Propeller Stern Lift
Posted: Tue Jan 05, 2021 5:34 am
I found [the forum here at CONTINUOUSWAVE] when searching for ways to modify a propeller for less stern lift.
An old topic from 2005 on the old forum had a good discussion about stern limit propeller and bow lifting propellers, and it prompted me to join [this forum] and to share a recent experience with this on my homemade boat I recently completed.
After my first test run of my boat, I found the engine over-revved and the trim angle was not right. My outboard has no adjustment in engine mounting height so spacers above motor board was used as temporary means to raise engine. The manual trim has six holes [ or stops,for adjustment of trim position]. Each postion provides adds 5-degrees [outward and upward] trim.
The boat was plowing the bow when [the trim stop was set at the first hole] along with over revving.
[Changing the trim stop to the next] hole caused the the bow to run very high. and the boat felt very unsteady.
The bottom of [Anti-Ventilation] plate was level with bottom of boat. I felt that [there were a method to get only] 2.5-degree more [outward] trim [from the first stop] then this propeller would trim about right on the boat, and the boat would feel safer to drive and spray less water spray from the bow.
Moving[the engine mounting height] up an inch on the test day did not seem to do anything except create [propeller ventilation when accelerating from a standing start at full throttle].
Instead of the 20-year-old beat-up propeller that came with the engine, I tried a brand new factory-style propeller. I tested again and the boat had the same behaviors.
For a third test propeller, I bought a so-called “stern lift” propeller, fitted it, and went back to the water to test.
To my surprise, the effect of this prop is extreme stern lift or rather extreme bow down, whichever way you look at it.
Where the boat was planing with original propeller on the second trim stop and had the bow pointing too high, now even on the third trim stop with this "stern lift" propeller my bow is lower than with the original prop tucked right in.
[Based on the] angle of thrust you would imagine coming from the propeller, the bow would be expected to be to be pointing vertical--which it does at very slow speed--BUT, nope. The stern lift prop has a trick.
After finding the old site here and the 2005 thread about stern lift propellers I think I have figured out how they work.
A propeller doesn't just push water in a straight line out the back of prop, but rather--if you imagined the propeller as a sphere--the propeller pushes water in all directions from up,down, sideways, and sternwards.
Since propellers slip, that slipped water flies off prop blades at 90-degrees and at full [engine speed], that water has a lot weight flying off sideways in all directions creating higher pressure in those places.
Now since our outboards have a big flat [Anti-Ventilation] plate directly above the propeller, that pressure cannot escape upward and must go sideways or down. That is [the stern-lifting propeller's] trick: stern lifting propeller use the deflection of slipped water centrifugally to create flow of water in all directions except upwards due to the [Anti-ventilation] plate. This extra weight of water slipped lifts stern--somehow.
Whether [the stern lifting propeller is[ causing an extra inch or three-inches of higher pressure water directly at transom by raising water level under rear of boat like a wave due to high pressure zone there which floats transom higher, or whether it is simply lifting the transom because of down thrust of slipped water, I am not sure.
I think bow-lifting propellers slip the water differently by holding it on blades and slipping it off at a less severe angle like 45 rearward or less and this misses the [Anti-Ventialtion] plate, thus preventing the down thrust being dominant and having a very narrow thrust cone which puts the trim back on to the actual trim position of motor rather than [at an] angle of slipped 90-degree water being flung off a stern-lifting prop. This seems logical.
I’ve seen it with my own eyes on the water. The engine is trimmed up but it is plowing bow. Believe in stern-lifting propellers.
Now, do I get the grinder and trim [the engine's Anti-Ventilation] plate to get a bit less stern lift? Haha.
The boat hull weighs around 88-lbs. The engine adds 75-lbs. In addition there were five litres of fuel. My body weight is 121-lbs. The boat under discussion is a light boat with a tendency to be stern-heavy. The boat is 8-ft long and 4-feet wide with a hydroplane hull. [Because of these factors] any amount of stern lift or bow lift from the propeller is quite noticeable.
