Is there a reason why Balsa is not

stop avoiding the original question

Is the transom construction for a stern drive boat important ??
is the stern drive thrust taken on or through the transom??
is the thrust of the stern drive pushing on the motor ??:idea:

 

Need answers !!!

Quote! -- Straight inboards, where is the prop thrust absorbed here?? through the transmission, and the engine, into the structure of the boat.
A stern drive also transmits some of these forces into the hull, not all because the transom comes first, but is buttressed by the engine and mounts. unquote !

Is there anyone out there that will agree with either of these statements ??? And of you do agree answer me why !!

 

push and leverage whats the differance ???

How about they exert the same amount of push but the stern drive is nearer the bottom of the transom and there is less leverage but the same push as a outboard of the same hp would give !!:idea:

 

I beg to differ on this opinion = anywhere below the waterline it is not a good choice.

All the 36 foot charter launch's we used to make all had balsa cored hulls from the chine to the edge of the turn into the keel 20mm thick balsa ,stem to stern also the transom was balsa and of course the topsides of the hull to !! The balsa was bedded into s slurry mix and pressed well into it so it came up between all the blocks every where , after the whole lot was surface ground and edges bevelled and rounded then was sealed with a brew of poly resin and applied with a paint roller . to have a glossy resin rich look and when it was hard we started glassing !
Never ever had one problem with this ever ! but in saying that all of us had worked in the industry for many years and there was never ever any doubt of anyone's workmanship or skills and that's what makes a big difference !!

I do remember seeing a boat that belonged to one of the resin companies preps and he had used a hole saw and drilled into the cored side of his boat to see how far the sea water would penetrate over a given length of time this was below the waterline so was totally wet all the time . naturally the water penetrated straight into the inner skin in seconds but surprised to note it did not soak any more than 10 mm sideways around the hole . even after 5 months was only about 12 mm !! so as for whole panels getting water logged its got to be the method used to install and poor workmanship not the products fault!!water will travel with and along the grain of balsa but wont go sideways !!
I did some testing myself to show management about 6 months ago with using uncatalysed resin and a 300 mm x 300 mm square of 20 mm balsa. The resin went straight through from top to bottom but again never travelled side ways at all over a 24 hour period !!
I do my own testing a lot of the times to prove and make a point if there is any doubt about the usability of any products !!

 

Not all balsa cores out there have been bedded in epoxy to seal each cell. A great number of earlier layups had balsa panels within which water could migrate laterally unimpeded by any barrier. This is not to say I would personally trust a correctly done modern below-the-waterline balsa-core construction. It seems ridiculous to build a hull this way, where WATER can possibly destroy a thing that is meant to go in the WATER.

 

Alan, hundreds of epoxy sandwich cats have been built with balsa core bottoms, to my knowledge there are no more insurance claims than with monolithic hulls. It is not that ridiculous when you can save 10% of light displacement build only on this part! And a monolithic structure is much more subject to fatigue than a sandwich one.

Nevertheless, there are still builders around using balsa or foam cores with ester resins, this is stupid.

But this debate will always go on.

As for whether the transom can take the load, it is a simple structural calculation: calculate the thickness of the skins needed to take the stress with a minimal safety coefficient two.

 

I will allow that modern balsa core construction is fairly well developed. I consider multis to be performance boats, in that they must above all be light. Most boats are monos and they seldom would need balsa cored hulls to be good boats.
As you say, polyester resins won't prevent lateral water migration. Therefore, it makes sense to say that buyer beware a balsa cored hull if he doesn't know the boat was laid up with epoxy encapsulated balsa.

 

Tunnels, with regard to your transom loading question... The difference is that an outboard is bolted to the transom only. In this case the transom needs to not only take the thrust, but also the weight of the motor itself. This creates additional dynamic loads as the boat pitches and slams in a seaway and the inertia of a heavy motor bouncing around, so your transom has to be strong enough to handle the thrust + these additional loads which are very significant as dynamic forces would be an order of magnitude higher than just the static weight.

With a stern drive, i dont subscribe to the notion that the thrust is taken by the motor. The thrust is still taken by the transom where its bolted to the outdrive, but thats the only load it has to take as there is no heavy motor hanging off it bouncing around...

 

The weight is a very good and valid point and very true , my point In this discussion is I am surprised how people see the loadings taken on any transom specially sterndrives and proves a point that people really don't have the foggiest idea what the hell is going on inside there boats !!
Thrust on a stern drive is similar as what a out board exerts ,sideways fore and aft and up and down !
Its no surprise that people get into so much trouble because of lack of understanding what they dealing with and how things work , or should work !:?:

 

Two questions for you then:

Question 1--- What's the maximum force designed for on each of the motor mounts of a 454 magnum sterndrive? Figure worst case full throttle on a rough day.

