Thread: What designers worry about

PAR I wish you all the success you deserve. Let us know how it all works out.

Cheers, Kev.

She floated on her lines and ran out surprisingly well. No hump or noticeable bow rise, just lifted even and uniformly, throughout the acceleration phase. Trimmed just right (3 degrees) without tabs, though this will likely change with the installation of the race engine, which has considerably more power. Picture shown is about 3/4 throttle. She maxed out a 62.3 MPH on a 450 HP test engine. She'll do much better with the 700 HP mill.

She won her first race Saturday, then promptly cracked her block, so she's done for the season. The last race of the season is next week and she's not going to be ready, but that offers all winter to get her going.

Congratulations, is that an open class or did you need to comply with measurements.

No she's a class boat and most everything is measured, from engine output to the location of the rudder. I had a really cool skeg that would have shaved some drag off her, but it got shot down by the rules guys, even though it technically fit within the rules. The main advantage of my design over the competition is two fold: the CG location and the hull form used in the after sections. My boat turns better then the others and can run slightly faster, but is slower to accelerate to speed. I'm figuring the loses off the line can be made up in the turns and at the top end. This was the comprise I accepted while addressing the design features to take advantage of, during the development phase. I know Mik doesn't believe in these compromises, but I had to trade off something to get the other stuff, so I gave away the hole shot for turning ability and a slight top end edge. In winning her first race, she was beaten off the line (expected), was catching up at the first pylon, turned faster, then was pulling away as they crossed the finish line. I could have focused the other way, maybe getting enough of a hole shot, to make up the loses in turning and top speed, but I elected to go for the 2 out of three deal, mostly because I noticed the boats that turned well, usually won.

Good call. Good work. Good result.
All that worry (after the build complete) wasted ! ;-)
Are racing the boat yourself ?
Alan J

No Alan, I'm just watching from 2,000 miles away, but the builders have been keeping me up to speed on the project.

Par, a beautiful design, and they executed the build very well, good to see they won the first race it was entered in, pity about the motor,.

jeff
vk4

Race day. See, she's really wood, but has been accused of being another material. They did change the sheer line a bit. If you look closely, the top stringer shows what it would have looked like, had they not elected to cut it down. I'd drawn a reversed sheer, which most don't like, including these guys. I don't like reversed sheers much either, but in this case I considered it warranted, as it permits you to see over the bow when up and scooting along. They did leave the heights at the bow and stern the same, so vision is still good. I have to admit I worked hard to get the reverse sheer to look good, but it doesn't look bad this way either.

SPS-8.jpg

An update on this race boat. She ran undefeated for the first races of the season, but has finally been bested in qualifying heat. The series she runs in has a unique race setup, with an initial "scramble" where they all (the whole class) start about a mile off shore and blast in towards a finish line. This "heat" determines who will compete against who, in the double elimination, qualifying rounds to follow. She came in second on her latest scramble, but continued on to defeat all in her qualifying rounds, eventually beating last years champion in the final heat, by 2 boat lengths. Technically, she's still undefeated as the scramble portion just determines the race order in the qualifiers and isn't a race, but it was a fun run while it lasted. In other words, she's not lost a race yet, but has lost a scramble.

The competition was concerned about her over the winter, so several boats made significant changes and one wholly new build. Knowing this, I held a few tricks in reserve, waiting until the competition started to catch up. I've just sent up a new rudder and skeg plan, that conforms to the rules, but should offer a couple of MPH over her current top speed (she's done 91.3 MPH or 146.9 KPH so far). Rule changes and a new engine have her sitting on a 750 HP, 460+ CID (7.5 L, but really closer to 8 L) Ford engine. I designed her to reach longitudinal instability at around 95 MPH (152.8 KPH), so she'll be pushing real close to the edge, once geared and propped right.

For those in the know, it's a wedge style of rudder and if you look at one, it seems like it would be a draggy beast, but it's not and is very effective at high speed, though admittedly, truly sucks at low speed. Picture an ax head, viewed from above. This is pretty much what a wedge style rudder blade is shaped like, a very fine leading edge with a thickening taper aft to a very blunt, wide, squared off trailing edge. The trick to these blades is the angle the two "faces" are separated at. My angle is in the 4 degree range, but the actual angle is a guarded secret. The new rudder will be milled from a 1" thick (25 mm) hunk of solid stainless.

I'll post some pictures of her new paint job and running at speed, when I get a chance.

Thanks PAR, very neat stuff. Do you have any build photos of the craft going together? Is it cold molded?

Cheers

Simon

Yes, I have a few build photos, which I'll dig up.

She's a double diagonal plywood molded build, over widely spaced ring frames and fairly closely spaced longitudinal stringers. The diagonal planking offers the slamming load stiffness, while the stringers provide the longitudinal stiffness. It's a racer so it's ridiculously light at 2,400 pounds (1,090 kg) carrying 750 HP. At 30' long (9.14 m) this is pretty darn light, especially considering her engine and drive are a 1,000 pounds (453.8 kg) alone.

The diagonal planking is difficult to make out, as it has a biax skin over it, but you can see the general idea of how she's built. I have some photos of them laying the skins over a jig, but haven't found them yet.

SPS-2.jpgSPS-1.jpgSPS-6.jpgSPS-8.jpg

The stringers are pretty tightly spaced on the bottom, but get progressively wider as they approach the sheer. If you look closely, you'll note the stringers get a tighter spacing as they go forward, to improve slamming load tolerance at speed. Ditto the plane patch areas aft. The engine shown is a mule, just to check fit, locate mounts and check bolt on stuff.



This is the engine currently installed, though the intake is now a duel quad tunnel ram, not the single plane, single 4 shown. Also the "zoomies" on the trial motor (photo above) have been replaced with a set of long tube, large diameter, upswept headers w/ a short collector. Iron block, aluminum heads, stroked, punched, the usual stuff . . .

Found a bottom shot as they're starting to fair it up. You can see the outer diagonal planking,



and that she's a long, lean beast too. Naturally, there's a biax schedule and filler over the planks, but the seams are visible. This shot is deceiving, in that is looks like she hasn't a lot of deadrise, but she does have about 10 degrees, which is fully carried from midship aft.