H170 and the dreaded rotten wood in the motor mount area, my findings to date.

[Shorefun]

As usual I am in uncharted areas. I am just reporting what is happening. What I am doing could fail.
I have mentioned before I am using Get Rot and saw dust to see if I can make the motor mount good enough so I do not have to gut it and put in new wood like others have done.
I did some checking and found it is generally ok to allow epoxy to get too cold and it will come back to life and harden. Yes there are certain negative effects, but that should not be a problem for this use. I am mentioning this because it got colder a bit quicker then the epoxy set (it is designed to set slow with Get Rot). So 3 weeks later the sawdust mix with Get Rot was still fairly soft.
So I remembered, Duh, get out the goose neck lamp with a 100 watt incandescent light bulbs (am I allow to use them?). Well this got it toasty and today I checked to find the saw dust hard as a rock.
Now the piece of wood was still damp. This dampness inhibits the epoxy from spreading further, it was in the manual, so I realized I should do something more. I drilled a few more holes and figure the 70 degrees (when 30 out) to 90 when 40 outside would help dry it out more.
After a couple of weeks of heat and holes the wood has less moisture.
My plans are to let the lamp stay on the area for a couple of weeks longer. Then wait till we will start seeing more days in the 40-50 range to do the next step.
I plan on using small drills to drill out some from the vent holes radially. Then let the epoxy saturate the areas. With the temps being cold I figure the epoxy will stay wet longer and capillary action will take it further into the wood. Once it has set well I will drill a few test holes and see how hard the wood is using large pins.

I am also thinking some kind of safety cable to the engine just in case.

 

[JimInPB]

I didn't put any wood back in when I did my motor mount repair. That upgrade was documented in a 2-part write up that I did. I went with all glass/epoxy & foam.
https://hunter.sailboatowners.com/mods.php?task=article&mid=11&aid=100114&mn=170
https://hunter.sailboatowners.com/mods.php?task=article&mid=11&aid=100115&mn=170

I would be interested in knowing what the wood in your motor mount looked like. I do not know if the thin sheets of veneer that I found in mine were factory original or the remnants of a previous repair attempt.

Thanks,
Jim

PS, a safety cord is a GREAT idea. I highly recommend it. If nothing else, keep a couple of feet of 1/4" line tied to the handle of the motor & just cleat it off to a stern cleat when you mount the motor. A nearby dedicated tie-off point with a short line is better, but anything is a big improvement over none.

 

[Shorefun]

I read your repair with interest.
My idea is if this epoxy does not give me something solid then I will proceed with a repair in your style.
So I have wood there, but it is not in the best shape. I can take an awl and push it in all over with no problem. Well I should be clear I press it in and it goes like an inch into the wood. So something is there, but it is not structural. I figure I have a sponge I need to make hard. So I started with just filling in the screw holes with saw dust as I get the area wet. As I did that I figured I would try making a few holes around the screw hole and then filling them too.
I let it cure. Now that it is cured I have a better idea for how much wood I have made hard.
I had an error in my ways as the wood was too damp for the Gitrot to soak into the veins. I then realized life a dufus I need to put a 100 watt bulb on it for a long time.
So a few weeks later the wood is now dry and my probing found I got a little area around the holes hard. I plan on going back and drilling some more holes with a smaller drill and work on saturating the wood more.
We will see in time.

 

[JimInPB]

I’ve repaired several rotted decks & transoms. The trick of drilling holes & filling them with modified epoxy works for small areas. It can work for larger areas if you drill a lot of holes but that gets unsightly. If you are going to do this, you need to get the core as dry as possible. I have left a blow dryer on a set of holes for weeks & gotten only fair results. It also helps to have a very non-viscous epoxy.

On older wood-cored boats, the water has usually had time to work it’s way pretty far back into the wood & it then seeps back out very slowly when you try to dry out the area by the hole. Fortunately, in the case of a H170, the wood doesn’t go that far back, so the water penetration isn't likely to be more than 6 or 8 inches, no matter where you start from.

If the wood is only soft near the point of intrusion (screw holes), then the fix is pretty easy. You drill out most of the soft area, then scrape around inside with a bent awl to get the rest of the soft stuff out, then dry it, then fill with your favorite blend of new material. If the wood is soft all the way through, then you have a few choices. You can make a bunch of holes & do a bunch of injections, or you can cut the whole area open & replace the whole chunk. In the second case, you can put new wood back in there or you can go with a non-rotting material like I did.

If you are doing a lot of holes, then the bent awl can be a time consuming tool to use for digging out all the soft wood. In that case, I bend a nail & put it in the chuck of a drill, so that I can power dig at the soft wood. A vacuum gets all the chewed up crud out afterwards.

