notched transom
 

For a better understanding of the hydrodynamics of high-speed powerboats, I kindly ask for your answer:

There's no doubt that a notched transom helps achieve higher speed, but I'm interested in exactly why. I don't buy the explanation about calmer water — it doesn't really hold water.

1.) A notched transom helps achieve higher speed because it shifts the center of gravity of the planing surface further aft, requiring less trim. This allows for the use of a propeller that lifts the stern, resulting in a higher-riding boat and thus less drag.

2.) Let’s say we have a powerboat that already has sufficient bow lift even without a notched transom. In this case, does a notched transom still help achieve higher speed?

3.) When does a notched transom have just the right depth?

maybe less wet running surface,so a slight less drag scenario.

I've always tried to have longer hull lengths for better rough water performance.

If you have hull length to spare, not such a big deal.

Same factor with hulls that have very short pointy bows. (Not the Fountain Beak) Not a lot of workable hull in the water, or the attitude of the hull is lower when it does make contact. Why some shorter hulls are able run with larger hulls is likely due to how similar they are in the aft 2/3s of the hull shapes.

The faster a hull moves through the water...the more the Dynamic COG moves aft.

A transom notch does't cut back the entire transom wetted surface, or the trim tab locations. So the "hinged" point of movement likely splits between the notch and the transom.

Any turbulent water the shears of the transom has a bit more time to recover. At 60mph, water shears off the transom at 4° rise. Faster = shallower rise. It's a time vs distance factor.

The drives need to be raised on a notched transom, just to keep them at their at transom propshaft depth.

All hulls are different. Stepped hulls even more so. Tuning requires an i dividual assessment. How much rocker is on the hull? Where is the staric COG?

It comes down to not creating a scenario where forces are competing to stabilize the hull. Obvioulsy, that's impossible all fonthe time with all of the variables, but the intent is to minimize the issue.

Don't solve the symptoms, solve the problems.

If notched transoms increased speed by anything significant, then wouldn't all performance boat companies use them??
Whether they help, hurt or do anything depends on the make of the boat.

Of course, it contributes to higher speed — I don't doubt that at all.
It's not about building or filling in a notched transom; I'm interested in the details of why and how it works.
I understand and believe everything written above, except for the part about "cleaner" water. That doesn’t make sense to me, because, for example, a stepped hull is supposed to glide over a layer of water foam — and if that’s true, that foam doesn’t just disappear immediately behind the stern. Stepped hulls can be very efficient...a nd a surface-piercing propeller is essentially operating within an air-enveloped flow anyway.

Hypothetically speaking, for whatever reason, without a notched transom on a hull with a pad keel, you can achieve enough bow lift to use a propeller that lifts the stern — for example, a 5-blade cleaver.
Would a notched transom still be beneficial in that case?

And does anyone know how the depth of a notched transom is determined numerically? I know, light transom-deeper notch..., but in details?

Apologies if I'm being a bit persistent, but I'm genuinely fascinated by the hydrodynamics of planing boats and would really like to understand it as thoroughly as possible.

I may get some chit for this but in my mind a notched transom is nothing more then a cheap way raising the X-dim without putting standoff boxes on the boat. It potentially can net some higher speed if properly dialed in...but what mfg is going to take the time to dial in every boat that's made for optimal performance unless its a race setup? Keep in mind that unless its a cookie cutter boat like a bayliner that get mass produced..custom boats are not all built the same. So what may be fast on one boat may not be as fast on the same boat built at a different time...
There are people who have installed notches on their boats after the fact and it has worked...but most are basing the notches on other models of the same family where they can start with a good baseline.
If you don't all-ready have a notch then get standoff boxes.
If you do have a notch then i would ask others with the same boat for setups as a stating point to help dial it in.
But i don't think i would take on the task of putting in a notch in a boat that never had one....unless the transom is gone and needs to be replaced and the boat was either free or its your dream boat that you just can get over letting go :D

It's not so much to raise X-dimension. It's to set the props back, in "smoother" water. It's basically a cheap setback box.

Search "35 Sonic porpoise" here on OSO. It's notorious for the porpoise issue BECAUSE of the notched transom. The boat gets on plane, and as you trim out, it starts to primarily use the back of the hull. And then it "runs out" of hull and the nose drops. Then this process repeats.
You have to either drag the tabs when in the higher RPM range...
...or get drastic and fill the notch back in (which was done on a few boats).

