Supercharged pleasure boats?

[Mark42]

I was wondering if it was possable to make a big boat with small motor perform like a big boat with a big motor.

What do you think of taking a common 24' cabin cruiser with a 305 V8 and remove the 8 and install a 4 cylinder with a supercharger to effectally double the HP. I know if you ran the 4 cyl at WOT all the time the life span will be greatly shortened. But if the boat is used for pleasure cruising, and only operated at part throttle speeds and cruise at 20-25mph, the 4 cylinder might make sense from a fuel economy point of view. The supercharger will let it produce big horsepower to get on plane, or hit higher speeds for short periods of time. But throttle back and let it sip the gas to extend fuel milage.

There are very simple and inexpensive bolt on centrifugal blowers that will double the motors hp. With water injection and proper tuning, they can be very reliable. And they are not exhaust driven like turbochargers, so all the plumbing and oil cooling issues are eliminated. Of course, they do take a toll on the motor, but thats what you get for burning the candle at both ends.

Just a thought.

 

[QC]

Re: Supercharged pleasure boats?

A Supercharged 4 cyl at double the power and half throttle is the same as a standard 4 cyl at WOT all of the time . . . The bottom end, rings, flows temps, etc. will equal the amount of bhp developed, or better said, the amount of fuel used and horsepower developed is indicative of the same load and strain on the engine. The supercharger allows it to do it, but it doesn't make it feel any better about it . . .

With that said, there are a lot of higher speed small engines, especially in Europe, with turbos and superchargers that could make good and reliable marine engines. You could also argue that a V6 OB, 4 stroke power head is exactly what you have described. The Merc Verado is Supercharged. Higher RPM is the other trick to accomplish this and used by all of the OEMs that build these types of engines.

 

[FlyTSi]

Re: Supercharged pleasure boats?

I drive a 87 Conquest everyday which has a 2.6 ltr turbo charged 4 cyl. I would think that the engine would have problems pulling a boat that size without being in boost, I know my car runs like a banshee if your in boost but not so much if your not. Another thing to consider is that boosting affectively increases the compression ratio by a considerable amount so you will have to run high octane fuel, plus on my car you will use twice the amount of fuel when your boosting. Thats just my thoughts, will be curious the see what everyone else's point of view on this is.
Cheers

 

[xxturbowesxx]

Re: Supercharged pleasure boats?

ive owned many turbocharged cars hence my name but feel that you would have to stay in boost to much to warrant any fuel savings.. to achieve the power you would need at lower rpms out of boost it would have to be a high compression motor and high compression + boost= boom..and if you have a supercharger staying out of boost is all but impossible.. i feel that a modern v6 would be a better choice because it would have all the power of the 305 and better economy..a turbocharger and supercharger for all out performance would be a great addition but im not sure they are very applicable for fuel savings..

 

[Mischief Managed]

Re: Supercharged pleasure boats?

It takes roughly .085 gallons of gas per hour to make one HP, supercharged or not. The amount of power necessary to move a given boat at a given speed does not vary at all so there would be no change in fuel consumption other than a slight drop for the lower weight of the smaller motor.

 

[External Combustion]

Re: Supercharged pleasure boats?

MM does the theoretical Carnot cycle thing on fuel economy. Sorry MM. The reality of gas engines is if you get below .32 pounds of fuel per horsepower hour, you have a good engine. Yes they have a few diesels that put this figure to shame, but the best in the world at this moment is a 100,000 HP two stroke Sultzer at .24 lbs/hp/hr.

Supercharging a smaller engine will give higher HP at a very large increase of technology and fuel quality requirements. Twice the HP of a naturally asperated engine will cost you big time in reliability if it was not designed for the load. The bearings will be loaded at twice what they were originally designed for as well as the other parts of the engine.

Thermodynamically, the larger the cylinder diameter, the more efficient the engine is due to lower heat loss. The shorter the stroke and the faster the RPM, the higher the thermal efficiency. I think the thermal aspects would be a wash at full bore running (WOT), but at lower load the larger engine would have greater economy. Just an opinion with no specific facts.

I ran test engines for many years and I wish that blowing one was the answer. Each day we get closer to it and each day I am am happy for the guys in the trenches, working on engines and blowing them up in an attempt to get better fuel consumption for all of us.

Just some thoughts from down here in the Nations.

 

[QC]

Re: Supercharged pleasure boats?

The shorter the stroke and the faster the RPM, the higher the thermal efficiency.

But the Sulzer is rated at 102 RPM . . . I agree with all else you have here, the WOT thing drives most people nuts.

