Thinner Footy?

Footy Class General Discussion (Footy)
61

as promised some photos I need to do a bit of mild bondo work. it got too hot while I was warming it…and it wrinkled on the port side near the forward cross member
the black rectangle sin side are just some old velcro from various battery and Rx holdowns

loa 12.3"
max beam 4.5"

since it uses the same amount of plastic it should weigh the same as a v-12 hull. but I have no idea what that weight is.I used 1/4" carbon/nomex for the transom and the rudder blocks. fits in the box corner to corner level to the top.

as I said, has a sort of “speed boat profile”

62

Bill,

Your hull looks great. I’m looking forward to hearing about your progress. Be careful, foam has some very addictive qualities. :slight_smile: The freedom to do a complex shape like yours in a very short period of time keeps me coming back.

Please post if you discover any tricks for working with it as you go along. Butch Bragg’s tip about cutting holes for mast tubes and rudder posts etc. with a sharpened tube is a great example of a simple solution that would never have occurred to me.

I think some great hull development will come from the ease of working with foam.

Take care,

Brent

63

Thanks, Brent.

One thing I think I’d do differently next time would be to pull the paper plan off the foam sections before gluing them together. I left the paper on as an aid to aligning the sections…which worked well and saved me from having to draw more alignment marks. I found that the paper sanded a bit differently from the foam, though…so the paper between sections stands a little proud of the foam surface. Maybe that can be corrected depending on the finish I use…or maybe I’ll have to claim that it’s a revolutionary technique to reduce hull drag!

Bill H

64

I have used Dap lighweight spackling with decent success as a filler on my hulls, though anyone who has seen them at a distance of less than 5 feet would attest to the fact that they are more go than show… Once it dries it sands well enough and doesn’t weigh too much. Unlike the foam it does need some form of sealer before sailing though.

The initial drive to do BC3 at 2"/50mm beam was because I could do it from a single block of pink foam. I have since been the recipient of some 3" thick foam which would allow a more Rubinesque hull design…or 2 BC4 1.5"/38mm hulls from the same block.:wink:

Take care,

Brent

65

my heat treated v-12, weighs in at a portly 65grams, thast bondo’d primered and sanded…:eek: that does include the rudder trunk and tube(but thats all carbon…it was 58grams prior to the bondo activity

my only problem with the narrow hulls is you start running out of room for the sail control arm…are you going tripple purchase on these??? with a bermuda or swing rig, you need a bit more sheeting…than on a Mc Rig…

66

Why not put the servos on a cassette??
Rgds
AndyT

67

Hi Andy

Can you clarify your "Cassette comment?

68

Marc,

I have tried two options so far to deal with the lack of space for a servo arm.

  1. Run an exposed servo arm that sticks over the side when the sails are out as in running or broad reaching. As Angus has wisely reminded us you want the servo arm in line with the load when you are beating, as this is when the servo is under the most load.

  2. The length of the servo arm required is directly proportional to the distance from your sheet fairlead and attachment point to the pivot point of the rig (for a swing or McRig) or the boom/club pivot (for a conventional rig). I have successfully used servo arms as short at 1-1/2"/38mm to allow full 180degree rotation of my swing rig. I just had to make sure the fairlead/attachment point were sufficiently close to the rig pivot point.

For light air it might be better to use a shorter arm and associated short pivot as the load on the sheet itself will be greater. This should help the rig to pull the line through the fairlead/servo arm more easily under those conditions. In heavy air the added friction will not be desirable.

Take care,

Brent
“Life is too short to sail fat boats.” Anon.

69

Bill,

Thanks for posting your pics of the foam sandwich - looks very interesting. Could you tell us what you glued up the slices with? Reason for asking is that it has clearly sanded well

Are you planning to sail the foam, or wrap it in something and take away the foam you first thought of?
I think we are generally agreed that weight is important, but not crucial (if that doesn’t start an argument I will be surprised)
When I reached this stage with Voortrekker (a foam plug) I had great hopes that I could make a light hull using galss fibre and acrlyic varnish as the bonding resin - it worked and was clean, but produced a “soft, floppy” shell which weighed nothing but was not workable.
Perhaps I should have persevered and made a modern version of papie mache

the acrylic shell is the RH one - I was looking at it last night - and wondering
andrew

70

Hollowing foam
I have made many aircraft using hollowed foams of various types.
It is normally recommended to hollow foam parts using a dremel and sanding drum - this produces serious dust, which shouldn’t be breathed (cos its dust, not because of the material its made of)
I have found that a loop of copper wire (salvaged out of a house power cable) formed into a loop and held on the bit of a soldering iron with a twist of wire does a good job of removing either most of the foam or places to fit servos, batteries, etc. (While working in Hong Kong with no tools I heated a bit of coat-hanger wire in a gas flame - worked well!)
Don’t breathe the fumes - because they are fumes and acrid
andrew

71

brent,

thanks. it just seems that the desirable locations for the fairlead happens to be where the hatch cover is…

I have not given much though to putting the fairlead on the bow and controlling the sail from the forward half of the swing rig, as opposed to the Main sail side.

