Re: hull extension in progress with picks
Re: hull extension in progress with picks
The only way to turbo a marine motor is to have it custom built... as far as I know there are no stock turbocharged marine motors. Mostly this is because a turbocharger works off of the engine's exhaust pressure... in cars the exhaust is used to turn an impeller that drives more fuel/air mix into the engine.
This is fine for cars, but on a boat it's more complicated because you usually have wet exhaust, so you have to run the exhaust through the turbo then out through whatever the rest of your exhaust system is, EG wet manifolds or through-hulls (with mufflers if you're anywhere that needs them). The simplest system would be to assume a dry exhaust like a through-hull system. But then you have to have a "special" turbo... the turbocharger is powered off of engine exhaust, so it gets really hot. Not a huge problem for cars since it's under the hood, but in a boat the temp is a fire hazard, and it exceeds the coast guard's temp limits for inboard boat engine parts. So a turbocharged boat won't be USCG certified, and may be hard to insure, unless it's listed as a race boat, which is another can of worms.
Renny_D said:
Thanks that makes sense - Oops' original response confused me as road race cars I play with are often under full load until braking. I still say turbos are the replacement for displacement however. With the right bottom end you can more than double the horsepower yet when off boost have a nice sedate motor. Hotrodders in europe are getting +300 hp/liter and that is not the crazy f1 big money guys but guys in their garage on weekends. That would be the like make 1500 hp from a 302. Tell me how you do that with displacement. Also a marine cam, as you described it, would work really well with a turbo especially if you are trying to make power at lower rpms and carry that torque through to redline. Low rpm turbo power is all about small turbine sections for fast spool as they begin limiting themselves due to excess exhaust manifold pressure ratios very early. Again I say the best of both worlds - hp when you want it and good gas mileage smooth running when you want to putt around. Having played on both sides of the displacement game I prefer forced induction or as my friend said "If your motor doesn't have forced induction then it 'sucks'
Well, your point of view is similar to a lot of car type folks I've seen that are new to the boat world. It's apples and oranges though.. here's why:
A turbocharger can be designed to provide a low end boost to a point, but it's only a fraction of its rating, IE 1% addl HP or so. Look at a dyno graph of a turbocharged engine vs an identical non turbo engine and note where the power is mostly provided... power increases exponentially as RPM increases, because the engine is providing more pressure (in the exhaust) to turn the turbo. This is limited by materials, exhaust pressure, and the max turbo RPM of course.
Turbos run off of exhaust gas, and at low RPM there just isn't enough exhaust pressure to provide a noticeable boost.. in a turbo powered car there's usually an engine RPM where the "turbo boost" kicks in. If you're not ready for it in some cars it may even cause a safety issue since you suddenly get a kick in the pants.
In a car, you use a transmission to absorb your engine's power and transfer it to the wheels at a useful RPM.. for instance redlining the engine at 9000 RPM in top gear you're usually converting the 9000 down to 1300 or so, since you can't really turn the tires at 9000 RPM... part of that conversion is in the transmission, part of it is in the differential. So gaining power by increasing RPM is useful... you can live with lower torque if your RPM increases because your transmission can convert the RPM into torque at high RPM, and you can downshift to get moving quickly at low RPM.
Additionally, tires can run efficiently at a large range of speeds... unless they're slicks, or running in wet/icy conditions, pretty much one rev of the tires moves the car one tire circumference.
On a boat, it's really a different system. I'll use an I/O as an example since that's what I know best.
The engine runs at its factory RPM usually.... for a chevy small block that means WOT is about 4200-4600. It doesn't make much sense to go higher for several reasons related to power (see below) but another major reason is the usage graph... if you graph a car's engine RPM over time, the graph will show spikes as you shift, speed up, etc. A boat's engine typically is revved to a certain RPM and left there... for hours at a time. Imagine how long your hopped up car engine would last if you got in the car, started the engine, left your driveway, then floored it and held it down for 6 hours? Boat engines do that all the time.
The engine output runs into a sterndrive via a rotary coupler. The sterndrive is basically a pair of right angle gear transitions that move the rotating output power down about three feet and reduce RPM by between 2:1 and 1:1 or so. Only one gear ratio is available, although you can shift forward/reverse and neutral. It's not built with more gears for reasons related to the prop (below), but another reason is the use curve again. In the above example driving your car, which component would die first, your transmission running redline all the time or your engine?
