Is there a happy Prop medium? specs included.

[rothfm]

Experimenting with props. Spec's below. Original prop runs good. WOT is in range, hole shot good. Speed not bad. I'm looking for a bit more speed so I can back off the throttle for cruising speed(is this possible?). Currently I'm "crusing" at about 5k rpm which seems high. Curious If I can maintain a decent crusing/plane speed and reduce some of the RPM's for better economy If this makes sense?

Ok, I have:

19'6" Sunbird neptune 202 Walk-Around Cuddy cabin appx 2000lbs
1993 115 Johnson (1993) 2-stroke 310lbs
WOT with current setup is 5400 rpm
Hole shot good
Speed "not bad". I dont measure speed overall, just RPM
PROP= 13 3/4" x 15Pitch Michigan Alum.
I typically carry about 20-30 Galls.

The only other I tried was a OMC S.S 13 x19". Man, It ran like a dog on the boat. Thought something was wrong. No Hole-shot, no speed and could only maintain WOT of 4200 top.

I have spares of 13 1/4 x 17" and a 13.375 x 17". Do you guys think the 17 Will be benefitial for what I'm looking for. No Skiiers, just fishing and cruising to fishing spots.

Appreciate the input.

MR

 

[steelespike]

Re: Is there a happy Prob medium? specs included.

Re: Is there a happy Prob medium? specs included.

Without your gps speed we can't evaluate your setup.Some cells have a gps app.
I can tell you that a 17" pitch will knock off about 300-400 rpm. It may slow hole shot might add a little speed.
Your cruise rpm does seem high,another area where your speed would be very helpful.Does it drop off plane if you go lower.
Do you adjust your trim for lower speed?
What is your typical load?

 

[rothfm]

Re: Is there a happy Prob medium? specs included.

Re: Is there a happy Prob medium? specs included.

Guessing my top speed is maybe 30mph, if touch slower. Going from the 15 to 19 pitch, it dropeed 800-1000 rpm for sure. It will stay on plane. Just seems like little speed cruising unless i keep about 4500-5000 on the tach. I trip up, and I do have tabs installed.

I get a lot of spray in the back, some from the tabs, and maybe some from the leg. I'm going to consider carefully coming up a hole on the transome. Its a 25" lower, Maybe too much in the water.

The 13 1/4 x 15 does seem like it runs pretty good. Maybe I'm overthinking it

 

[steelespike]

Re: Is there a happy Prob medium? specs included.

Re: Is there a happy Prob medium? specs included.

Is the boat intended for a 25" leg? If not then your surely dragging too much through the water
You have the 17" props give then a try with the smaller diameter first.

 

[jestor68]

Re: Is there a happy Prop medium? specs included.

Is there a happy medium? Yes; the happy medium would be a 17 pitch.

If you're running 5400 rpm with a 15 pitch, you would run about 5000 rpm with a 17 pitch and pick up a couple mph and better, more economical cruising, since the motor is running less rpm all the time.

Since your Johnson 115 is rated 115 hp @ 5000 rpm, running it at 5000 rpm WOT is where it's likely to produce the best all around performance.

Your boat shows a dry weight of 2700 lbs with a 115. A quick run through the Mercury prop selector gives a 17 pitch.

A reasonable outcome(using the slip calculator) would be 2.0 ratio, 17 pitch, 5000 rpm, equals a theoretical 40 mph, with 10% slip equals 36-37 mph.

 

[rothfm]

Re: Is there a happy Prop medium? specs included.

Just got back to my thread, Thanks for the input Jestor68 and others.

I'm going to try the 17" pitch, Seems it will drop some in rpm but still be in range and maybe give me a bit more speed. Seems I'm always cranking near the max 5400 even when "cruising" as I dont have too much speed.

Will post results.

Thanks again.

 

[rothfm]

Re: Is there a happy Prop medium? specs included.

Prop Update and new question about "Lugging".

Tried the 13 1/4X17", as expected it dropped my rpm down to 4200-440000 wot...Speed was up a bit, to appx 30mph. Hole shot was slow but not real bad. It seemed "sluggish" when applying throttle.

MY NEW QUESTION IS THIS: With the original 13 1/4 X 15" prop....I had good response, hole shot, 5450 RPM and maybe 27 mph. This setup shows the motor is dialed in good. It runs WOT nicely. Would running the 17 with less RPM have any adverse affects. I"ve read this makes the motor LUG? Seems the 17 I can cruise at the same speed but with less RPM and maybe save a tad of gas...But even at 4k rpm its almost at the upper end of its throttle range.

I may just stick with the 17 a bit, less response doesnt bother me, I just dont want to damage the motor in any way if it only produces 4200 WOT and I my cruising speed it close to 4k.

Maybe I need a bigger motor Liked my 88SPL, plenty of power and better on gas!!! Miss it.

 

[Sea Rider]

Re: Is there a happy Prop medium? specs included.

You need firstly to match best leg/transom height and then play with prop pitch or size all you want for your current boating application.

Happy Boating

 

[etracer68]

Re: Is there a happy Prop medium? specs included.

