Boat Project II

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liki
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Re: Boat Project II

Post by liki »

TomW1 wrote: Sun Jan 29, 2023 2:43 pm Liki I would love to run the naumbers through my calculators if I could get some idea of the weight of the Mangusta from BarraMan.

BarraMan do you have any idea of the weight of your Mangusta as it leaves the dock with 3 people. Then how much do you think the 10% addition increased the weight by. Thank for any help you can give.
Yes, it would be really nice to perform some power estimations and estimate the feasibility of downscaling AB23 plus consider the suitability to my layout plan 8)

But I'd need hull weight (est.), deadrise, chine width, displacements, freeboard, and frame locations for that and not all of those figures are provided in the study plans. Measurements I can roughly guesstimate from sketch for a good-enough power estimate. Fuzz has a very valid point above, load capacity diminishes much more in relation, displacements changes to the power of 3 and structure weight abt. to the power of 2 as mostly panel areas change. Equipment even less so.
For my use abt. 200kg for preferably 115-140hp OB plus controls, 150kg for fuel tanks, traction batteries, bow motor etc. .. Add 200kg more?

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Jaysen
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Re: Boat Project II

Post by Jaysen »

A missed “variable weight” in my opinion is “beer weight”. I put the variable weight in quotes as it isn’t really variable as it stays in the people for a bit of time meaning it could be on the boat the whole trip. This is different than fuel weight which turkey is variable yet we account for it.

To be clear, I consider “beer weight” to be anything human consumable. Water, food, etc. around here beer is the largest item in that category. Hence the name.
My already completed 'Lil Bit'. A Martens Goosen V12 set up to sail me to the fishing holes.
Currently working on making a Helms 24 our coastal cruiser.
“Mark Twain/Samuel Clemens” wrote:Eat a live frog first thing in the morning and nothing worse will happen to you the rest of the day.
Jaysen wrote: Mon Apr 29, 2019 3:44 pm I tried to say something but God thought I was wrong and filled my mouth with saltwater. I kept my pie hole shut after that.

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BarraMan
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Re: Boat Project II

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Jaysen wrote: Mon Jan 30, 2023 5:41 am A missed “variable weight” in my opinion is “beer weight”.
Not on my boat! 8O Beer is a post-fishing activity! :lol:

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Jaysen
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Re: Boat Project II

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BarraMan wrote: Mon Jan 30, 2023 6:24 am
Jaysen wrote: Mon Jan 30, 2023 5:41 am A missed “variable weight” in my opinion is “beer weight”.
Not on my boat! 8O Beer is a post-fishing activity! :lol:
I was recently repairing a larger than most here vessel and commented that “cleaning this locker after ever trip would help with the bugs… less beer to attract them” to which the captain replaced “that was just yesterday afternoon’s charter”. Working on the marina docks awhile back I can attest to you are the rare beast in the boating world. I thin most “accidents” are preventable in the Sri is leading up to the event.

That said, I’ve killed my fair share of cans while floating looking for fish.
My already completed 'Lil Bit'. A Martens Goosen V12 set up to sail me to the fishing holes.
Currently working on making a Helms 24 our coastal cruiser.
“Mark Twain/Samuel Clemens” wrote:Eat a live frog first thing in the morning and nothing worse will happen to you the rest of the day.
Jaysen wrote: Mon Apr 29, 2019 3:44 pm I tried to say something but God thought I was wrong and filled my mouth with saltwater. I kept my pie hole shut after that.

fallguy1000
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Re: Boat Project II

Post by fallguy1000 »

The size constraints really limit the greatness of the boat.

6.5m ~ 21' L
2.2m ~ 7' 2" B

This means a boat with a relatively narrow beam. The L/B ratio is a measure that can be overlooked in some sizes, but I prefer not to..mostly for efficiency and ride.

The L/B here is >3.

The only boats I would consider after reading all your constraints is the Panga 20 or OD18. The Panga20 is a fast, economical build, designed for excellent seakeeping. My only reservation is there are some rumors it can be a wet ride and I hate that, but maybe you can keep it at a decent speed and keep the spray behind you. I also wondered about making a Panga with a side helm, still fishable, but ahead of the spray some. The OD with a center console wpuld be drier, but not sure if you can fish the way u like from cc.

The slip size doesn't sound like it even has allowance for 6" fenders on each side. Either of the boats I mentioned would be great. The OD can even run on a 30hp motor, so probably cheaper unless you spent a lot to remote steer.

My 2 cents..
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rick berrey
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Re: Boat Project II

Post by rick berrey »

Years back when you saw a post on scaling it was a +10% length only with -10% length sometimes , stated as the rule of thumb . Some boats were already scaled up such as the DE25 . A 15% scale down I think would be a big risk , with the Panga 20 you know what you have from the start .

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Re: Boat Project II

Post by fallguy1000 »

rick berrey wrote: Mon Jan 30, 2023 12:42 pm Years back when you saw a post on scaling it was a +10% length only with -10% length sometimes , stated as the rule of thumb . Some boats were already scaled up such as the DE25 . A 15% scale down I think would be a big risk , with the Panga 20 you know what you have from the start .
rick is correct; scaling down is a crapshoot; high risk idea, sometimes the plywoods won't bend and the bulkheads are wide

Years ago, I shortened a Gil Gilpatrick Laker canoe 3" and narrowed the strips and the entire bottom had to be modified for those two reductions as the strips would not bend. The modification wasn't awful, but I was pretty worried for a week or two before I determined corrections.
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BarraMan
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Re: Boat Project II

Post by BarraMan »

Apologies in advance if anyone is offended by this post - that is not my intention!
Its just that my scientist brain won't rest once it has seen something that doesn't make sense - and some stuff around scaling and L/B ratio in the post above doesn't make sense to me! :doh:
So I did a bit of research, as one does, to try to get a better understanding of the issue.

