Thursday, September 15, 2016
Designing skiffs, my process
I have several design commissions for new skiffs and two sailboats now. I thought I would share a few tips on how I get my hull lines on paper. Truth is I use a variation of a very old method. In the past when building big vessels one of the methods was to screw together a stack of boards and shape your hull with these planks. When your half hull looked right you would then un screw and draw right angle station lines across these waterline lifts. You would then measure out to the ends of these and along with your traced out line of the half model you could then redraw this vessel full size.
This is a tried and true method.
Herrshoff and others refined the method by using a jig to take the lines off of their half models from the outside. This you can see in Herrshoffs books.
My method is a variation of these as I glue my model together and when satisfied with its looks I then cut it up into station sections and trace the hulls curves from these. I do it this way as my hull designs can be very complicated at times especially with my power skiffs. With the other methods there would have to be many thin planks to get all the details or I would have to have Herrshoffs set up. I like my method better. Plus I can do it anywhere. To use a Cad program I need to be plugged in. I like living and designing off the grid. In the end after my design is drawn I then glue my hull station sections onto a thin plank and I have my model back in one piece. At any time I want a station section reference I can just slide a sliver of paper in the slot between the stations and then trace.
Here's some pictures of this past week getting ideas from my head to paper.
I have shown this process before on this blog. It's under designing Tom Gordons Element skiff design.
But I will go over it here again and try to explain some subtle details.
The first thing you have to have is a client or an idea. Here we will look at a clients wishes. John wants a shallow polling very sea worthy dry running skiff that is different in looks than anything out there now.
He used to run a Hewes Bonefishing skiff so this is my reference as to performance. The Hewes is a very old design that was adapted from a ski boat. I know it well. John is a big guy as are his fishing buddies. He's 230 lbs. I figure his buddies are too. The engine he will use is a 150 hp outboard.
I add all this to my known weight list. Now I can come up with a loaded hull weight.
My estimated worst case total is 2,400 lbs. this is what I will have to design to float in 6" of water for poling after fish in the Florida keys and the US coastal waters. He would like to keep the skiff at 18'.
This is a big list of needs. Here goes.
I draw my ideas out on brown paper, it's cheap, erases well and I like the feel of it when fleshing out a design. Once I have my design idea down with rough measurements I do a quick displacement calculation to check the draft. It's very easy to just draw a good looking skiff, build it and then see where it will float. This does not work in the technical skiff market where an extra inch of draft is bad news. It takes me an hour to calculate displacement with these small skiffs. I use a $8.00 calculator to do my math and make sure I draw out by hand all my additions, divisions for reference when going back if needed. Remember widening your design changes it tremendously and adds very little volume. But adding skin thickness add lots more. Lots to think and play with at this stage.
With my drawing done I now transfer its main points to my glued up block of wood. I just use my navigating dividers for this by poking holes into the wood block transferring the main points like the chine level and such. When done I use a ships curve to draw the lines out.
Vola ! It's in there.
My workshop. I use high speed Makita 10,000 RPM angle grinders with 24 grit 5" discs. At this speed they can cut very clean lines along the chine lines and fair this hull up very quickly.
I have made over a thousand half hulls so it's very easy for me. If you take your time you too can get the same results. It might take 4-5 hours with your first try.
Bottom chine , the rest has to go. You could use a block plane too. I'am to lazy for this so I use my electric grinders.
Starting to look like my drawing design.
Stop many times to draw in pencil these station section lines. Look at your design many times.
I use my ships dividers to check widths going back and forth as I get closer.
Take your time.
Once I'am happy with what I see I use a Porter Cable random orbital sander to sand my hull smooth.
I start with 80 grit to get the grinding disc marks out and when done I go to 320 grit and quickly smooth off. Now I have something to play with and feel up. This you cannot do with a 3-D computer image. I love turning these hulls over in my hands caressing them as I would see the water flowing past at different trim levels, angles and speeds. Many times at this point I pencil in little spots that I don't like and go back to the bench and shave these away. I always walk away at this point and go to some other idea. This clears my head of the infatuation of this recent love. I want to think over our relationship.
I cheat and go to one of my many other lusts. These designs are casting about in my head. Some are for shallow water, others are for living aboard and will cross oceans.
I have a collection of my past relationships still with me. I compare my latest design to those that are for similar needs. I know from past experiences with all of them how they performed. I've been a very lucky man to have been able to have had these times together.
