Propellor loading VS shaft RPM

[parkland]

I have a few propellor questions.
I've been reading this section quite a bit, a lot of discussions are over my head, others end in disagreement.

If I had a 50 ft displacement boat, with dual engines, dual props...
Lets say they are 24" and 24" pitch.
If the transmission is 2:1, at 2000 engine RPM, that would mean 1000 RPM shaft speed, which would net a theoretical 2000 ft per minute, which should be just over 20 mph.

What is a customary or "close" number to use for propellor slip at certain RPM's?
At 1000 RPM shaft / prop speed, would the slip be enough that the boat would still be at displacement speed, or would the pitch need to be reduced so that the ratio is lower?
( I know this boat's displacement speed would be less than 20 mph, but what I'm asking, is how much slip generally occurs with props that size.)

What I want to do, is gear this sucker so that it can cruise at a fuel effecient speed with the engines around 1000 - 1200 RPM, maybe 5-7 kns.

Generally, how much slip would a 24" x 24" prop have being spun at 500 RPM ?

Just from reading etc, it seems like the 2:1 ratio trans would be the way to go.

 

[parkland]

According to a propellor calculator,

2000 engine RPM,
2:1 trans
24" prop pitch
Slip 0.3
15.9 MPH


And then, changing the RPM to 1000 with the same values,
we get 8.0 mph.

So does this plain out mean that a different ratio should be used,
or is that slip calculation way to low?

The engine is rated 185 hp or so at 2700 RPM.
I would like to run it at 1000-1200 for cruising,
1200-1600 ish for hull speed,
and 2000-2700 RPM would be like "emergency speed" lol .

 

[parkland]

And power wise, these dt 360's put down 160-190hp, with 450-550tq depending on year and calibration. I would get the lower HP ones as they don't require an intercooler.
So lets say it's the 160 HP 450 TQ engine.... The TQ peak is around 1600-1800 RPM.

How much ft/lbs of TQ will these props need @ 500 and 1000 RPM shaft speed?
At a 2:1 mechanical advantage, the prop could get about 800-900 ft lbs around 900 RPM shaft speed, at full output power.

I'm hoping that at low speed, this setup would load the engines about 20-30% TQ at 1000-1200 RPM.
In other words, hoping the loading would end up around 90 ft/lbs to the engine, 180 ft/lbs on the shaft, at 500 RPM, once the boat is moving. Does this sound remotely proper?
At that power draw, each engine would be putting out around 20 hp, for a total of 40 hp.

That SEEMS like a reasonable power to move a 50 ft displacement hull somewhere in the 5-7kt range, from what I've read so far.

 

[parkland]

A quote from another site:

"For example, the Nordhavn 40, which has an LWL of 36, according to the builder’s Web site, gets 7.7 nmpg at 6 knots (s/l 1.0) and 2.56 nmpg at 8 knots (s/l 1.34). This makes the trawler three times as efficient at s/l 1.0 than s/l 1.34. That’s a great reason for running this boat at 6 knots. And, by the way, this fuel flow, if accurate, means just 15 hp at the propeller. This Nordhavn can go faster — up to 9 knots — but it’s now running along at a fuel-hogging s/l of 1.5 and 1.29 nmpg using all of its 140 horses. Of course, “fuel hog” is a relative term, considering many 40-foot planing hulls get well less than 1 nmpg at cruise speed. All this just shows the influence on speed/length on efficiency. "

This is my goal, to cruise slow, and get huge economy.

 

[gonzo]

The slip will depend on several factors. For example, what is the prismatic coefficient and displacement of your boat. Hull shape will influence the calculations enormously.

 

[MikeJohns]

Slip will likely be around 40% but the wake factor is very important. The hull drags a load of water along with it so the prop sees a slower incoming stream than the adjacent free water velocity. The wake factor is affected by hull shape and displacement. We can provide a rough estimate from a few basic measurements.

Any real calculation needs the hull resistance curve.

 

[parkland]

The slip will depend on several factors. For example, what is the prismatic coefficient and displacement of your boat. Hull shape will influence the calculations enormously.

I believe OAL is 50, so waterline should be right around 44x14 or so..

 

[parkland]

Slip will likely be around 40% but the wake factor is very important. The hull drags a load of water along with it so the prop sees a slower incoming stream than the adjacent free water velocity. The wake factor is affected by hull shape and displacement. We can provide a rough estimate from a few basic measurements.

Any real calculation needs the hull resistance curve.

Right, but 40% at what speed? It constantly changes with speed and even prop RPM right?
Trial and error would likely be cheaper than any professional analogy.
I'm just trying to get a ball park idea.

 

[MikeJohns]

Yes slip ratio will be from 100% to around 40% for a displacement hull but you design to the prop power curve. That is in turn dictated by the vessel resistance curve.

You may be aware that the pitch power and RPM must foremost be matched to the engine output and rating curve.

Slip is only ever constant in smooth water in equilibrium conditions.

So the interest to you is slip at max RPM max power and max speed in open smooth water. The basic calculations can then follow.

Estimation of wake factor is built into the Moulded depth, beam, lwl, D inputs
Then speed the prop sees is boat velocity minus wake velocity and slip ratio is set simply by ideal prop advance and real advance.

Download the prop calc spreadsheet from the files section on this site. I think there is one there.
Play with gearbox ratio and diameter, speed power etc and it will give you a good enough indication.

 

[JSL]

a pair of 24" x 24" props on a 50' displacement hull (I assume of average displacement)
will have high blade loading and not be very efficient. Spend some time getting the right size and go from there.

 

[gonzo]

On a hull that is very full, the water goes forward at the stern, which changes all the parameters in the calculations. Parkland in not providing enough information for a proper answer.

 

[parkland]

a pair of 24" x 24" props on a 50' displacement hull (I assume of average displacement)
will have high blade loading and not be very efficient. Spend some time getting the right size and go from there.

I calculated that at a shaft speed of 1000 rpm (engine 2000 rpm) the traveled distance of the props would be 20 mph, with 0% slip.
With 40% slip, thats 12 mph. That is pretty close, probably slightly above hull speed for the boat, in fact online calculator says 9.8 mph for the hull in question.

Doesn't this sound close?
Why do you think blade loading would be abnormal?

 

[parkland]

On a hull that is very full, the water goes forward at the stern, which changes all the parameters in the calculations. Parkland in not providing enough information for a proper answer.

Lets say it's this boat:
( But assuming that the shaft tube angles are relaxed to a normal angle)
(The back end kinda sits high, so I doubt any water would be dragged backwards. )

 

[JSL]

24" might work but check it out. Dave Gerr's books (Nature of Boats, Propeller handbook) are a good start.
On line calculator may work. There is one I know of that is quite good but specifically raises the matter of blade area, which is does not allow for.... nor cavitation.
I am in the middle of a 50' semi disp. repower and the existing 24" (3 blade, over-square)props are being replaced with 28" 4 blade (better, 30" dia. would have been too intrusive).
Power is up from 2 x 300hp to 2 x 370hp. so the max speed should reach 18 knots... up from the present 14
Lastly, props push the boat but they also stop the boat. A heavy boat with undersized props can be a handfull.