Displacement

[sir-vivor]

I've been reading a lot on the new OB, because I will be repowering Sir-Vivor II with a new 200hp this spring.
I need some help here understanding DISPLACEMENT and its RELEVANCE!
I'm a pretty simple , let's go fishing kind of guy and engineering is like some far off galaxy terminology, so please keep it simple.
Please can some one help here, do I have to consider it ??

 

[tubber]

If you filled a gigantic measuring cup with water, then gently lowered your boat into it, some amount of water would spill over. If you take your boat out again, you can see how much volume of water was displaced, then multiply this amount by 1kg/liter. That's your displacement. That weight, is the same force that the water pushes back up to float your boat.
A displacement hull is one that ploughs along without planing. Like a freighter, a tug, or many other slow boats, or even Dorman Point.

All that matters for you is if 200hp works for your boat.

 

[sir-vivor]

If you filled a gigantic measuring cup with water, then gently lowered your boat into it, some amount of water would spill over. If you take your boat out again, you can see how much volume of water was displaced, then multiply this amount by 1kg/liter. That's your displacement. That weight, is the same force that the water pushes back up to float your boat.
A displacement hull is one that ploughs along without planing. Like a freighter, a tug, or many other slow boats, or even Dorman Point.

All that matters for you is if 200hp works for your boat.

Thanks Tubber, I got that but , why do the mfg (Merc and Yam for example) put so much emphasis on displacement. I might be overthinking this but I don't see why it is that relevant when considering a new engine and why they put so much emphasis on it. From what I've read its like , Whats the rating for your boat and some whats the weight , not whats your displacement (especially with a 21ft boat lol)

 

[gungadin]

I don’t know if you are refering to hull or engine displacement, However:

Looking at just displacement in popular 200 hp outboards it can run from just under 5 litres in a v8 to under 3 litres in a four cylinder.

To put it simply, you need to know the type of boat your engine will be pushing, do you need lots of low end torque to get up on plane? If you sre using a 4cy that produces 200hp, it may have a very high compression ratio and require a premium fuel, while a V8 may have a lower ratio and be fine with regular fuel. A four cylinder may even need a supercharger to reach the advertised 200hp.

Your boat may be just fine with a smaller displacement v6. Other things that you have to take into account would be weight and gear ratio of the lower end. For instance if your boat was originally rated up to 200hp years ago wen two strokes were the norm, you may need to lower your hp to stay within the max allowable weight.

Assuming that the engines will rev to around 6000 rpm you may find that the gear ratio of one engine would allow (with the matched propeller) for a higher top speed, but at the expense of slow speed if you use it for trolling.

Other factors can also come into play, many of which, I probably no little about.

 

[Rain City]

All I know is allegedly there's no replacement for it...

 

[Old Fisher]

I think you are asking about engine displacement ( the volume of air/fuel displaced by the piston as it travels from bottom to top of its stroke times the number of cylinders) and its effect on engine performance. You have probably encountered the fact that engine manufacturers (Yamaha for example) now offer a 200 hp outboard in a 4 cylinder or 6 cylinder version. The smaller 4 cylinder engine is cheaper and lighter but they are very different engines. Both are rated at 200 hp but remember that these ratings are their maximum power capability and you almost never run an engine at max power. The 6 cylinder engine will produce far more torque (the power that really matters) than the smaller4 cylinder engine and therefore will be able to swing a bigger prop making for more efficient cruise capability and better hole shots and load carrying ability. Even though the 4 cylinder engine is smaller (less displacement), it may not provide better fuel mileage because it will have to work harder to achieve the same cruise speed.
Your choice will depend on the type of boat you have and how you use it. If you are running a relatively heavy fishing style boat, like most of us, you will be happier with the bigger engine. IMO. As has been mentioned: There is no replacement for displacement

 

[beemer]

Displacement can also be represented through block and cylinder size. A big block 4 cylinder can often spin a bigger prop than a light weight 6 cylinder. When choosing an outboard look at the entire line that is usually derived from the same 4 cylinder block eg Suzuki 150 , 175 ,200. are the same block with an ecu computer telling the motor to feed more fuel to the higher horsepower motor. You need to look at the whole package , gear ratio , prop size etc.

