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Old 15-04-2021, 14:13   #1
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Question: filling a hollow transom of a plastic tender?

I have a plastic ( probably polypropylene) tender that has approximately a 30mm transom which is hollow between the inner and the outer shell.
I wish to fit a small outboard.
How can I fill this hollow to make a solid transom that an outboard can be clamped on? There is a small inspection cover as access.

Any advice would be appreciated.
Steve
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Old 15-04-2021, 14:33   #2
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Re: Question: filling a hollow transom of a plastic tender?

My advice is to check with the manufacturer to see if your plastic tender is capable of being powered with an outboard motor.

Your motor could just collapse the transom when you open the throttle.
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Old 15-04-2021, 16:01   #3
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Re: Question: filling a hollow transom of a plastic tender?

This is a WAG.... You could try filling it with spray in closed cell foam. It will make it stiff and act also as flotation. However, I am not engineer enough to know whether it would be strong enough then to use even a one horse o/b on it. Nor do I know if you can place a 3/4" plywood plate over the transom to spread the loads, but it is something to consider.
If you try the plywood, you should seal it carefully with epoxy, first, then prime and paint it.

I have never tried either of the above for a plastic dinghy of unknown plastic.

Ann
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Old 15-04-2021, 16:10   #4
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Re: Question: filling a hollow transom of a plastic tender?

Quote:
Originally Posted by JPA Cate View Post
This is a WAG.... You could try filling it with spray in closed cell foam. It will make it stiff and act also as flotation. However, I am not engineer enough to know whether it would be strong enough then to use even a one horse o/b on it. Nor do I know if you can place a 3/4" plywood plate over the transom to spread the loads, but it is something to consider.
If you try the plywood, you should seal it carefully with epoxy, first, then prime and paint it.

I have never tried either of the above for a plastic dinghy of unknown plastic.

Ann
My fear would be that the spray in foam would expand and distort or even burst the plastic.
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Old 15-04-2021, 22:24   #5
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Re: Question: filling a hollow transom of a plastic tender?

I had the a similar problem with a poly transom and had made an aluminium beam from 2" x 1" RHS with flat plates on the end. The beam goes the full width of the dingy and the aluminium plates have matching plates on the outside which are riveted through the sides of the dingy to those on the end of the beam.

In your instance I would look at whether a U shaped beam could be fabricated to slip over the transom then be drilled through and rods welded in where the engine clamps go. The outer side of the inverted U could have flat plates which riveted through to the flat plates on the inner side of the inverted U.

The material thickness of mine is all 3 mm.

From a structural viewpoint the corners where the sides meet the transom are probably the stiffest and strongest pert of the hull to carry the forces out to.
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Old 15-04-2021, 22:39   #6
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Re: Question: filling a hollow transom of a plastic tender?

Quote:
Originally Posted by rwidman View Post
My advice is to check with the manufacturer to see if your plastic tender is capable of being powered with an outboard motor.

Your motor could just collapse the transom when you open the throttle.


Many thanks rwidman. I can stand on it without deflection so I think it will be fine if I can fill it to make it solid and stop any lateral deflection between the inner and outer shell. Unfortunately she is very old and the manufacturer is unknown. She it the hull for an Optimist sailing dinghy.
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Old 15-04-2021, 23:07   #7
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Re: Question: filling a hollow transom of a plastic tender?

Quote:
Originally Posted by JPA Cate View Post
This is a WAG.... You could try filling it with spray in closed cell foam. It will make it stiff and act also as flotation. However, I am not engineer enough to know whether it would be strong enough then to use even a one horse o/b on it. Nor do I know if you can place a 3/4" plywood plate over the transom to spread the loads, but it is something to consider.
If you try the plywood, you should seal it carefully with epoxy, first, then prime and paint it.

I have never tried either of the above for a plastic dinghy of unknown plastic.

Ann


Thank you Ann

I will look into the closed cell foam. I was thinking of pouring epoxy but concerned that the heat in the curing process.

I have considered the plywood option but still prefer filling and then covering with a stainless steel or epoxy plate. If I can fill and stop any compression between inner and outer shell she will be fine!

There is a small inspection cover of about 100 mm so if I turn her upside down and pour in something it will work so my question is really what substance might join the two and remain solid?
Thanks
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Old 15-04-2021, 23:09   #8
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Re: Question: filling a hollow transom of a plastic tender?

Quote:
Originally Posted by RaymondR View Post
I had the a similar problem with a poly transom and had made an aluminium beam from 2" x 1" RHS with flat plates on the end. The beam goes the full width of the dingy and the aluminium plates have matching plates on the outside which are riveted through the sides of the dingy to those on the end of the beam.



In your instance I would look at whether a U shaped beam could be fabricated to slip over the transom then be drilled through and rods welded in where the engine clamps go. The outer side of the inverted U could have flat plates which riveted through to the flat plates on the inner side of the inverted U.



The material thickness of mine is all 3 mm.



From a structural viewpoint the corners where the sides meet the transom are probably the stiffest and strongest pert of the hull to carry the forces out to.


Thanks RaymondR.
That makes sense. I will look into this option!
Rgds
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Old 16-04-2021, 03:22   #9
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Re: Question: filling a hollow transom of a plastic tender?

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Originally Posted by SteveG58 View Post
I have a plastic ( probably polypropylene) tender that has approximately a 30mm transom which is hollow between the inner and the outer shell.
I wish to fit a small outboard...
Your “plastic” dinghy is probably rotomolded polyethylene.
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Old 16-04-2021, 03:28   #10
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Re: Question: filling a hollow transom of a plastic tender?

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Originally Posted by GordMay View Post
Your “plastic” dinghy is probably rotomolded polyethylene.


Thanks Gord
Do you think I can pour in some epoxy to solidify the transom ?

Thanks
Steve
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Old 16-04-2021, 04:51   #11
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Re: Question: filling a hollow transom of a plastic tender?

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Originally Posted by SteveG58 View Post
Do you think I can pour in some epoxy to solidify the transom ?
I'd think expanding foam, and/or external reinforcement [as others have advised], would be a much easier solution.
Do you know which make/model tender you have?
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Old 16-04-2021, 05:50   #12
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Re: Question: filling a hollow transom of a plastic tender?

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Originally Posted by SteveG58 View Post
Thanks Gord
Do you think I can pour in some epoxy to solidify the transom ?

Thanks
Steve
Most epoxy formulations when poured in this kind of volume will get REALLY hot as they cure. More than hot enough to melt polyethylene.
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Old 16-04-2021, 05:51   #13
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Re: Question: filling a hollow transom of a plastic tender?

When you tighten the motor mounts and when torque is applied by the motor you will start to crush the foam which will be very poorly bonded to the transom. The foam will break down with each use and turn to powder ... how fast that will happen is anyones guess.
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Old 16-04-2021, 07:10   #14
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Re: Question: filling a hollow transom of a plastic tender?

Quote:
Originally Posted by GordMay View Post
I'd think expanding foam, and/or external reinforcement [as others have advised], would be a much easier solution.
Do you know which make/model tender you have?


Many thanks Gord

I don’t know the make. It is an old optimist sail dinghy!
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Old 16-04-2021, 07:12   #15
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Re: Question: filling a hollow transom of a plastic tender?

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Originally Posted by boatpoker View Post
When you tighten the motor mounts and when torque is applied by the motor you will start to crush the foam which will be very poorly bonded to the transom. The foam will break down with each use and turn to powder ... how fast that will happen is anyones guess.


Thanks boat poker. That is my concern too!
I will check with the boat yard when I am back there next week!
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