Transom strength on 1981 lund mr pike 16ft. Motor setback

Discussion in 'Boat Design' started by Nate167, Feb 16, 2013.

  1. Nate167
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    Nate167 Junior Member

    I'm wanting to put a aftermarket trim tilt unit on my 1981 lund mr pike 16ft. I am going to replace the wood in the transom this week and after that I want to put the motor back on with an aftermarket Power trim tilt. The motor is a 1974 Johnson 75hp. It is a manual tilt and when I fish shallow waters with my kicker it's becoming a pain to keep lifting and lowering the motor. My question is will the transom be ok with the motor being setback 7"? The transom is 1" 3/4 thick and has aluminum brackets below the wood for support to the floor. I have attached a picture of the transom. The motor in the pic is not the motor I have on the boat now.

    Also I have searched the web for alternative material for the transom instead of wood and I am thinking of using hdpe plastic. The transom has a cap on it so the plastic would not be exposed to sunlight and is protected. What are your thoughts?

    Thanks for any feedback!
     

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  2. PAR
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    PAR Yacht Designer/Builder

    HDPE wouldn't be a good choice for a transom core. You can't bond it to the hull shell, which is critical for load transfer, plus other issues, such as weight.

    A 1 1/2" thick transom for a 75 HP outboard and kicker is pushing it a bit. Consider increasing thickness to 2" if possible. The reason plywood is so commonly seen in transoms is it's cost to weight/strength ratio. There are other materials where you can have inert or rot free transom cores, but you pay out the nose for them.
     
  3. Nate167
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    Nate167 Junior Member

    Par- Thanks for the feedback. I don't think I'll be able to increase the thickness to 2". Due to the way the boat is built. The wood core is completely encased in an aluminum box on this boat. What do you recommend for a good epoxy sealer for the plywood? Also what should I use to bond the plywood to the aluminum?
     
  4. PAR
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    PAR Yacht Designer/Builder

    All of the major epoxy formulators have products that are physically similar. My usual recommendations are Marinepoxy (bateau.com) and Progressive Epoxy products (epoxyproducts.com), mostly based on cost and mix ratio.

    Applying epoxy to aluminum can be tricky. What I do is to sand the aluminum with 100 grit on a DA, then immediately wipe with a solvent soaked rag. As soon as the solvent flashes off, straight epoxy goes on. This means sanding with mixed epoxy at the ready or a two person operation, with someone chasing you're sander with a rag and epoxy. This technique insures no oxidation. There are other techniques, such as using wet sanding paper, in the freshly applied epoxy, literally using the epoxy as the "wet" portion of the equation. This is the best technique, but is a bit messy.
     
  5. Nate167
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    Nate167 Junior Member

    Thanks for the recommendations on the epoxy. I was thinking about using the epoxy for in between the pieces of plywood and then using 3m 5200 for adhesive between the wood and aluminum. Or would it be a good idea to just use the 3m in between the pieces of wood as well? I don't think epoxy will work now because I would have to disassemble the box to sand the insides of the aluminum and I'd like to keep as many original rivets in place as possible.

    I gutted and rebuilt the whole boat last summer. This is my first boat and it was definitely a learning process. It all went pretty smooth and now that I want to put an aftermarket trim tilt on the boat. I figure now is a good time to replace the transom wood as well since I'll have the motor off.
     
  6. PAR
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    PAR Yacht Designer/Builder

    3M-5200 is a sealant with good adhesive properties, but shouldn't be confused with a real adhesive. The peel strength of 3M-5200 on aluminum, is measured in the hundreds of pounds, while epoxy is measured in many thousands of pounds, so you decided which you'd prefer. The same is true of wood to wood bonds, though better, nothing even close to epoxy.

    A flap wheel on a drill extension will get inside the box. Glue the plywood together with epoxy and bond the core (plywood) inside the aluminum box with thickened epoxy, after good surface prep on the inside of the box.

    Can you post pictures of this box and surrounding areas?
     
  7. tunnels

    tunnels Previous Member

    W hen you do one thing to you boat other things change with it

    Just to add a couple of cent to all this ! when you remount you outboard check the motor height shifting the motor back 7 inchs you could raise the motor a little . im not thinking of the shallow water but more from a performance point . At present the underside of the plate above the prop should be level with the bottom of the hull not below as the motor get shifted back it could go up a 1/4 of an inch at least . Makes it steer easyer and the motor perform better . also as the motor goes out the back it changes the balance of the boat so you might notice it looks a little down in the stern ! wont be much but just reposition something further forward to counteract if its noticeable .:D:p
     
  8. Nate167
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    Nate167 Junior Member

    Par, Attached are the photos of the transom. The wood is under the aluminum plate on the top of the transom and goes under the two corner pieces. Also the grey splash well is what I was talking about being riveted to the transom. aluminum. Underneath you can see where the wood only comes down a little more than half way and is met by the supporting brackets. Also I think I'm going to adhere the pieces of marine ply together with the epoxy. I've done some reading on it today and some people suggest mixing the epoxy without the hardner in between the sheets but also mix in some wood flour. You have any thoughts on this?

