Propeller size suggestion for Arima 21 Hard top with Tohatsu 150

[Camano Joe]

I've got a 21 hard top with a 2017 Tohatsu 150.  I get 5700 RPM trimmed up running a 3 blade 14.25x17 stainless prop.  Max speed is 30 MPH.  It seems a little slow.  I was thinking a 3 blade 15x17 or a 4 blade 14x19.  Any suggestions out there?

[Markshoreline]

Maybe a 14x15?
In my 21 HT with Yami 150 I run a 15 pitch 4 blade prop but only get up to 5200 rpm.

[davidsea]

   Both you guys are overpitched - you should be able to turn at least 6000RPM at WOT with a normal load in the boat.  Don't know about the Yammi, but the Tohatsu is rev-limited by the computer around 6300, so there's no chance of over-reving and hurting the motor. Some folks think that overpropping saves money, since you can cruise at a given speed  with less RPM.  The fact is that running at a given speed takes a certain amount of horsepower - regardless of propeller RPM - and horsepower is directly related to fuel consumption. Running at 25 MPH is going to burn about the same amount of fuel, no matter how fast the engine is turning to reach that speed.
   The real downsides to overpropping are poor acceleration, difficulty staying on plane at lower speeds, and stressing the motor.
Having experimented with 4 props on my 19 Hardtop, I settled on a 4-blade 14.25 X 17, and a 3-blade 14.5 X 17 as a spare. Both are cheap aluminum props, nothing fancy.  I used to always run SS props, but on sub-40 MPH Arimas, I'm not sure they're worth the money.  Plus, I'd much rather shred a $115 aluminum prop than bust a lower unit with a much  stronger $400 stainless prop.
   Granted, my 19 is pretty lightly loaded, but on smooth water and trimmed way out, I can make 38MPH @ 6000 with the 4-blade, and just touch 40 @ 6100 with the 3-blade.  If I were running heavier, I'd want to drop an inch or two on each one.  m2c

[Fisherdv]

   Both you guys are overpitched - you should be able to turn at least 6000RPM at WOT with a normal load in the boat.  Don't know about the Yammi, but the Tohatsu is rev-limited by the computer around 6300, so there's no chance of over-reving and hurting the motor. Some folks think that overpropping saves money, since you can cruise at a given speed  with less RPM.  The fact is that running at a given speed takes a certain amount of horsepower - regardless of propeller RPM - and horsepower is directly related to fuel consumption. Running at 25 MPH is going to burn about the same amount of fuel, no matter how fast the engine is turning to reach that speed.
   The real downsides to overpropping are poor acceleration, difficulty staying on plane at lower speeds, and stressing the motor.
Having experimented with 4 props on my 19 Hardtop, I settled on a 4-blade 14.25 X 17, and a 3-blade 14.5 X 17 as a spare. Both are cheap aluminum props, nothing fancy.  I used to always run SS props, but on sub-40 MPH Arimas, I'm not sure they're worth the money.  Plus, I'd much rather shred a $115 aluminum prop than bust a lower unit with a much  stronger $400 stainless prop.
   Granted, my 19 is pretty lightly loaded, but on smooth water and trimmed way out, I can make 38MPH @ 6000 with the 4-blade, and just touch 40 @ 6100 with the 3-blade.  If I were running heavier, I'd want to drop an inch or two on each one.  m2c

^^^spot on.

Running over propped makes the engine temperature run hotter as well.

[davidsea]

  Forgot to mention,  above numbers are with a BF115 Honda, not a 150.  Same displacement and 95% of the torque of a 150, ,gives up 4-5 MPH on top end, and nothing else........ except $4000 - $5000 in purchase price.