Transom Reinforcement for Outboard Conversion

[Jim Williams]

I am seeking suggestions from those of you who have replaced an I/O engine with an Outboard on a bracket.

Boat: 1985 Bayliner Trophy 21'
Old Engine: Volvo-Penta AQ125 I/O 120hp (already removed)
New Engine: Honda 150hp four-cycle outboard (already purchased)
Outboard Bracket: "Stainless Marine" 26" setback (20"high x 20"wide) 1/4" marine aluminum construction
Bracket + Outboard = 540 lbs

The sequence of photos show how I propose to mount the new engine. The outside/inside transom plates are primarily to ensure that the plug stays in place and remains watertight, the inside plate also serves as backing reinforcement for the bracket bolts.

Outside transom plate = 1/8" thick
Inside transom plate = 1/4" thick

I hope to minimize any transom flexing by using a knee brace that is attached to the top Bracket bolts and the stringers in the bottom of the boat. I plan on using 3M 5200 sealant wherever metal contacts fiberglass.

Question1: Will this be strong enough?

Question2: Can I use marine grade aluminum instead of stainless steel for the plates and knee brace? (much cheaper and easier to work with)


Photo1-Outboard bracket temporarily bolted to transom.

Photo2-Outside transom showing hole where old I/O was mounted. Bolts are in new holes for outboard bracket.

Photo3-Proposed stainless plate mounted outside over I/O hole .

Photo4-Inside transom showing existing construction layout.

Photo5-Proposed I/O plug and stainless plate over inside transom I/O hole.

Photo6-Proposed knee brace. Brace will be in tension since forces on the top of the outboard bracket are pulling outward. This is just a rough drawing of the brace, it will actually wrap more around the stringers with bolts through the stringer side and top.

[Jim Williams]

Transom plug finished...next step attach OB bracket and inside plate.

[funkyblues76]

Jim, that looks great! Im pulling whats left out of my transom on Monday, hopefully if it's not rotten I'll be working on a plug next week. Thanks for the pictures and help!!! I purchased a 12" setback bracket, but still no motor yet.
Be sure, I'll have some more questions next week. Thanks Again!

[Loveofsea]

Unless the plug is integral to the transom (multi-layered glass faired on the inside and out), you will still have a hole on your transom...You want the additional load of the engine to transition past the hole, not be drawn directly to it...

It is not good enough to simply fill the hole. the entire transom must become, in effect, a solid piece as it was before the original cut out for the I/O.

With the I/O, the load was never directly on the transom as it will be with your bracket. The old I/O was bolted to the stringers which took a major load off the transom.

I like the braces in #6 that tie in your transom to the stringers, but in order to get the maximum benefit from those braces, you should coordinate the holes in the O/B bracket, both inside and outside plates, AND the braces. Have as many bolts as you can, grip the bracket and both plates. it wouldn't hurt to make the plates, as large as you can make them in order to capture and tie into more of the existing structure--it's all about distribution of the load, as you well know...

personal opinion, i wouldn't use the bracket--- it exaggerates the moment of the hull beyond it's design characteristics...it takes the most substantial weight in the boat (the engine) and moves it aft a good 4ft of the original hull design...That bracket is going to apply a leverage the transom that it has never experienced.

if it were my boat, i would repair the I/O hole in the transom, make a cutout for the outboard, beef up the corners of the transom by fabricating (a splashwell with) intercostals that attach the transom to the sides on a lateral plane.

i do not understand the advantage of extending 500+lbs over 2 feet aft of the transom...

love your graphics!

[funkyblues76]

What did you do with the original gas tank, cap and hoses? Did you remove or just build around? If so, are there any special steps removing and pluging these open spaces?

thanks!

[Jim Williams]

I didn't change any of those items. The bracket location didn't interfere with any existing structures.

[awol46]

Before I went too far with the project I suggest you contact local mfg that has outboard mounts extending past transom. From the pics and the age of the transom I tend to agree that there will be way too much stress put on the transom with that much weight, not to mention torque applied onto a weakened structure. Did you think about restructuring transom to accept outboard directly, That would reduce the fles and weight, and give the most structural support.

[Bob S.]

I agree, too much load & stress focused on middle of transom. Would recommend at very least to use o/b bracket that is full width swimstep type. Also, how is wood in transom? Dry & strong or wet, spongy or rotten?

Looks like you are committed to current method. I would weld a heavy aluminum structure for the below decks reinforcement, the light bolt together stainless stuff gives me the willy's

[Jim Williams]

I should have mentioned that at the time I started this thread I also sent an inquiry (photos/diagrams) to "Stainless Marine", the manufacturer of my bracket. Their technical engineers responded that my design was appropriate for the HP/Wt and transom structure. I inspected the wood in all newly drilled holes and when tapering the inside of the I/O hole, and everything was dry/hard with no sign of rot.

