Planing hull - sail vs power

All else being the same, is there a difference in the ideal aft buttocks of a sailing dinghy vs a powered skiff? More specifically, can a sailing dinghy get away with a bit more aft rocker given the fact that its thrust line's vertical displacement already provides some bow-down trim?

As a hypothetical case I am thinking about a light skiff, say
Lp = 13ft,
Bp = 4.5ft
Displ = 200kg
V >= 12kts
FnL >= 1
FnV >= 2.6

 

Thats a good observation.

The V, hard chine transom on a sailing dinghy is very common, and when sailing, the idea is to keep it out of the water.

The BIG difference, is the hull surface for the main part of the underbody. Dinghys will usually have a convex surface to maximise slower sailing performance.

The power hull, benefits from a straight run from the turn of the bow.

One "combination" design I have considered is illustrated below.

 

So "ideal" for what speeds ?

 

12kts or faster.
I appreciate the requirement for sailing dinghies to also cater for low speed design, but for this hypothetical case it is not important.
It can be assumed that an equal amount of propulsive thrust is available and sufficient to achieve the required speed. The sail might just as well be an air-propeller.

I can rephrase the question as follows: if the thrust-line can be moved at will to provide optimum trim (for least drag) for each hull, one with straight buttocks and one with a bit of rocker, which hull would have the least drag at planing speeds?

 

Another question that may or may not be related: is there a difference in buttock design for a powerboat with transom mounted motor vs a motor mounted further forward in a well?
Aside from any trim implications, does the longitudinal location of the prop pressure differential have any influence on the local hull shape with regards to efficiency?

 

I doubt it will make much difference at 12 knots, except the rocker might create more porpoising. Maybe you will get more speed downwind, as the bows on the rockered boat will rise more easily to a wave, rather than plough, washing off speed. Once the boat is really up and running, beyond any "hump", you'd think the rockered bottom would have the advantage of a higher trim angle, and less wetted surface area.

 

The rocker at the bottom has a much more devastating effect on power. It forms severe drag by nature of the curved surface. That's why you never, ever see a planing boat with rocker.

 

Modern planing sailing dinghies all seem to be designed as pure planing craft, I guess you just sail them on their side to reduce wetted area when conditions don't allow planing.

 

How about a semi planing hull?

 

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"never ,ever" is a bit much. The Windmill, designed by Clark Mills, is a great planing hull design and has just the right amount of rocker.
Guillaume Verdier is one of the leading multihull designers and considers that the hulls of Gitana 17 are planing hulls. And, as such, make a huge difference in the ability of the boat to takeoff and fly.

Windmill-planes in 8 kts of wind:



Gitana 17 ama with a bit of rocker.

 

Totally different to a powered craft. For a start, the bottoms of those hulls are flat, with some rounded edges. next, the length.width ratio is hugely different to small hull that has to get enough surface to plane
Finally - its FOILING craft. It doesnt Plane, it Wings it.

The effect of Rocker on Power craft is so basic, I am surprised you even try to dis it.
"For maximum speed, a boat bottom should be as flat as possible in a fore-aft direction (longitudinally) for approximately the last five feet (1.5 m) (Figure 7-18). .... Any hook, rocker, or surface roughness on the bottom, particularly in the all-important center-aft portion (critical bottom area) (Figure 7-21), will have a negative effect on speed, often costing several miles per hour on a fast boat." Hull Shapes Review http://www.killcaremarina.com.au/commonly-asked/63-hull-shapes-review

"Although any hull will plane if enough power is provided and enough speed is attained, a hull designed for operation in the planing realm is sometimes distinguished by a flat run aft. In other words, in side view, the bottom is more or less a straight line towards the stern" Planing (boat) - Wikipedia https://en.wikipedia.org/wiki/Planing_(boat)

"Planing a sailing dinghy was first popularised by Uffa Fox in Britain. In 1928 Fox introduced planing to the racing world in his International 14 dinghy, Avenger"
Check out the complete lack of rocker.

 

I didn't even consider "rocker on powered craft" so I couldn't have dissed it.
On Gitana 17: while it flies, the planing hulls come into their own in the transition between displacement sailing and foiling-that is, leading up to takeoff. And also in rougher water where the hulls make intermittent contact with the water.
A flat run aft is planing hull 101-but it's a lot different than saying: "Thats why you never,ever see a planing boat with rocker".

 

Pretty sure you will find planing boats with rocker, but it is a matter of degree, of course. The original Cigarette racing boats did have rocker, the rationale being they were less likely to "stuff" (drive the bows into the back of a wave), a sometimes deadly situation. The rocker made them less stiff in pitch, and allowed the bow to rise, rather than punch through, at high speeds. This typically happened when the boat landed on the front of a wave, after being airborne. Tabs could be used to limit the porpoising caused by the rocker. But obviously not applicable to this thread ! I would say it might be helpful to have a little rocker, but not much, in this little boat.

 

Yes, it is a matter of degree. The point I was making is that any curve in the buttocks detracts from optimum planing ability, and is a design decision.

This is in response to the OPs actual request "one with straight buttocks and one with a bit of rocker, which hull would have the least drag at planing speeds?"

"The tests conducted into warped hull forms ........ conclusions given by Savitsky and Brown suggest that the drag of a warped hull is markedly increased compared to a hull with parallel buttocks,"
https://pdfs.semanticscholar.org/684c/1eb356659eea427d9dc1bad8b254095aaa17.pdf

 

Over the years, I had two 14 foot alloy boats, supposedly the same boat brand and name. but one had rocker, the other a slight hook. How that happened, I don't know, must have been rough-house building practices, but it wasn't from rough use dishing or deforming the bottom. The one that had rocker was drier and handled better, a little bit of rocker doesn't hurt.