Old hobie cat rigging, new boat.

thanks, i do have the will and the skills, talking to eric will be my next step.

 

Good one Matt!

 

Matt; The C-scow is a good pospect and so is the M-20 scow which is about the same size as a C. The M-20 has a tunnel bottom and it will really fly. I once owned one of those and it is one of my all time favorite boats.

If you want to make things simple and still very fast, you can think of a plain old flat bottomed skiff. When correctly designed they can be startlingly fast, they can be built light, cheap, and quickly. A sharpie like design is sailed to windward with about 12 to 15 degrees of heel, whereupon the underbody becomes deliciously efficient. Standing up, as on reaches and runs, it will plane like a demon.

There are a jillion flat bottomed skiff designs out there. Some of them won't do what I just said they'd do. Look for one with the forefoot elevated a little above the waterline and the after part of the bottom going upward toward the transom in an almost straight line. The deepest part of the bottom should be a little foreward of amidships, and the widest part of the bottom somewhat aft of amidships. Viewed in plan, the forward section of the bottom should show very little curvature. Not a straight line but nearly so. A boat of this sort at about 20 feet LOA can weigh somewhere in the 250 pound range if you select materials carefully. Keep the boat as narrow as you dare.

The flattie will not be as quick in light air as something like a Thistle, C-scow, M-20, or similar. It won't be a dog either as long as you keep it light. One of the appeals of such a boat is that it does not look as fast as it is. In a breeze you can embarras some pretty good boats.

Now all you guys can attack me for having the audacity to suggest such a fundamental boat. I will remind you of the old time Sandbaggers, and similar hot rods. This boat is not likely to humiliate many FDs but it will go very well indeed. You can use a trapeze or a sliding plank if you like. That'll be exciting.

(Doug, dont be telling Matt to fit foils to it. Not yet anyway )

 

i've been in love with foils since i saw them and started reading about them, but this is just a project that i want to keep simple, i'd rather make a new boat than restore the trashy old hobie 16 hulls.

i'm getting most of what your saying, but would you mind dropping some links with photo's of your ideas? there are TONS of "flat bottom skiffs" and "sharpie's" out there

 

lets also keep in mind the high aspect of the hobie 16 main sail, this thing is definantly going to heel

 

Flat bottom sharpie?

Hi Matt, thought that I would just drop my 2 cents worth after the last poster (messabout) suggested a simple flat bottom skiff as narrow as you dare. I beg to disagree with this view, as apart from being simple to build, seems to have only disadvantages. The H16 rig is fairly tall and powerful, and will likely overpower this type of hull even in moderate winds, unless all the crew were hiked out on trapezes or planks.
Secondly, the bottom shape has less efficiency than either a slightly curved V bottom or a C class scow shape, and would not handle as well.
Thirdly if you are seeking a hull shape with sufficient RM to resist the sail forces of the size of the H16 the scow shape which is truly designed to be sailed heeled upwind, (and I dispute that the sharpie skiff is), will work better in higher wind strengths.
Fourthly, in lighter winds, the M scow shape (if it has a tunnel bottom) will plane a lot earlier than the sharpie skiff. Nothing wrong with the C class scow shape either for early planing. I still think that the Moth lines that I refered you to earlier on, is a good basis for say a 20 ft. scow.
Best of luck with your final decision.

 

Doh! i didnt see that pdf until just now, looks really slick! thanks frosh.

 

another update, check my user gallery for an image of the HUGE mast upright with an outline for a size comparison ha 20ft LOA, hahaha OMG ITS ALL SAIL!

 

sail size

Hi Matt, I just looked up the Hobie web site, and if you have the full size rig, then it is around 210 sq. ft. in area. This should work fine on a scow approx. 20 ft. length by around 7 ft. beam. I also had a look at the scantling rules for the 20 ft. C-class scow and they seem very thick and heavy suggesting that they were drawn up a long time ago. With newer materials and techniques a lot of weight can come off.
I also noticed that the E-scow (38 ft. version) has been clocked at over 30 knots, which is a fantastic speed, probably as good as the Tornado cat can acheive under optimum conditions.

