Hull Design

Rcsailing.net General Discussion
121

The boat is looking really good.

Now for my rude questions as a curious fellow builder.

Looking at the aft deck, the cockpit sides appear to be balsa. If so, are they glassed over? Both sides? With what weight? (My entire aft deck is 1/16th balsa sandwhiched in light glass on both sides).

The rest of the deck looks to be ply - what thickness did you use? (I used 1/16 aircraft ply for my foredeck, but probably could have gone lighter)

Have you weighed the hull and decks? What is the weight?

What have you used as the screw-top hatch? It looks good.

Will your mast be stepped on the cockpit sole, or through the deck to the keelson?

Have you made the keel fin - or did you purchase an professionally made fin?

It’s hard to tell from the picture, but is that framing under the foredeck the support for the sail servo? If so, what are you using?

122

Muzza,

You are correct. The deck sides are balsa. 1/16 and the rest is Plywood 1/16. All the deck is laminated with light FG on the top only with the exception of the areas of high stress which have small pieces of plywood plus FG lamination. The nuts and washers for the spreders and other parts, are glassed inplace.

I have not weighed the hull. I’m waiting because I need to do some more sanding.

The screw top hatch was a great find. I went to Micheals (craft store) with my wife and there they were for about $1.50. Luckly it fits perfectly in the space. They only had this size.

The mast will be step on the deck. I reenforced the forwart part of the keel box to support the mast. This will give me about 3cm for the mast to be moved.

I made the fin my self. I used 1/16 plywood sandwiched between Unidirectional CF. Then 1/16 of balsa on both sides and another sandwich of CF cloth. It think the waight was good and it is very stiff. When I lift the bulb with the fin there is minimal bend.

For the Sail servo support. I just used hard wood placks glued together with CA and then resin. I might have to add a little thin FG to areas were the framing touches the hull.

I hope this helps.

123

Very similar approach to my own.

A couple of thoughts, and you may have them covered already, I build my servo supports in two parts - the part that must be attached to the hull (actually four small “shelves” on my boat), and a plate that is screws (with very small brass screws) into the shelves. Thus if I need to change my servo to a different brand, size, shape, or even reposition it slightly athwartships, I just have to remove the plate and reuse the built in shelf. I’d rather not fix any support to the hull proper - but rather to the mast/keel box and main bulkhead. Unfortunately I could not do this effectively given that fact the I use a Hitec digital servo/sailarm forward of the bulkhead.

Secondly - in order to get the sailarm in the right position relative to both the servo and the through-the-bulkhead sheet fairlead, I ended up positioning my servo at an angle. Thus the pivot point of the servo sits off the centerline, but I still have at some some of the weight of the servo across the centerline.

When the sailarm is in the close-hauled position, it sits a little under 170 degrees away from the bulkhead fairlead. I can program the servo to go further if I need to, but this is enough to take a lot of the load of the servo to windward.

My batteries are on the centerline, but my rudder servo is offset on the otherside in an effort to correct some of the weight imbalance (pretty ineffective as even a standard rudder servo is relatively light compared to the sail servo). Additional weight to bring the boat up to 4kg with the A rig is also offset to correct the difference. In a lighter boat this would be all that is needed, but mine is a little heavy and so not much weight needs to be added with the A rig. The next one will be much lighter.

I sealed all exposed balsa inside with an epoxy sealer (Everdure) which certainly added weight. I sealed all exposed plywood inside with thinned down one pot varnish - also added weight. Two of my other models do not have any form of sealing of bare timber within the hull.

124

Muzza,

I’m glad some one has a set up similar to mine.

I have two winches an arm and a drum. The opening on the mounting board fits both. Although I’m planning to use the drum. and the placement is centered on the boat.

I tried to balance the sail which with my batteries and rubber servo. I still have a little room to play with the placement of them. I can eather place the batteries infron or behind the rudder serbo and both are center on the center line of the boat.

As far as water proofing, I have not had water intrussion problems before so I desided not to seal the inside.

This is the first of two hull I’m building from the plug I made. This one is my test one. I’ll try to balance all the parts.

125

I was experimenting (mostly unsuccessfully) with cold-molding when I read Dick Lemke’s posting about using self-stick veneer. I did it a little differently; three layers: 5 oz kevlar, 1/64 cherry veneer, 2 oz glass. Came out at 9 3/4 oz for a hull 36 in LOA and 8.75 in beam, which is plenty good for a hull this tough.

The veneer adhesive, 3M 200MP, is pretty impressive stuff, waterproof and very strong.

You could probably shave an ounce or two off this with lighter kevlar, but this is for a free sail boat that has to hold on to her 10 lb keel in a full-speed collision with the side of Spreckles Lake :slight_smile:

The three pictures show 1) the kevlar layer stretched and taped over the mold, and given a coat of Aerospace Composites epoxy 2) the diagonal “planks” partly laid, and 3) the finished hull prior to wet sanding.

Cheers,

Earl

126

Thas a great looking hull Earl, makes me want to put down the metal working tools for a while, and break out the wood working tools. :cool:

127

Hi Earl -

that is going to be a one fine-lookin’ yacht!

