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Old 27-02-2011, 17:26   #1
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Prop Pitch

hello
i have an endeavour 40, with a perkins 4.108, the manual say i need a 17X16 prop. But with these prop, the engine only reach 1800 rpm and the boat speed is 7 knots. My question is does anybody have a similar boat, what is the criuising rpm, what is at this rpm the speed of the boat and what prop do you use.
Thanks
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Old 27-02-2011, 18:48   #2
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Re: prop pitch

16 pitch does seem like a lot, but with 51hp? Have you tried securing the boat to the dock and running it up. The prop will cavitate/vortex, and if the RPM's do come up above 1800 then I would consider the prop as the problem.
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Old 27-02-2011, 19:30   #3
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Re: prop pitch

The Perkins 4.108 is rated at 51 hp at 4,000 rpm. So if you can only get to 1,800 rpm something is very wrong. The most typical reasion is a badly fouled prop.

Also check to see what rpm the engine will run up to in neutral at WOT. It won't hurt the engine as the governor will keep it from overspeeding. You should see 4,200 rpm +/- at wot. If less, maybe the throttle linkage has slipped and isn't opening the throttle fully.

I ran your boat's parameters through boatdiesel's prop calculator and with a two blade prop and an assumed transmission ratio of 3:1 it says a 17x15 prop would work. So you are not hugely overpropped and 1,800 is very, very low at wot.
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Old 27-02-2011, 19:35   #4
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Re: prop pitch

A two or three bladed prop? That's the question.
You can't even begin this discuission without knowing that.
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Old 27-02-2011, 19:48   #5
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Re: prop pitch

We have a Nicholson 38 (it has a short waterline, 27 feet) that has a hull speed of just under 8 knots. It has a 4-108 with a 2:1 reduction transmission.

The nominal operating RPM for the 4-108 is 3200RPM. I also know from other Nic 38 owners that the slip factor is about 50% at higher speeds. When you do the math, the optimal size is 16 x 11 or 12. Interestingly, the area required is quite large and the recommendation is for a 4 blade prop. Won't do that of course because of the cost and drag when sailing.

The fact you cannot get over 1800 RPM implies you are over-propped. Of course, this is not doing your engine any good.

It's not that hard to work through the calculations for prop sizing. Nigel Calder covers this in his Marine Diesel Engines book (there is a typo in the equation for blade area on p230 though - it should be A = 100 SHP / (hs^1.5) where hs is hull speed).

You'll need to know the Shaft HP (SHP) which is the Brake HP (BHP = 50 for the 4-108) minus various amounts depending on what you have connected. Probably safe to assume between 5 and 8 HP are lost to those, making your SHP around 42. If you have a honking big alternatior, take off another HP or three.

So for instance, to calculate theoretical diameter:

D = 632.7*SHP^0.2/ShaftRPM^0.6 (Calder attributes this eqn to Dave Gerr)

SHP is shaft HP, and Shaft RPM = RPM divided by reduction ratio of your transmission. ( y^x means y raised to the x power )
As you can see, this does not depend on the dimensions of the boat, only the engine and the transmission. If your engine SHP is 42HP and your transmission is 2:1, then you need a 16" diameter prop.

To calculate pitch, you need to know what the slip factor for your boat is. But you can just assume a normal amount (see the book), say 45% or 50%. Then, at 8 knots (or your hull speed) at 3200 engine RPM, you'll see the pitch works out to be about 12 inches. There are lots of online pitch calculators at prop shop web sites.

Cheers,

Van
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Old 27-02-2011, 19:57   #6
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Re: prop pitch

Prop performance is a black art. Live with it. Always has been.
If anyone had a fool proof formula there would be no prop problems.
Go to the highest tech, most expensive powerboats. It's a black art.
Look at the unlimited hydroplanes. They do not use formulae, for long.
Live with it.

Call Max Pop in Seattle they will tell you exactly what you need. They do it every day an they don't need no stinkin' formula. It's all about experience.

I need to know the reduction gear and how many blades we are talking about.
Then we can guess. I know that engine pretty well having owned two of them.
My guess will be very close.

Meanwhile I'll start calculating from books written by people who have never in their lives designed a boat. NOT!

Yes, as a matter of fact, I am always like this.
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Old 27-02-2011, 20:05   #7
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Re: prop pitch

We have 43' Gulfstar with a 4-108 in it. We swing a 17x13 4 blade prop. Don't really know what max RPM's are, as we run at 2200 RPM's and make 6.5 kts and burn less than 1 GPH.
Have 2 spares with same dimensions the PO has, a 2 and a 3 blade prop. I'm staying with the 4 blade as I get ood power at low RPM's and don't have over heating problems...
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Old 27-02-2011, 20:28   #8
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Re: prop pitch

My experience has been that the common prop. calculation methods give too much pitch for my boat. 36' WL, 15 tonne.

