a prop's rake

It may be night time for you but in Us its still early morning.

He did say tonight he would take notes.

But your in the Med at the moment are'nt you.

 

No, I'm in Japan....and after a glass of wine with my meal, i do get a bit confused about time differences..hic!

 

Here's more data...

With the nibral 13 x 13 prop:
rpm - mph (gps)
500 - 4.7
750 - 6.1
1000 - 7.0
1250 - 7.6
1500 - 9.9
1750 - 12.3
2000 - 16.2
2250 - 21.2
2500 - 26.1
2750 - 29.0
2900 - 31.1 (wot)

One thing to note about is that the tachometer is mechanical and bounce 100 to 200 rpm sometimes...

Also found some old data from the brass 12 x 14 prop:
rpm - mph
375 - 3.3
500 - 3.9
800 - 5.5
1000 - 6.2
1500 - 7.8
1750 - 8.2
2000 - 12.6
2250 - 14.4
2500 - 17.3
3000 - 26.3
3200 - 28.6

Another thing I noticed today...
The boat needs to slow down to around 12 or 13 mph for descent steering. (nibral prop)
The brass prop would let me steer at more like 22 to 25mph

 

Looking at some old charts i have of prop data etc, assuming you have around 10% slip, the prop ratios are in the order 12x14 to 12x16, so you're in the right ball park, again assuming certain conditions, such as the rpm being correct, the engine power being correct etc...which it all probably isn't, but the range of 13x13 does seem about right. If it wasn't quite right, you would just change it slightly to suit your exact conditions (since it is bit of a guess anyway), as you have from the 12 x14 to the 13x13, just minor trial and error.

So this comes down to the boat.

the power to weight ratio 'predicates' a speed around 28~29knots, so that's about ok.

So, it must be down to the weight distribution on the boat, since just changing a prop, would not have such an affect, there must be some other effect going on, or something caught up underneath when running.

Can you run the boat with the 13x13 and move as much weight aft as possible run it, even add some small weights aft to get the LCG far aft and then repeat with as much weight fwd as possible?

See what happens. This is most odd, but interesting.

 

Frosty
"...Which is what i said in the first place- post number 2 from just plane experience..."
Didn't say it wasn't, just need more facts to eliminate what it is not.

"..how you work out slip without knowing that?.."

If i may para-phrase...just plane experience.

Without seeing the lines and the fact he has not said a g/box is used (which i asked for, so hence non), and lines of boats like these and the position of the shaft angle and prop from endless previous vessels (and old predicted data) indicates around 10%. (These are from old series data of boats and props at various speeds rnageing from 5 to 80knots).To err on the side of caution 10% value i used, and it seemed to stack up with his figures. That's it..no magic. If you wish to disagree, please say so and why.

 

Gearbox ratio again: 1 to 1

These are the responses that made me give up on trying to get a nibral prop that make my boat preform like the brass one several years ago...

Even the prop repair shop people looked at my funny when I ask them to change my nibral prop's rake forward; instead they convinced me to make my 13 x 13 into a 13 x 11 and add a cup. The 13's only gives me about 1.25" of clearance under the boat.

I can visually see that the blades on the brass prop are raked forward...
Is there a way to measure this?

I've found places to order props with different rakes, but which do I order?
I don't really understand their terminology or what the number mean...

How do I spec out a prop?

 

Sindel

It is not an exact science as such...just lots and lots of old charts showing which is 'best' and then interpolating between them. But other factors come into play as well, hence the iterative nature, as every boat is slightly different. The more boats you do the more of a "feel" one developes for a particular hull or arrangement that is then given as 'experience'. Personally, i wouldn't worry about the rake of the props or cupping..other factors are at work here.

 

Well you tell me how you work out slip without knowing gearbox ratio, and ille tell you how I knew his problem was weight distribution. Which by the way has not yet been proven before either of us claims success.

"About 10% slip" will just about cover most low speed planing light weight pleasure boats.

 

".."About 10% slip" will just about cover most low speed planing light weight pleasure boats..."
Eaxctly...told you there is no real magic to it. Just confirmation from my old data, that's all.

 

Ok lets look at this again . You say your boat is dangerously high at the front when planing. Why is it dangerous? Have you driven others?

 

Are you guys more interested in each others discussions?
need a ruler?

Anyways, when I put the 12 x 14 on my boat it lays almost level once you get to around 22 mph.

This prop's blades are raked forward to remove water from directly under the boat bringing the nose down...

It has never been necessary (in the past 10 years) to move weights around in this boat to make it ride correctly.

Only when I install either of the two 13" props is there a problem with trim.

Many commercial working boats (ie. tug boats) use a prop with forward rake. The applications are there, I just need to know how to use them...

My brother-in-law has a 17' sportsman with a very similar hull. He runs a 12 x 12 prop. I tried this prop years ago and it did not cause the trim problem just excessive rpms.

When I say the boat rides dangerously nose high, the boat can quickly be turned port or starboard 90 degrees by a strong gust of wind.
The 13" props lift so much of the boat's keel out of the water that the boat rides on only the flat part of the bottom. (aft of trailer tires)

 

I think mr boatpride is just trying to advertise his alternative site Boatpride.com ..hey frosty its like your motors ... crap

 

Well, lets look at this technically then.

What is rake?
When the face profile of a prop is not normal to the axis, this is called rake. It can be raked fwd or aft and the only reason why this is ever usually done is to improve tip clearance, and sometimes aids flow into the prop to provide a more uniform flow, nothing else.

Hence the questions.

 

Sindel ,is that yours or your brothers? the engine is far back is it a V drive gearbox?

Do you have a small fin under the boat. can you take some more pics, one of underneath and one of the engine installation and one of the prop that gives you trouble.

Ide like to see a pic of the boat planing in its "dangerous" situation. but!!!

If your brothers prop works well but gives you too much RPM have that one made with more pitch.

Im running out of ideas!!!!