Outboard Transoms - Let's Talk

Questions about modifying a design

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Iggy
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Outboard Transoms - Let's Talk

Post by Iggy »

Hi builders!

I think pretty much every outboard boat builder has, at one point or another, wondered:

"Do I modify/reinforce my transom?"

I bet even the most novice of boat builders can recognize that the transom is a place where a lot of weight and stress gets transfered to the boat from the engine. Its not hard to imagine your outboard motor shaking the seams apart or even getting ripped off the boat if the motor leg hits a rock or sunken log.

I've heard (or read) it several times that "There is no such thing as 'too much' reinforcement on a transom.". It makes sense, as the last thing any builder wants is to deal with a failed transom during operation.

However, it does beg the question "How much?" If there is no such thing as too much, then where do you draw the line between 'effective' reinforcement and 'overkill'? I actually don't have that answer, and I am betting that answer is very conditional like:
  • Is the boat designed properly in the first place?
  • Are you making design modifications to the original plans?
  • Are you using the MAX recommended engine size/weight (or something larger)?
  • What materials are you using to build your transom?
  • What kind of water conditions are you expecting to operate under?
I am betting there are a lot of first-time boat builders out there, with limited (or no) boat building or even boat-operating/ownership experience. In my case, the first boat I've built will be the first boat I've ever owned and the first 'big' outboard I've ever operated. Others like me simply don't have the experience to know what works, what doesn't, and what we should do, if anything, to prevent bad things from happening.

In my case, I've done my best to pick a reliable boat designer (Glen-L of course), and build very close to 'spec', particularly on the transom and based on some of the design articles listed here.
In the case of a Glen-L designed boat, boat-builders like myself also have the advantage of seeing how other builders have made their transoms and, depending on the boat design chosen, even see photos of a handful or a dozen or more sucessful boat builds. In my case (Malahini Design), there are over a dozen customer submitted photo's of other builds that show a variety of engines and transom reinforcements.

There is also the Glen-L forum, where you can search the history and ask questions about what others have done or experienced. However, just like every other forum, its a good idea to know the specifics about any sucesses or failures before assuming they might apply to your boat build.

One thing that boat builders can probably learn from the most is the experience of other boat operators. Something I cannot offer as I am a novice builder and operator. However, there are members of the boating community with pearls of wisdom to offer in regards to transom design.

If you are such a person, and so inclined as to share your experience with the rest of us, then I encourage you to spend a few moments and post your reply here.

Replies can include success stories or failure issues. Try to include:
  • What design/model did you use?
  • What modifications did you make to the overall boat?
  • What modifications did you make to the boat transom?
  • Was your engine larger or smaller than MAX design?
  • Did your transom ever fail?
  • If yes, how did the transom fail and in what way?
  • If yes, how did you repair it, and did the repair work?
  • Any suggestions you might have in regards to transom design
Also, if any boat designers / naval architects are out there reading this, and can shed some light on the process of transom design, I am sure we would all benefit from knowing what is 'already covered' in the original plans and what is not.

Ideally, this post is to serve as a resource for future outboard boat builders seeking answers.
Last edited by Iggy on Tue Dec 13, 2011 3:50 pm, edited 1 time in total.
Ian (aka Iggy)
My Malahini Build

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thudpucker
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Re: Outboard Transoms - Let's Talk

Post by thudpucker »

I cant tell you all that stuff Iggy.
It's been too many years under the transom. :lol:
Make it all replaceable! That's a good watch word.

The wood comes apart if you tow the boat with the outboard mounted on the transom.
The Wood comes apart if you cant keep the edges dry.
The Wood get's soft (comes apart) if it's not braced well enough to keep the Outboard from whipping to & fro as you navigate over rough water.

Plywood plies work good. You just have to keep an eye on that stuff. Solid planks work good too. I put a 'skin' of Aluminum over the top of my Transom where the outboard goes.
The Clamps have a better hold that way, and the outboard don't make scraps outta the top of the transom as you remove/replace it.

gdcarpenter
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Re: Outboard Transoms - Let's Talk

Post by gdcarpenter »

It's an interesting question Iggy - one most of us 'conciencious' builders have in the back of our head - but seems a difficult question to answer 'quantitatively'. Perhaps if enough folks reply we can formulate some 'consensus' as to what's more likely to work and/or more likely to fail, so I'll kick it off.