As in life some vessels have their pros and cons. I have my favorites and keep my not so great past relationships with me to let me know.... I am human and life is not always perfect.
After returning from my distraction with others projects I have a clear head and can look at this vessel with clear eyes and head.
I am happy but will see on paper now with pencil, calculator and a new look at our possible future together. You see, once a design becomes public knowledge then there is no hiding and no going back, you have declared your inner soul, thoughts, hopes, desires for all to see.
First thing you do is draw a centerline and then a base line to layout your top view and profile. The hull half model is pretty accurate on the bottom to the bow. The sheers most times are not due to cutting it on the bandsaw. Don't worry about this as the sheer can be changed later. Once done cut your half hull up right along all the station lines. I have a 14" cheap bandsaw that does this with ease.
On my boat the HFM I use a back saw which is a very stiff hand saw. I also over the years carried a small 8" bandsaw with me to make hall models.
Now layout how you are going to show all the sections.
After you have cut up your beautiful half hull you will see that your boat when reassembled has shrunk!
No problem, these sections will just fit in between your traced hull lines and the perpendicular you have drawn. Different hull bow here.
Once my hull is cut up into sections I go to my brown paper again and trace off these sections to get a look at what I have come up with. Because I have had lots of past relationships and have kept all their measurements, secrets on velum I can now check them against my new one.
This helps me along in the game of trying to improve on my past. Because I have been able to be very intimate with my past designs having built them all on my own and run and played with them through all kinds of conditions I know their each individual moods, the good, the bad and well... I've had a good run... But some have had a tendency to spit a little more in my face that I like so I am on a life long quest to find the perfect bottom.
The different colored lines are from my past designs. I use these to check the hull curves at the specific hull station from past hulls. This skiff is going to have greater beam. I can slide the old hull shapes back and forth out to this new hulls skin. This helps in seeing if the water flow is going to be heading in the direction I want. You also have to consider the hulls weight. A light skiff will be on top of the water, where as a heavier skiff will ride better but with the right hull bottom it too can ride along like a lighter hull.
It helps that I have ridden in lots of other skiff designs other than my own. I have lived most of my life on or right next to the water. I am always looking at the relationship between the water and the hull that I'am in and how it's dealing with it.
Once happy then you start tracing. You must line up the glued up wood waterline glue lines on your grid. Dont worry about the sheer level line. You check where the beam is by marking it from your previously traced hull. This way you know where to line this block up. The waterline must be perpendicular to the centerline perpendicular. The centerline of the block might not be square. No problem.
This is a different hull section here.
When tracing the hull top and side the wood model will not be perfectly fair. Don't worry about this now.
Showing scale with the pencil. If drawn full size this wobble could be 3/16". When building a skiff from another's design I would loft the hull out full size to fair in the hull lines. If drawn this way there will always be a little out. I have found from my method I can get up to 1/8" out once the station molds are set up. I use a wood fairing batten when setting up and bevel the station molds using this batten as a guide.This is standard.
But if drawn on a computer then it all comes out perfect.
Hull traced and station section perpendiculars going in. Showing my " Grute" series of hull designs.
Once drawn you can now fair with a ships curve or a batten.
After drafting this hull out and doing all my displacement calculations three times I felt this skiff would be better off by being 10" longer. My client gave me the go ahead. I changed the sheer too. The chine heights and lots of things can be tweaked at this time when fairing your hull lines out.
This boat will be designed to be built in wood epoxy as per the clients request. She is my idea of what a large technical skiff should look like with tumble home and sexy flair added. I feel that any skiff designed or built today that has its chine going into the water is outdated. To be totally silent when fishing in shallow water it has to be out of the water. The next problem - question is how high above the water for it to work. If it's to low it works great but can catch some choppy slop and make noise. Too high and it doesn't work so well if at all.
The best way to know in advance is to design in advance with your known weight list.
If not you're just hoping for the best.
With my experience of past relationships I have a good idea of what's going to be happening with this one. There's nothing like having been around the block a few times. But you still have to worry.
I will not be building this design. Hopfully John will follow my plans to the letter. Added weight or taking away materials can kill a design. She will be in Johns hands now. I wish her well.
My Grute 17.8 an enclosed power kayak for camp cruising.
A simple to build all round skiff that will carry a load, be very shallow running, polling, and hopefully she won't spit in your eye too much.
I will be posting all these pictures on my Blog.... hogfishdesign.wordpress.com for better clairity.
Have fun with your design, I hope she is all you invisioned