An old topic from 2005 on the old forum had a good discussion about stern limit propeller and bow lifting propellers, and it prompted me to join [this forum] and to share a recent experience with this on my homemade boat I recently completed.
After my first test run of my boat, I found the engine over-revved and the trim angle was not right. My outboard has no adjustment in engine mounting height so spacers above motor board was used as temporary means to raise engine. The manual trim has six holes [ or stops,for adjustment of trim position]. Each postion provides adds 5-degrees [outward and upward] trim.
The boat was plowing the bow when [the trim stop was set at the first hole] along with over revving.
[Changing the trim stop to the next] hole caused the the bow to run very high. and the boat felt very unsteady.
The bottom of [Anti-Ventilation] plate was level with bottom of boat. I felt that [there were a method to get only] 2.5-degree more [outward] trim [from the first stop] then this propeller would trim about right on the boat, and the boat would feel safer to drive and spray less water spray from the bow.
Moving[the engine mounting height] up an inch on the test day did not seem to do anything except create [propeller ventilation when accelerating from a standing start at full throttle].
Instead of the 20-year-old beat-up propeller that came with the engine, I tried a brand new factory-style propeller. I tested again and the boat had the same behaviors.
For a third test propeller, I bought a so-called “stern lift” propeller, fitted it, and went back to the water to test.
To my surprise, the effect of this prop is extreme stern lift or rather extreme bow down, whichever way you look at it.
Where the boat was planing with original propeller on the second trim stop and had the bow pointing too high, now even on the third trim stop with this "stern lift" propeller my bow is lower than with the original prop tucked right in.
[Based on the] angle of thrust you would imagine coming from the propeller, the bow would be expected to be to be pointing vertical--which it does at very slow speed--BUT, nope. The stern lift prop has a trick.
After finding the old site here and the 2005 thread about stern lift propellers I think I have figured out how they work.
A propeller doesn't just push water in a straight line out the back of prop, but rather--if you imagined the propeller as a sphere--the propeller pushes water in all directions from up,down, sideways, and sternwards.
Since propellers slip, that slipped water flies off prop blades at 90-degrees and at full [engine speed], that water has a lot weight flying off sideways in all directions creating higher pressure in those places.
Now since our outboards have a big flat [Anti-Ventilation] plate directly above the propeller, that pressure cannot escape upward and must go sideways or down. That is [the stern-lifting propeller's] trick: stern lifting propeller use the deflection of slipped water centrifugally to create flow of water in all directions except upwards due to the [Anti-ventilation] plate. This extra weight of water slipped lifts stern--somehow.
Whether [the stern lifting propeller is[ causing an extra inch or three-inches of higher pressure water directly at transom by raising water level under rear of boat like a wave due to high pressure zone there which floats transom higher, or whether it is simply lifting the transom because of down thrust of slipped water, I am not sure.
I think bow-lifting propellers slip the water differently by holding it on blades and slipping it off at a less severe angle like 45 rearward or less and this misses the [Anti-Ventialtion] plate, thus preventing the down thrust being dominant and having a very narrow thrust cone which puts the trim back on to the actual trim position of motor rather than [at an] angle of slipped 90-degree water being flung off a stern-lifting prop. This seems logical.
I’ve seen it with my own eyes on the water. The engine is trimmed up but it is plowing bow. Believe in stern-lifting propellers.
Now, do I get the grinder and trim [the engine's Anti-Ventilation] plate to get a bit less stern lift? Haha.
The boat hull weighs around 88-lbs. The engine adds 75-lbs. In addition there were five litres of fuel. My body weight is 121-lbs. The boat under discussion is a light boat with a tendency to be stern-heavy. The boat is 8-ft long and 4-feet wide with a hydroplane hull. [Because of these factors] any amount of stern lift or bow lift from the propeller is quite noticeable.