I understand that the whole debate on 3 blade vs 4 blade, and that with 3 blade better top end and 4 blade better whole shot. I also understand that the rpms of the engine change according to number of blades as well as pitch. I currently have a 2018 tohatsu 50hp. It currently has an 11x15 3 blade prop. What I want to do is get the boat to run a little faster. I have found a 12x19 4 blade prop that will fit the engine. So what i am wondering is if y’all believe that size prop would make a big difference without over stressing the engine.
Do you have a tach? Tohatsu will give an rpm range for your motor a wide open throttle. Find out what that range is. If you have to much prop, pitch and/or blades, you won’t be able to get into the recommended range. To little prop and you’ll over shoot. In my opinion, there is hardly ever a need for 4 blades unless you’re having porpoising or other handling problems. Just go with a three blade and find a pitch that performs well.
Unfortunately, it usually comes.down to just trying a few props.
17’ Henry O Hornet w/ Johnson 88 spl
26’ Palmer Scott project hull
14’ Bentz-Craft w/ Yamaha 25
No unfortunately i do not have a tachometer. I forgot to mention this earlier but the issue i have with finding a 3 blade prop is the highest pitch prop that i can find for the engine is a 10.5x17. So i don’t know if giving up half an inch in diameter to gain 2 inches of pitch will really make that big of a difference.
When dealing with props, there are only general guidelines given all of the variable in design (# blades, blade shape, blade area, diameter, pitch, venting, interaction with hull, mounting height, etc.)
With that said, going from a 11x15 3 blade prop to a 12x19 4 blade will not work. The increase in diameter alone is too much for a 50 HP outboard.
Regarding the 10.5x17, it is quite common to reduce diameter as pitch increases. Both diameter and pitch have an effect on engine loading. The reduction in diameter is to offset the increased pitch to keep low to midrange engine loading (especially during acceleration) at an acceptable level. If the diameter was not reduced the result would be sluggish acceleration and can also limit top end because the engine won’t have enough torque at higher speeds to swing the larger blade area.
Again, there are just general guideline. I recommend calling Skips prop shop local or one of the larger online prop retailers for their advise.
as stated, you have to know your rpms.
with that said, you can figure out a few things.
you are currently turning a 15 pitch, don’t really care about the specifics.
unless you are hitting the rev limiter at 3/4 throttle, you do not have enough horsepower to turn anything larger.
you only have 50hp
if you want to go faster, you have to get lighter or stronger.