 

aye, i hear ya, 38 feet is a LOT of boat for the first time river project

 

Frosh:
I Respect your gentlemanly disagreemnt with my Flattie scheme. The thoughts were to have a new boatbuilder keep things as simple and practical as possible. Cost considerations, building time, and general utility are worth some attention. The 20 foot flat skiff can easily accomodate 4 or 5 passengers which Matt says he may sometimes have. A scow of the C or M type is not so commodious. Incidentally the 38 foot scow is called an A-Scow and they are indeed tremendous performers. The E is smaller at around 28 feet. The E is also a go fast machine.

A most interesting exercise can be done on behalf of flat bottomed skiffs or any other body form for that matter. Try this..... Lay out a body plan for a simple three panel boat. Draw in a waterline. Examine the underwater sections. Find the centroids (center of area) of each section. When the boat is upright the centroids will lie on a straight line and the plan view of the waterline will be sort of boxy. If the forefoot is below the WL the box will be pointed in front. Not the way to do it. Better to have the forefoot up. Now heel the boat to about five degrees and examine the centroids. Notice that the foreward and after sections will have the centroids displaced to leeward. Lee helm will result. It should be obvious that the boat is displacing water to leeward in foreward sections and requiring the water to come back toward the centerline near the mid ships sections then back to leeward in the aft sections. A condition to be avoided I believe. Now continue heeling the body plan and locating the individual centroids. Notice that the centroid points begin to align themselves. At somewhere around 15 degrees there will be perfect alignment. Moreover the plan view of the waterline will appear as a very elegantly streamlined narrow shape. The waterline is also lengthened as you can readily see. At the same time wetted surface will have diminished nicely. When in this attitude the underbody presents a somewhat larger lateral area that is useful. When these things are working right you can raise the the centerplate about a third without sacrificing to leeway. In this attitude the flattie is an impressive performer. Continuing to heel toward larger and larger angles the centroids at the ends will begin to move to the windward side causing serious weather helm. Indeed the flattie has an envelope of best sailing angles. So do boats of other configurations.

A vee bottomed or rounded bottom behave in quite a different manner. Do up a body plan for either or both those types and you will probably be persuaded that zero heel is the better plan. We are not talking keel boats here. To be sure some of the big boats are purposefully designed to optimize centroid positioning and waterline length when in a heeled attitude. Small boats can be designed to accomplish the same goals. As a matter of fact nearly all the inland lakes scows use that premise. They have optimum heel angles. That is why most of them have twin bilgeboards and twin rudders that are angled outward at the bottoms. The downside of the simple three panel construct is that the sharp chines make more eddies and surely more drag when sailed upright. Unfortunately, one can not have it all. Who can refute the claim that a flat bottom will plane more readily than a vee or round bottom ?

As for righting moment, square chines contribute appreciably more than round or beveled chines. A flattie with a 48 inch bottom beam will generate well over 600 foot pounds of righting moment while heeled to 15 degrees and displacing about 500 pounds. Two or three hearty fellows hiking in the typically strenuous way may well increase RM beyond four figures.

 

Hi messabout, some of what you say is is true particularly the first bit of your argument. However I believe that you are suggesting a shape like the 1911 year of origin Star Keelboat. Not too bad a shape building wise and for righting moment, but importantly only good at displacement sailing when heeled. The wetted surface and form drag of the flat sharpie shape is fairly high when heeled. In recent times light high power to weight centreboarders can plane to windward, and this is what I have had in mind with the scow hull shape if built at light weight, and crew hiked well out, preferably with one on trapeze or even two if sailed three up in moderate to fresh winds.

