Mahogany runabout first time build questions

My first boat building project is going to be a challenge. I am an engineer by profession, and I am confident I can make the right design decisions, however I have only build model boats in the past and I want to hear from those of you with more experience building and maintaining wooden boats. I plan to build a scaled down (14ft) chris-craft style runabout. Here is a mock-up picture I made.
http://nbehlman.com/mockup.png
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I have a number of questions...
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- I have seen arguments for 3M 5200 vs. epoxy. I don't care how smooth the ride is, I just want my boat to be well preserved and light. What should I go with?
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- Does a polyurethane clearcoat protect as well as epoxy?
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- I think I saw someone lay lightweight fiberglass over the outside of a mahogany hull. I know it's glass, but is it really that transparent? I would like to reinforce the bottom with fiberglass. Should I glass it over on the outside?
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- I would like to save cost by using a plywood in lieu of mahogany. I have seen people on the internet use ribbon stripe tiama plywood and it looks pretty good to me. My concern is that the veneer is very thin and I will ruin it by sanding too low. Since I will be planking diagonally first, I think I can do all the fairing, sanding, filling etc. on the subplanking and lay the outer planking down very carefully. Does anyone have experience with this type of veneer plywood?
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- I was planning on using okume marine plywood for the subplanking. How should I bond the outer planking to the subplanking?
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- What kind of fasteners should I use? Bronze screws? Stainless?
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- Can anyone recommend a good source for buying mahogany and/or marine plywood?
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I would like to hear your input! Thanks.

 

Your first step is to get a good set of plans with construction details. The planking is part of the structural design, which will determine thickness, etc.

 

I am designing the boat myself. I used to be a structural engineer in the hull structures department of a major defense contractor. I am more interested in hearing people's first hand experiences with the materials I mentioned and double planking.

 

Here's a good place to start looking for plans.

Glen-L Marine - Inboards

Their forum is also worth a visit.

Lurvio

 

Again, I'm not looking for plans. I am more looking for firsthand construction tips.

 

"- I have seen arguments for 3M 5200 vs. epoxy. I don't care how smooth the ride is, I just want my boat to be well preserved and light. What should I go with?"

Epoxy is the durability choice. If preformed correctly both methods can produce a fine hull, but 3M 5200 is a sealant/adhesive, while epoxy (in this application is an adhesive. If used with proper encapsulation techniques, far superior then a typical 3M 5200 bottom job. In fact, with epoxy, you can greatly reduce the internal structural element requirements.

"- Does a polyurethane clearcoat protect as well as epoxy?"

In a word, nope. Polyurethane clear coatings are just that a clear coating. It doesn't seal the wood, just offers abrasion protection and UV resistance. Epoxy absolutely must have UV resistant coating applied over it (paint, varnish, polyurethane, etc.) as it can't stand alone against UV for very long. Epoxy is a coating and an adhesive, but not a finish. Polyurethane is a finish.

"- I think I saw someone lay lightweight fiberglass over the outside of a mahogany hull. I know it's glass, but is it really that transparent? I would like to reinforce the bottom with fiberglass. Should I glass it over on the outside?

Any 'glass cloth under 4 ounces, will be completely transparent. If you use 6 ounce, you'll see the weave. These light weight sheathings do not add any stiffness or strength to the planking, just abrasion protection.

"- I would like to save cost by using a plywood in lieu of mahogany. I have seen people on the internet use ribbon stripe tiama plywood and it looks pretty good to me. My concern is that the veneer is very thin and I will ruin it by sanding too low. Since I will be planking diagonally first, I think I can do all the fairing, sanding, filling etc. on the subplanking and lay the outer planking down very carefully. Does anyone have experience with this type of veneer plywood?"

Meranti plywood (Lauan) is a common choice in these builds. Yep, the veneer is thin, but craftsmanship can over come much of this concern. Simply put, you build needs to be about as precise as practical on these types of runabouts. They are the height of boat building art, so delving into this means you need to preform at the same level, or it'll look like something else. It's assumed you'll fair the layer below the final, exterior layer, other wise you need to paint the hull to compensate for the unfairness. You could employ a solid veneer as the last layer, which will permit some fine tuning of the fairness, though adjustments to the planking scantlings will be necessary.

