Sunfish Hull Speed...?

Davidasailor26 · Jan 9, 2019

The hull speed of a displacement hull is 1.34 x the square root of the waterline length. If the loaded waterline length of your sunfish is about 13' (LOA is 13'9") then the hull speed is under 5 knots. However, a sunfish will go up on plane, so it's not really limited by the hull speed. It takes some extra energy to get a hull out of displacement mode, but it can be done. Many racing keelboat designs will do this as well, often under Spinnaker in a decent breeze.

WayneH · Jan 9, 2019

I think the HULL speed of a Sunfish is way lower than your friend says. BUT, the Sunfish is a PLANING hull and will exceed hull speed easily.

David beat me. LOL

jon hansen · Jan 9, 2019

Davidasailor26 said:
The hull speed of a displacement hull is 1.34 x the square root of the waterline length. If the loaded waterline length of your sunfish is about 13' (LOA is 13'9") then the hull speed is under 5 knots. However, a sunfish will go up on plane, so it's not really limited by the hull speed. It takes some extra energy to get a hull out of displacement mode, but it can be done. Many racing keelboat designs will do this as well, often under Spinnaker in a decent breeze.

+1
every sailor should understand what hull speed means.

Brian D · Jan 9, 2019

A Sunfish will ride the bow wave especially down wind. I raced a Sunfish against a 32' on the New River in NC. Anything north of a broad reach the 32 beat the pants off of me. But downwind, the 32 could not keep up. So is the SF capable of speeds in excess of "hull speed"? Oh heck yeah. But since the SF is such a small boat, speed perceived is perceptive at best. Unless one has some kind of measurement device then one can say the SF flies like an eagle. I think downwind the SF could reach 6+ knots. JMOH

Will Gilmore · Jan 10, 2019

As everyone is pointing out, the term "hull speed" isn't a limit. It actually refers to a condition where the wake generated by an object's progress through the water is equal to the length water line (LWL). It is at that point that the efficiency of force to speed ratio begins to change dramatically. Any faster and the boat is having to sail up hill because it will be climbing its bow wake. This requires much more force than lower speeds when the hull is essentially riding flat. Hull speed is incorrectly referred to as theoretical. It's not a theory, it's a definition. The wave dynamics behind the term are well understood and consistent.
Once enough force is applied that the hull makes the top of the bow wave, the hull is mostly out of the water and efficiency drastically goes up again. You can feel it in a powerboat. As the boat climbs her wake the engine labors. When she reaches the top of her bow wake, she flattens out and takes off. You have to pull back on the throttle to keep her from bursting suddenly into high speed. This is an indication of the increased efficiency of both reduced wetted surface and not having to expend energy displacing water, moving a volume of water out of the way.

Sunfish are light for the sail area they have and they are flat bottomed. They are also a crab claw rig, which are one of the more efficient rigs. Exceeding hull speed should be fairly easy for them. They do best where the hull remains flat to the water. On a heel, they lose their planing shape.

If using a GPS to measure speed, be sure to account for current because hull speed strictly refers to speed through the water, not speed over the ground.

-Will (Dragonfly)

Hunter216 · Jan 10, 2019

Will
Thanks for that synopsis, I’ve read a few technical descriptions of this before but always got lost in the detail.

Check this out if you want to think about “monohulls” planing!!

Can’t wait until these become available on EBay <grin>

Will Gilmore · Jan 10, 2019

Simon Sexton said:
Typically, when a sailboat exceeds its maximum hull speed, it runs the bow into the water and causes the boat to pitchpole or potentially sink.

By the way, this is not typical. There are instances of old Clipper ships driving themselves under the water, but it usually takes much more than a building bow wave and heavy winds to sink a mono-hull. Catamarans are a little different story. They don't have as much forward flotation, but still, it would require heavy winds and a large wave to do that. Maybe catching a lobster pot on a fin keel could "trip" the boat under the right conditions. Surfing down a big wave into the trough with the wind really driving the sail could potentially do that to any boat. I sailed my hobie 18 into a 6' sea in 20 knot winds once. I had to put on a dive mask so I could see. We were under water almost as much as on top of it. However, never did we come close to pitch poling even though we were definitely exceeding her 5.6 knot hull speed.

-Will (Dragonfly)

Brian D · Jan 10, 2019

Hunter216 said:
Will
Thanks for that synopsis, I’ve read a few technical descriptions of this before but always got lost in the detail.

Check this out if you want to think about “monohulls” planing!!

Can’t wait until these become available on EBay <grin>

This reminds me of a lizard with hot feet.

Wonder if the AC75 folks used this concept for designing the foils?

Hunter216 · Jan 10, 2019

Will
There is an urban myth that there is a large cache of beer on the bottom of Lake Ontario where I grew up. One of the possible sources might be that there was a bunch of guys from a high school football team on a pontoon boat one day and a couple of the bigger guys decided they should run forward just as the captain went for a full plane to neutral. Of course I don’t have any first hand knowledge of said event!!!

Hunter216 · Jan 10, 2019

Simon Sexton said:
They just seem like a big flying safety hazard to me.

