Hulls that are prone to spilt turn.

**NZ Sidewinder** · 07-05-2025, 04:19 PM

Down here at the bottom of the world in circuit racing we are seeing a few instances of hulls grabbing at the bow and spinning violently.
Sometimes in a turn and Sometimes in the straights.
These are 80mph mono hull boats of around 17ft long, running 200hp v6 2.5 mercs.
There are a number of theory's being put forward as to what causes this in a hull.
Some theory's blame the driver and others blame the hull.
It's for the most part a one hull class.
Does anyone on the forum, particularly mono hull racers have an opinion on what might predispose a race hull to do this and what if anything can be done to remedy it.

**Jim Speros** · 07-05-2025, 04:59 PM

I've raced v boat offshore boats mostly twins but, SVL single too. The SVL single was a 525 in a 30 phantom. I raced velocitys with twins spinning in. And I raced step bottom fountains and a skater v bottom 38 and 39 twins spinning in. Twins spinning in with small skegs like bravos can swap ends if you let them get any out of shape.

On a single I would guess when you put a stern lifting prop and jack it to the moon it could be liking to easily swap ends. We ran the IMCO drive once with the prop higher then usual and I could feel the stern walk over as hull released. It went 96 ish. It was faster but, a handful too. We never swapped ends but, one bad hop could have ended badly.

Props are getting set higher and higher. At some point you will lose handling. Outboards don't have a very large skeg. Especially non CLE with nosecones.

My 2 cents. I hope it is helpful.

**NZ Sidewinder** · 07-05-2025, 05:31 PM

Thanks Jim
Yes, this idea has been put forward down here as well.
One proposal is to indeed, restrict propshaft height to a particular number.

**FUJIMO** · 07-05-2025, 07:43 PM

Originally Posted by NZ Sidewinder

...It's for the most part a one hull class...

...this one hull design, has a bottom issue...the way all these boats are rigged...is just one of many things it could be...

**FUJIMO** · 07-05-2025, 07:45 PM

...stock, unaltered, consumer 2.5 V-6 gearcases, or something else?...

**FUJIMO** · 07-05-2025, 07:51 PM

...can be something as simple as a particular design propeller, that everyone takes for granted to use, because it "performs" the best...but has a characteristic of providing too much transom lift at your favorite installed engine height, for that great performance...as an example...

**FUJIMO** · 07-05-2025, 07:55 PM

...can never have enough skeg. never change its size & shape. wedged trailing edge skegs are the best...

**FUJIMO** · 07-05-2025, 07:57 PM

...tell us about your zinc trim tab, direction its pointing, shape/size, etc., or lack of it, bolted to the bottom of your cavitation plate...

**FUJIMO** · 07-05-2025, 08:01 PM

...transom water pick-up, pitot tube, or anything else bolted to the transom, or hull bottom, thats disturbing/aerating the slipstream? lifting strake aeration to gearcase?...

**FUJIMO** · 07-05-2025, 08:07 PM

...have built many a performance hull, that people take for granted have perfectly flat bottom panel areas, right out of the mold, but are not. check the entire boat bottom with a known straight edge, such as a long aluminum extrusion...

**NZ Sidewinder** · 07-05-2025, 08:08 PM

Typically nose cone with lwp and a 25p prop.
I'd estimate that the average propshaft to pad number us 2inch
Most common hull has a long delta type pad.
Entry is fine.
Generally run driver plus 1 crew but there is the occasional tunnel deck.

**NZ Sidewinder** · 07-05-2025, 08:10 PM

There is a range of props but maybe the 25 trophy is popular.

**NZ Sidewinder** · 07-05-2025, 08:12 PM

Typically the hulls are very straight and true.
There is the odd bent one out there but they have other issues like chronic chine walk

**NZ Sidewinder** · 07-05-2025, 08:14 PM

Na, overall these hulls are true.
They are fast and fly on the stern 8 inches.

**joyrider** · 07-06-2025, 05:10 AM

NZ/Aus Bullet's are known to do this. This happens because of the design shape of the stem forward of the plank. When the water has a slight to ruff chop the bow digs in these holes driving the boat sideways.
Hull design vs water conditions.
I have seen this phenomenon happen watching boat racing and also had a nasty race accident because of it. This force snapping the engine mounts as the boat turned violently left.
The only way to minimize this is more trim out, but can't be maintained due to ruff water and the need to turn left racing.