Fitting low water pick up to outboard engine

[Glenno]

Hi phill,

I have been pondering on the good questions you have raised about surface props etc.

I will try to explain what I have found with surface piercing props through the years I have raced, I have been told I explain thing like crap by my apprentices over the years so bear with me.

1) You need to think of the water not at rest but at speed, yes its the same water but it responds differently at speed.
example: If you fall off a water ski at 25 mph you sink below the surface very fast, even the initial hit leaves a big hole in the water but if you fall off a water ski at 70mph the water is like concrete and you barley scratch the surface till you slow down.

2) Its very true what you are saying about props blades being fully submerged get better thrust (I can't aggree more) but also at speed- having that much gearcase and prop blades in the water is not eficient for creating better speed because of the drag factor. At a decent speed if you are getting enough thrust with only 2/3 or 1/2 the prop blades in the water why would you bury the motor deeper creating more drag, losing rpm and making the motors job harder, then you would have to lower the pitch of the prop blades as there would be more drag (more potential thrust yes very true but you only need so much for a light race boat) you would end up back with the standard factory set up as that is best for thust and every day boating. I have found with a surface piercing prop set up corectly you can run around 40 to 50% bigger pitch and sometimes smaller diamiter works better (on a light boat set up well) than a the factory props that come on the outboard while hitting better rpm than the factory props. I have also found once you get the surface props up to speed they grip very well and on one of my boats I am only getting 2.1% slippage.

3) A snow mobile can drive on water at speed, so can a motor bike car- look on you tube, good examples of water at speed!

I hope this helps,
Glenno

[Fast Fred]

30 feet of depth = one atmosphere of presure, 60 feet deep = two atmospheres, and so on. this is 3rd grade stuff 45 years ago. every bodys a tuff guy sittin at some key board hidin at there desk usein a fake name. i am Fast Fred, hear and ware i'm standin.

[Tim Kurcz]

How about water pickup at the bottom of the skeg? This allows positioning of the gearcase bullet at/above the water, eliminating drag almost entirely without risking loss of cooling water. From that position, all prop theories can be safely tested.

Tim

[Aeroliner]

A pressure of 1 atm can also be stated as:
≡1.013 25 bar ≡ 101325 pascal (Pa) or 1013.25 hectopascal (hPa) ≡ 1013.25 millibars (mbar, also mb) ≡ 760 torr [B] ≈ 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available [B, C] ≈ 29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available [C] ≈ 1.033 227 452 799 886 kgf/cm² ≈ 1.033 227 452 799 886 technical atmosphere ≈ 1033.227 452 799 886 cm–H2O, 4 °C [A] ≈ 406.782 461 732 2385 in–H2O, 4 °C [A] ≈ 14.695 948 775 5134 pounds-force per square inch (psi) [D] ≈ 2116.216 623 673 94 pounds-force per square foot (psf)

I beleive that 33.83 ft of fresh water is the corret number.

Alan

[BJuby]

Thanks Alan, I was going to state 30 feet (which I know in passing from my father who dives). I knew 6ft was not 2 atm. You gave the actual calculations, much better, haha.

[phillnjack]

Glenno
i understand exactly what your saying, and yes i understand that surface props can be larger picth due to only being half in etc at speed, that makes good sennse ofcourse.
And yes your right, when i do fall of ski's (very often ai do sink almost straight away.
what i cannot understand is the blade causing drag while its actualy turning and pulling through the water.
From my understanding of surface props and i have only run them on a small scale is that they are made to pierce the water far better than a submerged prop.

Im not being thick or trying to be smug, its just that this all seems to be going completely against
everything ive been told about propellers before.



phill

[Glenno]

Hi Phill,

I dont think your being thick or smug, I ran through all the same questions when I started racing, as the way race boats props run was also against everything I had learned over a life of being around outboards.

Quote: "what i cannot understand is the blade causing drag while its actualy turning and pulling through the water."

I think maybee another word to use would be load instead of drag - deeper gearcase definately drag but prop more load when deeper. So what I reckon is - if your getting enough thrust with only 1/2 prop in, if you were to lower prop into the water more it would load it up more (more potential thrust - bite, if the motor could spin it yes).

The whole idea of surfacing the prop is to unload the motor so it can rev more and spin a greater pitch prop.

On one of my boats with low pick ups the centre of the nose cone is around 5/8 above the water when the boat is at full go and the prop is only 7inch diamiter by 10inch pitch- I will try to upload footage to you tube as you can see full nose cone out and at least 1/2 prop out, still with great bite. When I bought that boat it was doing only 56mph and then I raised the motor 3/4 inch and it felt like it freed the motor and released its shakels and now it reaches 71mph.

If there was a motor that could have the power to turn that 7 x 10 prop fully submerged at the revs I am getting while srface piercing, i dare say it would hit the same top speed, if you ignored the extra gear case in the water drag. But you dont need that much bite, thrust, load on the lite tunnel hull its on. But if you where to put my motor (surface piercing) onto a regular every day fishing boat it would be totaly useles, not enough thrust to get it on the plane.

Any way I warned you I have been told I can't explain things, I will try just the same.
The Glenno

[Ron Hill]

How about water pickup at the bottom of the skeg? This allows positioning of the gearcase bullet at/above the water, eliminating drag almost entirely without risking loss of cooling water. From that position, all prop theories can be safely tested.

Tim

My dad made my brother a gearcase for his "A" Hydro in 1949 and my brother beat 32 "A" Hydros in the Hearst Regatta.

Because gearcases are in my DNA, I have wanted to make a stainless steel gearcase and part of that concept was to have the water pickup in the skeg.

A stainless steel gearcase could be made very thin and the skeg could be larger but thinner...

Maybe, in my next life Tim.