# Resistance factors, planing hull at low speed

Discussion in 'Hydrodynamics and Aerodynamics' started by Mr Efficiency, Dec 6, 2010.

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### Mat-CSenior Member

Quick question from a layman....

If we accept that the resistance of a planing hull at low speed is twice that of a displacement hull without an immersed transom, does that mean that it will require twice the hp to push it at the same speed?

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### AlikSenior Member

Effective power EHP [kW] is resistance R [kN] multiplied by speed v [m/s]; i.e. EHP=R*v.

Required shaft power is SHP=EHP/PC, where PC is propulsive efficiency.

In reality PC will not be same for displacement and planing boats, as it includes 'hull efficiency' that depends on hull shape and position of propeller relative to hull. So the answer is NO if we take it precise; but for rough estimate is YES.

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### Perm StressSenior Member

Part of heat in this discussion might stem from the fact, that dramatic resistance differences only occur for short and beamy hulls, as I did find out for my surprise.
So, for catamarans, with L/B well above 5, difference indeed was about 30% only.
While for L/B ratios ~3 -twice the difference is not unusual.

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### Perm StressSenior Member

To Alik:

By the, way, extending resistance graphs downwards is not extrapolation but interpolation between known point of "0 resistance at 0 speed" and known data at some higher speeds.
Pure mathematics here.

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### AlikSenior Member

This pure mathematics usually does not work for resistance curves. I mean one can join first point of data with 0,0 but this will not represent resistance curve shape.

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### AlikSenior Member

Have You compared Rn given in Donald's reply with original paper?

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### Perm StressSenior Member

It work a little better as looks like at first try:

*we know trend above minimum measured data
*we know "0" point
*we know approximate trend at "0" point
*we know the general shape to be expected
*we can eliminate exotic shapes and big jumps

all in all, quite a bit of info could be "created".

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### Perm StressSenior Member

I am short on time currently...

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### AlikSenior Member

You can join it, but it can not be taken seriously

The trend should be quadratic at very low speeds, in theory. But in practice it is not because flow formation is different at these extremely slow speeds. From my experience, it can also depend on pre-history of process.

Then, the curves presented in paper are re-calculated from model to full size. These results used to contain some factors, such as correlation factor CA that is usually a constant added to frictional resistance. So the results are greatly effected by this technique.

You can also note the discrepancy of points in original paper, at low speeds. How to join them smoothly with 0,0 - is big question.

For Rn - PBB says it is e06, but in original paper e05, as I remember.

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### parklandSenior Member

Well,
hate to bring up an old thread, but it seems like a good resource to me.
I do have a question that I haven't seen discussed in this thread.

I noticed on some planing style boats, that even at low speed, the props create enough thrust that the water flows out level from the bottom of the boat.
How does this come into play?
I mean, if the water isn't touching the transom, how can the transom be calculated into drag equations?
I noticed this happens with a planing boat with the propellors under the boat, but the ourboard engines seem to drag water behind the transom way more.

Thoughts?

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### daiquiriEngineering and Design

That's an interesting observation. Never payed attention and never thought of that kind of propeller-hull interaction, but it does sound plausible if props are designed to accelerate water to high speeds. Probably restricted to rather few boat types, but, if your observation is confirmed by others, it would imho invalidate all the low-speed range of towing-tank data for hulls where this phenomena is present on full-size vessel.
It would also open a non-trivial question about how to accurately and reliably simulate this flow behaviour on scale models in towing tanks.

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### sparky_wapJunior Member

The transom is

Out of the equation when you raise it out of the water. At least that's what I found out by shifting my weight forward in my little wooden boat. A great reduction in drag even at low speed like 6 mph.

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Next time you're in the bath/swimming pool/sea, try this.

Orientate your hand, with all fingers crossed and fingers pointing down into the water, like a paddle. Then place your hand into the water so the water level is up to your wrist.

Then move your hand increasing the speed at which you move it until the back of your hand has no water around it.

Compare how it feels or how easy or hard it is to moving your hand just before the back of your hand has water all around it, which would be just slightly slower than when it has no water touching the back.

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### parklandSenior Member

So then, my next question would be, has anyone else noticed this, and on what type or model of boat?

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### sparky_wapJunior Member

hull

Look at the picture in my profile. My wooden row boat is/was about 200lbs empty and a little over 12' at the water line. The 'square transom' is not effective for displacement speeds.

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