Strength of new boat transom?

[reelfishin]

I was wondering how strong is strong when it comes to a new boat's transom?

What got me wondering is a couple of things I ran across this weekend. One was a fairly new boat, less than a year old in which I noticed a fair amount of flex in the transom of an almost new glass boat with a 3 cylinder Yamaha installed. For some reason it didn't seem like it was all the way down and I was pushing and prodding at it to see why. What I noticed was movement in the transom itself. Not a lot, but movement. I could lift the motor and with the motor under my arm and in the locked 'UP' position, see visible movement in the transom. I could flex the transom about 1/4" or so overall when I was trying to get the motor to move on it's hinge. (It turned out to be defective new steer tube and bushing which was most likely damaged during installation of the cable and covered up).
Then yesterday morning I was at a marina/dealer getting parts and watched two guys fighting with an outboard on a small glass boat. Apparently they couldn't get to or were too lazy to go under the splash well to unbolt the two lower motor bolts and were flexing the motor up and down by hand apparently trying to snap the bolts. What finally gave was the whole transom, a load crack/pop sound could be heard as one guy lifted the motor as hard as he could under his arm against the transom. I was surprised that the transom gave way with far less force than I would have thought.

This made me double check my own boats, but there's no way I'd put that much force on my transom 'Just to see what happens'.

How much deflection and how much force is enough or too much?

I'm a pretty big guy and could feasibly also do damage if I tried.
The old test was always that a good transom will not move when you use the motor as leverage, but apparently that's not the case if you use enough force.
On my boats, I can put nearly all of my 290 lbs on the outdrive with no noticeable flex, but I never tried lifting the motor beyond it's limits.

A buddy was with me when we saw that display of mechanical genius, he went home and checked his new boat, and indeed, if you lift the motor and apply force, the transom will move almost a 1/2". No movement can be seen when pushing down or applying weight to the lower unit. We took that boat out for the first time two weeks ago, no transom movement is seen on the water and it is a brand new boat.

On all of the larger boats I've checked, there's little to no give in the transom, but on most smaller boats, even brand new, I can flex the transom to some extent fairly easily. We're talking new boats on the dealers lot.

On any of my boats which I have replaced the transom wood, I laminate a combination of wood and glass mat encased in epoxy, which makes for a very stiff transom, but even those will show some movement with enough force.
I did go out back and compare movement on an old Mako I have sitting at the shop, the boat and motor are junk so I could do as I pleased and not worry about ruining a good boat. I tilted the 1975 Johnson 3 cylinder all the way up and lifted, I could lift the boat off the trailer with no signs of transom flex, and that boats been sitting in the weather for years. I finally snapped the bracket off and the motor fell onto the splashwell with me holding the lower unit. It broke at the bracket hinge pin. Only the tilt guide and lock held the motor from dropping to the ground.

What it boils down to is how much force should a good transom tolerate? Even a solid transom will flex if enough force is applied. Every transom other than that Mako had some movement to some degree.
I'm not talking about a floppy rotted transom just movement. How much is normal?

 

[Driven1]

Re: Strength of new boat transom?

A good bit of a tale and an interesting question. I don't have an answer for you but I do have a couple of thoughts. I would expect that the transom's total area would be designed to handle about twice the rated horsepower for the boat in question.

It may also be possible that some amount of flex is designed into newer transoms to take some of the strain off the back of the hull in sort of a shock absorber action. Kind of like back in the day when cars were built like tanks for crash protection. Later finding out that it was better for the car to absorb as much of the impact as possible but keep the occupants mostly unharmed. Manufacturers may have found transom stress failure rates were lower if the transom was allowed to absorb some of the stress.

I've lately found myself pondering if a transom core really needs to be glassed in (or adhered to) the hull at all. I think it could just as easily be left unadhered and just held in place from the inside with screws to keep it cotacted to necessary contact points and from bouncing around. Think about it. While a boat is under acceleration in forward movement it's really the inside edges of the transom tabbing, the stringers, and the floor that transfer the energy of the motor through the transom to the rest of the hull. The transom is just an energy distributor. With that in mind, you'd think that it's not actually necessary to adhere the transom core, it just needs to make contact to those points. Think how easy it would be to replace a transom if it weren't glassed in!

