Prop Pitch Question

[MAKC]

Im helping a friend look for a new prop for his boat and as im searching around, i realize i dont quite understand the relationship of the prop pitch to performance. The prop he's looking for is a 15.5 inch prop with a 13pitch (3 blade). His boat is an older 25 foot cabin cruiser. Would a higher pitch give better top end performance or bottom end performance? Any help would be great. Thanks!

 

[Asureyez]

Im helping a friend look for a new prop for his boat and as im searching around, i realize i dont quite understand the relationship of the prop pitch to performance. The prop he's looking for is a 15.5 inch prop with a 13pitch (3 blade). His boat is an older 25 foot cabin cruiser. Would a higher pitch give better top end performance or bottom end performance? Any help would be great. Thanks!

Mike, at the risk of making your eyes bleed, let's answer your question this way ... by first looking at what a prop is actually is doing.

Your friend is looking for a 15.5 inch diameter wheel ... ie the distance across the outside sweep of the blades as they turn. The pitch is the combined distance of the column of water that is pushed in one revolution of the prop. What is meant is the combined distance of the leading edge of a blade to the trailing edge added together. The three blades have a total of 13 inches or each blade is 13/3 wide.

Hence the col of water being pushed is (15.5 X Pii (3.14159) times 13 inches (635 cubic inches of water per rev.) the prop will push a col of water equal to a cubic foot of water every 2.7 revs. A cubic foot of water (fresh) weighs about 62 lbs. Thus the thrust of this prop is about 23 lbs of water per rev. and is equal to about 3 gallons.

Whats this mean ... if the engine revs at 2000rpms and is gear reduced to 1.5 to 1 then the wheel turns at 1,333 rpms and push a col. of water 1,444 ft long per minute with a thrust weight of 535 cu ft of water times 62 lbs or 33,200 pounds. This leads us to the physics of "every action has an equal and opposite reaction." Throw 33,200 pounds in one direction push 33,200 in the opposite direction (makes rockets go up right).

The match of the horse power of the engines, the gear reduction, the dimension and pitch of the prop and the weight of the boat have to be balanced to utilize as much HP from the engine as possible without over loading the engine.

The wider the prop or the deeper the pitch the more thrust but more strain and less total possible revs. Add weight to the boat and the dynamics change as well.

IN general when the dia is the same, less pitch allows the engine to rev up higher faster and getting the boat up on a plane more quickly, BUT with the loss of top end speed. More pitch will slow the engines spooling up, to power the boat up on a plane, but will give higher top end speed. Increase the diameter ... similar issues have to be calculated and their effects determined.:huh:

The prop he is seeking may have been determined as being the "sweet spot" for his boat, but if he is just guessing he could be making a mistake, a big mistake by either removing resistance at cruise rpms and having to allow the engines to over rev. to ge tto speed, or by adding to pitch causing the engines to work harder to achieve the needed thrust to move the boat as designed.

There is software out there that will design the right prop for his situation.

To find a used prop for the boat you'll need additional facts.

The shaft size, the key size, and the type of shaft end ... ie. tapered, length of taper or straight. These will matter in order to find a correct prop.

 

[Tail Chaser 661]

Adding more pitch will lower rpm giving you more top end speed.

Assuming you have enough power and strong enough drive to push it.

A bigger diameter prop will do the same thing.
1" pitch increase drops rpm by 200 rpm
1" diameter increase drops rpm by 100 rpm

I had not noticed ,
Asureyez had already gave you the answer. :smt021

 

[Lazy Daze]

Read through what Blue Eyes said at least twice. There's a lot of good info there.

Here's my attempt, albeit less scientific. However, I have run quite a few boats in my days and played around with changing props quite a bit, too.

In theory, "pitch" = forward movement. However, there is "slip" as the blades turn through the water so 15" of pitch doesn't exactly mean 15" of forward movement.

The first thing to know is what is the WOT of the engine. Unless you tell me differently, I'll take a stab that it's a V-8. More than likely, that will mean he should be able to get it to 4400 - 4800 rpms. Typically, you'll be looking for this number with 1 person, minimal gear and a half tank of gas. However, for practical purposes, it's best to equip the boat the way he is going to run the boat most of the time.

Once you know his WOT, you can figure out where to go from there. As stated above, keep in mind lowering the pitch will increase RPM's. Plus, if he follows my advice on how to figure out WOT, he'll need to remember that when he is running light (and WOT) he could over rev the engine - just something to be aware of.

So have him give you more specs - year, engine, drive. And also the WOT that he can attain - trimmed out - as fast as he can go.

By the way, somewhere on Mercury's site is a pretty neat prop selector "wizard".