LB26

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jacquesmm
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Re: LB26

Post by jacquesmm »

I am following this.
The concept of a "fuse" part is what I use in several places in my designs, in particular on sailboats. To have a cheap part that breaks before provoking major damage is a good idea.
For the swim platform, think of what will happen in a collision: either it is built as a part of the hull and will take an impact or it is designed to fall off and separate from the hull. I prefer the second one and I would have a solid bolt on swim platform with the bolts as the fuse component.

If any, most of the collisions will happen at slow speed during docking maneuvers.
Rounded corners will help but keep in mind that the swim platform has another function: it is also a chine fender. It protects the tumble home of the side to hit a dock. If you change the shape, add a chine fender.
Jacques Mertens - Designer
http://boatbuildercentral.com

fallguy1000
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Re: LB26

Post by fallguy1000 »

Many systems on boats must be designed for failure.

If your neighbor in the marina comes driving into you real hard; an ice pick shape toward his hull moght be nice, but obviously impractical.

Everytime I am building something on my boat, I am always trying to be wise about how will it break. Never do we design or engineer a solution with the notion, it cannot break. Oh, there may be a rare case here n there, but generally, not.

I was at the marina one day and I watched a fellow back a rather large boat into the slip. He was a bit fast on the throttle and he hit an inflatable dinghy at the back of the slip. For him, the dinghy was the failure point. Should he hit the slip too hard, the dinghy received the swim platform impact and the swim platform, while still detachable was unharmed.

If you trailer the boat someday and another car hits it from behind; the swim platform will take a significant impact. And the M200 transom may be smashed by the swim platform. If your band of protection is stronger than m200; it will not allow an m200 swim deck to penetrate. However, M200 coaming, M200 platform top, M200 transom will have unknown results.

Designing the swim platform to break off the boat is the most sensible. If someone drives up onto your swim deck; where do you want shear? Because your swim platform cannot withstand another boat driving up onto you. But it can happen.

You have to consider how it will break or what will break.

Again, propose your idea on boatdesign.net and you'll get some interesting fellows like JM to reply.

I don't know the shear calcs with glass, so I am not much use to you, but I can offer your band of protection should be stronger than the transom itself and a bit higher if the platform breaks upwards. And the platform itself should probably be made with a lighter core with bolts that are designed to go through inserts because the bolts need to be tightened and cannot in light core.
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fallguy1000
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Re: LB26

Post by fallguy1000 »

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les2021
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Re: LB26

Post by les2021 »

Morning Guys

Here we go again :D

Thanks Dan / Jacques

Dan interesting read. Very complicated and involved, am I right in saying that platform was not meant to fail I could not see a break off point at all. It was also a beast of a platform, have to say it did look fab when completed.

Back to my problem..... once again excuse my layman's terms and thoughts !

From what I have read from you guys and previous posts is that there is disagreement over should it break away or should it be part of the boat.
Jacques / Dan you seem both to be in the break away camp and as you have more knowledge than I think I have time to learn I am inclined to go your way. BUT there are things that I can not get straight in my muddled head....

Dan your sketch seemed to suggest a type of rebate section to fix the platform to the transom using s/s bolts 6mm, this I can go with. Slightly tricky on the glassing side but I can deal with that. Might need more advise on bolting direct through glass, washers/ plates etc.

But as mentioned before, for me the whole danger point is not so much the platform it is the brackets.

If you go the s/s bracket route with a calculated bolt size and an internal backing plate, with a side swipe hit I can see the bolts shearing same as with a down force, but this a very tricky calc. If the calc is wrong what is to stop the bracket twisting and breaking out the transom ?

What if there is a direct impact from the rear ? the platform flips but the impact continues and pushes the s/s bracket point load straight through the transom ?

The same principle I suppose would take place with the foam/glass bracket ?

So it seems the market place has many different designs, the question is which one gives the best appearance and safety aspect ?

Jacques can I be annoying and push this one over to you :D I am really engrossed in building this fab designed boat but this is one I can not just
build the way I would like, it does not make safe sense.

Your design includes a swim platform so there must be a way you envisaged attaching the platform to fail and still look good. Your design is exactly what I want to build hence my " wonderful sketch " ( nice comments only please :lol: )

Can you sketch up something that I can use for a guide ?

My preference is foam/glass brackets this is only for looks so will bow down to your advice.

If anybody else has thoughts please chip in.

Cheers

Les

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Re: LB26

Post by fallguy1000 »

The bracket attachment points or pads also need to be made with something strong. They can be made of solid frp as can the edge of the hull.

You just laminate say 4-6 pieces of 600/225 tapes for the edge and for the pads. These are stronger than core. If you don't want the bracket fastening to penetrate the hull; make the bracket pads thicker.

I believe Penfield, on the link I gave you disconsdered failure, but designed the fastenings for 1000# minimum. His was solid frp; so he didn't have to bush the fasteners.

The most likely impact is for the platform to get driven onto or as Jaysen says, clipped. To reduce the likelihood of corner clipping; break the edge by the amount of side overlap. So, if the platform is 3" wider than the hull; just angle it back to the hull width aft. It'll look sexier that way as well.