I'm not a vessel engineer but I did stay at a Holiday Inn Express once.

This should make an interesting conversation.

 

[Willyclay]

Re: Strength of new boat transom?

I'm on my fifth project boat. Never owned a new boat. They range from a 1960 model to the current 1996 model. The only transom problem I ever experienced was a rotten plywood core in a 1965 LoneStar 16.5ft Medallion II. Great boat except that I had to remove the transom core with a shovel. Why the 1968 V-4 Johnson 100HP mounted on it did not fall off, I don't know! FYI, it was 1&5/8 inches thick not including the aluminum skin with seventeen plies.

The four other boats were all fiberglass construction and had ZERO transom problems with little or no movement. To me the most amazing transom design/construction was a 1960 Glasspar 16ft Avalon. It had NO splashwell, stringers or knees and the transom was only 1.5 inches thick. It had a 1962 V-4 Johnson 75HP on it when I got it and was solid as a rock. I later cut the transom down to accept a short-shaft 1958 I-6 Mercury Mark 75A. The only notable part of the design of the transom was a slight bow in it. That may have been the source of the rigidity since it was thinner than the typical transom then or now. My two cents!

 

[reelfishin]

Re: Strength of new boat transom?

I have a Glasspar Tacoma here, very similar to the Avalon model. Same curved transom design, two of the MFG boats I have here, an Edinboro and the other what I believe is a Niagra, both have curved transoms as well. The Edinboro is pretty solid but the Niagra has issues and most likely will need a transom job. I did the transom over on the Glasspar last spring, it was rotted in the upper corners but still solid down low. The Glasspar was curved far more than the MFGs are.

All of the transoms I've seen have been 1 1/2" give or take the thickness of the fiberglass. Most all are made up of plywood layers that make up a 1 1/2" core.
Most are two layers of 3/4" plywood, those with curved transoms are usually made with layers of either 3/8" ply or 1/4" ply. When I redid my Glasspar I used 1/4" plywood laminated with a layer of glass in between each layer of plywood.
It turned out rock solid but I am sure if I used the leverage of the motor when raised I could break or flex it. The boats that seem to have the most flex are those either with no splashwell or those with a bolt in splashwell like the MFG or Glasspar. Both have no inner gussets or support other than a thin bolted on splashwell. It does make for easy transom replacement since the top cap doesn't need to come off but I think it's a liability when it comes to strength.
I can understand an older boat having transom issues but I would have thought that by now nearly all boats would have composite transoms.

 

[oops!]

Re: Strength of new boat transom?

I can understand an older boat having transom issues but I would have thought that by now nearly all boats would have composite transoms.

youd think they would.......but even donzies are still wood......i think some mannufacturers are starting to go composite...but 90% are still wood.....


as far as the question...its best left for the naval archetect that designed the hull........but (imho)...it really depends on the legnth of the boat and the power attached to it.

the transom is actually the transmission of pushing force from the drive to the boat.......the transom gets pushed on....the force is transfered to the stringers, that are attached to the hull......the hull takes the pressure from the force of the water and distributs it evenly across the hull.....

think of a dragster.... the rear wheels being the transom and the long frame being the stringers....when the dragster takes off at the line....its tires hook up with the pavement.......(again....the slicks being the transom).....the force of acelleration is transferred to the frame....

now lets pretend the wheels dont turn...but slide across the pavement as a transom does in a boat..........as the dragster wheelies away from the lights....all the force is on the joint between the wheels and the frame..
(the transom and the stringers).....(thats also why kneeing the stringers is such a good idea).....but in the case of a boat....the hull is allso coming into play....so the stransom hull joint is most critical.

you dont want much flex in that area at all........

