Engine support.

Nasher · 21 June 2006, 19:53

I’ve been spending a lot of time and effort on my trailer lately, it’s now sprouted twin matching axles, bearing savers etc.

Even though I know its not good practice, I’ve always towed it around with the engine just raised up part way on the hydraulics.

Thinking about it, whether its on a lump of wood inserted between the saddle and engine, or just on the hydraulics, the forces involved as 200kgs of engine wobble about are all transferred to the transom through the relatively small area of the saddle, which I know happens anyway when bouncing about over the waves.

So I’ve considered making up a ‘Y’ bracket that braces the gearcase to the trailer, but I’m not happy that any movement of the boat on the trailer will transfer unacceptable forces through the gearcase and hydraulics.

So I’m considering putting some pads and eyes near the bottom of the transom that will take the feet of a small ‘A’ frame, the apex of which will then strap to the gearcase via a rubber block. I’d attach the frame to the eyes and power the engine down onto it.

Question is, by locking it all up solid, am I going to be putting undue stress on the area of the gearcase that touches the rubber block?
The hydraulics at least act as a shock absorber.
Any other comments?

Nasher.

matt h · 21 June 2006, 20:05

what i do is to raise the engine fully then down on to its engine stop, get a small say 500 kg ratchet strap and connect it to the towing eye on the transom, then around the engine, mid section then to the other towing eye, do it relatively tight, that will take the stress off the hyd's,

Nasher · 21 June 2006, 20:15

The drop down stop should not be used for towing. and anyway if I use mine the engine is almost horizontal.

The saddle and small area of transom are still taking all the bending forces though. I'm trying to spread the load over a bigger area.

Nasher.

antiqueboy · 21 June 2006, 20:16

it dosnt seem that this would acheive much, the weight now is on the transom spread quite well with the engine,transom and trailor rubbers absorbing pressure
your suggestion with an a frame on the transom,puts opposing forces on the transom,and gearcase.the rubber absorbing little i suspect it would stiffen everything up too much?
the a frame also puts a higher pressure in another direction on the transom which it is not built for this could act as a swinging pressure motion weakening the transom further
im no expect but save your money and time would be my advice

jyasaki · 21 June 2006, 23:50

The reason for supporting the motor while trailering is to minimize the pendulum effect and subsequent wear on the hydraulics and the transom.

While you're in the water and running, the force on the transom is generated from the prop. It is usually in a forward direction, with some twisting force from the arm created by the distance from the mounting bolts to the prop. In any case, while the forces can be fairly substantial, I think they are a much more gradual type force than the bouncing you get while trailering.

Most of the "transom savers" or "motor support brackets" I've seen have the lower unit supported in a block of some sort, and the bracket ties in to the trailer in some fashion (often to the rear-most keel roller or to a bracket on the rear crossmember.)

What this does is two-fold: It minimizes any kind of rocking action on the motor and its pivot points, and it transfers some of what does occur to the trailer itself, rather than channeling it all into the transom.

There are other methods (which I am skeptical about, at best): some people use a "M-Y Wedge", which is a small plastic tube, for lack of a better description, that slips over the shaft on the trim ram, thus taking the strain off the hydraulics; others use a piece of lumber placed between the motor and the mount, and relieve strain that way. Neither of these address the pendulum forces that the motor experiences as you go over bumps and what-not.

If you want to go with a quick and easy (note I didn't say "cheap") solution, take a look here:

http://www.swivleze.com/prod_ts.ht

jky

Andy_Rs600 · 22 June 2006, 00:09

Does anyone actually know where the centre of mass is for a large (150hp say) outboard? I started drawing a free body digram for this to take a look at the forces in a support, and then realised that i didn't have a clue. If you pick up a small outobard, the CoM is almost in the engine block, but i don't have a clue how that changes as the engine gets bigger.

