There is a discussion about outboards for the 26S with some reference to high thrust props which is why this post but I think its interesting enough to have a separate thread. I could be wrong on the stuff below... but am sure of it enough to post.. Caution.. somewhat technical so dont bother to read unless you have some time.
Generally high thrust props for these small outboards have a much better reverse than a conventional prop. I personally find the rest of the "story" about these much easier to understand if you look at the equations of power keeping in mind that if you have an outboard rated at 10 hp, you are not going to get any more power out of it than 10 hp but you might get less with the wrong prop.
The straight line equation for power is
Power = force * velocity = thrust * velocity
The rotational equivalent equation for power is
Power = Torque * revolution per second
So the idea for needing the high thrust prop is easy to see from the first equation Power = thrust * velocity. If you are trying to get the maximum power from an outboard and your velocity (speed) is higher, you need less thrust. But to get the same power output at a lower speed, you need to increase the thrust.. So you need a "high thrust" prop on a vessel that only goes slow for that amount of power generated.
Ive used both high thrust and conventional props on my 26S and could not see the benefit of the high thrust. But at some low enough speed, you would according to the simple physics or the equation for power. So where do you draw the line?
Last winter I took some data with a 26S and conventional 3 blade Diameter (D) = 8.5 and Pitch (P) = 8 (on the left in the picture below) and a high thrust 4 blade prop D= 8.7 P=5 shown on the right that actually came with the outboard (9.8 Nissan X long shaft).
The plot below shows the speeds I achieved with my Mac 26S for the two different props vs the RPM the outboard operated at.
In the real world test I did, I actually had higher boat speed for the conventional prop and both props pushed the boat well up into the theoretical hull speed limit. So for this hull shape and a 10 hp outboard, you are really not yet into the lower speeds where you would get a benefit from a high thrust prop.. Except in reverse where you speeds are generally much slower.
But, the hull speed for the 26S is also not much over the limit of where the high thrust prop would have a benefit. You can see this by the RPM where the conventional prop achieved its highest speed. That is about 5000 RPM. Go back to the second equation for rotational power which is Torque * RPM. Almost all of these small outboards get their rated HP at the max specified RPM of 6000 (as many explosions per second as possible at the highest torque the outboard can achieve that rpm). So in this case where the outboard at the torque the prop presented could only get up to 5000 and not 6000 left some hp on the table. Of course since the speed was already way into the theoretical hull speed limit, there would have been almost no speed increase by getting to the slightly higher rpm.
Also interesting is the high thrust prop does get to 6000 but still achieved a lower speed. For these experiments, I had also looked at the electronic ignition timing and the outboard has a rev limiter that starts to really screw up the timing over 6000 RPM. So Im fairly sure the high thrust prop had lower peak speed because the outboard was rev limiting.
So.. high thrust props certainly have a use but for a 9.8 hp outboard on the 26S, the boat speed is still a little too high to need the high thrust prop. As you either drop in hp for this hull or go to a heavier hull, the high thrust prop then may start to have the edge. I personally like the conventional prop better on this boat as it still get to the rated RPM range of 5000 to 6000 rpm but normal operation is at a lower and less noisy RPM than a "higher thrust" prop.
However, considering reverse where speeds are low, the high thrust prop just kicks axx.. plus in reverse they generally allow the exhaust to exit out the back rather than going over the blades like the conventional prop.
But regarding that really great reverse.. be careful. My old Honda had the reverse lock down damaged as the outboards mechanically are much better for handling forward thrust.
Generally high thrust props for these small outboards have a much better reverse than a conventional prop. I personally find the rest of the "story" about these much easier to understand if you look at the equations of power keeping in mind that if you have an outboard rated at 10 hp, you are not going to get any more power out of it than 10 hp but you might get less with the wrong prop.
The straight line equation for power is
Power = force * velocity = thrust * velocity
The rotational equivalent equation for power is
Power = Torque * revolution per second
So the idea for needing the high thrust prop is easy to see from the first equation Power = thrust * velocity. If you are trying to get the maximum power from an outboard and your velocity (speed) is higher, you need less thrust. But to get the same power output at a lower speed, you need to increase the thrust.. So you need a "high thrust" prop on a vessel that only goes slow for that amount of power generated.
Ive used both high thrust and conventional props on my 26S and could not see the benefit of the high thrust. But at some low enough speed, you would according to the simple physics or the equation for power. So where do you draw the line?
Last winter I took some data with a 26S and conventional 3 blade Diameter (D) = 8.5 and Pitch (P) = 8 (on the left in the picture below) and a high thrust 4 blade prop D= 8.7 P=5 shown on the right that actually came with the outboard (9.8 Nissan X long shaft).
The plot below shows the speeds I achieved with my Mac 26S for the two different props vs the RPM the outboard operated at.
In the real world test I did, I actually had higher boat speed for the conventional prop and both props pushed the boat well up into the theoretical hull speed limit. So for this hull shape and a 10 hp outboard, you are really not yet into the lower speeds where you would get a benefit from a high thrust prop.. Except in reverse where you speeds are generally much slower.
But, the hull speed for the 26S is also not much over the limit of where the high thrust prop would have a benefit. You can see this by the RPM where the conventional prop achieved its highest speed. That is about 5000 RPM. Go back to the second equation for rotational power which is Torque * RPM. Almost all of these small outboards get their rated HP at the max specified RPM of 6000 (as many explosions per second as possible at the highest torque the outboard can achieve that rpm). So in this case where the outboard at the torque the prop presented could only get up to 5000 and not 6000 left some hp on the table. Of course since the speed was already way into the theoretical hull speed limit, there would have been almost no speed increase by getting to the slightly higher rpm.
Also interesting is the high thrust prop does get to 6000 but still achieved a lower speed. For these experiments, I had also looked at the electronic ignition timing and the outboard has a rev limiter that starts to really screw up the timing over 6000 RPM. So Im fairly sure the high thrust prop had lower peak speed because the outboard was rev limiting.
So.. high thrust props certainly have a use but for a 9.8 hp outboard on the 26S, the boat speed is still a little too high to need the high thrust prop. As you either drop in hp for this hull or go to a heavier hull, the high thrust prop then may start to have the edge. I personally like the conventional prop better on this boat as it still get to the rated RPM range of 5000 to 6000 rpm but normal operation is at a lower and less noisy RPM than a "higher thrust" prop.
However, considering reverse where speeds are low, the high thrust prop just kicks axx.. plus in reverse they generally allow the exhaust to exit out the back rather than going over the blades like the conventional prop.
But regarding that really great reverse.. be careful. My old Honda had the reverse lock down damaged as the outboards mechanically are much better for handling forward thrust.
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