I agree the wrap around screen is not easy. My one nestles in a deep groove on the upper edges of the cabin sides where it is cemented in place.
As I said I had to replace it and delayed as I did not fancy the job, so when I eventually came to it the glue had failed and came away as crystaline particles.
It is getting the fit over the curved roof which is a slow fittting job.

At my scale the window frames were impossible and mine are thick paint.
I carefully masked everything and then about 6 or more coats of paint and leave to dry for several days.

I know planked decks are nice but of the several I saw they had a canvas deck so I just painted them as per.

Now I have her running I can see that there is a small take up of water, probably spray, I will have a think about that!

Roy

Right I have some data for you. First thing is she planes nicely. But not until I moved the drive battery 20mm back.

The power needed was 19.25 Watts. Measured under load and my guess would have been 20 Watts.
The beam is 13cm.
The amount of hull in the water when planing is 25cm.
LOA 40cms,
However, the deep vee surface is a bit over 14cms. (7cms each side including 3mm spray strip)
The weight is 791 grams or 1lb. 12 ounces.

I used a powerful 380 geared down 1:2 on a 35mm diam plain plastic 2 blade prop.
This motor is rated at 40 Watts so does not get hot. In fact when first planing I had an X prop and I changed down to the P pitch and she went a little faster.

The battery is a home constructed 6 cell 4/5th sub C, 2800mAhrs 6 cell battery welded together. When charged it has about 20Watt,Hour capacity.
Next time out I will check on endurance time.

Anyone with any formulas on planing and weight and surface area perhaps they fit in with theory?
Regards
Roy

Thanks for the information Roy. I thought I'd got most, if not all of them covered but you never know.

I've got the windscreens and pulpit rails to do which I'm a bit apprehensive about, but having said that, I was about other things, like the planked decks, but they turned out OK in the end.

Like most builders I've gone for single props as well and would have liked to do a twin. My Swordsman would have been a good candidate as it's got plenty of room but as it was only my second build I wasn't ready to tackle that then. I could modify it but the build has taken too long as it is, being left for long periods of time, and so I'd like to get in finished as it is.

Chris

Hi Chris the Sparkler was a free plan from possibly the RCME magazine I think the Bobby was a later and slightly larger derivative.

The superstructure was constructed from a side photograph and constructed of thin 1mm ply. The difficult bit was creating the wrap around screen, lots of cereal packets on a trial and error basis. the sides have a double thickness with slots left for the screen to be retained.

What would be the framing in metal of the screen is simulated with several layers of white paint with the screen having masking tape for the shaping.

The handrails are quite strong as they are soldered tubing supported by slightly flattened oval shape tubing and etched and painted to resemble wood. The pullpit has been damaged several times but is soldered tubing and brass hard wire and small washers at the base and aluminium paint.

It is over 40 years old now but has survived well. The spray rails are a little exaggerated but do the job.

So not a kit, just a one-off model. I think it works out to 1 : 24 scale, sort of looking for crew as well.
Back when building our son was with me and we found the full size boat moored up and after a little conversation we were invited on board and if I remember correctly this was the colour scheme.
I liked it so used it. By the way the couple had hired the boat for a week.

It was some time after I realised the Huntsman had 2 props, if I had known it would have had 2 props, bit annoying that!
Roy

Hi Roy

So it was essentially a Sparkler, which I believe was one of Vic Smeeds designs and you modified the superstructure?

I hope you don't mind me asking but I've written something on all the Fairey drawings and kits I know about and a 16" Huntsman 31 from the 1980s isn't one I knew about.

Chris

Hi Chris thanks for the mention. My Huntsman is essentailly a Sparkler hull from way back I had a powerful 380 with a 2 : 1 reduction gearbox and an old 6 volt camera battery driving. It planed no problem and as I said barely controllable.

I have been trying to make it plane but not go as fast but this has become the sticking point.
I have some lighter 7.2 volt batteries 4/5th sub-C, and I shall be trying these also an esc that weighs about an ounce. Fairly soon I would think. Will keep all informed and if it works I will send a picture.

The battery pack I have, I made myself as I recently bought a mini welder and nickel strip and 2800mAhr cells.
Roy

Roy - the plot thickens or at least changes! I had a look in Your Harbour and I see it's a Huntsman 31 and only 16" long. You made a lovely job of it for such a small model though.

