Trying to design my own cat.

Why rocker

Yes Richard you are missing something

Most cats have transoms and are not double ended. This means that they need to have the transoms bottom near the waterline. This means the stern must slope up near the transom.

Some cats have had less rocker - Hitchikers and earlier Wharrams (double enders ) are in this category. They have more trouble tacking. Rocker is actually a safety feature - not a racing feature. Racers want less rocker usually. Fast cats and tris have pretty straight keels and slower more burdensome boats need more rocker to keep the hull volume up so they can carry weight and get the transom at waterline.

So no conspiracy at all - just common sense. At the risk of repeating myself ad nauseum - get out on a few different boats. A bit of real world sailing will help you see why boats are the shape they are. No conspiracies just well worked out solutions to problems. I think your boat will be slow, really hard to tack and wet and on top of this almost impossible to sell. I think you should buy a plan from Richard Woods or someone similar. I find your lack of knowledge really frightening if people are to trust their lives in your boat. You are making some really simple mistakes that all design books and most builders could talk to you about. The internet is not where all the knowledge lies - it is out in the real world.

Cheers

Phil Thompson

 

Hi Phil.
According to Godzilla software, my hulls with one ton displacement will be pushed through the water more easily if the hulls are narrow and deep with flat rocker.....so I'm not sure that all of your sailing experience has led you to the right 'knowledge'. Maybe it has, because michlet doesn't account for wind waves.
If you can explain to me why my boat will be slower than it should be....that would give me a clearer picture.

If you want a transom that sits at waterline....then you want a hull that provides lift at higher speeds. So perhaps the slow boats that you are talking about, are not really that slow and burdensome. If they are, then why have the transom and rocker?
You say that rocker is actually a safety feature. Safer because it allows more buoyancy?? My boat has plenty of buoyancy. Not sure what you mean by 'safety feature'. Could you elaborate?? I don't want to be making simple mistakes that could easily be avoided without hours of modelling and testing.

I am a bit odd perhaps, in the sense that I don't care about resale value. Nor do I care about tacking ability, and nor do I care about getting wet on a hot summer day. I don't care if my design is not popular. I'm more interested in the hard science. No-one so far has explained to me why or how a hull with rocker and transom, is a better solution. I can't find out by sailing a bunch of small cats unless one of them is like mine.
I still haven't bothered to read any books yet...I've been kind of hooked on the internet.....so you are right about that.

 

Something like that Richard, most boat design considerations are a compromise between achieving performance criteria - - tweak one way and loose on another - - fat bum/pointy bow allows benefit in some seas/conditions/points relative to wind/swell.... Flat rocker (reduced banana as hobie called it? makes change of direction less rapid) therefore on the "hobie surfcats" lots of rocker to weave in and out of waves/breaking water for max manouverability in messy situations.... John Hitche's "XIT" - images on the BD net as well as in my gallery? of posted has NO rocker, is wasily driven and sort of holds a straighter course (most seas)

Hey, all I do is look at other boats and try to figure out what makes them perform differently and what do I want and is that the best way to achieve that performance / behaviour objective???? Confused? I am now.....

 

off your rocker

Dragging a transom is slow, turbulent and looks very ugly - that is why rocker is introduced in the after sections (often more rocker than the forward bow to amidships sections) to get the transom clear. Canoe sterns that drag are not quite so bad as wide transom designs - but dragging any transom, repeat, slow and ugly. Most boats are overloaded hence more transom dragging - then it becomes a Catch 22 situation, more weight, more drag requiring even more rocker to get the overloaded transom free, then because of more weight and designed in more rocker to counter weight, more pitching. Round and round in circles we go. The only solution is to have a clear view of what you want and then stick religiously to it, no compromising, no excess weight and junk carried aboard, get the drift? Weight is the dirty word. Keep that down and your boat will sail sweetly. That is why everyone here has been telling you you can't carry a big crew on your small boat. And no matter what you say, if your boat sails like a dog, you and your friends will not be happy. Have I repeated myself? - quite possibly.

 

This is what I'm also saying at the risk of repeating myself

It's just going to take a little time... for a LOT of piece of mind.

 

Gary
I can relate to most of what you say. The point on the canoe stern is incorrect though. Think about, if it was such a detriment to performance you would never consider having a bow deeper than the waterline. Canoe sterns offer the most efficient form for energy recovery.

If you want to see efficient hulls then take a look at rowing sculls. They offer the best speed for power input irrespective whether they carry 2, 4 or 8 rowers. No flat raised transoms amongst this lot.

Taking some logic from your points above and the correct understanding of a canoe stern you can deduce that an overloaded hull with a canoe stern will be more tolerant of the overloading than a hull with a wide flat transom. Maybe Richard is onto something.

Canoe sterns have recently been rediscovered with power cats so it could be Richard will set a new trend amongst sailing cats.

The older cats with canoe shaped were built in a time before composites were well developed and sails could develop the efficiency they can today.

