TOPIC: Need help determining Proper Pitch for prop.....

Boat is 15.5 Arrowglass Tri Hull - fiberglass approx weight is 900lbs (+/-)

Mercury 500 (50hp)....have power trim now....passenger weight? maybe 310 lbs..then also have to add 12 gal of fuel and battery?

current prop is 10.38" -13p...(10 3/4")

Not sure what the rpm range is on full throttle...

Talked to local Marina they thought the 13p was plenty for my set up?
Also when talking it seemed to me they recommended me going down in pitch for more speed? up in pitch for better hole shot?
now I'm a bit confused???
kind of looking for both....with the 13 seems like it just doesn't push the nose down?......if w/ using the trim..

Looking in the manual at differnt recommendations for this size of prop shows 16-17 pitch....

am i crazy for thinking I need to change

I think I found my answer...

Your engine has a recommended wide open throttle rpm range. Let take an example of an 18' boat with a 125 hp engine with a rpm range of 4500-5500. In most cases you purchase a boat with some sort of propeller on it. Let's say this motor has a 21 pitch prop on it and at wide open throttle under a normal boat load, the engine turns 4900 rpms. Well it's in the range so it's the correct prop, right? Well yes and no. For the health and longevity of the engine it's okay, but for overall performance, you want to run a propeller near the top of the range. Well how do you change the size to get that?

Remember this rule of thumb: For every 2" of propeller pitch, rpm's will change approximately 400 rpm's. As you drop in pitch, rpm's increase, and as you go up in pitch, rpm's decrease. So if we want to raise our rpm's we should choose a lower pitch prop. Following the rule, going down to a 19 pitch should raise our rpm's to about 5300. That would help the hole shot and also gain us some more speed.

As you follow the rule of thumb, remember that sometimes rules are meant to be broken. This is not an iron clad rule, sometimes varying 2" of pitch will only change rpm's by 200. Switching prop brands or manufacturer, it can sometimes throw off the rule too.

At this point the most important thing to understand is that less pitch means more rpm's, more pitch means less rpms. For your boat to perform at its peak, you need to run at the upper end of your engine's recommended rpm range. That's it. Once you're there and still want more speed, handling, lift, etc, that's when choosing the right propeller brand comes into play.

Here's a simple adjustment you can make to improvement performance under varying boatloads: If you normally run with one other person on board, use that boatload as your measurement when determining your wide open throttle rpm's. But if you sometimes load the boat with 2-5 more people or try pulling a water skier or tube, get another prop 2" of pitch lower than your normal prop. It will compensate for added weight and drag and make a huge improvement on performance in these circumstances. But remember to keep an eye on you rpm's when running a lower pitch for these circumstances.

Most props have the size marked somewhere on the prop, either on the barrel, on the blades, or in the front or back of the propeller. Use that pitch that you're currently running as your starting point. If it's already running correctly then that pitch is your pitch.

matt wrote:

am i crazy for thinking I need to change

Hard to say for sure without having a tach reading at WOT...but I'd say a 13p prop would be pretty close to where you should be. You'll never know for sure till youve got a tach though...

depends on the year and ratio but i would think 11in or 12in pitch would be better but a tach is the best way to know for sure.

motor is a '76...by my manual guessing it 2:1 ratio....
hopefully I can get out in time and test it out again...

I still think 13P is going to be a good starting point given your HP, the boat and your load. You go to a 16-17P prop and i think your going to be over propped a bit and ultimately will kill your holeshot, might slow you down and will lug your motor which is hard on it. If it was say, a 80 or 90 horse motor, 17P would probably be a good starting point...or it if was a much smaller, lighter boat. Going up in pitch(say from a 15 to a 17) is going to slow your holeshot but give greater top speed...assuming your engine has the power to spin the prop. Works the other way around going down in pitch.

All this in theory though...all bets are off unless you have a tach to properly determine where you are at RPM wise at wide open throttle.

As far as pushing the bow down...trim the motor in more. It will slow you down as your pushing more of the hull into the water but will usually result in a smoother ride. Trim up and get more of the hull up out of the water if you want to go faster. There are props that will lift the bow or the stern but thats another can of worms...

I'm a green horn to all of this..I'm use 2 running a 16' alum fishing boat with a 40hp tiller on the back...when I open it up the bow is up and planes out quickly....
now my tri hull...a lot heavier...when I hit the throttle the bow hardly lifts ..my boat is a shallow hull boat..maybe its just me...will have to get it out for a couple test runs before vacation....will get it out and report back I guess...thanks....glad I didn't go dump some green on a bigger pitch...thanks everyone..