Southampton Solent Model Yacht Contest

Bob Perry

Super Anarchist
31,943
1,338
"Bob is on Facebook!!??"

Yes Ed. I have as of this morning 1,498 friends. Beats the half dozen here by a mile. I have been cataloging the carbon cutter build there as well as posting Jeff's log entries. Will is a FB friend.

 

JustaBoat

Anarchist
Wow, BJ you must be proud. Will (and all) does all the data points on sail models = accurate data when scaled up? I can see (sort of) how it might work with sail and hull design but seeing the scale between the hull rig and keel makes me wonder. I would guess these would be considered ultralite design and not displacement design? For a better understanding, how would a functional model of RP's carbon cutter help in understanding its possible performance? Am I getting way to detailed and not looking at this at a higher level?

cheers

 

willp14335

Member
408
98
Anacortes WA
I think because you don't have material restrictions in the same way with a model boat the proprttions differ to take advantage of the fact you can go for deeper draft and larger sail area. This in turn requires a hull shape with different geometry to account for the higher amount of power (hence the hull runs at a higher speed to length ratio than a scaled down big boat with proportionally smaller rig and lower draft). Because of this, models will generally be narrower, more full ended and deeper than a real size vessel. Additionally, I believe form stability also does not scale in the same way as the rest of the boat necessitating more ballast on a deeper fin to allow the sail area to reach full hull speed.

Scaling down Bob's CF cutters would probably work better than most boats because those have an exceptional ballast ratio but I'm pretty sure it's still not 69%.

 
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SemiSalt

Super Anarchist
7,815
294
WLIS
Designers have used models for a long time, of course. It started getting scientific in the late 1800s, and real precise in the 20th century. Now the computer models are the hot ticket.

I suspect that towed models and tank tests are best at comparing two similar designs, not so good at comparing two very different boats. And the degree of precision required for race boats is very high, maybe better than the model tests can deliver, so the designer can be fooled. See: Brit Chance.

 
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Ease the sheet.

ignoring stupid people is easy
20,349
2,342
I think because you don't have material restrictions in the same way with a model boat the proprttions differ to take advantage of the fact you can go for deeper draft and larger sail area. This in turn requires a hull shape with different geometry to account for the higher amount of power (hence the hull runs at a higher speed to length ratio than a scaled down big boat with proportionally smaller rig and lower draft). Because of this, models will generally be narrower, more full ended and deeper than a real size vessel. Additionally, I believe form stability also does not scale in the same way as the rest of the boat necessitating more ballast on a deeper fin to allow the sail area to reach full hull speed.

Scaling down Bob's CF cutters would probably work better than most boats because those have an exceptional ballast ratio but I'm pretty sure it's still not 69%.
Things like wetted surface area get weird when we get to smaller boats too.

I remember reading illingworth and he mentioned the need for smaller boats having to have proportionally greater sail area than larger boats.

Of course, there are more knowledgeable people here that can explain it.

 

Rantifarian

Rantifarian
You can use dimensional (dimensionless?) analysis as a way to start comparing how dimensions scale. It is a really interesting tool when you start making sense of it, and very useful in real.world investigations.

Should be covered during one of your fluids courses later on

 

willp14335

Member
408
98
Anacortes WA
great thread.

brilliant work, Will.

I'd be interested in hearing more on your thoughts as to the benefits of an immersed transom.
My original thoughts regarding the transom were to design something with the lowest possible drag so I put in a slight overhang. Looking at the boats now though, I think in anything but the lightest air the flatter buttock run aft with a slight immersion could potentially be faster if one was to max out the allowable sail area. I think I'd try it with the transom just touching the water with lots of flare aft to avoid a big corner of the transom dragging when the boat heels rather than the slight overhang I had on this boat. I think it could potentially allow for a slightly better max speed as long as it doesn't effect pointing too badly.
I also think a little more bow knuckle would have been better on this design. She made a smaller bow wave trimming by the stern with the stem out of the water rather than in.

On the note of scaling, I read the owner of Mirabella V has three RC models of the yacht on board. Who wants to bet the keel is deeper proportionally than the real yacht?

Mirabella V, Athena and Maltese Falcon Comparison.jpg

 
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A

Amati

Guest
When fooling around with my RC's:

They sail upright only in very light airs. Double ended low prismatic would be a happy condition here?

Otherwise, over on their ear. Higher prismatic here? Immersed stern might start to pay off, but maybe a clean release is good too? Does your class plane? Forced mode?

While round bilges are nice for wetted surface (kind of), square (ish, even) is better for wave making. But as as they tip, wave resistance does goes go up. As much as rounder bilges or a round hull :unsure: ? As a chined hull tips over, it can get kind of like a v hull, so what kind of flare? (Did Pandora's box have chines? Or was it more round? )

Look at the last skinny low rider Moth designs. Pretty good in light air. I think that's why there are lower wind limitations for racing the foilers? You might check out International Canoe designs too. The Maas hulls kill in medium to heavy air. Not so much in the light. And of course, the open RC classes. Heck, any open class-

(What do you think?{ _} Or not.... :) { _} Check one please )

Just $.02 from an 'idiot with a computer' (tm!) who who has built some sailing canoes, a few RC's, and enough windsurfers to develop sensitivities to epoxy. :blink: You're on a neat adventure. Remember to check the safety sheets on all the chemicals you're using!

