Cold Molding Greavette Streamliner style runabout

Hello,

I'm working on a design for a 16 foot runabout with curved surfaces like the Greavette Streamliners. Looking at planking such a design is quite daunting. Thick planks must be used for covering boards and sheers that are carved to shape. Instead, I'm considering cold molding.

Would two layers of 1/8" fir, marine plywood with one layer of Mahogany veneer be a good skin? I would use a full frame with stringers to support the skin.

Would the inner layers be bonded to the frame structure with 5200, then subsequent layers with epoxy resin be appropriate?

Looking for some ideas for this project. I have never cold molded a hull but have built a few boats and have carpentry experience.

 

Although we don't know anything about your hull shapes, since runabouts often have undevelopable surfaces, it might be more practical to consider strip planking.
Before knowing if the layers must be of a certain thickness and the number of layers, it is necessary to know the entire distribution of the structure that you have in mind.
Can you show us what the hull is going to look like?

 

There was a student at the Lyme Boatbuilding Academy, built an old Pettersson design in just the way you are considering; so yes it is possible, but you may need to configure the frames/stringers and planking.

 

This is a 22' Greavette off the web.

I would say strip planking would be more fun, but so much depends on the final look.

Based on your description; you want to end with a bright mahogany finish. I assume you want that layer to look like horizontal planking.

I happen to own a 1960 Carver Commander cold molded all mahogany 16' runabout. It is nowhere near as lovely as the Greavette. Mine was designed with painting as the planned finish. It has 5-6 layers of veneers on the sides and 10-11 on the bottom and the boat is still in service today, but needs a two week stem repair. I actually removed all the paint and bright finished it despite the diagonal planking.

The bottoms get paint, so creating a veneer bottom is really overrated, in my opinion. I would probably go for strip planked bottom and cold molded sides, but not sure how you'd transistion. The bottom must be substantially thicker than your proposal. And I think you are a hair light on the sides at 3/8".

As for 5200, it is likely too flexible.

Consider me as a friendly source, but not someone who can truly spec a cold molded layup.

Here is a pic of the Carver before she was launched. I just realized I have no pictures of her. I think they were on a different phone or my pc..

Carver in garage https://flic.kr/p/cGnHmd

 

Cold moulding will produce an excellent boat,probably a better boat than the originals.I would avoid fir plywood as I have never seen any that had a good surface,of a quality that wouldn't require lots of glue to fill the tears and wild grain.Meranti would probably be a better choice.As for thickness,it would help to have an idea of the amount of power intended.You could do a lot worse than getting hold of Dave Gerr's book on boat strength for some pointers.I would epoxy the entire structure,avoid glassing the topsides and use a good 2 part varnish, leaving Sikaflex for bedding hardware.

 

As others have said, producing that round edge is easier with strip planking. That doesn't mean you have to strip the entire boat, the rest can be whatever you choose. The real tricky business is veneering the rounding, I suggest using steam bending then a vacuum bag.
Cold molding with ply or veneer is pretty common for runabouts. Depending on the shape there are also mixed approaches, with the more complex surfaces cold molded in two layers and the easier ones from a single layer of ply.
What you don't do is use 5200, just glue the skin to the frame with epoxy.

 

Thanks to all for replies. The picture Fallguy posted says it all. Love the rounded surfaces!

As others have stated, I'll definitely make the bottom thicker. The bottom should be easy, a simple, V-bottom runabout. But that shear is going to be difficult with rounded surfaces. I have looked at pictures of Dan Danenberg's planking of one of these Streamliners and a lot of thick, 3D carved planks are used on the rounded surfaces.

Getting the shape with several layers of cold molded plywood shouldn't be that difficult. But planking the mahogany with horizontal orientation will be tough. I was thinking with thin veneers carefully spiled and narrow enough on the curve, I could avoid compound bending of the veneers. Vacuum bagging may be the way to go. I used to make glass over foam model airplane wings this way.

The idea of strip planking the hull, then laying on veneers may work. I completed a strip planked sailing dinghy two years ago (in five weeks) and am now sailing it off my cruising boat in Grenada (Carriacou). I'll return to Milwaukee in spring to build this boat as my next project.

