To Twin Rig or not

[NZskiracer]

Hello all,

I have a 20ft ski boat which I have skiraced with a Mercury 300x outboard motor. Speed about 85mph.
In the search for more speed and fun, I am looking at putting 2x 250HP 3 litre mercury motors on the back of the boat. ( estimated top speed of 110MPH towing two skiers )
Rather than bolting the engines on the back, what calculations can I do to see if it will still float as engine weight goes from 210kg to 420kg plus the weight of extra rigging , batteries etc, fuel load will double from 125 litres to minimum 250litres. maybe I need to add extra floatation pods to the transom?
I have already taken the top of the boat to install a new floor and redo the transom, so prepared to strenghten and do major rework to the hull.

Looking for a bit of direction , and apply some science to my madness!

[marshmat]

H NZskiracer, welcome aboard boatdesign.net :)

May I begin by saying, 500 horses on a 20 foot ski boat is simply nuts.

Now that that's out of the way....

The easy way to see if the boat will have trouble with the extra weight is to pack sandbags in the lazarette to roughly the weight of the extra motor and gear, and see what new behaviours the boat develops. Unfortunately, with your deck and transom in pieces, this is not a viable option.

So you're stuck with actually calculating the centre of buoyancy, centre of gravity, moment to trim one inch, etc. Or, just being happy with 85 mph (already an insanely high speed for any boat.)

[dockdave]

Who wants to ski at 85 mph?

[NZskiracer]

Nuts.. Nuts... absolutely haha.
Water ski racing is a fantastic sport. A lot of people can develop the fitness and skill to ski at that sort of speed quite quickly.
Circuit races are slower, but I want to build a river racing boat. Race lengths are anything from 80kms up to 120kms long and at 85- 100MPH they go pretty quick.

The boat I have is quite long and narrow, so a single engine it is very on the edge as you "fly" the boat and balance against prop and engine torque. But a counter rotating twin rig setup would settle the boat down nicely and just be pure magic hauling down the river.

Is there a simple excel spread sheet you can enter data such as length , engine weight etc.
I have never done any calculations for bouyancy, but willing to learn.

[Luckless]

I wonder do design stacked, Contra-rotating propellers for such applications not exist? Gives you the counter rotating balance benefit of twin engines without the drag of multiple shafts.

[rwatson]

Like Donald Campbell said just before he flipped for the final time "Hmmm, I dont think this configuration is workable"

500 + extra kilos is going to need a bigger hull for sure, In fact a longer hull might get to 100 + with a lot less extra horsepower.

You aint gunna get a any 'simple ' spreadsheet to tell you the answer. Is your life worth some professional design help ?