Total conjecture here, but you know with some models you have to increase the beam or freeboard to make them perform satisfactorily and I think your model might be suffering because of its small size? The Huntsman 31 has a quite long narrow hull with a deep vee and not very supportive bow. Whilst it is a planing hull, but being a deep vee rather than flat it will never plane at a particularly low speed. My Huntress is quite beamy for its length and a shallower vee but is still going quite fast when planing. I doubt you could reduce the weight by much and doubt it would make much difference anyway? Spray rails might make a difference? I mentioned in my previous post that they can suffer from a bow down attitude and I've read of some folks having handling problems with the boat flopping into turns like PT boats are prone to.

Chris

Talking of weight, out of interest I weighed the model of the Fairey Huntress that Dave Milbourn built and it came to 1186g plus the battery giving 1409g. Full size weighs around 2500kg which for 1:12 gives a scale weight of 1446g (2500/(12x12x12)) which I found surprisingly close given that the full-size has a heavy diesel engine etc. The weight distribution would be completely different though with the motor sat further back and with fuel tanks and freshwater tanks etc.

I assume that the scale weight is calculated in the way that it is so that the model sits on the waterline? And allows for the fact that water isn't to scale and so offers more buoyancy?

When working I used to do structural calculations (steel and timber) for building works but wouldn't contemplate doing any for boats, and especially models, as I can see so many complications, especially from the positioning of the motor, battery and RC equipment etc. and the shape of the hull and would do others do and arrive at an answer by trial and error.

Myself and ToraDog did make contrasting posts but that was more from me being lucky to some extent with my models sitting on the waterline due, as I said, to the buoyancy of the hulls because of their size and weight. So with those, no trial and error involved as they sat as they should from the off! I haven't tried my Huntsman 31 yet, as I need to get some paint on it, but I think I may have a slight problem with that as they tend to sit bow down a bit due to the design of the bow and so I may have to put some ballast in the stern.

ToraDog was correct in saying that CofG is important but I still think that with planing model boats (unless very small or marginally powered) that the overall weight, (as I said within reason) is less so. I don't think that building light or heavy will really make that much difference?

Dave M. built his models light by using Liteply (I'm not a fan of it) and I build heavier using birch ply and thicker frames etc. Be interesting to see what my Huntress stern-drive model weighs?

Roy's Huntress should have no problem getting on the plane and I think it's due to lack of power/ too low a revs rather than weight. Spray rails/strakes do provide lift but that is more important on full-size boats that don't have such a good power to weight ratio and a need to be more fuel efficient. My Club 500 doesn't have any and gets onto the plane easily with it's brushed motor.

Hi Roy,
Re: PS I think Billions of Yen is not all that much!! Last time I looked it was 147 to the GBP.

I wouldn't sniff at 1/147 of a billion Yen = 6.8 Million quid or €6.01 million😮😋🤑
So if you have a few billion ¥ lying around ... 😁

BTW At today's rate ¥1.000.000.000 = 5.120.260GBP. Exchange rate 195.
😎

Chacun à son goût Alessandro.
(Ognuno secondo il proprio gusto).
I'm too impatient to mess around with hypotheticals and number crunching just to be able to prove myself right.
I want practical results and ASAP!😁
Ciao, Doug😎

Good evening naval modelers.
An interesting discussion was born that I enjoyed.
If I'm not mistaken, Toradog and ChrisF's positions are contrasting, although each has contributed valid arguments and this has made the exchange of ideas very pleasant.

In naval modeling, calculation errors or lack of calculation do not lead to tragedies and disasters, therefore calculations are not essential.
In naval modeling you can successfully build your own model even with limited or zero knowledge, doing tests and experiments.
However, there are those who also enjoy doing calculations, trying to see if their predictions were correct, if their project was valid and functional even before the test in the water.
I am one of these.
Those who have read some topics on the subject will already know it, they already know what my opinion is on the matter.
This does not mean that I blame those who rely only on tests and do not try to do any calculations.
I repeat: for me (very personal taste) even doing the calculations is part of the game and I enjoy it.

My only exception to pure 'suck it and see' is when I am contemplating doing a Plastic Magic conversion of a standard Airfix, Revell etc type kit.
Then I tape over all holes in the hull, shafts and rudder etc, float it in the DTTF and load it with water or lead shot to the waterline. Then I know the maximum weight it can carry.
Including all internal gubbins, decks, superstructure, and fiddly bits which are estimated at ca 60% of the total sprue weight.

