Hull speed

[Mitch]

Hello all-
I was towing a friend’s kayak instead of the usual dinghy behind my Typhoon last night and got to wondering about hull speed. I was once told that one could roughly calculate the hull speed of a boat by taking the square root of the boats waterline. That got me to wondering about the hull speed of a dinghy. One would think that towing a dinghy would slow a boat down, but would it slow the boat to the dinghy’s maximum hull speed? Or would the length of the dinghy be added to the length of the boat to give the boat an increased hull speed? I am very confused. Could anyone straighten this out for me or do I have to be a physicist to comprehend this mystery?

Mitch Feeney
Ty Galatea

[Craig Curtis]

Mitch,

As I'm not a physicist either and I'm sure others will post more thorough follow-ups, however, I'll give it a shot. In regards to the hull speed, you're close but missing one part of the equation. It's the square root of the waterline multiplied by approximately 1.3. As to the dingy question, I believe most dingies would plane and would therefore only have a minimal amount of drag.

Craig Curtis
1990 CD40 "Coalescence"



Craig_Curtis@prusec.com

[matt cawthorne]

Mitch,
Hull speed would be calculated for your Ty and it's waterline alone. Typically when towing a dinghy you can pull it out of displacement mode and make it plane if you are going faster than it's hull speed. However if you fill it full of gear and weigh it down you may find that you can not get it to plane and you would have to put a great deal of force on it to make it go as fast as your Ty. Any boat can be made to go faster than it's hull speed, it just takes a lot of force (translates to power) relative to its weight. If the dinghy stays in displacement mode it can really slow your sailboat down. Soft bottom inflatables can be really bad because they do not plane well. Usually if your boat is much bigger than the dinghy the effect of towing it is not too great. Try towing it at a distance that puts it in position to surf on your wake. This can really cut down on the drag.
Matt



mcawthor@bellatlantic.net

[Rich]

I used to tow a cheapo inflatable behind my Chrysler Mutineer, mostly as a fun way to cool down on a hot day. I would put one or two people in the "dinghy" and set sail. With that much weight, we would reach hull speed for the dinghy pretty quickly, the effect was to pull it underwater, thereby giving a wet ride to the occupants. I assume that is what would happen to any non-planning craft towed beyond hull speed, it starts to sink as it can no longer displace the water in front of the hull fast enough.



rpassmore42@hotmail.com

[Mitch]

Typically when towing a dinghy you can pull it out of displacement mode and make it plane if you are going faster than it's hull speed.

I'm not sure if the Ty is fast enough to plane out a dinghy! The wake surfing idea is a good one and I try to tow it empty when I can (there's not much space below decks on an overnight) but I still don't think I could plane out the flat bottomed dink I'm borrowing. When I'm sailing along in a good breeze and tug on the painter I am astounded by how much force there is. Cyclops (the dinghy) is really slowing me down. Does anyone have any suggestions? Maybe I just need a bigger boat!

Mitch
Ty Galatea

[John Vigor]

Mitch: Maximum hull speed applies only to displacement hulls, that is, non-planing boats. The maximum speed of a Typhoon is 1.34 x the square root of the waterline length. A Typhoon is 13 feet 6 inches on the waterline, so her theoretical top speed is 1.34 x 3.687 = 4.94 knots. Say 5 knots. She will of course go faster temporarily when she's surfing down a wave.

Any attempt to make her go faster in still water requires that she dig herself out of the hole she has created for herself--she lies in the trough of a wave with crests at her stern and bows. It takes an extraordinary amount of extra power to get her to start climbing over her own bow wave.

The top speed of a planing hull, on the other hand (one that is fairly flat and shallow, and able to generate dynamic lift from the water) is restricted mainly by the power-to-weight ratio. The more power, the faster she goes. The average planing hull will do 25 knots if there is about 40 pounds of weight for every horsepower. If you can drop the weight to 10 pounds for every horsepower at the prop, she'll do 50 knots.

Your kayak is a planing hull, so she has no maximum hull speed and the drag will be proportionate to her weight, the amount of gear you stow in her, and the speed you tow her at. The drag of a kayak really shouldn't amount to much at the speeds a Typhoon is capable of, but it will slow the parent boat down, of course.

Incidentally, as a rule of thumb, the speed in knots at which a hard dinghy will start planing can be found by multiplying the square root of the waterline length in feet by 2.2. So a 17-foot kayak would start planing at about 9 knots. An 8-foot hard dinghy, on the other hand, will start planing at about 6 knots.

