Help with power/sail cat hull design.

What Alik said. The difference between the sail boat and the motor boat is mostly about weight distribution and propulsion. For an OB powered cat, you need to keep the props in the water. Hull efficiency isn't very important if your props are spinning air half the time. So the pitch center tends to be/ought to be/stuff works better if the pitch center of the motor boat is further aft. The rig of a boat such as an AC 45 has a big effect on where the center of pitch rotation can be. The motors and fuel and tankage have a big impact on where the pitch center of the motorboat will be.

The sailboat hull will experience large sideforces overall. How these are handled varies, and the impact on the hull can be minimised, but one still needs to ask the question when the size of the missing side force can be about ten times the propulsion force.

It seems somewhat silly to go to all that effort on the hulls and slap a couple (not particularly well suited) OBs on it. A concrete monohull weighing 4 to 6 times as much if powered by a single small diesel could probably match you on efficiency.

 

Second page already and all the usual can't do it, shouldn't do it rubbish. ..

The main issue I see with this is the ac45 hull has a full transom stern which is not ideal for 10kt cruise, even less appropriate for a 6 - 8kts cruise. The ac45 sails around at 16kt minimums upto 30 plus in the right winds... A non transom stern will be more efficient at the pproposed cruising speed. I would still keep the high length to beam ratio.

The other issue is the volume of the hull. Ac45 Sailing Weight Is A Little Over 1000kg AFAIK And The Weight Is Concentrated Aft to account for the strong forward thrust pitching moment generated by the very tall rig. The volume in the bows is quite low and doesn't contribute a high restoring moment which was balanced with the rig response to give a low pitching characteristic in inshore waters where they operate the regattas. If you change any part of the boat as a whole or the operating speed or the typical wave encounter frequency or height, the likely response of the hull in a seaway will no longer be favorable.

So you need to consider the accommodations you require and the estimated weight of the ship and the operating sea state environment before looking at the hull shape , volume distribution and hull dimensioning. ..

 

Awww cmon - be fair.

"cant do it" - without proper plans.

You are talking about the strategy, when the original proposition thought they could just easily get some simple lines and guesstimate the scantlings.

if thats your idea of "shouldn't do it rubbish. " I dont know why you chimed in with "So you need to consider the accommodations you require and the estimated weight of the ship and the operating sea state environment before looking at the hull shape , volume distribution and hull dimensioning. .."

There's nothing been submitted that didn't add extra valuable info on the OP's problem.

 

Conceptcat,

Nice set of hulls you posted !

You can do everything you said and it will work, however there are (expensive) problems coming from what you want to do.

I think most of the problems have been mentioned (one way or another), the thing to remember is that a boat gets designed for a specific purpose, when you mix purposes there are trade-off's which are not always desirable.

A sporting sailing hull is not designed to be loaded heavily like live aboard. It will float though, unfortunately it may not float as you expected

Most cats have outboard motors, and they all work. The stern of a sailing boat however acts different "out there" (as to in a harbor's calm waters) compared to a power boat hull.

Unless you want to go racing, then the hull's design is not so critical.

As someone said, you start by calculating the weight you want to tote around with you, including the boat's weight, water, fuel, pots and pans, batteries and and and, and put everything in there. Then ask the wife what she wanted along, then triple her list weight Then you want a bit of reserve weight for the things you forgot to add or change your mind about later

This weight will determine your draft, hull(s) size, shape, etc. Once you start adding things up you quickly realize why a cruiser can weigh 8000kg's... that's EIGHT TONNES

Gawd knows what people put in those things that weigh so much, besides structures and stuff to keep the boat intact in bad weather. No boat designer want one of his boats breaking because it will hurt his reputation.

SO, what everyone is saying, design something that is going to work for your purpose. It will not work well if you take a set of sleek sailing hulls and use it for something else. A bit like using your car to plow the land instead of a tractor...

If you like the idea of a cat with living space in the middle, you may want to look at trimarans. They can be motored too, and some of them are bloody nice !

