Trying to find the right propeller...

[papagallo]

Hey everyone! First ever post here. I am purchasing a 27' center console from Judge Yachts and I am curious as to how I might find the right propeller. It's obviously a planing hull and the boat is supposed to weigh around 4500 lbs. if that matters. Judge Yachts states that it can take up to 300 hp (Twin 150's). I really don't feel that I need that much power, and I also feel that I could go plenty fast enough with a 200 hp single 4stroke. I plan on also carrying a little 20 hp kicker for a backup motor. But hey, I'm fairly new to boating, and I am relying on past experience with the mexican panguero's who haul butt with just a single 120 hp motor.

Now, all that's left is to find the right prop for the outboard. Most of the time will be spent trolling @ around 8 - 12 knots depending on what type of fishing I will be doing, and I really don't see myself exceeding 30 knots very often at all.
In addition, if the 200 hp outboard is too much feel free to chime in on that as well. I really don't know, maybe a 150 hp outboard will do the trick.

Thanks in advance!

[kjell]

This is the speed you will obtain.
20 HP = 9 Knots.
60 HP = 15 Knots
120 HP = 21 Knots.
200 HP = 27 Knots.
300 HP = 34 Knots.

[marshmat]

A boat's maximum power rating is generally a lot more than it really needs. Putting a smaller, more efficient motor on and sacrificing a bit of speed might be a good idea in your case. A single 200 will probably be more efficient than twin 150s for speeds up to the low- to mid-twenties.
As for props. Do your trolling on the kicker engine, and size its prop accordingly. The prop on the main engine should be optimized for your standard on-plane cruising speed. The boat builder will probably have some advice on where to look for props. A good prop shop will do some calculations for you (based on weight, power, hull geometry, target speed, etc) to come up with a starting point, then let you test a few until you find one that performs well in your conditions. It's impossible to calculate a 'perfect' prop fit without testing a few on the actual boat. (Calculations can get you an approximate diameter and pitch, but what rake, cup, shape, etc depends on the boat and how you use it.)

[tom28571]

Based on your intended use, I would think that a max diameter prop would be best. You can calculate a trial pitch by assuming a propeller slip of about 25 percent. Use the middle of the mfg's top RPM range and the lower unit gear ratio to get the theoretical (zero slip) speed.

Or you can just try different pitches to see which one puts the RPM within the Mfg's WOT range. A too low pitch can damage the motor from overreving and a too high pitch will lug the motor and shorten its life. (Although the too high pitch may actually give more speed)