Design Question: What characteristics make for a good planing hull for a scow

[Cybernaught]

Hi Folks,

I have been researching scows quite a bit and that is what got me interested in the PDRacer build. So this question is for Michael or anyone with an opinion on this subject.

What makes one scow hull shape better than another. I want to try and understand the basics of hull shape as it applies to the shape of the boat's rocker.

How did the PDR rocker get it's shape and would a different shape be more efficient? It looks like the max buyancy is about one third the LWL from the back. I am thinking primarily in terms of the most easy shape to get it on to a plane, if there is one. I also mean for a scow in general not just the PDR.

Is there a formula or ratio to determine maximum depth of rocker versus length of hull?

Is there an ideal length to beam ratio?

I'll leave it at that for now and see if there is any interest.

Good sailing to all,
CN

[Daddles]

The PDR bottom shape is laid down in the rules, you can't change it and I don't know how or why the originator of the rules designed it.

As for the rest of your answer, I'll be sitting back and waiting to be educated :D

Richard

[Boatmik]

Is this for a sailing boat or a motor boat.

If sail - how much length and how experienced do the sailors need to be.

If Motor - how much power.

MIK

[Cybernaught]

Daddles Said:
"The PDR bottom shape is laid down in the rules, you can't change it and I don't know how or why the originator of the rules designed it."

What I'm getting at is the reason for the design shape and how those reasons relate to performance. Is there an optimal format or set of ratios that works well for a planing scow type hull?

Michael,
The idea is for a sailing scow, probably in the 12 to 14 foot range with a single crew and occasional guest of moderate skill level. Performance level "fast daysailer" without acrobatics and with traditional rig. Ends may be pinched or not. Sort of a "Super-Duck" What makes one rocker shape work well and plane easily while different shape may only produce marginal performance? One design rules not standing.

Hope this gives you and idea of what I'm looking for. What's the best underwater shape (generally speaking) for a flat-bottomed plaining sailboat hull?
CN

[Boatmik]

Howdy,

YOu can't really look at planing in isolation. Much sailing time will not be planing time unless it is a real out-and-out flying machine. Even then it will spend 20 percent of its time not planing.

The general idea for flat bottom boats is (looking in profile) the first third of the hull should be almost straight. Same for the last bit of the hull - maybe 20% - but people overdo it usually thinking a "long straight run" is ideal for planing - this idea dates from the 1800s.

So most of the curve is in the middle.

However this leads to a boat that might go upwind OK and might plane OK but have a slow transition between the two.

So I would take the above with a very big grain of salt and try to follow the general trend described but just soften it all a bit by making sure the curvature was more distributed into the straighter areas.

But I would make sure the last foot of the bottom was pretty straight.

As far as side shape - the fast scows duplicate the bottom curve in the side curve - because they are sailed heeled to windward to reduce wetted surface and get more power (heeled maybe 10 degrees).

One trick pioneered by moths is to pinch the stern in narrower suddenly in the last couple of feet. It helps prevent nose diving.

Michael

[Cybernaught]

Thanks Mik,

That's exactly the kind of information I was looking for.

So the hull needs to be good in displacement mode as well in order to get up to planing speed efficiently. That makes a lot of sense. The PDR maximum depth is about 2/3 back from the bow in a larger boat would this probably more centered? In reading about design, on the web, I have read that, the prismatic coefficient should be around .57 or .58 generally speaking. In your opinion is the prismatic coefficient a good indicator of potential design performance or efficiency?

I have read about the idea that the curvature of the side of a sailboat should match the rocker of the boat to be more efficient when sailing heeled. Less eddy currents or something if it's sailing like a V hull and the sides are equal in form.

You said:
"One trick pioneered by moths is to pinch the stern in narrower suddenly in the last couple of feet. It helps prevent nose diving."
Is the idea to reduce stern bouyancy so the leverage downward on the bow is less?

Thanks for the input.
CN

[Boatmik]

Prismatic coefficient is not a predictor of speed. It works the other way. You estimate the likely speed for the boat and then choose the prismatic coefficient.

A note - it is very hard to get the right coefficient in flat bottomed boats and keep them on their lines.

There are lists that give the target Prismatic coefficient for different speeds.

You would never choose a prismatic coefficient that represents the top speed of a boat. It is best to choose one for the average speed for the conditions in which it will be sailed.

Michael