Radon Transom Replacement

Questions about boat repairs with our resins and fiberglass: hull patches, transoms and stringers, foam, rot etc.
fallguy1000
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Re: Radon Transom Replacement

Post by fallguy1000 »

Apologies. Thought you were showing us the final. People think they can get away with a little fiberglass.

Fun for u, this is a 13' Whaler that had some minor repairs.. hehe. They fished the engine up by the cables and saved it. Guy told me he saw a few cracks in the corners and did some minor repair!
44753ABA-C3D5-459F-B61C-160FC30E9604.jpeg
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bigtalljv
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Re: Radon Transom Replacement

Post by bigtalljv »

Well, I’m going to ask the angels in the head of a pin question, how many layers should I use on my new stringers?

A little background is my stringers are four different heights from various rebuilds the boat has been through. Best I can figure the original stringers had about 1/4” of roving on them, which I know roving isn’t great for that. They were 3/4-1” wide wood cores and about 8” tall. I have built new stringers using two layers of marine ply, so 1-1/2” x 8”. (All the people I have talked to who are rebuilding these boats are making the stringers about that size. LVL’s are popular because one guy used them and was vocal about it. I had the plywood so that what I’m using).

So how much glass? I know specifics are frowned upon and not usually offered so let’s talk generalities. 6 layers of 1708 seems excessive, I think that’s about 1/4”.

For plywood stringers is there a point where more layers stops adding usable strength/stiffness and just add weight?

Is there a acceptable conversion between plywood and foam core layups? I’m happy to just grossly overbuild it and build foam thickness over plywood.

Can stringers be TOO strong?

That’s too many questions already so I’ll see if that gets me anywhere.

Thanks
Jason

fallguy1000
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Re: Radon Transom Replacement

Post by fallguy1000 »

So, what speeds are expected?

And the stringers are 3/4" plywood?

I'm not really qualified to answer, but that doesn't mean I won't.

Stringers tranfer engine loads in the forward direction into the hull. Duh, right? Well, not so. Four engines or two or a single!

Get me these specs. I will plug them into Gerr's formulas tonight or when I have time.

Length
Vertical height, just use gunwhale to vee bottom average.
Beam
Speeds planned.
Stringer thickness.

If you have Gerr's text, page 43 is the stringer spec.
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bigtalljv
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Re: Radon Transom Replacement

Post by bigtalljv »

fallguy1000 wrote: Mon Aug 01, 2022 11:27 am So, what speeds are expected?

And the stringers are 3/4" plywood? - Yes, 2 x 3/4" = 1-1/2"

I'm not really qualified to answer, but that doesn't mean I won't. That's the spirit, I'm not qualified to do this either!

Stringers transfer engine loads in the forward direction into the hull. Duh, right? Well, not so. Four engines or two or a single! - 2 x 200hp outboard is the current plan

Get me these specs. I will plug them into Gerr's formulas tonight or when I have time.

Length - 27'
Vertical height, just use gunwhale to vee bottom average. - Ok, so lets say 40". BUT, I'm going to raise the gunnel about 12". the deck/sole is at about 26". Is taller or shorter stronger? I'd calc for the worst case.
Beam - 8'
Speeds planned. - from another guy with the same hull and a single 300hp outboard, he had 40mph top speed and 25mph cruise
Stringer thickness. - 1-1/2" (2 x 3/4" Marine ply)

If you have Gerr's text, page 43 is the stringer spec. - I do not. I guess I Should but that kind of stuff isn't my strong suit and my 4th year mechanical engineering student would rather build rockets than help me understand boat calculations...

fallguy1000
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Re: Radon Transom Replacement

Post by fallguy1000 »

bigtalljv wrote: Mon Aug 01, 2022 11:45 am
fallguy1000 wrote: Mon Aug 01, 2022 11:27 am So, what speeds are expected?

And the stringers are 3/4" plywood? - Yes, 2 x 3/4" = 1-1/2"

I'm not really qualified to answer, but that doesn't mean I won't. That's the spirit, I'm not qualified to do this either!

Stringers transfer engine loads in the forward direction into the hull. Duh, right? Well, not so. Four engines or two or a single! - 2 x 200hp outboard is the current plan

Get me these specs. I will plug them into Gerr's formulas tonight or when I have time.

