What larger design for lake use in NZ - PH22, AB23 or something else?

[jmacnz]

I'm in North Island, so up around Taupo and Rotorua lakes mostly.

I thought there were some differences between hull designs on OB and C series. Like the 17 and 19 versions of both having their differences?

Hows your boat coming along OneWayTraffic?

[OneWayTraffic]

https://www.fishing.net.nz/forum/c17-ne ... age13.html

viewtopic.php?f=12&t=64694

I'm getting there. I'm a Maths and Physics teacher so have all the Math skills but I'm a bit of a wood butcher. I built the transom twice, stringers three times (voids, used weights instead of screws), and glassed half the hull twice (heat causing bubbles working in the tunnel house. If you search for my posts there's a few threads.

Despite all that and the occasional other SNAFU I'm getting through it. My glassing has gotten better, I put a heavy duty groundsheet on the tunnel house to keep direct sun off, and I've learned to make the temperature swings work for me rather than against me. Anything can be fixed with epoxy and glass. I'm not after the best of finishes, but I want it to look like I cared, and have a good finish where it matters. Most of all I want it to be unsinkable and comfortable when I take it off the coast.

For Northern lakes I'd build one of the 19' craft to be honest. Even a 19' is a big project, and no scaling required. But any of the OB, C series will do well. I can't speak to the larger sizes, but I do know that the differences between the OB17 and C17 are not that great. There's not much one can do that the other couldn't.

[Browndog]

The C series of boats have more freeboard, are deeper and require two panels for each side of the hull compared to the OB series. The C series hull will be heavier, a little harder to build, use more materials and labor, and likely need more horsepower and fuel to operate. The C series are also wider and have more capacity than the OB series.

I built an FS 19 and can attest to its seaworthiness and quality of ride. The OB 19 should be as good or better than the FS 19 in this regard.

The lines of the OB 19 and C19 are also different and some may prefer one over the other just because of the design.

In the end, your requirements and preferences should determine your choice. Regardless of what you choose to build, any of the plans discussed will likely more than meet your needs.

Best wishes on your project.

[BarraMan]

There is a formula you can use to get an idea of max hp - I’ll post it when I stop travelling.

When I used it on my boat, it came out at 265 hp - so I hung a 250 on the back, which worked out pretty well. It has lots of grunt with no vices.

Having now driven it in all sorts of conditions I have no doubt it would take 300hp just as well!

[jmacnz]

Thanks OneWayTraffic, Browndog & Barraman for your replies. Very helpful group of people here.

Will be interesting to see that formula when you get to it Barraman, cheers.

Well done with your FS19 Browndog, that's a beautiful boat. All the best with your C boat project OneWayTraffic, well done on progress so far.

[BarraMan]

To calculate max. recommended o/b power:

1) Calculate the FACTOR for use in calculations by multiplying the overall boat length in metres by the transom width in metres. The transom width shall not exceed the widest measurement of the transom in that part which is below the point of ingress of water. If spray rails act as chines or part of the planing surface, they may be included in the transom width, but otherwise fins and flare shall be excluded. Where a boat has a rounded stern, for the transom width substitute the maximum width below the static float line measured at a point one quarter of the boat length forward of the stern.

In the case of my boat, that would be: FACTOR = 2.45 x 6.7 = 16.415

2) Then, for a boat with remote steering and 500 mm transom or equivalent, and a FACTOR >=5, power capacity in kW = [(16 × FACTOR) – 67]†, which for my boat is 195.64 kw.

To convert kW to hp, multiply by 1.34102, so 195.64 x 1.34102 = 263 hp, which can rounded up to the nearest 5 hp = 265 hp.

That is from Australian Standard AS 1799.1—2009 "Small craft Part 1: General requirements for power boats" - which I am pretty sure is a direct copy of the US standard.

[OneWayTraffic]

I should probably emphasise that the designer has stated several times to take those USCG maximum figures with a grain of salt. They are for production boats, and ignore all sorts of design factors, such as weight and deadrise. For each of his designs he posts a recommended and usually max hp. Many builders have used engines a bit above that max without major consequences, but the designer will not usually recommend it for liability and safety reasons. However if you do overpower the boat, the transom will usually be absolutely fine if built to plans, but you may need to beef up the glass on the bottom and stringers.

