Pick Your Brain

http://greatstuff.dow.com/

I considered great stuff, and intend to use it on the rest of the boat, but really wanted something denser and stronger for the transom, which will bear the most weight, torque, and vibration.

This wasn't your question.......but I have to mention, I don't know where you are, but with the holidays here and so many people out of work, you can pick up a bass boat dirt cheap. Have you checked craigslist?

I think rather than using thin walled steel and then filling it to make it stronger, you would be better off using a thicker gauge of aluminum pipe.........no filler needed.

If you're not worried about an issue with weight, go with galvanized pipe.

At the motor mount, how about thicker tube (maybe .090), and/or 1" X 2" size. If weight allows for it, probably worth considering the entire transom from larger/heavier tube section. Even .060 steel is going to be pretty heavy anyway... you cannot use aluminum tube everywhere?

At the entire transom, add a minimum of 3/4" marine plywood, and at least one more layer of plywood within 12" or so of the motor mount plate. Probably not wise to glass over the mount, would probably crack.

Are you thinking of a spray-in type of foam for the filler?

Have already installed the transom frame, and it is the 2 X2 construction around the entire frame. Yes, I do intend to double up on 3/4 marine grade plywood for the insert. My main concern is not so much the strength of the material that I used, but some sort of a plastic or rubber type filling to absorb vibration, and add some rigidity to it. I am not familiar with aluminum welding procedure, so chose steel construction. Weight should not be an issue. I have done all of the math, and my boat should weigh no more than a boat off of the line.

OK, I am with you now. I misread "...four pieces of the tubing into a 2 X 2 square ..." as a 2'-0" X 2'-0" square, not your clarified 4 pcs 1" tube to make 2" X 2" OA section. Makes more sense now.

Well, with all of the tube walls acting like ribbing inside of your four 1X1 welded together, I would think that pretty rigid.

I agree with you on the strength aspect, but would feel more comfortable knowing it was full of something that would help take out some of the fatigue factor. The transom is usually the weakest link on a boat, and the reason a lot of them are sent to the junk pile. Just want some insurance for peace of mind.

"At the entire transom, add a minimum of 3/4" marine plywood, and at least one more layer of plywood within 12" or so of the motor mount plate. Probably not wise to glass over the mount, would probably crack."

Absolutely right on the money!

The plywood as stated by Doorman is the only light weight product (lighter than steel) that is going to give you a dimensionally stable platform to mount your motor.

Marine plywood is and has been used in fiberglass boat transoms since they began being produced in the 60's.

As far as vibration, I would not be too concerned, most outboards are balanced very well.

Besides, it's a boat, unless you are on a glass smooth body of water, you are going to get more vibration from just cruising over the water.

GA

I appreciate all of the suggestions, and will take them to heart, but we have gotten way off track. My original question was that I am looking for a material to fill a capped steel tube with that will set in the absence of air, be semi-rigid, and not cost a small fortune. Prefer a plastic or rubber type consistency that will absorb vibration such as a motor mount material.

This would be ideal. I'm not sure about curing in the absence of air. But, I've used the Sikaflex self leveling polyurethane concrete joint product, and once the tube is opened, forget it, the whole thing will be cured within a week if it's not used. It dries into a hard rubber. Tough as hell, but pricey.

Sounds like a fun project!

http://www.sikacorp.com/ind-auto/auto-product/auto-product-flex.htm

Give them a call

http://www.sikacorp.com/contacts2.htm

What about dux-seal, a clay and silicone based (inert) maleable sealing compound used a lot in the electrical biz. It never dries out or shrinks or expands at normal ambient temps.

It can be lubed with some inert mineral oil and pressed or rammed into the end of the tubing to support the framing, almost creating a semi-rigid bridgework of the transom.

It's available in small to medium quantities at most hardware, and all electrical and plumbing stores.

Well, if I was going to go el-cheapo I would get on down to my local Home Depot and get a few cans of foam insulation. This would offer some crushing resistance and will help dissipate some of the induced noise along with helping in reducing harmonic vibrations.

To what degree, only a what the heck I'll try it will answer that question. Any other solutions would be a quantum leap in price.

Hello... Hello... Anyone else have a better solution out there?

I cannot comment when logged in as 'Flee to me remote elF', but perhaps this will be helpful even coming from a lowly 'guest'.

Atlas missles used by the US airforce relied entirely on stainless steel skins that were about the thickness of a dime for structural integrity. They were able to get away with this by pressurizing the skin/body. Just as you can stand on the very thin walled coke can when it is pressurized, the atlas missles could withstand much more abuse pressurized.

If you connect, seal (this will be easier if you are tig welding or bronzing), and pressurize the tubing, you will likewise have made the structure more rigid. If you attach a pressure guage, you will be able to monitor the pressure which could provide some warning should a problem develop. (If the tubing is very thin and there are bends in the tubing, there may be noticable straigthening when enough pressure is pumped into the tubes.)

That said, fatigue is not typically a huge concern with steel, at least not to the degree it is with aluminum. Is this cromoly? When you say 'thinwall' what wall thickness is that exactly?

Ultimately if you are concerned about failure in an area, additional steel in the form of a u channel fitting over the existing tubes, and somes additional tubes creating some small strong triangles to connect to the remaining frame, will be hard to beat.

The wall thickness for my 1" tubing is .0830. I have attached triangular bracing in 4 places on both sides of this transom beam. Understand your thinking of pressurizing, but seems this would be getting way to technical for my purpose. This 2" x 2" beam is made using 4 pieces of the 1" tubing, and is 6' long with both ends capped. I feel that it is plenty strong for the application, but just feel that it would be even stronger, but mainly that I could dampen vibration and engine torque by filling this assembly with a plastic or rubber type filler. May not even be a benefit, but I would feel better knowing it was there, as it will be inaccesable once the boat is skinned. This could not be anything but a benefit in some way, even if it does not help my cause in any way.

Judging by the last suggestion by 'guest', post 14, I take it that you're not planning on blasting into space anytime soon; just gettin' you and your son to your favorite fishin' holes and troll for some finned fine eatin' flappers.

I previously suggested dux-seal in post 12, as it resembles in it's properties of a 'plastic' sealing compound that won't cause corrosion while providing vibration dampening characteristics. But I'm not to push the point.

By the way Jeff, are you adding any flotation buoyancy (semi-submerged, overturned contingency elements), into your design?

And do you have to register a home made boat (with length, beam width, weight displacement, horsepower rates, etc.) with the local waterways navigational authoritative bodies for a seaworthy licence, as we have to up here in Canada? ^{(or just GO FOR IT and have have some fun!!!) -}

Your assumption is correct in that I am not building a rocket ship, although I do plan to mount either a 150 or 200 hp engine. Yes, plenty of flotation will be built into the hull for safety sakes. I am required to register my boat in New Mexico, which is no prob. as I have tagged a home-made boat in the past. Not against your idea of Dux-Seal, just don't know how I would get it in there at this point, as my ends are sealed. Appreciate all the input I can get. Thanks

You might consider a low viscosity epoxy such as "Gluvit." It's kind of pricey...Or you could lower the viscosity of regular epoxy resin with the addition of 5% laquer thinner.