I don’t like to use sawdust in my epoxy mix. I prefer to cut up scrap pieces of fiberglass into sections about ¾” square, then pull them apart & use the loose fibers for fill. I think that it makes a stronger & more durable finished product.

In cases where the soft wood is only around the screw holes, I sometimes drill out a hole around 1/2-3/4” in diameter, depending on how far back the soft wood goes, then I fill the hole with a “tootsie roll” of spiral wound fiberglass that has been soaked in epoxy. This still requires that the wood surrounding the hole be quite dry, so that the epoxy will soak into it & get a good bond. Some people will point out the difficulties in avoiding voids in the tootsie roll, but I have had pretty good luck with this method. Screws tend to grab back into that spiral glass plug real well.

 

[KeukaSailer]

Probably a little late here but...what I used before, on wet wood to give it stability is plain old gorilla glue. It's a poly glue that cures in the presence of moisture. Dries REALLY hard. My 146'd motor mount was a little soft on one side...so I drilled a few holes thru the decking into the wood. Pumped gorilla glue into it and sealed the holes with screws to keep the expansion limited to the wood. This had the effect of driving the glue into the wood before drying. The poly permeated thruout the wood. Worked like a charm!! Motor mount is now more solid that is was new...IMHO. Holds my honda perfectly

 

[BobbyFunn]

This thread has Jim's links in it as well as some common repair strategies. Its my turn to make this same repair and am following Jim's plan. Thanks for the repair guide!

My motor mount was fully attached but had a couple cracks around the left and stern side of raised section. There was no luran delamination. I've noticed the mount wiggles and with movement could see the cracks enlarge.

Autopsy:
I dremeled the luran layer to capture the cracks trying to keep as much curve intact. I needed to aggresively pry tbe luran off.

Underneath the luran was an intact piece of solid very dark brown epoxy. It requred me to dremel tbe perimeter again to get under this.

At this point its obvious why most temp repairs seem fixed, and also exacerbates future cracks.

The fillers are simply grabbing the epoxy layer under the luran and providing a grip for threads. This epoxy layer serves as a backer plate. The problem for the future is the rotton wood underneath allows flexing of the luran since thete is nothing attaching the backer epoxy to the core or the FRP.

Motor mount:

Factory Epoxy used to stick wood to luran

Internal wood backer scraped out with a screwdriver. I removed most traces of the factory epoxy.

Cleaned out down to the FRP. There is more rotten wood on the other side of FRP in the hull. No effort will be made to get rid of it or replace it.

The motor mount and rudder mount construction is the same. Metal, seal, luran, epoxy, wood, foam, FRP, wood.

Couple things ill do different from jims plan is i will undercut the foam by about 2 inches.

Lastly, ill implant a G10 board as the mount surface and cover that with lots of FRP. I wont reuse the luran.

I never want the motor to hurt the boat so ill use recessed nuts (anchored to the foam and G10) Im Ok with the motor falling off, just dont take boat pieces with it! There wont be a G10 backer in the hull.

Edit: This is as very easy project so dont hold back on this one. By dinner i had a sngle new layer of FRP down, and the core replaced.

 

[JimInPB]

On my boat, the 4 screw holes from the motor mount did not go all the way thru to the inner skin of fiberglass. Yours appears different in that way. Thank you for the pictures.

 

[Hunter216]

Wait a minute, where is the 4” round hole saw cutout???

As usual impressive work!

 

[Hunter216]

On my boat, the 4 screw holes from the motor mount did not go all the way thru to the inner skin of fiberglass. Yours appears different in that way. Thank you for the pictures.

I wonder if the inner layer of wood that BobbyFun mentioned is there for the screws to bite into. Maybe on your boat they used shorter screws or the sandwich was just thicker and they never made it to the FRP layer to make the holes???

 

[BobbyFunn]

Wait a minute, where is the 4” round hole saw cutout???

As usual impressive work!

Hilarious

Im saving it for the "move the bow eye one foot lower" project.

 

[Hunter216]

Hilarious

Im saving it for the "move the bow eye one foot lower" project.

Good idea, just leave the hole open and see how fast you would have to sail to completely flood your 17 foot bathtub!

 

[BobbyFunn]

I wonder if the inner layer of wood that BobbyFun mentioned is there for the screws to bite into. Maybe on your boat they used shorter screws or the sandwich was just thicker and they never made it to the FRP layer to make the holes???

Heres a picture of the wood under the motor mount. The shot was taken from the hole in the rudder. Note more factory brown epoxy. Looks like there was another hole in tbe FRP that got missed.

Its definately a backer for screws.

Ive seen pictures where the motor mount ripped off the luran like wallpaper. Not happening on my watch..

Im researching nuts of some sort that would be sunk into epoxy bedding and G10 backer.

