outboard bracket redesign

[Toepfer Marine]

Hello friends I have a 1984 Rybo Runner built under the Rybovich label These hull are 30' x 11' stepped hull bottoms (ahead of its time)they originally came with a few power potions one was the OMC sea drives. the path in power was modernized and Rybo built its own fiberglass engine bracket built very much like a modern aluminum floatation style bracket it has an exterior bolt flange. The bolts penetrate the transom as one might imagine. the bracket I have has two interior structures I would call stringers.
The Rybo folds added two large Aluminum plates locally to bolt the engines to the bracket, my desire is to remove those and add few pieces of a Coosa like product glassed on to the bracket aft section. The swim deck will fit on top of the bracket with a finished curb
My intention is to connect the bracket to the hull by extending the stringers through the transom and glassing them to the Hull stringers, The flange is useful for added stiffness, I wish to add glass to the inter of the existing transom making the structure somewhat monolithic.

I will also add a modern looking swim platform to the bracket providing additional glass contact.
I will follow up with pictures. I hope this inspires folks to jump in and offer suggestion. The anticipated power will be two 300 HP fours stroke engines Thanks in advance

 

[Toepfer Marine]

 

[fallguy]

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No, to your idea on the coosa. Based on my interpretation of the attachment, the aluminum serves as a plate that can withstand compressive forces. Coosa does not share that property, but G10 does get closer if you want a single finish.

Coosa will crush under bolt pressures and overtime. The typical compressive strength of coosa bw26 is 2000psi and aluminum is 40,000, or 20 times stronger. To stay in composites, g10 plate is about 50,000 psi. G10 can be embedded into the neighboring laminate and basically disappear, if that is the goal. Hard to comment more without pics.

 

[fallguy]

A 12x48 sheet of 1/2” thick g10/fr4 is $235.

https://www.professionalplastics.com/G10FR4SHEET

 

[fallguy]

I will add, I own a 22’ Bluewave boat with a coosa transom. The motor is bolted on with an aluminum back plate with the Bluewave logo. The plate is about 28 square inches which disperses the engine stresses (compressive bolt pressure) across more of the coosa than say a 2” round washer… Interestingly enough the plate is about 20 times bigger than a simple washer would be on the top two bolts.


If I have misinterpreted the use of the plates, let me know.

 

[Toepfer Marine]

I will add, I own a 22’ Bluewave boat with a coosa transom. The motor is bolted on with an aluminum back plate with the Bluewave logo. The plate is about 28 square inches which disperses the engine stresses (compressive bolt pressure) across more of the coosa than say a 2” round washer… Interestingly enough the plate is about 20 times bigger than a simple washer would be on the top two bolts.


If I have misinterpreted the use of the plates, let me know.

All good points Fallguy, in the history of the Rybo Runner they did not have access to a material like Coosa, my plan will be to laminate the complete rear surface if the Bracket brining the height up to what the existing Alum plates as well as applying tapered triangle pieces toward the boat to support the higher curb, I should this 100oz. of glass either side will aid in the compressive strength

 

[ondarvr]

I will add, I own a 22’ Bluewave boat with a coosa transom. The motor is bolted on with an aluminum back plate with the Bluewave logo. The plate is about 28 square inches which disperses the engine stresses (compressive bolt pressure) across more of the coosa than say a 2” round washer… Interestingly enough the plate is about 20 times bigger than a simple washer would be on the top two bolts.


If I have misinterpreted the use of the plates, let me know.

I had a Bluewave, we built two with experimental gel coat for test purposes. Owned it for 18 years.

 

[fallguy]

All good points Fallguy, in the history of the Rybo Runner they did not have access to a material like Coosa, my plan will be to laminate the complete rear surface if the Bracket brining the height up to what the existing Alum plates as well as applying tapered triangle pieces toward the boat to support the higher curb, I should this 100oz. of glass either side will aid in the compressive strength

I would disregard the glasswork in calculating the forces on the bolts.

Here is a bolt load calc to compare to the compressive strength of coosa. (From chatgpt) ..

You see the 6600 pounds? Using a 2”x2” washer reduces it to 1650, but this number is too close to the crush rating of the coosa. I’m not sure my inside backers now inaccessible are sufficient, thus I advise you to consider significant backing plates. Since the top bolts are relatively close to the transom top, another good plan is to radius and overwrap the transom with each sides laminate. Thus for say a 64 oz per side laminate; the top is 128 ounces. This way, the core does less and less and the fiberglass thickens to get closer to something like G10. Of course, a composites engineer would provide a much better detail than me. For me, I just recommend larger backing plate…mine on the bottom bolts are probably a bit small and I regret it and wish they were 3” square and I’d not think about it ever again. I erred and used only the engine weight for the calc.


Half inch bolt clamp pressure:


A common engineering approximation is:


F = \frac{T}{K \times d}


Where:

• F = bolt preload (lb)
• T = tightening torque (in-lb)
• K = nut factor (typically 0.18–0.22 for lubricated stainless, about 0.20 is a good estimate)
• d = bolt diameter (inches)

For example, with a 1/2-inch stainless bolt torqued to 55 ft-lb:

• 55 ft-lb = 660 in-lb
• K = 0.20
• d = 0.50

F = \frac{660}{0.20 \times 0.50} = 6,600\ \text{lb}


So each bolt clamps with roughly 6,600 lb of force.