UPDATED: I had a feeling I was over-propped...

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

wouldn't you expect a change from a 23p prop to a 25p prop to drop the RPM by 200-300 RPM and knock top speed down a 2-3 MPH?

I would definitely expect it to lower the RPM, but not necessarily affect the top speed. Top speed depends on how much power you're putting into the water, which is determined by where you're at on the horsepower curve for your engine. If it's a relatively flat horsepower curve (usually true with a stock big block), then the difference of a few hundred RPM will make a minimal difference in power output and little or no difference in speed. 400HP is 400HP, whether you're turning a particular prop at 4500 RPM or a 10% smaller prop at 10% higher RPM.

If you change prop styles or brands you could have even more effects (in either direction) caused by prop configuration/efficiency differences.

Hopefully it'll work out for you, nobody can guarantee it. Never hurts to have a spare prop anyway.

 

[Philster]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Thanks. I can grab a mint 23p Mirage Plus (3 blades of course) off someone locally for 300.

I will admit my ignorance to hubs, so any hub issues between Mirage and Mirage Plus?

45Auto, good points... and very relevant now that I will try a Mirage Plus, which differs from the Mirage in that is has some cupping. I will report back. Might not be this week, but soon.

I generally agree on the point about horsepower. Ideally I'd like more HP to turn my current prop faster = more speed. However, just going by some rules-of-thumb around here tells me I need a little less pitch. Okay, maybe it is not a 'must', but I want to be in the max RPM range at WOT. Plus, I need a spare prop and money tends to burn holes in my pockets.

Thanks.

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

A harsh critic in this thread tells us he never did a WOT run in his very own boat.

LMAO - gotta be talking about me! Cool, I've got a reputation as a critic now! Even a "harsh" one, that's even better!

It's true, I've never held my boat at WOT and let it climb to as high as it could. Never done it to any of my street cars either. I'll try to explain why to you.

Highest RPM I've ever seen on my boat is 4500 RPM, then I backed it back off, called it close enough. It was putting about 405HP into the water (see dyno chart below) at that point and showing 66MPH on the GPS:

Holding it wide open longer could have possibly got it up to 67 MPH or so with the additional 10 or 15 HP available at 5000 RPM, but what's the point? Acceleration is the first derivative (the slope or "steepness") of the power curve. The steeper it is, the faster your boat accelerates. You can see how it's flattening out over 4500 RPM. Since the power output is climbing slowly, the boat is accelerating slowly at that point. It might have stopped at 4550 RPM for all I know and never went any faster than 66. Again, what's the point? You'll have to make your own decision as to what's close enough for you.

If I was going to race it then it would be tuned and propped ($$$$$$) so the HP peak exactly coincided with the peak RPM attained under load to eke every available MPH out of it, just like we discussed on this thread:

http://forums.iboats.com/showthread.php?t=225803&highlight=secrets

But I haven't raced anything in over 20 years, have no intention of racing my boat or any of my vehicles, don't need the bragging rights for speed, must be a zillion bass boats and go-fasts around here that would smoke me without trying anyway. As I said earlier, I seldom even push it above 50MPH. But I do enjoy the acceleration I get from all the available torque, makes the boat fun to drive in the curvy bayous and rivers around here. I just don't feel the need to hold my boat at WOT just to see what it'll do any more than I do my Suburban that tows it.

 

[Philster]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Hey, you are a harsh critic! Which is fine! As long as we get to the numbers/issues at hand.

Listen, take all your shots as I am fine with it (to a point). I stuck with it to digest all the numbers/points/angles. Trust me, if you are casting me as some speed freak, don't do it. I am not the type to lean on it... and lean on it... and lean on it. But, I do know that a WOT run is in order, to see how fit/health everything is. I just want to land in the max RPM range, take note of the speed and call it a day. I quote my cruising speed more often than anything.

There are sick, insanely fast boats all around me, too. There is some serious coinage here in the NE USA, being that I boat with people from Philly, NYC and Washington D.C. You'd think they were giving away 500+ HP engines with the Bajas, Fountains and other barges around here. They are everywhere. I don't even have an open exhaust, as it goes through two big waterlift mufflers (stock setup). I could rip them out in 15 minutes. I enjoy talking and hearing my radio.

Nice exchanging posts with you. Happy and safe boating.

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Dont see to much bantering in prop forums...... but looks like fun it could be fun...


