Prop size gurus, I need some help, 3 blade to 4 blade

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Rat

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Ran the boat today to get a good set of numbers. I made 6 two mile runs (one into the wind one with the wind); each lap's speed was averaged from my GPS log (using the water speedometer instead of the GPS speedo). Here are the results:
Motor specs: 1984 Evinrude, 150 HP, V6, 15 Spline shaft
RPM Range: 4500 - 5500
Propeller specs: 3 blade, 14.5" diameter, 22 pitch, stainless
Weather Conditions: 90 degrees, sunny, wind SSE 10-15 with gusts to 20
Lake Conditions: Three boats on the whole lake, slight chop (<6 inches)

Run One average: 63.7 MPH @ 4900 RPM
Run Two average: 66.8 MPH @ 4900 RPM
Run Three average: 63.8 MPH @ 4900 RPM

Total average: 64.76 MPH @ 4900 RPM

I can stand to go a little higher in my RPM range so...
In looking at the Solas charts I could use the 14 x 21 x 4 prop (aluminum) and be about right?
Solas has a 14 x 22 x 4 in stainless but I think this would keep my RPMs where they are now or maybe even lower them some. Plus, I'm just not ready to plunk down almost $400 for a new stainless prop right now.

will the 14 x 21 x 4 prop keep me in the RPM range I'm in now or do you think it would bring my RPMs up a little? I'm okay with the RPMs staying where they are but I would really like to bump them up a few hundred; but I don't want them to drop below what they are now.

So propeller gurus, is the 14 x 21 x 4 prop a good place for me to start?

FWIW, I usually cruise around 3900-4000 RPM which is real close to 40 MPH.
 
OKay, the speedo is relative anyway; if it's wrong now it'll still be wrong with the new prop. I can run new numbers this weekend with the GPS instead of the water wheel. Regardless, it is still relative...

So what about the 14 x 21 x 4 prop? Good place to start?
 
Your middle set of numbers looks a little funny to me only because speed always changes with rpm. Your boat must be fairly light to achieve those speeds. Regular speedometers are OK but test data should be from a GPS. Make sure your tach is accurate too. Check it with another tach. Pole switch on the back can be set wrong for your engine.

There are a lot of really good prop options for that engine. Solas wouldn't be my first choice.

OMC props are usually the best bet. Trick is to find earlier ones that weren't outsourced. The ones I keep coming back to are the Raker series for fast boats. If four blades are a must, maybe try a Cyclone.

As far as pitch is concerned, you pretty-much know where you stand. Try to find a friendly dealer to borrow from and test. When you settle on a good prop, buy from that dealer. When you return a borrowed prop, make sure it's shiny clean. Tell the dealer exactly how it ran. Dealers can use this information.

If you have to buy and test, then you have to be pretty careful. Stainless props make pretty expensive garage ornaments. Some online forums can be helpful in swapping props. Check out Scream n Fly. Hot boats and props are familiar to most all who frequent there. There is a prop selling forum there too.
 
All else being equal, adding a blade will drop your rpm. I would want to drop more than just one in pitch especially if you want to pick up 600rpm. Make any errors in the less-pitch direction. You can always throttle back a little if your prop is too small. A prop on the small side gives a much better hole-shot. Smaller props are easier to re-sell too, generally.
A prop that is too big does nothing for you. The boat is sluggish out of the hole and the engine is overloaded at full throttle.
 
That is good advise 2fast. Generally when you add a blade you want to drop your pitch by 1". You need to find accurate performance numbers before you start playing with props.
I had a 1995 Stratos with a hull weight of #1300. I had a Johnson 150 pushing that and I could go 62mph @5600 with a 24" Raker prop.
I suspect that your speed and rpm's are off. If I give you 6% prop slip your speed should be 48 with that prop and rpm's.
Generally 4 blade props have a better hole shot and are more efficient at cruising speeds. You will loose top end performance though, because of increased drag.

Answer these questions and maybe we can be of more help.

What style of hull and weight is your boat? How fast does your boat get on plane? What performance issues are you trying to correct?
 
She sits low in the stern so I am looking to get some stern lift and she handles like a wet noodle (slides REALLY bad in corners at speed). The 4 blade prop will take care of both problems; I just want the RPMs to be the same or a bit higher.

She is 17'-9" OAL, 80" beam and 58" chine to chine at the transom; flat bottom. I have no idea on weight, but she is built heavy. She gets on plane fast, maybe 3-4 seconds with considerable (but not crazy) bow rise.

I will be using a local prop shop for the testing; I haven't found one yet but we have several around. I'm sure I will be limited to the brand they carry.

Here is a shot on the trailer:
IMG_1059.JPG


And here she is in the water:
20130203_101552.jpg
 
Why don't we optimize the initial set up first?
Second........what kind of prop is the 22? Raker by chance? If so you have one of the best performance props out there already.
I would start out by going up on the transom and let that prop work. If it is a Raker (one of the only props OMC made that has even number pitches) then it is designed to be a surfacing propeller anyway.
With that old cross-flow you don't have to go up much in RPM to get back in the safe operating range.
Think you are putting the cart before the horse at this point.
 
Engine is as high up on the transom as it will go and the AV plate is running out of the water while on plane; I think the height is good. Jack plate would be the next step there though...

Prop has this written in it, it looks like someone used an engraver to write it though.
Precision Propeller, Turbo, 14 1/2 x 22. No other markings.

Again, I am looking for more control in cornering and stern lift.

Will raising the engine get me those two things? If so, I will get a jack plate instead of a new prop.
 
I think the cornering problem is a function of the hull, not the prop.
That sure looks like a lot of motor for a flat bottom jon boat. Looks like something I'd do...
 
The jack plate will let the motor run in cleaner water. But is your transom up to it. It put a lot of stress on it. That alone can make the boat run better. If you do get a plate, get a water PSI gauge as well. You do not want to run out of water. There is nothing wrong with that prop you are running now. Yamaha owns them now if I remember right. Turbos make really good 4 blades as well. If it were me, I would get a Mercury prop with the interchangable hub and get a Trophy 4 blade.
 
Assuming your readings were all correct, I'd drop back to a 20" 4 blade SS and try that. I personally like the 4 blade myself. Lets you run the motor shallower, hooks up in a chop better, more stern lift (generally...some 4 blades are build FOR bow lift on a bass boat), holds plane at a lower RPM, gets out of the hole better, smoother running, etc.

The boat sliding, that's a function solely of the hull and not so much the prop or motor mounting. A 4 blade will sometimes help slightly but do not expect it to solve it.
 
if you go from a turbo or raker down to a basic solas, especially an aluminum, i dont think you'll be happy with the performance - with any pitch. i'd drop down to a 20p raker or a 21p viper and call it good. a prop shop can always adjust your pitch if it doesnt get the rpm numbers you want.
 

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