Prop size stamp looks X'd out??

[jefndeb]

Boat is out having bottom paint work done, having propglide applied as well.

I noticed that it looks like the original prop size stamp 26x19.5 has been X-d over?

Does this mean that the pitch has been adjusted maybe?

 

[SoWhat]

It has been re-pitched. Looks like 21

 

[High Wire]

Yes exactly. It might be a faint 18.

 

[Comodave]

Usually people take pitch out of a prop because they can not develop full RPMs.

 

[jefndeb]

Ok, thanks, where do you see a 21?

 

[jleonard]

Very faintly stamped below the original number

Put a piece of paper over the whole area and do a pencil rubbing.
Keep it in your log book because you'll forget what it says in a year. LOL

 

[psneeld]

If you care, one school of thought would have you reverse the order of nuts holding the prop on.

 

[jefndeb]

OH ok yes I see it now, anybody recognize what looks to be the manufacturers name there?

 

[rslifkin]

OH ok yes I see it now, anybody recognize what looks to be the manufacturers name there?

Looks like HyTorq to me.

 

[koliver]

When I had mine re-pitched, the old numbers were replaced with new in a way that no trace of the old remained and the new is in the place and font of the original. My log records the original pitch and the date of re-pitching.

 

[skyhawk]

I see the 21, too.... it's pretty clear.... but can't be sure there's not a decimal and trailing numbers that have been sanded off
I have seen areas like that carefully polished to reveal stampings more clearly....

 

[FF]

Most any prop shop will toss it on the pitch blocks for free in hopes of getting some work.

Why do you want to change the pitch? 26 R 13 is how I read it.

I am sure someone has a similar boat and can advise the stock prop dimensions.

 

[Comodave]

Agree, when I took mine in the scanned them for free and then I had them repitched. Had .5” pitch taken off. $800 per prop.

 

[jleonard]

I am sure someone has a similar boat and can advise the stock prop dimensions.

I looked up the data file I had from the old Mainship Yahoo group that I was a moderator on.
One 2006 400 listed his prop as 26 x 19.5 and said it was a 5 blade.
A few others listed their props as 26 x 19.5 and did not specify the number of blades.
They listed their engines as Yanmar 6LYA STP and their WOT varied from 3300 to 3450 (the 3450 was the one who specified 5 blade)

Hope this helps.

 

[koliver]

Agree, when I took mine in the scanned them for free and then I had them repitched. Had .5” pitch taken off. $800 per prop.

In my naïvety, I simply took mine in without asking the price. Lucky for me, the prop shop changed the pitch by 4" on each of 2 props, shined them up, took out edge roughness and charged only an unremarkable low price. At the following year's haulout, knowing the price, I did it again with the other set. I would have been shocked had the charge been anywhere in your price range.

 

[FF]

The usual Rule of Thumb is at 1000shaft RPM the boat will go 1 K for each inch of pitch, , but prop slippage must be deducted.

Its a great rule for noodling while the boat is covered with snow.

A 2-1 gearbox with a 2000 RPM engine would have 1000 Shaft speed, with a18 inch pitch , if the boat goes 9K the slippage 50%.

 

[skyhawk]

1K = 1 knot? 1 kilometer?

 

[FF]

This is a boating board so a K is a nautical mile .

 

[Bacchus]

The usual Rule of Thumb is at 1000shaft RPM the boat will go 1 K for each inch of pitch, , but prop slippage must be deducted.

Its a great rule for noodling while the boat is covered with snow.

A 2-1 gearbox with a 2000 RPM engine would have 1000 Shaft speed, with a18 inch pitch , if the boat goes 9K the slippage 50%.

This is a boating board so a K is a nautical mile .

This isn't exactly my wheelhouse but...
I thought the various prop calculators dealt with speed and not distance?
You are using RPM which is a measure vs time...
Isn't the resultant nautical mi / hr or a knot?
In my circle k = knot, nm = nautical mile, sm = statute miles....
Yes it relates to nautical mi but it is nautical mile / hour?
Am I in a different universe?
View attachment 122513

 

[skyhawk]

...or in the context of this discussion...revs per MINUTE
so K might = NM per minute
obviously not the intent....

I've seen Nautical Mile abbreviated as NM
I've seen Nautical Mile Per Hour abbreviated KT and also KN

regardless....it makes sense to use distance traveled over time to roughly approximate prop pitch...just over 6,076 feet per NM
and that relationship didn't jump out at me right away..... so interesting food for though, FF. Thanks

so in the example

A 2-1 gearbox with a 2000 RPM engine would have 1000 Shaft speed, with a18 inch pitch , if the boat goes 9K the slippage 50%.

running the math to wrap my head around it....
(1,000 revs/minute)(18 inches/rev) = 18,000 inches per minute = 1,500 ft per minute

(1,500 ft/minute)(NM/6076.12 Ft) = 0.2469 NM per minute

(0.2469 NM/Minute)(60 minutes/hour) = 14.812 NM/hour = 14.8 Knots
*considering zero slip of course....
so 14.8 Kt/18inch pitch = 0.822 knots per inch of pitch
so pretty close to one knot per inch of pitch

so a valid and interesting rule of thumb.... although I don't have any feel for how much slippage one typically will get...probably depends greatly on several variables

still...my gut intuition would have lead me to round the speed down for slip....
0.8222 kt per inch might round to say approx 1.2 Knot per inch of pitch... or maybe 3/4 Knot per inch of pitch

 

[koliver]

The usual Rule of Thumb is at 1000shaft RPM the boat will go 1 K for each inch of pitch, , but prop slippage must be deducted.

Its a great rule for noodling while the boat is covered with snow.

A 2-1 gearbox with a 2000 RPM engine would have 1000 Shaft speed, with a18 inch pitch , if the boat goes 9K the slippage 50%.

My BW trans are 2:1. With 17" pitch and 2000 rpm I get 8 knots. Less in snow.

 

[skyhawk]

My BW trans are 2:1. With 17" pitch and 2000 rpm I get 8 knots. Less in snow.

cool...a real world data point...
so I assume that means 1,000RPM at the prop
(1,000rev/min)(17 inch/rev)(1ft/12in)(1NM/6,076.12 ft)(60 min/hour)
= 13.98 knots
8/13.98 = 0.5718
1-0.5718 = 0.42813
so what, about 42% slip?

 

[rslifkin]

As a point of reference on slip, here's what I get for slip on my boat a few different speeds. My engines top out around 4200 RPM, 2.57:1 reductions, 22x25 props (twin engine).

• Idle (750 engine RPM / 292 shaft RPM): ~4.3 kts / ~28% slip
• Slow cruise (1300 engine RPM / 506 shaft RPM): 6.4 - 7 kts, 39 - 33% slip (faster speed / lower slip with lighter fuel / water / stuff load)
• Fast cruise (3300 engine RPM / 1284 shaft RPM): ~17 kts, ~36% slip
• Max continuous (3400 engine RPM / 1323 shaft RPM): ~18.5 kts, ~32% slip
• WOT (4200 engine RPM / 1634 shaft RPM): ~25 kts, ~26% slip

In my case, I think a little more prop blade area would help at fast cruise. That, or the boat is really a little underpowered, as there's a noticeable drop in slip (indicating an efficiency gain) by pushing that last bit to max continuous.

 

[ProMaritime]

Yes, typically that is what the X means.