"- I was planning on using okume marine plywood for the subplanking. How should I bond the outer planking to the subplanking?"

Okoume is a fine planking stock, light, strong for it's weight, etc. Bonding the layers together falls into the traditional/3M 5200/epoxy debate. Once you make this decision, the rest is determined.

"- What kind of fasteners should I use? Bronze screws? Stainless?"

Bronze is the choice for 3M 5200 and traditional build methods, with stainless being suitable under epoxy.

"- Can anyone recommend a good source for buying mahogany and/or marine plywood?"

Much depends on your area, but these grades of plywood can be special ordered for most lumber yards or mills.

Lastly, how did you down size the runabout and are you aware of the implications of mechanical similitude, in regard to the scaling process? Assuming your back ground permits your understanding of this, it's pretty important to get things in proper proportion for the new scale, which is the basis of my question and other poster's concerns.

 

I am basing my design on a 16ft Chris craft runabout. Half breadths are pretty much scaled linearly. The heights above baseline are tweaked a bit. I wrote a code that calculates deadrise, hull weight, VCG, LCG, CB, etc. I used that to do savitzky's speed predictions and porpoising stability calculation. I will be using a jet drive, so I do not have any control surfaces. Is there something I'm missing? Is there a good technical reference I should have? I am going to 87% length of the Chris Craft which I don't think will put me too far off.

 

Hi nbehlman the build I'm working on at the moment http://www.boatdesign.net/forums/wooden-boat-building-restoration/new-boatbuilder-32783-2.html is plywood skin and then planks of mahogany and then coated with 4oz cloth and 4 coats of west system epoxy it will then be coated in varnish for uv protection it is very hard to see the weave in the 4oz cloth. In the build most of the ss screws were taken out after the epoxy had set so I used only a few in the end , they say that stainless steel screws need air to keep them from corroding so you should not cover them in epoxy.

Best of luck with the build.

 

That's not correct Sean, stainless covered in epoxy, in a bonded hole, will not corrode. I've cut open my own work from 20 years ago, to find perfectly pristine stainless screws, buried in epoxy. This said, if the fastener isn't in an encapsulated wooden element and bonded, it will likely corrode.

 

I think that using epoxy is easier than 5200; cheaper too. You can build the hull with double diagonal and then use 3/15 (4.5mm) veneer to make it look like traditional longitudinal planks. If you are an engineer, it should be easy to calculate the scantlings.

 

If it was me, I'd use a plywood inner layer and skin externally with veneer, of course with epoxy. I wouldn't sheath the boat and would store it in doors out of sun light. Most of the aft bottom can be done in large pieces (plywood) on the diagonal, while ripping them down in width from midship forward.

The 3M 5200 bottom job, is a left over of a bygone era, that some just can't let go of. These bottom jobs where the answer to the leaky traditional methods, before epoxy took the industry over. Some still swear by this method, but it's just simply that that can't accept that it's been superseded by epoxy. So, it depends on what you want. If it's a real traditional build, then the 3M 5200 approach can be helpful, though epoxy is more effective if used in the role instead of 3M 5200. If you elect to toss 3M 5200 over the side, in favor of a modern build (epoxy) then you can also toss most of the structural elements in the drink with the 3M 5200, making huge savings in weight, assembly costs and effort too. Nope, it's not a traditional build, but it'll be stronger, tighter, lighter and as a result will preform better to boot.

 

This is good info, thanks. Personally, I think epoxy is a fantastic material, but I saw some people talking about 5200. Having never built a full size boat, I had to ask. Epoxy is always great on models.

I am wary of the cost of mahogany vs. the ribbon stripe tiama. I will look into it. Thanks again.

 

Meranti is a reasonable substitute for mahogany, though it you know the two species, it's easy to tell the difference, but 99% percent of the folks looking, would know the difference between meranti and mahogany, if it hit them on the head.