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24' long?
anyway you might like to try a place like this for construction ideas:
http://www.duckworksbbs.com/

 

A solid foam hull is heavy. The foam is at least 30kg/Cu.m. A hull will have about 0.25Cu.m. So before you start giving it any strength you have 7.5kg. You should be aiming to build the entire hull for this weight.

Unless you make the glass covering reasonably thick (and heavy) it will pull away from the foam because it will have pinholes that allows water behind the glass and it will pump the glass/foam bond and it gradually fails.

A low cost hull can be made from 4mm ply. Typically any external ply with waterproof glue is good enough if the boat is stored under cover and you do not intend to take it across an ocean.

It is very hard to get a more efficient hull than flat panel. Going to all the trouble of forming a rounded chine will not give much benefit. At higher speed it is a disadvantage.

If the hull is really slender you will need stabilisers and making these can be used as a training exercise for the main hull.

 

I think I'd tend to agree with you, Rick. I've done a fair bit of reading up on flapper-type propulsion system, and all the reputable authors seem to conclude that overall propulsive efficiency isn't that high.

Mind you, the overall propulsive efficiency of a typical propeller drive system (not one like yours, optimised for the boat, speed and powerplant) isn't that good, either. It's probable that the Hobie drive is around the same, or perhaps slightly better, than the efficiency of something like a typical outboard motor set-up.

It may be that the reason some have a view that the Hobie is so good is that they are comparing it with sub-optimal version of other drive systems.

Jeremy

 

Jeremy
The reason people think they are efficient is that they are told that:
http://www.hobiecat.com.au/support/kayaks/miragedrive.html
I have attached an extract that highlights how the term efficiency is misused.

I have also attached my assessment of the efficiency based on some reliable measurements that a friend in the US did.

The Hobie Adventure gets along OK at 8kph. It is an easily driven hull at that speed. The poor efficiency of the drive does not matter as it provides a nice load. It may get tiresome for long duration.

An untrained person getting in a Hobie would do better in it than in a paddled craft. It takes skill to do well in a paddled craft. Technique is important to keep the boat moving freely. Pushing pedals back and forth takes little skill although it would get easier with training as well.

The over-centre action of the bottom pivoted Hobie pedals is not biomechanically efficient either. It is awkward compared to cranks. Top mounted swing arms are far more biomechanically efficient - even better than cranks. I have modeled them and done actual efficiency comparison against cranks in a recumbent position.

 

One more point that is worth noting.

The difference between 8kph and 10kph does not seem like much until you try to sustain 10kph in the Hobie. John is a reasonably fit cyclist, able to hold 25mph on an upright bike for hours but could not get the Hobie to 6mph (9.6kph) for very long.

Greg K also has a Hobie. He was not inclined to try to set a distance record in it.

 

Interesting Rick. I'd not read all the advertising on the Hobie site, but I can see how their apparent claims could be interpreted as stating that their drive is efficient.

Some of what they're saying is correct, but they are confusing force and efficiency to some extent. It doesn't look as if they are actually lying, but they aren't exactly telling the whole story either. It looks as if they are being careful to make claims that can be substantiated (I've no doubt a poor paddler could be beaten by a novice Hobie paddler), whilst letting readers conclude that their product is, perhaps, more efficient than paddling, no matter what.

The tug-of-war claim is pointless, as everyone should know that bollard pull is only something tug boats designers need to worry about. Just as in aircraft, static thrust from a propulsion system tells you very little about the overall performance, plus it is a near-zero efficiency test, as the boat isn't moving at any significant speed.

I think you've hit the nail on the head with the training issue. It takes skill to paddle or row efficiently, but I'd guess that there is very little technique needed to get a Hobie drive working well. For the majority of occasional users this might well make a very significant difference in perceived efficiency.

Jeremy

 

The effect of power required being proportional to the speed difference cubed, something that few seem to full appreciate

I've lost count of the number of people who've asked me how fast their aircraft will go if they swap out the 80hp Rotax 921 for the near-identical size and weight 100hp Rotax 912S. They seem to think that an extra 20hp will give a 20 - 25% improvement in cruise speed and get quite disappointed when I have to tell them that the difference would be really very small indeed.


Jeremy

 

Jeremy
My sister was quite happy with here Hobie when she first got it. After a while the weight made it a nuisance even with the purpose built car lifter aand the wheels for getting it to the beach. She also got a bit of a fright in currents when she found it hard to make way.

She was surprised how much faster mine goes and she wants one. A job that I start soon.

 

Hi, Rick and Jeremy.

One thing I wonder about on the Hobie is the long term effect on knees and ankles, since it is not circular pedaling. Is this an improvement or detraction compared to regular cycling? Maybe we will be hearing about "Hobie" knees equivalent to tennis elbow issues as the number of users grow. OTOH may not be an issue...

I don't see weight and portability being being an issue for the Hobie compared to other pedalled craft, as one can always go with the inflatable model. Efficiency of the inflatable may be even lower than the hards, though.

Vic

Porta

 

Vic
I had the opportunity to use a Hobie Adventure a couple of years ago but I did not have any way to transport it without getting help to lift it. Unless you have some sort of loading rack they are very difficult to transport unassisted. Weight is a key feature of this type of craft and rigid ones can be made lighter than the Hobie but probably cost more.

 

This is the Hobie which I'm thinking could be easily handled:

http://www.hobiecat.com/kayaking/models_i12s.html

Fits in a car trunk, or can easily be lashed topside fully inflated without scratching car's paint surface. There are now 12v high speed inflators that can inflate and pressurize to the normal required pressure of 2.5# in under 3 minutes for that size boat.

V.

 

You know the saying with airplanes,
If it looks like it can fly ......
but this thing looks like a tub !
did you see Wizard's little 9 foot tub in the 1200 mile florida UFC?
you'd not think that was going to do very well either, but it did!
and the hobie ai tandem of pelican's didn't finish, too bad.
different boats for different things and different people ....
they all got their place ...

 

Are you still planning to do a direct drive, or use the belt drive?

Michael Lampi

 

Since last September (2009) I have been without a fast boat for racing. The so-called racing times posted for me since then have been in a Nauticraft Escapade, which is a great photography platform but in no way, shape or form a speed demon.

The top speed of an Escapade is about 5 mph, with cruising speed at about 3.8 to 4 mph. On the other hand, it can carry three adults and is comfortable to pedal in freezing temperatures and keeps you dry in 3 to 4 foot seas.

Michael Lampi

 

Direct drive with the 1:2 Mitrpak box. If George does not get an answer to me on the cranks by the end of April I will buy the standard box and make cranks.