 

[Totally unexpected]

No replacement for displacement...simple as that lol

 

[Wild Bill]

So many variables! Assuming you are starting with naturally aspirated engines an engine with a longer stroke will usually give you more torque, so likely a 4 cylinder over a 6 cylinder if displacements were similar. Then you have to factor in the lower unit gear ratio, the 6 cylinder could end up with more torque at the propeller.
Naturally aspirated versus supercharged and compression ratio also come into the picture.
I would go with making up your mind listening to people on here who have had experience with the engines you are looking at, performance, economy, weight, price, warranty, longevity etc.
I prefer lower stressed components so I would probably pick a lower compression 6 cylinder with more displacement.

 

[Tyee Fighter]

A quick tale of displacement...
I took out a 5.0 fi vortec and replaced with a 5.7 vortec, much more hp and gobs more torque. The hp doesn’t matter for top speed, as both engines reached max rpm and there was no prop change but the time to get there is significantly less. The boat planes out in a hurry, and is a pleasure to drive as the throttle doesn’t get touched much, it’ll power through waves and wake without coming off plane and having to constantly adjust for a nice ride. In the ocean I think torque is king for load capability and for fuel economy as well.

 

[ericl]

Horsepower is the product of torque times rpm. All other variables that affect torque being equal (same compression ratio, same number of cylinders same valve timing) the engine with larger displacement will produce more torque at a given engine speed (RPM). Drive a car with a large engine up a hill & you may not need to step on the gas - drive the same car up the same hill with a smaller engine you will be more likely to have to step on the gas. If you are in your boat & encounter a large wave the larger displacement engine may not slow in RPM due to the increased load, but a smaller engine will need more throttle to maintain speed. If the loss of speed in the boat with the smaller engine causes the boat to come-off plane, you must know apply more throttle to get back onto plane. This will use more fuel plus be a major PITA.
All the other good stuff above is true; running at low RPM increase engine life.
Let me just add that there are other factors that can increase torque besides displacement, compression ratio, largest probably being Direct Fuel Injection and electronic control of valve & ignition timing.

Sadly O/B mfgr's don't like to publish detailed specs such as torque & horsepower curves on their engines ; probably makes them look bad compared tp other engine types (especially diesel).

 

[Rayvon]

I'm no mechanic either ,but to answer your question,"Do I have to consider it (displacement)? The answer would be Yes.I think you may have confused hull displacement with engine displacement in the beginning,others have sorted that out for you.I'm a big Yamaha fan and if I was buying in the 200 HP range,I'd go for the 6 cylinder over the 4 cylinder (big engine displacement difference)because the 6 would not work as hard and they have a very long history of being almost bullet proof.Not that the 4 isn't a good motor too.Like others have said and I don't think you've answered what boat you have will determine which motor is best for your use.

 

[Viking]

Horsepower is the product of torque times rpm. All other variables that affect torque being equal (same compression ratio, same number of cylinders same valve timing) the engine with larger displacement will produce more torque at a given engine speed (RPM). Drive a car with a large engine up a hill & you may not need to step on the gas - drive the same car up the same hill with a smaller engine you will be more likely to have to step on the gas. If you are in your boat & encounter a large wave the larger displacement engine may not slow in RPM due to the increased load, but a smaller engine will need more throttle to maintain speed. If the loss of speed in the boat with the smaller engine causes the boat to come-off plane, you must know apply more throttle to get back onto plane. This will use more fuel plus be a major PITA.
All the other good stuff above is true; running at low RPM increase engine life.
Let me just add that there are other factors that can increase torque besides displacement, compression ratio, largest probably being Direct Fuel Injection and electronic control of valve & ignition timing.

Sadly O/B mfgr's don't like to publish detailed specs such as torque & horsepower curves on their engines ; probably makes them look bad compared tp other engine types (especially diesel)

Horsepower is the product of torque times rpm. All other variables that affect torque being equal (same compression ratio, same number of cylinders same valve timing) the engine with larger displacement will produce more torque at a given engine speed (RPM). Drive a car with a large engine up a hill & you may not need to step on the gas - drive the same car up the same hill with a smaller engine you will be more likely to have to step on the gas. If you are in your boat & encounter a large wave the larger displacement engine may not slow in RPM due to the increased load, but a smaller engine will need more throttle to maintain speed. If the loss of speed in the boat with the smaller engine causes the boat to come-off plane, you must know apply more throttle to get back onto plane. This will use more fuel plus be a major PITA.
All the other good stuff above is true; running at low RPM increase engine life.
Let me just add that there are other factors that can increase torque besides displacement, compression ratio, largest probably being Direct Fuel Injection and electronic control of valve & ignition timing.