 

Frosh:
I think that you'd be a genuinely good adversary in a debate about the relative merits of various floating objects. I wish I could buy you a few pints and have a friendly go at that subject or whatever else we may choose to examine.

A Star boat in my mind is a very well developed, elegant tub. (I hope there are no Star boat types reading this) The Star is by no means a Flattie skiff although it has flat paneled skin components. It has A slightly convex bottom, a weighted keel, and too much rocker for a go fast boat. It is indeed a displacement hull and is sometimes capable of surfing but not planing. The 110 is another example of displacement boat, cum keel, that is flat paneled. It is a double ender that is quite narrow. A displacement boat too, albeit a pretty fast one.

If you have a basic computer program that will do flat panel boats then do the heeled condition research. In fact the hard chine boat at heel angles of more than about 12 degrees has very impressively small wetted surface. It is difficult to get a light weight boat to do much better than ten or eleven pounds per square foot of wetted surface. The flattie will easily exceed 13 pounds per square foot in heeled attitude. Heavily loaded boats will do much beter with weight to wet surface ratio, but heavy boats are not generally considered fast boats. Imagine an underwater shape similar to that of the old Piver designs. Piver trimarans such as Nugget. They were pretty slick in their day. A good skiff can duplicate that shape and aspect ratio. They will not likely plane to windward but neither will a C or M scow despite the lies of their afficianados. As a former scow fiend I know first hand about these things. The M20 has a 27 foot mast, big jib, and spinnaker. It was plenty fast to windward but did not actually plane in the usual sense of the word.

I reckon a 49er will plane in any direction but then that is a pretty wild and perilously over canvassed boat. I reckon some or all of your Aussie 18s might do that too. An anvil will plane to windward when you put a square kilometer of sail on like the Aussies do. Wet surface be damned. Any way wet surface is a primary concern when sailing at or below hull speed but of diminishing consequence when going like a bandit.

Matt: Forgive me for hijacking this thread to indulge in an argument with a down under friend. There is actually some value in arguments of this sort, because it exposes you to various schools of thought and occasionally some technical aspects that you may wish to apply. I'll try to contain myself henceforth.

Here is another idea. Dial up the Y-Flyer web site. The Y is a scow that is built of plywood. It is not as big as the boat we are looking at here, but it could be scaled up a bit to do the job. Ys are pretty lively but they have an unrealistic weight rule that makes them less hot rod and more family boat. They need not be built as heavy as the class rules specify. It's a fairly easy build, and it makes a good looking boat too.

 

Matt: Forgive me also for hijacking this thread a little, but I must make a response to messabout re his last posting. However something good might come out of the debate.
Messabout, I do not have a computer program that will do 3 D analysis of boat hulls, however the problem under discussion is really too simplistic to require a computer at all.
I have in the past studied underwater sections for racing kayaks which being human driven are very critical for even the slightest improvements in drag components.
This situation of the flattie is absolutely comparable to the kayak situation.
Ignoring stability issues the slowest cross section would be a simple V shape as this would have the greatest surface wetted area. The lowest surface area cross section is a semi-circle however this is not the lowest drag shape at sub-planing speeds, but it is not far off. My understanding is that for the kayak the best low drag shape is semi-circle but decreased in radius with flared out topsides.
My understanding of the shape you suggest, and you also say that the flattie will not be planing upwind is a shape very close the simple V that I described above. How do reconcile this apparent contradiction?
Getting back to Matt's decision, I agree with you that the Y Flyer is an easy build and also looks as if it would perform fairly well. Probably needs scaling up though, and built of around 4 mm Okume marine plywood.

 

i've learned alot from the debates and by all means i dont mind you guys talking about diffirent things at all, its all somehow related and is good.

mess, i have done a few sketches of a 3 panel boat, and found the 15+ heeled wetted surface to look rather like a nice airfoil.

However, i'm not entirely sure about the correct shapes you were trying to convey, here is what i came up with.

As i draw these pictures after i post this, i'l be checking out the y flyer