Glen-L.com

Mr. Hot Rod:
Thats a cool table. Thanks. I can print that out and take it with me on the next run.

Cool.

Ken,



I got home and drank a Beer! You are correct! 1.14 mph minus slip.

So, if you reduce 1" of pitch and add 100 rpm to the shaft there is no speed change.

My pump guy said it's all about the effective angle of attack of the prop the water. When the prop first starts to turn, the angle is the angle 90 degrees from the shaft minus the the blade angle to the shaft. As the boat starts to move the 90 degree starts to reduce. Ideal would be when the difference goes to zero.

According to him, if you go much ether side of zero you get cavitation. Add to the fun the equation it constantly changes with speed, boat weight, ...

In short, Some Wild Ass Guess.

I learning stuff from this that I hope I need. SOON!

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Bill:
The blade angles are also explained similar way in Dave Gerr's book "Propeller Handbook". This book is a must read. Since you are already a engineer, you could breeze thru his graphs & tables, it is already in easy to read format, even for me.

"So, if you reduce 1" of pitch and add 100 rpm to the shaft there is no speed change. " Yeah, that is about exactly what I am getting. So the steeper pitch props are getting higher slip figures, backs up what the above has stated.
Apparently, the only way now to increase speed is to reduce slip.
Acme now is offering me a 14.5x14.25 4 blade and a .80 DAR. I might try that.

Ken,

UNCLE!



I'm learning! What is DAR?

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Bill,
You got to get Gerr's book. It has all the stuff in it.
DAR is disc area ratio, the total area of the blades as compared to a circle's area of same dia.
The higher the DAR the more power it can handle without cavitation.

Ken,

So, you could not see through a 1.0 DAR prop.

It sounds like a prop to try.

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Yeah, I said try it. I can already see differences in low speed handling with these different props I'm trying. So, what handling would the small dia./high DAR prop have? I got a gut feeling that would be the sweet ticket for top end speed, but it may suck for docking. Not that I am paranoid about docks or anything like that.

Ken,

You see 4 blades on competition ski boat and I'll bet wake board boats. So, lots of thrush to diameter.

The low speed handling may improve. The smaller diameter may decrease prop-walk. The lower pitch will lower the speed at the dock (You're in a displacement hull mode.).

Just thinking. I may be full of ....

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Just seeing 4blades on a competition boat don't mean crap. Those are production made boats at the same time. Production dictates cheapest costs to build = smallest shaft/strut/rudder/hardware. Smaller dia can be run on a smaller strut = less cost to produce.
you have to stand back and look at that whole picture to tell.

Kens
Just another thought here. Do you have a cabin on the boat or plan on putting one on it? I am wondering if you are running bow high all of the time without the extra weight forward if you do not have the cabin on.
If you don't have a cabin I was wondering if it would be worth the effort to add weight forward and see if that helps. Gotta be an answer some place and I am sure it is bugging you good.
Jimmy

_________________
Dreams are free, Reality cost money.

Jimmy70:
No plans for a cabin at this time. The bow is not running high. As a matter of fact, bilge water accumulates under the engine when running, so the keel is absolutely level when running. It likes weight in the center of the boat, I've had people walk around while running.

Ken.
My 2 cents'
That engine is normally used in a 16-18 ft runabout. I had one in a 18' searay bowrider. It would turn up 4400-4500 rpm and run about 42 MPH.
I think that you may be getting all that the hull and torque allows, you probably have 3 x the wetted surface of an 18 ft runabout.
Are you using fuel consumption figures for a normal installation ?
I would try the biggest diameter with low pitch on that boat to minimize the load on that little engine.
Hey 28 MPH is doing great with 135 hp in a boat that size.
Good luck, boat looks good
Bob

_________________
I just don't understand, I cut that plank twice and its still too short
http://www.narduccimarine.freehosting.net

I pulled that engine out of a 19' Mercruiser runabout. Yeah, I got a lot of wetted surface as compared for sure. That is why I went with deep gear ratio. Those mercruisers were geared about 1.6 or 1.75:1. I got 2:1 velvetdrive.
Something else I thought about is my raw water pump, it is bigger than I need for the motor. By the part number, this pump can cool a V8. I may lose a couple HP by driving a too-large pump. Probably takes a little more to turn the VelvetDrive compared to the sterndrive's lower unit also.
The fuel consumption I am getting is actual, I have been making runs on my 3 gallon tank to see how far it goes. Right now I am buring 10minutes/gal. Still overpropped too. I going to the propshop in the morning for my weekly tweek. I guess by now that I will end up with a 15x15, or thereabouts..........
Oh, and the motor load on that small motor in a 25' boat. I weighed my boat, 2480lbs with gas onboard in running shape.
The 19 footer I took the motor from.........2000lbs stripped of the motor, outdrive, transom gimbal, gas tank, and floors. My boat is larger, but a whole lot LIGHTER than the hull the motor came out of. I know what the old hull weighed, I paid by the pound to get rid of it.

Let me state this about that. If you increase RPM, then you also increase horsepower, very simply. But, the planing hull speed chart only uses HP/weight ratio to arrive at speed. So my boat weight is constant, and HP is going up with RPM, right? All gas engines get higher HP with higher rpm, very basic and accepted.
So as rpm goes up with reduced pitch, then my speed should go up too, because my HP has gone up.
Right?

I'm not sure if increased rpm ALWAYS mean increased HP. If you look at a dyno chart for a car engine you will notice that peak HP does not always occure at peak RPM and quite often if RPM continues to increase after peak HP has been met the HP levels will decrease until you reach an rpm level that causes valve float.

So in other words increasing RPM does not always increase HP

examples below were created using dyno200 so while not created on a real dyno I believe the point I'm trying to make is still valid which is there is a point of diminishing returns as far as HP are RPM are concerned.


355 SBC


427 SBC


396 SBC


And yes these are all SBC it wasnt a typo