the screws.

What kind of screws are best to fasten the starboard down. I've heard reference to machine screws and am not sure exactly what type of screws would provide the best tapping.

Also what kind of screw to fasten transducer bracket to starboard.

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