Force exerted by outboard on transom

Discussion in 'Outboards' started by RogerWilco, Sep 6, 2013.

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DCockeySenior Member

That gives the moment about the location on the bracket used for the distance. The moment can be used to determine the forces, satisfying both sum of forces and sum of moments equals zero with the thrust of the propeller included in the sum of forces.

But that provides the forces due to thrust but not the dynamic forces. Dynamic loads may be more important than the load due to thrust.

2. dskiraPrevious Member

How you will calculate the dynamics load from no starting measurement?
In my mind the motion of an outboard is necessary to start a dynamic load, which mean to start a motion you need a thrust.
Without the propeller turning, you have no dynamic forces.
Now in top of the thrust you can add dynamics load from the hull, and that is a good point, the load the propeller has to push. But that can be find in the necessary thrust to push the whole hull. The resistance of the hull at a given speed for a given thrust of the outboard motor.
So yes I see David what you are getting at. It make sense.

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TANSLSenior Member

I think we need to distinguish between the propeller thrust, which does not depend on boat's speed, but is what generates the ship's speed, and engine weight that, due to inertia, can exert a force 3 or 4 times greater than its value.
But what I think some would like to know is how to calculate the thrust generated by the propeller. Is it to ask the manufacturer?
Then we'll talk about the efforts on the transom, the size of the console and all those issues. But without knowing the thrust, the rest is useless.

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DCockeySenior Member

By dynamic loads I've meant the loads on the transom from the inertia of the engine due to the hull moving other than at a steady (or close to steady) speed. The movement of the hull could be caused by waves, such as slamming. They will also occur if the boat is trailered by the trailer going over bumps in the road. Any suggestions on a different name for these types of loads? They can be very significant for a transom with an outboard mounted to it.

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TANSLSenior Member

Slamming is a phenomenon that occurs in the area near the bow of the boat due to causes now I can not say because I do not remember (possibly very specific overpressure). It has nothing to do, in my opinion, with the loads on the transom.
The more effort that gets transom are when the boat is trailered with the engine in the transom. Greater than any of those produced by the waves. Comparable only when the engine hit a floating rigid object.
Inertia forces are that, forces of inertia and can not be called otherwise.
But how to calculate the propeller thrust?

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tom28571Senior Member

I guess I thought that was all clear.
Actually This should all be clear. Are we complicating a fairly simple issue by over talking it?

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PARYacht Designer/Builder

I agree Tom, this isn't a difficult subject, both mathematically or in a practical sense. The OP hasn't returned to this thread yet, possibly over some of the banter. It's not a difficult thing, particularly in light of the countless tens of thousands of transoms, built to these relatively standardized thicknesses. As to Geer's spec's, well they're a little heavy, but certainly safe.

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Concur.

First port of call seems to be the last these days in this super duper electronic highway age. Just consult a good ref book when in doubt, if unable to calculate the thickness yourself:-

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Mr EfficiencySenior Member

You really wonder how much redundancy there is built in to standard transom construction, many boats finish with half-rotten transoms that still seem to hold up, though of course not without the odd catastrophe.

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PARYacht Designer/Builder

From the spec's I've seen in Geer's books and the other obvious texts (like the one John has shown), I'd say you're right. 1.25" (32 mm) as the minimum for 5 HP and lower, is just silly, as is 1.75" (44 mm) for a 12 HP. I think mine (previously listed) offer more rational thicknesses, particularly in small craft where weight savings could be as much as 50% over some of the other recommendations.

11. dskiraPrevious Member

I have for my four stroke 15Hp a transom of 1-1/2" thick lumber, not plywood.
Work very well.
It depend mostly how the transom is connected to the hull, the area without reenforcement where the bracket is attached, well as usual it depend of the whole construction and not one dimension.
So the thickness should be also practical for the clamps on the bracket if its not bolted.
I prefer my 'obvious text" than any other one. It serve me better in most cases.

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TANSLSenior Member

When the boat is required to meet certain requirements, be guided by the "recommendations" can be very risky. For example in the case of a boat you have to get the CE mark to be the flagged for a country of the European Community, which therefore must comply with ISO 12215, the thickness of the plywood in the transom, for a 12 HP engine, should not be less than 35 mm. And besides, should wear both inner/outer fiber layers whose minimum thicknesses are also established.
What "always worked" continues to work, but may not be valid.
NOTE: a good technician usually has a habit of figuring things out and not be swayed just by the "recommendations". It is often dangerous not see where you are. It is always necessary to rigorously justify decisions

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PARYacht Designer/Builder

Many of the CE requirements are absurd, as the 1 3/8" transom thickness on a 12 HP engine suggests. Hell, you can't even build a "real" sharpie, if trying to conform to CE spec's and as we all know, those damn sharpies all sank on launch day and were just lousy, unsafe sailors.

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TANSLSenior Member

Well, everyone can say what he likes but when one is required, for whatever reason, to meet regulations, he must comply with and opinions not based on anything concrete or the way we have worked for many years, can not be suitable for anything.
All rules, including rules that the EC issues have their purpose and there is no reason to say they are absurd. Or at least, if you say this, you must explain why you consider them absurd.
My experience tells me, as I said on another discussion in this forum, on a boat that I've studied, I got a lighter hull with ISO 12215 standards tan with SSC regulation of Lloyd's Register. Taking into account, of course, the added weight of the transom.

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rxcompositeSenior Member

My experience with outboards are;

1. The mounting/clamp throat bracket limits the thickness of the transom. No use making the transom thick if the clamp/bracket will not fit. From this thickness, the design load is transferred to the central girder, primary longitudinals, and primary sides.
2. The bracket has a shear pin in case the leg hits a log or a rock. No worry about overbuilding the transom.

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