Keel on foam boat

I suspect the answer has a lot to do with what you decide to do about the immersed transom. The immersed transom causes a considerable amount of drag, which will make the boat tend to point down wind. If you leave the transom as it is, you will probably have to move the rig further back. How much further, I honestly don't know. I suggest making a scale model. That is probably the only sure way to get it right.

The people who built cobles and dhows were probably illiterate, but they had generations of experience to build on. All I have to offer is theoretical knowledge in its place. Of the two, I definitely believe experience is the better.

This being said, it's better to have the rig too far aft than to far forward. If it is too far aft, it will be harder to steer it down wind. If it is too far forward, it may be impossible to steer it upwind at all.

As a wild guess, I suggest moving the rig aft about nine inches, if you leave the transom as it is, and maybe three inches aft, if you put rocker in the stern to move the bottom edge of the transom out of the water.

 

I have taken your advice and did a calculation. I planned 2 inches rocker depts on bow, 4 inches rocker depth on the stern. I looks like that mid section is almost flat. Attached is the spreadsheet screenshot of the calculation.
I will do another calculation based on your latest suggestion. I am curious to see how does it relate with the previous one.

Cheers.

 

The spread sheet is good but you have some incorrect dimensions listed in the second column. Depth dimension is to be the depth of immersion, not the distance above the base line. The middle sections 4, 5, 6, will have the largest depth numbers because that is the part of the boat that is most deeply immersed. It is the area of the individual below water line sections that we are looking for. For example, if you are using a preliminary estimate of the draft at four inches, then the first section entry should be 4-1.25 = 2.75 inches. Fix the rest of the depth dimensions and re-run the spread sheet. The numbers in column F will become much larger. The reason is that depth of immersion at sections four through six will be 4 inches, not zero.

When you get the new sum of column F entries the sum will be larger. Multiply that by 12 as you have done, then divide by three as you have done. The resultant number is the number of cubic inches of displacement of the boat.....not one third of it. When you know the number of cubic inches of displacement you can use the constant 0.0361 as a multiplier which yields the displacement in pounds.

It is conventional to have the bottom curve drawn in such a way as to have little or no flat area like in your mid sections. That generality applies to small boats not ships. That there is a flat area implies that the angle of entrance and angle of exit is more severe than is ideal. If you want to be technical you can imagine how a water particle must accelerate when the boat is moving. A steep angle will, at least theoretically, cause the particle acceleration to be higher. If we can believe in Newtons equations then think of F= ma. If acceleration is increased then force must be increased in order to balance the equation. That argues for having the least abrupt curvature that will fit into the required arithmetic of the boat. All this is getting a bit sticky isn't it? You could have a flat bottom end to end and simplistically no acceleration would occur at the bottom. You have to push water out of the way to make the boat move. The pushing and shoving would be at the sides of the boat where there are two angles of incidence, one port one starboard. Even if your design and execution of the build is near perfect, a major part of the resistance to movement through the water will be skin friction. This is going to be a boat that does not move fast. Twelve footer sail boats are not fast boats...................except the ones that those crazy Aussies build and race.

An often used estimate of the potential maximum speed of a boat is by taking the square root of the waterline multiplied by 1.34. The result of that calculation is the maximum speed in knots. One knot is about 1.15 miles per hour. If your boat has 11.5 feet of waterline length then we can calculate sq. root of 11.5 times 1.34 times 1.15 = about 5 MPH. Do not figure to achieve that blazing speed often. This estimate method is not carved in stone and there are frequent exceptions but this is a good ball park estimate.

The work that you have done so far is preliminary. You may have to tinker with the draft dimension in order to arrive at the 250 pounds displacement that you have suggested. Only then will you get a fair assessment of the amount of rocker that might be ideal.

Who knew that there are so many things to consider when designing and building a simple little boat? Have at it BCer.

