Boat Project II

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fallguy1000
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Re: Boat Project II

Post by fallguy1000 »

That was a grammatical, meant <3. The l/b is a measure of efficiency. When faced with a tight slip; all I meant to suggest is that a narrower boat can be a good thing; perhaps it did not come across that way. His slip at 7'2", doesn't have room for fenders even, so rather than fight it; embrace the constraint! A couple 4" fenders puts him at 6'6" beam. Which boats are those? Of course, the flare of the hull also becomes a factor here..

Scaling down is indeed a crapshoot.(if the panels won't bend). And then a person messes with bulkheads.

https://boatbuildercentral.com/support- ... -plans.pdf

Also, I think the boats are listed at oal. The engine then adds say 18", so I think he has to scale most boats down pretty far.
My boat build is here -------->

viewtopic.php?f=12&t=62495

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BarraMan
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Re: Boat Project II

Post by BarraMan »

An interesting article but not particularly relevant to this discussion. It’s focussed on up-scaling while we are talking about down-scaling!

It does, however, say:
Our designs (Mertens-Goossens Yacht Design) can very often be scaled by 5 or 10% without major problems. In many cases, no major changes to the plans are required but consult us anyway.

TomW1
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Re: Boat Project II

Post by TomW1 »

A correct downsizing is doable because everything is reduced by the same amount. But only by 10% or if you need a little more 11% do not go to 15%. You must downsize all dimensions. There is a reason Jacques limits downsizing or upsizing to 10% as more than that means a redesign. Use Okume which bends easier. One thing you can do is raise the sides a little more to allow for more interior depth inside the boat and allow the raising of the deck for more height above the waterline.

I would reconsider the PG20 liki.

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

liki
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Re: Boat Project II

Post by liki »

I think the 1. option is the MG20 (MG it was and not PG) with only modified deck & layout, but I would like to get the power estimation calculated for that. But if the hull structural weight is about the informed 400kg I think it should work with the intended power. The 2nd would be downsizing AB23, and then the "lesser" sizes hull-qualities wise modifying C17 with lower sheer would look the most promising. It should be doable with under-sole fuel tank and traction batteries per what I read here somewhere.
Oh yes, and the slip; It has limiting spars that are exactly 6,5m long and have rounded, soft bumpers at the ends 2,25m apart. On the run there's a little more room but the boat has to fit "in" between the bumpers and in the long run is guess it will not do good if the hulls sits too tight between the bumpers. Very much like these: https://www.a-laiturit.fi/tuote/venepuo ... uotekortti
Last edited by liki on Tue Jan 31, 2023 10:37 am, edited 1 time in total.

rick berrey
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Re: Boat Project II

Post by rick berrey »

I would think at 15% i would want Jacques 2 cents before I spent my money . If you scale L/B you will get the same boat , but may be not the same abilities as per designed . Scaling up if there are weight issues you can always add more , most builders do without scaling up . Scaling down 15% you may have weight issues to contend with , so are the scantlings now to heavy , will you have to shift people , tanks , batteries , ect . to keep C/G , will things fit where intended ? Weight is only one possible issue

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Re: Boat Project II

Post by jacquesmm »

I did not see this thread but will read it and reply in detail if needed.
In the mean time, here are some figures and rules of thumb.
Any or almost any of my designs can be scaled up or down by 10%. They can be scaled more but in that case, will require some design changes like extra framing, scantlings changes and maybe, for open boats, move the cockpit up or down to keep it self bailing.
If you scale 10%, that means multiplying all linear dimensions by 1.1, all surfaces by 1.1^2 and volumes by 1.1^3.
1.1 exponent 3 is 1.33. In other words, it means that with a 10% scale up, your volumes increase by 33%. The boat will float much higher IF weights stay the same but they are not.
Your weight of materials is close to a function of the surfaces (ply, glass, resin) and will increase by 20%. 1.1^2 = 1.21.
A larger boat will be relatively lighter but there is a limit, that is why I say 10% is safe.

Speed is a function of the HP, weight and hull shape. Shape stays the same by a scaled up boat will be heavier and require more HP or, it will go slower with the same HP.