I run an 16ft glass boat, with a 90 johnny, and 12 gal of gas. With two people, and loaded, I carry a lot of stuff, even a 4hp kicker. I run about 36mph (gps) @ 5100 rpm, with a 17 pitch. If I add another 200lb adult,and pull toys, I will put my 15 pitch on, then it runs 35mph@5400rpm. Rips out of the hole, with either prop. Are you sure your speed is gps? I know my boat is dry, you sure your not carrying water logged foam. I would think your 115HP would do a bit better, and handle the 17 pitch.

 

[jestor68]

Re: Is there a happy Prop medium? specs included.

Prop Update and new question about "Lugging".

Tried the 13 1/4X17", as expected it dropped my rpm down to 4200-440000 wot...Speed was up a bit, to appx 30mph. Hole shot was slow but not real bad. It seemed "sluggish" when applying throttle.

MY NEW QUESTION IS THIS: With the original 13 1/4 X 15" prop....I had good response, hole shot, 5450 RPM and maybe 27 mph. This setup shows the motor is dialed in good. It runs WOT nicely. Would running the 17 with less RPM have any adverse affects. I"ve read this makes the motor LUG? Seems the 17 I can cruise at the same speed but with less RPM and maybe save a tad of gas...But even at 4k rpm its almost at the upper end of its throttle range.

I may just stick with the 17 a bit, less response doesnt bother me, I just dont want to damage the motor in any way if it only produces 4200 WOT and I my cruising speed it close to 4k.

Maybe I need a bigger motor Liked my 88SPL, plenty of power and better on gas!!! Miss it.

Are you sure you didn't put another 19 pitch on there? Because that's the same rpm you said you got with a 19 pitch.

You should have gotten 5000-5100 rpm with a 17 pitch. Your boat/motor seems to be defying the laws of physics. You're not supposed to get a 1000+ rpm change with 2 inches of pitch change.

 

[dazk14]

Re: Is there a happy Prop medium? specs included.

His boat uses a 25" shaft.

If he's from Boston and fishing, then he's in the Hahbah(harbor) with salt water (read years of bottom paint).

He's well into the 3k's with a 115, with a lousy bottom, likely some extra water weight, so ~32 is probably a reasonable real world #.

Questions for the OP.

Where do you fish? inner harbor, outer harbor. The reason I ask is it can get darn choppy out there and usually your teeth start rattling much over 25mph. Unless you're an early bird.

How many are normally aboard?

You indicated your tabs are throwing spray. Do you have Bennetts and are they fully retracted? Do you use them for planing or to minimize porpoising? There are some big wakes out there...

Do you have GPS speeds. I know some were mentioned, but not confirmed as GPS and what speed you're looking to cruise at.

Are you using a michigan wheel "Vortex" model?

Jestor is on the money here. Something is off on the numbers and 17 seems darn close.

I think what you are looking for is doable.

 

[rothfm]

Re: Is there a happy Prop medium? specs included.

Thanks,

I run all salt water, outter Boston Islands, Martha's Vinyard, and Block Island sound. Can be tuff water, never flat for sure. It it was flatter, I'm guessing I'd be up 5 more mph. Speeds were from a new Humminbird GPS installed recently. I may drop it in a lake for better testing.

I get a little more rpm with the 17 vs the 19...and a bit more speed. Cant be sure about water in the foam. I've looked toward that close-Except for the circular hatches here and there, I cant see the whole insides but all appears dry and around the fuel tank. Boat sits level also.

I run about 50gals of fuel, 30+lbs Ice, 6gals fresh water, a gazillion rods, 2 anchors, spare prop, coolers, tackle ...you got the point, alot of gear. Sometimes alone, but sometimes 2 to 3 adults on board.

Like the prior poster...Seems in some conditions the 17 might be ok (me alone and less weight), other times when more loaded, the 15 is better. I'm caught in between the two I think.

All in all, either seem ok, but surely not the 19.

Thanks all.

 

[dazk14]

Re: Is there a happy Prop medium? specs included.

A Solas Amita4 13"x15" 4 blade fits the bill. sku 2413-130-15 $111.99 delivered

It'll pull your rpm's down a bit. Lower slip. Improve rough water handling/tracking.
Allow you to plane at a lower speed for when it gets really rough - which is darn near always in the waters your talking about.
Accelerate quicker. Improve fuel usage.
Real world usable speed will improve at lower Rev's. Trimmed out 30 mph would be my estimate, but the wild card is the large load you carry.
Often on those boats with 4 blades, you can trim up a fair amount and use a bit of tabs to settle the bow movement. The added stern lift of the 4 blade along with positive trim, tends to lift the whole boat.

There are other good prop guys tuning in on this and I'm sure they'll weigh in.

 

[rothfm]

Re: Is there a happy Prop medium? specs included.

Thanks Dazk, appreciated..I have bunch of older omc std props i've been messing with. I just may try something more modern and new at some point.

 

[Sea Rider]

Re: Is there a happy Prop medium? specs included.

Rothfm,

You'll be lugging the engine only if running at wot at less than min rpm factory stated parameters for your engine. Weight/prop issues. Best is to run 1/2 to max 3/4 wot parameters range, will have better hole shot with slight less top speed. Can't have best of 2 worlds with just same prop.

Happy Boating.