Length - 27'
Vertical height, just use gunwhale to vee bottom average. - Ok, so lets say 40". BUT, I'm going to raise the gunnel about 12". the deck/sole is at about 26". Is taller or shorter stronger? I'd calc for the worst case.
Beam - 8'
Speeds planned. - from another guy with the same hull and a single 300hp outboard, he had 40mph top speed and 25mph cruise
Stringer thickness. - 1-1/2" (2 x 3/4" Marine ply)

If you have Gerr's text, page 43 is the stringer spec. - I do not. I guess I Should but that kind of stuff isn't my strong suit and my 4th year mechanical engineering student would rather build rockets than help me understand boat calculations...
Scantling Number Calc
Sn=27•8•40/12/1000 =0.72

Basic Hull thickness in solid frp

Inches = 0.25 times cube root of Sn
=0.9•0.25= 0.225"

Estimated top speed in knots 35

Increase laminate thickness by 1% for each kt speed above 10kts.

25% increase

Basic thickness, solid frp minimum lower topsides
0.28"

Hull bottom... 0.322"

Longitudinals (based on 10)

Stringer core dimension (for

3.12•Sn^0.28

3.12x0.91= 2.84"

Stringer height minimum 1.42"

Stringer laminate minimum = 0.17•Sn^0.38

=0.17*0.88
0.15 inches

Tabbing runout

1.5" minimum

Chine reinforcement for hard chines

35% of hull bottom or 0.11

Translation...

Gerr makes things complicated regarding stringers. The Gerr calcs use a basic scantling number and other assumptions. Changes to those change the calcs.

As you can see, Gerr is using a much thicker stringer, but the glass thickness is what you wanted. 0.15" of glass is about 4 layers of 1708

I must admit some interpretation failure for me on the stringers. Not sure why they would want so many stringers. Hard chines reduce them by one per side, but 5 is nuts. He is suggesting a thicker stringer and for as large a boat as yours another 3/4" plank might be wise.

Gerr's scantlings tend to be more than enough.

This is enough for something to talk about.

I accept no liability for the work or the correctness of the calcs. You can go buy the Gerr book for like 30 bux!
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bigtalljv
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Re: Radon Transom Replacement

Post by bigtalljv »

what service, that was quick! Thanks.

3.12x0.91= 2.84"
Stringer height minimum 1.42"

Is that saying it wants 2.84" wide stringers that are only a minimum of 1.42" tall??

I thought tall was better than wide for stringers? I guess this is why it's just so hard to get answers for these questions. That's a "right" answer but it's kind of different....

Ok, on to Plan...probably past B....J?
So the best advice is to rebuild what was there, correct? If I go the same height and 1/2" wider with at least 4 layers of 1708, I shouldn't be too far out of the ball park of what I had before? I guess we could argue the inherent strength and rigidity of solid wood vs marine ply....but I can make that up with more glass if needed?

For the layers of glass when talking about stringers does the tabbing count or that's separate? X layers up and over then Y layers of tabbing?

fallguy1000
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Re: Radon Transom Replacement

Post by fallguy1000 »

Tabbing is separate.

Like I said, I don't know how to interpret Gerr on the stringers.

I'd tab those in with 3 tapes and I'd go 3x3/4" on the stringers. For tapes, I'd go 6-6-4 or 8-6-4 cuz 6-6-4 is a pain bcuz if you go 2-4,4-2, the 4" tape is a hardspot in the middle.
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fallguy1000
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Re: Radon Transom Replacement

Post by fallguy1000 »

Solid wood versus ply is all about availability. But softwoods are not ideal. If you are using softwood, I'd laminate 4 pieces together.

Plywood can be less, but I might still go 3 pieces based on Gerr. Again, my interpretation is less than great. It is a weak area in the book, tbh.
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bigtalljv
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Re: Radon Transom Replacement

Post by bigtalljv »

Thanks again. I have Okoume marine ply. Three layers of plywood is easy enough. And I can just throw some more glass on it….. this is how internet boats just keep getting bigger :-).

I either have 12-8-6 tape or 8-6-4, I ordered it some time ago and need to go dig it out but either way I have lots and good overlaps.

fallguy1000
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Re: Radon Transom Replacement

Post by fallguy1000 »

3x 3/4"
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