Ventilation? With picture....

[lakelover]

I had my maiden voyage in my project boat in November, and I had a little concern that at WOT, it was topping out at about 6200 rpm instead of the recommended 45-5500. There is some discussion on the rpm's in my thread, starting at post 1147 on p. 46:

http://forums.iboats.com/showthread.php?t=309879&page=46

After I got her back on the trailer, I noticed that there was some paint gone off the prop that had been there when I launched. Since I had lightly scraped bottom with the prop for a couple seconds when I returned to the launch, I just assumed that is what took the paint off.

In thinking about it more recently, I wonder if it was that, or caused by some ventilation? Looking at the pattern of the missing paint, I'd think it would be scraped off more on the tips of the blades than where it is.

Any thoughts? Here's the photo:

 

[Bondo]

Re: Ventilation? With picture....

I had my maiden voyage in my project boat in November, and I had a little concern that at WOT, it was topping out at about 6200 rpm instead of the recommended 45-5500. There is some discussion on the rpm's in my thread, starting at post 1147 on p. 46:

http://forums.iboats.com/showthread.php?t=309879&page=46

After I got her back on the trailer, I noticed that there was some paint gone off the prop that had been there when I launched. Since I had lightly scraped bottom with the prop for a couple seconds when I returned to the launch, I just assumed that is what took the paint off.

In thinking about it more recently, I wonder if it was that, or caused by some ventilation? Looking at the pattern of the missing paint, I'd think it would be scraped off more on the tips of the blades than where it is.

Any thoughts? Here's the photo:

Ayuh,... That's dredging damage...

Ventilation don't pop the paint, cavitation does, but it don't look like that...

 

[lakelover]

Re: Ventilation? With picture....

Thanks, Bondo...I should know this, but in a nutshell, what's the difference between ventilation & cavitation?

 

[UncleWillie]

Re: Ventilation? With picture....

Ventilation is the sucking in of air from the suface; the reason for Anti-Ventilation Plate above the prop.
Much like how the drain in the sink will suck air.

Cavitation is the water changing to vapor, Boiling due to the low pressure from over driving the prop.
The "Steam" bubble collapses catastophically.
The effect is like micro detonations with a lot of energy focused in an extremely tiny spot.
It will remove paint and the metal of the prop if allowed to continue.

Cavitation occures on the low pressure (Front) side of the prop.
The paint would be missing on the front of the prop.

 

[robert graham]

Re: Ventilation? With picture....

Ventilation is the sucking in of air from the suface; the reason for Anti-Ventilation Plate above the prop.
Much like how the drain in the sink will suck air.

Cavitation is the water changing to vapor, Boiling due to the low pressure from over driving the prop.
The "Steam" bubble collapses catastophically.
The effect is like micro detonations with a lot of energy focused in an extremely tiny spot.
It will remove paint and the metal of the prop if allowed to continue.

Cavitation occures on the low pressure (Front) side of the prop.
The paint would be missing on the front of the prop.

Thanks for a darn good explanation!....Wonder how fast/RPM's prop has to be spinning to cause cavitation?

 

[lakelover]

Re: Ventilation? With picture....

UncleWillie, great explanation!! Thanks.

 

[steelespike]

Re: Ventilation? With picture....

Cavitation can happen on almost any prop and if you look on the back side of any prop you may see some evidence of cavitation.
It can be an indication that the prop is very efficient especially if it appears in roughly the same area on each blade.
If you tell us your motor,prop size and wot rpm and speed we can tell if your setup is working.

 

[lakelover]

Re: Ventilation? With picture....

If you tell us your motor,prop size and wot rpm and speed we can tell if your setup is working.

50hp

12.5 x 15

6200 rpm @ wot

However, at this point, speed is unknown, estimate with 2 people between 35-40(guess).

(Estimated gross wt. with batt., fuel, equip and one person = 875 lb.)

