Repowering my P-29
 

So after running 73mph and coming out of the water like I'm dragging my anchor, It is time for more power. The 300hp/350 Mags that came stock in the P-29 are just not enough.

I'm having a pair of 500hp/383's built. Thinking about keeping the Blackhawks. What can I expect to see for MPH? Also, I am hoping this will cure my constant around the dock stalling. These BH drives are super fast, 7mph with only one drive engaged at idle(700rpm). If I turn down the idle any more the motors simply won't run. As it is, when putting the drives in gear, I tend to stall alot.

With the additional Torque of the 383's I hope to overcome that initial stall when putting the drives in gear and make make more than enough power on top to see some pretty significant mph gains.

Rich

I'm not sure on the mph gain, but I would think you would be in the 80's. I thought the Blackhawk boats were faster than that out of the box? Have you had any issues with your drives? I've heard they were somewhat problematic, and unreliable. That's why they quit making them.

Out of the box people have hit 76mph. In perfect conditions light load and no fuel I might see 75. Unfortunately, I don't go boating by myself. I haven't had any reliability problems with the drives just tired of the idle speed manners.

I didn't realize that my post yesterday was my first post on OSO. Been lurking a long time.

I would like to see some before and after pic of the boat, and keep us updated on your progress. I LOVE P-29s. They are one of my all time favorites.

What kind of idle speed manners do the b-hawks offer? I would have thought they would be straight as an arrow given they are duo-prop drives. Shows you what I know. I have thought about picking one up for my boat to see what kind of performance improvment I might get. But I can't justify 2+ Gs on an experiment like that.

I was under the impression that is why they ran EFI motors, so the idle was low enough to make it manageable at no wake speeds. what about running one motor at low speeds?

I have not had any before/after data to check this equation, but I have read changes in speed can be estimated using the following:

mph = square root (hp/weight in lbs) x hull constant

Assuming:
- weight (fuel, people, etc.) equal 6500lbs
- engine power at the crank of 320hp, 640hp total
- top speed of 73mph

That gives you a hull constant of 233.

If I take 500lbs out of your boat (fuel, people) that puts your speed at 76mph.

Now, if I put the weight back in and only changing the power from 640hp total to 1000hp total you jump to a 91mph. My guess is that is a little high. But, again, I do not have any real data to support that.

IF anyone here has speed/power/weight data for the P-29's pease post it up and I will curve fit/calibrate those equations to give you all a better tool to predict with.

Just for grins, I modified that equation to read:

mph = cube root (hp/weight in lbs) x hull constant

So, for

640hp, 6500lbs, 73mph the hull constant is 158

Using that constant for the above gives

640hp, 6000lbs nets 75mph
1000hp, 6500lbs nets 85mph

Does that sound more accurate? I'm not sure if that function should be a squared or cubic function.

Again, if you guys have the data I can do that math.

I would stick with the first equation, i ran 500 horse 427's for a couple years and ran 95ish at 6000 rpm. Now i have 650 horse 434's with pro chargers. I'm still running the blackhawks without a lot of problems except for docking. I use one drive around the dock and have the nuetral safety switch bypassed. I slip into gear at 850 rpm and feather the throttle til i'm idling again at 850,slip out of and i'm at 1800,

I'm pretty sure that with the original fuel injection when you put it in gear,the fuel injection automatically keeps the idle set so you don't have to play with the throttles.Im hoping to put FAST fuel inj on over the winter.

Nice! Do you have an accurate weight number for your boat with the 427's?