Four blade prop

[rodbolt17]

like I said.
4 strokes fire every other piston stroke.
two motors.
both develop 115 HP at the rated RPM.

both have roughly the same gear ratio.

with the same size prop which to you think is going to make it to WOT"hole shot" first?

[zenoahphobic]

like I said.
4 strokes fire every other piston stroke.
two motors.
both develop 115 HP at the rated RPM.

both have roughly the same gear ratio.

with the same size prop which to you think is going to make it to WOT"hole shot" first?

Depends entirely on which engine produces the greater torque at the idle to medium rev range.
Traditionally two strokes in whatever are weaker at lower revs because they are more dependent on exhaust tuning that cannot be designed fully broadband so tuned towards max power at maximum revolutions.

Having said that there will be an overwhelming corris saying Fourstroke is inferior in the up and go stakes as far as O/B s are concerned not shared with rest of Fourstroke propelled vehicles. Why that is so I don't know.

The every power pulse as opposed to every second is totally irrelevant. Smaller four strokes used to out number the number of cylinders anyway. The lesser power pulses will have greater combustion pressures to compensate.

[dngharvey]

rod bolt - point? OP is unhappy with 115 2 stroke, I am happy with 150 4 stroke - bet he would be ecstatic with a 150 2 stroke!

Actually, I'm not at all unhappy with the package overall. My question was whether a four blade would help her over the bow wave when loaded heavy.
I've gotten tons of information which is much appreciated. Rodbolt never disappoints with his wit and knowledge and other posters have been very helpful. I'm not alone!
Found a real clean 13 x 15 Solas four blade very reasonable to try out and will post results when there is a chance to try it.
Thank you, everyone.

[99yam40]

[QUOTE=zenoahphobic;116577 Smaller four strokes used to out number the number of cylinders anyway. [/QUOTE]

What does this mean?

[rodbolt17]

what I am saying is that before the advent of single body EFI and variable cam timing the 4 stroke VS two stroke of the same approximate displacement the two stroke would walk rings around the 4 stroke.

the same still holds true in the tiny mite portable stuff.
on identical rigs a a 25 two stroke will hit WOT way before that 25 4 stroke yet both are two cyl carbed engines of a similar displacement.

it comes down to the two stroke has a power stroke EVERY time the piston comes up, the 4 stroke every other.
another way, it takes the 2 stroke 360* of rotation to complete a power stroke for all cylinders regaurdless of how many.
it takes a 4 stroke 720* to do the same thing.

the two stroke generates twice the heat,its why they are way more suspectible to cooling system degradation and lean fuel mixs.

at or near the rated HP rpm both those 25HP motors will burn about the same amount of fuel.

[Zara Spook]

like I said.
4 strokes fire every other piston stroke.
two motors.
both develop 115 HP at the rated RPM.

both have roughly the same gear ratio.

with the same size prop which to you think is going to make it to WOT"hole shot" first?

On that note there is this video:

https://www.youtube.com/watch?v=_-7I00cZUE0

[zenoahphobic]

What does this mean?

To be clearer, when manufactures like Honda began to make 4st O/Bs they typically had more cylinders than the 2st they replaced. (it was only later that Yamaha introduced multicylinder 2st i.e. 3 cyl).

So I am not sure that Rodbolt's argument about one powerpulse to two, stacks up. Surely the argument is lost (apart from the other reasons I have given) by a simple illustration: when you have a two cylinder 4st compared to a one cylinder 2st; they both have one power pulse on every cycle.

I am not sure about the implied 4 blade argument either!

Rodbolt you state at the same HP at WOT both 2st and 4st use about the same fuel. This is true and fundamental. However you state 4st produce twice the heat - absolute nonsense. One cannot get more heat out of the same amount of fuel; that is also fundamental!

Your meaning with this has nothing to do with why a 4st you claim is: "slower to WOT". Again if one can magically create more heat out of the same amount of fuel then one creates more horsepower. 4st are more efficient in burning fuel and therefore offer greater horsepower for the fuel input. So this is argument to support 4st are more powerful, not so?

Good video, but absolute BS! The thrust is entirely dependent on the propeller used given same weight as claimed. If both props are identical, then I would like to see this demonstration performed again. Oh America the land of dreams!

[99yam40]

To be clearer, when manufactures like Honda began to make 4st O/Bs they typically had more cylinders than the 2st they replaced. (it was only later that Yamaha introduced multicylinder 2st i.e. 3 cyl).

So I am not sure that Rodbolt's argument about one powerpulse to two, stacks up. Surely the argument is lost (apart from the other reasons I have given) by a simple illustration: when you have a two cylinder 4st compared to a one cylinder 2st; they both have one power pulse on every cycle.

I am not sure about the implied 4 blade argument either!

Rodbolt you state at the same HP at WOT both 2st and 4st use about the same fuel. This is true and fundamental. However you state 4st produce twice the heat - absolute nonsense. One cannot get more heat out of the same amount of fuel; that is also fundamental!

I read where he said 2 strokes produced more heat not the 4 strokes.

I follow you where same amount of fuel burned, creating same amount of heat, but maybe heat build up would be more accurate
And I can see where that would be true if the one cylinder fired every cycle where it has less time to cool off. Where the 4 stoke has more time for the heat to be removed.


And I still do not get how you wrote this statement
"Smaller four strokes used to out number the number of cylinders "

How can 4 stroke motors out number the number of cylinders?

[99yam40]

To be clearer, when manufactures like Honda began to make 4st O/Bs they typically had more cylinders than the 2st they replaced. (it was only later that Yamaha introduced multicylinder 2st i.e. 3 cyl).


