Thinking about building a bay boat

[BarraMan]

My recollection is that the standard Mangusta 20 is built with a 1/4" ply core bottom with 2 x layers of 12 oz biax glass either side.
Jacques' recommendation to me was that I put an extra layer of 12 oz glass in the exterior bottom, but I think that was because of my 10% upscaling rather than any issue with the potential speed of the boat.
The standard M20 had a recommended max 250 hp O/B, and that's what I have on my boat.
My boat will do 50 kts so I think the std M20 would be significantly faster. I rarely go over 30 kts, the exception being when someone is silly enough to try to overtake me !
After 6+ yrs in service, I cannot detect any deterioration in the structural integrity of my hull. Before every annual extended trip away I go over the bottom of the hull with a small hammer, testing for 'soft' spots!

[fallguy1000]

I'm not going to reply in detail, but Gerr recommends an extra 1% of glass thickness for each knot of speed over 10 knots. You can run the math on it, but for a boat like the PH22, or MG20 I'd probably try to step up the hull core, and/or put an extra layer of glass on it, if going faster than recommended or scaling it up. Especially if going up in core is doable when you plank it, I see no downside.

An alternative would be to look at the scantlings of a boat of similar size and speed, and extrapolate from that.

Based on Tom's speed expectation; the scantlings get increased 10% then.

[OneWayTraffic]

My recollection is that the standard Mangusta 20 is built with a 1/4" ply core bottom with 2 x layers of 12 oz biax glass either side.
Jacques' recommendation to me was that I put an extra layer of 12 oz glass in the exterior bottom, but I think that was because of my 10% upscaling rather than any issue with the potential speed of the boat.
The standard M20 had a recommended max 250 hp O/B, and that's what I have on my boat.
My boat will do 50 kts so I think the std M20 would be significantly faster. I rarely go over 30 kts, the exception being when someone is silly enough to try to overtake me !
After 6+ yrs in service, I cannot detect any deterioration in the structural integrity of my hull. Before every annual extended trip away I go over the bottom of the hull with a small hammer, testing for 'soft' spots!

With that much glass, there is essentially very little difference between that layup, and a pure glass one, except that yours would be significantly lighter.

I must admit to being curious as to why he did not specify a 9mm core.

[OneWayTraffic]

I used Diab groups sandwich calculation tool to compare a 6mm core with 1.5mm glass each side to a 8mm glass layup. There's essentially no difference at all. I needed to use H250 as a core but that would have a minimum impact on global stiffness and strength. The plywood core would be more resistant to penetration and better in other ways at a weight penalty.

https://www.diabgroup.com/tools/sandwic ... tion-tool/

I'd attach a PDF of the results but the attachment feature seems not to be working.

[TomW1]

Browndog thinking about things I would go ahead and add another layer of glass inside and out no matter if you go for the 150 or 200. That covers the 14 mph increase from the 115. About 7mph for each motor. With 4 stingers that the PH22 has the boat will be plenty strong with the extra glass. Good luck on your build.

Tom

[fallguy1000]

A 14% increase, based on other remarks..I did NOT review the SN numbers, etc. And I don't have the schedule; this is an example.

Total skin increase, if 4x12=48 ounces start is 7 ounces, or each side would go from 12/12 to 12/16 versus (12&16)x2 = 56, 6x12=72

For a hand laminated boat, 8 ounces extra in skin is 32 ounces per yard with resin. For a boat like PH22; bottom skin area, roughly 3.5x2 yards or 7 yards is 16 pounds, or almost an extra gallon of resin.

And the thing is; if you build a beefier and beefier skin; nothing is gained but safety margins. The specs already include that...

So, don't be too quick to oversimplify adding another skin vs increasing one...

I happen to be someone who overbuilt a bit, so this is coming from some experience; not hubris.

As OWT remarks, another direction is to move upwards in the core thickness which may affect bending. I'd want to look it all over. Resin and glass are lotsa time and cost....

