Design for worst case scenario

[Mik the stick]

In my quest to learn about design I have seen many craft designed with different transoms. I think I know that a displacement hull is the best type for a smooth ride in rough conditions. A rounded stern is also probably best in rough conditions. My design attitude to designing anything is to design for worst case scenarios. But every design is a compromise. My question is how bad is a flat transom on a 36'+ Displacement hull type boat. With such a transom a 35' boat could have nearly as much room in it as a 40' boat, the rudder could be hung outside the hull giving a bit more turning effect.

But there must be a downside, will it handle really badly in a following sea.

 

[PAR]

. . . My design attitude to designing anything is to design for worst case scenarios . . .

You've run smack into a common misconception that novices trip over, which is to attempt to design for eventualities that will never happen. You will at some point have to absorb the principles, dynamics and engineering necessary, to accept the realities of placing humans, in unnatural environments.

Flying along at 35,000 feet is an unnatural environment for a person, so appropriate safety margins and redundancies are incorporated, knowing full well that in spite of your best efforts, you could still find yourself falling from that altitude. Some acceptable risk is necessary, or you'll never leave the shore line. If in a craft, father from shore then you can swim back to, the same rules apply.

The worst case scenario yacht just doesn't exist, as any can become overwhelmed and have. Most often it's the foolishness of it's skipper, not the design decisions that cause the inevitably sad events. So, if you want to do something about the worst case scenario, get some training in both design and vessel operation.

 

[michael pierzga]

In my quest to learn about design I have seen many craft designed with different transoms. I think I know that a displacement hull is the best type for a smooth ride in rough conditions. A rounded stern is also probably best in rough conditions. My design attitude to designing anything is to design for worst case scenarios. But every design is a compromise. My question is how bad is a flat transom on a 36'+ Displacement hull type boat. With such a transom a 35' boat could have nearly as much room in it as a 40' boat, the rudder could be hung outside the hull giving a bit more turning effect.

But there must be a downside, will it handle really badly in a following sea.

Yah..its a mistake to design for the worst case. You would end up spending your would life in a tank.

A small boat needs a tender and access to the dock. The transom on a custom build should perfectly match your tender, davits and boarding arrangement. The thing you use everyday.

Your whole boat should be designed the same way. How you will use it.

Naturally you must obey the rules and produce a safe boat.

A safe boat at sea , is a fast boat. When the weather forecast comes and its not looking good, the best way to defend yourself is to get the hell out of there. Storms are small in diameter. In the ocean ,altering course and dropping 2 degrees in latitude to escape the danger zone , as fast as possible, will be the difference between force 10... we are all going to die... and force 7..coffee spilling, saltwater on the windshield. This is what seamanship means .

Safety downwind in a sailboat requires perfect control of the boat with its steering . An effective rudder and directional stability so the the boat goes in the direction you point it. . The same must be true of motorboats.

 

[Mik the stick]

My engineering training is in electrical/electronics. The avoidance of weather is just one more thing I don't know about. I know the trade winds blow in a certain area and direction but don't yet know where that is. Areas of ocean where winds are lighter or generally non existent are another don't know. I will have many happy hours finding out.

My design philosophy is based on the fact that If I or anyone else designs a safe boat/ship then you have a better chance to survive long enough to learn from mistakes.

My only real experience: I once rowed to Inchgarvie island on the river Forth. the round trip took about 90 minutes in an 8 foot dingy. It was a very calm day and with hind sight I think I was luck to survive. On the way back my friend and I were hit by the wake of a very big ship heading toward Rosythe.

An 8 foot dingy is I'm sure a very safe vehicle, but not the best choice for the middle of the forth river.

 

[michael pierzga]

Personally I think your idea for a transoceanic 50ft motorboat is ill advised.

A seaworthy coastal motorboat that can cover 300 miles per day at cruise speed, with a 500 mile range is more logical.

When its time to cross oceans, put your boat on a ship.

