Higher speed with drag!!

[Verytricky]

OK,

The F1 ski racers hit 130mph towing a skier, but when they return to shore after the race they cant go faster than about 80mph without loosing it.

The drag of the skier assists in the boat stability.


So I was thingking.....

( oh shuddup! )


Design a cat type boat that exceeds the design stability you would normally expect, and 'drag' a remote leg to the rear of the boat. This would be flexable, but would increasingly oppose the angle if say the cat started to lift in front. This drag weight would hold the boat down, preventing the flipover, so allowing the angle of attack to be closer to the danger point, and allowing more power than the design should have, increasing speed.

[Willallison]

Can we watch when you try it for the 1st time...?

[stonebreaker]

Wouldn't it be easier to just make the boat longer?

[Verytricky]

Longer would mean heavy?

If you had a light weight boat and a drag tail you could dispence with the additional weight of a longer boat?

[yipster]

still got me puzzled tricky
easy answer offcourse is
downwind is always faster

[RANCHI OTTO]

lower dynamic trim angle=more wetted surface but.... parallel thrust to water level = more prop's efficiency = more speed.

My ASD (Arrow System design) hull shape has more wetted surface but the increase in resistance is much lower than the propulsive total effiicency...

A 13 m boat with 2 x 425 hp + surface prop's Trimax and 11.5 t displacement in tropical condition.....all 12 boats have reached speeds in excess of 40 knots!

[Verytricky]

Yipster - I forgot about you for a while....

Ranchi - The speeds I am looking for are more in the region of 120 knots, and as such a whole lot of things change.

Having not been faster than 90 knots myself, I am unsure of what to expect. I can definately see the change in the way a boat behaves between 70 knots and 90 knots. I can only assume there are more changes to occur when I pass 100 knots.


My aim is to go really fast offshore and set some long distance world records. So I am reading everything I can find and this throws up many posibile ideas. When I have such an idea, the best place to bounce it is in this forum..

[Willallison]

If you are talking offshore, how are you going to ensure that the 'skier' remains in the water?

[stonebreaker]

Well, while I can see what you're driving at with the drogue idea, it's really not all that efficient, is it? I mean, the drag is there all the time, and you really don't need it except when the boat veers, do you?

What about just moving the rudder a few feet aft of the stern, on a boom arm or something? Seems like surface drives do this already. That way you move the center of pressure a few feet astern while keeping the center of mass in about the same spot.

[yipster]

ah but wait, your talking waterski boats, minimum displacement, low wake, made to tow etc, iow special boats! checked the speed record at over 140

[Tim B]

This is a case of keeping the longitudinal stability up, so you're looking at skegs, hard strakes, and quite possibly assymetric hulls on a catamaran.

All a drogue does is to damp the longitudinal modes to an extent where they are controllable. Class 1 offshore powerboats get to just shy of 200mph, and do so without a drogue in sight.

Boat stability (in all cases) is the solution of a six-degree of freedom 2nd order differential equation. Two results are attached for the case where there is no resoring force. Note the effect of damping!!

Cheers,

Tim B.

[Dan Ellison]

We drag parachutes to slow down, does that count? Thats from 260+. And we do it in 1320ft.

[marshmat]

Comparing the F1 ski racers to a 'normal' high-speed boat is perhaps not the best way to go about this.
The ski racers are designed with the knowledge that they must haul the extra drag of a skier around. The hull shapes are engineered with the skier drag in mind. As a result you end up with a hull that performs very well as engineered- with the skier drag- but when that drag is removed it is no longer being used as it was designed to be, and so becomes unstable. The boat is designed around the fact that it will be towing, and so is a very different boat from what you'd get if it were just meant to be a fast boat.
In a boat that's just meant to go darned fast, the preferred solution would be to engineer the hull for the normal running condition- full range of fuel levels, normal crew, no towed object. The boat would be designed to be stable and fast in the most efficient condition you could run it in. Adding extra drag is not necessary if the boat is optimized for normal running.
Take a look at Unlimited hydros for inspiration, perhaps also SST tunnel boats.

[Jimboat]

Getting performance catamarans (tunnel boats) to run effectively is usually a matter of proper designing for dynamic stability. This is sometimes a complex design issue, but most always just a matter of doing the design and analysis work. The balance of hydrodynamic lift/drag and aerodynamic lift/drag is changing at every velocity, so it truly is a dynamic issue...which is why many tunnel hulls behave oddly at some speeds. Never the less, a properly designed catamaran can be designed for optimized dynamic stability throughout it's velocity range and normal running conditions.

[Verytricky]

Originally Posted by marshmat

Comparing the F1 ski racers to a 'normal' high-speed boat is perhaps not the best way to go about this.
The ski racers are designed with the knowledge that they must haul the extra drag of a skier around. The hull shapes are engineered with the skier drag in mind. As a result you end up with a hull that performs very well as engineered- with the skier drag- but when that drag is removed it is no longer being used as it was designed to be, and so becomes unstable. The boat is designed around the fact that it will be towing, and so is a very different boat from what you'd get if it were just meant to be a fast boat.
In a boat that's just meant to go darned fast, the preferred solution would be to engineer the hull for the normal running condition- full range of fuel levels, normal crew, no towed object. The boat would be designed to be stable and fast in the most efficient condition you could run it in. Adding extra drag is not necessary if the boat is optimized for normal running.
Take a look at Unlimited hydros for inspiration, perhaps also SST tunnel boats.

Yes, this makes sense.

I am unsure of tunnel boats in the offshore area. Fine for coves and bays and inshore/inland, but there is swell and waves offshore, and IMLE tunnel boats do poorly in these conditions. I race boats that do 100mph inshore, yet I beat them offshore when I only do 75mph. On occasion they zoom past me, yet for most of the time they are poor handling, poor performing in the real sea.

I always thought that for a reasonable amount of money and power, the monohull was the best for offshore?