But my concern using the bow is the propensity for the footy’s to dive…

My plan is to use the rectangle batty pack. 2x2 stacked this gives me space on the sides fro the rudder servo and rudder linkage on one side. and put the sail servo in front of the batty and just behind the keel and run my sheets on the other side and then out the stern end of the boat.

72

Andrew, I’m on the road…so can’t look at the details on the can until home Saturday. I think it’s a 3M product…a spray glue kind of like rubber cement. If you spray one surface, the pieces can be separated…it makes a tack, but not permanent join. If you spray both surfaces, make sure you put them together correctly, because it’s a permanent bond.

Bill H

73

Marc,

This is not an ideal solution but before I switched to above deck sail arms I ran the sheet out the back near one corner of the deck and then forward to a bow mounted fairlead. The advantage is no hole in the foredeck, the downside is one extra turn for the sheet.

I’ve always sheeted forward on a swing rig because I can use the last bit of sheet trim to tilt the rig forward if I need it. Having your sail arm mounted above the deck makes a bow fairlead simple to use. I typically mount the sail servo just behind and to one side of the mast. The arm is athwartship for running, nearly fore and aft when close hauled.

Take care,

Brent

74

Andrew, I’ve been thinking about whether I’m going to sail the foam or make a shell…and I think the answer might be “yes”

If the foam works well as a boat, I might take a hull from it…maybe even use it as a plug for a mold so I could make some for friends…but I like the idea of being able to test the boat without going through all that work.

I might not bother if I don’t want to duplicate the hull, provided the weight comes down enough when I cut out places for electronics. Although I agree that weight is not crucial, that’s in the context of the tradeoffs between light weight, strength to hold up in regatta situations, and performance. Pursuit of light weight at the expense of other important factors doesn’t make sense to me. One thing boat design quickly teaches us is that there’s no free lunch!

Could the acrylic shell for Voortekker be resurrected by some foam reinforcements?

Bill H

75

Let’s not forget to include positive flotation when considering important factors regarding whether it’s worthwhile trying to save weight by gouging out a foam Footy hull!

Regards,
Bill Nielsen
Oakland Park, FL USA

76

Bill H wrote:

“I might not bother if I don’t want to duplicate the hull, provided the weight comes down enough when I cut out places for electronics.”

This statement leaves me wondering. What weights are people building hulls to, including deck, mast step, keel box, rudder support and radio mounts?

Bill, I’m guessing your hull won’t be much over 45 grams if you use aluminum for the mast step and rudder tube. By the time you cut out for the RC gear it may be less. My hulls, being narrower, are down in the 30-35 gram range. Is this heavy compared to other techniques?

Thanks in advance for any feedback.

Take care,

Brent

77

Okay a new question to show my ignorance.

What is the purpose of rocker in a sailboat hull. What conditions demand more or less? What behavior does it provide or prevent?

Angus, Flavio, Niel…anyone with design experience please enlighten me.

Take care,

Brent

78

Dear Brent, a good question is sometimes better than a good answer…:wink:

My opinion has been required about two or three points relevant to “scientific” footy model design

So, I will try to express my point of view - as far as possible - in an easy and clear way

1 - narrow versus broad hull : which one is the best ?

1.1 - computer calculations

I have tried to apply naval architecture method to estimate hull drag of our little models
I have used an “indirect” method ( professionals call it regression ) to guess drag based on hull main parameters ( example : lenght,beam,draft, displacement and so on ).
This is not a true measurement ( like a towing tank test ) but is ONLY a useful design tool to give a reliable preliminary estimation of drag.
I have modified frictional drag equations in order to take in account footy small size and speed ( laminar flow )
Even if a full agreement with full size test of footy has not been achieved ( yet ) I feel confident that comparison between different choices is reliable enough ( winner boat is the best on water and on computer screen )
A good confirm of the method is that both my “folgore” ( based on calculation) and the well known bobabout2 of brett mcCormack ( based on result of more than 30 different models ) have similar dimensions.