I'm a bit of a heretic here because I built my transom out of solid 1 3/4" thick white oak, which I know has it's downsides. (FYI I'm building a ZIP.) Being a carpenter by trade and a long time boat user I've seen more than a few rotten transoms, and most seem to be from water intrusion, and most of the worst damage was is ply constructed transoms. This is my own personal experience. My theory is that solid wood is more resistant to water seepage, and the more 'plys' or 'layers' you have the more the risk of 'opportunistic' water intrusion.

I used two transom knees instead of one, each knee was a tad shorter than the specs, and obviously not mid ship - but my thought was with the 'stiffer' slab transom there would less 'flex' than a ply transom, and yes the blade of grass can give while the might oak may fail analogy has not been lost on me. On the other hand 'gluing' ply to a frame then perhaps gluing a veneer over the ply to cove the knee bolts seems too many 'invitations' for water seepage or separation. My knee bolts are my transom eye/u-bolts so no concealing required, and there are the two 'shanks' or 'legs' of the u-bolts to double the knee bolt thingie.

My understanding is that a ZIP with a 20" transom height would arguably be rated for a 40 hp max. I'll be starting off with a 17" tranasom height on a '59 Mark 35A 35 hp so will be at or near theoretical max rating for the lower tansom height.

My thoughts on reinforcing the transom has been mostly my motor/splash well construction. The GlenL motor well plans are simple but I do not think they add any strength to the transom and are kind of bulky. I used blind sliding dovetail joints to connect the back ends of the motor well sides to the transom so they are 'intimately cojoined' and literally cannot be pulled apart. My theory is that once the front of my fully assembled motor well is secured to a crossmember it will form a sort of 'box' section and stiffen and add support to the transom. That's my story, time will tell. Then again, I built my transom/motor well assembly before the frames were mounted on the jig!
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Iggy
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Re: Outboard Transoms - Let's Talk

Post by Iggy »

Lots of views, but just 2 replies so far. Even if your reply is 'followed spec', its good to know what has been working for others instead of making assumptions on our own.

Has this been discussed too many times before?
Ian (aka Iggy)
My Malahini Build

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vupilot
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Re: Outboard Transoms - Let's Talk

Post by vupilot »

As I got closer to finishing I was very nervous about mounting the heavy motor and trailering the boat for miles and miles of pot-hole laden roads. I had nightmares and about the entire transom seperating so I understand your concerns but I feel these concerns are just us overthinking ourselves. When equiped and built as designed I feel you have nothing to worry about. I think its the ones that experiment, overpower, put on too heavy a motor or think they are smarter than the designer that give the rest of us worry.

Built mine as spec in the plans and not even with the updated transom sides that join to the battens at the floor, just the basic closed in motorwell. I did use pieces of 1" square stock to beef up the corners in the joints of the deck framework and transom to sheer/chine joints. My motor is max that was originally recommended/designed for.

Its only been 2 years but I've got about 2,500 trailer miles on mine and one huge pot-hole that sent the entire trailer, boat and motor a foot completely in the air at 65mph and no sign of any wear yet. I do use a transom saver to support the motor when tilted for trailering and I feel that helps take a bit of load off the transom and transfer it to the trailer.

upspirate

Re: Outboard Transoms - Let's Talk

Post by upspirate »

My first build was a TNT rated by Glen-L at 15HP on the web site.

On the plans it states "Under BIA regulations the maximum power for this boat is 25 HP.We (Glen-L) feel this could be safely exceeded by at least 10 HP without problem".

I built mine using lumberyard pine and a-b exterior plywood,resorcinol glue,no epoxy or cloth. The only mods I made to the planed design was to double the twin knees from 1/4" plywood as drawn to 1/2" plywood.

the TNT uses two 1/4" plywood knees on either side of a cleat on the transom and underside of the motorwell as well as the sides of the keel....I doubled these to 1/2"....probably not necessary,but I did.

On mine, I ran a Mercury Mark 35 and Mark 55 with no problems.I later ran an old 4 cyl Mercury 80HP for a couple of runs with no structural problems.(Yes,alcohol was involved in THAT decision as I had the motor, hull, and beer all laying around,and No I don't advise it as I never could get it sorted out and it wasn't safe!)

My second TNT was built as designed,with mahogany frames,marine fir ply,and epoxy glue and saturation (no cloth....I don't recommend this,use cloth! ). this one was powered with mostly a 10HP 4 stroke Honda,a 25 Johnson for a while, and a 35 HP Mercury for a while.