 
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JustaBoat

Anarchist
"I think because you don't have material restrictions in the same way with a model boat the proportions differ to take advantage of the fact you can go for deeper draft and larger sail area. This in turn requires a hull shape with different geometry to account for the higher amount of power (hence the hull runs at a higher speed to length ratio than a scaled down big boat with proportionally smaller rig and lower draft). Because of this, models will generally be narrower, more full ended and deeper than a real size vessel. Additionally, I believe form stability also does not scale in the same way as the rest of the boat necessitating more ballast on a deeper fin to allow the sail area to reach full hull speed.

Scaling down Bob's CF cutters would probably work better than most boats because those have an exceptional ballast ratio but I'm pretty sure it's still not 69%."


---Damn!!, answered in a very clear can concise way. Damn. This is what working with RP can do for your intellect? Sign me up... Maybe there should be a RP institute of naval arch, audio and music. Oh, not to mention the RP Culinary Institute. Did I forget one?
 

willp14335

Member
408
98
Anacortes WA
When fooling around with my RC's:

They sail upright only in very light airs. Double ended low prismatic would be a happy condition here?

Otherwise, over on their ear. Higher prismatic here? Immersed stern might start to pay off, but maybe a clean release is good too? Does your class plane? Forced mode?

While round bilges are nice for wetted surface (kind of), square (ish, even) is better for wave making. But as as they tip, wave resistance does goes go up. As much as rounder bilges or a round hull :unsure: ? As a chined hull tips over, it can get kind of like a v hull, so what kind of flare? (Did Pandora's box have chines? Or was it more round? )

Look at the last skinny low rider Moth designs. Pretty good in light air. I think that's why there are lower wind limitations for racing the foilers? You might check out International Canoe designs too. The Maas hulls kill in medium to heavy air. Not so much in the light. And of course, the open RC classes. Heck, any open class-

(What do you think?{ _} Or not.... :) { _} Check one please )

Just $.02 from an 'idiot with a computer' (tm!) who who has built some sailing canoes, a few RC's, and enough windsurfers to develop sensitivities to epoxy. :blink: You're on a neat adventure. Remember to check the safety sheets on all the chemicals you're using!
These boats are too heavy to plane but my bigger boat will in the right wind. The next design I hope to build someday is 125 cm so that should plane easily with a light displacement and large sail area.

 

Bob Perry

Super Anarchist
31,943
1,338
B/D for the cutters is 43%. Could be 45% if I used a light ship displ.

The BP school of yacht design closed when Will graduated. I have had maybe a dozen requests since Will but he raised the bar and I think I'll just let it end there on a high "C'. ( the musical note, not the grade)

 

Oregonarchist

Anarchist
751
0
Oregon, USA
great thread.

brilliant work, Will.

I'd be interested in hearing more on your thoughts as to the benefits of an immersed transom.


My original thoughts regarding the transom were to design something with the lowest possible drag so I put in a slight overhang. Looking at the boats now though, I think in anything but the lightest air the flatter buttock run aft with a slight immersion could potentially be faster if one was to max out the allowable sail area. I think I'd try it with the transom just touching the water with lots of flare aft to avoid a big corner of the transom dragging when the boat heels rather than the slight overhang I had on this boat. I think it could potentially allow for a slightly better max speed as long as it doesn't effect pointing too badly.
Strikes me as a question of wind (boat) speed; your original design would be faster in lower winds, but once powered up the clean release of an immersed transom, even at sub-planing speeds, would be faster. I like your idea of the transom just touching the water; perhaps you could set the hull trim the transom higher for low winds, and lower for fully powered-up conditions?

I also think a little more bow knuckle would have been better on this design. She made a smaller bow wave trimming by the stern with the stem out of the water rather than in.
I think you already mentioned this before, but what about going as far as a scow bow, which seem to have proven fast on minis (and scows, obviously...)? Seems to be the logical (extreme) extension of an above-water line knuckle and fullness in the ends.

On the note of scaling, I read the owner of Mirabella V has three RC models of the yacht on board. Who wants to bet the keel is deeper proportionally than the real yacht?
I'll bet it is...
 
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Steve Clark

Super Anarchist
I am particularly pleased by the absence of radio control.

I never had the time or patience for it myself, and paddling or rowing around after the model is more than half the fun.

RC can disguise flaws in balance.

Were you able to have articulated rudders or were you limited to fixed skegs?

SHC

 

Speng

Super Anarchist
4,988
13
Cincinnati, OH
One of the kids a couple years behind me (his dad's a famous French multi designer) designed a very close winded boat that auto-tacked on headers (he claims it wasn't on purpose) but of course it went upwind like a bat outta hell.

When you look at pictures of the boat you can see it is grossly disproportionate if scaled up. The same thing goes for my other model boat, 14" draft, 36" LOA.

My Cp was 0.55. I think I could have gone to 0.58 and been faster.
Yes most certainly. 0.55 is a decent Cp for something like a TP52 which I don't think is as "fast" (Froude number wise) as a model yacht

 

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