question about pdracer

[ancient kayaker]

Stability is roughly proportional to beam cubed; the boat will lose 58% of its stability at 3/4 beam. My own sailboat has a 4' beam and is stable enough to walk around in without problems. My canoe was 3' in beam and was very tippy even sitting down. Other factors have an effect of course but I don't think you're going to like a 3' wide PDR.

[messabout]

Terry; I agree that a 36" PDR is going to be tippy. too tippy for anyone short of ballerina status. Tinkering with the beam cubed ratios, I get conflicting results when compared to actual static RM calculations. I think that this might be a can of worms that challenges my modest abilities. It is fun to contemplate however.

Just for giggles I compared the PDR with the NZ Firebug. Firebug has the same 8 foot by 4 foot planform as PDR. It is also a scow or punt type. There is a really big difference in initial stability calcs. Firebug has 24 inch ends and beveled chines. It is not as butt ugly as PDR but will be much more demanding of its' skippers agility.

[ancient kayaker]

Originally Posted by messabout

. . . Just for giggles I compared the PDR with the NZ Firebug. Firebug has the same 8 foot by 4 foot planform as PDR. It is also a scow or punt type. There is a really big difference in initial stability calcs. Firebug has 24 inch ends and beveled chines. It is not as butt ugly as PDR but will be much more demanding of its' skippers agility.

I didn't know about the Firebug: nice looking little boat. The beveled bilges and narrower ends are going to make a big difference to the primary stability but it will stiffen up nicely as it heels. I am tempted to say it will go better than a PDR in a light wind or under oars due to lower wetted surface, but my own 10 x 4" flattie is so good in both those departments that is hard to know what to believe anymore.

It is amazing how good a simple boat can be. Boat design is largely magic and if thought it was safe I'd ask the designer (our own PAR) if he is related to Harry Potter, but I don't want some humongeous snowy owl showing up on my doorstep - the neighbours worry enough about me as it is . . .

[JRD]

Originally Posted by messabout

Terry; I agree that a 36" PDR is going to be tippy. too tippy for anyone short of ballerina status. Tinkering with the beam cubed ratios, I get conflicting results when compared to actual static RM calculations. I think that this might be a can of worms that challenges my modest abilities. It is fun to contemplate however.

Just for giggles I compared the PDR with the NZ Firebug. Firebug has the same 8 foot by 4 foot planform as PDR. It is also a scow or punt type. There is a really big difference in initial stability calcs. Firebug has 24 inch ends and beveled chines. It is not as butt ugly as PDR but will be much more demanding of its' skippers agility.

A good point. The firebug is a kids learner boat, from what Ive seen they arent having too many problems keeping them upright. When they do capsize the boat is easy to bring upright again and becasue of the wide side decks hardly any water ends up in the cockpit.

The Firebug was one of the last designs of the late John Spencer, known in NZ as the plywood king due to nearly all of his designs being hard chine, but ussually embarasingly fast. He designed the firebug to encourage parents and kids to build their own boats, which has been reasonably succesful in NZ and Australia. They have been used in a number of school and community building programmes here.

[Petros]

obviously you have to make a larger door from your basement. You have to get your priorities right, boat or house?

build a trimaran or a catamiran, join the hulls after it is out of the basement. way more fun than an ugly PD racer.

[WhiteDwarf]

JRD The Firebug is no mere "kids' boat" . I've sailed one for the last 5 years. Won our handicap class two years running,etc. I weigh 90kg and have sailed for over 50 years. Alot of fun, is thr Bug.

As the acknowledged 8 footer guy at our club, I was asked to try our a newly built PDR for a builder. It was the worst boat I have ever sailed by miles. The heavy rig made it unstable, there was no visibility, under the sail and poor manouverability for so small a boat. From that one experience only, I see the PDR as a device to discourage people from sailing.

