Still having problem understanding pitch

[acdc96]

I've searched on this forum and a little online and still not clear on how the pitch of the prop effect performance. My Nissan 18hp has a 9" pitch prop on it now. The range of pitch goes from 5" to 11.5". I want a little more speed and better low speed steering. It still has the original aluminum prop on it now. I would like to get a stainless steel prop, but might go with aluminum again due to cost. Can anyone explain pitch to me or recommend a prop for my outboard?

 

[Starcraft5834]

ac... pitch= for every revolution of your prop your boat will travel a given distance, increased pitch= more distance travelled per revolution.. every motor has a recommended WOT range.. not sure what your outboard is.. someone else will chime in.. once you reach your WOT range and stay there..it can be assumed your in the right ballpark for prop pitch. Once you get that.. you can play with it some.. ie- going to a 4 blade for better hole shot for heavy boats, folks that put to much blade pitch on. ie- say a 21p prop on a heavy 3.0 engine boat and cant get on plane quickly, or at all, or cant get to recommended WOT rmp's means that to much pitch for that boat and motor.. there's a couple of experts here that will likely help more... that's my understanding of what pitch means....

 

[acdc96]

According to the manual the wot is between 4,750 - 5,500 rpm.

 

[acdc96]

The manual says wot is 4,750 - 5,500 rpm

 

[Sea Rider]

The manual says wot is 4,750 - 5,500 rpm

Have not stated your current boat type, size, load, N° of passengers, max HP transom rated for a proper answer. What are you trying to accomplish. Are there any letters attached next to 18....?
Pitch is the blade angle against water, as when a propeller engine or helicopter rotates its blades to vary wind angle when at take off, cruise, landing

You seem to have the old model, newer same ones runs 5200-5800 wot rpm. Could be tunned depending on current timing settings.

Happy Boating

 

[H20Rat]

I want a little more speed and better low speed steering.

Don't we all! Problem is, those are two different objectives that don't play well. Low speed steering likes a big prop with low pitch. High speed favors a smaller prop/high pitch. Have to pick one, either more speed, or more torque/low speed capability.

 

[steelespike]

We really need your boat brand/size, dry weight typical load and present wot rpm and gps speed.
Low speed steering do you mean maneuvering around the dock?
Imagine the prop pitch as a single speed in your car transmission, lower number pitch is like a lower gear lots of power but not much speed
Higher pitch is like a higher gear more speed but less power on top of that the prop needs to be able to get the motor within its rpm range at wot
lightly loaded. All props have a certain amount of slip in the water so your 9" will actually move forward less than the 9" pitch indicates in one revolution.
Very generally slip will be lower as prop revolutions increase.
Get us your wot rpm and gps speed. An induction tach is reasonable and easy to install. Your gps speed alone will be a little help.

 

[acdc96]

1991 Sea Nymph 14K 14ft Dry weight: about 300lbs Max Hp: 18hp GPS top speed: 25-27mph Cruising speed: 20mph Number of people: 1 (210lbs) Motor weight: 81lbs Gear weight: around 150lbs (battery, gas tank, fishing gear, safety equipment, etc.) Tachometer: unknown (don't have one) Engine: 1988 Nissan 18hp NS18C2 I would like more speed. I would like to top out at 30mph.

 

[steelespike]

First of all you can't just pick a number and make the boat do it.
Speaking in generalities a 18 hp is good for about 27 stock on a reasonably light,fast boat.
It's nearly impossible to evaluate your setup without a tach.
Guessing 5% slip (usually would be higher) 25 mph 9" prop,2.15 gear ratio I come up with 6,639 rpm
Slip is usually higher 13% that would make the rpm even higher.Guessing rpm is like russian roulette with the chamber full.
Its hard to argue with a gps speed but I think its nearly impossible to get that kind of speed/rpm with the weight your moving.
If that rpm is accurate then there may be room for more speed.
​ For the health of the motor you need to get an induction tach.Reasonable,easy to install.
Max rpm is 6100.

 

[pecheux]

Here is a sample of pitch vs performance on my own boats. My present boat is similar to yours (avatar) with a 15 hp low performance OB. Prop pitch is 6 and alone in the boat I get approx 19 mph top speed anywhere around 5500 rpm

Bought a spare prop 8 pitch and tried it. Result = The boat simply crawls, and motor will not reach it's normal rpm, and top speed is slower. So you see more pitch on an 18 hp may not automaticly give you more top speed.

On the other hand my ex 20 footer boat which was light for it's size, with a 115 hp motor equipped with a 19 inches prop gained way more top speed when I changed the prop to a 21 inches pitch which lowered the rpm from 6200 to an acceptable 5600. simply because the motor had the power to push the boat with either prop. I dont think there is any mathematical formula for proper prop pitch and stellespike gave you the right guidance when he wrote "russian roulette".

Good luck

 

[jestor68]

A Solas Saturn (SS) 9-1/4 X 11 should work well to get your rpm down closer to the 5500 limit and give you a few more mph.

It's only $120. It'll out perform the aluminum 11.5 pitch.

His WOT rpm is 4750-5500. Go to the Nissan site and look it up.

 

[Frank Acampora]

Pitch is very much like gears on an automobile: low pitch is like low gear. You start the car in low gear and when the engine reached its maximum RPM, you shift to a higher gear. Except that with a boat, you don't have another pitch to shift up to. Simply speaking, for a given nominal diameter, pitch will set the maximum RPM of the engine at wide open throttle. --given that the engine has enough power to swing that particular prop.

Now to make things a bit more complicated, different brands of props in the same diameter and pitch will have a different "slip" ratio and will perform differently. Thus they can not be compared by the numbers alone.

 

[Sea Rider]

Just a simple question about pitch, rpm, top speed.

Engine data :

2 stroke 18 HP engine
Prop pitch 9"
Max wot rpm 5800
Max wot achieved 5790
Top speed 22 MPH
Very lightly loaded boat
Ideal set up and water cond

If going for a 10 pitch with same diam as current 9? Is it possible to gain more MPH with less rpm and more pitch ? As understood dropping rpm down will drop max speed accordingly. Is that so ? Don't plan buying a 10 pitch to check that out, that's why asking, pretty happy with top rpm and nice speed with my standard factory deklivered prop/pitch.

Happy Boating

 

[steelespike]

In your case assuming a 2.15 gear ratio and gps speed it appears slip is about 4% Possible but not likely.
I wonder if the tach may be off a little low.
Being essentially at max rpm a simple pitch change will probably net less speed.
If over revving Then an increase in pitch may net more speed.