New guy seeking V drive/BBC info

[U.F.O.]

Hello everyone, Im new to the board and this is my very first post. Im not sure if this is the best place for this...but I hope so..so here goes.
Id like to start off by saying Im not actually a boat guy....Im a street rodder myself, so bear with me if you could please. But, I do have a "thing" for the big chevy and Im tryin to help a guy out on a boat project. And, I heard this was the place to be for boat info....I just hope you guys like street guys hehe.
Anyways, the boat in question is a V drive flat bottom with a "lake lay up" hull. The hull is said to weight about 600-700lbs and overall weight is 1500 lbs. He is hoping for around 110 MPH and beleives he will need around 650-700 horses. Thats about all I know about this boat. Oh, and he is building a 496 stroker(4.25 stroke) and mainly needs info on the engine setup.
Well, here is my dilemma. As I said, I dig street cars and making that kinda power at a streetable rpm(Around 6300-6300) with around 500 inches is no big deal on pump gas with good parts.....I could spit out combos for that all day. But, he told me that many boaters have said to him that with the light V drive boat he would be better off to shoot for a high revving engine(like 7000rpm plus). Im not disagreeing....I just ask why? Is it due to the V drive setup and the prop and all that? Or, if youve got the power needed and the right setup, are you just as capable running just as fast at a lower rpm? Just as an educated guess, I would think it would be great to build as much power as you can at as low of an engine speed as possible. This would add to engine life if you can help to "spin all those parts around" at lower engine speeds.
But, if the V drive setup would prefer less of a high powered low rpm grunter, and more of a high rpm deal.....than we could go that route to. I just wanted your opinion cause I personally would want to build something that spins 6500rpm at best making around 700 horsepower....but I dont want to give the wrong advice. Sorry for being so long winded guys, and I appreciate all of your time. Thanks again!

[Morg]

Welcome to the bourds.
Let me start by letting you know that in the past posts have shown up like this that turned out to be a seasoned poster, who had nothing to do & thought they would spice up things. So be ready for alittle ribbing.
But I'll stick my neck out for ya, Being the holiday season & all
To run 5000-6000 rpm's & get the speed you are looking for your gear & prop. combination will make the boat an absolute dog out of the hole & take a mile to get their, If it does. Part of the speed deal is to get the boat out of the water & freed up. It would be so lazy out of the hole that it would never get free of the water.
You can also make more power at higher rpm. Search for a video with sound of a flat/runner leaving the line. You will here the motor RPM out of the hole (prop. slip) then pull down as the prop. grabs water, then build rpm to the lights.
Good luck,
Lots of invaluable information here, & lots of Playa's
Morg

[superdave013]

find out a little more about the boat. Is it a flatbottom or is it a runnerbottom. Runnerbottoms has small tunnels (5/8 ~ 7/8").
Morg is right about the get up out of the hole deal. Going 110 is no big deal but if a 75 mph jet boat gets 3 boats out front at the start that's not good. That's why short gear ratios and high rpm wins races. But what's he really going to do with it is the question.
BTW, some of the nastiest flats out there are running de-stroked deals and turn big time RPM.

[Jim W]

Morg and Dave,
Hoping you can explain this to me?
Isnt torque what gets you out of the hole and spinning the prop as Morg says. If the motor is a high RPM/hi HP deal where is the torque to get the thing launched and "freed of the water"?
Isnt that why a blown set up works so well? The instant "hit" out of the hole (low RPM torque) and the high RPM/high HP run up with boost all the way to feed the engine and make HP?
If HP and RPM is whats needed to be fast , wouldnt an old style 302 SBC turning 9,000 RPM be the hot set up????
Not trying to argue with the experts at all, just trying to figure out the HP/torque balance thing.
Here in NY there are about 3 flats in my area. So You guys are an invaluable wealth on information.
Thanks, be good, Jim

[Fiat48]

I'll just throw this food for thought in.
Unlike a car...a boat is a dyno....a waterbrake as such. How much water the boat pushes is a major player in how the motor is built for it.
Motors built to make hp at high rpm belong in free boats..or you'll never see that rpm to get that hp.
So..the boat is a major player. First thing Superdave said. But that is why we call him Superdave.

[Jim W]

Fiat 48,
UFO did say a flat bottom boat.
My boat, Hallett flat bottom v-drive just wont seem to air out. It runs about 85 MPH and playing with the t-handle it either porpouses (sp?) or seems really wet to me. A friend of mine who does circle races says I need more MPH to get it to air out. The little 396 just aint got the ass to do it.
To get a certain MPH do you build for that HP at a RPM to get it done and the torque follows along or what?
Thanks, be good, Jim

[Fiat48]

I would build biggest inches I could, as much compression as I could get away with for the fuel you can afford to burn. Cam, gear and prop the thing to run 6500-6800 rpm all in all done. Stump puller. That's what I do for heavy older flats that do not have the bottom design to get free.
Turn a bigger prop and more gear in a heavy sticky boat.
PS: It's all about cylinder head and the big inches use the head up faster. The cylinder head becomes the governor.
Speed is so much about boats and bottoms..even similar designs. A boat I had would not budge over 110 mph no matter what was tried. Removed the motor and dropped it in a new Canyon and it went 117. And opened a whole new world of tuning.....it responded to motor tune changes the other boat did not. Later years this same Canyon received a motor from a struggling racer with a very nice Cole Tr-2 which would not run over 120 mph. His motor dropped in the Canyon ran 132 and change on it's first lap.
And cylinder heads:
A 541 in this Canyon would not run much over 119 mph. Cams, injection, various tunnel rams,carbs etc were tried. Along with prop and gear combinations. Cylinder heads were worked and reworked again by "the best in the business." All to no avail. In fact, the speed got worse as the heads were ported larger by each "expert."
With the right cylinder head in the end...that motor went mid 135 and change. The big motor simply used up the head quicker.