Rodbolt you state at the same HP at WOT both 2st and 4st use about the same fuel. This is true and fundamental. However you state 4st produce twice the heat - absolute nonsense. One cannot get more heat out of the same amount of fuel; that is also fundamental!

I read where he said 2 strokes produced more heat not the 4 strokes.
And I can see where that could be true if the one
cylinder fired every cycle where it has less time to cool off. Where the 4 stoke has more time for the heat to be removed.
maybe not heat generated but heat build up.

And I still do not get how you wrote this statment
"Smaller four strokes used to out number the number of cylinders "

How can 4 stroke motors out number the number of cylinders?

[zenoahphobic]

I read where he said 2 strokes produced more heat not the 4 strokes.
And I can see where that could be true if the one
cylinder fired every cycle where it has less time to cool off. Where the 4 stoke has more time for the heat to be removed.
maybe not heat generated but heat build up.

And I still do not get how you wrote this statment
"Smaller four strokes used to out number the number of cylinders "

How can 4 stroke motors out number the number of cylinders?

Sorry 99Yam40 my error, just replace in my sentence 4st with 2st; the argument remains the same.

I can agree in part with your argument about heat dissipation in 4st. This is largely due to greater displacement (larger cylinders) offering greater surface area for heat transfer (all around cylinders including exhaust port), and to a small extent the more efficient purging of the exhaust gases (the non power producing stroke).

However the inherent inefficient burning of fuel in 2st aided greater cooling ability - the unburnt fuel takes away a substantial of heat. It is fundamentally untrue to associate leaning mixture problems having any relationship with a 2st producing a burn every cycle. This destructive heat problem is due to the difficulty in con*****ing fuel mixture, being a lesser problem today with engine management. Lean mixtures "take out" just as many 4st's today.

I am not sure what you are getting at in my "multi" cylinder argument.
If you go back over a century the first 4st engines were single cylinders; very large cylinders. The initial problem with them having a non powering stroke was that there was not enough energy left to make the next compression stroke; but this was easily fixed by adding inertia to the crankshaft. Indeed very large flywheels were added. A two stroke needs less of a flywheel.

This difference was the major factor why 4st are heavier than 2st. To get this weight down 4st began to appear with more than one cylinder! It is the flywheel (sum of all rotational mass) that determines the torque of an engine low down, but it also determines the rate at which an engine can accelerate to WOT (a lot of the engines output gets stored as energy in the flywheel). So maybe Robolt's argument about slower 4st has it's roots here??

[rodbolt17]

I said two stoke make MORE heat.

its cause they fire EVEREY time the piston is at or near TDC.

4 strokes every OTHER time that piston is at or near TDC.
just how it works.

on an F250 V6 it takes 720* of crankshaft rotation to fire all 6 cylinders.

on an sx 225 it takes 360* to do the same thing.

on the 4 stroke exhaust cycle the piston rings and cooling system have twice as long to dissipate the heat.

that is simple physics.

[99yam40]

I am not sure what you are getting at in my "multi" cylinder argument.

Ok, read this slowly and see if it make any sense

four strokes used to out number the number of cylinders

[zenoahphobic]

Ok, read this slowly and see if it make any sense

four strokes used to out number the number of cylinders

Oh no you're picking on my Engrish. I write very truncated, if I were to write much more precisely, there will be many more words and explanations, qualifications etc.

I'm not as brief as some.

Perhaps that sentence should be read as: "smaller fourstroke engines used to have more cylinders than equivalent two strokes of the same horsepower. (hence outnumbered).

There was a time when (still probably is) say in the 50 to 100 Hp range fourstrokes were all of 4 cylinders compared with many twostrokes of equivalent HP having less cylinders. Count them.

[zenoahphobic]

I said two stoke make MORE heat.

its cause they fire EVEREY time the piston is at or near TDC.

4 strokes every OTHER time that piston is at or near TDC.
just how it works.

on an F250 V6 it takes 720* of crankshaft rotation to fire all 6 cylinders.

on an sx 225 it takes 360* to do the same thing.

on the 4 stroke exhaust cycle the piston rings and cooling system have twice as long to dissipate the heat.

that is simple physics.

Yes keep it to simple physics. (you introduced the word physics).

Your first statement is wrong and your supporting argument does not follow!

You also introduced thermal dynamics.

One of the laws of thermal dynamics is that heat travels quicker the hotter it is to the surrounding area (keep it simple). Another law is that by expanding space the temperature drops (principle behind refrigeration).

As I understand the F250 is a 4.2 litre engine and the 225 is a 2.6 litre engine. Clearly the F250 has larger combustion chambers (cylinders).

On the one hand the F250 has to make up it's power over two cycles dealing with that heat over those two cycles. But it also has a far larger combustion chamber, to firstly make more power (more fuel and air into chamber); and secondly has greater wall area (if you like) to dissipate this greater fuel air burn heat.

So this might explain your theory, as your half the number of strokes does not.

[99yam40]

Oh no you're picking on my Engrish. I write very truncated, if I were to write much more precisely, there will be many more words and explanations, qualifications etc.

I'm not as brief as some.

Perhaps that sentence should be read as: "smaller fourstroke engines used to have more cylinders than equivalent two strokes of the same horsepower. (hence outnumbered).

There was a time when (still probably is) say in the 50 to 100 Hp range fourstrokes were all of 4 cylinders compared with many twostrokes of equivalent HP having less cylinders. Count them.

Now that make some sense to me