First some definitions etc:
Definition: L/B = length divided by beam.
Units: Dimensionless.
Usually, the waterline dimensions LWL and BWL are used for monohulls.
What it's used for: Performance
Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a given length.
If a boat can plane, smaller L/B often suggests more efficient performance at low planing speeds. The balance generally tilts in favour of high L/B for fast boats.
Typical ranges of L/B are: 2 to 4 - Small to mid-size planing powerboats.
The ratio of length to beam at 3:1 is considered a "classic" proportion, and as an example, the Boston Whaler Outrage 22 is precisely at this mark.

Then some observations:
As I understand it, the L/B ration is usually calculated from the length and breadth at the waterline. However, as we don't have those dimensions for the plans on this site, I have used overall length and breadth, which should do for the purposes of this exercise. All metric dimensions (M).
AB23 (the main subject of this thread) 7.02 (L) x 2.45 (B) so L/B = 2.87
OD18 (mentioned) 5.50 x 2.15 so L/B = 2.55
PG20 (mentioned) 6.20 x 1.85 so L/B = 3.35
MG20 (mentioned) 5.95 x 2.15 so L/B = 2.77

AB23 x 0.9 = 6.32 x 2.21 so L/B = 2.86 (vs 2.87 above) - so scaling down doesn't significantly alter the L/B. 8O
MG20 x 1.1 = 6.7 x 2.45 so L/B = 2.74 (vs 2.77 above). My hull is not quite identical to the MG ie deliberately slightly higher and therefore wider.
Interestingly, I also have a significantly scaled down MG hull, ie MG20 x 0.1, ie a 1/10 scale model = 595 mm x 215 mm so L/B = 2.77 (vs 2.77 above) - no surprise when you think about it.
I can take the dimensions at the waterline on the model: 510 mm x 185 mm so L/B = 2.76

Image

And finally some comments on the posts above:
"The size constraints really limit the greatness of the boat.
6.5m ~ 21' L
2.2m ~ 7' 2" B
This means a boat with a relatively narrow beam. The L/B ratio is a measure that can be overlooked in some sizes, but I prefer not to..mostly for efficiency and ride.
The L/B here is >3"


No, the L/B = 2.96 - right in the "classic proportion" range!

"A 15% scale down I think would be a big risk"

Why? :doh:

"scaling down is a crapshoot; high risk idea"

Again, why? :doh:

"sometimes the plywoods won't bend"

I would be pretty confident that 1/4" ply would bend as required for this build! 8O

"and the bulkheads are wide"

No, the bulkheads would be scaled down by the same amount and therefore will be in proportion to the rest of the hull! :doh:
Last edited by BarraMan on Tue Jan 31, 2023 12:06 am, edited 1 time in total.

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BarraMan
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Re: Boat Project II

Post by BarraMan »

Jaysen wrote: Mon Jan 30, 2023 6:35 am
BarraMan wrote: Mon Jan 30, 2023 6:24 am
Jaysen wrote: Mon Jan 30, 2023 5:41 am A missed “variable weight” in my opinion is “beer weight”.
Not on my boat! 8O Beer is a post-fishing activity! :lol:
I was recently repairing a larger than most here vessel and commented that “cleaning this locker after ever trip would help with the bugs… less beer to attract them” to which the captain replaced “that was just yesterday afternoon’s charter”. Working on the marina docks awhile back I can attest to you are the rare beast in the boating world. I thin most “accidents” are preventable in the Sri is leading up to the event.
That said, I’ve killed my fair share of cans while floating looking for fish.
We have generally never mixed alcohol with boating and as the "skipper" I never do! That said, in the Northern Territory (Australia's Last Frontier!) where we fish every year, its almost illegal to NOT be drunk in charge of a fishing boat. 8O A couple of years ago they tried to introduce random breath testing but there was such an outcry from the NT fishing fraternity that the proposal was quickly dropped! :lol:

liki
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Re: Boat Project II

Post by liki »

Hah I am sort of "with" Barra here .. Our fishing is generally very "wet" but boats are usually smaller and slower, sometimes only row boats. +35kn motor boat is no place for the captain to be drunk.

But yes, form issues would arise if trying to adjust only e.g. length or width, then the form of potentially all panels would change as the result of a little change. Adjusting everything only the size changes for everything and while it certainly has risks they are different than having to adjust the forms.

LB 3 is not a problem at all, it is higher than for "normal" modern boats of this length but ~100 years ago 4 would had been usual for 6m length. PG20 would work rather well, yes, it should be able to handle waves adequately. And water temps are usually around 5 C while we fish and we usually wear dry suits, wet rides are not that bad issue. 8)

I have been tinkering quite some with the idea for this project while gathering experiences using the earlier much smaller one and waiting for the "correct" time to get started plus now I am finally getting the mooring for a larger enough boat so it wouldn't have to sit only a trailer any more. Actually this old Norwegian boat, Hydrolift F-20, made me think more about ~6x2m size and higher top speed instead of ~5x2m: http://www.hydroline.se/MARINE/F20.htm

Modifying one with bow and aft casting platforms would be easy and also an option if I happen to come across a suitable project hull but those are rather rare.

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