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[GG]

Here's a little problem for you, try explaining why a planing model boat goes through different attitudes when accelerating from rest up to full (plaining) speeds.
I thought I knew the answer but have seen a quite different explanation.
Glynn Guest

[raflaunches]

Going from my aerodynamic training I’m guessing that it’s to do with hydrodynamic lift working in a similar method to aerodynamic lift.

[SteamboatPhil]

Well that's my homework for tonight 

[tonyH]

I punched in du Cane and this newish paper shows the attitude change/pressure variation hump. Might be useful since it's on model research.

[roycv]

Hi all, I was testing out my 16 inch loa Huntsman this morning I had slowed it down as many years ago it travelled at insane speed with about 3 inches of the rear hull in the water but it was not controllable but was relatively stable in a straight line. 

I now have a 380 (40 watt) motor geared down  1:2  and an X prop 40mm.  Last time out a week ago I had a 3 bladed brass (prop shop) one 40mm dia.  This only planed for a minute or so then she slowed down and just went off the plane but at an angle in the water pushing more before it.  Nominal 6 AA cells at a load of 2.2 amps.

Hence the increse in prop pitch now current measured at 2.8 amps in the water.  I had other problems with weed due to weather which made life difficult I will continue in a few days.
Initially I used a sub-C 6 cell pack but in trying to save weight I went to some new 2.3 (2300) Ahr AA cells I am sure I got some chattering from the motor due to the voltage drop on the battery and when I applied rudder i.e. extra power taken to operate servo it got worse.  Looks like I will have to go back to the sub-C pack as the extra weight does not seem to affect it too much.
I am not going to buy LiPos as I know it works without.  I also had changed from a cheapo esc to a bandolonean tiny esc which alegedly will cope with 10 amps.  I will have to go back to, one change at a time, to see the affect.

 My objective is to get a minimum planing speed and lowest power consumption.  The answer is out there......
regards
Roy

[GG]

Well I left this post alone for a few days in the hope that someone might have ideas about a weird thing I had recently come across.
It was suggested that the reason why a planing model's stern tends to sink when accelerating from rest is due to its Center of Gravity (CG) moves rearwards.  On reaching the planing condition its CG has then moved forwards...?
This puzzled me as a model boat ought to be considered as a "rigid body", that is all its parts are fixed in place and do not move relative to each other.  Hence its CG is in a fixed position all of the time.  I'm sure many have witnessed the results in a model boat where the internal parts are not firmly secured!
The only thing I can think is that someone has confused the effects of Inertia when accelerating.  You know when sat in a car that starts to accelerate forwards, you feel pushed back into the seat.  Nothing to do with the CG moving, it's just you being pushed back into the seat so the seat can exert a push forwards and allow you to have the same acceleration as the car.  Same with elevators, when you start to accelerate upwards and so "feel heavier".
Thanks for the responses though.
Glynn Guest

[NickelBelter]

I would imagine it has something to do with the location of the planing lift (which is due to water being deflected downwards by the angle of the hull) moving from the bow towards the stern as the hull rises out of water

[raflaunches]

Hi GG


I think the only time Centre of Gravity could be taken in to account would be in the Coastal Motor Torpedo Boats by Thornycroft in  WW1 where the torpedos would be launched off the stern at speed. On my 1/12th scale CMB we have tried different variations of torpedos fitted and the model will not plane with the torpedos removed. I’m guessing that something was moved or the boat was built to plane before the torpedos were fitted whilst ours was built the other way around.

[roycv]

Hi all, visualising what was happening with me, the hull increases its angle to the direction of travel and driving a small wall of water ahead of it.  I imagine the prop now is facing more downwards and trying to climb over this hill of water.  Any ideas?
Roy

[Jonty]

  I don't know if this is relevant, but you certainly get rearward weight transfer in a car as it accelerates. That's why a rear wheel drive car is better at going up muddy or snowy hills than a front wheel drive one.

[John W E]

Hi


Quick explanation of the bow rising and the stern sinking.   Think of a see saw.  The bow wave is a central pivot - as the bow of the vessel climbs the wave - there is no support for the stern, as it is generally more narrow than the bow on a planing vessel.  This is because on a planing vessel, the widest part should be roughly 1/3 back from the bow - even less in WW2 vessels.    As the vessel starts to increase in speed, the centre point of the vessel begins to move towards the stern obviously.


This is obviously a very basic explanation because there are all sorts of factors which govern why and how a boat planes.   Mainly the boat hull shape.


John

[SteamboatPhil]

I have a copy of Peter Du Cane's book (High Speed Small Craft, second edition 1956---first published 1951)
Doing my homework....my head hurts, sorry chaps still no answer........more homework 

[ScottW]

Might there be anything useful in conversation about full-size boats here, maybe?
https://www.boatdesign.net/content-search/10321977/?q=planing&o=date

[warspite]

My take (and very tongue in cheek) is that as the prop pushes the water back, this creates cavitation at the rear, the hull then sinks into this cavity of flowing water as she moves forward, the start of the effect of flow is somewhere along the hull, i.e. the water is stationary and is then sucked into the cavitation and then into the stream of water flowing over the prop, so the bow rises as it see saws, the faster the vessel goes the further the effect moves towards the rear, the step in a planning hull assists the lifting of the hull by cavitation - like I said my take on it. 

[grendel]

with a displacement hull, hull speed is reached when the length of the wave created perfectly matches the length of the boat, when this occurs the front half of the boat is floating on the crest of the wave, and the stern in the trough, effectively, now the boat is travelling uphill, hence the bow rises and stern sinks, what happens when you get to planing speeds, i have no idea.

[GG]

Left this for a while, actually had a short "holiday" which means no access to the Internet!
Lots of differing ideas but I think one common problem is not treating the model as a "free body", that is just considering the forces acting on.  This can lead people to remembering Newton's 3rd Law of motion (the easiest one to remember) and combining action and reaction forces (which appear to cancel each other out) when each only affects one body.


Looking at it this way and I feel it's the thrust line, acting below the models Center of Gravity (which thankfully no one has suggested moves rearwards) which causes a turning Moment that raises the bows as soon as the motor starts.  This moves the Center of Buoyancy rearwards to create an opposite turning moment.  As the boat accelerates forwards, Hydrodynamic Lift supplements the Buoyancy's effect and reduces the immersion of the hull in the water.  In fact at high enough speeds very little of the hull is immersed.  To be honest, a flat bottomed hull maximizes this effect, something that I've seen with the few such models I've built.  But, unless conditions are calm, a "V'ed" bottom gives better handling.


Glynn Guest   

[RST]

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