So the power goes engine->sterndrive->prop. Here's the real difference between boats and cars (like you didn't know, right?). The prop isn't friction based, it's a reaction drive. The blades shove water backward, and the boat goes forward.
Due to the physics of propellers (they're simple machines, basically archimedes' screw) they have exactly ONE speed and a small range of RPMs they work most efficiently at. Speeds (defined as forward movement through static, non churned water) below that limit they still work, but rather poorly. Speeds above that they still work, but deliver no extra thrust no matter how much faster they turn. RPMs below that range still work, just poorly, and RPMs above that range don't work at all (see below).
The no extra thrust at high forward speed is a limit imposed by the prop pitch.. basically the prop (and boat) are moving through the water too fast for the angle of the prop blade to get "ahead" of the water flow and push backward... if the prop isn't able to shove water backward faster than the flow around it, it's effectively standing still (think about it). Increasing the angle the blades grab water ahead of them at increases the prop's pitch.
Why the RPM limit? Water isn't solid. It cavitates, IE transitions to vapor under enough pressure. Also, a boat floats on the water's surface, so the prop is always near the surface. Boats can push air under themselves and into the prop sometimes, or the trim mechanism can move the prop out of the water. If the prop is not designed to pierce the surface (some are in fancy Arneson drives) then this is called "ventilation"... it means there's bubbles of vapor or air around the prop that are affecting thrust. The prop on a boat doesn't push air very well, and it provides not much push with bubbles. Higher RPMs have the potential to create more bubbles faster, up to the point where you're pushing almost no water, just creating bubbles.
So in addition to one forward speed where any given prop works best, there is pretty much a small RPM window where it works at all. Below that window its thrust backwards doesn't exceed the turbulence created by the blades (imagine a prop turning in neutral) and above that RPM it starts to ventilate and thrust drops.
So then... the gears in the sterndrive lower the engine RPM at WOT from 4200 or so down to (in my V8) about 2800 RPM maximum or so. That's a good range for my 14.5" prop to spin at... not too much ventilation. I can still adjust prop pitch by replacing props, which changes how much water gets shoved backward with each prop rotation, and sets my max speed. I can't go above a certain pitch limit though, because the engine has to have enough torque to overcome the resistance of the water to the pitch I choose. Resistance goes up as pitch goes up (to go faster you need a higher pitch prop, to use a higher pitch prop you need more TORQUE!).
Now there are a couple other things to consider here. First, boats need a good amount of thrust at low RPM to get on plane... this is another difference between cars and boats. Cars have a linear required power curve from 0 to wherever drag starts to have a significant effect. You can speed up to 20 or whatever, more with the wind, using a certain amount of torque (once inertia is overcome) to get moving, then add RPM to go faster. Hence the multiple gears in a car transmission.
A planing boat must first push itself OUT of the water, so most of its hull is in air, in order to go faster than a "displacement" hull whose speed is governed simply by its length. A displacement hull does not go faster than its design length period, no matter how much power you add. So to go fast we need to get our hull out of the water. This reduces friction from the hull and we speed up.
That takes a lot of thrust.
So we throw power at it... we go WOT to turn the prop at the top end of the RPM it'll make power at (remember it works better and better until it reaches optimum RPM and forward speed) and hold at WOT until we're on plane, at which point the decreasing drag and increasing forward speed causes us to speed up a LOT (faster speed means the prop works better, which means more thrust, which means faster speed...), and we cut throttle back to hold the speed we want on plane.
We could just use a lower pitched prop, that works better at low forward speed, to get on plane quicker, right? But you need a high pitch prop to go fast! So it's a trade off... high top speed vs. good "hole shot" or getting on plane in the first place.
With me so far? Take a break, get some coffee, I'm not done yet
So what happens when we add a turbocharger? Well, it adds HP (which is a measure of work done over time) by adding RPM, which is part of the HP equation (it's actually HP=(torque(lb/ft)*RPM)/5252). If you look at the equation, it makes sense that you can increase HP by increasing RPM (how fast the motor spins) or by increasing torque (how hard it tries to spin).