Lets try this..What type of prop do you have now..Im assuming ss..merc..3 blade.... 4 blade....and your gearing what is it? How heavy and what type of load do you run. What is your elevation and is your tach on within a 100 rpm..

Rather than banter about HP and wot theroy's.....can we go to slip and blade area....thats the final end product of HP. How heavy is the boat and do you have tabs...Will it benefit from stern lift... bow lift or does that even factor in......Lets hope so we can cheat a little maybe..

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

can we go to slip and blade area

It would be nice to go to slip and blade area, however it seems that very few people on this forum understand the difference between slip and efficiency in relation to a propellor. I've used up the thousand-character limit on posts several times trying to explain the difference. Most simplified explanation that I've found that ignores end effects and shows the relationship between slip, thrust, power, area, and efficiency is on Wikipedia, about halfway down the page. It's been my experience that it's not very productive trying to discuss it here. Lots of seat-of-the-pants ancedotes, usually contradictory, but very little understanding of the physics of how a propellor works. Hard to get far without some knowledge of fluids and math to explain the basic concepts.

They start with slip, go into lift and drag (which are affected by blade area), then show how thrust is determined, and from that efficiency.

http://en.wikipedia.org/wiki/Propeller

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Its much eaiser to just get that boat hull on top of the water with tabs...probably a strongly cupped 4 blade stern lifter and she how fast she will bounce.. bounce.... bounce..I once asked Hwsiii to calculte how much lift a boat hull would produce..He passed..

You seem to have a very good grasp of the process 45...Think you could calculate how much lift is generated by a hull catching air.... say 22' of hull with a beam of 7' @ 55mph...Hmm wing effect in short


Opps to the thread starter..What type of motor does that boat have

 

[Philster]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Dont see to much bantering in prop forums...... but looks like fun it could be fun...


Lets try this..What type of prop do you have now..Im assuming ss..merc..3 blade.... 4 blade....and your gearing what is it? How heavy and what type of load do you run. What is your elevation and is your tach on within a 100 rpm..

Rather than banter about HP and wot theroy's.....can we go to slip and blade area....thats the final end product of HP. How heavy is the boat and do you have tabs...Will it benefit from stern lift... bow lift or does that even factor in......Lets hope so we can cheat a little maybe..

See post 18 insert. Do the math and get back to me. If you are serious (or nuts). BTW, it's now running a 25p SS 3-blade Mirage. That is the only difference between the bone stock factory boat and mine. Factory-equipped it was a 23p SS Mirage.

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

You seem to have a very good grasp of the process 45...Think you could calculate how much lift is generated by a hull catching air.... say 22' of hull with a beam of 7' @ 55mph...Hmm wing effect in short

Easy one there. How much does the hull weigh?

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Easy one there. How much does the hull weigh?

5000lbs in this case....

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

See post 18 insert. Do the math and get back to me. If you are serious (or nuts). BTW, it's now running a 25p SS 3-blade Mirage. That is the only difference between the bone stock factory boat and mine. Factory-equipped it was a 23p SS Mirage.

Not really crazy.......lets just say to much talk by some very bright people...how about the gearing and motors..Lots of options there...I take it does have tabs..

Post 22 seems to indicate a 502 with a bravo drive ok lets assume and correct me if i am wrong here thats 1.36 gearing

That 21% slip with the 23 and and 25%@ with the 25...Nothing to shocking there ...But 25 pitch prop thero speed at 4400 is 77 and at 4800 it is 84...seems rpm is a big deal

You need blade area dude 21% slip is a bit much....Heres a ideal
Turbo 2+2 TH Series

24 pitch..300 scoots..have it labbed and call it a day... couple hun for the labbing

 

[Philster]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

It's 1.5 gearing.

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Think you could calculate how much lift is generated by a hull catching air

Easy one there. How much does the hull weigh?

5000lbs in this case....

Then at steady state conditions (neither rising or falling) it's generating exactly 5000 pounds of lift.

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

You can always go to the Mercury prop slip calculator and fool around with all kinds of numbers if that amuses you:

http://www.mercuryracing.com/propellers/propslipcalculator.php

My boat (Bravo 3 drive) shows negative slip numbers under some conditions, which is impossible. Just means that the effective angle of attack is less than the apparent angle of attack.

