fitting ply core into transome of new hull

[Fishy Phil]

Hi All
just wondered if you guys might like to advise me on at which point I should bed the plywood core into the transome of my 18 ft fast fisher.
I used Dave Gerr's scantling rules to arrive at my laminate schedule, I will have an overall base thickness of 5.2 mm before adding thickness to bottom, chine etc. I intended to bed the ply in at 5.2 mm then covering it in with 3 to 4 layers of CSM to reach a total thickness of 51 to 53 mm.
I will be laying up my first hull this weekend after many months of my spare time designing and building the moulds for this project. I would be very gratefull for any advice regards the transome and thank you all for the information i have already gleaned from various threads posted on this forum
Very best regards Phil

[Fanie]

Hello Phil,

Personally I hate any kind of wood in a boat, be it ply or whatever other kind - too many problems with it in the past, and water ALWAYS get in.

If I ever have to replace my boat's transom I would either use a material that would structurally be strong enough or make a structure using glass to do the job. In both cases you would gain on some weight.

[Fishy Phil]

Hi Fanie
thanks for your fast response. like yourself I don't like the idea of too much wood in the boat, but feel a ply core in the transome will be my best option in achieving the structural strength I require.
Regards Phil

[Fanie]

Hi Phil,

It's up to you. As I said I would rather go to the trouble to work out a glassed structure capable of handling the outboard. It is a lot of work, but I would rather do that than having to do it in a few years time all over again.

Such a structure would have to have vertical and horizontal structure for much strength and rigidity.

They say experience is something that enables you to recognize a mistake the second time you made it

Of course just glassing a piece of wood in there would be easiest and fastest...

[PAR]

Fanie, you may have had some bad experiences, but you'll find wood is hard to beat in structural applications, pound for pound. The worst cared for polyester runabout, with a plywood transom, will provide a generation of service before it drops the outboard in the drink. With reasonable care, the transom can last longer then a lifetime.

The basic problem is people don't do anything anymore. They don't care for their boats, they just park them in the yard, let the rain fill them up after the tarp, that they should have replaced the year before, tears and lets in the weather. They aren't kept dry, or well ventilated nor are regularly maintained. Most folks have to charge the battery each time they use their boat, of course after washing off the months of accumulated goo, that has built up on its surfaces from lack of attention.

This doesn't mean wood is a poor material, it just means it, like any other building material, has been neglected. Don't care for your 'glass boat and it will craze, oxidize and possibly develop blisters. A metal boat will rust or otherwise oxidize, suffer from other forms of corrosion and eventually meet the same demise. Generally, people are the problems with building materials not the material types.

The industry standard is 1 1/2" of plywood for moderate outboard power (say up to 150 HP). Saturate the crap out of the core material and use mat between the layers. I use two layers of 3/4" plywood canted at 11 - 12 degrees (to the LWL) so I can increase the cross grain strength. Layup everything "green" so you'll have the best bond you can have and tab it well back into the hull shell. If using a lot of HP, especially on a relatively small transom, use three layers of 1/2" plywood, again canted, but with the center layer parallel with the LWL or consider increasing the thickness of the transom. Plywood is much stronger then convention 'glass lay-ups and steel of the same weight. This isn't my opinion, but simply fact.

[Willallison]

I think you may also find that you need rather more than "3 or 4 layers" of glass to build up the required thickness.
If you're using Gerr's book, follow its rules to the letter - it's a simple method to follow, but the engineering behind it is quite complex, and changing any one aspect can effect others that you may not have considered.

[Fishy Phil]

Hi Par
I had intended to use three layers of birch marine ply to a thickness of 40 mm (1 1/2"), which I was going to glue and press prior to bedding it into the transome green. Very interesting idea to tilt the ply to take advantage of the cross grain, I'll look at doing that. Maybe i should piont out I am fitting a mercruiser inboard with alfa 1 leg and so I'm tied to an overall parrallel thickness of between 51 & 54 mm to suit the motor and leg. I have managed to persuade three friends along with my project so we will all be fitting mercruiser units but with differing horsepower from the 120hp diesel to the 220 hp 4.3 MPI.
Regards
Phil

[Fishy Phil]

Hi Willallison
From my reply to Par you will probably note that I am restricted to a total thickness of 51 to 54 mm and 40mm of that is going to be marine ply.
Taking into acount gell coat basic hull thickness and a wet CSM layer to bed the ply I will probably have about 7mm of hull thickness behind the ply leaving between 4 and 7 mm to complete my overall thickness.
When using Dave Gerr's scantling rules to arrive at the hull laminate schedule I did follow his rules preciseley although if any rounding was required I rounded up rather than down. I do take onboard your concern on changing any aspect of the design and possibly causing problems elswhere. Information is pretty vague in the book regarding the possition of plywood within the transome laminate hence my original question.
Regards
Phil

[PAR]

Phil, your laminate schedule sounds fine and rounding up is always good, though usually not necessary using Dave Gerr's scantling rules. Balancing the laminate is generally preferred which for you, would be around 6 to 7 mm on each side. If you have to skimp on one side, to keep the total transom thickness within the limitations of the drive assembly, then cheat the inner skin, maybe 7 mm outside and 5 inside.

Attached is the canted plywood layout I mentioned, of course showing three full size sheets on a typical 18' boat transom (with outboard cutout, which you don't need). Will, does this look familiar?

[Willallison]

Yes, it does Paul - and as Phil noted - a good idea that probably not many think to incorporate.

Phil - sorry, I got the impression from your earlier post that you'd be using thinner ply. As you say, Dave's book can be a little light on for detail in places, but at the end of the day, I think you can be assured of long-lasting conservative structure if you follow them.

[Fishy Phil]

Hi Par
thanks for your advice on the transome laminate, just one of those things where I thought I was heading in the right direction but i just needed a bit of confirmation from an experienced builder like yourself. Also thanks for the sketch on canting the plywood small changes can make a big difference.
Phil

[Fishy Phil]

Hi Willallison
my appologies earlier for not setting out my question as clearly as I could have. Thanks to yourself and Par my concerns are now resolved. With out the aid of Dave Garr's book I would have really struggled to arrive at a suitable laminate schedule. By the way I laid up the first hull this weekend, when I pop it out of the mould I'll load some pictures for your scrutiny.
Phil

[Fanie]

If I'm not mistaken Sea Cast makes an artificial transom 'insert' you could have a look at. I couldn't find the original informations again, but at the time I looked at it it was something I would have preferred over the wood.

[Fishy Phil]

Hi fanie
i'm not really looking for an alternative to the ply for the transome, I was more concerned with its position within the laminations. Thanks for taking the time to try and help.
Phil

[Fanie]

Well, my own boat has wood in it's transom. At the time it was built all these fancy materials and stuff wasn't available like it is now.

Wood is truly hard to beat in some ways, if only it was a bit lighter and wouldn't rot so easy eh ! Better plant some more trees...