Limey Linda · 22 June 2006, 00:59

Don't make it too complicated. The effective center of mass is the centerline of the centerline of the horizonal engine pivot. That is where the load is taken regardless of tilt angle. This pivot point is typical 12 " aft of the transom. Using hydraulics to secure the engine is a bad idea; after a few bumps you will force the oil back though the cylinder valves and it will become sloppy. Using the lift catch is a really bad idea. The old trick of lowering the engine on a piece of wood and then using a heavy duty bungee to strap the skeg to the trailer, this will stop upward/downward motion and compensate for small movements of boat on trailer: really works. Unless your driving a dragster all boat movement will be forward due to braking so make sure it is a good strong bungee Remember it is the bouncing that does the damage not the weight. Ever gone over a speed bump ( sleeping policeman , as I remember) too fast and felt your arse lift off the car seat????? and your in the vehicle. Imagine what is happening to your engine which is pivoted off the back maybe 25-30 feet feet away. Your arse moves up/down a little but your engine wants to move one hell of a lot more.

Hope this helps. T.

Andy_Rs600 · 22 June 2006, 08:17

Quote:

Originally Posted by Limey Linda

The effective center of mass is the centerline of the centerline of the horizonal engine pivot. That is where the load is taken regardless of tilt angle. This pivot point is typical 12 " aft of the transom.

my little brain doesn't understand........

The load is always applied in the same place as long as there is only one support. In that case the posiion of the CoM only affects the moments(bending loads) applied to that support. If there are 2 supports that changes. The distribution of forces and moments between the supports depends totally on the position of the CoM.

As it is probably the bending moments that do most of the damage to the transome, the ideal support would provide enough stiffness to rotation of the engine that the transome bracket did not have to carry much bending load. Alternativly, you could make the normal mounting floppy enough that it didn't carry load, but that would probably involve bleeding the hydrualics. There are a few simple ways of making the joint between transome and support (or trailer and support) such that it can not transfere any bending load to the boat/trailer either. If this were all set up right, the only loads the transome would have to take (almost) would be pure compression. Being as fiberglass is stiff in compression, there isn't much fatigue action associated with that loading, so you're boat doesn't break

DavyBuoy · 22 June 2006, 10:51

Started something there didn't you Nasher !!

Limey Linda · 22 June 2006, 22:45

http://www.m-ywedge.com/ This is new and makes sense to me. I would still use a heavy bungee to stop upward movement. If you get one let us know how it turns out. Watch the demo video.

Nasher · 22 June 2006, 23:02

Why should I buy something?

Did I not make it clear I would be making any solution myself?

Nasher.

Hightower · 22 June 2006, 23:22

I wouldn't worry about the stresses caused on a trailer going over a bumpy road. They will be nothing like the stresses created when wave hopping.

Yes. Thrust from the prop is a factor in design of outboard brackets but IMHO the stresses caused by the beformentioned pendulum effect would be cause for concern as Transoms have the strong knees on the inside and not the outside of the hull.

If you follow the RIB manufacturers recommended weight of outboard on transom you can't go wrong.

PS...Trailers have suspension to aborsb shocks....Boats do not!

I wouldn't worry about it Nasher, go spend your money down the Pub/Tank of fuel instead.

Limey Linda · 22 June 2006, 23:43

Just re-read this thread. I would probably do the same as Nasher and build my own support system. However: please note that hydraulics do NOT provide any shock absorbsion. The ram is a cylinder full of oil and liguids do not compress, only gases. It does not work like a car shock absorber. Still think a stick and a bungee is the simple solution. Although I would be tempted to buy one of those slip on ram thingees for 20quid , just to see how well it worked.

Hightower · 23 June 2006, 00:18

I know how hydraulics work! I'm an engineer and work with this sort of stuff every day.

But I don't think you get it.

Last year I broke my transom......I didn't break it on the back of my trailer. I broke it going over the wake of a large high speed ferry. That proves that there are larger forces acting on the transom whilst under-way, at Sea.

Surley if the Hydraulics can take the force of 200hp and support the weight of a 200kg outboard going at 50knotts a simple road trip isn't gonna hurt it. I've never had a support and I haven't yet damaged anything.

Nos4r2 · 23 June 2006, 00:49

Quote:

Originally Posted by Hightower

I know how hydraulics work! I'm an engineer and work with this sort of stuff every day.