In order to get a millimeter thicker in 12 ounces glass; that is a layer, so 9mm ply above 6mm ply is like adding 3 layers of glass..with far less weight penalty. Point is, might be wiser to go to 9mm bottom ply and keep the original schedule..

[TomW1]

duplicate

[TomW1]

OWT and fallguy the reason for not specifying 9mm is because of the bending factor of the bow. Just like the MG20 uses 1/4" ply and 2 layers of fiberglass this is the same situation. The PH22 has a sharp entry that needs to be bent on the frames. Jacques did things for a reason. Browndog if you want to go with a 150 or 200 double your glass. only way to do it. You are in MG20 speeds so you need to build/glass The PH22 like it.

Tom

[BarraMan]

I am NOT a naval architect! and I am confused.

If I use the Australian design standard at the time I built my boat, AS 1799-1.2009, which I understood at that time to be essentially the same as the USCG/ABYC standard, to calculate the recommended max hp for my boat (MG20 x 1.1) I come up with 269 hp - rounded out to 270 hp! Jacques specified max hp for the MG20 as 250 hp, so in the same ball park. I went with 250 hp for my up-scaled MG hull cause I figured that was heaps for my purposes and matched the Yammy 250 Vmax SHO that I had in mind for my build.

While the PH22 is a different design from the MG20, the dimensions for the calculation of recommended max hp as per AS 1799 are essentially the same as for my hull, so AS 1799-1.2009 would suggest 270 hp max. Interestingly, Jaques recommends 150 hp max for the PH22, although he does say that the USCG/ABYC specs would indicate more. I assume that his recommendation is based on the style of the hull and the specified scantlings and glassing etc.

However, if I use what appears to be the latest AS 1799 (USCG/ABYC) standard (AS 1799-1.2021) the calculation comes out as 125 hp for both boats.

Something seems wrong here!

While 125 hp may come close to Jacques recommended max 150 hp for the PH22, my boat with 125 hp would be a 'dog'! My boat handles the 250 hp with ease and would be quite happy with 300 hp. On the other hand, the PH22 with 250-270 hp would be a beast!

The other test for determining max hp specified in AS1799 involves running the hull through a 90o turn at max speed within set dimensions. I am pretty confident that my boat would pass this test - but I am equally confident that a PH22 with 250 hp would fail. I think the PH22 would power slide, or worse, outside the required specs!

The maximum horsepower capacity must be computed as follows:
(1) Compute a factor by multiplying the boat length in feet by the maximum transom width in feet
(2) Locate horsepower capacity corresponding to the factor in Table 183.53.
(3) For a boat with a factor over 52.5, if the horsepower capacity calculated in Table 183.53 is not an exact multiple of 5, it may be raised to the next exact multiple of 5.

Table 183.53—Outboard Boat Horsepower Capacity [Compute: Factor=Boat Length X Transom Width]
If factor is over 52.5 and the boat has remote steering and at least 20″ transom height, horsepower capacity is (raise to nearest multiple of 5) = (0.8 X Factor) −25

So, [0.8 x (22' x 8.5')] - 25 = 124.6 hp

Am I going wrong somewhere?

[OneWayTraffic]

OWT and fallguy the reason for not specifying 9mm is because of the bending factor of the bow. Just like the MG20 uses 1/4" ply and 2 layers of fiberglass this is the same situation. The PH22 has a sharp entry that needs to be bent on the frames. Jacques did things for a reason. Browndog if you want to go with a 150 or 200 double your glass. only way to do it. You are in MG20 speeds so you need to build/glass The PH22 like it.

Tom

I figured that as a possible reason, but the PG20 had 9mm and as much of a bend. Every builder struggled with it and some cut slits. Just pondering really. I'd rather spend more time kerfing slitting and bending thicker wood than laying up extra glass. But that's because working with wood is almost pleasant. Glass not so much.