John Vigor
CD25D "Jabula"



jvigor@qwest.net

[Dennis]

Someone showed me that trick of having the dink "Surf' Down the wake.
When you do it, notice the amount of tension in your line with the dink behind the wake as opposed to infront of the wake. You'll be suprised!

Dennis

Mitch,

As I'm not a physicist either and I'm sure others will post more thorough follow-ups, however, I'll give it a shot. In regards to the hull speed, you're close but missing one part of the equation. It's the square root of the waterline multiplied by approximately 1.3. As to the dingy question, I believe most dingies would plane and would therefore only have a minimal amount of drag.

Craig Curtis
1990 CD40 "Coalescence"

[Joel]

John,

With the large overhangs all Alberg-designed craft exhibit, wouldn't the length at the waterline increase as she heels, thus increasing theoretical hull speed?

Joel
Pokey II
'73 Ty #459
Bayside, NY



bondy_joel@hotmail.com

[Neil ordon]

>>I assume that is what would happen to any non-planning craft towed beyond hull speed, it starts to sink as it can no longer displace the water in front of the hull fast enough.<<

It's not a matter of displacing the water in front of it. If that was the case, the shape of the bow would be what determines hull speed. Actually, waterline length is all that matters unless you plane or sink.

A displacement hull going through the water creates waves, starting at the bow. The length of the wave is a function of speed, the faster you go, the longer the wave. Problem is, once the wave reaches a certain length, the stern is trapped in the trough and can't excape.


Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167



cdory28@aol.com

[Neil Gordon]

>>With the large overhangs all Alberg-designed craft exhibit, wouldn't the length at the waterline increase as she heels, thus increasing theoretical hull speed?<<

Absolutely!


Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167



cdory28@aol.com

[M. R. Bober]

The bigger boat idea has merit. Persue this line of reasoning.
Mitchell Bober
RESPITE
CD330

[John Vigor]

Joel, it's true that immersed overhangs theoretically increase speed, but on small boats it's a very small amount. If you increased the waterline of a Typhoon by two whole feet, from 13 feet 6 inches to 15 feet 6 inches, the hull speed would go up by just 0.3 knot from about 5 knots to 5.3 knots.

But let's consider when the extra waterline length comes about. It doesn't occur to any great extent under power, or on the run, because you're not heeled over. And on the beat, although you're heeled over, you're not likely to reach hull speed anyway. So while you will hear a lot of glib talk about overhangs increasing speed, it's only on truly competitive racing boats that it really makes much difference, and then for only a small part of the time.

Incidentally, not all Alerg designs have large overhangs. The 25D has practically none aft.

John Vigor
CD25D "Jabula"



jvigor@qwest.net

[John Nuttall]

.......more to do with form stability than waterline increase anyway?

Isn't this feature one of the reasons these Alberg designs (and similarly designed boats) feel somewhat tender then harden up as they heel over?

I realize this is a complicated topic, with no simple answer, but it's incredibly interesting. So, if you wouldn't mind John, could you launch into a little discussion about it?

Regards,
John Nuttall
s/v Aimless
CD31 #28
Oriental, NC

ps- Please be sensitive to my Lancashire "daftness" and use small comprehensible words...



Aimless@nc.rr.com

[Neil Gordon]

>>Isn't this feature one of the reasons these Alberg designs (and similarly designed boats) feel somewhat tender then harden up as they heel over?<<

The overhangs only provide fore and aft stability, by adding buoyancy to the bow and stern. The tenderness/stiffness in heeling is a function of the hull shape and ballast.

Absent the keel, the hull is essentially rounded. Some designs are broader and more flat. Compare a log in the water to a board... the log has no stability and rolls easily... the board is relatively stable. So until the weight of the keel comes into play, the boat is easy to roll.

For the first 10 degrees or so, heeling the boat causes the keel to move mostly horizontally and a little bit vertically. That doesn't take much effort. The next 10 degrees is a little less horizontal movement and more vertical. Fighting gravity takes more effort... so the force needed to go from 0-10 is a lot less than what's needed to get from 10-20. Compare this to pulling a playground swing (with a very heavy friend on it). The initial horizontal movement is relatively easy.


Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167



cdory28@aol.com

[Mitch]

OK, so a towed dinghy does create at least some drag and slow me down. If I haul it up on deck (not like there is room on a Ty) it would increase the weight of the boat, causing her to sit lower in the water, increasing the waterline and therefore increasing the hull speed, right?
I still think the answer is to get a bigger boat! Anybody want to trade their larger CD for a Ty? Neil?

Mitch
Ty Galatea