Sadly O/B mfgr's don't like to publish detailed specs such as torque & horsepower curves on their engines ; probably makes them look bad compared tp other engine types (especially diesel).

Torque x rpm ÷ 5252 = hp

 

[Viking]

Horsepower is the product of torque times rpm. All other variables that affect torque being equal (same compression ratio, same number of cylinders same valve timing) the engine with larger displacement will produce more torque at a given engine speed (RPM). Drive a car with a large engine up a hill & you may not need to step on the gas - drive the same car up the same hill with a smaller engine you will be more likely to have to step on the gas. If you are in your boat & encounter a large wave the larger displacement engine may not slow in RPM due to the increased load, but a smaller engine will need more throttle to maintain speed. If the loss of speed in the boat with the smaller engine causes the boat to come-off plane, you must know apply more throttle to get back onto plane. This will use more fuel plus be a major PITA.
All the other good stuff above is true; running at low RPM increase engine life.
Let me just add that there are other factors that can increase torque besides displacement, compression ratio, largest probably being Direct Fuel Injection and electronic control of valve & ignition timing.

Sadly O/B mfgr's don't like to publish detailed specs such as torque & horsepower curves on their engines ; probably makes them look bad compared tp other engine types (especially diesel)

Horsepower is the product of torque times rpm. All other variables that affect torque being equal (same compression ratio, same number of cylinders same valve timing) the engine with larger displacement will produce more torque at a given engine speed (RPM). Drive a car with a large engine up a hill & you may not need to step on the gas - drive the same car up the same hill with a smaller engine you will be more likely to have to step on the gas. If you are in your boat & encounter a large wave the larger displacement engine may not slow in RPM due to the increased load, but a smaller engine will need more throttle to maintain speed. If the loss of speed in the boat with the smaller engine causes the boat to come-off plane, you must know apply more throttle to get back onto plane. This will use more fuel plus be a major PITA.
All the other good stuff above is true; running at low RPM increase engine life.
Let me just add that there are other factors that can increase torque besides displacement, compression ratio, largest probably being Direct Fuel Injection and electronic control of valve & ignition timing.

Sadly O/B mfgr's don't like to publish detailed specs such as torque & horsepower curves on their engines ; probably makes them look bad compared tp other engine types (especially diesel).

Torque x rpm ÷ 5252 = hp

 

[Viking]

The bigger displacement engine will be under less load and last longer . Offer more torque as well. Boats need torque. The more of it you have the better. Like a diesel, the more weight you add, the less the rpm will be pulled down.
The bigger displacement engine will cruise a lower rpm because it will have more torque.
Fuel burn . Who really knows lol
Drag cars need horsepower.
Hope that makes sense

 

[Foxsea]

If you can, look at torque and h.p. curves for each engine. Rated horsepower is usually achieved near WOT. That rated horsepower is what we pay for. But for most of us, it is irrelevant because we rarely run there. Looking at the curves, check the torque and h.p. output at the rpm you would normally cruise at. You may find that the larger displacement engine provides more of both at lower rpm's. As Viking mentioned, that will often equate with less wear and tear and better durability. You can do many modifications to get more horsepower from a lighter, smaller displacement engine but for a boat, that's not often the best choice.

Pretty good article on this topic:

Best Outboard Motors for Your Boat | Sport Fishing Mag

Here's how to decide between four- and six-cylinder 200 hp outboards for your style of boating.

www.sportfishingmag.com

 

[Old Fisher]

If you can, look at torque and h.p. curves for each engine. Rated horsepower is usually achieved near WOT. That rated horsepower is what we pay for. But for most of us, it is irrelevant because we rarely run there. Looking at the curves, check the torque and h.p. output at the rpm you would normally cruise at. You may find that the larger displacement engine provides more of both at lower rpm's. As Viking mentioned, that will often equate with less wear and tear and better durability. You can do many modifications to get more horsepower from a lighter, smaller displacement engine but for a boat, that's not often the best choice.

Pretty good article on this topic:

Best Outboard Motors for Your Boat | Sport Fishing Mag

Here's how to decide between four- and six-cylinder 200 hp outboards for your style of boating.

www.sportfishingmag.com

The article in the above link is excellent, in easy to understand language

 

[ericl]

Torque x rpm ÷ 5252 = hp

Thank you