Displacement or Semi-Displacement Hull

[PAR]

I have a 28' design that offers standing headroom within it's pilothouse, sleeps 4 comfortably, though two will be in the pilothouse on a convertible settee. and is available in both displacement and semi displacement hull forms.

As Tad has pointed out, you idea of 180 degree stability isn't practical and likely based on some misunderstanding of the dynamics involved. In other words, you could live with considerably less maximum positive righting stability and still feel very secure in your yacht.

I believe you are at the point of needing to talk with a professional, for a sit down to work out a design brief to meet your goals. There are several stock designs that seem to fit your requirements, from most designers, including plans houses too. Have you looked at Glen-L's offerings?

[waynep]

The self-righting objective dream comes from knowing very little about lifeboat design as you mentioned. RNLI Lifeboats are self-righting, and fast too! If 180 degrees is not feasible for me, then the largest positive righting moment I can get will have to work.

Hopefully we are still within the confines of Displacement or Semi-Displacement selection when discussing stability, crew comfort, and seaworthiness. There must be a good hull design out there that provides the right balance for these objectives.

I also have some minimal, and well known, design requirements that I would like to be met, especially regarding crew comfort and seaworthiness. I believe seaworthiness and stability are basically the same thing, while crew comfort is another matter dealing with the dynamics of the boat.

PAR, as you mentioned, I should look at some of the designs that are out there. I can then look at each design paramater at a time to determine if the boat is what I am wanting. Stability analysis on computer looks very optimistic when analyzing a watertight hull, especially with a large superstructure. A good Vcg analysis should provide a better idea about stability of the vessel. A great deal about cabin layout can be learned in this process as well! I just hope I don't have to pay a fortune getting the information I am looking for! In the end, will the hull be a displacement hull or a semi-displacement hull Most likely it will be a displacement hull - the larger the better. Maybe a heavy keel towards the stern? Some self-righting sail boats won't self-right until the mast breaks off away from the boat!

[Tad]

Originally Posted by waynep

I believe seaworthiness and stability are basically the same thing,

Certainly reasonable stability is required in a seaworthy boat....but so is much more......

A poorly built boat is not seaworthy, a boat built of bad materials will not be seaworthy for long, a poorly maintained boat is not seaworthy, and an ineptly crewed boat is not seaworthy.

Wayne I think you need to slow down on the analysis and go to sea for a while......practical experience will help.

[Ad Hoc]

Originally Posted by waynep

There must be a good hull design out there that provides the right balance for these objectives.

There are, but each one is based, like all boats, on an SOR. A boat you may like, the RNLI for example, has a totally different SOR to yours. Your SOR is unique to you, but with a few compromises, can be fulfilled. You just need to accept where those compromises are, to move fwd.

As PAR said, spend more time at sea, many things will then become more obvious to our own SOR.

Originally Posted by waynep

I believe seaworthiness and stability are basically the same thing, while crew comfort is another matter dealing with the dynamics of the boat.

Tad covered that one well. But there does indeed appear to be a misunderstanding of the word “stability”, since the word covers many aspects of boats behaviour, not just the one that most people initially consider. Most seem to equate stability with how the boat rides in waves and will I feel sick! That is just one aspect, but this realm is called seakeeping. The seakeeping will tell you how ‘stable’ she is.

Originally Posted by waynep

I just hope I don't have to pay a fortune getting the information I am looking for!

If you’re worried about spend some money up front first, then perhaps you’d better think again about this whole project. It is far better to spend and investigate now, than waste far far more money later one buying a pig of boat that doesn’t fulfil your SOR. On another thread, Apex, said it best, money spent now is an investment and a saving, not a waste.

Penny wise, pound foolish comes to mind.

[waynep]

Thanks for your comments. Attached is a 3D Image of the boat design I am planning on. Will need about 4 ft of headroom under the forward deck. This would be higher than the recommended value of freeboard for this size vessel.

[waynep]

I would not mind spending time at sea in a vessel similar to the one I am working on. That would be beneficial in regard to how she would handle. As far as building a strong vessel that will withstand the forces of the sea, I'm not sure how much time at sea will help in designing structural members. I'm not an architect, but I am an engineer. I have taken many courses in structural engineering, including structural dynamics and FEM for structural design - I do understand what stability means thank you very much. As far as a pig of a boat is concerned, even a small cruiser will need at least 4ft of headroom under the deck to be marginally comfortable, or the pig of a boat will need to be longer than the minimum lengths I am talking about here.

[arthor]

Originally Posted by PAR

I have a 28' design that offers standing headroom within it's pilothouse, sleeps 4 comfortably, though two will be in the pilothouse on a convertible settee. and is available in both displacement and semi displacement hull forms.

There are several stock designs that seem to fit your requirements, from most designers, including plans houses too. Have you looked at Glen-L's offerings?

Hello Par,
I have the study plans for Glen-Ls Klondike and argosy. Both look lovely boats and I haven' read anything about them to think they are otherwise. They both are written up as Semi Displacement. Are these the ones you mean?? How are they regarded for coastal work or the occasiona Irish Sea/English Channel hop?

I am also interested in the 28' you mention. Could I see a picture please??

arthor

PS - I have been pondering hull shapes and am coming to the conclusion that a SD is not much use really. Doesn't have the speed of a planing hull and is a poor subsitute for a displacement hull when it gets rough. Why do people have them??

[rasorinc]

Do you want to be able to trailer the boat? Most states have a limit of 8'-6" however, with a permit you can trailer 10' or even 12' with out doing anything else except buying the yearly permit. Check out your states wide load permit requirements as this could influence your decision. The only restriction in Calif., Tenn. or Oregon is getting the permit and not traveling on certain days and certain narrow roads with narrow bridges.
Just a thought for you since you mentioned 6 month continuous use and 6 months no use. Saves on paying rent for a space.

Most of the Glen-L hulls 27' to 32' say semi displacement but quote a speed of 7.5 to 8.5 K and all show a good size skeg.
I lived on the north pacific until a year ago--not an area of calm water or weather and these boats could handle it. I am not SAYING a STORM, just average weather and wind.

[FAST FRED]

Most states have a limit of 8'-6" however, with a permit you can trailer 10' or even 12' with out doing anything else except buying the yearly permit.

True but if you plan to travel (not just store at home) EVERY state you go thru must be contacted and paid for the use of their roads.

FF

[tom28571]

Originally Posted by FAST FRED

Most states have a limit of 8'-6" however, with a permit you can trailer 10' or even 12' with out doing anything else except buying the yearly permit.

True but if you plan to travel (not just store at home) EVERY state you go thru must be contacted and paid for the use of their roads.

FF

Fred, an earlier poster reported that there was an organization that would do all this for you with one fee. That sounds good to me for occasional use.