Atkin's Happy Clam's propeller

This has been asked here before several years ago, and the most likely answer implied that the test was only conducted in one direction with a strong current and a tailwind. I would like to think that Atkin was better than that. I suspect that the propeller pitch may have been misstated.

In the plans and pictures the prop diameter does look like 10". According to the description:
"Her speed came up to full expectations. She was accurately clocked over the official measured mile off Lloyd's Neck in Long Island Sound, making 14.8 mph with a Palmer 5*-hp. Baby Husky marine engine turning 2,250 rpm. The Columbian Type E two-bladed propeller is 10 in. in diameter and has a 6-in. pitch."
*The description on the Mystic website says 6 hp. That write up also says "Happy Clam should do 14 mph with 5 hp., 20 mph with 10 hp."

With zero slip, the 10x6 prop could screw through the water at 12.8 mph. Assuming 25% slip, the boat should make 9.6 mph with the specified 10x6 prop. 25% slip seemed like as good a number as any to start with based on pure ignorance and very little reading. I would have to say that it was not possible for that boat to go close to 15 mph in that place with a 10x6 prop at 2250 rpm. A 5.2 mph current seems extremely unlikely off Lloyd's Neck where the current is currently running at 0.3 m/s about 3 hr after high tide with a 6' tidal range. The current speed is from ventusky.com.

My knowledge of prop design consists of "ask an expert", which brings me here. My question is whether a Palmer 6-hp. Baby Husky marine engine could turn a similar looking 2 bladed 10 inch diameter prop at 2250 rpm with enough pitch to make 14.8 mph? For example, a 10 x 9.3 prop at 2250 rpm would give 14.8 mph and 25% slip, on paper. I am not looking for an exact answer, just whether it looks reasonable to assume that the performance reported could have been real had the prop been optimized for the boat and if the engine had the torque to turn it.

While it is hard to mistake the diameter of a prop, mistaking a 9" pitch for a 6" pitch might be possible.

When you find the flaw in my math, try to correct the mistake rather than informing me that I am an omphaloskeptic and not a naval architect. I won't dispute the assertion, but the math would make for a better answer.

 

I’m calling bs on the pitch. When you run the numbers with a 10 pitch, slip is 30%. Of course, 6/9 is an ages old problem.

 

Given the size of the flywheel on the Palmer compared to a modern outboard, I would guess its possible. I read nothing about if the motor was direct drive, or a reduction drive, which would help. A Yanmar 1GM10 with a 3-1 reduction can throw quite a prop.

 

All the Palmers were direct, from my quick reading.

 

What difference to the output power do you think a flywheel makes?

 

That's what I thought at first. Not so much power to do work as ability to keep moving slowly under load. You need enough momentum to keep the speed up between power strokes with a low power one lung low rpm motor fighting a lot of drag torque. Too little rotating mass and it will slow down too much between power strokes to get past the next compression stroke, especially getting started.

Ever watch a big one lung make and break engine idle without a load? Whump, puff puff puff puff whump... I missed the Rollag steam threshers reunion this year. More big old stationary engines in one place than I have seen in my entire life. Even a room full of sterling engines, all running. You can really get a feel for how massive flywheels store energy and smooth things out.

 

Torque. Not a suggestion that it gives more power, only the ability to drive a prop with greater pitch.

 

Yes, low rpm requires a lot more torque for a given horsepower, but it is the energy stored in the flywheel that keeps things moving smoothly between power strokes.

Torque and power are inexorably linked to speed. There is no reason a high speed, low torque engine can't be geared down to produce enough torque to turn a slowly rotating shaft while generating a lot of torque. There may be any number of practical reasons not to try it. As you said,

Having read far too many badly paraphrased interweb plagiarists' failed attempts to succinctly explain a simple concept, I have settled on this one. This is the part of the explanation that none of the rest used:

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Big heavy flywheels were an old solution to reduce vibration. Modern engines can turn at much higher revolutions and often use a counterweight shaft or a damper to reduce vibration at a much lesser weight than a heavy flywheel.

 

Stationary steam engines were big users of very heavy flywheels, particularly those which drove generators.
The flywheels were used not so much for vibration reduction as much as the need for a fairly constant RPM.
The fly-ball governors are always just a bit behind the curve, and the flywheels smoothed-out the highs and lows.

 

Good day to everyone ,

May I do an input here ?.... in respect to all above mentioned ;
on subject of this discussion , should be noticed that a Propulsion plant has a driven propeller (or an impeller some times) aft to the boat and
on Main engine's output a flywheel . When the boat runs both are turned and exert power in line from the source torque provider ; the main engine.
These two are coupled in between them by a transmission gearbox with Reduction ratio -R from flywheel to propeller by a line shaft ..... as we exam the case ,
we can say that the angular speed of flywheel is -ω1 and the angular speed of propeller is ω2 , the center of mass to transversal axis of rotating disk running
in a radius -r1 for flywheel and -r2 for propeller and also the mass of them is m1 for flywheel and m2 for propeller.
My thought is that we should care - somehow- the product of flywheel capacity ( R x m1 x ω1 x r1) to be greater > than (m2 x ω2 x r2) propeller's rating charge .....
adding that is also well known subject , that vibrations at low frequencies are generated at the propeller's disk when the propeller reacts all the time to the Main engine's
torque loading order and the nature of real open water flow.....turbulent and vortexes
.....in action Mass and Inertia are existing simultaneously .....as far Force applied to mass , inertia reacts to acceleration.....
or when mass of flywheel accelerated due to exerted power of Main engine , propeller resists.....
its a matter to overcome the magnitude of flywheel mass acceleration , to the propeller's inertia extension.

 

We might also note that that every time the 2-bladed prop goes vertical behind the deadwood it enters an area of turbulent flow.
So its torque loading is constantly being interrupted every 1/2 revolution.

 

It is not interruption , it is a variation of propeller's blade loading , that produces low frequency vibration.

 

If the RPM is not constant it causes vibration. Smoothing out high and lows is preventing vibration.

 

One need not be pedantic, It's still an "interruption" between clean flow and turbulent flow.
And after ~60 years in boats I'm quite well experienced with the vibrations it can cause.