MG20 questions to designer

[OneWayTraffic]

As far as answers are concerned I can say that the specs I have seen for foam core match fairly closely with those above for a boat of the size of MG20. I have taken Dave Gerr specs from his book Elements of Boat strength into a spreadsheet and can post them here for a boat the size of the MG20. The book has a lot of other details on the structure but these are the basic specs.

[fallguy1000]

I think it’s worth noting that the OP specifically stated “at this time” on the cheaper front. Consider his statements.
1. In Poland. There’s a war on his border hampering supplies of some products.
2. He has more availability of foam as marine ply is reserved for pro builders.
3. He has experience using it in other projects.

Under normal conditions I think we would all say “avoid foam”. But these are not normal conditions and we should answer the question if we have the answers. I do not. I just think we are being a bit less helpful than we could be given the unique circumstances here.

Foam is not cheaper. It takes about a year longer to build. The panels need to be preglassed or stations need to be doubled. Preglassing each panel, done correctly, is with vacuum. More epoxy, more supplies, a vac table all drives cost. The list is essentially endless, not to mention, the laminates are much heavier glass and resin use also higher.

Laminating errors cannot occur on foam and happen easier in heavier laminates. To do the bottom well is also vacuum.

Consider a simple case of a foam bottom; glass and resin demands are about 4-6 times that of plywood, discounting workmanship difficulties entirely. Then add supplies like bagging, bag tape, peelplies, bleeder film, breather media. This stuff all adds up. Then, foam usually at some point requires stapling. Just the plastic stapler is $250..

If someone wants to build in foam; it is not done to save money or time. Sorry.

When you conpare a foam panel to a ply panel; the cost difference is only a piece of the story.

[Jaysen]

Fallguy, if you can not buy ply because it isnt available or it costs more per sheet than foam then foam is cheaper. That is the OP problem.

I 100% agree with what you are saying from the standards you and I use here in the US. Foam does not make sense here due to laminate costs, laminate failure impacts and such. Prices are all upside-down compared to our experiences when looking at costs for building in other countries. I can’t explain it but some how it works out. Especially in the future east in Europe you go.

Anyway, the OP can likely come out on top of the cost curve compared to ply because he’s in Poland even though we can’t here in the US.

[OneWayTraffic]

I think the time difference is still significant though.

OP exactly what foam and resin do you plan to build this with? There's foam and then there's foam.

[fallguy1000]

Fallguy, if you can not buy ply because it isnt available or it costs more per sheet than foam then foam is cheaper. That is the OP problem.

I 100% agree with what you are saying from the standards you and I use here in the US. Foam does not make sense here due to laminate costs, laminate failure impacts and such. Prices are all upside-down compared to our experiences when looking at costs for building in other countries. I can’t explain it but some how it works out. Especially in the future east in Europe you go.

Anyway, the OP can likely come out on top of the cost curve compared to ply because he’s in Poland even though we can’t here in the US.

The problem is generally economies of scale. If he decided to use vacuum; he needs a pump, lines, valves, a vac table that he must build for the panel sizes for the 20 which is about ?22 feet. Unless he can find someone or installs ribbands on his stations. But then the boat is flimsy and cannot he removed without distortion. There are just so many more complicated factors that a person needs to commit to foam, expect longer build times, and greater expense.

Even buying breather media usually is best done as a whole roll, so I am sorry, but if he thinks he saves money using foam; he is very likely mistaken when you factor in all things.

How do I know? Buying a roll of vac film for $400 which is probably now $500, and totally unneeded for ply. Vac pump $250, and we are at $750. Then let's say resin and glass for ply is $500. This means foam is $2000. So, we are at a low estimate, not counting building a vac table or other supplies... I am at $2250 extra cost on a boat that uses ?10 pieces of plywood or $225 extra for foam per panel plywood...

It is not cheaper. But lighter, done right.

[rick berrey]

There is foam from China that is cheaper in some countries , foam can be hand laid just like ply , there is no right or wrong between hand laid and vacuum . Vacuum is going to be lighter -v- hand laid heaver , but each will meet the structural demands if the wet out is right . I am willing to bet hand laid is quicker than vacuum . Light -v- heavy is why there is a 25' rule of thumb for foam , twice the glass and 60% epoxy hand laid wet out for a beginner , but over 25' either method starts to have a payout in weight savings .

[fallguy1000]

There is foam from China that is cheaper in some countries , foam can be hand laid just like ply , there is no right or wrong between hand laid and vacuum . Vacuum is going to be lighter -v- hand laid heaver , but each will meet the structural demands if the wet out is right . I am willing to bet hand laid is quicker than vacuum . Light -v- heavy is why there is a 25' rule of thumb for foam , twice the glass and 60% epoxy hand laid wet out for a beginner , but over 25' either method starts to have a payout in weight savings .

Vac is 60% epoxy, but the only way to use less resin is infusion. Wet bagging requires speed and wetouts are done at 100-110% of glass weight. Then the excess resin is removed into breather media through bleeder release film. Bleeder film is used to hold some of the resins back so not to starve. Hand lamination is always 100%, an amateur can work their way to say 90%, but it is really hard to do without air entrainment.

Glass is more than 2x, and hand is fine, but for the MG20, no voids on the bottom are allowed.

[Matkuz]

Thank you for all your replies. I know that plywood build would bw way easier. If everything will go right propably i will build my MG using plywood but i wanted to know opinions and laminatiom scheldules for foam version.
For my latest project i was using Herex 100kg/m3 and Epoxy LG385 and biaxial glass. This combination is good to laminate both hand, vacum or infusion. Ive got everything need for vacum.
I agree that it won't be easier and cost will be similar or propably even bigget but we will see.

Lets go back to second question which was about reverce chine and lift strikes. I was thinking about 25mm/~1inch height.
What do you think about adding pad do MG ?

[TomW1]

The reverse chine and lifting strakes are fine. Put two strakes on each side about 1/3 from the chine and keel. Just make sure they are not under the stringers as you need the stringers to rest on the trailer's bunks, so move them outside if needed. I would make the reverse chine 2-21/2" at the transom then taper to 0 about 1/4of the way to the bow. They don't need to be all the way to the bow as the bow will be out of the water when on plane.

As for the pad no. You need the full length of the keel to provide control at speed. Pads are sort of obsolete and you will not find them on high speed boats. If you want a jack plate get an 8-10" setback one which will put your prop in the water coming up from the keel and use a 4-blade prop to gather the maximum amount of water.

Tom

[BarraMan]

My view of the world - for what its worth!

I used to race a 17' Bullet ski boat that would top out at about 60 mph.



It had 22o dead rise at the transom and a runner pad, but no trim tabs or reverse chines. At speed it was inclined to chine walk - quite stable but could be somewhat unnerving none-the-less!
You need excess power to put the boat up onto the pad but once there, I suspect that it may be a little more economical to run than a comparable boat without the pad.

My 22' Mangusta hull has 18o of dead rise and no pad. It has trim tabs and reverse chines. The reverse chines are 2 and 3/4 inches at the transom and run to about halfway along the hull where they merge into 1 and 1/2 inch spray rails.



A mate who used to race power boats once said that he would like slightly wider reverse chines but he liked the look of my hull.

My 22' Mangusta hull is clearly more stable at speed, to me at least, than my 17' Bullet hull was, although it never gave me a scare!

I talked to Jaques about both reverse chines and a pad on my Mangusta hull, but it would be fair to say that he was NOT enthusiastic about either.