Prop pitch question

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NJ-boatbuilder57
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Joined: Fri Aug 25, 2017 10:59 am
Location: New Jersey

Prop pitch question

Postby NJ-boatbuilder57 » Sun Mar 25, 2018 8:43 pm

I'm finishing up my engine rebuild (it's a frankenstein made from the best parts of a 35HP '57 Golden Javelin & a 35HP '59 Gale. Basically the same engines)

The stock prop for either engine is a 10-1/2" dia x 13" pitch, for which I can't find a new offering. Michigan makes a 10-1/2 x 12 which is close...

I know that a higher pitch will give better top end, and a lower one better hole shots....but I always understood that to mean extreme differences: i.e. 14" pitch for speed, 9" pitch for skiing, etc. But for general, every day use, does the 1" difference between the stock prop and the one I can buy make much difference?

Thanks!

EDIT: I suppose some info about the boat might help, right? It's a 14' skiing / utility....built from Glen L plans. Weight without the engine is about 600 lbs. Plywood hull over frames, slight V design.

Image

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Dave Bernard
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Joined: Wed Mar 11, 2015 12:00 pm

Re: Prop pitch question

Postby Dave Bernard » Sun Mar 25, 2018 8:48 pm

all depends on the boat you use. 1in pitch will change the RPM's by about 200-300 RPM's.

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20mercman
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Location: Kenosha Wisconsin

Re: Prop pitch question

Postby 20mercman » Sun Mar 25, 2018 9:31 pm

I have found that having too much pitch is much harder than too little. Of course, too high RPM can be an issue, but if you are within the RPM range at WOT with the 12 pitch, then you are going to like that prop much more. In my limited experience with the OMC motors, they seem to be able to tolerate load and over pitch more than a Mercury can. If you over pitch a Mercury, it will never give the performance it has the potential to deliver. On our many 20hp Mercury's, the difference between an 11P and a 9 is like night and day if you load it down.

Steve

FrankR
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Location: Florida

Re: Prop pitch question

Postby FrankR » Sun Mar 25, 2018 10:05 pm

The 12" has proven to be the best prop for the average boat the motor is likely to be used on. At least, that has been my experience.

outbdnut2
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Re: Prop pitch question

Postby outbdnut2 » Sun Mar 25, 2018 10:42 pm

I have several 35HP OMCs and run 13 inch porps on my 35's with a couple of 14' aluminum boats weighing approx 230 pounds (without the motor), so I think the 12 inch will be ideal for your 600 pound boat. Your Glen-L boat looks great! Nice job!
Dave

Here is my 1959 35 HP Lark on my 1957 Crestliner (13 inch prop):


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Jerry Ahrens
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Location: Lake of the Ozarks Mo.

Re: Prop pitch question

Postby Jerry Ahrens » Mon Mar 26, 2018 7:07 am

I also think the 12 pitch will be the best for your boat. I run a brass 13 pitch Michigan on my 14 ft. Alumacraft (aluminum} runabout, with a 35 Gale. It's much lighter than your rig. A 12 pitch is to light for my boat, unless it has 4 adult passengers. I would start with the 12p prop, and check the WOT RPMs with a tachometer.. then you will know for sure. And yes, a one inch pitch change can be the difference of night and day on most boats. Lugging the engine from to much pitch, will cause a drastic increase in fuel consumption, and is generally hard on the engine. To low of pitch, may result in over revving the engine, and cause poor performance.
Jerry Ahrens AOMCI Member MAPS Chapter President

fleetwin
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Location: Rhode Island

Re: Prop pitch question

Postby fleetwin » Mon Mar 26, 2018 8:38 am

The issue that gets over looked here is that the outboard does not have unlimited power to swing a larger pitched prop to achieve those higher boat speeds everyone speaks of. The easiest way to relate this is to think of how a manual shift car performs. What would happen if your Toyota corolla was in fourth/fifth gear while coasting at 25mph, then the driver stomped on the gas to pass another vehicle? Yes, the car would stumble, vibrate, ping and knock while hardly accelerating at all. The other thing to keep in mind is that cars have the ability to coast, rolling tire resistance is a whole lot less that water resistance on a boat while running at high speeds. In other words, boats slow down pretty quickly when engine RPM is backed off quickly, almost like someone had applied the brakes. Cars can coast along nicely on flat surfaces when your foot comes off the gas.
So, it is not uncommon, for an outboard to actually perform a little better (high RPM and boat speed), on a lower pitched prop, especially if it was over propped to begin with. The outboard does not have the luxury of "shifting gears" to help develop power either. There are many cases where a particular rig might need two different pitch props to perform nicely for all boating activities (cruising with light loads vs skiing with heavier loads).
The outboard trim can be adjusted as well, but it is tough to switch trim pin positions while underway.
So, your best tools here are a good shop tachometer (it must be accurate) and a GPS. You will have to adjust your trim pin to a compromising position that allows easy planning and minimal hull drag while at higher speeds. Be sure to have a normal load in the boat, along with normal fuel tanks and gear. You must know the operating RPM range of your engine also. Generally speaking, you will want to prop your rig at least to the middle of the RPM range. In other words, if the RPM operation range on your 35hp is 4000-5000RPM (not sure if this is correct), you will want to prop the engine to at least 4500RPM. Different engines have different power characteristics, some need to be propped right up to the upper limits of their particular RPM range.
This issue became a real issue when outboard companies started selling engines to boat companies so they could "pre rig" boating/engine packages to be sold to dealers/customers. Needless to say, the boat builder wanted to present his rig/package as a great performer, so they are often over propped. Dealers are left holding the bag, and usually are unwilling to water test the rig and supply the correct pitch prop for their customer.

outbdnut2
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Joined: Wed Apr 27, 2016 2:46 pm

Re: Prop pitch question

Postby outbdnut2 » Mon Mar 26, 2018 10:33 am

Good point about getting the trim pin right. As Fleetwin said, try various settings - one step different on the pin makes a big difference - like shifting gears on a car. I've found that on most boats I've run, when pulling a skier, or tuber, dropping the trim pin down one notch from your normal riding setting is necessary for optimum performance, and getting the skiers out of the water easier.
Dave

NJ-boatbuilder57
Posts: 248
Joined: Fri Aug 25, 2017 10:59 am
Location: New Jersey

Re: Prop pitch question

Postby NJ-boatbuilder57 » Mon Mar 26, 2018 4:29 pm

Thanks, guys! Good info all around.

Pardon my ignorance, but it sounds like a tach is pretty important to getting everything dialed in. But as far as I know, my engine has no provisions for a tach. What's a fellow to do?

FrankR
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Joined: Tue Mar 10, 2015 6:32 pm
Location: Florida

Re: Prop pitch question

Postby FrankR » Mon Mar 26, 2018 4:49 pm

Tiny Tach


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