most of the stregnth of a transom (as mentioned) comes from the tabbing of the transom into the hull......in new production......after the transom is installed there is a coating of glass either sprayed onto the transom....or hand layed and tabbed into the hull.........i like hand layed for this application...but a lot of the price point boats are sprayed with a choppergun.......(i saw chris crafts that were hand layed with really thick glass and fiber glass cords wrapped around the transom to the hull joint.)

if your transom is weak to the point that you think it will break off......get in there and tab some glass between the transom and hull.....a couple of tabs of 1708 and the transom will stop flexing....as the weight distribution will be displaced farther along the hull....and not just at the back 2 inches of the glass hull......

in the hull extension thread....on the big ray......i not only tabbed the transom in...but ran a full layer of 1708 up the entire hull and transom, and then layed another peice of transom over the wrap......the transom is truly a peice of the boat and the strongest part of the boat.

i hopes this helps you and if any other members have thoughts join in!

cheers
oops

 

[reelfishin]

Re: Strength of new boat transom?

I agree with tabbing in the transom as you describe, but the 'flex' I refer to is actual flexing of the transom panel as a whole, not movement at the edges. In other words, is two layers of 3/4" plywood which often has to span a 60" length of transom enough to hold up to years of use and the weight of the motor.
I don't doubt that any of the transoms in question will hold up fine for years to come, it's just that the idea of a transom being solid like a rock doesn't seem to be the case. I tore apart one boat, an older Starcraft that had what to me seemed to be excessive movement in the transom at the motor. After pulling the upper cap, the wood was dry and solid, it just wasn't a tight fit in the hull cavity. It spanned the whole opening, but didn't fill the entire void between the inner and outer glass. The flex was the motor clamps just no being able to compress the glass layers together onto the wood. When I put it back together, I added an additional 1/4" of width in the way of several layers of glass both in between and on the new transom wood. It could have still taken a few more but I didn't want to make it too wide. I filled any remaining voids with chopped glass and epoxy paste. The result was a very solid transom but I can still make it move if I lifted on the motor. But just the same, it's only as strong as the wood core and the few layers of glass mat.
What really got me thinking about the overall strength of the wood core was that I had a 40 HP motor with a seized or stiff tilt pin, I was trying to work it loose on a motor stand when it the stand's wood panel cracked. The wood mounting panel was a sandwich of tw 16" x 16" plywood panels 3/4" thick laminated with epoxy with a double layer of woven glass mat inbetween and single 3/16" aluminum panel on each side. The aluminum bent, and the plywood just snapped. I would have thought that the plywood alone would have been stronger, but it wasn't. The plywood was left over marine plywood from a local supplier. I would have bet money that the motor bracket would have gave way long before that motor stand would but not so.

It also goes to show the amount of force a tilted motor can put on a transom, especially when towing. Any of the transoms I looked at and saw flex in exhibit no movement with the motor down and I can't put enough force on the lower unit downward to cause any movement, it's only in the tilted position. When the motor is down and locked, the force is not multiplied as it is when the motor is free and the tilt hinge point is a fulcrum point which amplifies any force applied. I'm sure there's some engineering term for this but its not that important other than the force when pulling up on a motor apparently is far greater than the force of the motor just pushing directly against the transom or the weight of the motor hanging on it. A motor in the down or latched position has 10 or 11 inches less of fulcrum force or torque capability on the transom as it does when it's up. The leverage point is changed in effect from the point of the lower bolts to the point of the hinge when its raised.

 

[brownies]

Re: Strength of new boat transom?

I've replaced several transoms. My technical method of checking them was to raise the motor (outboards) and stand on the cavitation plate and jump up and down a few times. Always been under the impression that there shouldn't be any "see-able" flex.
These were all fiberglass bass boats running 85hp and up though.
Skeeter has an add where they show one of their boats setting on what looks like an engine stand. Bolted to the transom with the weight of the boat hanging in the air. From what I have seen though, IF that's the reason you buy a Skeeter, you better buy THAT boat. I have several Skeeters and if you ever took one apart, the transom to stringer support area wouldn't impress you.
Ranger sort of incorporates a jack-plate style transon into many of their boat models. The actual transom only big slightly larger than the the motor bracket, and this transom is set back from the main body of the boat. What's inside almost apears to be a non adjustable jack-plate. Makes a person wonder how it'd ever hold up, especially if you ever see one that's had the transom torn loose (from a wreck...I've not seen one damaged from normal use, but.....It does amaze me that I haven't).

