A buddy of mine is a big time boat guy and has a 25' boat that came with a chevy 305. It now has a 350 in it rated for 260-270hp. I saw a boat in a magazine with a ford 302 rated for 260hp.

It came to me that I don't understand how these figures are so high, I kind of feel like they're very over-rated.

The redline on these engines are no more than 4000 rpm due to being setup for tq. They can't be pushing monsterous torque as they're limited by displacement, the 302 I'd say doesn't have over 330 ft-lbs and the 350 probably doesn't have over 360-370. These seem like high estimates to me but regardless, how are these engines rated so high in HP when the RPM band is low and the tq doesn't seem high enough to compensate?

Anyone able to set me straight?

 

Mostly they are of higher perfomance engines that currently dont have any pollution control on them.Also they are most often operated near wide open throttle...where as a car or truck you barely use 1/4 to 1/2 of the throttle.Accually most boats operate in the 4500-5500 range which is done by the amount of pitch on the propeller. Also pond for pond an outboard will put out more power than an comprable inboard motor.

 

Yup, they are set up to operate at high RPM wide open, just the opposite from land vehicle engines. They won't operate worth beans at part throttle.

 

These engines that I am speaking of - don't run past 4000 rpm. My other friend has a 40ft Cruisers with twin 454's that don't go above 3800 rpm.

My 2 stroke 115hp I-6 merc outboard runs to 5500. That to me is a little low for a 2 stroke - but it's a large 2 stroke in a boating application...so it makes sense as it needs tq. Not sure what the displacement of it is, but probably in the 175-200ci area. As opposed to my light snowmobile that runs to 8200 rpm (700cc twin cyl 2 stroke).

Another has a 35-40ft'r with twin Chrysler 318s...probably very similar to my motorhome's 318 (72 2bbl) that runs to 3800-4000.

They're not run at full throttle constantly - the most fuel efficent point is about 3/4 throttle depending on the engine/prop/boat.


These engines are very similar to RV style engines, if not identical minus cam specs and marine gear.

Smog equipment doesn't rob very much horsepower.

 

3800 RPM is a lot different than 2000 RPM. Engines can be set up to operate efficiently at any particular RPM or power setting (within constraints). Things like induction and cam are what determine the power band. Check any camshaft site.

 

Ok.

My understanding is their efficiency has to do with their application too, such as giving it more throttle just makes these large boats push in the water vs get up and out/plane.

I think I understand what you're saying...

I'm just having a hard time grasping my understanding of how they're making these rated hp figures.

Most people know horsepower is how quickly work is done which is relative to RPM. With such low RPM, (like a diesel that has low hp ratings), I can't grasp how they're ratings are so high.

My only shot at it is that they're making enough torque to overshoot the low RPM figures and keep the hp ratings high. But I don't see how that would happen when they're not anything rediculous in torque. If one of these 302s was making 400 ft-lbs I'd understand the higher HP rating because....HP is a function of torque and RPM...the higher the tq the higher the HP.

An engine that produces low RPM torque is one that will generally have a low hp figure because the tq peak is low in the RPM band and the overall RPM range is low, 4000-5000 or so. When you increase the hp figure, you shift the tq further up in the RPM band and typically increase the RPM range to over 6000 depending on the application.

I'm just a little ignorant when it comes to the details such as this.

 

Does anyone know if they use the same testing methods on marine motors as they do on "land" apps? Is it SAE net hp?

 

Probably crankshaft HP not net.

 

Crankshaft hp would be - the hp the engine produces w/o losses from accessories right?

Net would be including accessory loss?

Boats don't have much for accesory loss, they have an alternator and water pump, possibly something else depending on the application.

 

Marine HP is measured on a dyno and at the crankshaft. When you measure HP at the prop a safe loss number would be around 15+ HP. Propshaft horsepower is what you would refer to as net to the marine industry.

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Marine engines are designed to make the most power at lower RPM because of an inherent weak link………..the drive system. Marine drives don’t like to turn 8K

 

No, they certainly don't like to turn high RPMs.

But I don't believe they make the engines like that because of the drive system...they make the engines like that because of what work they're doing. Just like an RV, they are typically lower in compression for more constant heavy loads and are not as concerned with acceleration. These engines are made to hum along at whichever RPM and torque is the main concern. Whether it's a gas engine or diesel. Diesels are perfect for boating and RV applications because they do a better job than a gasser. You can build a gasser to be a low rpm torque monster and act more like a diesel but a diesel is nice in these applications. You wouldn't want a higher RPM engine in this application because the torque would be made too high in the RPM band. You'd naturally want a low RPM tq monster.

