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Does foam make boats stronger?

Old 02-19-2020, 11:25 PM
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Default Does foam make boats stronger?

I pulled off my old teak step pads on the gunwale of my Mako 171 that I'm currently restoring. Under these step pads are holes which allow me to see in the gunwale itself. One side has foam all the way to the top and the other is only half full. Should I be concerned about this? Does stepping on the side with less foam risk cracking of flexing the fiberglass? My father thinks it's a big deal. I don't. Who's right? Look at the pic to see what I'm referring to.

Less foam

More foam
Old 02-20-2020, 06:09 AM
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Foam can increase rigidity. Think of it as a network of “microstringers”. None of them is individually very strong, but in combination...
Old 02-20-2020, 06:16 AM
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On many of the newer boats, the foam is part of the structure and strength. Can't speak for yours specifically.
Old 02-20-2020, 06:50 AM
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Yes it adds strength and rigidity. I dont think you need to be concerned at all. The deck you speak of was designed to support your weight.
You could always add some if you wanted to if it makes you sleep better.
Old 02-20-2020, 07:01 AM
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I have to disagree. Foam has no strength at all. It is mostly used to provide a form so that fiberglass can be molded into the desired shape such as fiberglass stringers. I do agree that if you do not have stress cracks in the hull I would not worry about it.
Old 02-20-2020, 07:01 AM
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I agree that in the overall structure foam can add a good bit of stiffness, rigidity and strength; however in the upper portion of the side (below the gunwale) the effect is probably minimal at best. From the waterline and down is where it would really do something.
The conversation as to the benefit from a reserve buoyancy perspective is different.
Old 02-20-2020, 07:05 AM
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It's most likely the foam has been 'missing' since the boat was built and the boat has done fine up till now - I'd replace the step pads and not give it a second thought.
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Old 02-20-2020, 07:05 AM
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Originally Posted by NedLloyd View Post
I agree that in the overall structure foam can add a good bit of stiffness, rigidity and strength; however in the upper portion of the side (below the gunwale) the effect is probably minimal at best. From the waterline and down is where it would really do something.
The conversation as to the benefit from a reserve buoyancy perspective is different.
Foam in the upper portion of the side can help prevent oil canning and deaden sound. Agree with others who have said, if you're not havering problems, move on. But if it keeps you or your dad up at night, it's easy to add.
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Old 02-20-2020, 07:06 AM
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Originally Posted by DJF View Post
I have to disagree. Foam has no strength at all. It is mostly used to provide a form so that fiberglass can be molded into the desired shape such as fiberglass stringers. I do agree that if you do not have stress cracks in the hull I would not worry about it.
I think you will find that foam can add a tremendous amount of stiffness and strength. Try taking two 4’ sheets of 1/8” FRP and laying them across a couple of saw horses, they will barely be able to support themselves. Take the same two sheets of FRP and fill between them with 2” of expanded foam and you will be able to stand in the center of the sheets up on saw horses. .... simple analogy.
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Old 02-20-2020, 07:09 AM
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Originally Posted by mshugg View Post
Foam in the upper portion of the side can help prevent oil canning and deaden sound. Agree with others who have said, if you're not havering problems, move on. But if it keeps you or your dad up at night, it's easy to add.
completely agree with you about the oil canning,....... I was thinking of only the upper 3”-4” as missing, not enough to allow oil canning, but your point is well taken.
Old 02-20-2020, 07:15 AM
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I have two foam filled boats and have had another that were foam filled. I am not sure I would want any of them if they were just built with the outside fiberglass...foam does add rigidity when used properly. That said when I was a kid we had a 17 Mako (like a 78 or something) and a 21 Mako (same era, 70's) and from time to time you would see a small piece of foam show up in the bilge.
Old 02-20-2020, 07:16 AM
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Foam comes in different densities. 2lbs is USCG compliant for buoyancy - It fills a void so water cannot be in that space which decreases your chance of sinking. That foam is not providing any real strength to the area. It's only 30-40 psi strength. There is 16lb foam which is hard as a rock and provides 10Xs the strength. Urethane Foam , Expanding Marine Polyurethane Foam

For what it's worth, foam isn't really applied to a 17' - 20' boat with much precision. My 2004 Sea Hunt had foam between the stringers and the gunnels under the floor. I don't think it went entirely up to support the floor. It was also thinly applied to a gunnel but that was probably overspray.