Then I can start rummaging in the parts boxes for suitable motors and electronics.
Tricky bit is the battery of course; trade off between mAh for endurance and weight.
Only Calculation involved is addition; of the weights of the individual parts until it reaches the established maximum. Then the head scratching and more rummaging usually starts🤕
Cheers All, Doug😎

Me 2 Len👍
Suck it and see. Works every time😉
It only gets a bit tedious when the damn thing won't fit in the bath.😠
Oops, sorry I mean of course - The Domestic Test & Trials Facility (DTTF)😁
For my 1/72 H class destroyer it just fits if I stuff up the overflow and fill to the brim (minus displacement depth, Eureka!) For Graf Spee and HMS Belfast - no chance so off to the lake with a box of tools and bits and a couple of gottles of geer😁.

PS to TD: 'My pint' is a half litre of fine Bavarian Bier. Prost😋
PPS: I am (was?) an engineer; electronics and Naval COMMS.
😎

ToraDog.
Works for me. That's the way I do it. Len

Sacrilege! But I like it!
Roy

OK, I admit it. I have never done a weight calculation that made any difference to me. Every model I have built, going back to a Lindberg destroyer, has been based upon putting the hull in the water and seeing where it sat, and how it ran. I honestly believe that 99% of us do the same thing. If it does not work right, adjust and try it again. After all, that is engineering, just without the whiz bang computers and slide rule. Don't believe me, how many time have you not read that after trials a new ship had x many tons of ballast added to make her stable? I think that the Japanese had this issue in the 30's just prior to the War.
We all do it.
I have a 1/16th ELCO PT. She should be running VEE drives, but where to buy them and do I want to pay the $$$$? NO. So the motors are way further foerward than they should be. Almost all the weight, except for fwd armaments, were after of center line. Fuel+ engines, torpedoes. i have placed two batteries against the transom and one forward of the center motor. All guess work, but she sat at a scale WL and she planes off nicely. She has a tendency to nose dive on a tight turn, but turns were not tight on PT's anyhow.
My pint is to leave the calculations to the engineers, Titanic, the Space Shuttle, should I go on?
We are modelers, we make things work.....

From reading various contributions every one makes good points and all valid.

I usually settle for the motor, battery, esc combination with a sink full of water a prop shaft and propeller, as well as a meter and a rev counter I made myself.
Then off to the club lake with a handful of propellers to do the fine tuning. I love that part, working out minimum power for best speed.

Then I start thinking about which of the boats I am going to build next.

Roy

Hi ToraDog

Aircraft yes, and yachts, but we are talking biggish planing hulls here and in my experience the hulls are so buoyant it almost doesn't matter. Having said that, I always try if I can to even out the weight along the hull with the battery towards the stern but with my Huntress stern-drive model that was difficult to do. I was so relieved when I tried it in the bath!

The River Cruiser is basically the same hull but with more freeboard and has a conventional prop shaft and sits about the same.

What you don't want is nose down but even then it will plane with enough power. Like skimming stones all you need is a flat surface and motive power!

This is what this forum is all about, discussion and different views and experiences. Long may it continue!

Chris

ChrisF,
I have to disagree with you. Weight and location of the center of weight make a huge difference.
One example of that that comes mind is fuel tank location. Most builders will try to locate tank at the center of weight(or gravity) in order to take advantage of the smaller change in that location as the fuel is used. Aircraft, especially, use this approach. The wet wing approach helps keep the center of gravity very close to the center of lift.

You’re absolutely correct Roy. Use experience & common sense for models. Throw the hull in the water with your best estimate of how it will plane or handle. If not pleased, change one variable at a time until you achieve your aim. But, change only one variable at a time; changing more than one at a time means you won’t know what really made the difference.
To me, this turned into quite a good discussion.

Bill

Within reason with planing model boats I don't think the weight matters that much (takes cover! 😀).

From my experience building Faireys the hull type is usually very buoyant and I think building heavy helps with how the boat performs i.e. doesn't bob around as much, sits well in turns and creates a good wash.

