boat engineers ??

[amess]

I'm building a designed boat from plans but am making changes.
First - to cutter rig with bowsprit
Second - 1800# full keel to fin (bulbed) keel

Obvious advantages in sail plan and handling/speed.
Anyone help with placement, changes to keel based on new sail plan would be appreciated.

I know - why mess with a good plan? Because I can.

[donald branscom]

I'm building a designed boat from plans but am making changes.
First - to cutter rig with bowsprit
Second - 1800# full keel to fin (bulbed) keel

Obvious advantages in sail plan and handling/speed.
Anyone help with placement, changes to keel based on new sail plan would be appreciated.

I know - why mess with a good plan? Because I can.

I can help with part of your project.
If you are going to use a cutter rig move the mast back 22%.
Make the appropriate changes to the interior.

The CB should remain the same. Take a cardboard cutout of the side view of the hull ,to scale, and stick a pin in the middle near the top. Keep moving around while it is hanging on the wall untill you get it to balance. There is your CB.

The CE of the sails you will have to compute.
Bulb keel? well that will put more stress in a smaller area of the boat and it will not be spread out like with a long keel. You will have to improve the framing and hull around the place where the keel is attached.
I personally do not like this part of your changes. If you have been in the ocean with a fin keel in a rolling sea and 2-3 knots of wind you have to constantly have to steer the boat. With a long keel it would stay on coarse. The bulb keel is vulnerable for many reasons. It is a racing thing to reduce wetted surface. Racing boats have lots of corporate money to throw around on this type of silly stuff.
Bulb keel boats even when designed by people who are supposed to know what they are doing often have problems like cracks,delaminations and bulbs have been torn off.
They look good in the boat yard though.Even when no boat is attached to them.

[Ray Frechette Jr]

Let's see, you're building a boat with 1800 lbs of ballast soa reasoable guess is thatthe boat will weigh at least 5-7ooo lbs. And most boats come out to a pretty close price per lb basis.

So, given all the time and money you will be spending on this boat I would suggest the services of a Naval Architect to engineer it for you.

Yeah, you can probably end up figuring it out yourself with help on the forum, but for the money invested, I think having a Naval architect engineer it for you would be money incredibly well spent.

[Todd Bradshaw]

...and to not have professional help in making these changes on a boat that size is just plain stupid. Sorry, but it really is. There are two or three (and only two or three) forum regulars who are qualified to answer your questions and they do it for a living (for money). Advice from anybody else is worth no more than you are paying for it and your future passengers deserve better.

[djswan]

...and to not have professional help in making these changes on a boat that size is just plain stupid. Sorry, but it really is. There are two or three (and only two or three) forum regulars who are qualified to answer your questions and they do it for a living (for money). Advice from anybody else is worth no more than you are paying for it and your future passengers deserve better.

That's the best advise I have heard here. I say the same thing to folks building houses.

Derek

[Varna]

;) Yeah go for it-'cuz ya can! That's the best reason I've heard to alter a full keel profile to a fin...nice.
In fairness almost anything CAN be done, but if your asking here you shouldn't do it...but you can!
Fin/Bulb has particular merits and if done correctly yields the desired results. The high dollar race designers and owners know they are on a safety factor edge so failures are accepted-that's part of the rush of it with the massive loadings on tiny parts. At those levels of boating it makes sense and also we all benifit down the line with evolving consumer end mat's and techniques.
If you do it without competent help just go alone always until it is proven reliable, just like the test pilots do. Or be wise and get a pro, that is if your serious, and not trolling...

[donald branscom]

I agree about getting expert help.
That said, be careful who you get. If you want a cruising boat get help from the right expert.
If you want a fast boat same thing.
I can tell you from personal experience that some advice they give you won't actually work.

And remember if you are the captain you are responsible for it all.

One more thing. Read books on the subject. I reccomend Howard Chappelle's "AMERICAN SMALL SAILING CRAFT."
And the BEST book on keel design.... CA Marche's "THE HYDRODYNAMICS OF SAILING"

[paladin]

and pre-warn any potential passengers of your engineering changes...

[Ray Frechette Jr]

Well, in some respect it is heartening to see all of the fairly strongley worded critiiques. It means mine is one of the gentlest suggestions. And I was worried mine might come across a bit strong.

Realistically though I doubt engineering help form a Naval architect would run more than 1 or 2% of the total value of the boat.

For a home built small craft such a s a 16-20 foot daysailor, home devised soutions canbe contemplated as corrections can be obtained more easily and cheaply. And codging an alteration and or solution can be part of the fun.

However on a 6000 kb boat, the economics change in a big way. Saving a grand on engineering would very possibly appear to be a bad bargain once the boat is in the water.