Also, 100 bhp per liter is pretty common these days and reliable . . . so half of a 5.7 would be 285 bhp . . .

 

[External Combustion]

Re: Supercharged pleasure boats?

The Sultzer would have higher efficiency if it were ran at a higher speed, but the metalurgy does not allow it. The reason higher speed is more efficient is it does not allow as much time during the cycle for heat to transfer to the walls, piston and head. Shorter stroke also cuts down on wall heat transfer.

Yes there are many long lived supercharged engines. They were initially designed for the additional loads of a super charge in the cylinder. Blowing an engine not originally designed for it shortens it's life simply by overloading the bearings and generating more heat than the setup was designed for. Short durations of supercharged use will not materially affect the life of a naturally asperated design, but constant use of positive manifold pressure will.

I spent two years behind a turboblown 12-71 Screamer. The only thing it seemed hard on was my ears, but it was designed to have 14 pounds of boost originally. It is also somewhat hard to call an old Detroit "naturally asperated".

I am not against blown engines, afterall my favorite is Offenhouser and I would toss it in the face of others that derided my car as being "just a four cylinder". An engine for a five hour race is not necessarily a good engine for a boat though.

Bolting on a blower is not a simple affair. First you have to know the pistons and rods can take the load and the heat. Cast pistons have to give way to forged ones with the crown designed not only for the load, but also to allow the oil to cool them. Sometimes better ring technology is needed. Is the charge going to be intercooled, giving a greater charge at a lower pressure? If so that cooler must be designed to fit the engine. The valve timing must be altered to take advantage of the greater charge in the cylinder and the stems and heads beefed up to handle the greatly increased heat lest they melt in half. Sometimes the water passages in the engine have to be redesigned to cope with the additional heat generated.

Most of the blown engines I have seen have a relatively sharp torque curve, which is not good for a boat with no transmission. This could be designed around though,but you would still be lagging behind a naturally asperated engine in flexability.

My old professer once said "remember boys and girls, there is no replacement for displacement!" It still seems true today.

 

[xxturbowesxx]

Re: Supercharged pleasure boats?

a turbocharged engine if tuned right can be a substitute for displacement but not replace.. what determines how much power an engine can make is its voumetric efficiency (the amount of air/fuel and engine can take) 5.7 liters engine take a maximum of 5.7 liters which would be 100% efficiency which in theory is possible but most engines run well below 90% but when you add forced induction into the mix you can increase efficiency well past 100% therefore a 2.2 liter engine would require 182% efficiency to produce 300hp where the 5.7 would only need 87% to produce the same number..


granted cylinder pressures on the 2.2 will be much higher than the v8 it can produce the same hp with turbocharging..but to get towards the post you are squeezing the same amount of air and fuel in the 2.2 that you are in the 5.7 so fuel efficiency is going to be the same at wot. the 2.2 will be more fuel efficient if you arent boosting the engine(probably around 2000rpm) but with no transmission you wouldnt even be able to get on plane without running boost to the 2.2..progressively after 2000 rpm the v8 becomes more efficient..


the amount of boost it would take the 2.2 to run to produce 182% considering most turbocharged engines run about 110 to 130% would be to much to be considered reliable and your engine would eventually go boom on the water..and i realize many 2.2 engines reach over 300hp but this is done with high compression ratios which will further hurt reliablilty and because you can better control an engines rpms with a transmission a marine application will be much less reliable running..

 

[getinmerry]

Re: Supercharged pleasure boats?

What about Rotary (Wankel) engines? I've been thinking about trying to adapt one to boat use. They have a much higher RPM range, more HP and torque, are much smaller and lighter, and seem to be well suited to marine use. Plus, they're practically indestructable.

Read up on them.

Chuck

 

[xxturbowesxx]

Re: Supercharged pleasure boats?

im well aware of the wankel rotary motors i had a 88 turbo rx7 a few years ago the apex seals go bad on those motors quickly and arent easy to replace.. it got so bad i had to pour power steering fluid into my motor to lube them enough to get the motor started.. also the rotary even though its 1.3liters in most cases it weighs as much as a 5.7.. i sold that car to a freind and we swapped a 383 stroker in it and it only added 60lbs..ive seen a turbo rotary in a boat and it hauls but id say you could build a v8 to put out alot more performance and tourqe you will never get out a rotary.. but ill have to disagree with the indestructable part of the rotary ive seen many blow up..but as far as economy i think the rotary wouldnt be a good choice it would be a cool motor for all out speed but still you could get more out of a supercharged v8 and it would probably be cheaper..
heres the boat http://youtube.com/watch?v=jNl0GoNPPAw

 

[getinmerry]

Re: Supercharged pleasure boats?