My acknowledgments on the build. I hadn’t thought about narrower strips. They would lend themsleves to a bit more adjustments to take out any seam holes. I look forrward to any photos of the completed boat.

128

Thanks, guys :slight_smile: The “concertinaed” Rip Tide hull is pretty complex; the planks had to be tapered along the middle 1/3 of the hull because the angle at which a plank lays flat in the afterbody is significantly different from the angle at which they lay flat in the forebody. A full “blood and gore” description will appear in the next issue of the USVMYG newsletter.

Cheers,

Earl

129

Here is the final result. The boat is ready to sail. The boat hull is meets all rules but the rig does not. I used CF to build it. I would like to thank everyone for all the help.

The last thing I got to do is give the hull one more rub. I got a little dirty when I was attaching the bow bumper.

I sailed it in a small pond near my house. I was very pleased. It tracks very well. I don’t need go make any corrections with the rudder in a closed hulled. It seams to be much more responsive than my Mistral. It accelarates quicker. When I sailed it, the was wind was changing all the time. I can’t wait to take it to a larger pond.

I weighted the boat. This included all electonics and rig (ready to sail). The total came down to 4.35 Kg. I was over my target but I think it will do.

130

very interesting forum. I also have a great interest in designing an IOM that can cope with gusty conditions without savagely "rounding up " I have noted all the discussion about getting the LCB and LCF close together, not only at upright but also having them stay close together when heeled . I have been able to get Maxsurf to make a design that gives good results for the positions when upright but cannot find how to check them when heeled . Can you do this in Maxsurf ? If not how do you do this ? I can see how you would do it in Hullform but cannot get a model from Maxsurf into Hullform. Have tried DXF but it doesnt want to work . Any clues ?

Thanks

John

131

Hi John.

You can do it with Hydromax from maxsurf.

132

I’ve been corresponding with Eric Sponberg, a naval architect and designer of full size yachts, about the Turner analysis and in particular about my conjecture that Turner’s method boils down to a fancy way of getting the Longitudinal Center of Flotation (LCF) to coincide with the Longitudinal Center of Buoyancy (LCB). Here’s what he had to say:

I agree with you that the evidence in your text boils down to a round-about way of looking at LCF and LCB. I had learned a long time ago that L. Francis Herreshoff had designed boats that, when heeled, the LCF would stay in the same fore/aft position or move forward slightly. The reasoning is that as the boat heels, it necessarily picks up displacement, and since the weight of the boat does not change, the boat must rise out of the water slightly so that boat weight and displaced volume are equal (condition of floating equilibrium). If the LCF moves forward at heel, that means the forward waterplane, and likely the hull sections, are fuller, and so the bow tends to rise. This is borne out by your discussion in your paper. The bow rising necessarily points itself more to windward, the direction the boat wants to go. This rotates the hull sufficiently to ensure the proper angle of attack to the keel to create lift to the windward side. This makes the aero and hydrodynamic forces complement one another.

If the LCF moves aft when heeled, then the stern waterlines are wider and sections fuller and so the stern rises to windward. This causes the bow to sink away from windward, away from the direction the boat wants to go, and at least reduces angle of attack on the keel. Taken to an extreme, the angle of attack can go to the other side of the keel so that lift is generated to leeward. Leeward lift tends to right the hull against the pressure of the wind, and so makes for an unruly boat and one that is harder to control. The aero and hydrodynamic forces are working against each other.

I daresay your continuing analysis of your models will show this kind of favorable movement of the LCF, as you surmise. Evaluating the position of LCF is far easier and quicker than going through a Turner analysis.

On my designs of late, I have been very careful to make sure that LCF maintains a good position at heel, being either in the same fore/aft position or slightly forward. My design Saint Barbara was launched this summer, and the owner reports that it is “silky smooth through the water” and “the smoothest boat I have ever sailed.” His previous boat was a J-105 which he thought of as one of the best built and sailing boats that he had had up to that point. Saint Barbara excels the J105 by a long shot.

Both “Rip Tide” and my ABQ 65 RG65 designs balance out according to Turner’s method, and their behavior in gusts bears out Eric’s analysis: they tend to pitch up and jump forward like they’re trying to leap out of the water.

The upright LCF is calculated by DelftShip, which calls it the “Waterplane Center of Flotation” Heeled LCF can be easily found by drawing the heeled LWL shape on a piece of cardboard, cutting it out, and seeing where it balances in the fore and aft direction.

Cheers,

Earl

133

Earl

Tried this out one one design iteration that I am working on and got an LCF shift forward of 12mm ( on an IOM one metre ) when heeled compared to upright . How much shift forward ( deemed to be a good thing by Sponberg in his quote) is good ? Obviously too much will cause violent bearing away to leeward which is not good. Seems to me that none to a little would be right , but how much is a little in % of DWL , and how much is too much I wonder ?

cheers

John

134

I don’t really know, and I don’t know of anybody who does for the size boats we’re talking about. My guess, and it’s just a guess, is that you’re close enough to build the boat and see how she does.

