PG25 or GS28

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OneWayTraffic
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Re: PG25 or GS28

Post by OneWayTraffic »

You can read his opinions on them in a sticky on the Power boats section. Adding them with strips of plywood on the sides would be difficult. Easier to use a spray rail or fill in the chines as I indicated.

cracked_ribs
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Re: PG25 or GS28

Post by cracked_ribs »

I've done them on a couple of boats...they aren't rocket science or anything.

My current boat, I just sliced a 1x3 diagonally, lengthwise, and shaped it from there. It's in my build thread if you want to see what I mean.

At GS or Panga speeds, doing the math is a waste of time - in fact lots of expert builders don't bother trying to work out the specifics on multi-million dollar customs, because the basic effects have been worked out on thousands of boats through experience, and it's pretty easy to replicate. In the boats you're looking at, you're going to see your best performance without too many sacrifices with a 4ish inch wide step, somewhere between flat and 10 degrees of reverse. They probably won't do much to your speed but you will plane off a little quicker. If the boat has a narrow bottom and angled sides, you'll probably find it buys you a little more initial stability, too.

If you want to run at sixty knots, or figure out a way to plane a 16'x6' boat with a 9.9, then it's worth getting into the details of the step design, but honestly...these boats will plane off fast anyway, you might get them on step a knot lower or so. A bit of reverse to the step might make it a little more efficient at speed but it'll be subtle. If you build light and don't have much with you, you won't notice much effect, although they do keep the spray down fairly well. Not so much as the monster spray rails you sometimes see, but pretty well. But a really light boat that runs with the step barely touching the water, doesn't benefit much from its existence, obviously. If you load them down for offshore trips, that's when you'll find the advantage.

Anyway, if you like them better than spray rails, I say go for it. They're easy to build and personally I don't like the look of most spray rails...very tacked-on looking to me. I put them on my current boat and on a boat for a friend of mine, after the build, because he thought it was a wet boat and didn't like rails either. And then another guy local to me building an FS17, I did them on his because he was really inexperienced and wasn't confident. But they're pretty easy.
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Fuzz
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Re: PG25 or GS28

Post by Fuzz »

Adding chine flats is one way of increasing the beam and not changing the bottom. But you are now placing flat areas on the bottom and that comes with both good and bad. Take the idea all the way and you have a flat bottom boat. We know those upsides and downsides. What you get is some of each depending on size. There is no magic when designing a boat.

cracked_ribs
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Re: PG25 or GS28

Post by cracked_ribs »

That's a very good explanation...you put flats on 5 percent of the running surfaces, your boat will act 5% more like a flat bottomed boat.

This is why I like a bit of reverse to the step...you still get some of the slap but you also pick up a bit more lift: any water that gets thrown out the side instead of back or down is wasted energy. But no such thing as a free lunch; any efficiency you pick up comes at the expense of a firmer ride.

This is also why I think it's pointless to invest a ton of effort into designing the "perfect chine" - there isn't one. It's all trade-offs. Replicate a typical one and rock on.
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TomW1
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Re: PG25 or GS28

Post by TomW1 »

cracked_ribs wrote: Thu Jan 07, 2021 2:36 am That's a very good explanation...you put flats on 5 percent of the running surfaces, your boat will act 5% more like a flat bottomed boat.

This is why I like a bit of reverse to the step...you still get some of the slap but you also pick up a bit more lift: any water that gets thrown out the side instead of back or down is wasted energy. But no such thing as a free lunch; any efficiency you pick up comes at the expense of a firmer ride.

This is also why I think it's pointless to invest a ton of effort into designing the "perfect chine" - there isn't one. It's all trade-offs. Replicate a typical one and rock on.
Well said!
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Johnston
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Re: PG25 or GS28

Post by Johnston »

Thank you - I am learning a lot from reading this website.

One thing I am also curious about is that there are plans for an FAO Panga online - but I believe this Mertens Panga had its aft section modified from the original concept so that it can take a larger engine and run on the plane.

I read somewhere the designer wrote this modification doesn’t make it “quite as sea worthy” as the original non-planing version.
I am trying to understand why?

Am also trying to understand how much efficiency at low speed does a planing type Panga loose compared to the original design for very low HP?

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Re: PG25 or GS28

Post by cracked_ribs »

Anything the designer says should supercede anything I'm about to say but in general, planing boats are always a little less seaworthy than similarly sized displacement boats, just because of the way the flatter, stiffer stern interacts with the water. Maybe a way to picture it is like this: if you put a shoe box on the water, it'll float really well but you could tip it over in a small breaking sea. If you put a volleyball on the water with a weight glued to one spot, it'll basically withstand anything and keep coming upright. That's not a perfect analogy and let's not get bogged down in the details of how that's not a perfect representation but it illustrates the sort of motion of displacement boats vs. planing boats - one rolls around easily but really resists going over in bad conditions; the other resists rolling in most states, but flips in extreme situations. Does that make sense?

At low speed where the bottom six inches of transom is immersed, the planing boats will lose some efficiency because the water will sort of "suck" against the transom; the FAO design would be more like a rowboat where the aft curves up and gently allows water to depart. So at 5 knots or something it'll be super efficient; it's floating on the water like a duck, just kissing the sea and you could push it around with a feather.

But the planing boats need to get enough speed to ventilate the transom - that water has to be slipping off the bottom and heading straight back before they get efficient.

Jacques' panga probably does that at 9 knots or something which is really low for a planing boat; my big boat doesn't dry the transom untl twice that speed. So from 5-8 knots the FAO boat is practically free to push around, but that's all it'll ever do. From 8-25 knots, the PG25 is burning fuel...but very little relative to other 25' boats. I bet it burns a quarter what I burn at 25 knots.

That's my take, anyway.
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Johnston
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Re: PG25 or GS28

Post by Johnston »

Thanks for the explanation. Very clear. My assumption is still that there won’t be a huge difference in seaworthiness/ economy at the low end for the planing Panga - otherwise the design would not have evolved to be as popular as it is?

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Re: PG25 or GS28

Post by cracked_ribs »

That's probably fair to say. If the FAO can be pushed to 10 knots max, say, that's probably just within the planing speed of the PG25, so at that overlap I actually wouldn't be surprised if the PG25 is more efficient. I'm not sure, but regardless of hull shape you still have to push a given mass around, and running the FAO on the pins is probably more thirsty than running an efficient outboard way below its max output.

I don't think the seaworthiness would be much of a factor, personally. There's maybe some theoretical advantage to the FAO but it would so insignificant relative to other factors... it's like "would I catch more fish if I had a man as a fishing partner, in case I have to haul a big fish on deck and men are generally stronger than women?"

Maybe in theory if you're comparing averages on a giant scale but you aren't taking the theoretical median man or woman fishing. You're comparing specific individuals, and their sex and average strength is so much less important than other factors, I just wouldn't let that factor concern me.

I'd be totally confident in the performance of the PG25 and I'd way rather be offshore in one of them than the FAO.
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OneWayTraffic
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Re: PG25 or GS28

Post by OneWayTraffic »

The FAO Panga may or may not be more seaworthy, but you will need all that stability should you get caught offshore. A planing boat gives you a chance to get out of the way, while the displacement boat just has to take it.

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