 

I was concerned about the amount of foam I need to scrape therefore I focused on the 'empty' volume calculation. By reversing the numbers in depth column (4 - current value) I arrive to an immersed hull volume - 6.61 cubic feet.
4 inch draft -> 6.61 cubic feet * 52 lbs = 343 lbs.
3 inch draft -> 4.54 cubic feet * 52 lbs = 236 lbs

The drawing used with the above calculation. Length wise section are foot long, height wise 2" per each layer.
Aan additional 1.5 inches of foam on the bottom will bring me in the desired displacement range with a sufficient rocker.

 

The dagger board trunk is an interesting problem considering that the hull is made of foam.
Here is the design: a plywood box of required dimensions (sides and spacers in between). Attach 1x1" reinforcement 'square ring' around the bottom, flush with the opening of the case.
In order to fit this in the hull and make it strong - carve the hull opening to accommodate sliding in the entire assembly (the case all the way thru and the reinforcement 1" in) while remaining flush with the hull bottom. This assembly will resist any pulling force exerted on the trunk.
In the cockpit 1x1" 'square ring' at the bottom of the case visible from the cockpit, carved flush into the cockpit floor, and then the entire case surrounded by 6mm plywood secured to the reinforcing frame and distributing the force over a larger cockpit floor area. With an additional 'square ring' - the plywood 'washer' will be sandwiched between two 'square rings'.

 

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This may help:
-fresh water 62.4lbs per cubic foot
-saltwater 64lbs per cubic foot

 

62.4 - 64 lbs of water per cubic foot of air. The buoyancy specification for rigid insulation foam I have used for hull states 52 - 55 lbs per cubic foot. Considering that immersed portion of my hull is made out of solid foam I have used the foam spec as the calculation parameter.

 

BCer. You have used the specific weight of foam in the above calculations for the flotation capacity of your boat. That is not the way to do it. Try not to think in terms of the weight of the material you use for calculating displacement.........such as foam. It does not matter what material is used, what does matter is how much water the shape of that that material will push out of the way. A boat that weighs 250 pounds will make a hole in the water. Now, Think about the hole in the water not the boat or the material t is made of. . No matter what shape that hole has, it has made a hole in the water that pushed 250 pounds of water out of the way. It is the size, or volume of the hole that determines how much flotation you have . Try Wikipedia to explain Archimedes principal in more detail. There are concrete boats that float just as well as a boat made of wood, fiberglass, or foam. It is the total weight of water that has been pushed out of the way that determines the amount of flotation (buoyancy) that a specific size can provide.

You have estimated the weight of your boat, its contents, along with your own weight at 250 pounds. Your boat needs to make a hole in the water that pushes 250 pounds of water out of the way. If water weighs 62.4 pounds per cubic foot then 250 divided by 62.4 equals 4.006 cubic feet for the size of the hole your boat must make. You have seen by calculation that a four inch draft provides more displacement (hole in the water) than you need. So lets let the boat float a little higher (the hole is not as big) and see what the displacement would be. Three inch draft is closer to what we are after. Now you are better able to calculate how much rocker to put in the bottom. Keep in mind that the Material that the boat is made of has no influence on the size of the hole. It only determines part of the total displacement....or weigh that the boat must support.

The reasons for using foam to form the body of the boat are many. Perhaps the most important is that the weight of the boat will be small compared to the weight of a heavier material like aluminum or whatever. Foam is easier to form, it is possible that it is cheaper than conventional boat material. Foam is a satisfactory, if not ideal, material to use. You boat will be lighter than one made of heavier material and that will just be part of the advantage in that it will not contribute as much to overall displacement as a boat made of wood or fiberglass.

Forgive me if I have been guilty of presuming that you may not be familiar with all that I have written. It is not unusual for those who are not physics majors to assume that the material has some relationship with Archimedes principal. It does not. We are interested in the volume of that hole in the water not the material that causes it. You have the tools to calculate that as explained previously. .

Keep up the good work and lets get that boat built. Cheers.