For getting driven onto or impacted; the platform will break off in a downward direction, so you design everything to fail downward. The brackets will bend or the fastenings will fail, but the pads are at the hull edge.

You'll have to have bushings made or find something available and cut them down.

If you want to make it all solid core and not breakaway; then I'd at least not make the bottom brackets like spears into the transom. This can be done by making them strong enough to support 2000#, but of a material weaker than the transom...so light glass does that...

consider my comments conversation please

Also, consider trim tab locations when bracketing.
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les2021
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Re: LB26

Post by les2021 »

Morning All

Cheers Dan.

This does seem a subject that is difficult to get to the bottom of. I understand all the arguments for and against, but for a first time boat builder or even someone who has built before, a concrete answer from a marine engineer / designer is the only way to safely move forward. I can not find any structural details / sketches on this subject probably because all boats have different construction / lamination schedules.

I do not have the knowledge to start performing shear loads for bolts, fibreglass etc.

I could just use my own thoughts on this and beef up the lamination schedule more glass on the transom / topside / bottom connection. But would this then just divert the problem to a different location.

So at the moment things have stalled slightly.....

Dan, I am taking that FRP's are fibreglass, resin, pads ? and bushings do i take it these are backing plates / washers ? Sorry for the lost in translation or my lack of understanding.

Cheers

Les
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fallguy1000
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Re: LB26

Post by fallguy1000 »

The key is the word solid. Solid frp means no core. And that combined with proper mountings and brackets will protect your transom which also protects the boat from sinking. Now, I have not forgotten you are foam. But I do not know if your offsetting foam weights are enough to keep you from sinking or sinking below the engines if holed.

So, yes, what you have drawn is what I would build. I would also make a solid band of frp where the platform butts to the transom.

But, not with the foam brackets. I would use steel brackets. And so the solid frp pads would go to the hull edge, bearing in mind trim tab plans. I would reserve some space for them, but I can't advise absolutely. It is not my expertise and I would need to ask. Seems like a foot reserved on each edge would be great plenty.

If you insist on foam brackets; just make sire they are not made with M200 and make them light.

By the way, you can also make one of these pads for a transducer unless you are locating in hull. But again, must consider trim tabs. The benefit is no holes in the hull and only holes in the pad. So, 3/4" thick or 15mm perhaps pad. Not every detail belongs to the designer. I can tell you from my build, many elements were done by me doing the engineering. For example, if you only wanted to build a small swim platform for two people; it may be different than the designers. And for me, I engineered the roof for snow load here.

I understand how this can be frustrating, Hopefully, JM can provide some guidance. Otherwise, I can help as well. Mertens might be cruising on his own ship as he is retired and enjoying time at sea sometimes and internet is uncertain for him.

Also; you need to consider typical. Typical swim platforms I see all the time are made from teak. They are not integral to the hull. Here are commercial brackets. Don't buy these, they are made from crappy 304 and will rust for you. The pivoting ones are 316, though. These will also give you ideas as they have 3 styles.

https://www.teakmarinewoodwork.com/plat ... ackets.htm
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fallguy1000
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Re: LB26

Post by fallguy1000 »

Also, if you need vending help on 316 hardware, I recommend McMaster Carr. They do not vend to foreign accounts, but I would help you if needed.

Here are some 1/4" 316 bushings. You may need to grind them to length.

These are 1/2" od, so they would reduce the total need and allow you to build a foam platform. These guys also sell cup washers and other 316 hardware. I was a little disappointed in the 1/4" cups. I could tell they were made on very old, shit tooling, but they look okay on the boat. Just up close, they are shit rippled.

https://www.mcmaster.com/stainless-stee ... -size~1-4/
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fallguy1000
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Re: LB26

Post by fallguy1000 »

If you insist on making it integral; then I suggest you make the solid frp section wider. Bond it to the hull with thickened epoxy well. Then make the fiberglass tapes the weak point. You could achieve this by making tapes with 200 gram woven glass top and bottom. So, for a 24mm platform, you'd need to build the solid frp band 130mm wide (or 150mm if you have the materials. Then the light woven tapes can be the failure point. But this is a bit hinky because the bonding area of the platform of 24mm is going to make it really strong, so you'd almost need to not bond it..or spot tack it only..

Consider it conversational...please.. the solid glass piece glued to the hull would be 130mm min to 150mm practical. The platform joint might need more thought..but a light 200g tape top and bottom won't allow your hull to be damaged... okay

Give JM time to reply.
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les2021
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Re: LB26

Post by les2021 »

Dan

Very useful reply and good info thanks.

Stay with me I am learning as fast as I can. I have attached another couple of my wonderful sketches.....

I think I am with the bush thing and platform, as you will see I have not attempted the hull fixing as not quite clear yet. I have attached an idea of mine so please do not laugh ( oh alright laugh ) It would, I hope protect the hull and be a break off point. Let me know what you think..... But I think there may be a better solution ! The idea is a s/s strip embedded in the FRP with no direct bolting to the transom, the question is would it be strong enough.

Cheers

Les
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