Converting from Jet drive to Stern drive

[gearhead496]

I have a 1979, 21' Omaga Mini Day Crusier jet boat. It came factory with a 460 Ford, and a Jaccuzzi WJ jet pump. Top speed new was 47 mph, and the boat weights 3600 lbs.
I would like to convert it from a jet drive to a stern drive. with a top speed of at least 80 mph. Mods done to the boat as of this time consist of removal of all interier, fuel tanks, steering, gauges, engine, and jet pump. That I done myself.
The boat was taken to a fiber glass repair shop, were they removed the 3/4"floor which had soft spots, about 300 lbs of foam blocks, the rotten beams, and the 3/4"transum which also was rotten.
They installed new beams and extened the engine runners 10" towards the bow in case the engine needed to be moved forwards for balance , sprayed expandable foam under the new 1" floor, and install a 2" transum, and dryed in the jet pump intake hole.

Now, at this point I seam to have may questions.
1) Has any body ever done this before?
2) What engine, big block, or small block, would work best? (I have excess to both, Chevy only.)
3) What make and type of stern drive should I use, and that is most flexable for gear ratio and prop swap?

So if there is anybody out there who has any information, and that can help me in my goal, please feel free to throw me a line.
Thanks, gearhead496

[erikgreen]

If you're serious about 80mph, you'll have to have:

1) A big block
2) Some serious performance mods to the big block to up the horsepower
3) A bravo 3 or Volvo penta drive certified for the HP/Torque of the block above.

and be aware that the hull may not be designed for that level of power, and may be really, really unstable at speed. So make sure your life insurance is up to date.

Gear ratio can be changed in any drive, it's expensive to do in almost any drive. You probably want a 1.5 ratio anyway for a big block V8.

Erik

[rasorinc]

I personally think for high hp and high speed the only outdrives that can really hold up are Konrad Marine. take a look at them. Stan

[erikgreen]

Those look a lot like an MC Bravo clone... which isn't necessarily a bad thing. Have you used them, and with what engine?

Erik

[gearhead496]

I really dont have my heart set for 80 mph, but it would be nice to come close. Yes I relieze that Im going to be out side the box with what Im doing to the boat, But do you really think that it is going to make it that unstable?
As for engines, I have got 3 big block roller motors to chose from that are ready to go.
Since this is my first boat build, which engine would you suggest?
1) 454 BBC 450 HP 450 ft/lb torque
2) 468 BBC 480 HP 500 ft/lb torque
3) 496 BBC 500 HP 520 ft/lb torque
The 460 ford that was in the boat had 360 HP and 380 ft/lb torque, but that was with the old engine mounts. and floating ribs.
The new engine bay has been framed in and reinforced for a stronger engine, plus I added slid track motor mounts for moving the engine forwards for a better center of gravity if needed to compensate for the stern drives weight.
I do believe the the bravo unit would be my best bet, in case I need to do gear and prop changes.
So, if 1.5 is a good starting point for gear ratio. What kind of prop pitch do you think I should start with?
But just in case all fails, Im upping the life insurance. lol

[erikgreen]

You're probably going to need the 500HP, and depending on the hull shape you have that may or may not get you the speed you want.

The drag curve for a boat's water resistance varies by hull shape. One big commonality between the curve for most boats is that it increases its slope exponentially at higher speeds.

What this means is that at 55+ mph you need more than 10hp increase for every 1 mph increase in most cases, and more as you go higher. 80mph is a tall order for any boat, and it will require you to keep the boat light and balanced. Whether it's unstable or not is a question for a naval architect... it depends a lot of the shape of your hull and what part of it will be in the water going that fast.

If I were you I'd be looking to reinforce the transom and stringers some, but that's just a gut reaction from me. Again, consult a naval architect. Really.

For a prop pitch (really the last thing you need to worry about at this point) you're going to be looking around 30" of pitch, but that may vary quite a bit depending on engine torque, WOT, slippage, etc.

If this is your first build, I definitely recommend you go over what you plan with someone who designs boats. A boat is not a car... a car breaks down and you're walking, a boat breaks down and you're swimming... most people can walk farther than they can swim.

Erik

[gearhead496]

Erik an Stan, I was looking at the Konrad units, and if Im not mistaken they use a
shuttle flight transmittion for Fwd and Rev.
This could be a big problem, do to adding more weight to the stern.

[rasorinc]

Where is your 150 gallons of fuel going? can it be moved to help cb? I don't think you are locked into that tranny but I may be wrong. I gave them my boat and engine data and they provided speed data etc. Rather pricy for me
but were quite helpful and have been around a long time. I would not put a Merc on 500 hp unless you buy them 3 at a time.

[gearhead496]

Wow, I had know ideal that there was so mush to figure out to going fast in a boat. I think that I might of set my speed goal a little to high, an my safty factor a little low, for this boat.
I think that after talking to you guys, you have really opened my eyes.
I dont mind being out side the box, but I think that Im going to far out.
So, since the boat came with a big block ford with mid 300 HP and a weak ass jet pump, that wont brake 50 mph.
I think what Im going to do to keep it safe, have fun week end after week end, and keep the cost fairly down. Plus have a cool boat that is a little different.
Is to find me a salvage boat of the same size (21'), with a merc quick silver 350 with an Alpha 1 drive unit. Refresh all the running gear, then install it all in my boat. Hopefully I can brake 50 mph, lol
This way if I like what I have built, after working out all the bugs. I can always up grade her, with a more power and a stronger drive unit.
Thank you guys very much.

[rasorinc]

Last thought--if you have a powerful engine don't use the outdrive to trim your boat. A pair of SMALL electric trim tabs cost $ 5-700.00 and will save an expensive outdrive. Stan