Hull Speed Revisited

[Don Maurer]

I was just wondering if the cross sectional length of the rudder should be used in calculating the waterline length of a boat with a pointed stern (at the waterline). It would seem that a closely fitting rudder with a square leading edge to the bottom of the skeg or keelson and a foil shape to the trailing edge would be a natural extension of the hull just as the fine bow and stern of a kayak are. If this is the case, rudder design would play a significant role in the theoretical hull speed of a small dinghy. Opinions?

[Ian McColgin]

Well, sorta.

A dink with a truely pointed stern might well squat and lose more speed that way than ever it could gain from waterline length.

If you take the average of (1.3)(square root LWL) as hull speed, then adding 1.5' to a 10' hull could bump your speed by .3kt if nothing else were happening.

The effect with kayaks is enhanced by the fact that narrow hulls produce small waves. Even when you hit "hull speed," the point at which only two waves are on the hull and the after wave is beginning to lag behind the stern, thus making a hole in the water over which the boat must climb to maintain speed and which grows larger as you add speed, with a thin hull this climbing out of the hole matters little. With kayaks, extending the ends produces more gain from smaller wave formation than the gain from hull length and both outweigh added skin resistance.

It would not really help a displacement dink that makes a pretty big wave for its size. To get speed in a dink, go for a planing hull.

Quite an aside: I've lost my table book, but you can make the wave length to speed ratio by manipulating the common formula for "hull speed" of 1.3 times the square root of the waterline.

Some hulls produce smaller waves and thus might have a formula 1.5 or so times. Some tubs it might be only 1.2 times. And I can't recall the formula for water - take salt ast STP.

Anyway, close enough lets say that knots to crest to crest wave length are about:
2kt = 2.5'
3kt = 5.5'
4kt = 9.5'
5kt = 15'
6kt = 21'
7kt = 29'
8kt = 38'
9kt = 48'

The correct version of this table gives two things:

You can easily estimate the speed of any waves you are in; and

Observation of bow to sternwave seperation is a finastkind instant speed log, especially if you mark it with observation points over prominent hull features seen from one place at the helm.

[PVanderwaart]

I don't think the rudder is going to make very much difference. The real issue is buoyancy. When the trough behind the bow wave is under the middle of the boat, the fat part of the boat can't do the floating, and the stern has to hold the boat up. When it can't, the stern sinks and resistance goes up a lot.

Fast boats of all types have the bouyant part of the hull well aft and have wide sterns.

[Ian McColgin]

Regarding the bouyancy of the stern, I believe there is a range depending on the propulsion method and the general beam to length ratio.

Thin boats with little stern bouyancy as a seperate bouyancy - these example have plenty of overall bouyancy - that are exceptionally narrow have virtually no practical hull speed because their wave formation is so gentle. Granuaile (sail) and some commuter express boats (power) are good examples. Kayaks are another.

Small sailing boats of more normal beam need some stern bearings to handle their propulsion but rowing boats of similar capacity benefit from a narrower, less bouyant stern. That's why a Whitehall model is so slick under oar and a generally miserable sailor. If you take a whitehall's foreward half but draw out the stern to give an aft raking point of maximum waterline, making the buttocks a bit convex, you'll hurt the rowing performance and improve the sailing.

Two very different sterns optimize speed under power. If she stays displacement, you'll want the least stern that still keeps her from squatting with the prop thrust. If you want to plane, a nice broad flat bottomed thing unsuitable for sailing or rowing is just the ticket.

G'luck

[Dan McCosh]

The wave-making capability of a vertical plane (such as a rudder) is virtually zero. You can see this if a hull had a beam of zero. It would push the "formula" for hull speed to infinity. The rudder, or a zero-volume stem extension, should have little or no effect on hull speed--one reason why it isn't rated in handicap rules. What would be intesting is a bulb extension below the water line, similar to the forward extensions used on some commercial ships.

