NV23 scaled down 10%

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Matt Gent
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Re: NV23 scaled down 10%

Post by Matt Gent »

TomW1 wrote: Tue Jun 15, 2021 4:48 pm Let's go over a couple of things first. As the NV has a monohedron hull so as it reaches planing speed it just continues on with its speed increase. There is no hump to break through that a V shaped hull has. The hump is not there. So any way here are some numbers I have come up with.
This has been posted over and over and it is still not true. Monohedron just means constant sections - no warp or twist. It could be flat, or super deep V, and still be a monohedron. a monohedron may or may not have a hump. My pontoon is also a monohedron.

My boat most certainly has a hump in the performance curve - I have measured it, both in trim angle and in efficiency. The other DE and NV designs I imagine would act the same. Its not bad, and not nearly as bad as some heavier production boats, but it is there. The boat still has to climb its bow wave to plane off. It gets worse with increasing load. In order to make it really go away the boat would have to be truly semi-displacement design, which would maybe not be a monohedron form. And the Lcb would be further forward (like the Nina). Or be very light.

Image
The trim and mpg are scaled to fit on the plot so consider them non-dimensional. The hump is the yellow curve bumping up, and the green curve dipping down. Other boats could be much worse, and mine may be exacerbated by the setback bracket.

Here's a (monohedron!) production boat (18' Bay Reef with Yamaha 115) for reference, with a bad hump (efficiency cut in half at dip vs. barely planing)
Image

Based on fuel consumption, my boat would go 10mph with 25hp, as loaded for cruising.

Given that, I still would agree that a scaled down NV with 25hp would likely meet your needs. Maybe some other designs which were built around those requirements might do better (B&B OB20). At minimum if you want to sit in the NV trunk cabin you will need to raise the top a few inches, particularly after scaling. Same issue for standing under hardtop after scaling. Maybe raise the cockpit sole an inch or two also. I would draw it up to scale and see what it looks like.

TomW1
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Re: NV23 scaled down 10%

Post by TomW1 »

alex2293 wrote: Tue Jun 15, 2021 8:59 pm Thank you Tom, that is exactly the boat size, speed, outboard power and information I was looking for.

I have one or two last questions. I'll try to be as clear and concise as possible.

In the NV23 study plans, the displacement is 2500lb and PPI at DWL is 560lb. The NV23 is self-bailing up to what displacement? At least 3500lb from what I understand.

When a boat is self-bailing, is the bilge completely dry? Does it have a bilge pump in case something happens? A drain plug to drain any water from a leaking inspection hatch?

Thanks again, this is amazing to see the amount of support on this forum.
Alex
t
First take Matt Gent's observations as his on his boat as his observations. Believe what Jacques says as the designer that the NV23 will transition to plane smoothly.

Yes 3500 sounds like a good number, the -10% would be around 2200-2300lbs maybe a little more.

The bilge area is sealed by the deck but you do need a bilge pump and I recommend a 2000gph as it more reliable and tougher. The bilge compartments will be buoyancy foam to provide flotation in case of an accident. You can make it totally upright with foam sheets under the gunwales. Ask Jeff what you need when you order the plans. While you don't need foam in all the compartments under the deck you can use limber holes in the frames to allow the water to drain to the bilge pump. The plans should show you where you need foam.

A drain plug is common at the transom when you put her on a trailer.

Hope this helps. Ask any questions you have.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

alex2293
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Re: NV23 scaled down 10%

Post by alex2293 »

Thanks for this valuable information Tom.

Regarding the water evacuation. I think that I read that the sole of the NV23 is one inch higher than the water level.

It means that with a displacement up to 3500lb*0.9^3 it is self-bailing. Rain water at the dock won't be an issue.

If once in a while I have a higher load, I have to plug the side scupper because they'll be partially or completely submerged?

In this case, how does the water reaches the bilge pumped with a sealed deck?

I'm not sure if I like the idea of the flotation foam. I understand the safety advantages but is it possible for this foam to become waterlogged?

I am planning a minimalist build. Portable fuel tanks above the sole, below side benches. No wire or cable under the sole, everything under the gunnel or along the sides. The bare minimum amount of wiring and plumbing.

