Outboard Engine Thrust

In order to design the transom and brackets, some magnitude of force must be known/determined.
I have never seen, and I have asked, any data that indicates the thrust range that any given horsepower engine is capable of generating.
I guess another way to determine is the resistance of the hull.

Any insight is appreciated.

Thanks, Mike

 

At a rough guess i'd say well over 1000 lbs for a V6 outboard, but obviously rough water and steering changes will impose extra, and differing loads on your mounting structure. Another thing to consider, the shaft length of the engine, an extra long long will be different to a shorter one, as the drive line is further from the bracket.

 

Rule of thumb

1 Gal an hour of gas burned is 10 HP..

each HP will create about 20 lbs of thrust if fitted with efficient prop and reduction gear.

 

I would think that you would want to design for the loads encountered during unusual events - say hitting a log or sandbar. Or towing another boat off a sandbar.

Technically, any engine can develop any thrust.

 

Build for 2000 pounds thrust and you'll be fine.

 

You make a good point here, hit an "immovable" object at speed and you can put extreme loads on the transom, opposite to the normal direction.

 

If we all designed things to encounter unusual events we would be driving a round in things that resemble a train and live inside a mountain..

Every thing has its limitations and can only do so much. A tug boat capt knows how much his engines can pull and how much the bollard or line can take.

Designing it so it can run in to sand bar is ridiculous. Don't run into a sand bar and it will be fine, as most boats are.

 

Put it this way, it is probably a common experience to have an outboard powered boat decelerate much more rapidly than it ever accelerated under power, by running into a hidden sandbar, so presumably the force involved is greater than the propulsive one, albeit turned around 180 degrees.

 

Typically, you have an idea of the maximum power requirements the transom will be asked to endure. The transom core is sized to this thickness and related reinforcements sized appropriately. There are very general rules of thumb in this regard (transom core thickness) which include a safety margin. If the reinforcements (stringers, knees, etc.) are also scaled appropriately, the laminate schedule becomes obvious. Depending on application the safety margin may accept significant leg impacts, groundings, etc., but this is application specific, in as much as you wouldn't have much margin on a racer, while having a considerable one on a rough water cruiser. The SOR will establish these margins.

 

Thrust depends largely on the hull size, weight and propeller. A manufacturer does not possess that information, so he could only supply bollard pull data for all props that fit a certain model. To the average buyer such numbers are useless, he is interested in performance figures, not in static values.

You can calculate thrust from performance data as published in boating magazines. For transom and bracket design the moment acting on the engine mounts is much more important than thrust.

 

Thrust is HP, gear ratio and prop pitch.

The size of the boat and its weight has nothing to do with thrust.

Your confusing thrust with acceleration.

 

Frosty believe me, I know what thrust is!
The actual thrust an engine can develop depends on the object that is pushed forward, HP, RPM and pitch just set the upper limit.

 

Thrust is from the engine it has nothing at all to do with what it is thrusting.

No you don't --you are talking acceleration.

I said in the post above it was HP, Gear ratio, and pitch.

You can bolt it to a wall and get max thrust, the weight of the boat only comes in if you need acceleration.

 

**For a guess at thrust at the prop, assuming it is pitched well** (another topic)

HP x 746 W/hp x 0.5 (prop efficiency, another topic)= power delivered to water
power delivered to water in Watts/ speed in meters/sec= thrust in Newtons.

Example:
100Hp outboard at 30 knots (about 15 m/sec)

(74,600 Watts x 0.5)/(15m/sec)= 2487 Newtons.

There are about .225 pounds force in a Newton. Call that figure 560 pounds of force.

In this case (my boat) I would like to think that after some prop selection, my efficiency is a bit higher, in the low 60% range, with a corresponding thrust higher than 3000N. But I don't know for certain.

With different props of different pitches, areas and efficiencies, and speeds limited by physics like engines, hulls, etc. you can achieve different thrusts.

P.S. someone please correct me if i goofed any formulae from memory.

 

Not bad, FishStretcher.
But on your friend's boat, twice as large and heavier, the same engine would need a different prop, reaches a lower speed but provides more thrust.
The OP wants thrust figures to calculate required strength for his transom and bracket. Without info about the object to be propelled that is not possible.