Cheers,

Earl

135

Earl

Just when I thought I might be getting a handle on this , I was looking on the Boat design forum and found a thread about "rounding up to weather in gusts " and what do I find but

[i]Originally posted by Stephen Ditmore
I think the method referred to by Coozeman is the same as that discussed by designer Cyrus Hamlin in his book “Preliminary Design of Boats and Ships”, pages 195-197. His method is to calculate heeled LCF (center of the waterplane) at 30 deg (25 deg for flat bottomed/shoal draft hulls) without allowing the hull to retrim from static upright trim. He then takes the distance between the upright and heeled LCF’s as a percentage of waterline length. Hamlin recommends that the LCF should shift aft as the boat heels, but the shift should be less than 1% of DWL.

Cy Hamlin was a lecturer at The Landing School when I was there, and explained that his method was a simplified variation on the method referred to by Ian Ward involving wedges of sections. BTW thanks, Ian, for the reference concerning the term "metacentric shelf.

This is the exact opposite !!! seems hard to believe

I dont feel comfortable with their argument , as with models anyway it’s the ones that bury their bows in a gust that seem to have the problems , so I can see how the bow lift with a forward LCF shift would help and a rearward shift would cause to bow to sink in and make it worse. Regardless of any effect on the heading they don’t steer well with the stern in the air!

I have a boat which is a real dog at this sort of behaviour so I might see if I can recreate its lines in Maxsurf and then do the cardboard cut out heeled waterline trick and see what it’s LCF shift is ! If nothing else it will show me what not to do .

I made an error at the transom when doing the heeled waterline area plot. When I corrected this I got a forward shift of the LCF of 10mm or 1%

One percent of the DWL would seem about right as this is about what I have used when moving the keel to retrim and it gave a slight bow up trim , but nothing radical,so wouldn’t hurt at all to have the bow lift by that much on heeling .

I wonder if anyone has ever collected the hydrostatic data on various designs and compared the correlation between LCB, CF positions , the shift of these when heeled and the actual on the water sailing characteristics ?

As you say the next step is to build it and see.

cheers

John

136

Hi Earl

I’m sure Eric is a fine gent and highly knowledgeable and experienced. However, what he has to say here unfortunately makes little sense. A heeled boat always makes a positive angle of attack. If it didn’t, it wouldn’t sail any kind of course to windward. There is no sense in which, when the stern rises out of the water, the keel experiences a reduced angle of attack, never mind generating lift to leeward! The angle of attack of the keel (ie the leeway) remains more or less constant, as it must do if the boat is to keep a course to windward. The best source for this explanation is Ross Garrett’s “Symmetry of Sailing”.

137

Well, I’m not going to get in the middle of this :slight_smile: I think as a matter of practical advice to designers, what appears to be the case is:

a) heeled lcb close to upright lcb necessary but not sufficient

b) heeled lcf close to upright lcf probably necessary and sufficient

c) b often results as a side effect of a, but not always.

I’m going to do the lcf analysis on Satanita, the most notorious (and fatally) unbalanced hull in history. Will take a while, stay tuned.

Cheers,

Earl

138

Hmmmm
The plot thickens. The point I was trying to make Lester was that the two statements are completely opposite . Eric’s theory as to why, may or may not be correct , but the statement that a forward shift is beneficial is the bone of contention . The real issue here is which direction should the LCF move on heeling to stop rounding up - forward or aft?
We can worry about trying to reconcile the theory with the facts later.

My thoughts are that models will always be more sensitive to this than full size for various reasons , eg
much faster occurrence ,
helmsman can only see it happening not feel it so less warning
limited means of countering
wind speed variation in gusts much higher
more violent effect ( not many full size boats get regularly blown flat)
more variable wind direction ( lakes compared to open sea)
models are regularly overdriven

Empirical evidence would seem to suggest that the wide stern boats are less likely to have a forward shift , and also seem to be the worst offenders. To me this would tend to suggest that a forward shift would be the right direction.

I await Earl’s test on Satanita with interest.

The only consensus seems to be that zero, or very small shift can’t be wrong .

Anyone else have any thoughts on this?

John

139

Hi John

I think we do know that, as the wind rises, the rig CE must move forward to retain balance. This is what we do while sailing, by either raking or moving the mast forward. The implication, therefore, is that the boat’s centre of pressure should move aft with heel if the mast is to stay unchanged… Does that make sense? (I’m not too sure that centre of buoyancy enters into this, since the key measure is the lead, the gap between sail CE and hull CP…)

140

[QUOTE=Earl Boebert;47983]

I’m going to do the lcf analysis on Satanita, the most notorious (and fatally) unbalanced hull in history. Will take a while, stay tuned.{QUOTE]

Just as a matter of interest the first UK registered Marblehead K1, Gentle Ladye, was reputed to have been derived from the lines of Satanita. I have no record of her performance in competition, but she must have been a brute to free sail, especially in the Braine gear era. The boat is, I believe, still in existence and I saw her on the water a long time ago (1979?), but I recall very little of what she did, as I wasn’t then obsessed with older boats. I’m not sure who has her now, but it might be worth finding out and having another look at how she sails.

Russell