A little more about this project: Last summer's project was to take a Glastron Jetflight and morph in a Seadoo GTX jetski drive. The 22 degree deadrise matched the jetski hull. I simply cut out the jetski bottom that contained the engine pan, intake and nozzle and glassed it into the Jetflight. It worked perfectly. Now I want to build a classic looking runabout with a jetski drive. I'll install stringers in the bottom of the hull to attach the jetski hull pan with screws and 5200 sealant. As with the Jetflight, the drive won't even know it is in a new hull. I plan to use a 130 hp Rotax 951 DI engined drive. The Glastron used a 110 hp drive, very fun to drive and had a 45 mph max speed. I expect the 16 foot mahogany hull to be a bit heavier.

Living on a boat is great but I miss boat building projects. Now we are snow birds and I have 5 months a year for these projects. Also pictured is the mahogany dinghy cradle I built on Uproar during the Covid lockdown in Tahiti. I was fortunate to have all materials on board for this.

 

That was a genius idea to cut off the bottom and use all the ready made mounts, etc.

 

I see some downsides to strip with a veneer.

Wood likes to move and if you encase it all inside and out in glass; it'll be less likely to move, but I'd still be nervous about the veneers doing their own dance.

So, for me, I'd strip plank the entire boat. You can run full length strips on the sides or make 22 degree bevels and offset if too hard.

The strips at the gunwhale/sheer get thicker to allow for major fairing. I could do it, but it is not ez. Then when you get in far enough toward the top; you rabbet or dado for the deck and step down to more reasonable sizes.

Then, I'd glass the entire thing with 6 oz like a big giant canoe. The glass is invisible

It'll need some framing..but that is all done after glasswork, save perhaps keelson.

 

I can't tell you how easy this was. The only difficult parts were hooking up steering, throttle and reverse cable. I used an old Evinrude control that has separate throttle and shift levers. Other than that, it was plug and play. My cousin really wants this boat. I'll give it to him to make room for another project. Total cost was $500 for the Jetflight and $2500 for two jetskis. One is intact and I'll sell with trailer next spring. Very cheap project. Still needs some construction for hatch cover, outboard well cover and total paint job. This will be more work than the initial project.

 

I would steam the veneer in place. Put it in a plastic bag, hook up a wallpaper steamer and staple the bag to boat. Then use some straps or thin ply with screws and gradually tighten until it conforms. After it dries glue it on, then trim it with a router. I would use veneers that are approx. 1/8 to have some sanding room, if you are confident you can go thinner (1/16), those you can probably bend on with a clothes iron and a wet towel. Vacuum bagging will give a better finish, and is almost mandatory with 1/16 veneers.

 

If that Greavette is similar in shape to the intended boat,I don't see that many problems with veneering the deck edge.This might cause screams of outrage and gnashing of teeth.However,from the windscreen aft,there isn't a huge amount of curvature and the first three feet or so is only a little curved both ways,The more or less cylindrical portions won't,be be too challenging and if broken down into smaller segments,the severely curved section needn't be insurmountable.There will be some veneer wastage and I would think it sensible to have more on hand than the bare minimum.I would approach it by laying out my leaves of veneer in advance to establish a grain pattern that doesn't show too many mismatches since you don't want wild figuring next to an undistinguished section and I would try to arrange for the plainest pieces to be allocated to the area of maximum curvature.

I would use a soft pencil to put alignment marks across the sections I selected for the severely curved areas and then treat the area a bit like trying to plank the outside of an orange.That is to say I would expect each veneer segment to need careful fitting on both edges and I would regard it as desirable to stagger the butt joints at the ends of adjacent strips of veneer,the pencil marks would ideally work out to be quite close and careful fitting ought to make them just about invisible.The more cylindrical sections beyond the zone of maximum curvature won't be a problem.Holding the veneer down while the glue cures might be interesting and I'm not sure how simple a vacuum bag would be.You can use staples through veneer scraps but pulling out a couple of thousand staples is pretty tedious even without the probability of a few breaking a leg off.Done well,it could look outstanding and as an aside,could somebody explain the common use of sludge brown stain on a lot of this type of boat?Is it to obscure the colour range in the planking or to achieve what the customer believes is the shade of mahogany?I've always found it to be pink when freshly cut and to darken with age.

 

Those colors are all to deal with UV.

After you put 20 coats of varnish on a deck; it starts to take the hue of the finish.

 

The pic of the Swedish runabout in post #3 seems more or less typical for powerboaters.
Underway with hanging/trailing fenders.
Sorry, couldn't resist.