Torque, Thrust, and Force

[Texasmark]

Read a couple of comments yesterday and I think Q.C. and Walleyehed to name a couple, were involved and it reminded me of a personal curiousity that others might also be interested in.

Marine Engine Design 101: Ok, let's say that my job is to design a 50 hp outboard motor. We go through all the design requirements and finally get to the "push" mechanism.....gearbox ratio and prop dia/pitch determination. How do we work through this part?

One of the reasons for the question is that I have had the Merc Tower of Power 1n 115 hp. with a 2:1 gearbox while Merc offered the same engine choked down to 90 hp with a 2.3:1 box. Today I have a 3 cyl Merc 90 hp with a 2.3 gearbox while Merc adds a 4 th cylinder to the same engine putting out 100-125 hp and uses 2:1 box.

What's the difference in the 115 with a say a 19P prop to do a job and a 90 hp with say a 23P to do the same job (didn't do the math, just looking for the concept)? I know the answer is in the thread title, but I don't know which part plays which part.

Oh and to keep the playing field level, both engines are throttled to put out 90 hp. So like the 115 may be rated at 5500 and the 90 rated at 5000, both will be at 5000 when looking at the answer.

Thanks,

Mark

 

[SuzukiChopper]

Re: Torque, Thrust, and Force

If I remember my physics correctly, force covers both torque and thrust. They are both forces being acted upon a certain object. Torque is a rotational force (prop spinning) and the total amount of torque at a certain point on the prop will be determined by the diameter of the prop. Thrust is a linear force applied to an object which will be determined by the pitch of the prop as well as the 'working' surface area of the prop.

Seems to me that both forces will have a bearing in answering your question.

 

[jeffnick]

Re: Torque, Thrust, and Force

I?m not going to pretend to be an expert, but here?s how I think about things:​

HP is the amount of work that can be done​

If work done is via a rotational source, torque tells where in the rpm range the HP can be found/utilized.​

Thrust is a product of mechanics (gearing, pitch etc.)​

Force = Mass times Acceleration and could be a measure of thrust? ​

Your 50HP could be mechanically tuned for high thrust (holeshot), top speed or anywhere inbetween. You'd be better off with torque at a low RPM for the holeshot...​

As you play with gearing and pitch, you are in effect assigning the RPM of your motor to a given speed...ideally putting the motor's torque where it is most desirable.​

 

[Silvertip]

Re: Torque, Thrust, and Force

1) Prop diameter has little to do with thrust or torque. Pitch does. The prop doesn't deliver torque. It provides thrust based on torque applied to it by the engine.
2) Marine engine designers don't design a a xx HP engine and then seek a prop or gearset for it. They study the duty the engine will endure, determine optimum engine displacement that will provide the economy and performance characteristics they are shooting for and they then look at combinations of gear ratios and prop designs to accomplish those goals.
3) Throttling an engine to 90 HP is not a realistic comparison between a 115 and a 90.
4) The 115, because it has more power, can better utilize a 2.0 gearset than a 90 HP motor in most applications. But that is not always true. The 40/50/60 HP Merc triples can be fitted with either 1.83, 1.64, or 2.33 gear sets depending on application. Is it not surprising that the 1.83 ratio is the standard. 2.33 goes on Big Foot motors and the 1.64 went on 60 HP versions with the small gearcase. Nothing else changes. The 1.83 and 1.64 are small hub gear cases. The 2.33 is the same as that on the 75 and 90 HP motors. Prop diameter and pitch are adjusted accordingly. Prop diameter is a function of gearcase hub size and clearance between trim tabs and AV plate.
5) Engines are designed with propeller speed (rpm) in mind. A large displacement engine can spin more prop pitch than a smaller displacement engine using the same gear ratio. It's all about leverage.
6) Case in point. My daughter bought one of my older boats that was originally purchased with a 40 HP Evinrude (two cylinder - 747 cc). I kept the motor when I sold her the boat and it was repowered with a 40 HP Merc (3 cylinder - 967 cc). The Evinrude was far quicker out of the hole than the Merc even though it had significantly less displacement. The Merc would outrun it but not a run away by any means. The difference in hole shot is due to the Evinrude 2.43 gear set and a 12-1/4 x 15 prop. The Merc runs a 10-1/2 x 13.

This discussion all boils down to propeller speed and pitch and the engines ability to twist the combination.

 

[Frank Acampora]

Re: Torque, Thrust, and Force

And let's not forget that sometimes it boils down to money, design be damned: Remember that most of the times the engine manufacturer buys gear sets from major gear producers. A 50 cent difference in component price is a 5 to 10 dollar increase in finished product sale price. Sometimes the designer is told by corporate penny-watchers and pencil-pushers which component to use.

My personal opinion is that that is the reason for regressive gear ratios on old Chrysler and Force engines.

 

[45Auto]

Re: Torque, Thrust, and Force

Marine Engine Design 101: Ok, let's say that my job is to design a 50 hp outboard motor. We go through all the design requirements and finally get to the "push" mechanism.....gearbox ratio and prop dia/pitch determination. How do we work through this part?

First we have to know a few of your basic requirements.

What kind of boat are you designing for? Could be anything from a 8' lightweight racing boat to a 50' house boat.

Does your 50 HP engine have a broad powerband (makes 50 HP anywhere between 1500 and 6000 RPM) or is it a very lightweight, high-strung racing motor that makes 10 HP below 9900 RPM then makes 50 HP between 9900 and 10,000 RPM?