Unless there is flex I wouldn't worry about it. A metal backing plate across that area would be far better a supporting a plate vs. any foam.
Old 02-20-2020, 07:23 AM
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Empty bucket sideways, can crush with your foot. Same bucket filled with foam and try to crush it with your foot, do you think you can?
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Old 02-20-2020, 08:19 AM
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We all make stupid comments time to time, don't we?
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Old 02-20-2020, 09:50 AM
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Originally Posted by DenmarkSC View Post
Foam comes in different densities. 2lbs is USCG compliant for buoyancy - It fills a void so water cannot be in that space which decreases your chance of sinking. That foam is not providing any real strength to the area. It's only 30-40 psi strength. There is 16lb foam which is hard as a rock and provides 10Xs the strength. Urethane Foam , Expanding Marine Polyurethane Foam

For what it's worth, foam isn't really applied to a 17' - 20' boat with much precision. My 2004 Sea Hunt had foam between the stringers and the gunnels under the floor. I don't think it went entirely up to support the floor. It was also thinly applied to a gunnel but that was probably overspray.

Unless there is flex I wouldn't worry about it. A metal backing plate across that area would be far better a supporting a plate vs. any foam.

Pretty much agree with Denmark, here.
I'd just add a little. Considering that even the 2lb flotation foam will carry 40psi, that equates to quite a lot per square foot. 40psi x 144 si is greater than 5000 lbs per square foot. So, yeah. It is only intended for flotation. But, when it is added to a structure, if it is taking up a compression load, it can add a LOT of strength.

All depends on how and where it is used.

If I understand where this foam is in the boat, it is likely there to aid in stability when the hull is inundated. If all your flotation is under the floor, when the hull is full of water it 'wants' to turn up side down. The weight of the motor and anything else above the floor (where the flotation is) will want to flip over below the flotation. To meet USCG stability requirements in boats under 20', builders add foam to areas as high up in the hull as possible. The tops of gunnels would be one such place.
Old 02-20-2020, 10:36 AM
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Originally Posted by DJF View Post
I have to disagree. Foam has no strength at all. It is mostly used to provide a form so that fiberglass can be molded into the desired shape such as fiberglass stringers. I do agree that if you do not have stress cracks in the hull I would not worry about it.
Hey dumb dumb, you ever held structural floatation foam? You can stand on it, jump up and down on it, or hit it with a hammer and nothing will happen. Stop spreading misinformation.

- Structural Floatation Foam is very strong. It is used to provide structure to boat hulls.

- Not sure if it’s designed into the structure of this boat but it’s cheap enough, just buy some and fill her up.
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Old 02-20-2020, 10:49 AM
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OP, you now have every answer to choose from, so enjoy. Lol

That 17 Mako was designed with a floating floor. If you took it apart, the deck is not connected directly to the hull. The hull has stringers, the deck has stringers, and it is foam filled in between. The deck “floats” on the foam filling the space, and is definitely important under the deck.

Voids in that foam are bad if they are in structural areas of the floor—forcing the floor to be entirely self-supporting over a large area—or allow water to pool and get held in the foam. However, the inside gunnels were (optionally) cut out for rod holder inserts. The lack of foam up the hull sides would not bother me at all. In fact, the foam up the sides makes it a real pita to add rod holders, and prevents water from getting aft and into the bilge if it gets into rod holders.
Old 02-20-2020, 10:54 AM
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Originally Posted by Romeomikehotel View Post
Hey dumb dumb, you ever held structural floatation foam? You can stand on it, jump up and down on it, or hit it with a hammer and nothing will happen. Stop spreading misinformation.

- Structural Floatation Foam is very strong. It is used to provide structure to boat hulls.

- Not sure if it’s designed into the structure of this boat but it’s cheap enough, just buy some and fill her up.
While you're points have some merit, I don't think a guy that smears bondo all over an unprepared surface or thinks that he can create a fine performing craft with minimal marine aptitude and a pile of construction grade lumber is in any position to refer to somebody else as "bumb bumb"...
Gerald
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Old 02-20-2020, 11:18 AM
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Simple answer (all are correct) yes, no, it can, sometimes, possibly, it varies.
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Old 02-20-2020, 11:39 AM
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If it is an older Mako it was built in an era where foam was used to provide positive flotation, not structure, meaning the boat was built with enough structural integrity to not require foam as a support.

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