CcanDo

The boat had #5 drives on boxes and iron head blower motors.The new set up is #6 dry sump W/O boxes and aluminum head blower motors.The #6 set wt. change adds 160 #,plus 150# for a shelf and knee braces.The motors are 60# lighter per side.The driveline was 27 inches.The boat planed good ,flew balanced and landed level without water in the ballist tank. With 7 gallons it landed a little nose low.The driveline can range from 20 to 30 inches.Presently,the target point is 8 inches behind the last running step.Any comments will be appreciated.

Dueclaws

CCanDo-
Wish I could be of help, but the only additional information I have came from this well-connected and educated commercial airline pilot and his equally competent son. They're like walking encyclopedias.

Here's the formula:
engines are typically placed 10% behind 1/2 of the boat's center of gravity (CG). IE: if the boat is 40' and the CG is 20', then the engines would be placed at 10' less 2', or 8' ahead of the transom to centerline (CL) of engine.
(1/2 CG x .9 to engine CL)

Keep us posted with your progress.
And how about some photos of your boat, engines, & trailer ?
Due

Dueclaws

CCanDo-
Forgot to mention that, most likely, we'll use the existing engine mounting locations. When it was originally setup as a race boat, it had #6's and "similar" horsepower engines, but my engines/transmissions probably weigh more.
Due

CcanDo

Dueclaws,what are your thoughts on transom flex? In other words,as the top of the gimble rotates rearward when power is advanced how much will drive toe in be effected? Logically,as toe in might change, the CG relationship to the rear running step is effected proportionately.

Dueclaws

CCanDo
Lots of variations to take into consideration, IMHO.
I can see toe-in changing if one side of the transom flexes more than the other. If they flex exactly the same, wouldn't the toe-in stay the same and the trim angle of the drives change?
How will you know, unless the transom develops a stress crack or you use an instrument to measure the flex while underway?

With your power, this will be a bigger issue than my boat (hopefully!)

Due

Brad Zastrow

I just went through this with mine. Blower motors, Huber transmissions and Number Sixes. The setup as told to me that works is the back of your transmission (flange) will at the last bulkhead. I am using a 30" driveshaft. My last setup was actually further ahead because I was using two speed transmissions which are longer. Engines came back about 4" which I feel will be better balanced. Full of fuel and four people on board it felt bow heavy at times before.

CcanDo

Hi Brad,stripped,we wiegh 3,440#,X diemension is 22.25 inches and Y is 60 inches.The trailer has to be rewieghed empty for boat CG verification.However,it presently sets at 65 inches from the outside of the transom.Therefore, the driveline will be 23 inches.-----Would you know the wieght and X&Y numbers of your Motion?

Katdrvr

How did you determine the CG?

mike

Brad Zastrow

Not sure on x. The center of the prop shaft is 4" above the bottom. I was told that is perfect. I think your drvie shaft is too short. Sounds like you are going to a lot of trouble to make this a science with this CG. I simply found out what works. Larry Goldman is the expert at Extreme Marine in Miami. Ask him.

CcanDo

Katdrvr,the method includes,trailer only(boat offloaded) wiegh only the trailer axles,then wiegh only the trailer jack (trailer is unhooked from vehicle), then for verification,using a platform scale wiegh the axles and jack together.The axles and jack wieghts should equal the total wieght.Then,with the boat loaded do the same procedure.Ideally,the tires will be at the same pressures,the equalizers will be lubed and the trailer frame will be leveled.Then,measure the distance from centerline of the jack to centerline of the trunnion. The trunnion is half way between the axles on a two axle trailer or the center of the hub on the middle axle on a three axle trailer.Then,measure from CL of the jack to the transom.Now, we can find your CG.Then,we can measure from the CG to such points as CL of the fuel tanks ,back seats,front seats and etc. to create datumn points.With that information you can create an envelope that shows CG shift as fuel load or persons load changes.