Hull forms by designers like Nigel Irens accomplish this

http://www.nigelirens.com/ldl/yachts-product.php?id=1&idcat=1#

 

[PAR]

Mik, you need experience, much more experience, before selecting a design (you're not going to design a 40' motor yacht, trust me). Once you get a real handle on the things that are actually important, rather than preconceived, novice notions; you'll be much better equipped to establish a reasonable SOR, that will be not only be possible to build, but also affordable. Get rides on boats of every shape and configuration. Rent boats, steal rides in the middle of the night, maybe a bare boat charter for a week or two. Only real sea legs time, will define the you ideas of "a boat" and that's as simple as it gets. Dreaming is great, but a whole lot more fruitful, if you have the experience to support the logic behind them.

 

[WestVanHan]

Mik- marinas around the world are filled with boats that never get used,and the few that do usually get less than 100 hours a year of use.
And IMO 99% of these these are coastal boats that cost a small fraction of the trawler/ocean crosser types.

Buy a decent coastal boat, and invest the rest of the money wisely and your boating costs will be paid for by the interest.

 

[SamSam]

I guess this is why I get banned from other sites.

My question is how bad is a flat transom on a 36'+ Displacement hull type boat. ....will it handle really badly in a following sea.

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

 

[thudpucker]

This the water I want to fish in. It won't matter what Transom I had.
But I really would like to experience one of the Round end transom boats before I hit the bottom of the box.

 

[Richard Woods]

I would agree with the other comments.

You are being too specific too soon. First decide what you will really use the boat for. Then decide what are the "worst case" seas that that use implies. Then use the RCD to decide on what stability and other standards you need to meet

Boats aren't just for going places or surviving rough seas. You need to live aboard, carry a dinghy etc (as Michael said). You want a cockpit to enjoy good weather. That all compromises seaworthiness but makes the boat practical

Even boats like all-weather pilot boats (like the Dutch one on another thread) are compromised. The stern of a pilot boat is usually curved, not because it is more seaworthy, but because it causes less damage if it hits a ship while turning away after dropping off a pilot.

Many fishing boats compromise seaworthiness because of the need to handle pots and lines

But please, please don't design anything until you have first gone out to sea to see what its really like. You wouldn't design a car if you'd never been in one, never mind didn't know how to drive

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[messabout]

Thudpucker; That's a spectacular picture. I want to fish there too, and I'm not even much of a fisherman.

 

[Submarine Tom]

Anyone, anyone, SamSam...

 

[SamSam]

Anyone, anyone, SamSam...

Yeah, these threads annoy me. I shouldn't comment because these threads annoy me. I shouldn't comment because these type of threads are annoying. It's none of my business. It doesn't matter anyway.

****. Here I go.

Yes, a big flat transom on a displacement boat will handle badly in a following sea.

Even though worrying about the worst case scenario is foolish, which you would realize if you would only spend years of training in handling boats and large amounts of time and money learning design, in the end, yes, a big flat transom on a displacement boat will handle badly in a following sea.

Even though fast and displacement boat don't go together, fleeing hundreds of miles is the answer, as following seas only happen in storms, never in any other situation. But, if you happen to catch the tide wrong, or the depth shoals up, or running 150 miles is not reasonable when you live just on the other side of the harbor inlet, and you do have a following sea, yes, a big flat transom on a displacement boat will handle badly in a following sea.

That you have engineer training in electrical/electronics is wonderful, but you must have been sick the day electromagnetic compatibility, the piezoelectric effect and big flat transoms on a displacement boat handle badly in a following sea was the subject.

Give up thinking about it and just buy a used boat as they are a dime a dozen now. But keep in mind when you are looking for a boat to buy, a big flat transom on a displacement boat will handle badly in a following sea.

 

[michael pierzga]

My experience is that a wide transoms don't negatively affect handling downwind.

An immersed wide transom does.

 

[FAST FRED]

My experience is that a wide transoms don't negatively affect handling downwind.

An immersed wide transom does.

No truer words were written!