How fast are beachcat hulls ACTUALLY CAPABLE of going with lots of motor power on the back?

Supposedly beachcats are displacement boats and therefore have a maximum hull speed, since they are not capable of really planing.

We have a Hobie 18 that has been converted into a powerboat (no mast) on it. We put a fiberglass "pod" on it that is fastened to the main and aft beams. The outboard motor mounts on the aft end of the "pod," and the front end of the pod has a console with steering wheel and controls for the motor.

Originally, we had an 18-horse motor on it, and the boat would go just under 25 mph (verified by radar gun). That motor got stolen, so now we have a 30-horse motor on the back, and Rick thinks the boat goes about 30 mph (on flat water).

This is including the "pod," which weighs about 150 pounds, and the motor, which probably weighs over 100 pounds, plus a 200-pound person on the boat.

So what I am curious about is whether this means the hulls are capable of going that fast (even with at least 450 pounds of "crew" weight) and, therefore, they could potentially go that fast under sail if they had enough sail area, and with the center of effort positioned correctly.

Obviously, with a heavy outboard motor on the back, the sterns squat down, and the bows are up and there is no tendency to pitchpole.

I just think that if you can prove the potential hull speed of a sailboat by using a motor, you can then figure out how to design a sail plan with enough equivalent power to get it to that hull speed without pitchpoling. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Am I wrong?

No disrespect, but I think you are, unless you are designing for a very narrow wind range. It's a fact of life that if you want to be powered up in light air, you'll be struggling in high wind, and vice versa...

I've been clocked by the Michigan State police going 90 pulling a beach cat. Very expensive time trial! I believe I have additional undocumented runs around 95.

Obviously, I am not explaining this well.

There have been past threads on this forum, sometimes hotly debated, about how fast a beach cat can go. Some have said that the top potential speed is slightly under 25 mph (or maybe it was knots, I can't remember). There have been anecdotal reports about going as fast as 30 mph. People have been trying for a long time to get gps readings that confirm speeds into the mid and high 20's, but most are just brief spurts, like when surfing on a wave.

Some have said that the limiting factor is that most catamarans have displacement (rather than planing) hulls.

So the question is, what is the upper limit of the potential hull speed?

In almost all cases of beach cats going very fast, the thing that ultimately stops them is the bows digging in and causing a pitchpole.

Therefore, based on speeds with outboard engines, it seems obvious to me that, even under sail alone, if you can get enough weight at the back of the boat, keeping the bows up out of the water, you would be able to achieve much higher speeds than have so far been recorded.

Like, when it is blowing about 25-30 knots, strap some 100-pound bags of sand on the back of the boat, rake the mast back, put up a spinnaker (to lift the bows) and see how much faster you can go.

Also, I wonder how much faster it could go if you leave the mast off and use a kitesail. <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

1997 Bermuda World Tornado Championship, during an event called the Bacardi Blast sprint. A timed run between gates that were 500 feet apart.

The Record stands at 27 MPH or 23.5 knots average over the 500 foot course. Completed in 12.53 seconds.

I'm sure you have all seen it, but here is the short video of the event.

http://www.youtube.com/watch?v=do7nWUgijZs

Yes, that is one of the events I was trying to remember. Do you know what the wind was for that?

I have never heard what the wind speed was, but by judging the water in the background and considering it is a protected section of water, I would venture a guess of 18-20 knots. This could be a good discussion on its own (how much wind is in this picture?) That would be good practice for goobers like me to judge conditions. I know that my normal stomping grounds will start to white cap at around 12-13 knots if the wind is against the tide. But I have seen it blowing over 20 knots with no white caps and light chop when going with the tide. I have almost put my behind in a sling a couple of times trying to figure out the conditions. Other than erring on the side of safety or knowing the conditions will get worse, I could have had an unscheduled appointment with rescuers. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

modern hull designs will plane.

Some of the older ones would as well (see warbird's Tigershark project.)

Let me caveat this message with the fact that I am not any kind of engineering expert. I just read a little... plus I stayed at a Holiday Inn Express once.