Of the transoms I have replaced. My method of checking for weakness is explained above. After replacement...none moved/flexed. And I've never added any extra support or made any transoms any thicker than they came.

 

[reelfishin]

Re: Strength of new boat transom?

I've replaced several transoms. My technical method of checking them was to raise the motor (outboards) and stand on the cavitation plate and jump up and down a few times. Always been under the impression that there shouldn't be any "see-able" flex.....

I suppose this also has a lot to do with your size and weight? At over 300lbs, 6'4" tall, I've not yet found a transom that won't flex to some degree with the motor up. I have a run stand I built at the shop, it's made of 12" 1/4" wall C channel with 4x6" 1/4" uprights. It don't flex. But I have had motor brackets that were stuck or corroded either break of flex when lifted by hand. I had one the other day snap at the hinge area on the port side while trying to work loose a seized tilt tube. I would have thought for sure I'd break the lower unit off before the bracket broke. The motor wasn't corroded thin or anything, just the tilt pin had seized from a few years of non use.
Most boats are made with plain plywood, usually 3/4", a material that you can take and break over your knee with some effort. So I don't see how a span of say 60" across the width of a boat and two unglued layers of such a material can ever be flex free. The shorter or narrower the transom, the less obvious flex. I went down to a local boat dealer today, they have several brands, most advertise composite transoms, some do not. Every boat on that lot with a motor had some flex with the motor lifted up.
I'm not talking about standing on the outdrive, I'm talking about grabbing the outdrive under my arm and lifting. I'd venture to guess that I could apply enough force to any one of them to either break the transom of the motor bracket. I could easily stand on any of them with the motor down and they would show no flex. I can stand with all of my weight on the transom of my 1983 Renken 900, but yet if I lift on the motor, the bracket and center of the transom will move about 1/8" but the unsupported span of transom on that boat is only about 27" wide or so. On my Sport Craft, the transom spans the entire width of the boat, as do those on my 1962 Glasspar, and 1965 MFG. I stood on the lower unit of one new boat and watched the gunwales move up near the middle of the boat, another 16' boat lifted the bow completely off the bow stop with no flex. A buddy has a less than one year old Mako, and even that has some flex when it's 115 Suzuki is lifted against it's upper limits while raised. That boat was brand new this past spring.
I am not sure whether the 171 model has a wood or composite transom, but they do advertise the deck as being balsa wood cored, so I suppose the transom is also wood cored. That boat is one that actually scares me when doing the lift test, it feels like I can rip the motor right off. There's not much support and lots of flex. It's a trailer launched and garage kept boat as well, so event though it's less than a year old, it's also very unlikely there's any rot in the transom. It also has no splashwell.

At what point is the stand on the outdrive or wiggle the motor up and down test too extreme? I would venture to guess that I can apply at least 350 lbs of force to the lower unit by lifting it, and that would be amplified by the length of the motor and the leverage it applies through the transom bracket. I'd guess the force would be close to 1,000 lbs or more? There's no way two pieces of 3/4" plywood could hold up to that with no movement?

I'm not saying that a transom won't give some, but at what amount of force does it become an issue to really look into? I guess I'm more looking for an engineering answer than anything else here.
I was told that a transom redone with Sea Cast was as good as it gets, but yet I checked one out at a boat show and was able to flex that as much or more than a wood one. (If I hadn't known it was a new transom done with resin, I would have probably begun removing the cap to prepare for a transom job). While one done with Sea Cast by a neighbor on a much larger boat had no noticeable give at all and holds his twin Mercs just fine.
I'm not knocking any particular boat or design, I was just wondering how much is too much force or too much movement.