Atkin's Happy Clam's propeller

I had expected when I asked if it was possible to use the 10x6 prop and a 6 hp motor to power the boat, that the answer would be based on the available torque and power of the engine to turn the prop. What I got was that it may be possible for the boat to make that speed with only 6 hp, and that the prop pitch could not have been 6". According to the nomogram in Dave Gerr's "The Nature of Boats" a 6 hp engine should be able to turn a 10" prop at 1800 rpm. The nomogram also indicates that the prop should be 8" diameter if it is turning 2250 rpm. The Fn for a 17' (4.1 m) hull moving at 14.8 mph (6.6 m/sec) is 1.01, so it is not a displacement hull and the 20-30% slip estimates may be too high.

Next question, does the following argument work at all as an initial estimate of the pitch needed to attain the observed speed?

The known values are prop diameter and boat speed. The gear ratio and pitch are unknown, but need to be determined to make the boat perform as advertised. I thought that it would be routine to estimate the thrust for a given hp and speed and a trivial matter to estimate the requisite pitch. All I found was one rough estimation of thrust at a given speed on this forum posted by Daiquiri. Slip could be accounted for in Newton's action/reaction law by multiplying the thrust of the prop by the volume it passes through in 1 second to find how fast that volume should be moving backwards in the immediate vicinity of the prop. That should account for the fact that slip decreases at higher speeds. I varied the pitch by trial and error to find where the swept volume accelerated by the prop was moving at the same velocity as the slip. The assumptions being that slip may be accounted for by the fact that the water the prop is interacting with is moving in the opposite direction, and the slip is zero relative to that imaginary cylinder.

Using these calculations:

If the Happy Clam does make 14.8 mph (6.6 m/sec) at 225o rpm, the slip should be a function of the prop thrust and mass of water accelerated the prop.
1. The The 25.4 cm diameter propeller moving at 6.6 m/sec will act on 335 liters (speed x blade area) of water.
2. If the prop is estimated to be 50% efficient, the thrust works out to 356 N (79.8 lb)
3. The water acted on by the prop in one second is accelerated to 1.1 m/sec (356/335).
4. Making the rash assumption that the water the prop is acting on is moving backwards at 1.1 m/sec, and the effective slip is 0, a pitch of 8.1 inches would produce the observed speed of 6.6 m/sec with 14% overall slip. Shifting gears, as suggested by Gonzo; at 1800 rpm with a pitch of 10.1, the water is still moving backwards at 1.1 m/sec with a 14% slip.

 

Your speed estimates are wrong; the fluid is accelerated from the incoming value (Vp). The velocity in the propeller disc is the average between Vp and the final exit velocity (Vj).

 

Keep in mind an Idea for a Quick-Check on the solved method you try.
Start from Diameter of the propeller from the attached Nomogram, and as you see 7''-2/3 is the optimum.
Next see that the optimum Diameter for shaft speed 2000rpm is 8'' , 1600rpm is 9'' and 1300rpm is 10''.
So you have to choose a gearbox reduction ratio :
1. R=1.125 for 2000rpm - D=8'' ,
2. R=1.40 for 1600rpm - D=9'' and
3. R=1.73 for 1300rpm - D=10'' .
Then find the Cb [block cofficieent factor] of hull to calculate Va ; Va is the speed of water flow that is exerted from hull and before the suction of the propeller.
On Va value basis you able to choose the P/D [Pitch to Diameter] ratio at the point where the efficiency factor [where the slip is minmum] of the propeller is the highest.

 

I think that may tell more about how the prop interacts with the fluid than what the prop did to the fluid in the end. I was only looking at the work done, not how it was done. How much the thrust should accelerate the mass of water that passed through the prop. The relationship between prop diameter, speed, and thrust is the work done on a volume of water. It does seem to explain why a larger prop slips less and why slip decreases as speed increases.

I was thinking along the lines of a rocket in space. When the rocket motor is fired, the center of mass of the exhaust moves one way and the rest of the rocket moves the other way while the center of mass of the system remains motionless.