But I don't think you get it.

Last year I broke my transom......I didn't break it on the back of my trailer. I broke it going over the wake of a large high speed ferry. That proves that there are larger forces acting on the transom whilst under-way, at Sea.

Surley if the Hydraulics can take the force of 200hp and support the weight of a 200kg outboard going at 50knotts a simple road trip isn't gonna hurt it. I've never had a support and I haven't yet damaged anything.

It'd be interesting to see the stress analysis on it if it was possible to gather the relevant data to do one

Limey Linda · 23 June 2006, 01:39

Quote:

Originally Posted by Hightower

I know how hydraulics work! I'm an engineer and work with this sort of stuff every day.

But I don't think you get it.

Last year I broke my transom......I didn't break it on the back of my trailer. I broke it going over the wake of a large high speed ferry. That proves that there are larger forces acting on the transom whilst under-way, at Sea.

Surley if the Hydraulics can take the force of 200hp and support the weight of a 200kg outboard going at 50knotts a simple road trip isn't gonna hurt it. I've never had a support and I haven't yet damaged anything.

It would appear that we should take Mr. Highpower's engineering advice and do nothing to protect transoms etc. while trailering. "you pays your money and takes your choice" I do not think so; but that is just an opinion from a 60 year old engineer that has owned 10 boats and trailered well over 10k. miles ?????

BogMonster · 23 June 2006, 11:50

I don't know anything about engineering but one might reasonably assume that RIB manufacturers make their transoms strong enough to take the specified engine weight in whatever situations the boat might be found in whether wave hopping or on a trailer? Lots of engines falling off would be bad for business....

What I can say is that there are no sign of problems with mine and it has been trailered by previous owners on roads that are much rougher than any found in the UK (unsurfaced gravel and clay), and it has even been towed quite some distance off road by the previous owner, which would be more thumping and banging around than most ribs would ever see trailered on tarmac roads. The trailer has no suspension and is fitted with four 7.50x16 Land Rover wheels which do not give an especially smooth ride. I think the guy I bought it from said he just used to lower it on to the locking frame and then put a ratchet strap around it to take the load off the hydraulics. No apparent problems (famous last words!)

Hightower · 24 June 2006, 00:01

Quote:

Originally Posted by Limey Linda

It would appear that we should take Mr. Highpower's engineering advice and do nothing to protect transoms etc. while trailering. "you pays your money and takes your choice" I do not think so; but that is just an opinion from a 60 year old engineer that has owned 10 boats and trailered well over 10k. miles ?????

HighPower.....I like it!

Let's just get one thing straight please.

I don't tell people what to do at all Linda. I let them make their own minds up. I can however offer advice though experience or knowledge or even offer opinions if it helps.

If people want to design and build or buy a peice of kit that they think will add protection to their transoms and outboards, that's fine. I however have not the need for such a device and have never had a problem with any boat I have owned.

I did however have a small fishing boat with no hydraulic T&T that I had to trail with a piece of wood jammed between the saddle and skeg (which is what Nasher is kind of talking about), but that was to give clearance between the skeg and road whilst trailering and nothing more.

jyasaki · 27 June 2006, 18:00

Quote:

Originally Posted by Hightower

But I don't think you get it.

Last year I broke my transom......I didn't break it on the back of my trailer. I broke it going over the wake of a large high speed ferry. That proves that there are larger forces acting on the transom whilst under-way, at Sea.

No it doesn't.

All it "proves" is that your transom failed while on the water, rather than on the road. Can you state for sure that all damage occured at that point in time, or could it have been weakened on the road, to fail when you hopped the wake?

jky

Limey Linda · 27 June 2006, 18:31

Quote:

Originally Posted by jyasaki

No it doesn't.

All it "proves" is that your transom failed while on the water, rather than on the road. Can you state for sure that all damage occured at that point in time, or could it have been weakened on the road, to fail when you hopped the wake?

jky

A VERY, VERY good point. When it comes down to it why take the risk of not independently supporting the outboard, particularly from bouncing. Some boats maybe stronger than others, but "who knows".