# Force exerted by outboard on transom

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

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### RogerWilcoNew Member

What would be the best calculation for lbs of thrust (maybe even psi) of the outboard when engaged?

So you have a X hp motor, that weighs Y amount of lbs. What is the force requirement of the bracket/transom to support?

It's been awhile since class.

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### TANSLSenior Member

May serve as a help to know that ISO 12215-6, Chapter 7.3 shows some formulas to calculate the thickness of the transom, in the case of outboard engines less than 100 kW.

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### DCockeySenior Member

Not the most accurate calculation but better than many estimates. Also doesn't work at very slow speeds.

propulsive efficiency = (thrust * velocity / (shaft power to propeller) (with appropriate units)

thrust = (propulsive efficiency) * (shaft power to propeller) / velocity

power rating or recent engines is claimed to be maximum power to propeller at the shaft

propulsive efficiency - rough estimate for an outboard is 0.3 to 0.6. At slow speeds propulsive efficiency approaches 0

consistent units are needed such as feet/sec, pound force and pound force * feet / seconds

Added - an estimate of thrust is not sufficient input by itself for designing a transom. See my note below.

Last edited: Sep 7, 2013
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### rasorincSenior Member

Every set of wooden Boat Plans I've seen and that is quite a few calls for 2 layers of
3/4" plwood for the transom. Only 1 set called for 2 layers if 1/2" plywood. This may be overkill because designers and architects might fear someone will up the plan HP from 30hp to 90hp. I can convert it to Steel or aluminum but not to composits.

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### DCockeySenior Member

If engineering a transom based on the loads an outboard exerts on the transom then you need to consider moments as well as forces. My guess is the moments will be the larger factor.

Forces and moments can come from:
- Static weight of the outboard with the center of the outboard offset behind the transom.
- Forces and moments due to thrust of the outboard over the full steering range in both forward and reverse.
- Dynamic (inertial) loads (forces and moments) due to movement of the boat in a seaway. For a higher speed boat these can be very large.
- Dynamic (inertial) loads (forces and moments) when trailering if the boat will be trailered.

Look at designs similar to yours and see what they use. Then consider if your loads will be the same or higher. Design accordingly.

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### Mr EfficiencySenior Member

The longer the leg, the greater the moment as well, both for thrust and for the leg hitting a submerged object and wanting to tear the motor off the transom. I think there is a thread on this forum about how much 1 hp equals in terms of thrust, but it is obviously dynamic loads that need to be considered too, if you look behind at the outboard cowl when moving fast in rough water you will notice quite a bit of movement going on, which is partially transom movement, and the rubber mounts of the engine being compressed momentarily. I don't know why thickness of transom gets fixated on so much, it is possible to build a very strong transom that uses internal structures to stiffen and strengthen the flat transom panel, which need not be very thick because of the additional support of it.

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### DCockeySenior Member

There is no simple formula equating engine power to thrust. Thrust depends on the shaft speed and propeller characteristics along with other factors. Two outboard engines can have the same power and produce different thrusts at the same boat speed.

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### Mr EfficiencySenior Member

Hmmm....not sure I agree with that. If it is the same boat going at the same speed, the thrust must be identical, or there would a speed differential, surely ?

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### DCockeySenior Member

Yes, if the engines are on the same boat and the boat was moving at constant speed and not accelerating.

But in my post above I didn't say anything about the engines being on the same boat. Just that it's possible for two engines to be putting out the same power at the propeller shaft and moving at the same speed to producing different amounts of thrust. If the thrust is different at the same speed then either the resistance of the boats is different or they are accelerating at different rates.

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### PARYacht Designer/Builder

I think it would be much simpler if Roger (welcome to the forum) would just tell us what the boat is and what he'd like to put on it.

Transom thicknesses have been standardized for some time. 1.5" is common, but the range is 3/4" through 2 3/4", depending on output.

No sense in calculating it, the generally accepted ranges are:

dinky to 5 HP - 3/4"
5 - 10 HP - 3/4" - 1"
10 - 50 HP - 1" - 1 1/2"
50 - 90 - 1 1/2" - 2"
90 HP and up as fat as you can, given the bracket grip range - typically 2 1/4" - 2 3/4" and incorporating additional stiffeners to get the loads to the hull bottom (brackets, stringers, etc.).

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### DCockeySenior Member

PAR's advice agrees with my suggestion above to "Look at designs similar to yours and see what they use. Then consider if your loads will be the same or higher. Design accordingly."

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### jonrSenior Member

As Mr. E suggests, forget about the standard load and worry about the transom ripping off when you hit a log.

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### tom28571Senior Member

The principle force on a transom is a twisting one about a moment defined by the tilting axis at the top of the mount and the support point (cross pin) at the bottom. Thickness of the transom at the top is not as important as how the backwards force there is supported by the boat hull. The equal and opposite forward force at the bottom of the motor mount is easier to handle because that force is spread over a greater area than the edge at the top.

Other forces from motor weight are less important except for trailered boats. In that case, the dynamic shock loads can be larger than in a seaway. Main thing to consider is that the trailer supports should directly support the transom or the hull will be distorted, which is very common and a cause of poor performance.

IF you know the thrust of the prop, the loads are easy to calculate.

A big thick transom all across its height and width is a waste of material and weight. Dave Gerr offers some recommendations on transom thickness but not on the other important points. Using Gerr's thickness recommendations has always been adequate for me.

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### DCockeySenior Member

Tom, I assume you mean the loads due to thrust are easy to calculate if the thrust is known. Dynamic loads on the transom due to accelerations in a seaway or while trailering will depend on the inertia of the motor and the accelerations, not on the thrust.

15. ### dskiraPrevious Member

Thrust of propeller multiply by the distance from the propeller to the bracket.

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