[DCockey]

Originally Posted by WhiteDwarf

JRD The Firebug is no mere "kids' boat" . I've sailed one for the last 5 years. Won our handicap class two years running,etc. I weigh 90kg and have sailed for over 50 years. Alot of fun, is thr Bug.

As the acknowledged 8 footer guy at our club, I was asked to try our a newly built PDR for a builder. It was the worst boat I have ever sailed by miles. The heavy rig made it unstable, there was no visibility, under the sail and poor manouverability for so small a boat. From that one experience only, I see the PDR as a device to discourage people from sailing.

Sail rig and area are open under PDRacer class rules, so what you didn't like were the choices of the particular builder, not PDRacers in general. Class rules here.

Would you condem a rowing boat because the owner selected oars which were too short and heavy?

[WhiteDwarf]

David, Sorry if I offended you, regarding the PDR. I did stress that "From that one experience only."

WhiteDwarf

[Collin]

If you cut out all the parts before assembly (glue chines, and bulkhead pieces together) you can literally nail it together in an afternoon and paint it. Let the paint dry overnight and you're done. The boat can live outside just fine.

You can even tack it together inside to make sure it all fits together, withdraw screws, and take it outside with screw holes to line everything back up. There's no reason to do any complicated half hulls or modify the design.

[gggGuest]

If you're going to make it narrower then you'll get some stability back if you can make it longer, and it will be a better boat in many other ways. 14' * 3'6 might be a nice size for a recreational craft.

I don't know that the PDracer is necessarilly a good model though.

The Firebug is an astonishingly clever piece of design. Spence was always an unsung genius, but that way he got such a sweet shape out such ridiculously simple construction features just boggles my mind.

[sharpii2]

In case you might be interested, I have a design that has never been built yet that is 3ft x 12 ft.

Unlike a pdracer, it has deeper rockers, so it can heel some before the transoms get immersed. By heeling some, it is supposed to present a 'V' to the water, instead of a flat section. I once presented the idea to Phil Bolger and he said he thought it would be fairly fast. I still have the letter.

It has a max design displacement of 480 lbs.

Your proposed 3ft duckette would displace about 457 lbs before the bow and stern transom start to go under.

My design was intended to be built inside a mobile home and to be driven around in my S10 pickup truck.

I'm thinking of shortening it by about 9 inches to make the stern transom deep enough to hang the rudder on and so less boat will hang out the back of the truck.

[sharpii2]

Originally Posted by CutOnce

Agree 100%

And ....

It will sit deeper in the water given the same crew weight, increasing wetted surface area somewhat - decreasing the beam increases immersed surface area and drag. Given the already very short length and limited hull speed, I would expect any possible light wind speed gains to be negated.
--
CutOnce

Not quite true.

I did do a math experiment drawing a very crude boat. I started with a L/B ratio of 2.0 and went up from there. I made the boat progressively deeper as I lengthened it, maintaining the same displacement.

I found whetted area continued to decrease until the L/B ratio was close to 10. The Beam/Draft then was about 2.0.

Further increases in length and depth caused the whetted area to start increasing.

At its best point the narrower boat had roughly 16% less whetted surface than the original boat. And that included boards and the rudder.

This may explain why motor ships have the proportions they do.

With sailboats its a whole different matter.

As the Beam increases, the whetted area increases, but so too does the sail carrying ability. In fact, the sail carrying ability increases in direct proportion to the initial stability of the hull. And that increases with the cube of the Beam increase, if the boat is kept the same length, and approximately the square of the Beam increase if the narrower boat is lengthened to make the B/D go down less.

This is for two reasons:

1.)The longer narrow boat has a greater B/D than the shorter narrow boat and
2.) the longer narrow boat can set up more smaller sails (which have low VCA's) than the shorter narrow boat can.

This all, of course, assumes no deep external ballast.

So, if a boat is made wide and shallow, it ends up with greater whetted surface, but a much higher S/D and, most likely a much higher sail area/whetted surface area as well.