Slip is nothing more than the necessary angle of attack of the prop blades. Some props have more, some less. Means very little by itself, though some people treat it as gospel. Make sure you read the disclaimer right beside the nice picture of the boat on the Prop Slip Calculator:

Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust.

The Mercury propellor fundamentals site also has a basic description of slip and angle of attack about 3/4 of the way down the page:

http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

It's 1.5 gearing.

That put your slip @ 16% on the 25...much more reasonalbe... the 23 is at 13%. Yes you are over propped.. By going to the that turbo 4 blade labbed your rpm should stay very close to your target of 4800 aka the 23 does 4800..24 should do 4600... labbing should increase your rpm by 200-300...netting 4800 to 4900. that should net 65 @ 4800 @11% slip

Some of the unkowns...How much drag will adding the 4th blade have..this is not a generic motor..how much more lift will the turbo provide which decreases the wetted surface area aka drag and using 45 example hull lift can be dramatic on the overall picture..Thats a 25 degree hull it should respond well to lift

Sometimes you take thoughts and run with them...its not linear that i know but there are just to many variables to play with as 45 is clearly demonstrating quite well..

In the end its about slip.. angle of attack (cupping) and hull dynamcis..But you are overpropped @ 25..

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Easy one there. How much does the hull weigh?



Then at steady state conditions (neither rising or falling) it's generating exactly 5000 pounds of lift.

45...bad boy...How much is the lift is the being generated by the hull catching the wind...Umm areodynamic lift..

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

this is not a generic motor

Its a stock 14 year old, 415 HP (when new), 502 cubic inch Mercruiser. About as generic a big block as you can get.

Sometimes you take thoughts and run with them

24 should do 4600... labbing should increase your rpm by 200-300...netting 4800 to 4900. that should net 65 @ 4800 @11% slip

Yeah, but it would be nice to be running in the right direction. Lots of "shoulds" in that statement. I think Philster will be lucky to get a couple of MPH out of the 10-15 HP that he theoretically isn't using yet to get his boat up to the manufacturer's claim of a real 61 MPH. Might see 65 or 70 on the dreamometer though, if that makes him happy.

I say go for it Philster, it's your money we're spending!

 

[45Auto]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

45...bad boy...How much is the lift is the being generated by the hull catching the wind...Umm areodynamic lift

All of it, under the conditions you stated. At steady state it's in equilibrium. Unless the bow is pitching up or down, then the offset between the CP (Center of Pressure) and CG (Center of Gravity) is compensating for the pitching moment due to the thrust offset. If it's not rising or falling then the weight is exactly offset by the lift. If it's rising at 32.2 ft/s^2, then it has 10,000 lb of lift. If it's falling at 32.2 ft/s^2, then it has 0 lift. Easy to interpolate to find any number in between (or outside the 32.2 ft/s^2 if you want to say it's rising or falling at more than 1 G) if you want to make it up .

Lots of other things to consider (speed, angle, drag, etc) if it launched off a wave. It's all easy enough to solve, just takes a lot of money (instrumentation) and time to gather the information. Very few pleasure boaters (actually none that I know of) have ever had the inclination to go through the hassle. Easiest (and cheapest) thing to do is just spend $500 or $600 on props and hope they do what you think they will - IE, go 65 MPH on a 14 year old 26' boat. If it only goes 61, so what, especially if it's not your money!

 

[Philster]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

Didn't we establish that I need a spare prop?

Also, money is no object, so I am not sweating 300 bucks on a Mirage Plus 23p that my boatel neighbor has.

Give me until July 2nd, by the time I get the prop and a chance to change it. My boat is two hours away and I won't see the Formula owner until next week. I won't be 'down the shore' until then (this is a North East thing).

You guys are definitely entertaining.

 

[Tail_Gunner]

Re: I had a feeling I was over-propped...

Re: I had a feeling I was over-propped...

45 when you get going...you are tenacious. Look he was looking for another prop..gong up in pitch and increasing blade is going to increase effiency... labbing is again going to get a increase in the rpm ratio...Assuming is it is done right..A should now...That turbo should lift the boat out of the water..It is a known overall lifting prop...i have a variant of one on a 3500 lbs of boat....The lift is so strong it chines walks a 55 the boat which is a I/O...it is running almost on the cav plate.. Now will it lift that scrab..yes just how much is the question..But certainly more than that 3 blade merc prop.. and less wetted surface generally means speed