Hull design differences between 2 & 4 stroke

Hi,

I've heard some newer boats are designed for 4 stroke outboards which presumably would mean changes to the strength of the transom to support the extra weight and changes to the hull toward the rear to give better flotation. Is anyone here able to comment or share any stories about this?

Cheers
Splint

 

4 strokes were developed to improve emissions and environmental acceptability.

2 strokes are much lighter weight for the same horsepower.

You can put 2 strokes on a boat designed for 4 strokes and improve performance, put putting 4 strokes on a boat designed for 2 strokes will usually degrade performance.

 

But why would it degrade performance? Is it because a 4 stroke with the same horsepower as the 2 stroke would weigh more?

I'd be interested to know if designs are different for 2 & 4 stroke outboards.

 

Borrowed without permission from www.southportboatworks.com .....

*****
It's no secret that today's big, new V6 four-stroke outboards weigh more than conventional two-stroke V6s. What's often overlooked is that most outboard sport fishing boats were designed far before four-stroke V6s were available. Consequently, those products were never intended to perform with as much as 1300 pounds of outboard weight on their transoms.

Southport's Hunt4 hull designs, on the other hand, were designed to utilize the weight of modern four-stroke V6s to provide a softer, drier ride, more stable platform and "sports car like" handling. In fact, Southport is the only sport fishing product line designed to maximize the potential of modern four-stroke power plants.
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How much of that is sales talk and how much is genuine Naval Architecture I'm not sure. I'd be interested to know if there is any significant design change between a general design and a four stroke design.

And yes the extra weight for the same horsepower is where the performance loss would come from.

Cheers
Splint

 

A transom certainly needs to be designed for the motor(s) it is to mount. True enough, but the biggest load on a transom is not the weight of the motor but the thrust, so power is by far the more important item. The buoyancy of the stern is important in weight considerations but it is not umcommon to see more motors stuck back there in parallel than the boat designers or builders ever intended. Weight of 4 strokes is always more but not enough to be the major factor.

If a pod is to be used, then the strength of the transom is even more important because of the long moment arm of the thrust.

 

In the 'early' days of 4-stroke o/b's there was quite a difference, not only in the mass of the engines, but also in their power delivery. These days, there's generally very little between them, so the design differences are correspondingly less.
You're more likely to find that weight becomes an issue when choosing between engine manufacturers. Manufacturers build one basic powerplant to cover a range of hp outputs. So for instance Manufacturer A might have a single basic engine with hp outputs of 150, 175, 200, 225 hp, whilst manufactuer B might produce one with outputs of say 115, 135, 150 hp.
If you happen to be shopping for a 150hp engine, A's is likely to weigh considerably more than B's - regardless of whether they're 2 or 4-strokes.

On the environmental front, the gap here has narrowed too. The newer direct injected 2-strokes are at least a match for the 4-strokes, though 4-strokes are still generally quieter than 2's