Effect of Draft on Seaworthiness

[Krawdad]

I believe the "effect of draft on seaworthiness" was your main concern and while I agree with all comments regarding ride and fuel issues I strongly disagree that a flat bottom combined with shallow draft does not affect seaworthiness. It most certainly does and anyone caught in truly severe weather in one has good reason to worry, I know I own one and have covered thousands of miles with it in all conditions. They are impossible to maintain course at the slow speeds severe conditions dictate and extremely easy to broach in a following sea. "Leave your trim tabs down and your sure to broach". Neither are any of them self righting and ask anyone who says they are to prove it. Neither is skidding a good thing as your being propelled down into the troughs. Quite literally they skid up and down the waves and are blown all over the lake in a storm. Ever watch some powerboats leaving a marina during high wind and noticed that some dog-tracked severely on their way down the channel? Inevitably it is a giveaway of a shallow draft vessel with a mainly flat bottom. Imagine how they would steer in a full blown storm! The disadvantages of a flat bottom are of course heavily mitigated in direct proportion to the increase in draft. Draft is everything in terms of safety in true life threatening conditions. Flat bottom, shallow draft, go fast boats are made for running from the storm, not weathering it.

[ViribusUnitis]

How many power yacts are truely self righting?

I suspect that many of them are not. Even releltivly accepteded passagemakers.

[roverhi]

This may not be relevant to this post but there was a bit of controversy about the fast smallish coastal patrol vessels of Germany and Britain in WWII. IIRC, the British went with round chine flattish bottom designs, sort of the traditional boat bottom. The German designs were V bottoms. The German boats were faster in flat water and into the waves. The Brit boats were more stable off the direction of the waves. The Brits could outrun the German boats by heading across the seas as the German's V bottom boats rolled terribly and had to reduce power in the same conditions. In the real world of the English Channel and North Sea, conditions were almost never flat. Can't remember where I picked this up but you might try and google WWII Brit and German Patrol Boats.

[Hydra]

roverhi,
I'm afraid you got it wrong: British MTB had hard chine hulls and German Schnellboote had round bilge. Just look at the pictures.

Alain

[RaySea Lady]

I have a 58 foot Bluewater which is a Shallow V tunnel planing hull with reverse tines, 32 inch draft at the keel and a long entry bow. I have over 1000 hours and she handles very well but at slow speeds, under 6K, you are engine steering or on autopilot. The roughest I have been in is 10 foot waves on Lake Huron. But, self righting, absolutely not.

[roger.waite]

If you want to answer your question, I can only suggest you go and look at modern lifeboat (e.g. RNLI) designs. But the bottom line (no pun intended ) is every hull form is a compromise.

Up to Force 8 with a fast, medium weight RIB design, my lifeboat unit talked to one manufacturer who would not put a skeg on their design because they felt it could trip the boat on a big wave. Personally, I was more concerned with leaving our prop on the shallow bar we have to cross!

But some of the new RNLI designs are relatively deep, and (by memory) some have skegs. As did the classic, heavier Waveney lifeboats. Slow, but you never wondered whether they would bring you home.

[capnorv]

Originally Posted by ViribusUnitis

How many power yacts are truely self righting?

I suspect that many of them are not. Even releltivly accepteded passagemakers.

All United States Coast Guard 44' motor life boats are self righting as has been proven many times. Unfortunately not always without harm to the crew

[roger.waite]

This shot gives a pretty good view of the shallow draft of one of the RNLI's newer all weather lifeboats. Note flat stern sections for a bit of speed ...

Barrow Tamar lifeboat launch

And the sharp bow (with big flare) for comfort (and buoyancy when needed).

RNLI - Royal National Lifeboat Institution

[roger.waite]

Originally Posted by capnorv

All United States Coast Guard 44' motor life boats are self righting as has been proven many times. Unfortunately not always without harm to the crew

Yes. Barrel rolls are not a barrel of fun. But at least most crew are now inside the barrel. (Waveney's have come up without their crew!)

As I understand it, there are three big issues Re: self righting ability.

1. Does she have the buoyancy up high to bring her back up?
2. Will she flood (esp. engine room vents) if too long upside down?
3. Will the engines still run, after they have run upside down? (Many will not.)

As you move down the list, you move towards what the USCG and RNLI look for when they talk about a self-righting lifeboat.

[CDunc]

I think to be sure what type of passagemaker are we talking about, If your talking about voyaging into the ocean I would not attempt it in anything not classified as an ISO Cat A or Cat B vessel. These are the the only Design categories for Open Ocean passagemaking. If you are planning on buying or using a Cat B or C vessel then it was not designed to withstand the rigors of the open ocean and are either near shore or protected water boats. If a boat is designed to Cat A or Cat B then they should be self righting, however as I write this I have not as of yet found an example of a truely flatbottomed power Yacht (6-23M Length) that is designed for the open ocean. The original post asked what guidlines are to be used as far as draft and I stand by that if the vessel is designed (and certified) for the open ocean then it is a matter of seakindlyness and stability and ultimately how shallow is the water at your destination.

[ViribusUnitis]

Question, what is defined as "open ocean"?

Is island hopping down the Bahama, down to the windward, and then leaword islands concidered open ocean? Clearly, that is diffrant than doing a run accrost the Alantic.

Edit in>>

A quick google search defines catagory A as designed for conditions exceeding Force 8, and 13' waves. Thats a heck of a blow.

Also, Island Packet suggest that their 41' PY Cruiser is A rated, self righting, and a grand total of 3' 8" of draft. Compared to all the 6' sail boats running around, I think I'd call that shallow draft.

[CDunc]

ViribusUnitis,
Here you go I hope this helps:

A. Ocean: Designed for extended voyages where conditions may exceed wind force 8 (Beaufort scale) and significant wave heights of 4 m and above but excluding abnormal conditions, and vessels largely self-sufficient.
B. Offshore: Designed for offshore voyages where conditions up to, and including, wind force 8 and significant wave heights up to, and including, 4 m may be experienced.
C. Inshore: Designed for voyages in coastal waters, large bars, estuaries, lakes and rivers where conditions up to, and including, wind force 6 and significant wave heights up to, and including, 2 m may be experienced.
D. Sheltered waters: Designed for voyages on sheltered coastal waters, small bays, small lakes, rivers and canals when conditions up to, and including, wind force 4 and significant wave heights up to, and including, 0,3 m may be experienced, with occasional waves of 0,5 m maximum height, for example from passing vessels.

[ViribusUnitis]

sorry about that CDunc,

I edited and changed my post after I engaged the brain and did a quick google search. What really occures to me is that if you follow a proper weather window, you could probably go all the way down the island chains, and never get out of the "C" rating.

Does anyone have a listing of requirements for the various ratings? Is there a listing of requirements online some place? As far as what is required to be done to get the vessel to pass each requirement?

Thanks!

[CDunc]

The vessel would have been evaluated by the design team and builder. You could ask the manufaturer for the certifications. The list of requirements is extreamly complicated having to deal with many items from the scantling rules, to the type of glass and pressures associated with their construction and details of how they are put together. You can lookup the ISO standards but you will have to pay for them and thats not reccomended as the cost is quite high.

[CDunc]

I personally wouldnt take the chance with a C boat, many weird things can happen with weather out there. Lots of people do, even sail or row across the atlantic in a row boat. Its just not my style