Navigation lights
Bilge pump(s)
Porta-potty
Chartplotter with a transducer
NFB steering

Thanks,
Alex

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Re: NV23 scaled down 10%

Post by jacquesmm »

Instead of scaling, please look at the OP21 and P21. They have the same hull type as the DE23. They will do well at displacement speeds. A P21 will move safely at displacement speeds with a 10 HP. It will need much more HP to get on plane. How much will depend on the weight.
Don't get an engine that is borderline sufficient, you don't want to run at max. rpm all the time.

In the first post, you mentioned the HDM19. That is a completely different hull, it will never plane but will be very efficient at slow speed.
Talking about slow speed and smaller boats, have a look at the HM19. That one will be easy to build, move well with a 6 HP but will need 30 to plane lightly loaded, 50 with a safety margin.

All very different boats. How about narrowing the choice by looking at the budget?
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alex2293
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Re: NV23 scaled down 10%

Post by alex2293 »

Thank you Jacques for your input.

I liked the Novi 23 because it has a long cockpit, a small cuddy and a short hardtop. The maximum outboard is also the minimum of the P21. I like this because I would prefer lower speed and better fuel economy.

Originally, I was looking to build a Widebody Tolman Skiff 21ft4in long with an 7ft6in beam. However, as a mechanical engineer I do not appreciate the lack of accurate plans nor the double layer bottom, the side panel scribing, etc.

All this lead me to scale down the NV23 by 8% to have a 21ft by 7ft6 boat that I could build in a 16x24 temporary shelter. I have a single car garage with woodworking tools where I'll be able to prepare all the parts, even during the winter.

By the time I'm ready to buy an outboard, 2 years maybe, I'll be able to afford a Tohatsu 60hp easily.

From your study plan I budgeted 10 000$ for all the plywood, fiberglass, epoxy, primer, bottom paint, bilge pump, steering, lightning, etc.

I prefer the minimalist approach so rigging won't cost an arm and a leg.

Thanks again,
Alex

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Re: NV23 scaled down 10%

Post by jacquesmm »

Good point about the double bottom. It is almost impossible to build that without air pockets unless you plank the second layer along diagonals. Even then, any air pocket is an invitation to delamination.

The max. HP I show is never the max. legal HP but is based on expected speed and weight. In almost any case, the builder can install larger engines if he increases the bottom and framing specs.

The topside layout is very much to the builders preference. As long as maximum distance between frames is respected, you are free to build the superstructure as you want.

Your budget is realistic for a 21' boat complete hull, without engine but with all what you need to cruise.

Pick a hull and we will help you with the customization once you get at that stage.
Jacques Mertens - Designer
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Fuzz
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Re: NV23 scaled down 10%

Post by Fuzz »

Good move going with one of the plans here. I live in the land of Tolmans so have seen a lot of them. plus here you get the designer help and if you start a build thread you will have lots of friendly forum advice.

TomTom
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Re: NV23 scaled down 10%

Post by TomTom »

Alex ... I have never been on a boat and or built one and thought “oh wow - I wish I had made this thing 3 feet smaller”....

I think that if the Novi 23 suits your needs - but is a little big .... I would be looking at ways to solve the “little big” problem.... can it live on a trailer? Can you negotiate a better mooring fee? Can you find a way to use it as a 23 foot boat”... Can I get a tent a little bigger to build under? Can I make do with the workshop space I have and build as is designed...

Because the reality is if you scale it down, you will have just as much work ... if not more....

You will have a boat that doesn’t necessarily work as designed....

You will have been much less efficient with your materials and your time; you will spend hours re-thinking things the designer already thought about; your proportions will change - so you will either have to find a way to stand back and eye-ball her - or you will need to spend time drawing her ....

Do you have space to stand back and eye ball her? Or is it easier to squeeze her into a tent and work around that in the knowledge that when she comes out, you know she is well proportioned?

When all this is said and done, you will have a smaller boat - which has lower resale value....

How bad are your mooring fees? What will you really have saved/ gained for all these efforts?

The boat is within the legal trailering width ... and as a person from Africa (who knows about bad roads) I can attest that having visited the US many times ... you guys have really really big, good roads.... everything is big in America!

So for me trailering, a 23 foot boat or a 20 footer on your roads is neither here nor there.... it will be an almost identical level of effort/ stress...

Also, those extra 3 feet in length don’t translate into a volumetric increase in build time/ effort/ cost... but they do translate into a volumetric increase in how useable your boat is!

So for what they are worth... these are my thoughts!!

Best of luck with your decisions....

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Re: NV23 scaled down 10%

Post by jbo_c »

All of Tomtom’s points are very good ones.

Jbo

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