 

My Mercruiser equipped Chris Craft Stinger has two 383 strokers. I had to select the cams to match the safe rpm range for the outdrives. The drives are designed for no more then 4,800 rpm's stock, but mine have high performance gearsets in them. I've held them at 5,200 rpm's for nearly an hour straight, but I have drive coolers that pull up water from by the prop and spray on top of the drive gearcase. Heat is the main enemy of drives. I also added drive gearlube reservoirs that triple the gearlube capacity. The only drive failure I've had since then, had something to do with not backing off on the throttle on re entry. The rear of my 28' boat was at least 15' out of the water (surprise wave), and the boat was nearly facing the sky. The splashdown was pretty soft, but obviously I made a driver error by not pulling back. Some drives have a higher tolerance for high rpm then a Mercruiser Alpha SS.

My factory 260's showed one at 235hp and one at 247hp on a propshaft dyno. The replacement 383's were built at the same time, same machine work and same parts as each other. One puts out just over 375 and the other about 390hp. I round it off to 400hp each when stating crank hp to people with questions. It's easier then trying to explain it all. The oil pressure is slightly higher on the weaker engine. I think that the 100+ psi (actually maxed out on my gauge) cold oil pressure may be stealing some power. My machinist stated that I should use the pink spring for my oil pumps (hv/hp). When I told him how much psi, he stated that "it's a little much for a small block". I then started running 5W/20 after break-in, and now I have about 70 psi at WOT.....As far as the low compression ratio? Not so. I'm running with .125 piston domes and 78cc heads. My temp stays around 150-160, except for idling after a extended WOT cruise (like during a poker run).

 

For all you guy’s into boats and performance marine, go check out the web site www.offshoreonly.com . Maybe the board gods here (if you’re listening) should talk to the board gods there and do a link swap. Fords are very popular in the offshore world and I’m sure a lot of guys here have some pretty nice boats.

The low compression of today’s marine engines is a direct result of the lack of availability and cost prohibitive price of quality high octane fuel. I used to shudder in fear when I would see the Sunoco truck pull up to my boat.

Unless you are running a surface drive like a Kamma, Arneson or Buzzi stay away from the RPM’s. Even then there is no need for excessive RPM’s. Drives like the Alpha, Bravo and Cobra DO NOT like to spin faster than 6000 rpm. They are shiftable drives (no transmission) and inherently weak for performance or race applications. Most of the time it’s the upper end of the drive that lets go because this is where all the gear movement takes place that controls forward and reverse. Hardened gears and the like make things better but never good enough. Even drives like the MerCruiser IV, V, and dry sump VI (external transmission drives) don’t particularly like high RPM.

Another weak link (although making great strides with technology) is props. The more RPM you put to a prop the greater the chance of metal fatigue (hydro-dynamics & harmonics) and ultimately prop failure. And when a blade comes off at 6000 RPM you then stand a real good chance of opening a whole new can of worms……….literally. I’ve seen prop blades imbedded in boats, and outdrives exploded from the blade hitting it. I always found cast prop’s to be more durable than a prop with welded blades.

Keep in mind that torque moves mass. You can have an engine that develops 580 / 730 (torque/horsepower) @ 7000 RPM that runs like a scalded dog in you’re 66 Chevelle. With this engine you can spin a 24” pitch prop at 6500 to 6800 RPM while you cross everything you can in hopes your drive and engine stays together and run 70 miles per hour in a given boat. Then take an engine that develops 650 / 650 (torque/horsepower) @ 6000 RPM and put it in the same boat and you will find you now have to run a 30” pitch prop which you can turn all day long @ 5800 RPM. 80 plus miles per hour and lots of smiles.


stevef100s

What you describe above is a definite no no. When you’re boat leaves the water and becomes airborne you’re releasing all resistance on your prop allowing everything to free-wheel, like dead revving your car. If you left the water spinning 6000 RPM and don’t pull back on the throttle your probably now well over red line or in the bump box. Imagine that prop hitting the water at now 6500 RPM. It’s like hitting a brick wall and hard on the drive train. The trick is to pull back some when you leave and hit it again as you feel the boat coming back into the water. It’s called throttling. Not doing so is one of the quickest ways of breaking something that is usually expensive. If you ever watch a rough offshore race you’ll see that it isn’t uncommon to travel 50 feet or so 10 feet in the air. What you will rarely ever hear is the engines in that boat over-revving or banging the bump box. All you’ll hear is a smooth transition, silence while in the air and the same RPM upon re-entry as during lift-off.

Best regards to all,
Eric<O</O

 

Marine are rated hp at the propshaft if built after 1980 fed law . most of the power comes from the induction & unrestricted exaust . The block on some engines are a little beefier in areas , but most are automotive blocks built for the marine industry by the engine supplier ford, gm ,cummins, ect..

 

Aiso that inline six likes to breath turn it on up to about 6ooo