FE last post has a lot of interesting stuff in it and I think Im understanding why he increased the prop pitch. It seemed like the wrong thing to do but that is because I use the outboard way different than FE. I use the outboard to get somewhere reasonably fast. When I get there, I shut the outboard off and sail. Often its somewhat like a ski lift. FE on the other hand motor sails. The outboard is running the whole time he is moving. Since he has a secondary source of power (sail and pedal), he will have a cruising speed (like 9 to 10 mph in that video) that probably does benefit from a higher pitch prop.
I looked at my own case with the Suzuki 2.5 hp which I think is a nice match for how I use the TI.
Just the outboard itself and with just me in the back seat gets the TI up to about 8 mph at full throttle. I have not measured the RPM required for this speed but I am going to guess that its about 5000 RPM based on knowing what 6000 RPM sounds like.
I did calculate the prop slip for the TI at 8 mph and for 5000 RPM (prop pitch is 5.5 and gear ratio is 2.15). FYI, the equations used are from here
http://www.go-fast.com/prop_slip.htmThe slip for the conditions above is about 35% and the prop theoretical “screw” speed is 12.35 mph. This means that at the pitch of the prop and at how fast it is rotating, the prop would screw through the water at 12.25 mph. Since the TI is traveling at 8 mph, this is how the prop generates thrust. Now if there is some other force was propelling the TI (such as the sails), the outboard could rev up to 6000 RPM which has a theoretical screw speed of 14.53 mph.
So my TI would need to be traveling greater than 14.53 mph before the water moving past the prop actually tried to spin it faster than the motor was trying to spin it. Well.. Im not at all worried about 14.52 mph.. pretty sure that will never happen while Im motoring or even motor sailing.
So currently my prop is pitched to give me close to 2.5 hp and I know that much power will drive the boat at 8 mph. In order to go faster than 8 mph, I would need to add additional HP say from the sails. If the TI follows a square law drag vs power, adding an additional 2.5 hp from the sails (for 5 total) gets the boat speed up an additional 2.8 mph for a peak speed of 10.8 mph.
But at 10.8 mph, my 5.5 pitch prop is now just letting the outboard rev limit. So I need to go to a higher pitch prop like FE has done in order for the outboard to still give 2.5 hp at those higher speeds.
There is of course a down side to this higher pitch prop. Say there is no wind or it is fairly light. The outboard is going to drive the boat and create an apparent wind.. and apparent wind can create lift. The problem is that the apparent wind for this condition is going to be almost directly head on. For the sail to create any lift, it must have an angle of attack and in this particular case of directly head on apparent wind, the sail lift force vector is actually pointing BACKWARDS – doing the same thing as drag.
You just cant get power out of sails in no wind and very little power out of sails in low winds. Ice boats are able to work with this as they have very low drag and don’t need much power to work. But the TI does.
Without the sail generating any power (which is the case for no wind or light wind), now that too high of pitch prop basically bogs the outboard down and since it needs RPM to generate HP, you would no longer be able to generate enough HP to get to 8 mph. In the case where I was motoring into a head wind, the too high of pitch prop would likely result in even lower peak speeds. In the no wind conditions, I actaully degraded my speed going to higher pitch.
In my case where I generally don’t motor sail, Im just trying to get some place where I want to sail (without the outboard running) as fast as possible, the Suzuki 2.5 hp with the 5.5 pitch stock prop is about perfect.
If you are often motor sailing and in strong enough winds where the sails are going to generate significant additional HP over the motor, you probably do need to go to a higher pitch and you probably can achieve some fairly high cruising speeds (like 9 to 10 mph, maybe more with two 2.3 hp outboards). But you also have the tradeoff of losing speed over the stock props when there is light wind, no wind or a head wind.