I used the Crouch formula for years and it was always within 5% of real world testing. There are some online calculators:
https://www.omnicalculator.com/everyday-life/boat-speed
and
https://continuouswave.com/calculators/crouchCalc.php
are the first ones i found,
Use a factor of 185, more for the MG20, less for many of my other designs.
Do not expect more than + or - 5% accuracy but you will anyway not build your boat within 5% of my calculated weights.

Start with that while I read this whole thread.
Once you decide which hull and scale I will comment on proposed changes to make it work.
Jacques Mertens - Designer
http://boatbuildercentral.com

liki
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Re: Boat Project II

Post by liki »

Hi Jacques,

I toyed the calculators with 180/185/190/195/200 coefficients, 115/140/150hp power and 1000/1100/1200kg weights and it adds quite a lot of confidence that the MG20 hull with 140hp should perform to the 35-40kn range top speed. 200kg for 140hp OB and 150kg for fuel (120l, give-or-take-a-bit), tank, controls, etc. and if the hull structure as-designed is estimated at 400kg it could be possible to stay at 1000kg light displacement with a modified deck as a fishing boat. And the seakeeping abilities should be sufficient with that hull. With LiFePo4 batteries additional fishing equipment might come at around 100kg, with SLA might become 150-200kg. This would total 750kg + 100-200kg leaving some room to err on the heavy side on buildup to keep to the desired trailering weight. MG20 should tolerate at least 1500kg total displacement I guess?

Would it be possible to get freeboard, chine width, and frame locations (I recall having seen the frame locations on the forum already ... somewhere) for the MG20? It remains the 1st option as it should also provide the sought seakeeping qualities and modifications would be limited to deck, sole, and layout. Built to designed size with deck and layout much alike Barraman's up-scaled one.

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Re: Boat Project II

Post by jacquesmm »

liki wrote: Wed Feb 01, 2023 8:33 am
Would it be possible to get freeboard, chine width, and frame locations (I recall having seen the frame locations on the forum already ... somewhere) for the MG20? It remains the 1st option as it should also provide the sought seakeeping qualities and modifications would be limited to deck, sole, and layout. Built to designed size with deck and layout much alike Barraman's up-scaled one.
The plans include a drawing titled stations. On that drawing, I show all the dimensions you ask for. Once you buy the plans, I will, for convenience, add an extra station between D and E. It is not really necessary, the lines are almost straight between D and E.

There is also a drawing titled "Expanded Plates" on which I show the stringers. They are part of the basic framing.

There are three layout drawings for inboards or outboards, a total of nine drawings with dimensions but if you don't find a dimension, I will help.
Jacques Mertens - Designer
http://boatbuildercentral.com

liki
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Re: Boat Project II

Post by liki »

jacquesmm wrote: Wed Feb 01, 2023 1:21 pm
liki wrote: Wed Feb 01, 2023 8:33 am
Would it be possible to get freeboard, chine width, and frame locations (I recall having seen the frame locations on the forum already ... somewhere) for the MG20? It remains the 1st option as it should also provide the sought seakeeping qualities and modifications would be limited to deck, sole, and layout. Built to designed size with deck and layout much alike Barraman's up-scaled one.
The plans include a drawing titled stations. On that drawing, I show all the dimensions you ask for. Once you buy the plans, I will, for convenience, add an extra station between D and E. It is not really necessary, the lines are almost straight between D and E.

There is also a drawing titled "Expanded Plates" on which I show the stringers. They are part of the basic framing.

There are three layout drawings for inboards or outboards, a total of nine drawings with dimensions but if you don't find a dimension, I will help.
Ok, I can well understand that and I guess I am confident enough to shoot on the MG20 already. Yes, judging by the few build pictures it looks like a frame or atleast some sort of additional support would be needed there. I'll do some close-enough estimates based on the study plans but it starts to look like this closing settled state. 8)

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BarraMan
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Re: Boat Project II

Post by BarraMan »

I look forward to seeing another Mangusta hull come together! :D

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