Hulls that are prone to spilt turn.

[John#24]

Hi The fastest drivers in that class drift their boats around the corner even on the tight river courses
When I raced those hulls we had 260Hp and able to keep the nose up by running a Bow lifting prop and the HP helped
Now the problem is the 200Hp hasn't got the grunt to lift the nose as quickly
Next problem is youthful inexperience if handfuls of trim are used and going down too much will bow steer, split turn, broach, front foot what ever expression you want to use Using too much up in the corner will loose drive dropping the nose
The majority of drivers have come from a youth class 12 to 16 years old and stock 30 HP ( A class of boat that is very forgiving and at much slower speeds reaction time to errors is by proportion easier to correct )
As the saying goes the faster the speed the bigger the mess
These guys race 6 ft apart and at 80 mph there isn't much time to correct errors in driving
Given that we can,t stop our sons from participating competitively we need to make the hulls more forgiving or change the model of hull
The latter isn't going to happen as there are over 400 x Bullet 1750s out there at very reasonable prices to get into

Back on subject of the variable deadrise and the way the pad blends in
These factors might be the biggest part of the problem ( Other than driver error )

As we will be blueprinting our hull
Any design suggestion on how the pad to vee placement and blending in would be appreciated
As for the variable dead rise we can remove a portion without major modifications
Is it worth our while to remove the variable deadrise all together?

Cheers

John

[NZ Sidewinder]

That white boat in the picture started out as a 2 seater and I'd drive with my Son as a passenger.
Then I converted it to a center steer and let him loose.
Single seaters are the way to go.
There isn't the need to find a crew member, there is only half the human risk in the event of a crash and
the boats fly better.
In the offshore racing, I'm the passenger with the other Son driving.
That is truly frightening.
He's young and has no fear, mainly because he hasn't crashed yet.
I spend most of the race telling him to slow down.

[joyrider]

As you know the type of boats in New Zealand are social ski-boats racing in either the South-Island boat marathon series (NZBMC) or NZPBA circuit racing. They all either have a plank/pad running surface hull with most engines mounted high on transoms. With these boats raced on the edge & under full trim they race with very little hull in the water on the straights and drop off the plank going round corners.

The problem is they are not race boats in the pure sense. We could all suggest any number ideas but unless the sport see's there's a problem from a driver safety aspect not much could be done other than giving more racing room. Cheers Wayne

[NZ Sidewinder]

Yep.
We got the starts sorted and the drivers do a superb job of maintaining lines, lanes and safe spacing through the first turn, then this latest crash out in open water, going in almost a straight line, just threw a spanner in the works.
I'm currently building a cell for the Mod vp boat but that is an incredibly expensive job.
No change out of 25k for that.
So cells are not the answer if we want to race vee bottoms.
This thread is intended to investigate solutions.
One possibility might be a pair of bullous strakes low and forward.
Something that tricks the boat into behaving like it has less deadrise forward

[NZ Sidewinder]

Would something like this work?
This is a Sleekline hull

[NZ Sidewinder]

I'm not sure what most guys do with their zinc tabs but I personally either use a blank plate or cut the fin off.
Sometimes they interfere with the prop, so they are a nuisance.

[John#24]

Hi The Bullet 1750 certainly is from a race pedigree https://bulletboats.com.au/ https://bulletboats.com.au/ and holds a world record today Cheers John

[NZ Sidewinder]

I owned one and got rid of it because it was dangerous in some situations.
My son drove it in a 40km river race with a 30kg bag of sand in the passenger seat no problem at 80mph.
But that was with a bow lifting prop and combined weight of him and the sand of about 120kg.
Trimmed up the whole way.
Put 140kg in them seats and get into a choppy patch of water in even a slight turn at 75 plus mph and they will bite you hard.
Check and see if the record holding bullets are 2 seaters or tunnel Decks, big difference.
Check if the race pedigree is for straight line kilo speed or roundy roundy.
I'm not making this stuff up, I have numerous videos of the boats going feral.
Don't ask to see the video as they are not shared outside those involved in our racing
There is no question, they are fast, perhaps too fast.

[powerabout]

use Dancrafts if you want to corner, Bullets are for straight lines, hence the large plank.
vee bottom circuit racing need propshaft height rule.

[NZ Sidewinder]

Thanks, you maybe absolutely correct, but the issue is that we have lots of Bullets here and ideally we need to find a $1000 fix for them so that we can host races without the expectation that a novice driver could kill themselves.
That would be a disaster at many levels.
Racing is dangerous, but these boats can be more unpredicable than what is reasonable to expect.