I ran across this: https://www.boatdesign.net/attachments/prop-demand-jw-herbert-pdf.66030/
It is from an old thread, but without knowing James Hebert's handle, I can't find the thread to see what folks had to say about it.

I don't know how to find data on different boats that includes power, speed, slip and prop diameter to fool with. If I could run the data against my hypothesis, I might be able to shoot this down myself.

 

Since your velocities are wrong, you end up with the wrong mass flow through the prop and the impulse calculation comes out wrong.

 

That nomogram is designed for 3-bladed screws, I think. The "Happy Clam" prop, which was the basis for this discussion, is an old type of 2-blade, probably with a blade area ratio >0,5, which makes quite a difference.

 

As for the original question "....could the 5 hp Palmer drive a 10 x 6 inch propeller with reasonable thrust and efficiency at a boat speed of 14,8 mph (~12,8 knot or ~6,6 m/s)?"; I think Fallguy nailed it already in post nr 2 of this thread. The prop would operate with a negative angle of attack, giving a low thrust.

Checking the numbers for the Wageningen B series screws, 2 blade and BAR 0,38 shows that the operational point is very close to its maximum open water efficiency (~close to 70 %) if the pitch is 9 inches.

 

Thanks. It looks like my hypothesis has more holes in it than just the velocity.

 

No, your constraint is the 6HP of the proposed engine. You need to calculate the resistance of the hull at different speeds (ideally make a graph with the curve power vs speed) and you will then calculate the propeller at the speed possible with the power available. Otherwise, the projected speed is a fantasy.

 

Atkin & Co. - Happy Clam https://atkin.mysticseaport.org/Utilities/HappyClam.html

 

I can only talk from my own experiments to "confirm" speed predictions on low power. There is no doubt in my mind from practical experimentation, that the base of the box keel works as a lift as speed is gained, so typical hull drag numbers do not apply.

There was an old thread somewhere of a guy who had an over-powered box keel boat, that was loaded with "lots" of lead ballast. The prevuios owner told him that the boat would rise and then drop over on one side or the other, so he added ballast........then a bigger engine. A case of a semi-displacement hull being driven by an owner wanting full planing speed.

On paper, the 26ft "Two Brothers" with its 12hp Palmber should by most calculations not reach its stated speed. I changed my mind about that after experiments. Although i could not confirm any prop specifics, the stated HP seemed accurate for the stated speeds. Box keel hulls can be deceptive.

 

Is there any data of a Happy Clam that was built and the speed measured with a 6HP at the flywheel engine?

 

If you will pardon a link to the WBF.

I would have said the same thing, but I figure that you're more likely to believe Eric. The only data available is in the link to Mystic. This link is in the link to Eric's comment. The Mechanics Illustrated article linked in the first post is the same boat.

There are a few other Atkin designs that have very good performance with low power, mostly Seabright type with box keels. Gerr has some observations worth looking at in "The Nature of Boats". Their ability to carry loads at speed in rough water resulted in a little 'friendly' competition between the rumrunners and USCG. Competition and cooperation as they were sometimes built side by side in the same yard. Nondisclosure agreements anyone? Bolger played with box keels, but, well.... The Rescue Minor and similar boats seem to give up a little speed for the extremely shallow draft and well protected prop. The attached table was cobbled together over 8-10 years. When it resurfaces on the laptop I give it a poke. The numbers are from the write ups that I found either in books (Farmer's Coyote) or online (Atkin, etc.) There are larger examples in the Atkin plans collection, but I drew the line at 25'.

 

Calc. Optimum Diameter for 2 blades propeller.
Troost L. Open Water test series with model propeller forms. Trans. North East Coast Institution of Engineers and Shipbuilders , 67 , 1952

D = 1,524 · (P)^0,2 / (n)^0,6
D in m , P in kW , n in rps

P = 4,41 kW = 6 hp
1. R=1.125 , n2000=33,3 , D=0.25m , D=9''-27/32
2. R=1.40 , n1600=26.67 , D=0.286m , D=11''-1/4
3. R=1.73 , n1300=21,66 , D=0.324m , D=12''-3/4

 

I went to the link. It has an estimate from someone that calculated with zero slip, and another from someone using 35% slip. There is no actual measured speed,