So engine ratings in HP are deceptive... two engines with equal HP may have wildly different torque ratings.
Setting aside the engineering problem of installing one and keeping it cool in a boat, a turbocharger adds power by adding RPM. Which it does very well above a certain minimum RPM level.
If we add a turbocharger to my V8 boat, what will happen is that when I reach a certain RPM, the power curve changes... smaller increments in throttle produce large RPM gains (at the expense of fuel use, of course). So what do I do with that RPM gain?
Nothing.
If I go above the prop's RPM limit, I'll get cavitation. So I have to move the throttle lever smaller amounts as I throttle up to get the same RPM. I could get the same effect by changing my hand throttle to require less lever movement for the same throttle setting.
Could I change the drive unit gears out for a different ratio? Sure! If I put gears in with a 3:1 ratio, or double what the V8 used, then I can keep my prop in its RPM range! But unless I change to a higher pitch prop, I can't go any faster through the water. So I'll have to do that.
But will I go faster than the non turbocharged boat with the same prop? No. As long as the non turbo boat motor has enough torque to spin the prop up to optimum RPM and maintain that RPM until we reach optimum forward speed, then the extra torque we get from the turbo + gear reduction does us no good.
Remember that we need only enough torque to overcome water resistance on a given pitch prop. As long as we have a minimum, we're able to spin that prop up to full speed. The turbocharger + gearing do add some torque to the prop over the non-turbo engine, but since our engine torque curve falls off at higher RPMs, it's a diminishing return as we increase RPM (the RPMs have to go faster and faster to maintain the same torque level).
A big block, on the other hand, has a flatter torque curve through the RPM range the prop is using because it's not using high RPM + gearing to make high power.
So we increase prop pitch to increase our boat speed, provided we don't move pitch too high to have a successful hole shot, and provided our engine can still turn the prop to full RPM we'll keep going faster.
Forever? No. Every hull has a maximum speed on plane. Without hydrofoils you can't get the whole hull out of the water. Even with a big engine, it's possible to have a high pitch prop you still can't get to full speed if you can't get your hull high enough out of the water.
Can we lower the amount of thrust needed to get our hole shot? Sure! You cut weight of the boat itself, you bring less people and gear, and less fuel, and you shape the hull to direct the water downward as much as possible, instead of pushing it sideways. A perfectly flat bottom will require much less power to plane than a V hull. Additionally, the less hull we can have in the water, the less drag there is on plane, and the faster we go for a given amount of thrust.
So the quickest boat to plane is a flat bottom, and the fastest boat on plane is one with very little hull in the water. Ever look at a hydroplane race boat? What if we found a way to remove the sterndrive lower unit from the water, so the drag from that goes away, and at the same time we changed from a propeller drive to something that works in air instead? We could really scream. Ever look at a hydrofoil?
So in exchange for our large $$$ investment in a boat-converted turbo and custom drive with custom gear ratio plus more expensive insurance, we got exactly the same return in speed as we did for just buying a new prop.
To get the best speed out of any boat you need to pick a prop that will push the hull to its max speed (usually the hull shape and weight decide this) and then pick a sterndrive (gear) and engine (torque/RPM) combo that has the minimum torque to swing that prop and generate enough thrust to get on plane. Once on plane you can accelerate to the top speed the prop can handle.
On 383s....a 383 stroker adds torque over a 350, just like a big block does. Typically they run at higher RPMs than a 350, which isn't too useful, but the torque helps. However, a 454 or bigger will have more torque than the 383 by a lot, assuming equal engine generations. The 454 will run cooler and last longer, and it can produce the same speed as a 383 with cheaper gas because it has a lower compression ratio.
One more thing to keep in mind when deciding on boat engines.... the drag curve in the water is (I believe) exponential, and the curve gets very steep above about 55-60 mph. In a car, modding the engine for an extra 20 HP makes a big difference in top speed. In a boat that already goes 60, that extra 20 hp might let you make 62 miles an hour. It doesn't matter how you add the power. So to go from 60mph to 90mph in a boat, assuming your hull can do that, you might need to triple your horsepower, preferably by providing more torque. One way to do that is by tripling your number of engines of course...
There's no replacement for displacement in a boat
Erik