Same could be said with some beer can races when the competion juices kick in. The people that decide to “fly” these things have to be a little crazyto even step aboard. I think I would rather be a spectator.

Hunter216 · Jan 10, 2019

Brian D said:
Wonder if the AC75 folks used this concept for designing the foils?

I used some digital software to overlay the two images and I think you are on to something. Not the first time man has skimmed an idea from nature!!

Will Gilmore · Jan 10, 2019

Hunter216 said:
There is an urban myth that there is a large cache of beer on the bottom of Lake Ontario

It must be a huge cache of beer for such a myth to persist. One could spend more than a small fortune trying to recover something that may not be worth that much just because of a myth.

https://en.m.wikipedia.org/wiki/Oak_Island

Just saying. :beer:

-Will (Dragonfly)

Hunter216 · Jan 10, 2019

Actually not a myth (well the story I told was!)
In prohibition there was a steady flow of alcohol from Canada (Ontario) to USA (upstate NY). They used to hang the bottles over the side in grain sacks and when the Coast Guard would approach too close they would just cut them loose and let them sink. Such a waste!!!! Just saying

Hunter216 · Jan 10, 2019

Will Gilmore said:
By the way, this is not typical. There are instances of old Clipper ships driving themselves under the water, but it usually takes much more than a building bow wave and heavy winds to sink a mono-hull. Catamarans are a little different story. They don't have as much forward flotation, but still, it would require heavy winds and a large wave to do that. Maybe catching a lobster pot on a fin keel could "trip" the boat under the right conditions. Surfing down a big wave into the trough with the wind really driving the sail could potentially do that to any boat. I sailed my hobie 18 into a 6' sea in 20 knot winds once. I had to put on a dive mask so I could see. We were under water almost as much as on top of it. However, never did we come close to pitch poling even though we were definitely exceeding her 5.6 knot hull speed.

-Will (Dragonfly)

I had an "interesting experience" windsurfing one day. Sailing overpowered in voodoo chop, stuffed the nose into a wave that just appeared with no warning and did an amazing imitation of a tether ball as I rotated on my harness lines. Also did one of those on a Hobie trapeze - We DID pitchpole!!! You would think that I would know better but just acquired a foil for my windsurfer as a 65th Christmas present to myself.

Hunter216 · Jan 10, 2019

Simon Sexton said:
a friend and I had an interesting conversation about Sunfish

Interesting how the initial conversation turned into a treasure hunt for sunken beer. Oooops that was me. Sorry!!!!

Joe · Jan 10, 2019

All these years this discussion has persisted and still so much confusion. Back in the day, the theoretical hull speed formula was commonly used to determine optimal steam engine size for a given size boat. It's predicated on the fact that full displacement hulls move through the water, not over it.. they must push aside the water to make progress. This movement creates a wave that essentially traps the boat within its length. The phenomenon places a predictable limit on speed through the water that cannot be exceeded without an exponential increase in propulsive power, requiring massive increases in fuel consumption, weight and lost cargo space to accommodate them.

A small flat bottom boat with a dagger board, such as the Sunfish, has no relevance to the theoretical hull speed discussion... it's designed to move over the water, rather than through it.

Will Gilmore · Jan 11, 2019

Joe said:
All these years this discussion has persisted and still so much confusion.

I would guess it is all in the name. People are looking for simple rules of thumb and fail to realize that "hull speed" is a specific term for a specific condition. Instead, they look at it as a limit and are pleasantly surprised when they discover they have somehow managed to sail faster than their "hull speed". For a displacement sailboat, it is a good measure to determine how efficient your sailing skills and rigging trim are.

Joe said:
hull speed formula was commonly used to determine optimal steam engine size...Sunfish, has no relevance to the theoretical hull speed discussion

what you say may be true except, all boats, no matter their designed bottom shape, move through the water until they have managed to climb that wave and get over the water. The discussion here is, in fact, about whether the term hull speed is appropriate to use for a sunfish. What would be an interesting discussion is about at what point has a boat stopped climbing that wave and reached a plane. All hull shapes can plane, but, while they have a hull speed based completely on LWL, they don't all plane at the same point, power wise. Their CG needs to clear the crest of their bow wave and different shaped bottoms require different amounts of force to get there. Perhaps, while it takes more or less force to climb their bow wave, the speed at which they are going when they achieve their planing speed is the same. I don't know the answer to this. But, if you are designing a planning hull, it would make sense to know how much power is expected to be needed to exceed hull speed. Or, is it enough to simply draw a boat with a flat bottom and put the biggest sail you can on it?

-Will (Dragonfly)

Hunter216 · Jan 11, 2019

Joe said:
Back in the day, the theoretical hull speed formula was commonly used to determine optimal steam engine size for a given size boat.

Yes but the times they are a changing as they say. We now have 100 foot foiling trimarans that ‘liftoff” in 15 knots with “speeds” of 30-40 knots that can “fly” above 4 metre waves that are going to be single handed around the globe.
I assume they also have a “hull speed” but the clear lines of definition of the past seem to be blurring.