Need some help on transom question

Hi all, new to this forum it looks like a great resource.

I am from Australia and own a Haines Signature 21 ft boat, which HAD a 200hp Yamaha fitted.

Late last year the top engine bolts were piercing through the transom indicating it was shot.

From there I cut out the rear skin, cut out all rotted wood, replaced with 2 x 3/4 inch ply core epoxied together, glued the rear skin to the new ply, bevelled the edges and glassed over the join. Finished it all up by sanding level and repairing paint.

Now since then, my last motor threw a piston and is now in the heavens. I have since purchased a new motor which will be on a jack plate as it is a 30 inch leg.

So now here is where I am stuck. The hull design and weight are limited to 235hp and 270kg. Weight of boat is 950kg dry, 210litres of fuel..

The motor I plan to put on is 225hp and 264kg, however on a jacking plate.

What would be the best way to test the strength so that I know the back of my boat will not rip away from the top due to the torsional effect of the larger motor?

Thank you very much for your help.

 

If it makes you feel better I put a 150 on a boat rated for 80 and then later a 225.

Any way you could brace it with knees if your not sure hidden into the well, or even brace with steel.

Testing would be to destruction and thats not much good to you.

 

If you are satisfied you did a good job and the top of the transom is stiff (ie the top of the transom gets pulled backwards) then do not worry ...you will know when you do your first hole shot.

 

Thank you for the replies.

Would adding braces help stiffen the upper section of the transom??

Also, where would a plate be best applied?? Splashwell side top bolts??

Cheers fellas! I really dont want to sink this weekend lol

 

Ok so I got the motor on and it flexes like all hell... The Ply is solid but yeah, theres plenty of flex there!!!

So Im guessing the weak point is the stringers??

Where to from here?? I am already in the process of getting the leg cut down to a 25 inch.

Stringers in the rear Im guessing??

 

Sounds like you need a couple transom knees. I wouldn't want any flex by pushing or pulling on the motor. I'd think about adding a couple of glassed plywood knees perpendicular to the transom and parallel with the stringer(s) with a good bond to the transom.

 

I agree with impish but most people ( sure you are not one ) dont realize that the top edge of the transom gets pulled backwards ..so if you can get a length of 3 in x2 in aussie hardwood screwed and glued across the top inside the top edge that will transfer some of the loads to the sides. the bottom of the transom is pushed in and is supported by the bottom of the boat and the floor where it meets the transom.

 

Thanks fellas!

Ok so,

Some internal Knee braces and maybe a 10mm plate along the inside to the edges??

How will I know that the edges wont shear out? Anything I can do to brace those?

Cheers for the help, much appreciated

 

The kness need to go down into the keel taking the load off the transom all together.

A 235 saddle is a bolt on but still has some small lugs to help in mounting, these will not go over the 3x2+ transom.

The knees could be horizontal, look for what ever you have that is strong.

However if you going to fit a bracket this will take the pull of the transom to some extent.

 

Ok the problem is that it has a very low splashwell which obviously limits the height at which I can knee brace it. Probably only have about 12 inches clearance from the deck, and maybe add 10 inches to the keel.

I'm thinking, is it worth cutting out the splashwell and inner skin, removing the ply and adding a thicker inner core with a raised section to suite a longer leg?

Since I will have the section cut out I will also have better clearance to the stringers.

There are two thick glass braces running along each side of the boat that I will also figure a way to brace to.

Thoughts?

 

Sounds like your getting the idea. I think 2 knees to the two braces will be enough. you could make them in St steel and arrange the bolt holes to align with your new bracket.

That would be easy and more than sufficient.

 

How will I connect the stainless brace to the stringers? Epoxy and glass them in? Cheers

 

Ok so what thickness knee braces if I were to go with wood?? Would two x 3/4 inch epoxied together be sufficient??

And same thickness for a horizontal brace be ideal??


Additionally, I plan to cut into the rear skin to brace it, OR, would it be ok to simply but the knee braces to the skin, and glass it in??

Your help is greatly appreciated.

Cheers

 

most big motors dont have a fitting that goes over and around the transom ...so a 75 x 50 across the top would be the go ...good aussies hard wood ..wood knee brace as described fine.

 

Added some Knee Braces.

After opening the floor It was a bit soggy but the stringers are very solid. So I am glad there is good encapsulation there

Plan now is to plate the top bolts, install a ski pole(disguised to be a ski pole at least), and have two structural pipes connecting the ski pole to this plate.

Below the splashwell this ski pole will be connected to a horizontal beam which bolts on to the side beams(Sorry, terminology?).

Heres some pics! The epoxy came out nice and hard !!!!