[IMPATIENT 1]

i'm not a v-drive owner but i'm surrounded by em.i own a 78 taylor sj jet with @750hp on the bottle.i can get 3 boats on most v-drives around me that have more hp just because i'm "dried out" and full power at 6k with button down before the v-drive can get into it's power range.
now, when i race the same v-drive i handed it to in the cove in open water he hands me my ass because he set it up for top speed instead of cove racing or short track.
i think most will agree, wether its a v-drive or jet,you build it for a purpose.do you want it smear someone on top end or bottom end?
i live near alot of lakes with coves we party in.i purpose built a cove racing jet with low rpm power.hope this helps. :idea:

[GofastRacer]

Fiat 48,
UFO did say a flat bottom boat.
My boat, Hallett flat bottom v-drive just wont seem to air out. It runs about 85 MPH and playing with the t-handle it either porpouses (sp?) or seems really wet to me. A friend of mine who does circle races says I need more MPH to get it to air out. The little 396 just aint got the ass to do it.
To get a certain MPH do you build for that HP at a RPM to get it done and the torque follows along or what?
Thanks, be good, Jim
If you're satisfied with 85mph, all you need to do is get the right gear and prop combo for the power you have and it will free up and not porpoise, and may even go a bit faster, right now you have to keep it glued to keep it from hopping because you don't have the power hold the boat on a set, just try dropping a gear and you'll see a difference. If you're looking for more speed, then you need to up the power!..

[Jim W]

Fiat 48,
What kind of curve for torque and HP is desired?? Flat, long and broad or peaking? I am talking lake/ pleasure use.
At what speed is a whirl a way a good idea?
Be good, Jim

[Fiat48]

I would want a whirlaway on just about any V drive. Certainly on the 110 mph you are trying to obtain. I pm'd the rest.

[U.F.O.]

Ok thanks for helping so far....and thanks for not beating me up for being a car guy. One more question(well, for now....maybe more). The quote about prop slip got me wondering. How does the drive setup actually work in these here boats? Is it just one speed(one gear ratio) or multiple gears as in a street car transmission? And, on the launch, is it just direct drive or is there some sort of clutch system or what? Basically, what causes the prop slip? Is the slip through the drivetrain....or are we talking slip in the water(like tirespin) before the prop grabs enough water to start pulling the boat along?
Im getting the impression that they dont have more than one gear to work with. Cause, when I relate it to a car...one of these big inchers can be built to make explosive torque at lower speeds and still make 650 horses or better at around 6000rpm. This is the hot ticket in a street car and even if torque is lacking at low speeds....having 3 or more gears to choose from really helps things out. However, if the boat on has one gear, which would be equivaltent to top gear in the car....than I understand much more why its good to be able to rev well and gear these things up GOOD. Even with all that torque a big incher can make, a street car would have plenty of trouble trying to launch in drive! I hope that made sense. Thanks again guys.

[Morg]

You are correct, we are talking about the proppeller slipping. As the speed increases clean water is "loaded" into the prop. causing more of a bite.
The proppeler & gear combo. will let you get into the power range of the motor at the hit of the throttle. The guys who have it right will get a very smooth & fast acceleration.
99% of lake boats are running direct drive no trany. I'm pretty sure the alky flats are running a lenco for clutch & another for 2nd gear.
The v-drive is an engage only with a prop. realease.
BTW- on the whirlaway, my old flat did 85mph, my new circle deal does somewhere a little above 100mph. The new boat has the whirlaway & is much "nicer" slowing down. In your application I would not consider the whirlaway as an option. I highly recomend it.

[U.F.O.]

"I see", said the blind man! Now it makes much more sense seeing that youve basically got only one set of gears to work with. One could compare it to a stick driven car(or fully manual auto tranny) who is trying to launch in top gear! I dont care how much torque you make....it aint gonna launch nearly as well in fourth gear as opposed to first!
So, it makes sense that you would want lots of gear reduction to help the launchas much as possible....but then of course you need to spin the rpms up enough to reach the speed you would like to acheive. You guys should become car guys....you cant imagine how much cheaper it is to build one of these motors for a streetcar. You dont need to spend all the money bulletproofing the motor for safe operation at 7500rpm....heck, you can turn 10's pretty darn in a big incher that doesnt need to break 6300-6400rpm, and its cheaper and easier to keep that motor together! Either that or consider trannies for your boats hehe.
Just kidding guys. Thanks a lot for all the info. Now its back to the drawing board so I can give some tips on re-engineering this combo to turn some revs.