I've depitched and cut down my prop to 21 x 12 (for 85 hp at 2500) and am still 200 rpm short of getting full power...
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Old 27-02-2011, 20:37   #9
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Re: prop pitch

If your engine cannot reach full RPM and your engine is healthy and your hull and prop are clean, then you are over pitched, over diameter or both.

Some engine manufacturers won't even warrant an engine that cannot reach maximum RPM, like Cummins, because it is so hard on an engine.

It's not okay to lug a Diesel and its not more efficient. Lugging a Diesel will never allow it to run into its higher RPM ranges where you can blow out the accumulated carbon. Lugging a Diesel will also shorten its life. If anything, it is better to be a little under pitched than a little over pitched.

I used the book by Dave Gerr when I re-engined the boat a number of years ago. The horsepower was more than doubled to 630 on a lobster type semi-displacement twin screw hull. I ran the books calculations and nailed the correct size for the boat...a four bladed 24 x 24.

I'm just saying the book worked for me and might be worth the investment if you need a rough idea of the number of blades, pitch and diameter.

Fewer blades are more efficient. More blades are smoother and quieter. That's the compromise there.
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Old 28-02-2011, 08:22   #10
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Re: prop pitch

I think Dave Gerr's book on props is a good one.
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Old 28-02-2011, 09:49   #11
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Re: prop pitch

With a 4-108 and a 2.5 to 1 reduct gear We tried a lot of props on the early Valiant 40's and ended up with :
18 by 12 for a two blade
15 by 10 for a three blade
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Old 28-02-2011, 21:27   #12
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Re: Prop Pitch

On the issue of number of blades, does anyone know if there is published data or any black art opinions on the difference in drag under sail for 3 vs 4 blades, assuming shaft is free to rotate? Should a cruiser care about the difference?

Basically, we think we have more noise than we like with our 3 blade 16 x 12, and the possibility of reducing that by going to a 4 blade prop is tempting. We think cavitation is starting well before we reach cruising RPM - another sign of lack of blade area. Also, Gerr's equations imply we should have a 4 blade prop for the area.

Cheers,

Van
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Old 28-02-2011, 22:31   #13
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Re: Prop Pitch

I've been told, by some experts, that allowing the prop to free wheel actually creates more drag then if locked in.

Quote:
MIT and US Naval Academy studies
which were widely quoted were initially focused on ship propellors with
the idea that an engine could be shut down to save fuel at sea. It was
later expanded to look at smaller propellers. The conclusion was
somewhat missreported. The findings of the study indicated that a prop
with more than two blades that was allowed to freewheel produced less
drag but only if the shaft was allowed to spin with minimal friction.
When friction was added to the shaft, drag increased dramatically and
quickly produced more drag than a prop that was locked. This has
implication for people who are considering a water driven generator that
would be powered off of the prop shaft. Locked three blade props produce
the least drag when placed in the 'Mickey Mouse' position (one blade
down and two up).

Two bladed props were found to produce less drag locked than when
allowed to freewheel and produce the least drag in the vertical
position.

As to your transmission, most small transmissions use a small
'splasher' of some kind to provide lubrication. I am not sure that is
true of small Volvos as they have always marched to their own drummer.
In any event, allowing the prop to free wheel takes a toll on the
cutlass bearing, as well as the engine seals, output bearing and
internal bushings. If you care about transmission longevity then locking
the transmission in reverse makes the most sense.

JeffH 5/4/06
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Old 01-03-2011, 01:18   #14
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Re: Prop Pitch

Quote:
Originally Posted by afmstm View Post
On the issue of number of blades, does anyone know if there is published data or any black art opinions on the difference in drag under sail for 3 vs 4 blades, assuming shaft is free to rotate? Should a cruiser care about the difference?

Basically, we think we have more noise than we like with our 3 blade 16 x 12, and the possibility of reducing that by going to a 4 blade prop is tempting. We think cavitation is starting well before we reach cruising RPM - another sign of lack of blade area. Also, Gerr's equations imply we should have a 4 blade prop for the area.

Cheers,

Van

If you're going to go to four blades, consider a feathering prop with adjustable pitch in forward and reverse. I've bought a VariProp for my new Beta 60 I'm installing this year. It's nominally 19 x 15 and four feathering blades, replacing a Campbell Sailor 18 x 13 fixed three-bladers.

The transmission will be a 1.88:1 ZF25A hydraulic. The purpose is passagemaking.

The crucial word above is nominally. I have quite a bit of leeway in adjusting that prop so that I find the "eco-cruise" sweet spot of about 2,200 RPM/5.4 knots (I estimate), and I can pitch it more severely for reverse stopping power, a concern with a steel boat weighing perhaps 17 tonnes loaded and in which I don't intend to put a bow thruster.

So on a bigger boat, that's an option. There are many decent makers of feathering props; I can't endorse mine...yet...because it isn't yet installed. But I went for one where I can adjust pitches in the water with a snorkel and an Allen key on a lanyard...

Hope this helps. I have a similar question regarding a folding prop I would have to have "custom bent".
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