Hydroplane or Rigger Center of Gravity, Center of Balance?

Aaron Lavigne · Jun 22, 2015

OK so I know how the CG is calculated on a hydroplane/rigger. What I am interested in learning is what affects does changing the CG have on the hull? Rule of thumb is 15-20% of the after plane from the back of the sponsons, so what exactly changes when you have a rigger with 15% and a rigger with 20%? My thought is if the CG (balance point) is lower, the rear of the boat is easier to lift and you might blow the rear end out more often?

I am curious because hydros and riggers are so similar, but I notice with sport hydros (like the Whiplash) the motor sits further ahead of the front sponson transom and it makes the CG% lower than on a rigger where the majority of the engine is behind the front sponson transom.

If you have rear sponson or skis on a rigger, would the balance point need to be higher % of the after plane? Most sport hydro don't have rear shoes so is that why the balance point needs to be a lower % of after plane so the prop can lift?

What affect does the balance point have on a hydro/rigger wanting to nose dive or rocket launch?

I have a rigger with a 19% after plane CG and if I only lengthen the tub 2 inches without changing anything else, the CG will naturally increase. If I then move the sponsons forward the CG will now decrease. Now the overall length is longer, will that be a better runner in choppy water than a shorter rigger with the same after plane %?

Just some questions to get you thinking, would love to hear your input guys...

Hammerhead · Jun 23, 2015

I run 18-20% with the best results on a rigger.

Your angle of attack also effects it.

I run 3° on the sponsons and 2° on the ski at 20% and I like the ride. Personal preference is also a factor.

Marty

Aaron Lavigne · Jun 23, 2015

Good information

Is it a gas rigger and how long is the tub and front sponsons? Or if not, what are the lengths anyways because I want to see how long the sponsons are compared to the tub. On my rigger the tub is 41" long, the sponsons are 20-1/2" but if you only include the planing surface it's about 14". I am picturing the rigger as sections and how they factor into the ride, like how much of the rigger is on each side of the CG.

Hammerhead · Jun 23, 2015

It doesn't matter how long anything is. What matters is the relationship between the AOA of the ski and sponsons. The distance between the trailing edges of the sponsons and the trailing edge of the ski and where the CG falls within that distance.