Boat Project II

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liki
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Boat Project II

Post by liki »

Ok,

Time for the 2nd project is at hand ... Let's start with the use:
- Pike angling, mostly protected bay and coastal waters with mostly up-to 0,5m(1,0m) chop but short and steep.
( This video showing an excellent MG20 build picked up here shows very similar conditions https://www.youtube.com/watch?v=Odz0j1mpgdQ )
- Configured like bay boat for at most 2+1 people, 2 fishing + captain. Shallow water operation not that important.
- Looking for 35-40kn top speed, more interested in maintaining decent planing speed at rough seas than top speed.
Reading the descriptions AB23 hull shape would be wonderful here, decent deadrise, easier "hump", sharp forefoot. Plus the higher displacement aids in seakeeping.

But then the realities:
- Mooring & construction room really limit the size to max 6,5mx2,2m with 6x2m more preferable. Building outside would not be fun here and even doing that the mooring would still limit the size.
- I want to be able also handle & trailer it alone and keep tow weight to about 1250kg(1500kg)
- I'd really hope to achieve that 35kn with 115hp OB-blocks, or 40kn with 150hp but not higher power than that.

Then the options I am considering and with some questions
a) MG20 ... Would build into a nice boat as in the video above but would it meet the weight limit, say about 900-1000kg light displacement, and would it perform at all with 115-150hp? With e.g. the light Suzuki 140hp.
(b) AB23 .. 90% would be still iffy-niffy and it would have to go down to 85% size. I read scaled-down discussions here and I wouldn't mind the resulting structural overbuild but it would still be around 1000kg light displacement and just maybe could run well enough with 115hp. Then again not exactly supported option plus would/could have issues with sole height and freeboard? But it would be nice. :))

Contesters C and D would be OB17 ang PG20 sacrifing some speed and/or seakeeping for easier handling and economy. Either could be built for the purpose in very straight-forward fashion.

Contester E would sit somewhere between C and D - C17 with lower sheer. I reckon it would be closer to 5x2,05-2,10m with a lower sheer and have a sharper forefoot than OB17. In the pictures it looks like there's some difference in transom deadrise and forefoot between OB17 and C17 but how much in reality? Has any C17 been built that way?

All comments appreciated.

TomW1
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Re: Boat Project II

Post by TomW1 »

I am going to make a suggestion here. Go for the C19 it has less draft than the Abaco or the Mangusta. while top speed with a 115 is in the mid to upper 30kts. Unfortunately for you any boat you choose exept the Manguta will require more space on the sides than the 2.2 meters you designate. The Abaco and C19 require 2.4 meters for the beam, all meet the width requirement except the Mangusta but then you run into HP probllems a 150 is probably as low as you would want to go and get your speed requirements. Once you get the deck down you can arrange things as you wish. As far as speed I can run the numbers through my calculators to ger what HP need.

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

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BarraMan
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Re: Boat Project II

Post by BarraMan »

liki wrote: Fri Jan 27, 2023 6:51 am Time for the 2nd project is at hand ... Let's start with the use:
- Pike angling, mostly protected bay and coastal waters with mostly up-to 0,5m(1,0m) chop but short and steep.
( This video showing an excellent MG20 build picked up here shows very similar conditions https://www.youtube.com/watch?v=Odz0j1mpgdQ )
- Configured like bay boat for at most 2+1 people, 2 fishing + captain. Shallow water operation not that important.
- Looking for 35-40kn top speed, more interested in maintaining decent planing speed at rough seas than top speed.
Reading the descriptions AB23 hull shape would be wonderful here, decent deadrise, easier "hump", sharp forefoot. Plus the higher displacement aids in seakeeping.

But then the realities:
- Mooring & construction room really limit the size to max 6,5mx2,2m with 6x2m more preferable. Building outside would not be fun here and even doing that the mooring would still limit the size.
- I want to be able also handle & trailer it alone and keep tow weight to about 1250kg(1500kg)
- I'd really hope to achieve that 35kn with 115hp OB-blocks, or 40kn with 150hp but not higher power than that.

Then the options I am considering and with some questions
a) MG20 ... Would build into a nice boat as in the video above but would it meet the weight limit, say about 900-1000kg light displacement, and would it perform at all with 115-150hp? With e.g. the light Suzuki 140hp.
(b) AB23 .. 90% would be still iffy-niffy and it would have to go down to 85% size. I read scaled-down discussions here and I wouldn't mind the resulting structural overbuild but it would still be around 1000kg light displacement and just maybe could run well enough with 115hp. Then again not exactly supported option plus would/could have issues with sole height and freeboard? But it would be nice. :))

Contesters C and D would be OB17 ang PG20 sacrifing some speed and/or seakeeping for easier handling and economy. Either could be built for the purpose in very straight-forward fashion.