Quote:
im well aware of the wankel rotary motors i had a 88 turbo rx7 a few years ago the apex seals go bad on those motors quickly and arent easy to replace

According to what I've been reading, they've changed the Seal design and material so that now they typically last 100,000 miles or longer.

I know there are drawbacks, but if Rolls Royce feels they're reliable enough to put them in modern aircraft, then they must be fairly bulletproof now.

Chuck

 

[xxturbowesxx]

Re: Supercharged pleasure boats?

the 93-95 had the same problems... if they updated the design where will you get this motor from? besides the rx8 1995 was the last rotary to be sold in the states... and your right about the aircraft motors but these arent the same as what they sell in cars and will cost much more..

 

[FlyTSi]

Re: Supercharged pleasure boats?

Be cool to have a boat with a blow off valve

 

[xxturbowesxx]

Re: Supercharged pleasure boats?

my dad used to bass fish for a living and his best freind had a 18' allison race boat with a 2.5 merc with nitrous injection its was a funnn boat.. im used to fast boats my dad had a stv race boat back in the day but my freind had never been on a fast boat and he was so exited i warned him it can get pretty hairy but he went for a ride in the allison and at about 120 the boat started dance and he threw up lol..

 

[getinmerry]

Re: Supercharged pleasure boats?

the 93-95 had the same problems... if they updated the design where will you get this motor from? besides the rx8 1995 was the last rotary to be sold in the states... and your right about the aircraft motors but these arent the same as what they sell in cars and will cost much more..

Mazda RX8 has the newly designed motor.

Chuck

 

[QC]

Re: Supercharged pleasure boats?

I thought Wankels had fuel efficiency issues. I also though they actually had a good power to weight ratio. I guess not . . . ??? I guess I need to search for some specs. I also had Mazda experience when I was young. An RX3 SP which was basically RX7 running gear with an RX3 body on it. A real sleeper, had a lot of fun with that car.

 

[xxturbowesxx]

Re: Supercharged pleasure boats?

ok i thought you were talking about the older setups but how much are you going to spend on a rx8 engine and turbo setup?


my 88 turbo II was horrible on gas but responded well to modifications it was a torqueless wonder my old turbocharged civic had tons compared to it but like i said the 383 v8 we swaped into it only weighed 60lbs more and some guys claim that ls1 swaps in the 93-95 rx7's save about 120lbs...now a turbocharged built ls1 capable of 7500rpms would be an awesome marine engine..

i have a freind that has a rx3 with a 95 rx7 motor and it pulls wheelies in 3rd gear.. i have ridden it in once at the track and it was pretty scary..

 

[QC]

Re: Supercharged pleasure boats?

It is also somewhat hard to call an old Detroit "naturally asperated".

Weird that they used to teach us that. They were scavenged and not technically blown . . . Also weird to me that with the Sulzer, and other mega 2 cycle engines, that they overcome the efficiency losses I understood were acssociated with scavenging and effective stroke reduction from open ports at the bottom . . .

 

[Mischief Managed]

Re: Supercharged pleasure boats?

MM does the theoretical Carnot cycle thing on fuel economy. Sorry MM. The reality of gas engines is if you get below .32 pounds of fuel per horsepower hour, you have a good engine. Yes they have a few diesels that put this figure to shame, but the best in the world at this moment is a 100,000 HP two stroke Sultzer at .24 lbs/hp/hr.

Supercharging a smaller engine will give higher HP at a very large increase of technology and fuel quality requirements. Twice the HP of a naturally asperated engine will cost you big time in reliability if it was not designed for the load. The bearings will be loaded at twice what they were originally designed for as well as the other parts of the engine.

Thermodynamically, the larger the cylinder diameter, the more efficient the engine is due to lower heat loss. The shorter the stroke and the faster the RPM, the higher the thermal efficiency. I think the thermal aspects would be a wash at full bore running (WOT), but at lower load the larger engine would have greater economy. Just an opinion with no specific facts.

I ran test engines for many years and I wish that blowing one was the answer. Each day we get closer to it and each day I am am happy for the guys in the trenches, working on engines and blowing them up in an attempt to get better fuel consumption for all of us.

Just some thoughts from down here in the Nations.

Perhaps I should have said .085 gallons per hour to make one propshaft HP in a commonly used application, like Mark42 described in the first post on this subject.

I'd love to have an engine in my boat that only used .32 lbs of gasoline per hour to make one propshaft HP, but I am convinced that there are none commonly available. My very common boat engine uses about .55 pounds of gas per hour per HP.