drive options; vee, stern, jet

[curtis73]

Ok, more research has been happening on the Thunderbolt project. I had recieved some mis-information that was cleared up recently. Its a bummer to say the least. I had seen on a couple websites that the design was intended for Vee drives, but had also heard that they could be ordered with provisions for stern drive or jet drive. Now I find out that its designed only for vee.

That's a pain in the butt for me. I use my boats in many situations, including Canadian lakes, inland salt water, and various other applications. Pulling up to an island or exploring backwater in any of these situations requires being able to trim the drive out of the water.

Now...as this applies to the Thunderbolt... I can see moving the powerplant back a few inches and moving the battery up front to balance out the hull, but adding a stern drive will add several hundred pounds hanging off the transom.

Will it make a difference that the hull is designed for an iron V8 and my engine will only weigh about 350 lbs?

Will it help if I use a racing style stern drive to keep weight down?

Might it be possible to add some weight to the front, or will I be adding so much that it will be counterproductive?

Can anyone suggest a boat design of similar style that is designed for a stern drive?

[Jeff]

Will it make a difference that the hull is designed for an iron V8 and my engine will only weigh about 350 lbs?

This will definitely help you if you're moving the engine location backwards or adding weight on the stern. Distance from the center of gravity * item weight is what you have to keep relatively balanced with the original design. Do you know how much a 302 or 350 block weights in comparison?

I have to see if I can find the exact weight for an Alpha Drive (which is OK up to 300 hp I believe and definitely fine around 250 hp), but it's a couple, not several, hundred pounds that you'll be adding to the stern. Moving the engine back will have an effect of course, but the transmission weight comes off since the outdrive is also the transmission.

When you bring up adding weight up front, it's better to think of moving weight than adding weight

[curtis73]

The engine I'm planning on right now is a vortec 4200 L-6 from a new chevy trailblazer. 270 hp, flat torque curve, all aluminum. I think I'll be able to shave several pounds off this by deleting the power steering pump, air injection pump, 170-amp alternator (in favor of a smaller 70-amp), and other associated bracketry.

You had asked about typical small block weights. I think a stripped down 302 (iron block/iron heads) with just an alternator and waterpump may come in just barely under 500 minus intake and carb. Many late-model low-output thin casting chevy 305s tipped the scales at around 535 from the factory with aluminum intake and carb. Buick SB assemblies are just slightly more than chevys, but Olds, Caddy, and Pontiac (generally in that order) get progressively heavier. A BB Pontiac with iron heads, factory iron intake, and heavy cast manifolds can tip the scales at 800 or more.

The 350-lb weight was a guess on the 4200, but it should be close. It shaves weight with aluminum, two fewer cylinders, OHC valvetrain, but adds weight with a heavy crank, structural cast aluminum oil pan and cast stainless exhaust manifolds. I may fabricate an exhaust header, if for no other reason than to be able to weld on a heat exchanging water jacket. (and to save weight)

I could design a fuel tank to go up front, but won't that adversely affect handling....I mean with 120 lbs difference between full and empty, won't that really change my plane throughout the cruise day?

Jeff, you write:

Distance from the center of gravity * item weight is what you have to keep relatively balanced with the original design.

Ok, that makes sense. What unit of distance do I use? If the net movement of the engine is 6 inches back, do I use inches, cm, mm? Or is it what ever unit I want as long as I keep it all constant?

Thanks alot

[Jeff]

what unit of distance do I use? If the net movement of the engine is 6 inches back, do I use inches, cm, mm? Or is it what ever unit I want as long as I keep it all constant?

Inches is more than accurate enough. All I was suggesting is that your setup should be 'close' to the original design in terms of overall balance. So if you add one weight aft, see if you can shift another forward a little. If you add a heavy weight close to the CG you can balance it with a light weight item far from the CG in the other direction (distance * added weight ~ distance * subtracted weight). That's all.
Unlike a race boat like a tunnel hull which is right on the edge, you have a fairly broad latitude in comparison. For one thing, (movable/moving) human weights in a boat under 25 or 30 feet make a big difference. In your boat you will feel the difference of 1 person or 4 people aboard right away. Also I'm not even sure the original design was balanced perfectly for an exact weight engine - depending on things like what type of manifolds or what engine was available, the weight might easily vary by ~200 lbs on different boats built exactly to the plans. In a one-size fits all production boat which might come with several engine packages from a single to dual V8's, trimming the outdrives slightly solves the problem. Now some of these sit better at anchor than others, some plane better because they're better balanced, and many probably are running with drives in a slightly less than optimal angle/depth/etc. My point is simply that the more you balance your hull weights and the closer you keep your boat to the original design balance, the better it will likely perform (and the safer you will be from having something unexpected happen) but at the same you have a fair amount of flexibility and if you add 50 or 100 lbs. of weight aft and shift the CG slightly you can compensate for this and will still have a boat that is as good as the majority of production boats on the water next to you.

I could design a fuel tank to go up front, but won't that adversely affect handling....I mean with 120 lbs difference between full and empty, won't that really change my plane throughout the cruise day?

You want to keep tanks close to the CG for the exact reason you have stated. Having a fuel tank up front would not be good at all.

P.S. Thanks for the weight information - I remembered the approximate weight of a 351 I had to change a few years back, but I didn't have it separate from the old cast iron manifolds which were also quite heavy. I sometimes have a habit of asking more questions than I answer - so thanks

[Shmoozo]

Originally Posted by curtis73

The 350-lb weight was a guess on the 4200, but it should be close.

Not a bad guess.



It's probably actually a bit more than 400 pounds in the form in which you'd install it according to the info on this web page:

Ward's Auto World info about the Vortec 4200 engine

From that web page:

Originally Posted by Ward's Auto World on their web site

Fully dressed, the engine - including all of its direct-mounted accessories and catalysts - weighs 430 lbs. (195 kg); using more-commonly quoted weight, in "as shipped" trim, the engine weighs 408 lbs. (185 kg).

You'd be leaving off some of the engine's accessories and saving a little weight that way, but perhaps gaining a few pounds with various mounts, coolant lines, and such that would be required in a custom built marine installation, or so it seems to me.

[PowerTech]

You might like this thing I saw it at a boat show it trims up and doun to.It's bad ass.http://www.pulsedrive.net/diagrams.htm