No structural problems here either.

I changed my skiff transom from an inside motor board to a full frame to frame doubled transom as it was easier than cutting notches in the cross brace for the motor board and I figured on using 18HP that I owned instead of the recommended 15 HP

Anyway my point is that Glen-L's designs are strong enough as designed when powered and used in the intended manor (In my opinion and experience)

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ttownshaw
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Re: Outboard Transoms - Let's Talk

Post by ttownshaw »

The transom knee does its job but I think we often overlook how much strength the motor well adds to the design. Built to spec it adds more vertical stability (IMHO) to the transom than does the knee. The athwartship member adds horizontal strength. When the decking is added it locks the upper edge of the transom in place. Proof is in the pudding, so to speak, you don't hear about transoms failing when built and powered within spec. The only issues I've heard about came about as a result of poor construction, removal of specified members (like the knee), and bad trailering practices (not using a transom saver).

Just my opinion.

Here's a detail of the motor well to review: http://www.glen-l.com/designs/outboard/motorwell.html
Bill

I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
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upspirate

Re: Outboard Transoms - Let's Talk

Post by upspirate »

ttownshaw wrote:The transom knee does its job but I think we often overlook how much strength the motor well adds to the design. Built to spec it adds more vertical stability (IMHO) to the transom than does the knee. The athwartship member adds horizontal strength. When the decking is added it locks the upper edge of the transom in place. Proof is in the pudding, so to speak, you don't hear about transoms failing when built and powered within spec. The only issues I've heard about came about as a result of poor construction, removal of specified members (like the knee), and bad trailering practices (not using a transom saver).

Just my opinion.

Here's a detail of the motor well to review: http://www.glen-l.com/designs/outboard/motorwell.html
You bet a motorwell adds to the strength!

The TNT design also has coaming/carlings that tie the sides of the transom to every frame up to the very forward one,and also a motorwell.

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jeffh
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Re: Outboard Transoms - Let's Talk

Post by jeffh »

I think most of us know the story: My boat is rated for 85HP. I bought a 90HP E-TEC which weighs 320-lbs. The transom is 1.75" thick. My motor well was built a little narrow to accommodate two rear hatches. The motor well sides are 1" thick but did not extend down to the battens. The center knee was oversized and connected to the first frame. In order to make it stronger, I built two of these (2" thick) which are epoxied and fiberglassed to the transom and the bottom. They also have 4 bolts (2 in the bottom and 2 in the transom) and extend to the first frame:
Image

Image

steering tube with motor well side mount
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2011 MALAHINI - KICKED IN THE HEAD
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upspirate

Re: Outboard Transoms - Let's Talk

Post by upspirate »


goatram
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Re: Outboard Transoms - Let's Talk

Post by goatram »

Iggy wrote:Hi builders!
"Do I modify/reinforce my transom?"


I will be installing a couple of 1/4" aluminum braces to what is already there
Engines will be twin Yamaha F250
Aluminum already engineered and built by a professional Boat builder Dale Bean from Clarkston WA
  • What kind of water conditions are you expecting to operate under?
Up to 5 to 6' wind max wind wave Pacific ocean off the Washington coastal waters 10' to 12' combined seas.
  • What design/model did you use?
Coronado
  • What modifications did you make to the overall boat?
Raised Cuddy Roof and a 8' Pilothouse
  • What modifications did you make to the boat transom?
The Original Hull was cut out from just fwd of the #2 Rib and then new plate was welded in to extend the Hull for a Offshore Bracket/Swimstep. The Transom has no motor well cutout
  • Was your engine larger or smaller than MAX design?
  • Did your transom ever fail?
not yet nor am I expecting it to
  • If yes, how did the transom fail and in what way?
  • If yes, how did you repair it, and did the repair work?
  • Any suggestions you might have in regards to transom design

I am new and since Iggy asked the Question I thought I might chime in even if it is a year old.
My Coronado hull was modified to run twin Motors. It is a hull extension with added floatation by enclosing the swimstep. Motors will be the new Yamaha 4.3L F250 fly by wire. Each engine weighs 600lbs wet. The 120 gallon fuel tank will be replaced by 2 ea. 104 gls tanks ste 20" fwd of the #2 rib running fwd 8'. The Fishbox is 48" X 63" wide and is placed 12" fwd of the #2 Rib running aft.