[Tom Lathrop]

What Todd and the others said. Dumb idea. Find a boat plan you like and build it. If you can handle what you plan to do, then you should be able to design your own. Of course you "can" do anything you like but the probability of success is very-very low. That is why boat designers make the big bucks.:rolleyes::D

[donald branscom]

Adding a bowsprit is a common theme. But I would like you to consider this:
If the design you have now has a balanced sail plan (and I am sure it does), just adding a bowspirit causes some problems that you may not want.
Boats are charged BY THE FOOT for slip fees. 6ft X $8.00 FT=48.00 per month !That is $576 per year !!! Read what Joshua Slocum a round the world sailor has to say about the subject.
Boats with bow spirits have to be watched more closely at the dock because in rough weather the bow spirit can bobb up and down untill it hits the dock or other things on the dock untill it is damaged. I have seen it happen many times.

Usually a bowspirit is designed into the balance of the boat by the designer.

By the way ....why is your on screen name A Mess????
Are you just playing with us??? cannot figure it out.

[paladin]

Just a simple story......I learn to draw boats because I thought it was fun.....I drove airplanes for fun...and worked electronics, so I should have known better.......
I started the useal design courses just as the HP45 calculator was introduced, before computers......I liked to draw pretty boats.....and then I would start working on the "engineering"....
One such boat was a 32 footer, a common sight by todays standards, very new in 1969. It used warped (diagonal ) plywood on the topsides and a bottom very similar to John Welsfords Sundowner today. It was about the 20-22 boat that I had designed....It had a 10 foot 4 inch beam, and the rig was a sloop, with the rig totally inboard, outboard rudder, but with a keel that today would be called a Scheel keel or a modified wing keel....as I had also studied a bit of aerodynamics when in school.
The person for which the boat was originally drawn was an American in Guam, who decided to have the boat built in the Philippines.......and then he decided to improve it. He did exactly what you are proposing, adding a 4 foot bowsprit to hank on a "cutter" rig. Then he discovered the molds would cost almost double to make the keel so he opted for a fin keel...the actual difference in price was about 400 dollars at the time. He also decided he wanted a world voyager so the 5/8ths total hull thickness became 3/4 inch, as did the deck.....and where 3 layers of vectra were specified, he used 3 layers of heavy fiberglass cloth....
needless to say the boat did not balance and the stability went the way of the buffalo.....and I was to blame because it was "MY" design....so from then on I made sure that I had control of any boats that I drew......and not for homebuilders. I did not want someone to die because of a poor design.

[willmarsh3]

At my marina one day I heard second hand about someone who built little (one person) wooden boats to his own design. The only problem was that they did not sail very well because he did not understand things like stability, center of effort, center of buoyancy, or how to balance the sails.
I once tried to design a boat and quickly realized that I would need to take courses in boat design at a well recognized school of naval architecture. I decided to buy plans and generally stick to them.
I sincerely hope your endeavor turns out well.

[mmd]

Things to check during design phase:

Lateral area - replacing full keel with fin will reduce lateral area, affecting leeway resistance and sail balance

Dynamic stability – redistributing ballast weight will affect ship’s motion in a seaway, possibly affecting crew comfort and seaworthiness

Static stability – adding to and/or altering sail plan will affect heeling moments acting on hull

Rig strength – adding sail area will add stress to rig components; changing keel CG will alter righting moment, which impacts the rig; changing keel CG will alter roll period, which impacts the rig

Structural strength – concentrating ballast keel loading will impact hull structure; changing rig loading will impact hull structure; changing rudder from skeg-hung to spade or semi-supported will alter loading of rudder and surrounding structure

Steering - changing rudder from skeg-hung to spade or semi-supported will alter steering geometry; will affect rudder stock size requirement, will alter steering effort required

Drivetrain – shaft unsupported length may change, requiring larger shaft diameter and associated equipment.

It is not a small task that you are planning to do. Be very careful and very thorough – the lives of your friends and family may depend on it. Personally, I think that you should heed the advice of the others above who have suggested that you either continue your search for a set of plans that more closely suits your needs and desires, or commission a competent and experienced designer to assist you.

[Oscarvan]

I say the same thing to folks building houses.

Hey, I resemble that remark.....But I've been a diy builder for a long time....and have done some reading here and there.... ;) BUT, there's a lot more variables in a moving boat than a static structure.....

So I think the advice above is valid. Learn how to do it right, which is a lot of work, or get someone to do it right for you.

[Jim Ragsdale]

I asked a similar question on the board a few years ago. I wanted to modify the deck structure and sail plan of a design. After listening to the comments of the people on the site here, I hired a naval arch. who came up with a similar design that was just what I wanted. I have been happy so far. I'll let you know when it hits the water :)

[Lucky Luke]

I know - why mess with a good plan? Because I can.

I would say that the general opinion, which I share, is that you can t.