1.2 how to compare apple with oranges ?
to start with I have carried out Ia methodic comparison between hulls with :

1.2.1 same displacement

-same waterline lenght ( 300 mm )
-same displacement ( 500 gr )
-different beam ( ranging from 150 mm, down to 80 mm)

speed has been increased from almost zero up to about 1.5 kts

all boats are with same longitudinal distribution of volume ( same prismatic coefficient )

  • narrow hull are deeper, beamier have a smaller draft -

narrow hull is winner on all speed range

1.2.2 same initial stability ( righting moment at no heel )

due the fact that reduced beam means reduced stability too, I have done a second comparison between hull of :
-same LWL
-same initial stability ( in other word, narrower hull must be heavier to have same righting moment a zero angle of heel )

narrower hull is the winner again, bat the gap between thin and beamy is smaller

1.2.3 same righting moment at given heel angle

due to the fact that when going upwind sailing boats - usually - are heeled, I have carried out a more complicated comparison between hulls with a 30° heel angle .
More details are needed to have a full picture of the situation ( for example beam on deck, and fin keel depth )
In this case , to compare apples with oranges, I have compared different hulls with same righting moment at same heel.

same heeling moment means that this is a race between boat with same rig ( or, at least same area and center of effort heeling arm )

the winner will be tha desing with minimum drag

Once again the thinner footy is the winner on all speed ranges

Both because no heel drag curve is an efficient one, as well due to side effect that additional drag due to heeling for thinner hull is much less than beamy and shallow boats

1.3 Diagonal boats

a comparison between a “standard” hull and a diagonal one ( few centimeters longer ) is a bit more complicated.

keeping constant the righting moment, longer hull have more wetted surface, but greater “hull speed limit”.

Results are very interesting because drag curves are x crossed ,to put it simple :

  • below one knot, standard hull is the best one
  • above one knot, diagonal hull is the best

+++

Conclusions :

even if my calculations are far from perfect, I have used all available engineering knowledge to perform a systematic study

current methods are not reliable enough to investigate very radical, or very unusual hull shapes, and for this reason the possibility of something “alien” is still to be considered as an open door ( planing ? submarine ? catamaran ? )

up to now I have not investigated beams under 80 mm on waterline
I will try to drop down to 40 mm, but reliability of results will be reduced too

hull beam is not everything
a winner should be also a boat a seaworthy and steerable model, capable to tack, jibe and race within a “good” range of wind speeds

a thinner hull seems to be the best ( thin vs broad ) from pure hydrodinamic point of view
a diagonal hull seems to be the best ( long vs short ) at high relative speeds

Flavio

Folgore ITA 5
Presto ITA 13 ( coming soon )

PS next time : few words about rocker and hull shape

79

Flavio

Thank you for your very clear laying out of the field and application of as much science as seems to be possible - I enjoyed reading it and Think that I understood it.

I do drive a narrrow boat - Angus’s Voortrekker (VT) - 80mm wide and very close to a foot (not a diagonal boat) and have noticed (I believe) a habit that it has and I wonder if what I observe is A)true and B)explainable

I think that VT mine and Mike v d Peet’s have the same habit of being fast when they are fast, and stopping sharply when (for any reason) the drive disappears. I compare it to sailing a big catamaran or a windsurfer - great speed and almost no inertia.

Do you think I am sailing VT too light - and losing momentum, or too heavy so the immersed hull stops quickly, or badly (change the nut behind the sticks)

You mentioned, towards the end, submarines! What a completely mad and impossible idea! This is a narrow thread so may I introduce you to ZBF - she is 40mm wide and is intended not to touch the surface at all

It is my firm belief that there is a footy spectrum of narrowness with dimensions of practicality, charm and winning ability. If we could locate Folgore at one pole of this spectrum, then ZBF will be found at the other:D

I aim to complete Sloice (one AA cell wide) in the near future, but I will add “bustles” to adjust the buoyancy so the overall width will be greater - perhaps as much as 25mm.
andrew

80

Flavio, thank you, as ever, for your masterly analysis!

Might I add a couple of thoughts.

  1. Since it is much easier to produce sea-kindly narrow hulls that do not change shape very much when heeled than it is with a wide hull, the maximum effective angle of heel of a narrow boat is propably higher than that of a wide one. In other words, if a fat hull has to start dumping heeling force and drive at (say) 25 degrees, a thin one can hang on up to (say) 30 degrees before out of balance forces and excessive induced drag outweight any advantage gained from ‘pressing’ the boat.

  2. As you are aware, but some may not, frictional drag makes up a much higher proportion of drag at medium speeds in a Footy than it does in a larger boat (cube-square relationship). It is therefore very important to take great care to reduce the canoe body wetted area in a diagonal boat. Easter Saturday has not been a runaway success: at this oint this may reasonably be ascribed to her never having been raced seriously with any sort of sensible fore-and-aft trim. After quite careful analysis of winf conditions in Florida (including WHEN people race, which seems to be important, ES has a finer-ended hull with more rcker and sections that are closer to a semi-circle throughout her length.

  3. I have for sme time being contemplating a diagonal boat with a comparatively long U-sectioned bow overhang with a plumb stem. This should reduce the wetted surface when the boat is upright but turn into useful length as she heels. It might possibly be combined with a 12 m type ‘chin’. A useful side effect would probably be to move the (static) centre of flotation further aft, therby increasing the moment required to trim 1 mm by the head.

Any thoughts?

:zbeer::graduate: