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PostPosted: Thu Dec 06, 2007 8:32 pm 
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Location: Thumbpit,Michigan
Steve
No that is not my boat in the archives. I think a displacement speed boat is best if you want to spend a lot of time on the water at the lowest cost. My boat does 7 mph. at 2800 rpm. It burns 3 qts. of diesel per hour. At todays price, that is about $2.70 per hour for fuel to run my boat.

Carl

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PostPosted: Fri Dec 07, 2007 10:59 am 
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Location: Lander Wyoming
Carl, what is your boat? Have you got a picture you can post here?

Steve


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PostPosted: Tue Dec 18, 2007 7:54 pm 
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Location: Coastal Georgia
I still have not solved the prop issues with my boat. I have talked to several other propshops, a prominant Marine Architect, and numerous boat gurus. I cannot figure where I am not getting any more speed, nor better fuel consumption. They all agree with 1 thing: my boat is EXTREMELY lightweight for its size. Then in the next breath they all say my prop is cavitating real bad. Well hells bells, if it is so damned light why is it cavitating so bad? THAT question stumps the experts. Now, I am getting amused at the situation. I have my questions written on a card & I call the different propshops and architects to read my questions. Lo and behold the different answers I get, and these guys are edumacated professionals!
Anyway........I was surfing other forums........found a forum on propellers & stern gear; and I ran across this one:

'A customer came into the shop several weeks ago with a situation similar toy yours. His dock buddy told him to try a certain set of props and they were not performing correctly. I asked what his buddy did for a living...he said that he was a gynecologist. I could only reply that he has seen some things that I have never seen and I have seen some things that he has never seen!'


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PostPosted: Tue Dec 18, 2007 8:30 pm 
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ROTFL!! That's funny.

Ken, I think you're going to be the resident expert on props soon. That's because you're blazing new trails here and obviously, you're going where none of these so-called experts have gone.

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PostPosted: Tue Dec 18, 2007 10:15 pm 
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Location: New Orleans
Prop cavitation and the weight of the boat have nothing in common, and are separate beast all together.

Cavitation is dependent on the prop configuration and rpm of the prop.


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PostPosted: Wed Dec 19, 2007 2:53 am 
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FDMSIV wrote:
Prop cavitation and the weight of the boat have nothing in common, and are separate beast all together.

Cavitation is dependent on the prop configuration and rpm of the prop.


What do you mean by "prop configuration"?


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PostPosted: Wed Dec 19, 2007 6:51 am 
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Location: New Orleans
# of blades, pitch, diameter, skew, etc...

If your prop was cavitating severely there will be little pits on the tip of the prop you were using. If I recall you haven't used 1 prop for an extended time. If you do have the prop you have used the most, check the tip of the blades and see if they are a little rough.


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PostPosted: Wed Dec 19, 2007 8:20 am 
Ken,do you have a deep keel in front of the prop that may be disturbing the flow of clean water to the prop?

Different problem,but when they put my sailboat in the water for the first time(it was the prototype) it wouldn't move.
They pulled it & two weeks later determined that the skeg in front of the prop was too thick & wouldn't allow the water to flow cleanly to the prop.They had to grind the sides of the skeg & taper back to the prop to get it to work.


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PostPosted: Wed Dec 19, 2007 12:59 pm 
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Location: Chelsea, Quebec, Canada
Quote:
I cannot figure why I am not getting any more speed

I feel your pain, Kens ! You've tested many props with very little speed gain . . . You're close to peak RPM so you're not lugging the engine . . . You've consulted with boat gurus to no avail . . . I'm not a gynecologist, but I think you've hit the proverbial brick wall : a ratio change ( :shock: :x :cry: ) may be required if you want more speed.

You've stated that your boat is EXTREMELY lightweight for its size. The book Inboard Motor Installations (I'll drag Glen-L into this :lol: ) recommends deep reduction gears for heavy boats. Your current prop slip calculations indicate that you're in the ballpark, except that you're still shy of peak RPM. If the tip of the blades are not pitted as FDMSIV has suggested, you're on the right track with this combo. As a result, your fuel consumption is high because the engine is spinning at high RPM while your propeller shaft is rotating at 1/2 engine RPM swinging a large diameter, more efficient low speed prop. It's like being stuck in second gear all the time . . .

For more speed in the midrange, the lighter hull could use a lesser ratio which would decrease fuel consumption compared to your 1.9:1 gearing. Another way of putting it is that you'll burn the same amount of fuel, but you'll be going a lot faster doing it. Spinning a 1:25:1 (with the same prop) or even going 1:1 (with a smaller prop) would be feasible. This would increase your speed, but it still won't help your fuel consumption if you keep your foot in it.

The table below suggests that a 1.25:1 gear at cruise speed would yield 30 MPH at 3000 RPM instead of 19 MPH with a 2:1 gear.

Code:
RPM = 3000
Pitch = 15

                 Shaft   Theor  ------------------ Slip ---------------------
   Ratio      RPM   MPH     10%     20%    30%    40%     50%
   ------     ------   ------   -------   ------   -------   -------    -------
    3:1       1000  14.20   12.78   11.36    9.94     8.52     7.10 
    2.5:1    1200  17.05   15.34   13.64   11.93   10.23     8.52 
    2:1       1500  21.31   19.18   17.05   14.91   12.78   10.65 
    1.5:1    2000  28.41   25.57   22.73   19.89   17.05   14.20 
    1.25:1  2400  34.09   30.68   27.27   23.86   20.45   17.05 
    1:1       3000  42.61  38.35    34.09   29.83   25.57   21.31 


I'm sure other aspiring gynecologists in the gallery will chime in with their analysis :lol:
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Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com


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 Post subject:
PostPosted: Wed Dec 19, 2007 2:13 pm 
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Location: New Orleans
Kens,

I don't think your prop is cavitating excessively. Cavitation not only pits your prop but would cause excessive vibration, enough to be noticed.

This may be the stupidest question ever axed, but is there any chance that something is preventing the shaft from rotating faster? During a recent "help a buddy move day" I was elected to drive the rented 26' moving van. I was not told however that the van was limited to 65 mph. Kind of strange considering the interstate speed limit is 70. As the van approached and reached 65, the engine didn't do anything differently (sounds/cutting out), but the damn thing wouldn't go any faster. The only time I really felt anything was after the move during play time with the van. I floored it and when it hit 65 there was a little bit of a jolt, but nothing serious. I am guessing the tranny was governed somehow.

I really don't know much about inboards, but it really sounds like you problem lies between the output shaft of the engine and your prop shaft. Is there anyway you could measure the shaft rpm?


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PostPosted: Wed Dec 19, 2007 2:59 pm 
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Location: Birmingham, AL, USA
FDMSIV,

Could a hydraulic clutch be slipping at about the same torque load on each prop?

Bill

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PostPosted: Wed Dec 19, 2007 4:34 pm 
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Location: Coastal Georgia
Changing gear ratios, I do not believe, will yield any more speed; less if anything. Here's why:
When you go to a shallower gear (1.5:1) in my case, I would turn a smaller prop, I would NOT turn the 15 pitch that I am now. I have run prop program in the past for 1.5:1, and it was something like 13x13, or, 14x12, about that size. Not only would I turn less pitch, less diameter too, so the prop gets more loading, not as efficient.
As a matter of fact, if you run the prop program, you get the fastest speed with the biggest prop= deepest gear ratio. UP to a POINT.
I do not feel as though I am at that point because; I am turning a nearly square prop. If I was turning like a 15x22, or a real high pitch ratio, then definitely go to more shaft speed the get a flatter pitch.
So far I got Professionals, Gurus, & Gynecologists telling me 3 different things wrong with my boat. The Gynecologists do seem to have as good ideas as anybody.
My boat dont run all too bad, it goes and it turns, but hell, it is supposed to do better, by everybodys prop program. And my fuel consumption proves it out.
But why?


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PostPosted: Wed Dec 19, 2007 4:37 pm 
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Location: Coastal Georgia
Bill Edmundson wrote:
FDMSIV,

Could a hydraulic clutch be slipping at about the same torque load on each prop?

Bill


No.
I can run any prop, WOT, turn the helm hard over to the stops, and load the engine excessively, and the tranny hangs in there.


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PostPosted: Wed Dec 19, 2007 4:44 pm 
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Ken,

Just a shot in the dark. :?

Bill

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PostPosted: Wed Dec 19, 2007 5:02 pm 
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Location: Coastal Georgia
Hey, I am open for any and all ideas.
I even went so far as to dump water overboard. Remember that when I built this engine, my raw water pump is by part number for a small block chevy V8; it just happen to fit my harmonic balancer. So I used it. Some of the Gurus indicated that I can get high exhaust back pressure by pushing that much water out the pipe. I am flowing enough water for 300hp through a 140hp engine. I thought this would be sound troubleshooting. So, I put a tee in the line for the (other) bank of cylinders that aint there and ran the excess water overboard.
.........no change..........


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