Hull Drag of Small Dingies/Skiffs

Does anyone have reasonably accurate (+/- 50% would be a start) formulae for calculating the hull drag (ignoring keel) both on- and off-plane? Or preferably, actual tow-tank numbers for any common small planing hulls ( Lasers, 29ers, etc.)?

I have been putting together a spreadsheet to determine aerodynamic and hydrodynamic forces on sailcraft, and estimate speeds. It takes into account most aerodynamic and hydrodynamic forces, and seems to give reasonable results. Hull drag is my last and most intractable problem--any help would be appreciated. I will share/post the spreadsheet if there is interest.

 

Clarification

I am most interested in resistance at planing speeds, 15 knots and up. Has anyone towed a Laser behind a speedboat?

 

I think there may be some graphs of hull resistance v speed for Tasar, Int Canoes and 18' Skiffs in High Performance Sailing, but I haven't got it to hand to tell you the numbers.

 

The DELFT series is actually pretty accurate at the scales you're talking about. I suppose for planing operation you could use a savitsky method, but it's on the end of it's applicability.

I doubt there's much tank work on dinghies, as testing it would cost a few times more than the boat would.

Best of Luck,

Tim B.

 

I wouldn't say the charts are really big enough to be usable. The Bethwaites have significant data, but they treat it as commercially sensitive, although I believe they've said that they're willing to share info with anyone who can bring a significant body of data to the party.

 

That, and the true power limit is the ability of the crew to hold the rig up in higher winds. Look at the IC's and 18' skiffs...righting moment and stability is the bug-a-boo.

Then again look at the differences in hull form between IC's and 18's. Hull form has really very little to do with max speed compared to the rig forces.

 

Hull form is a good point actually, and one that is more obvious on older dinghies moreso than newer dinghies. The Lark/420 comparison shows this well, at low speeds (and particularly in waves) the Lark wins easily because of the sharp bow. Once both are on the plane, the 420's trapeze allows you to hold a lot more power on. When planing on smooth water, there is very little to pick between them.

The modern thinking is to have a sharp bow and flat aft sections. You reduce the resistance by reducing the beam to the point that you have just enough stabilty (or none at all in some cases (IC and Moth)).

Cheers,

Tim B.

 

Thank you all for your help; it does seem that the hull shape becomes less important at higher speeds, with the wave-making drag decreasing as the skin friction increases, for a reasonably constant total drag force.
In my Vpp it seems the most critical factors at high speeds are the L/D ratios of the wing and foil, and aerodynamic drag of everything.
All the best,
Matt

 

My empirical observations would dispute that. Many, especially the older fashioned designs, just top out in speed. However handling characteristics are at least as important as drag at high speed.

 

Seems like it would be easy enough to generate "real" data for small hulls, towing behind a powerboat. I just got a "crane scale" - 3,000 lbs capacity - on ebay, that would be just the ticket. Indeed, I will be doing some analysis of my sharpie, to evaluate options for electric propulsion.

Sal's Dad

 

How do you mean 'not big enough'? For sure, its dodgy using one graph from a book without fully undertsanding how it was derived, but it's a reasonable start. I'd love to have access to the Bethwaite's data, unfortunately I don't have any to swap. :-( Maybe I should take some readings from my NS14 to give them. It would be interesting to quantify the improvement since the Dribbly (Tasar) hull was designed. I'd also love to pick Phil Morrison's brain to learn some of his ideas, but he seems a most elusive man.

Matth - I'd be very interested in seeing your spreadsheet when its done.

 

Sal's Dad: Yes, I think I'll be doing some towing. I do hope the forces will be a lot less than 3000 lbs!

PI Design: The spreadsheet will be ready soon; I am adding some explanatory notes. It is designed for Vpp of inclined-rig 'no-heel' boats (hence the high speed planing). The limiting factor on sail force in the spreadsheet is when the vertical component of sail force exceeds [an adjustable fraction of] total boat mass, lifting the boat out of the water. I can modify that to be heel limited, by adding an entry for approximate sail center-of-effort height, and max righting moment.

There are some other I need to add as well; right now I have induced drag of the sail but not for the keel; it is currently approximated with just a L/D ratio. This may be good enough, however.

gggGuest: You're right, the handling will be critical at high speeds; I was referring only the theoretical smooth-water hull drag being not overly dependent on hull shape. I suppose if the boat is pitching and porpoising through the chop then my calculations will go out the window!

 

That makes sense, below planing speeds; the wavemaking drag curve is exponential while the skin drag is linear. So at the low end of the sub-planing range the skin drag is much bigger, but the wave drag dwarfs the skin drag around the 'hump' speed.
Is it fair to say that the wave drag is the analogous to the induced drag of a wing or hydrofoil? Think of an airplane doing a takeoff run starting from rest; the weight of the plane is carried by the wheels (hull displacement) up until takeoff. The induced drag increases exponentially up to takeoff speed (hump), then decreases as the plane continues to accelerate up to cruising speed and the angle of attack of the wings (pitch angle of hull) decreases.
Hmm
Therefore, non-planing boats are like airplanes that never get off the ground, and just taxi around.

 

I did some "tank testing" yesterday; basically pulling things behind a little 14' aluminum 10hp runabout, and leaning out the front of said runabout to drag various centreboards, windsurfer fins, and canoe paddles in the water.

Very surprising results! Pulling a 14' BIC-410 dinghy up to top speed of the tow-boat ( 15-20knots est.) took about 60lbs force (est. using calibrated arms). I was expecting a 'hump' and then efficient planing, but the drag just built and built, until the hull was throwing a big bow spray, and the wake off of the back of the dinghy (no rig or crew, est.175lbs) was probably half the size of the tow-boat (2 adults, 6-700-ish lbs). It felt like I was pulling it through molasses.

Thinking this might be due to the poor planing shape of the BIC, I next towed a windsurf board, 120litre, about 20lbs, 24" wide and 8-9 ft long. This board feels very slippery and fast when ridden, and I expected it to skim easily over the water, especially as it was completely unloaded. Instead, the drag at around 20 knots was about equal to its weight--around 20 lbs! again it felt like pulling through molasses, a very viscous, steady suction. It was riding at a flatter attitude than when ridden, so more surface (about 2/3 est) was wetted.

I have a new respect for skin friction!

Dragging the centreboards and fins was also interesting, in that the drag seemed very low, even for large, poorly shaped, and in one case very warped test specimens. The breakaway from attached to separated flow was interesting, as was continued running past separation in the superventilated region. This was clearly less efficient, but at higher speeds it should in theory improve.

I will plug in some manual data to my Vpp, which is going to bring down the speeds considerably.