    Tunnels, thanks for the 2 cents on mounting the motor. :D I knew setting it back would give me performance gains but did not think about raising it. Is it able to be raised due to moving the motor back out of the turbulence of the water coming off the boat?
     

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  9. PAR
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    PAR Yacht Designer/Builder

    Who ever told you to mix epoxy without the hardener (or even changing the ratio or thinning it with solvents) should never ever be a source of information again. He's a idiot and frankly absolutely not familiar with epoxy or chemicals in general. Where did you read about this non-hardener trick?

    Adding a thixotropic agent, like wood flour is a necessity in this situation, though wood flour wouldn't be my first choice. I'd use a 50/50 mix of milled fibers and cotton flock, with some silica to control viscosity. Log onto www.westsystem.com and download their free user's guide, which will cover the basics about epoxy use.
     
  10. tunnels

    tunnels Previous Member

    Hello
    Moving a motor back away from the transom is always a good idea and there are small gains to be had .
    The best gain is the steering get easyer to turn the wheel . Do you have cables or push pull ? My own boat is way over powered and have a beast of a 115 yamaha so i fitted Hydralic steering ! wow i will never use anything less ever again !! its pure heaven and one finger control at any speed !!
    The further the motor goes back the more you can lift it ! in your case of just 7 inchs you could raise in about 1/4' or even to 1/2 to the underside of the plate !! run a straight edge along side the keel on the hull bottom ,block it there and slide the outboard up and down till you get it right and clamp it there . Also check its 90 degrees to the boat as well and that ist plumb in the middle . You would be amazed at the number of boat where motors are not in the middle and not straight up and down .A little extra time setting things true can make a completely differant boat of it , Handles better is just that little more lively and saves fuel at the end of the day because its not working as hard each time you click it into gear !! Good luck !!
    a 70 HP SURE NEEDS A 2" THICK TRANSOM But !!:D:p
     
  11. FMS
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    FMS Senior Member

    That is a recipe for disaster.
    It is critical to mix the correct ratio of Epoxy parts A and B (hardener) to get strength and cure.
    Part A alone is only a sticky goo that won't set. It won't cure later by top dressing.
    Mixing incorrectly results in part remaining uncured.
    It is miserable to try and scrape off uncured epoxy; it loads up sandpaper immediately. It is easier to start with fresh plywood than try to clean it up. Mix Epoxy as precisely as possible.
     
  12. tunnels

    tunnels Previous Member

    rEAD THE INSTRUCTION BEFORE YOU START !! NOT AFTER WHEN ALL GOES WRONG !!

    All and any epoxy resins used reguardless what they are needs a hardener !! Not only do they need a hardener but in the correct proportion as well !epoxy you never change the mixing instructions to slow it down you use a differant hardener to speed it up theres yet another hardener but the amount o hardener never changes
    Also what ever the brand you that what you use !!
    Never use someone elses product . they dont mix and certinly dont match , And if you do buy resins and hardeners read the instructions because not all epoxys use the same ratio and after you mixed its to late to change it and it may never go hard for the rest of your days . This is where so many people get into trouble thinking they know better when really they dont know stuff all and wonder why things dont work !!

    Weather and humidity will have a really big effect on the end result as well !So if you working outside and not under cover you could get into yet more trouble . Humidity is moisture in the air and if the ply wood is damp the epoxy wont cure and could stay soft and the aluminium could be wet and yet more problems Build youself a small tent and warm everything up !!,not only the back of the boat but the epoxy and the plywood you going to use ! an air circulating heater that moves air is more efficent than just a radiating heater . Most resins like 20c to 25 c and the humidity lower than 65% and with you correctly mixed slow epoxy you will have plenty of working time and elliminated a big percentage of possible problems that you never even thought about !!
    Who ever said dont use hardener is an idiot and wants sending for a long walk on the ice !!
     
  13. PAR
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    PAR Yacht Designer/Builder

    This isn't necessarily true. Most formulators can mix and match resins and hardeners assuming the same mix ratio. This said, you should have a clue about comparability before you try this stunt.
     
  14. tunnels

    tunnels Previous Member

    This is a person that is doing his own job and keeping him on the straight a narrow is the best path to follow . hes already had some mixed bad advice and i do not was to cloud the issue any more than whats alread happened !!Its why i suggest just use one brand and stick to it !! :)
     

  15. PAR
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    PAR Yacht Designer/Builder

    I completely agree Tunnels and you're right I shouldn't have brought it up.
     
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