 

[steelespike]

Re: Ventilation? With picture....

Prop calculator says with 7% slip the 15" prop does 33.8 Slip is a guess.
If you can get us the speed by a gps we can tell what the slip is better.
Food for thought a 17" prop would put rpm at about 5800 and speed at 35.9. with 7%
A 19" prop would put you at about 5400 speed at 37.3 again assuming 7%
Rpm is an estimate.
Your holeshot is excellent but as we increase pitch holeshot will slow a bit
But as I recall you figure to cruise and fish so no problem.
I have a 88 50 Evinrude, kissin cousin to yours, on a 18ft Crestliner Utility 375 dry weight.I run a 19"
Never clocked it or had a tach but it flies If it makes 6000 rpm at that 7% would be 41.5.
By the way we had a 70 Starcraft NOVA,a sporty little 15 footer With swivel bucket front seats.
with a 87, 50 Evinrude.
Unfortunately never clocked it but it would scoot pretty good with one or 2 aboard.
We had it 26 years.
What a great job on your boat I checked the project out.
I'm in upstate NY as well.Where do you boat?

 

[Texasmark]

Re: Ventilation? With picture....

I don't know about your 2 cylinder OMC, but 6200 on a Merc is a walk in the park, regardless of the fact that I have not found a stock Merc data sheet showing the recommended max operating rpm to be more than 5500. So forget how high the rpm's are as far as I am concerned.

 

[lakelover]

Re: Ventilation? With picture....

I have a 88 50 Evinrude, kissin cousin to yours, on a 18ft Crestliner Utility 375 dry weight.I run a 19"
Never clocked it or had a tach but it flies If it makes 6000 rpm at that 7% would be 41.5.

The weight I gave was a combination of everything, hull, motor, persons, equipmet etc. The hull weight alone is listed as 415#. Not sure what number you were looking for. When I get back out, I'll have to borrow a GPS & clock it.

That Nova sounds like a winner, Gibbles just found one of those.

 

[lakelover]

Re: Ventilation? With picture....

I don't know about your 2 cylinder OMC, but 6200 on a Merc is a walk in the park, regardless of the fact that I have not found a stock Merc data sheet showing the recommended max operating rpm to be more than 5500. So forget how high the rpm's are as far as I am concerned.

It really didn't sound like it was screaming or anythying at 6200, in fact it sounded pretty easy. It was around 58-5900, then trimmed out to around 6200. I just want to make sure all is well & I'm not overtaxing it or anything.

 

[steelespike]

Re: Ventilation? With picture....

50hp

12.5 x 15

6200 rpm @ wot

However, at this point, speed is unknown, estimate with 2 people between 35-40(guess).

(Estimated gross wt. with batt., fuel, equip and one person = 875 lb.)

I guess my gross weight is about 850 lbs including oars,anchor misc. junk and one 220 lb adult.allowing 200 for the motor.
Yours is probably about 220 with power trim.
As far as numbers go What you have given so far plus a gps speed and we can figure your slip.
With the gps speed and slip numbers you can confirm if it all adds up right.
Our weights are so close it would seem you could run a 19" prop as well.
The biggest difference would be with 4 or more people My larger footprint would get up on the water easier.
It used to plane easily with 6 adults and an old Gale 25.
My mothers Nova would struggle slightly with 4 adults and gear when compared to one or two aboard.
The Novas struggle was partly because the passenger space was pushed back due to the front deck like yours.
It was a a deeper V than your boat too.

 

[lakelover]

Re: Ventilation? With picture....

I guess my gross weight is about 850 lbs including oars,anchor misc. junk and one 220 lb adult.allowing 200 for the motor.

Our boats have just about identical specs it looks like. The 50 is 200 lb., and I guesstimated weights on the rest of the gear. Most use will be with 1 or 2 people, once in a while with 4, but that would be for a sightseeing type trip, no great need for speed there. The Jet Star bottom is real flat.