Barraman's MG20 was designed for 250hp max, and has the deadrise and design to require and handle that kind of hp. Even scaled up 10% (giving a 30% bigger boat in volume) performance is apparently stellar. I'm still waiting for a ticket to Aussie to see for myself.

My C17 has moderate deadrise and is designed for 50hp. According to the designer a 60hp or 75hp is fine, 90hp borderline and 115hp (as somebody asked) excessive. The size is very similar to the figlass firestar which is designed for a 90hp-115hp motor. Most glass boats in NZ use similar power. I could put that much on but why? I'd probably need to beef up the glass scantlings and have power that I wouldn't be able to use under normal conditions. The etec 60 I plan to transfer on to it from the donor boat will already give 50km/h plus speed, and that's more than enough for me in this boat.

Once you settle on a design, you will be able to have a look at what people have powered that design with and plan accordingly. But don't think that you will need as much power as a production glass boat. You won't.

[BarraMan]

The OP asked if there was a way to get an idea of max hp for a boat - and I replied. I don’t have a horse in the race - I’m just an ideas guy!

That said, provided it was under the max o/b weight for the design, I would always put the max specified hp on a boat as I can’t tolerate underpowered boats. I do understand that $$ can be a limiting factor!

Its one of the biggest problems I see in recreational boating retail here. Boats sold to unsuspecting punters with minimum hp power to keep the sale price down and therefore more attractive. They go for a test drive with min. fuel and just 2 on board and it gets up and goes well, but when they load up Mum and the kids, full fuel, cooler full of ice and drinks - it becomes a ‘dog’!

Repowering with more hp becomes a priority!

My boat will hit 50 kts! Its done that once in the four years since it splashed. It spends 99% of its time at or below 30 kts - but it will still do 30 kts loaded to the max.!

[jmacnz]

Great, thanks Barraman.

OneWayTraffic, I proposed the question in relation to the upsizing of an OB19 to 21. I haven't seen a scaled up to 21 built yet, but there's been multiple chat about it in the forum. Knowing what the OB19 power recommendation was, I was curiously wondering what you'd put on a 21ft version. Comparable maybe to the PH22 and C21 with 125hp max or GP21 with 150hp perhaps, but will see.

I agree with Barraman that there's nothing worse than underpowering. In my opinion underpowered automobiles are painful and asking for trouble, boats likely similar. I'd always prefer to default to the maximum.

I'm leaning towards an OB or GP at the moment, and if I go with the scaled OB21 then I'll be consulting the designer for further information after I've bought the plans, taking into account horsepower and maximum unit weight that can be hung on the transom. Various upgrades on the hull like extra glass to proof it for more horses would be futureproofing a build. Who knows where outboards will be in 20 years power to weight? (probably all electric?)

I was thinking when looking at Yamaha outboards that if a boat like the OB19 can take a 90hp Yamaha (166kg / 366lb) then the 115 and 130 Yamaha are not much heavier at (176/178kg and 388/392lb). So the 130 may be a good candidate for a OB21 (subject to checking in with the designer). The 150 Yamaha (228kg / 503lb) may even be suitable for the larger OB with the right hull strengthening in the larger build. Anyway, thanks Barraman for the formula, I like a little calculation. Will have a look at that and check in with the Jacques once I've bought the plans.

Cheers

[OneWayTraffic]

That's right I'd forgotten that you were scaling. Christmas holidays are rotting my brain.

I'd go with the power recommendations for the C21 as a guideline then. And I'd have no problem putting 130hp on that or an OB21, though I doubt you'd be using full throttle except on dead calm days.
Displacement of the OB19 is listed at 864kg. Scaling to 21 I get 864*1.105^3=1170kg.
By Crouch's formula that would work out to about 65km/h (40mph) at full throttle. That's good speed.

Though I have to ask: why not a C21? Is it the width?