[Harry Miller]

I know the context usually makes it clear but even so is it possible to change the font to one where STEM and STERN don't look like the same word? (stem/stern)

[Aramas]

The 'waterline length' of a hull from the water's perspective (the only one that effects speed) is its apparent waterline length - not a stretched out technical waterline length such as that made by hollow waterlines or appengages.The purpose of a hull is to push water out of the way and then put it back as neatly as possible. Water is not fooled by technicalities.

As a visual aid, imagine a sheet of ply attached to the stern of a boat, lengthening its technical LWL by 8 feet. The only effect it would have on hullspeed is to slow the boat with additional drag-producing wetted surface.

If hollow waterlines extend the LWL of a boat by 3 feet over what it would be if they're straight, then it performs just the same as the same hull with straight waterlines that is 3 feet shorter, with the exception that the 'extended' hull will have more wetted surface and hence be slower.

This also applies to the prismatic coeffeicient, so it should be clear that taking such numbers as gospel is somewhat misleading. ie, Take two boats of the same 'paper' LWL and Cp, one with straight waterlines and the other with hollow. The one with hollow waterlines will actually behave as a shorter boat (lower hullspeed) with higher Cp (more drag at low hull speed, less at higher speeds.

Don't take numbers at face value - they're just tools, and tools are not of much use without the knowledge of their limitations and peculiarities.

[Don Maurer]

I'm not sure I completely agree. As the hull passes through the water the water separates into two streams on either side of the hull. At the stern, the streams meet again. If they come together too abruptly, they create a lot of turbulence as the streams mix together. The smooth transition of a hollow waterline gives the streams time to straighten out before they mix together, creating less turbulence. It may not follow the same rule as the hull speed formula, but a dinghy with a long rudder should be marginally faster than one with a shorter one.

[Tom Lathrop]

I agree with Aramas that hollows in the waterlines should make hull speed less than convex or straight waterlines, assuming that both are smooth rather than abrupt. I doubt that a rudder that does not form a smooth transition of the hull waterlines and has a significant displacement relative to the adjacent hull has any positive effect on speed. Fish seen to work very well with that set up.

Of course nature does not work by consensus of forumites and cares not what we think so we could all be wrong.

[ 06-21-2005, 06:14 PM: Message edited by: Tom Lathrop ]

[htom]

The fish design, as well, is a compromise between "fastest fish for length" and "most powerful fish for length" and "most efficient fish for length" and ... and who knows what else.

[Boatmik]

Fish may be very smart about what happens underwater, but they know nothing about the water/ air interface where wavemaking occurs.

Note that fish don't have hollow waterlines at the bow either! :).

Certainly it is good for entry and exit angles not to be too great, but often ppl only look at the shape of the waterlines of a particular plan without looking at the 3 dimensional situation.

For example imagine a sharpie style boat with a flat bottom. A boat may have a wide half entry angle of 15 degrees when looking at the waterlines, but if the forefoot is clear of the water and the bottom of the boat meets the water at 5 degrees - what is the effective angle?

Somewhere between 5 and 15 degrees.

Bolger in his writings makes a lot of sense about this and keeps the forefoot of his boats clear of the water so the shallower angle of the bottom is more dominant.

Regarding a good hollow at the stern so the water "from both sides of the boat meets nicely" - generally at the stern it is easier to get a shallow exit angle - the waterlines are running relatively straight to the transom and the buttock lines are as well but generally with a much shallower angle.

Sorry I can't describe this really well - but the main point is to remember that the situation is 3D and that you can't look at what happens to the waterlines without thinking of what happens to the buttocks in the same area.

Cheers
Michael Storer
http://www.storerboatplans.com

[George Roberts]

I love the phrase "theoretical hull speed."

Using 3D computational fluid dynamics I would include the rudder and get a reasonable estimate.

Using (1.3)(square root LWL) the only way to get a reasonable estimate is to know the hull speed and length of a SIMILAR boat and use the formula:

Hullspeed2 = Hullspeed1 times squareroot(LWL2/LWL1)

For that it does not matter which length you use.

[paladin]

the outboard rudder on my old 31 foot trimaran definitely made the boat go faster.....that's where I mounted the outboard. :D