Personally I would consider (which is my humble advice) to make your rig sailable. It's fun, you can go weeks without refueling $$$ and so on. If you can afford hulls like those, what a pleasure ! but don't go live in them, those you sail until their bottoms wear out

Shop around, watch some youtube video's on the sailing and power boats, look at as many as you can and pay attention to the things you think may be of interest. The more you explore the more you will realize what you are after and what will work the best for you.

Btw, there is no such thing as a perfect boat. In a way they are a lot like wiemen, each has different features (I'm having trouble not going explicit here) and they all have drawbacks and hangups, and don't forget bloody expensive. It's all about what you can bear to stand and live with If it wasn't that boats don't talk back, most blokes would simply get one more wife instead

Good luck !

 

LOL. One of my bean counter friends quickly calculated the amount of flotation foam I used is far less than the water a filled-up hull could hold... so the boat would sink like a stone if water gets in

 

That cannot be true!! If you use that logic then a boat going backwards will be as fast as one going forwards, even if it had a large flat transom.

There is very little weight difference between different A class or F18 cats, and they all have the same sail area and length. Yet some designs are clearly faster than others. So it is changes in hull shape that make one boat faster than another.

Even if you are not racing it is nice to be able to get home before dark. And if you are in a powerboat a faster hull shape means less fuel is used.

So an efficient hull design is always essential

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard, I would say slenderness ratio or DLR is the primary parameter. Other parameters are of secondary important, giving change in resistance not more that few percent. So, hull design should start from table of weights.

 

Not sure what the argument is since boats don't sail backward.... even with or without large flat transoms ?

I'd say that makes their design critical. Of course it is the hull shape. And the sail shape among other things.

Of course. I haven't heard of anyone yet who wanted a slow, inefficient boat that can be as heavy and unpleasant to sail as possible yet.

Still, unless you want to go racing, then the hull's design is not so critical. That is why most commercial boats are heavier than they could be, hence they are slower and besides, not every one can afford a carbon fiber boat. Oh you can go racing in anything, how much good it will do remains to be seen.

 

I think the point is that considering coefficients blindly can give results that don't correspond with reality.

 

Since racing hulls are on their one hull most of the time (or supposed to be), why are they designed to function optimally upright only ? The hull bottom shape should be rotated outward and compensating for the additional weight (Alik) so the hull shape should be designed from upright to over 45 deg, and the dagger board should sit outward at an angle of about 15 deg.

 

I would agree, but, whatever the SLR, the actual hull shape is still important.

I didn't say it quite right, I meant if you rigged a boat with the blunt end forwards it will not be as fast as one with the pointed end forwards. So, taken to the extreme, hull shape does matter, even if it is an Extreme 40

Richard Woods

 

Well said Richard.

A hull doesnt have to be operating at its peak all the time, but at lower speeds, it will have twice the performance that a badly designed hull will have.

 

Rubbish. .. I did.

Remind me again, is this boat design forum or boat plan sales forum?

 

There is a belief that if one can take hull shape from racing boat, the new boat will also perform. But it will not! Start from weights study, with different DLR optimum hull parameters will differ. It is a big mistake to start the design with hull shape; though common for amateurs. "I will take proven hull shape from..." or even "I got the mold of ...".

Make estimate of weights including margin for omitted items and for growth, make sketch of general arrangement trying to fit accommodations and engines, and only then once will see the baselines to draw the hull around. In this particular case, I believe LCB should be shifted aft compared to racing boat, that means full stern.

Naval architect will design the hull purposely for the boat. It makes absolutely no sense to copy hull from somewhere, as it will end up a failure once load and its distribution is different.

 

Well "did it" is pretty relative. I have followed some of your build posts, and we can debate whether a 'proper' plan would have given you better results if you like.

And lets add how many years you have been around boats, compared this OP.

This forum is about getting the best result for the least dollar, and if you don't think adequate knowledge, proper planning and research gets best value, then you are in the .3% of 'constructionists' around the world.


Ps - oh, for those interested, look up the number of Threads started by Groper seeking advice on his 'ad hoc' build. eg http://www.boatdesign.net/forums/multihulls/catamaran-composite-beam-design-42873.html
Interesting reading.
Who did you end up getting insurance through in the end Groper ?