I extended my original 21' Northriver Seahawk hull by cutting away the small 18" X 24" extension and welding on a new full width by 30" Long extension. This gave me more floatation at rest and calmed down the porpoising that I experienced with this 2004 year Design. Northriver in 2006 redesigned the hull to include a longer and wider hull extension. They also made the bottom with less curve from front to back.
My Cornado with the hull extension and enclosed swimstep.
My Cornado with the hull extension and enclosed swimstep.
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the internals of the supports that are now covered
the internals of the supports that are now covered
This is my NR stern. Motor is removed. I had increased the original hull extension by 7". This was removed to install second mod
This is my NR stern. Motor is removed. I had increased the original hull extension by 7". This was removed to install second mod

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darthplywood
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Re: Outboard Transoms - Let's Talk

Post by darthplywood »

Personally, I doubled the thickness of my entire transom. THe plans called for 3/4 inch Marine plywood with another layer of 3/4 inch plywood around where the motor is mounted. Rather then just doing the extra thickness where the motor is mounted i doubled the entire thing.

Furthermore, i have a heavy aluminum motorwell (see picture) whith a thick aluminum plate where the motor through-bolts go to spread the weight of the motor.

To clarify,the main reason i did this is "Grandpas advice". My grandfather always did this when he built boats.....but also, motors are heavier now then they were. For instance, the 75 HP evinrude weighed only 250 lbs or so in the 60's but my new 2012 90 Evinrude weighs 320 lbs.
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Andy Garrett
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Re: Outboard Transoms - Let's Talk

Post by Andy Garrett »

I dont remember this thread when it was posted, but it still has merit, so I'll add my two bits.

I am one of those who had a donor boat very early on. I didn't seek to install an oversized engine, but it's what was available when I needed it. So my Zip was getting a long shaft 50 when it was designed for a short shaft 40. Knowing this was going to impart more wieght to the transom, I ordered extra bronze bolts with the intention of building horizontal knees where the sheer meets the transom. I thought that this was the point of greatest stress, so why not? Those bolts and knees never found there way into my boat.

I made the standard knee and a transom closer to minimum thickness (mine is 1.5") than maximum. I didn't want a boat that sat heavy at the rear while in the water, so I kept the wieght down. My motorwell is fairly standard, but anchors to the vertical rienforcement I installed on my battens--I got a bit more surface area for glue that way.

Once the decking is in place, mine will be a boat built pretty close to minimum spec while carrying a bit more wieght than it was designed for. That scares me a bit. To add to that, I installed to ski pulls on the transom in areas where it is only 3/4" thick (between the motorboard and frames). At first, this scared me a lot, but then I figured that the motor is pushing the transom while a tube or skier is pulling on the transom. No undue stress will be attempting to pull the transom away from the boat.

I have a transom saver and will not be trying to push the envelope of performance, so hopefully, my boat will survive. I will trust the design and my own careful attention to detail.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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Re: Outboard Transoms - Let's Talk

Post by Trackhappy »

You Guys need not worry with Glen-L designs... I have just had the (mis)pleasure of replacing the transom in a 16 ft Haines Hunter plastic fantastic. Rated at 114HP, and the transom is just two bits of 1/2" ply with about 8 layers of fibreglass over it and two 1/2" ply 8" x 8" knees. I REALLY had to stop myself from redesigning it. :cry:
By the time I have built a boat, I'll be ready to build a boat....

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Iggy
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Re: Outboard Transoms - Let's Talk

Post by Iggy »

So far, my 90HP Mariner hasn't done any damage to my 2" thick transom on my Malahini.

My transom is made of the outer 3/4" Meranti Ply, then I added 1/2" douglas fir and another 3/4" Meranti to the interior, everything epoxied together, to form the 2" transom. I extended the width of the reinforcement up to the sides of the splash well box.

I epoxied a 1/8" thick solid Sapelly lumber veneer ontop of the edge of the plywood to help protect it from wear & tear & water damage.

I have the standard 3x 3/4" built-up knee bolted to the keel & transom, plus the standard motor splash-well shown on the Malahini plans.

I've pulled a tube with it, and strapped down to it for shipping. I'll keep an eye on it, as I have yet to hid anything with my motor skeg or so anything violent to my boat that could add a lot of stress to the transom.
Ian (aka Iggy)
My Malahini Build

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