Question 2--- How much force could a weak transom put on those two bolt rods that connect the back of the engine to the sterndrive transom assembly before those break?

 

The two forces are different. An outboard pushes from a different point higher up because that's where its attached. Leverage tends to exert a twisting force on the mounting pad but more importantly the area just below the top of the transom is pressed forward (the very top is pressed backwards) requiring the construction of the upper transom to transfer that pressure to other areas such as the sides and any built-in or molded in bracing attached to the bottom.
More bracing is required of the outboard than the stern drive due to the higher position of the external parts and as said, the not inconsiderable weight of the outboard itself.
It simply a more efficient way to propel the boat because it uses less materials in order to achieve the same end. Never mind the obvious advantage of moving the engine forward which is far more seaworthy due to having less weight in the ends.

 

At last people are beginning to think about the stern drive as a unit and a out board as a different unit , but both AT 200 hp should be pushing the boat as close to the same speed as each other ?? YES ??
They are both pushing on the transom one near the top and the other near the bottom !!
So the transom really is a really important and vital part of the boat and needs to be constructed so !!
All these things I had to think about a while back because of the boats we were making at the time were all tail down and tail heavy and what could be the most practical way of shifting weight forward !!2 Motors first then big fuel tanks along with a water tank and batteries along with an oversized gen set were all the heavy weights sitting right at the back under the floor
while looking into this problem I discovered Mercruiser makes a kit so the motor can be shifted forward away from the transom !!, the kit only consists of a drive shaft and a exhaust extension !!, so I got to thinking about all this and everything !! Now I am trying to make you guys think about the loading on the transom when the motor is mounted away from the stern drive and the only connection is the drive shaft and exhaust rubber hose . so loadings and push on the transom needs to be understood more . no one has seen that all this is completely taken on the transom !!Be it high up in between of low down !!
ok to add to the confusion even more lets throw another 3 rd problem into the pot, what about a surface drive where is all the push to move the boat exerted ???

 

So if we all ignore the questions they will fade away in the distance and be forgotten !
A thick transom for a stern drive is still pushing the same on the transom as an out board right ?? but in a different place ! one near the top and one near the bottom there are other forces involved as well such as twist and leverage
The leverage on a stern drive is less because it closer to the water than a outboard that is further away its why in mentioned the old 16 inch, the 20 inch, the newer 25 inch ,and the extra long 30 out boards, but the push exerted over all on the transom is still the same !! right ??
Must be or the boat would not move forward at the same speed or close to the same speed .

 

Modern stern drives have fairly lightly loaded transoms. The loads are borne by the engine beds (through a progression of mechanical connections), which transmit the loads the the hull bottom and to a lesser degree the hull shell proper, which is where they need to go. Outboards transfer loads to the transom, which then spreads these to the hull bottom and sides, so understandably the two arrangements are handled differently. In most modern designs that are outdrive specific, you'll see just a single layer of 3/4" plywood (or other core) in the transom and probably not even full width, just for the very reasons I've just explained.

I'd like to see the drawings or parts you're speaking off Tunnels, as I have a digital version of the Mercruiser catalog right here and I clearly see a torque arm, attached to the inboard side of the transom assembly and bolted to the engine beds.



This image shows the direct relationship of the drive and engine. Nothing is floating or hanging on the transom, it's through bolted. The transom assembly is bolted to the inboard bell housing, which has the rear mount tabs on it. If you use the engine relocation kit, the torque arm(s) pick up these mounts and also receive the transom assembly through bolts. The inner and outer bell housings shown below clearly align for through bolts. Also you can see the starboard aft mount tab.



As to balsa, well it certainly can be used below the waterline and has, in all resin systems, though admittedly vinylester and epoxy are the only good choices, plenty of yachts have polyester/balsa hulls. Some have had issues, but this could be said of any core material. If well engineered and proper application techniques are employed, a fine, light weight core material. Personally, I'd never have a balsa hull, but I also know the huge numbers they've been produced in, which is telling in itself.

As for the speculation about potential torque loads and element design, well I think several have made good points, but the engineering understanding is lacking a bit, as it isn't that difficult. Inboard, I/O, extended I/O, outboard, it doesn't matter, it's pretty simple math really, though not the same for each, they all do get the loads primarily to the bottom and subsequently dispersed, to the rest of the hull shell.

Now before you ask Tunnels, there's no "Cliff's Notes" for these calculations, just common engineering problems, that need reasonable solutions, which are addressed on an application specific basis, with a the appropriate margins described by the application specifics.

 

Much differant to what we been using !!

What about this one !! this is what we were intending to use with a longer drive shaft and exhaust !!

http://wwwarchive.mercurymarine.com/uploads/6866/1905/drw43-007.pdf