Hatching Of The Wren: my A25 restoration thread

[jerridsc]

I will be using the original tank for gasoline in addition to another 15 gallon tank installed on starboard side opposite original tank for extended range. The PO removed and cleaned the original tank as described in a post which I will try to forward to you.

[nebulatech]

Interesting! Where are your batteries located? I can only imagine the 15 gallon tank would live where the original batteries were installed. Do you know your fuel consumption yet, or rather, how far will 35 gallons take you (at hull speed)?

[jerridsc]

Some answers and photos for nebulatech: 4 batteries will live where engine used to be. Fasteners for swim platform and engine mount are 3/8” ss with locking nuts. Two gasoline and LP gas sniffers located in aft berth (where my LP locker and fuel filters will be located) and in pilot house bilge area. Won’t be using blowers as they do for enclosed gas engine inboards. Not sure what my range will be. Hull speed in calm conditions will require @2200 rpm’s for my 25hp mercury bigfoot. Mercury website says at that rpm, @ .5 to .6 gallons per hour. We’ll see.

[nebulatech]

jerridsc, thanks for sharing those photos, that gives me some ideas!

And WOW, .5 gph at hull speed? That will give you considerable range with 35 gallons! Beautiful bottom paint btw

[nebulatech]

20210520_180155.jpg

Just finished milling the veneer for my rubrail. I also made a stack of 6" wide pieces to veneer the bulkhead in the cockpit under the helm.

[tribologist]

Sorry. Yes the front and rear cabin curved windows are 1/4 or 6mm. I was just suggesting to go 5/16 or 8mm if u replace the main windshield. (Don’t do that. . )

[nebulatech]

20210612_124148.jpg

Whew, it's been a while. I just reinforced the transom for my outboard motor mount. 5 layers of heavy woven roving. I had planned more, but ran out of vinyl Ester resin. I tabbed the corners by the motor vents and the floor with 2-3 layers of chopped strand in between roving.

[tribologist]

Nebulatech,

When you order more vinyl resin (Check out Mertons fiberglass, they are very good to deal with), get yourself a roll of 1708 biax/mat. So much nicer to work with than weave and builds way faster than mat. Way less mess too.

Are you not laminating in a piece of plywood into the transom? It would take a very significant amount of glass to built is strong with only fiberglass but with your mount there should not be that much load on the transom.

I'm so looking forward to see how this is going to come out. A 25-30 hp four stroke is a lot cheaper than a diesel... Would be very quiet too. Where are you on the paint project and the soft sand. I am going to work on stress cracks etc this summer and in the fall or next spring we are painting the deck. One of the things I really wrestle with is the non skid. We have some quite ugly areas that should be sanded flat and rebuilt.

[nebulatech]

Tribologist, thanks for mentioning the combi mat. My biggest problem with woven roving is the way the edges fall apart. I imagine the combi mat would be better in this regard. After much searching (and little tangible answers) I chose not to use a wooden core. I admit I'm not an engineer, and I'm not able to calculate loads to ascertain how thick my transom needs to be. If you care to weigh in, PLEASE DO. I would certainly appreciate it!

Here is what I came up with: Based on the information I could find, I decided to compare G10 to plywood, since G10 is essentially a fiberglass and resin product. G10 has a tensile and compressive strength of 40,000psi/60,000psi. I found that plywood varied a lot, but the highest numbers I found listed a tensile/compressive strength of around 9000psi/8000psi. Of course, G10 is probably stronger than the 40 year old polyester hull, and a lot stronger than the less-than-perfect layup I added using vinylester. I'm going to assume that my transom has a tensile/compressive strength half that of G10, or 20,000psi/30,000psi, just to be safe, and probably realistic. So assuming these numbers are accurate, fiberglass is at least twice the strength of wood. Boat transoms with bigger motors (150+ hp) are usually 1.5" plywood with fiberglass skin. My dingy is 3/4" painted plywood with a plastic piece added to it, making it about 1". It's probably rated for up to 10hp. With my motor being 25hp, weighing about 140lbs, I need something in-between, probably the equivalent of 1" plywood, or 1/2" fiberglass.

I found that the original glass where the rear ladder was bolted on was about 1/2", thicker than the hull in other words. I added close to 1/4" to this. The ladder rungs, with two bolts, were sufficient to hold a working load of at least 200lbs, which is more than my motor. I'm planning to use either 4 or 6 1/2" stainless bolts to hold my bracket to the transom. I don't think I'm going to have any issues with the weight on the transom. I'm principally concerned with the force of the thrust from the motor translating into the hull. The boxes for the motor vents present a challenge because the load is transferred into them instead of the topsides. In retrospect, reading over your comments, I am considering adding some stiffeners joining the transom to the bottom of the hull. Since I'm not quite decided, and hoping for your thoughts, I think I will simply avoid priming and painting this area for the time being.