I would assume that most catamarans plane since if they were displacement, they would be tied down by their theoretical hull speed. According to the Annapolis Book of Seamanship, hull speed is calculated as:

Hull speed = 1.34X (square root of waterline length in feet)

That would put the hull speed of a 20 foot beachcat at right around 6 knots. So anytime you are sailing over 6 knots, you are probably planing to some degree. Right?

-Rob

No; Cats, because they have long slender hulls do not obey of the hull speed rule.

Is this really part of the "Test"?

Okay, Pete, since you jumped in on this: If you put a 30-horse outboard on the back of your Tiki, how fast could she go? Or a 50-horse? Or a 70-horse?

This formula is derived from Froude's law. Froude's law is derived from how the wave created by the bow interacts with the wave created by the stern, as well as how the waves are related to the boat speed. If these two waves interfere contructively, you will get a big increase in wave drag.

The Froude's number is given by:

Fn= Boat velocity/ "the root of" (g*length in waterline)

Typical Froude's numbers:

Type of boat------------length(m)------speed (kts)------Fn

Oil carrier-------------300------------16,5-------------0,16
Catamaran(transporter)--35-------------35---------------0,97

As one can see the catamaran has a relatively very high Froudes number. These wessels are designed to carry people fast and safely between islands etc, and so that requires high speed, but also a not too big vessel that easily can be handled in port. The answer: Long, slender hulls in the water line. These hulls creates alot less wave drag as they "pierce" through the water. The slenderness however means loss of stability, hence why one have to have more than one hull.

When asked of what the max speed for a high performance beach cat was, my hydrodynamic prof. simply said: "there's no such thing as a theoretical max speed. If you have enough power, you can always go faster."

That been said, personally, I think the Tornado blast contest already mentioned here gives a pretty good estimate. Tornados are wide and have alot of power. In addition, the record set in the speed contest, was done by a highly skilled crew.

Expecting Wouter to fill in here as well.

There is always a limit at some point.

Why don't you post this question on boatdesign.net to see what answers that you get there as well?

It wouldn't matter. Max for that boat is around 12 knots, after that the rooster tail would just get bigger.

I'll bet posting that on Sailing Anarchy, with someone like Wouter to keep the pot stirred, would be quite entertaining.
:::Wicked Evil Grin::::: <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

Here is an image of the first run of the boat. Tiger Shark 18 foot Ron GIven design.
I sailed for two hours just checking everything that was wrong with the rig and boat (which was plenty).
The boat sailed faster than my Nacra 12sq and NEVER tried to pitch. The Nacra would have been a hand full at that stage and I would have been constantly altering trim with the bow an inch or two from the surface. My Hydra 16 would also have been pitching.
When I got onto this boat I stepped over the bows slopping sand spots on both hulls. The sand was still there on return to the beach.
The boat planed up wind and down.
most noticeable difference in driving the boat is a much lighter and more stable feel.

My reasoning is very simple here.
The TS was designed on the Paper Tiger basic platform and stretched to be a
30 foot twin trap rig.
WHile the hulls my want to pane thay cannot in the twin man guise as there is not enough boyancy to get it going as the power drives the hulls down into the water.
I have cut the rig down by more than 5 feet. I have square topped the old sail and taken some amount off the foot length. I have raked the rig back a lot.
I am at present one man up and so the power quickly pushes the hulls up and out of the water.

I have ben working getting my H17 together this last month as it is my daily ride and can now go back to this project.
Whirlwind are making new sails for it and I have to get into heavier weather.
Again, my stated goal is 25 knot runs over the distance the Tornadoes were running.

I guess in this case the bottleneck will become the structural integrity in high temperature/high pressure conditions.

If you provide progressively higher power and manage to keep the boat stable and in one piece (vibration, ressonance, etc.), the ultimate limit should be when the hull colapses and burns (or vice versa) due to the difficulty to dissipate the heat caused by hull-water friction. It shoud happen at a few hundred knots speed.

If this problem is solved, maybe with ceramic foils, about the same thing will happen at a higher speed but with hull and rig friction with air. I believe this is the actual limit for a space shuttle. Now we are talking a few thousand knots.

Sailboats are far from both limits.