Design Assistance

Hello,

I am new to the forum but not to the website, having lurked on this site for sometime to answer certain questions here and there. As of late I have been designing a Ketch that I would like to build within the not too distant future. The length would be 74' long with a 19' 8" beam with a fiberglass and marine ply construction. The boat would be designed with the purpose of sailing around the South seas while also allowing me explore various islands and nation. However, with this length comes the problem of draft. Many of the 70ft yachts have drafts in and around 9-13 feet. Which is far deeper than I would prefer. Preferably I would want the boat to have a draft of 6'6"-10". To this end I was wondering if a bilge eel would be able to provide the stability I require at sea, while giving me access to many of the best parts/ marinas of the South seas. Is my thinking too far off? Is there something I am not understanding? Basically I am wondering if a Bilge keel would be the answer to my problem and if so, why haven't companies such as CNB, Shipman, Amel, and Oyster not implement them into their yachts to increase marina access for their customers.

Jon

 

welcome to the forum dragoon119

Unfortunately there is a miss match. You are designing this yourself, yet seek basic fundamental answers to the stability and implications of such. If you are unable to satisfy yourself how what where and why..then I would suggest you engage the services of a naval architect/yacht designer to properly and technically answer those questions for you and establish the basics or DNA of your boat. Otherwise you'll end up in trouble on those South sea when the wind blows and the waves are not as expected!

 

dragoon119, let me simplify the thing a little. If you are sailing with the sails unfolded and a wind of a certain speed, that will generate a heeling moment, that the bilge keels can not counteract. If you are anchored in a marina, the bilge keels can decrease the amplitude and frequency of the swing.

 

I honestly didn't think this was basic, and I assure you that I tried to locate the answers myself but it was not connecting. Bilge Keels while I understand are less effective and create more drag, do reduce draft of a vessel. However as of let I believe the largest Bilge Keeler is a 54' Yacht in Washington State, not discounting the Ned70 power yacht. That was really were my thoughts came from as that vessel has a length of 70 foot, let a draft of only 3'4". Bare in mind that it has a self righting system. But it got me thinking that a Bilge keel may just be the answer to minimizing draft, while stabilizing the craft under sail.

 

However I was under the assumption that the entire point of a keel is to both add stability and counter act the heeling motion of the sails. Why would the Bilge keels not counteract the heeling motion of the yacht? Are you saying the total load on the sails combined with the mass of the boat would be more than the mass of the twin keels?
However I was also under the assumption that they were designed heavier than the yacht anyhow just for that reason?

 

You need to consider, among other things, the shape of the hull. It is possible to have proper stability without any external keels or ballast. These are basic principles of physics and marine architecture. The keel can be used to lower the center of gravity. However, the main point is to provide lateral resistance, and in some cases to dampen rolling.

 

Sorry but you're trying to dumb this down to a simple one liner. Doesn't exist in design. Everything is a compromise, and thus each vessel is designed 'that way' based upon its own SOR and merits. Thus what is good for one may not be good for another. You really need to design it, and by design, I mean, establish your parameters and arrive at a solution that satisfies those parameters in all conditions, beit stability, seakeeping, ergonomics etc..

 

I'm glad I was late to the party, have fun with this one guys . . .

 

bilge keels just deep enough to let it settle on the hard nice and level, and to protect the rudder and big (efficient) prop, and for enough lateral resistance for moderate sailing, then a center board for deep-water big-wind fast sailing.

I hear most non-motorsailor "true" sailboats still spend about 1/2 time under power without any sails.

 

Dragoon do you have any idea of the cost of a 74 foot boat? If you are very wealthy then you can logically consider having such a boat. If you are wealthy then you will be best advised to have the boat built by an established yacht builder. If you build such a boat yourself it may take ten years or more to complete.

 

Pardon my ignorance of marina access, but how many are unable to house or enable transit on a high tide, of boats drawing 10 feet ?

 

So what I am drawing from both Gonzo and Ad Hoc's replies is that without a 3D representation, and further data no one would be able to assist and or advise me as to whether my design direction is truly feasible. Understood, will be working on translating my sketches into a 3D model after work and will be uploading them to this thread to gain further assistance.

 

During lurking here I have seen many of your comments and know you are very respectable. I hope that through this thread I will make you regret that you were late to this party and happy you entered.

 

Yes, they range in the 2-6 million dollar range for a completed yacht with minimal to no customization. I believe I could build one with sufficient planning for much less. At a quicker time than you quoted.

 

I must tell you that it is not necessary at all that you make a 3D model. Many can give you a lot of help without the need for you to make a 3D model of your boat. If you want to do it, great, but it is not necessary. Let us not complicate life, nor complicate life for others, more than necessary, please.
What information do you have at the moment?