Regarding the crazing, my boat had it pretty bad. I was really concerned about it, so I understand how you feel. My shipwright friend suggested I either sand it out or use veil mat. I really didn't feel that my skill was good enough to use veil mat. I found that when I sanded to prep for painting, most of the crazing sanded away. I sanded well, and ended up going through the gelcoat in probably half a dozen small (less than 1") spots, but that's OK because gelcoat is not structural, and I painted over it. Honestly, just sanding out the cracks seems to work for the most part. After sanding and priming, the only areas that still had crazing were non-skid areas, and applying the new non-skid took care of that so my foredeck and coachroof are crazing-free (at least by appearances). We will see if that holds true for the rest of the boat when I paint it.

Regarding the softsand product, I was very impressed! It is very soft underfoot. I will take some pictures on the next sunny day, and send them in. I found that I top-coated too heavily and the sand doesn't translate as well as it could have in some areas, but it does seem to work pretty well. I should have bought the sample set with different grits to experiment with. I calculated that I would need about 2 quarts, so I bought a gallon of the stuff. I'm pretty sure that 2 quarts would be sufficient if you're careful, but I can be more precise after I finish painting. If you would like some to play around with, send me a PM and I'll gladly ship you some. You could use it on your front hatch, as an example, or do a sample board like I should have done!

[tribologist]

Without getting to much in to plate and shell theory... Think about cardboard... All the stresses are taken by the skins. The strength of the core barely matter at all. It's just there to keep the skins from buckling and to take shear loads. 1/8" of glass on each side of a 1/2" sheet of divinycell is probably about as strong in bending than a solid 1/2" glass panel. Lots of panels in aircrafts are thin skins on aluminum or nomex honeycomb.

A very familiar hull design is divinycell core and glass on in and outside.... Like the albin deck. The cabin roof is something like 1/8 on the top skin and 1/16 or 3/32 on the inside with a 3/8 divinycell layer in between. Fiber Glass Bob Ross eg Andy at Boatworkstoday on youtube has an insane amount of videos that are the most informative pieces you will ever find. are there other ways to do things? sure, but if you do what he does you will never fail.

He has loads of videos on stress cracks. Don't use weil (is that what you call the weave?). use 1708 or mat there too or lately people are using flexible epoxy. Both Andy and TotalBoat (jamestown) has videos on it.

Classic method https://www.youtube.com/watch?v=i9HbPP8A0Fo
With flexipoxy https://www.youtube.com/watch?v=PwNNnK-XBMQ&t=983s

The only time I would use weave is for glassing wood hulls or possibly to build very think layers using the super heavy weaves. I have yards of regular weave that might even throw away. I buy 1708 as 4" wide band and as full width. Mat I always by 10-15 yard at the time.

G10 is probably a little stronger but not much. Glass is glass. it has not changed much. There has been S and E glass for a long time. S is high performance glass. Boats are made from E and so is G10. The A25 drawing call for 30% glass content Not sure I believe this https://www.unicomposite.com/what-does- ... heet-mean/ 10%? in that case the albin is stronger...but I would think the fiber content of G10 is as high as a hand lay up but maybe not... G10 was never thought of as a structural material. Its for circuit board so it has to be strong enough and easy to drill...

You don't really need heavy bolts. That just makes it hard not to crush the laminate. What you need is large stiff washers to spread out the load to avoid stress cracks. 3/8 is plenty strong at around 6000 lb but you should try to get large, thick washers. Like Mcmaster 92844A143 they are 0.1 thick and 1.25" od

Do you have any sketches on how you are hanging the motor? I can help you with some basic analysis. We can do a zoom meeting or something?

jerridsc's design is really well thought out both from ease of fabrication and from a structural point of view. He spread the push loads along the transom and then the pull loads are in the stays from the top.

At 20 HP = 15kW at the prop 7.5 kt or 3.858m/s You have 15000 W/3.858m/s=3887N = 873 lbf force pushing at the prop at hull speed (that SI system makes physics easy...)

The further vertical spacing you have the lower the forces get. A ideal mount would mount way down on the transom and way up. If you did that you would probably not even have to reinforce.

Imagine the loads on the cleats during a storm or the load when one lupp the boat up on the trailer and look at those fasteners...

[nebulatech]

Tribologist, thank you for all of the info. I will be digesting it over the next week or two. Thank you also for the kind offer for a Zoom meeting. I will take you up on it. Right now, I'm building new window frames for the forward cabin. Pictures coming next week (hopefully)

[nebulatech]

Small update. Painted bilge, lazarettes, cockpit. Also sneak peak at new windows. I took pictures of the construction, which I will add later, possibly in a new thread...

20210820_123230.jpg

20210820_123255.jpg

20210820_123519.jpg

20210820_123524.jpg

20210820_123540.jpg