Re: hull extension in progress with picks
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.
I've been looking and you're right it will have to be a custom installation unless you were going turbo diesel which there are many of available. You're also right that there is a fair bit of heat but heat wraps would take care of that no problem. As far as water in the exhaust it doesn't come out of the head that way it is added at the mixing elbow so adding a turbo would require that the turbo be plumbed into the exhaust prior to adding water to it.
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.
Hi Erik at this point I would return your quip of earlier - these is the kinds of statements that people new to turbo charges and turbo charger theory make. Turbos run off of exhaust pressure ratio not rpm. A belt driven centrifigal supercharger is a lot more like what you describe. Anyone who has ever driven a turbo anything knows that without a load a turbo will produce no boost. I can sit in neutral and floored till I'm bouncing off the rev limiter and I'll have less than zero pounds of boost - this thing called vacuum. Why is that what's happening? With less load it takes less fuel and oxygen to push the piston down turn the crank etc. Place a load on it and more air and fuel are required to push that piston down. So turbos are load based not rpm based. Turbos add torque not horse power exactly what you want in a boat motor. If you look at dyno graphs and I've seen a lot of them turbos add Torque in the mid range and often in low to mid range. A properly sized turbo added to a na motor will have a flatter torque curve than what the engine had before it had the turbo but in general it will still follow the torque curve of the engine. The torque curve on piston motors is driven primarily by the cam. Also to the question of efficiency you mentioned earlier - again it comes back to sizing of both halves of the turbo. You size the turbine section for the approx spool time you want then the compressor section for the amount of airflow you need. With a properly sized turbo you will be at the begining of your efficiency island trying to come out of the hole and at the other side of the island at your fast cruise rpm that means you'd be a 90% of the efficiency the turbo can provide in the range that your boat operates 90% of the time. How does this happen. At the lower rpm setting the there is a big load on the engine therefore a lot of available exhaust gas to turn the turbine - remember the turbine was sized to the spool range we wanted - the turbo spools up to the boost setting it was set at and the waste gate opens dumping excess exhaust gas. Now the turbo stays at that boost setting (pressure in the intake manifold constant) and the cfm ingested rises as the rpms go up.. As the boat comes out of the hole the load lessens - less exhaust energy - and the wastegate closes some and as the load continues to drop the wastegate will be fully closed. Then depending on hull shape the boost may actually begin to come down. In a sense the turbo will begin doing what the driver will eventually do with the throttle. This is exactly what we are looking for. I'm in the process of trying to contact the company that built that turbo v8 boat as I expect that you are very wrong about how the boat operates - my guess would be if there is anything bad about how those boats come out of the hole. I would suspect it has nothing to do with the turbos and everything to do with prop selection.
I'll see what I have in my notes... unfortunately prop design is a very black art as far as the internet is concerned. It's an older field though, so you can find books on the subject here and there... the problem is that they're written for naval architects and aeronautical engineers, so you have to have a heavy math background to keep up. I really don't. A couple more things that may come up with regards to turbos, I just think I'll throw them out here for thought. First, I know there are a ton of turbos out there of many sizes, drive types, shapes, etc. with different lag/boost threshold. Even though you can get models that, like a supercharger, provide forced induction across the whole RPM range, they still don't add much, if any, torque.
That is all that turbos add - plain physics makes that so. It's turbo selection that decides where that is applied
Show me a turbo that provides an additional 10% torque or more and can still let the engine use 87 octane gas without additives and I'll be interested
If you insist I'll try and find documentation of that for you - I did some quick searches and just finding prop dyno sheets are a pain. The issue about fuel octane is all about ignition timing and retard and advance curves. That's like saying you can't run a high compression engine on regular - you can you just back off timing - are you getting the most out of your engine no - will it run fine and be happy there sure. I also know first hand that this is true. My car in na form has 92 hp and 105 ft lbs of torque, on 87 pump gas limiting boost to 12lbs I make 168 hp and 218 ft/lbs torque. Now run e85 or race gas and I'm now making 330 hp and 390 ft lbs torque. So it's not will my engine run on 87 but more of why would I. I know for the most part we eschew electronics on boats for reasons of corrosion etc but a knock sensor based timing retard will give you the max torque available for any fuel used - they even use those on high compression engines.
More coming - I had it all written out in one response but we're limited to 20k words. I didn't want to delete any of what Erik was saying both for fear of hearing my response was made out of context and out of respect for the time Erik took to respond. So call this part 1