Is the lower a totally new design (huge investment cost) or are you using existing components (lowers, gearing, propellers, etc)?

You can see how much difference the type of boat makes by just looking at Mercury's Bigfoot propaganda:

http://www.mercurymarine.com/engines/outboards/fourstrokes/pontoonpower.php

The Bigfoots use bigger, deeper props for low speed operation. However, they are putting the same amount of power in the water as the same motor with a normal lower. The Bigfoots use bigger gears and shafts because they are operating at higher torques than a smaller propellor. Contrary to the beliefs of some, prop diameter has a HUGE affect on thrust and torque. Why do you think big ships (ocean liners, tankers, etc) use 30 foot diameter props instead of 14 inch diameter props?????? Obviously, there?s not much affect going from a 13.75? diameter to a 14? diameter (a 1.7% change) as opposed to going from a 17? pitch to a 19? pitch (an 11.7% change), but you can look at the basic prop equations and see the effect. T is thrust, D is diameter, Q is torque, and V is pitch in the equations below (straight from Wikipedia ?Propellors? article, I?m not going to post the rest of the differential equation derivatives here).

You can see how the diameter squared is a major component of the thrust equation:

And if you want to know how much torque a prop can absorb, you have to worry about the diameter cubed:

Marine engine designers don't design a a xx HP engine and then seek a prop or gearset for it. They study the duty the engine will endure, determine optimum engine displacement that will provide the economy and performance characteristics they are shooting for and they then look at combinations of gear ratios and prop designs to accomplish those goals.

Actually, they don?t try to optimize it for any specific economy or performance characteristics. That would give them a VERY small market, and they would have to have a million different engine configurations. They try to make the performance characteristics as broad as possible for every engine displacement to cover as many markets as possible. That?s why you can sometimes enhance your engine application through the use of a specific propeller that optimizes some of the engine?s characteristics for your unique application.

In other words, your design requirements (that you haven?t stated) such as cost, performance, and market will drive your selection of drive ratio and prop diameter/pitch combination.

Today I have a 3 cyl Merc 90 hp with a 2.3 gearbox while Merc adds a 4 th cylinder to the same engine putting out 100-125 hp and uses 2:1 box.

In your specific example, the 4 cylinder motor puts out more torque, thus requires less torque multiplication through the gearbox, to put the same amount of power into the water. This also allows the 4 cylinder to use a higher pitch prop and deliver the same acceleration. For example, if you prop both engines for the same top speed, you?ll have to use a lower pitched prop on the 4 cylinder. That means that the 4 cylinder is going to have more torque on the bottom end and accelerate harder (pull up skiers better). If you prop them for the same acceleration, the 4 cylinder will be able to provide the same acceleration on a higher pitched prop and thus have more top speed.

This thread shows how you would optimize gearing and props for a specific application:

http://forums.iboats.com/showthread.php?t=225803&highlight=secrets

 

[Silvertip]

Re: Torque, Thrust, and Force

Your example (and formulas) for large vs small diameter props are valid. However, manufacturers must still design the engine for the expected performance characteristics most of the engines in that line will see. If a larger diameter prop is "always" best, then every lower unit in the midrange HP bracket would have the capability to use 12 - 13.5 inch props which is not the case. Merc again is an example and yes, that's why there is an abundance of models. Merc and Yamaha both make "high thrust" models. Small gearcases cannot accept the deeper ratios in the case of Merc. Johnson/Evinrude gives you a 2.34 gearcase on their midrange motors and and there is a wide selection of props to get the performance you need. It is about "design" and "expected use". How each manufacturer achieves their stated goal is what creates competion. Small diameter props spinning at high speed are fast but not very good out of the hole. Large diameter props spinning slower but with more pitch and deeper gears can push a barge and come out of the hole like a rocket but may not be very fast. Happiness is somewhere in-between which is what every manufacturer shoots for. It also makes no sense to install a huge and heavy lower unit on a small engine. The small engine will simply not be able to use the capability. Once an engine gets up into the three digit HP range gear ratio tends to settle in the 2.0 - 1.85 range and any desired performance characteristics can be handled with setup and prop selection.

 

[Texasmark]

Re: Torque, Thrust, and Force

Ok 45 and Silver. Guys, I have both your agendas in my messkit. Being a retired design engineer of other types of things, and a practical applications guy with 50 years of boating under my belt, I can side with both of you.

But 45 sir, you provided that for which I was looking. Thank you.

Just for the record. In high school, I spent the last couple of summers as a deck hand on a tug boat. We ran barges from South Texas over to NOLA via the Intercoastal Canal. We ONLY had a 400 hp Atlas diesel engine powering our tug, but it was a 6 cyl in line, 10' tall and 20 feet long monster....you could crawl down into the jugs. The torque this sucker could put out had to be staggering.

Like 45 said, it swung (?) a 20' diameter wheel with about a 1' pitch at all of about 100 rpm, but you could hook it to half a dozen barges of 60' x 135' with an 8' draft (many tons of weight each) and move them at a respectable 10 mph.....after you got it all going.

So, it's like he said, it's all in the design for the requirements.

So in my analogy, I really think that the 90 was out there to push large loads at a respectable speed based upon criteria developed from 45's equations but I still don't know why it's in the lineup other than to fill it in, maybe to satisfy BIA hp restrictions on certain smaller watercraft and give the owner enough power for that craft to do what he/she wants.

Again thanks for the input and the mental exercise.

Mark