 

[JimInPB]

Bobby, what year is your 170? I think that mine is a 2008

 

[BobbyFunn]

Bobby, what year is your 170? I think that mine is a 2008

2004

might explain some of the variations.

 

[Hunter216]

So - again not back seat driving just asking for your thought process.

The OEM layup was apparently designed for easy mounting of the motor mount, just plunk it down and screw it to the hull. Long screws into lots of layers. Likely would have been OK except for water getting at the wood and destroying the integrity of the layup.

As I visualize it the “ layers” you will end up starting from inside the hull are:
Old wood, factory FRP, new layer of FRP, foam, G10, layers of FRP, sealant, metal.

The “new” mounting attachment layers are G10 outward to metal.

You get strong structural sandwich from the inner FRP, foam and outer G10-FRP layers however the mount flex will be carried by the G10 layer outward.

Would the mount not be stronger if you took advantage of the full sandwich?

 

[BobbyFunn]

So - again not back seat driving just asking for your thought process.

The OEM layup was apparently designed for easy mounting of the motor mount, just plunk it down and screw it to the hull. Long screws into lots of layers. Likely would have been OK except for water getting at the wood and destroying the integrity of the layup.

As I visualize it the “ layers” you will end up starting from inside the hull are:
Old wood, factory FRP, new layer of FRP, foam, G10, layers of FRP, sealant, metal.

The “new” mounting attachment layers are G10 outward to metal.

You get strong structural sandwich from the inner FRP, foam and outer G10-FRP layers however the mount flex will be carried by the G10 layer outward.

Would the mount not be stronger if you took advantage of the full sandwich?

Crowd sourcing is awesome. Keep it going.

You are right. I was thinking it would fail right where you think it would, it it sounds less appealing each moment.

I wanted to get some "T Nuts" under that top level G10 and then bed it down on the foam, then glass over, hoping just the epoxy filler between the G10 and foam and the FRP on top would be enough to keep the the mount fixed, but I sound like an idiot just describing that. In that case I would not not use the G10 backer in the hull.

The right thing to do is get in the bilge and remove that rotten wood and replace that with G10, and have bolts go all the way through, and not use the upper layer of G10.

The catch is..I'll need to whip out the hole saw to place a backer. Since the rudder mount was foamed and glassed I can no longer get my arm to elbow under to the motor mount. I don't need the EMT's using the jaws of life on the hull deck joint either after pushing my luck

Then the electric trolling motor will have to be the weak point with those flimsy plastic screw threads.

 

[Hunter216]

Just another thought but could you keep your original plan but install toggle bolts down through the layers?

 

[JimInPB]

Crowd sourcing is awesome. Keep it going.

You are right. I was thinking it would fail right where you think it would, it it sounds less appealing each moment.
View attachment 165846

I wanted to get some "T Nuts" under that top level G10 and then bed it down on the foam, then glass over, hoping just the epoxy filler between the G10 and foam and the FRP on top would be enough to keep the the mount fixed, but I sound like an idiot just describing that. In that case I would not not use the G10 backer in the hull.

The right thing to do is get in the bilge and remove that rotten wood and replace that with G10, and have bolts go all the way through, and not use the upper layer of G10.

The catch is..I'll need to whip out the hole saw to place a backer. Since the rudder mount was foamed and glassed I can no longer get my arm to elbow under to the motor mount. I don't need the EMT's using the jaws of life on the hull deck joint either after pushing my luck

Then the electric trolling motor will have to be the weak point with those flimsy plastic screw threads.

The point you are at now leaves me thinking that maybe you might want to consider doing exactly what I did with the motor mount. Carve out the top 1/2" of foam & lay in glass, one sheet at a time, rather than buying the prefab G10. You can carve out the foam up under the existing Luran & pick up a large area to distribute the load. If you want to get fancy, you can taper the thickness of the glass towards the ends & only have full thickness in the area where the screws grab. The selfish part of me would like to see you do something different so that I can compare those results to mine & learn more, but the practical part of me doesn't want to see you pop any more holes in that boat to get the backing plate in.

 

[BobbyFunn]

Just another thought but could you keep your original plan but install toggle bolts down through the layers?

The toggle would cut through the bilge backer of rotten wood, then start nibbling at the really thin FRP. It would be hard to know when enough torque had been applied.

A jack nut style would need to be really wide so as not to ruin the FRP, but then it might be so wide it does't fully compress the rotten wood.

Learned about t nuts, brad t nuts, slotted jack nuts, rivet nuts, barrel nuts, but nut'n looks great.

 

[Hunter216]

Then the electric trolling motor will have to be the weak point with those flimsy plastic screw threads.

Yes it always comes down to weakest link doesn’t it. A friend of mine that I learned a lot from had a lot of “sayings”. One of his favourites while fixing something was “Your not going to fly in it, it’s good enough”
Maybe I should have kept my thought to myself.