 

Thank you for the explanation. I was not quite sure whether I was on the right track so I used the buoyancy spec for the foam which was lower and in my mind safer (not untrue).

 

Just think of the hole in the water. Think of how many pounds of water it would it take to fill that hole. Now you know how big a hole the boat has to punch into the water in order to support whatever weight to be supported.

Since we have become friendly corespondents, Tell us why you intend to use Titebond to attach the glass to the boat. Are you allergic to epoxy? Is Titebond actually cheaper as a bonding agent? Will it make as smooth a surface as epoxy?

 

Thank you again. I understand the concept of displacement. As the buoyancy is the upward force exerted on the submerged (partially of fully) object. From the material properties, the foam in this case, a cubic foot of foam will hold 55 lbs of weight before it is fully submerged in fresh water. At the same time it will displace 62 lbs of water. I suppose that the delta between these values can attributed to the weight of the foam itself. What would be the impact of the fact that the submerged part of the hull will be made of solid foam, if any? I would assume none since the displacement is the only value that counts.

I have used cotton drop cloth and titebond on plywood in a non-marine application. Worked really well.

I would like to use 8 to 10 oz pre-shrunk (washed, dried, and ironed) cotton cloth and titebond to 'glass' over foam. Titebond bonds to the foam well and it is hard when fully cured. Titebond is cheaper and easy to work with. The surface is fairly smooth. It can be sanded if needed and painted (exterior paint works well). Titebond III is waterproof but more expensive. Titebond II is not waterproof, the paint will take care of repelling water.

 

The myth of "titebond glassing" regularly comes up on this forum. Even its promoters confess that is only works for very short times in the water ( an hour or two then a week to dry out). If titebond worked and was as cheap as promised, then production boat manufacturers would not be using epoxy or polyester

I financed my university education building theatrical sets. I often glued cotton fabric over foam. It extended the life span from a month to a year.

I have followed this thread from its inception, Messabout, who is giving excellent council, consistently replied before I could. You have put considerable time and effort into your project. I hate to see you shorten its lifespan. Please use marine grade resins.

I like the ingenuity of your attachment bands. However, they will tend to move about. One frequently used technique is to inlay wood into the foam before covering with glass.

Best wishes
Paul

 

Thank you Paul. Can I use resin over canvas? I would prefer not to work with fiberglass cloth. As for the attachment bands - I was thinking of inlaying the straps in the hull surface, glue, fasten them in with screws (4" spacing") and then apply the next layer. Only the loop or possibly o ring would stick out.

The titebond application you mention, does it include the glue on both sides of the canvas, and does it include exterior grade paint? My boat will probably be in the water 2hrs a week.

I am wondering whether the glass is strong enough to keep the inlay wood from being ripped out?

 

BCer,

Epoxy will adhere very well to cotton fabric. But, it would be like preventing cracks in concrete by replacing rebar with straw. Glassfiber yarns are basically 10,000 times stronger than cotton yarns. In fiberglass composite construction, the resin mostly holds the strong fibers in correct orientation. Countless more layers of resin/cotton would be needed to match the strength of a few layers of epoxy/glass. Don't worry FG cloth does not produce the same ichy scratchy that FG insulation is renowned for.

It is common practice to use extra layers of glass were anticipated loads could concentrate, such as mast basses, rudder or other attachment points.

The theatre sets were mostly used indoors, so exterior paint would have been an oddity reserved for Shakespeare in the park. Several layers and/or weighs of cloth from cheesecloth, muslin, to heavy ducking, depending on how detailed or level of expected abuse. The cloth was always well saturated with glue. The work closely matches what i now do with epoxy. Yhe biggest difference: i now add winger instead od ammonia to my nightly overalls laundry.

 

I finally got to the rocker build. There will a lot of rasping. Since the rocker came up as topic after I had started rounding the chines the additional layer of foam will have a slightly smaller footprint than the original surface of the hull bottom.




Do I need a halyard if I can reach the top of the mast? If latter, I could just hang the yard on a shackle to of some sort.