CG on a rigger

Many times I have asked about the CG position on a rigger and have been told in almost all cases that it is not critical and only the turn fin position position relative to the CG is where it's at. I do not not buy that and finally I see Joe Warren mentioning optimum CG position in the threads about the CMB 27 boat. Of course I do understand that the turn fin placement relative to the CG is critical having found that by trial and error.

Having raced various monos (mostly European FSRV boats but also US style mono's) over the past 30 years or so I know that CG is critical and now I am taking riggers seriously I find the same thing.

For example: On my JAE 21 I was having problems with it running too loose and tried all the usual things such as varying sponson/strut/ski angles but ended up adding weight to the front of the boat and moving the radio gear forward in the radio compartment which improved it.

I then narrowed the sponsons by almost 30% and removed the weight and the boat was much more stable showing me that in fact the CG relative to the plan area of the boat was what had been the factor causing too much lift. i.e. Even though the CG had moved back because of removing the weight and the lighter front sponsons, the lift was reduced due to less area at the front of the boat.

My point here is that I when the boat leaves the water completely when running over wake or waves then I want it to stay at the same attitude and not go nose up or nose down. To do this the CG relative to the plan area/angle of attack of all surfaces has to be correct above all else.

If that is correct then why does no one want to say what the CG is of a particular design?

My JAE 45/VAC45 is a really fantastic boat in almost every respect but will fly,showing me the CG is too far back but has improved a lot by adding some weight to the front and moving radio gear forward. I built it to the plan,but followed the forum here and fixed the engine as far back as possible when in fact I think it should be all the way forward.

Comments?

Greg Robertson said:

My jae .45 has the engine all the way forward. It stays hooked up very well but if you hit a wake it almost always

Flys level and lands and keeps going. when I built the boat Aaron told me to go as far forward as possible and reduce the AoA of the rear ski. My jae .21 runs better with narrower fronts also

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My JAE45 ski is at 2.5 degrees. How does that compare?

That is too much on the rear ski. 1 degree is where I would try and end up. We built ours with the 65/35 rule with about 2 degress and they will hop the prop out of the water if you hit a wave at full speed. Moved the battery to the back and it helped. Zipp has already addressed this I'm told.

Hey Dave:

As far as I can tell It's all a balancing act relative to the speed range you're working in, just like your monos.

All factors are important, including surface angles and areas, prop lift, rudder lift, turn fin effects etc, as well as CG. I've found that some have more input than others and anything that touches the water is very important.

For a boat to "fly" level, the C/L (center of lift) has to be close to the CG so no big fore/aft moments are introduced when it does see air. The designers have seen to that, otherwise if it was way off you probably couldn't add enough weight to overcome the aero effects. Years ago I designed a boat with way too much lift in the front and it kept blowing off, I added a whole pound of lead to the front deck and guess what, it still flew!

My heat racing boats balance at about 70% of the afterplane. I worked with that over the years building different sponsons (front & rear including skis) until they "flew" right. Dave above mentioned his JAE is at 65% but has cut down the ski angle and moved weight around. His (and his Dad's) JAE's work very well!

I think what you're finding is you're probably getting more speed than what the boat was originally designed for and are having to "adjust".

BTW, what props are you running on your 20 & 40 boats over there?