[powerabout]

Thanks, you maybe absolutely correct, but the issue is that we have lots of Bullets here and ideally we need to find a $1000 fix for them so that we can host races without the expectation that a novice driver could kill themselves.
That would be a disaster at many levels.
Racing is dangerous, but these boats can be more unpredicable than what is reasonable to expect.

Get the propshaft down especially if guys running stock lowers as skeg is tiny
Can glass on reverse lifting strakes down aft, to help it hold on but correct design is going to take lots of testing

[Jimboat]

In your opinion, would an extra pair extra strakes placed forward and very low help with this?
I don't know you but did suspect that the bow entry was implicated.
I too have witness multiple accidents involving these split turns.
Here is a sketch of two hull forms.
One may be prone and the other may not be prone.
The sketch isn't to scale and is a slight exaggeration of the realative hull shapes.
The goal here is to find a solution to what I see as a potentially lethal problemAttachment 543838

Interesting thread...
re: deadrise question wrt chine walk... yes, increased deadrise has the effect of reduced lateral dynamic stability and is MORE prone to chine walk. Lower deadrise hulls are less susceptible to chine walk. Also adding outboard strakes (that still contact water at critical speeds) will help reduce chine walk. Problem with this approach is that the strakes help because they add 'outboard' contact and some additional lateral stability (roll damping), but they also add add'l drag - which is the reason for flat pads and low wetted surface contact in the first place, so...

Also, reducing CG height will help... but not that easy. Also moving heavy weights more inboard will help improve chine walk susceptibility (reduces roll inertia)

Problem with many chine walk 'fixes' is that these also often reduce top speed, so not so attractive alternatives to some.

Also, chine walk article here:

[NZ Sidewinder]

Interesting thread...
answer to deadrise question re: chine walk... Increased deadrise has effect of reduced lateral dynamic stability and is MORE prone to chine walk. Lower deadrise hulls are less susceptible to chine walk. Also adding outboard strakes (that still contact water at critical speeds) will help reduce chine walk. Problem with this approach is that the strakes help because they add 'outboard' contact and some additional lateral stability (roll damping), but they also add add'l drag - which is the reason for flat pads and low wetted surface contact in the first place, so...

Also, reducing CG height will help... but not that easy. Also moving heavy weights more inboard will help improve chine walk susceptibility (reduces roll inertia)

Problem with many chine walk 'fixes' is that these also often reduce top speed, so not so attractive alternatives to some.

Hi Thanks for your reply.
The issue in this thread isn't about chinewalking, it's about split turns.
However chine walking is another interesting aspect of fast boating that we all have had to deal with .
In my experience, hulls that have tall pads tend to walk excessively and require a lot of steering input to drive.
One hull that we race here came out of the mold with a pad that was about 1 3/8 tall.
That hull walked and to improve it, a small angular fillet was added to the internal corner where the pad wall met the hull.
That was a high deadrise hull and this mod made it drivable at speed.
I put everything except the motor on the center line.
Center steer, fuel tank, battery.
Motor offset 1/2 inch to starboard.
This boat is fast and races well, but in certain situations like when running a non ratcheting gearbox it will rock wildly when decelerating into a turn buoy.
If the driver was out in the normal position, this rock would throw him out.
Answer to that was don't slow down for the turns unless absolutely necessary.
But getting back on track, it's the high deadrise forward hulls that seen to bow steer the worst, and that's a nightmare for circuit racing.

[Jimboat]

NZ Sidewinder - right, i saw the mention of "chronic chine walk" and related the similar issues with hulls that have lower 'roll stiffness' and 'roll damping', like 'hooking', 'Chine Tripping', Bow Steering' and 'split turns'. as you point out, the wetting of high deadrise forward (bow) area when entering a turn can often trigger these behaviours, and hull with lower roll damping characteristics can behave poorly. sounds like you've been able to address some of it with center weight placement. Motor offset is a neat feature!

[NZ Sidewinder]

NZ Sidewinder - right, i saw the mention of "chronic chine walk" and related the similar issues with hulls that have lower 'roll stiffness' and 'roll damping', like 'hooking' and 'split turns'. as you point out, the wetting of high deadrise forward (bow) area when entering a turn can often trigger these behaviours, and hull with lower roll damping characteristics can behave poorly. sounds like you've been able to address some of it with center weight placement. Motor offset is a neat feature!

Another mod is using a pair, or possibly even a single small trim tab 1 foot to the side of the pad.
I've done this as a pair.
They are only 3 inches long and 6 inches wide and includes an adjuster screw.
The port side is adjusted down 1/8 inch and stb side is flat.
They are both well out of the water at speed, but deflect the shedding water..
They also manage the purpoise that can result from a rocker pad.
The port side tab helps to minimize the effect of prop torque.
On a powerful vee bottom, prop torque will lay the boat over on its side just like a drag race car.