Rick W

 

See Richard, we got all sorts of advice coming - all because you typed - - whatever - - a catchcry or two occasionally does work wonders.... and I bet there will be more advice coming....

 

I am not responding to the profanity. In fact I was put off by it in a public forum no matter how it was disguised. Just felt obliged to put the case for canoe sterns on efficient hulls.

And don't take that to mean I do not like raised wide transoms on sailing boats. I do not have enough experience with either type in a range of conditions to offer advice from experience.

I know a wide stern will power better above hull speed but I figure that is not where Richard wants to operate. He can buy a kite board if he wants to go fast under wind power.

Rick W.

 

Rick
The Australian C Class Quest cats had canoe sterns but even compared to today's designs in higher technology materials, those boats were still very light - and they went through the water with very little disturbance - but remembering English criticisms of that period, they observed the Aussie boats pitched too much. Now those Quests had very little (to almost zero) rocker if I remember correctly, the hulls were race boat fine and yet they did not handle UK sea conditions as well as the broader transom MacAlpine-Downie designs. Going further back with fine (asymmetric) hulls, canoe sterns were the CSK designs from Hawaii - and they had considerable rocker - and they pitched badly also in certain sea ways. Your point of canoe bow and immersed canoe stern being fast on rowing skiffs: okay, correct but I personally do not like deep bows on sailing multihulls - they can overcome or tend to affect steering if the bow is immersed too far - but I accept your point of the canoe stern (and pointed it out earlier) being less of a problem than a dragging transom design. Why am I writing this round the merry-go-round stuff - as long as it works, there are no rules. But the operative word is "works" - and the broad, asymmetric sterns of Richard's design are going to turbulate.

 

Gary
Agree that some flat sections fore and aft will help with damping pitch. I think there is good prospects for horizontal foils to do it more effectively.

I have not followed how the asymmetric hulls developed. I can understand the value above the waterline, been on a beach cat with asymmetric hulls and have played with asymmetric model hulls but I do not know what Richard aims to achieve.

Rick W

 

Rick, firstly thanks, - - - I think Richard, on an earlier post, implied, it was to get maximum effective beam on a 10 ft overall width build - if I have read and understand correctly....

 

Been away for a bit. Thanks for the replies everyone.

Just to be clear, I haven't done any analysis on the hulls I have drawn. It's just a sketch. When I said I was conclusive, I meant that there was nothing more I wish to conclude without the help of a NA. The only big unknown remaining for me is the final hull shape.

My proposed hulls are asymmetric on both the longitudinal and transverse axis.

1) The longitudinal asymmetry below waterline, is just a result of the flaring of the tunnel walls, and the desire to keep a wide floor to stand on. I thought a bit of longitudinal (hope I'm using the right word) asymmetry shouldn't hurt too much. In fact, it might help to stop the boat slipping sideways when it is heeling slightly...thus minimising the need for dagger boards.

2) As for the asymmetry on the other axis (wider stern and narrow bow), this was just a wild guess, based on comments made by Gary, and stuff I have read about canoes. As I mentioned in a previous post, paddled canoes go faster when the max. beam is slightly aft of centre. Couple this with the notion that a thin bow and fat stern will help the hull to negotiate waves, then the obvious experiment is to try it out. However, modern asymmetric canoes are powered by the alternating motion of paddles, so the reasoning for their shape could be irrelevant to my design.

3) There are two reasons for the asymmetric flaring above the waterline. The first reason is to maximise the BWL, as Masalai pointed out, within California's 10 ft trailer restriction. The second reason is that I wanted to devote the flaring to the tunnel, to maximise the boat's ability to lift up over the tunnel waves, which will be bigger than the waves on the outside. This is important because my clearance is little more than a foot. The centre of gravity must be kept low, the BWL must be kept wide, and the bridgedeck pounding must be kept to a minimum.

Gary has pointed out that the fatter stern will create turbulence, so the elusive equation is: total hull drag vs performance in waves. Remember, I am not trying to design a competitive boat. This performance equation is impossible for me to work out at this stage. This is why I have shown all my guesswork.....to see if people say "Richard, you're an idiot"....or "Richard, it is definitely worth testing". Good to see it hasn't all been "idiot".

Rick, do you feel like whipping up a visual suggestion? (I like watching your sketches popping up here and there in other threads). If not...I understand. I just thought there's no harm in asking. Obviously, I would need to take into account your lack of knowledge with waves. Perhaps I, or the NA will need to build and test two models - one similar to mine and one similar to Gary's.

Just one other thing....those of you who feel like you are repeating yourselves....please don't think I am being ignorant. I never skip-read anyone's comments.

 

I think I was supposed to say vertical axis...not longitudinal...sorry. You catch my drift.

 

Bloody hell, I keep getting mixed up with the different axis names, and axis directions. Again, I apologise if I have got it wrong.

 

Don't worry about it, you know what you mean and all the following posts will identify and clarify things hehehe (masalai is in a cheeky mood again/still)