For most of my builds the motor is in the cabin which I balance out by having the battery towards the rear. But with my Huntress, having a stern-drive and the motor right at the back I thought I was going to have problems with the bath test with it sitting stern down due to the substantial weight there, but no, it sat bang on the waterline as per my other conventional Huntresses! And it performs really well.

Good thing with models and particularly if using brushless you can always fit a more powerful motor.

With long thin displacement hulls weight and its positioning is usually more important if not critical.

Chris

Hi Chum, I think that is why they invented trial and error!

Did you know that a Japanese gent took 18 years working out what elements were needed to create a white LED? He was employed to do it. After 5 years they told him to stop, then they sacked him for carrying on but he did carry on.

Eventually he got there and needed a manufacturer, which was the company he used to work for. Then they claimed the invention was done in their time and cut him out of the equation!

Eventually the courts forced the company to acknowledge his contribution. The company still makes Billions from his perseverence.

So if you do come up with this formula do get yourself a lawyer!!!!!

PS I think Billions of Yen is not all that much!! Last time I looked it was 147 to the GBP.

Roy

Apologies. I made it sound like one formula can be determinant & was just about to say it wasn’t true. But wait a minute; I think one lengthy, complex equation derivation is likely possible. The formula derived could contain all the hydro dynamic factors that take into account hull design, weight, plus mechanical forces such as thrust, thrust angle, drag, slip, etc created by motors & props. Think about the complex equations for thermodynamics & fluid mechanics. It becomes a matter of knowing what the value of the equation’s factors (ex, weight, drag, thrust) are.
In reality another equation or set of equations are likely required to determine the value of the factors in the final equation.
Duh!! I guess I have just said that more than one formula IS required!
So net, net, it’s a complex question to answer.🤑
Which was Roy’ s original point!!

Quick illustration of variables: I own a 38" Cigarette with twin stern drives. She will fall off plane at 29 knots, with the stern drives fully retracted and the trim tabs fully down. She will not plane off below 29 knots with the tabs at neutral and the drives fully down. Leaving the drives in the neutral running position will prevent the boat from planing off at all.
The point I am trying to make is that there is no one set of rules for boats, or boat styles. For instance, if I change my props from the current 4 blade, which gives much fast stern lift, to a three blade, the boat take longer to plane off, but achieves + 2-4 knots in top end speed.
Too many variable for one calculation.

I forgot to mention that when doing tow tests, most humans pace at around 2-3 mph and the fastest, NO not us folks, manage around 27mph, faster than a car on a LA freeway at 8 AM.
I think that most of our models will plane at around 6 mph, although my 1/16th PT requires a bit more.😁

….. but this is a many faceted question.

TD, yes it is! Many, many hydro dynamic factors
come into play. I’ve ordered the book.

I don't want to be the fly in the ointment, but it seems to me that the maximum weight should be equal to the designed displacement. That figure includes the hull, electrics and battery. Subtract the hull, and fixed electrics( receiver and servo(s), maybe a tad for the shaft stuffing box, ect, and you have a figure that represents the weight of the motor and battery. Here is where the variable come into play. Type and RPM of the motor as well as torque, and type and sixe of battery to satisfy the motor's needs. One needs to bear in mind the appropriate propeller for the model as well. Shaft angle and position will dictate the size of the prop. Two blades are faster than 3 blades, 3 more so than 4, ect. Pitch figures in as well.
So, how fast do you need to go to get onto plane? How fast did the full scale prototype go? Scale that speed and you have an aim point. Another way is to tow test the hull with the appropriate displacement weight aboard. At what speed does the model get onto plane? Oh, BTW, it is a stepped hull? ? ?
I mentioned in a separate thread about a book on Model Propellers. In it, these topics are discussed.
Sorry if I rambled, but this is a many faceted question.

I suppose the speed to get on the plane is related to the area of the flat bottom of the hull versus the surface resistance of the area.
I think my Huntress weighs too much currently as she tries to climb a wall of water at full speed. I tried moving the battery to the bow but that did not work for me either.
If lighter she would get over the hump and plane. Another almost essential thing is to have spray strips which add lift and divert the water from getting on the deck.
Roy

Hi about planing calculations. A while back I rebuilt a 40 inch planing hull boat, from the 1950s to radio control. I did not have any calculations just a feel for what it would take to make it just plane.