Looking from a distance, I think the biggest advantage is that during fast planing the bottom go higher out of the water and because of the notch the drive is suddenly further back of the base center of grawity and the engine can even look over this new stern, which at that moment shifts the center of gravity back, which greatly improves the trim. Maybe even so much that you can put a clever propeller up and lift the whole speedboat higher, which in the end really helps a lot to higher speed. Without the notched transom it is probably not possible to do this because the drive would need too much trim, which is not the case with the clever and you dig deeper in the water.
So the notched transom indirectly helps to higher speed.

Am I right?

A notched transom can increase a boat's speed, but the effect depends on the boat's design and operating conditions. Here's a concise explanation:

• Reduced drag: A notched transom allows water to flow off the stern more cleanly, reducing turbulence and drag compared to a flat transom. This can improve hydrodynamic efficiency, especially at higher speeds.
• Lowered engine height: The notch often allows the outboard motor or lower unit to be mounted higher, reducing the wetted surface area of the gearcase in the water, which further decreases drag.
• Improved planing: By altering the flow of water at the stern, a notched transom can help the boat plane more quickly, reducing resistance and allowing higher speeds.

However, the speed increase isn't universal:

• It’s most effective for high-performance or lightweight boats (e.g., bass boats, racing boats) operating at high speeds.
• For heavier or displacement hulls, the notch may have minimal impact or could even disrupt stability if not designed properly.
• The benefit depends on proper setup (propeller, engine height, and weight distribution).

Studies and boater experiences suggest a notched transom can add 1-5 mph in some cases, but real-world gains vary. If you want specifics for a particular boat, I can search for relevant data or analyze posts from boating communities on X. Would you like me to do that?

Transom notches are not mainly about clean water
 

Even single engine cats where the propeller is behind the tunnel sometimes have notched transoms. It is about choosing leverage versus running surface, and the choice will differ depending on how much running surface you need and how much bow lift you need.

E.g. the nineties Norwegian Hydrolift Sportcat 22, a single engine cat based on a modified Chris Hodges (of Cougar fame) design, used a notch depending on engine choice:
1. If rigged with a JohnnyRude V8, it was built without a notch. The additional running surface was beneficial with the heavy engine.
2. If rigged with something lighter, e.g. a Mercury or Yamaha V6, it was built with a notch. The additional running surface was not beneficial with a lighter engine, and the notch gave additional leverage for bow lift.

The notch was created with a removable box in the Sportcat mold.

The notch, like an extension box, provides leverage for the propeller and the weight of the engine and/or drive behind it. This leverage is free. Using a propeller to create the same amount of lift takes energy. That energy could be used for pushing the boat forward, i.e. speed, instead.

Clean water for the propeller obviously does not matter at all in this single engine cat situation.

Even the GenAI did not suggest that it depends on the make of the boat :D

@Markus;
That's exactly what I'm assuming. And if create a surplus of bow lift with a notched transom, put a clever up at the back, which lifts your stern, and will fly through the air like a TUFF... almost like a catamaran. And in such case notch really help a lot.

Correct?

p.s.
Will never try to do something like this, just curiosity.

Correct.

Except the "stern lifting" cleaver, as far as I understand, does not really lift the stern even if we call it "stern lifting". It is neutral.

Lowered engine height?? If the outboard motor or lower unit is mounted higher wouldn't that raise engine height?? Cant trust AI for everything just yet :D

"Clean water" is exactly how I was informed by 3 different boat builders: Harry Schoell, Jay Ross, and Pat Haughey. At least that's the case for V-bottoms. It's a "cheap setback box" that works better in NOT smooth water. BUT in smooth water, that's when the "teeter of the totter" comes into play as I described above. Having owned a 26' Sonic, as well as being aboard several others; in which pretty much all of those hulls employ a notched transom, that's exactly the case.

THIS picture is of a NorTech hull, not a Sonic. But I think it tells another strong part of the story, as you can see via the location of the through hull pick-up; and virtually LACK of extension below the bottom of the hull. As soon as that water passes the aft section of the "wetted" hull, the water is going UP, which lends right back into the design purpose of putting the props in "cleaner" and maybe even a bit more "dense" pressured water as it's flowing upward into those drives/props as well.

https://www.offshoreonly.com/forums/...m-img_5103.jpg

Source:
https://boattest.com/view-news/6373_...Should-Know%21

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