I did not consider the swim platform option for my bracket because I use my boat primarily for fishing and those platforms just get in the way, they are also twice the price!!!

Due to material availability I have since increased the Knee Brace aluminum thickness from 1/4" to 3/8". I still think 1/2" stainless bolts are more reliable than welded aluminum for attaching the Knee Brace, especially for a structure that will be subject to flexing and vibration.

[awol46]

Sounds like you thought this through and verified the project. I believe the extra thickness and the stainless bolts are a good direction. Just ensure you seal the transon well so moisture won't reach the wood filler. Good luck and keep us informed on the results.

[bedforj]

Jim,

How did everything work out? I'm in the process of repowering my 23' searay with a cuddy, and trying to decide if I need twins or a single outboard.
and would welcome any suggestions and would like to know how your boat handled with the outboard bracket.

[naturewaterboy]

Jim,
Have you considered as an option to the metal braces, using triangular plywood pieces glassed in to the hull and transom? From the pictures, it looks like the only part of the transom that has much thickness (and strength) is the very center. It looks to me like there will be a lot of force trying to rip this section (to which your bracket is attached) off the boat. Although the metal braces will transfer the forces from the top of the transom to the hull, any twisting or torque forces may not be transfered as well, resulting in some flexing of the transom. Just my 2 pesos worth - I'm doing a similar project on a 34' Silverton - my whole transom is only 1/4" thick so I'm reinforcing the whole width to 2" thick with plywood.

[Jim Williams]

I am nearing completion of the knee braces and will post some photos soon. It is not difficult, just time consuming to get precise angles cut/bent using only hacksaw and bench vice. I also need to have them welded in a few spots for extra strength. I estimate another 3-4 weeks before I do some testing in the water. Some items I still need to do:

-New remote control with cable routing.
-Hydraulic steering installation (was not required, but thought it would be cleaner installation that rigid cable).
-Wiring harness for new engine with some new instruments.
-Fuel connections.

Naturewaterboy, I decided to use metal knee braces because I had read about others using them and also it allows me to connect directly to the bolts that fasten the outboard bracket. I think the 2" x 2" x 3/8" aluminum angle will be stronger than one made of plywood. As you noticed the center of my transom is two layers of 3/4" plywood with 1/4" fiberglass on each side for a total of about 2" thick. Outside the center transom it is only one layer of plywood/fiberglass with 1 1/4" thickness. I think by using two knee braces on each upper corner of the bracket bolt pattern there should be minimal torque movement to the transom.

[brittanydog]

Jim,

I’m amazed I found your post, I’m doing the same conversion, same boat design, and live in the same area.
You’re a little further ahead in your conversion. I haven’t order the bracket yet.
My boat is a 22’ Bayliner. Your transom looks from the inside identical to my boat. I removed a 4 cyl. Volvo with a 270 outdrive. I have sealed the hole with marine grade plywood and Fiberlay epoxy fiberglass mat. I’m in the process of building out the transom to 3”. I have two outboards (25" 200 hp Yamaha or a 30" 225hp Yamaha) that I can install on this boat, At this time I’m planning on installing a 200 hp Yamaha with a 25” shaft, but when I measure up from the bottom of the hull 24” on the transom (where the bracket will mount), I’m not sure the bracket will be high enough to keep the motor from getting swapped, but I’m also concerned about installing the 30” 225 Yamaha. My concern is moving the center of gravity of the boat. I’m guessing I removed about 800-900lbs in the I/O unit - motor and outdrive. All that weight was at the bottom of the boat, now I’m moving the weight up 24” or 29”. Did you do any calculation on the movement of the weight higher on the boat? Any concerns on your boat?
Please keep me posted on you boat project, and I’ll do the same. I’ll post a few pictures when it stops raining.. Here are few pictures of the boat before project started.

Louis

[Jim Williams]

Brittanydog,

What a coincidence! You must have the 2160 Trophy? 1983-84?

I didn't do any precise calculation on the shift in center of gravity. I reasoned that a 200# lighter engine/outdrive moving aft 3' would not be that significant compared to the 3000# forward of the engine. Same for trailering, after removing the old engine my tongue weight did not change that much, still around 300#.

I don't think you will have an a problem with sitting too low in the water and swamping your engine, you might sit 2-3" lower. A very rough calculation is about 300# water displacement for every inch lower your transom sits. What is the weight difference between your 200hp and 225hp? Are they four stroke? Where are you getting your bracket? Stainless Marine, where I bought mine, might be able to help you decide which engine to use. They are very helpful.

My biggest concern is the possibility of porpoising which is more likely with a shorter boat and a longer setback. But after doing some research I found it can usually be remedied with engine tilt-trim and/or trim tabs (which I already have). Hopefully it won't even be an issue.

All of my transom modifications are completed so tomorrow I'm having my engine bolted-on. I will be posting more photos soon.

Jim