I reckoned 30 watts and to fine tune it with a few propellers. As a period boat I installed a Marx Hectoperm in orange and black a sort of first edition! I used 6 cells and a 40 mm dia. prop. And runs for 40 minutes on a 3300mAhr battery

She went just nicely as it was for a young family and grandad's intended boat but never completed. It was called Dolphin, and is in My Harbour, just had to look fast but have some endurance

I am sorry not to be able to show any maths it is just an estimate after looking at plans etc.

I would suggest that for models just have a look at what works and take note!
I mentioned the Huntress and how fast she is, I am trying to slow her down but still plane, not yet succeded!

Regards
Roy

Chum,
"I just don’t know how to write the equation to make the calculation"
It's NEW math Bill. You are too old for that....🤣

BJ, actually I wasn’t asking about how to calculate weight. I was interested in how Roy calculated the maximum permissible total weight of a model knowing the speed required to get the hull on plane. If one knows the speed required the max weight can be calculated. He was providing an answer to a question of motor selection for a hull.
It’s a naval architectural question of calculating planing speed knowing the weight of the vessel.
Thank you for your answer & calculation examples but I was asking about a calculation for something a bit more complex.
I’m an engineer not a naval architect. I understand what factors go into determining hull speed, I just don’t know how to write the equation to make the calculation.

Thanks again.

Hi! CHUM, Fibreglass weight can be worked out by area X 'glass weight, + resin. Chop Strand Mat weight is 2 - 2.5 X resin X glass area, 'stitched fabric' & or cloth, & tape, are closer to 1:1, resin to 'glass. Resin is the 'week link', so the less U use the stronger the laminate, (as long as it's 'wet out') & lighter, less brittle. Continuous 'glass (Gun rovings & or Woven Roving cross strands), can fill the stringer gap & add stiffness & or strength locally, long ways on race boats. Any help? 🤔 'B J'. 🤞

Thank you Roy for a very good explanation. I thought you perhaps had developed a rule of thumb method. Guess I will have to delve into naval architecture if I really want to know how to make that calculation. Numbers don’t bother me but most of my boats aren’t planing hulls anyway.
Thanks again; Bill

That is an interesting question, I think it is because I have made quite a lot of models and restored a few and especially with yachts done lots of calculations as to scale and weight etc.

I am not the brightest of souls and have to look things up and study until I am familiar with the subject.
I read a very simple EUP (English University Press) book called Teach yourself Naval architecture.
I was a resident computer engineer back in the late 1960s. I had a lot of time on my hands as those above me assumed that a lot would go wrong but it didn't.

The book gives you an idea of volumes and general information and I found some formulas I could understand and wrote a computer program to give me a print out of models and associated parameters of the size models I liked.

Reading some of Vic Smeeds writings and estimates also helped, so estimating size and volume and power started to come without thinking. I can generally work out a ballpark figure for models upto about a meter long.

My first go at this in 1975, was a small yacht which was a 6 by 4 drawing in Yachting Monthly magazine. Yachts especially have to have quite a lot done to hulls to enable a model to sail.
In this case redrawing where needed, I had to double the underwater volume. It took me weeks of tweeking and fairing the lines to get there without distorting the hull.

If you look at My Harbour and Condor, this is a 1 : 12 scale model of Goosander which is with some irony for home construction. It looks like the prototype when sailing, so job done.

If you look at Nordfjord (My harbour) you see my failed build of a ferry boat, which had some character but would not stay upright.
I reviewed all possibilities and sawing it in two was the only solution and extending it. I then put the 2 parts on a table and sat back and looked it. I kept moving the parts further apart until they looked right. So my original 24 inch long model is now exactly 8 inches longer.

When you do this you are increasing the underwater volume where it is virtually a block, so you can put more lead inside to get her balanced correctly.

I have about a dozen yachts and twice that in electric powered model boats. Some have gone now and some were built for others.

I did wonder if I was a frustrated full size yacht owner but I have sailed on a few posh yachts as crew but more lately am Dad sitting in the corner! But I realised I wanted to see the yacht sailing which you can't do when you are crewing.

If you have not already done so, read up on the technical side of hull design there is only a bit of arithmetic so not difficult.

Incidently when working out scales, volumes and increasing size and decreasing size it is much easier to work in fractions. That's because you can do a lot without a calculator and you learn more as well.

Roy