Contester E would sit somewhere between C and D - C17 with lower sheer. I reckon it would be closer to 5x2,05-2,10m with a lower sheer and have a sharper forefoot than OB17. In the pictures it looks like there's some difference in transom deadrise and forefoot between OB17 and C17 but how much in reality? Has any C17 been built that way?

All comments appreciated.
Given that my boat features in the video at the start of this, I feel compelled to comment! :D
Let me say at the outset that I am NOT advocating that you build a Mangusta hull, but I can share some insights. It took me a long time and a lot of reading before I settled on the Mangusta hull upscaled by 10% for the hull of my boat. I have no regrets about my decision - its a great boat for what we like to do! We spend 95% of our time in sheltered water and only occasionally find ourselves in the conditions shown in the video above. I recently pushed the boat to the extreme in similar conditions and made a mistake, resulting in a near disaster (see below!). :help:

https://youtu.be/sBAYyYwxzrM
https://youtu.be/VrFbqHJ1ZHE

If I was regularly dealing with similar conditions I would build the hull with a self-draining cockpit and perhaps some sort of wave breaker up front.

I suspect that the AB23 hull may be better for you. Variable dead-rise with 17o at the transom. Given your size limits, I would downsize it 10% which would bring it down to 6.3M x to 2.2M ( I think, by my calculations!) - still a BIG boat IMHO. Not hard to do if you buy the metric plans. You say that running in shallow water is not an issue for you, but regularly I run my boat in less that 1M. Not real "skinny' but OK for a big boat.

Jacques says the AB23 will do 27 kts on 150 hp, so 10% downsized hull should go over 30 kts on that hp. Plenty of speed for that size boat. If you want to go faster - put a bigger motor on it. I've run mine at 50 kts (250 hp) but that is too fast for me and more than I can afford in gas (90+L/hr). My boat is very happy between 25 and 30 kts, and it will hold the plane down to about 12 kts.

Fully load for fishing (400 L of gas, ice, food etc) my boat has a tow weight of 2,500 kg (1,900 kg boat and (600 kg trailer). Using that as an example, I think you may struggle to keep your tow weight (boat and trailer?) down to 1250 kg, even for a lightly built 10% downsized hull. But that's just my gut feeling based on some rough calculations.

The joy of building your own boat is that you have great flexibility to build what you think will work best for you. I wish you well with whatever you decide and will enjoy watching the build! :lol:

liki
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Re: Boat Project II

Post by liki »

Yes, self-bailing and the fun of doing something "factual" again aside of a "thinking" job are things I am after - Otherwise I would have a near-perfect production boat to just go and buy. :o But they are surprisingly rarely self-bailing, likely to increase stability and cockpit depth.

Yes, C19 would be ok candidate aswell but it is too wide and AB23 should definitely have all the qualities I am after. But exactly width is a hard limit, my mooring has fixed spars 2,25m apart and the boat has to fit between them with some room. Length can be up-to about 6,5m but I already need a temporary extension to my garage for that and size and weight just have to be reasonable for practicality in use and also the effort put into building ( I "invested" 260h into a total rebuild of a 4,2m skiff earlier for similar purpose ) - More power just costs a bit more and is the easiest one to overcome and actually adds surprisingly little to operational costs compared to size. :lol: Fuel economy of MG20 with 115/150/200hp would not be that different.

I'm definitely confident I can scale, modify sheer & layout to my suit but if it excessive and starts require changing sole, freeboard, spacing between structurals goes off it is no longer "just scaling". And AB23 I would really need in 85% size. :( MG20 should do well as-is to the sheer line and judging comparable designs could get the expected performance in 115-150hp range but study plans alone do not provide enough data to perform e.g. Savitsky calculation. Maybe I could quickly draft a similar hull shape and estimate based on that.

Construction-wise I don't see that much of a difference between MG20 and the "5x2m class" C17/OB17, all should require 2 joints atleast for the upper panels (c17 would build with 1-part upper panel this way), likely a frame more and a lot more area to laminate and fair. One more thing to consider is how well frame locations suit the intended layout and I need to decide would it be a casting "deck" or platforms a little below gunwales. The smaller class would be more practical .. but .... But. :D

OneWayTraffic
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Re: Boat Project II

Post by OneWayTraffic »

Being nearly at the end of a three year project building a C17 I can say that it is indeed a fair bit of work. Elephant trunk scuppers can clear water faster than any pump, and these boats have a sole level above the designed waterline. Most of us can't keep to that designed waterline, but in bigger boats it's easier to at least have self bailing.

TomW1
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Re: Boat Project II

Post by TomW1 »

I did some calculations for you this morning and the Abaco reduced to 90% meets your size requirements. I came up with 6.31 meters x 2.205 meters for the beam. Good luck which ever boat you build. :D

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 »

Yes it would become really close to the limits at 90%. In fact the more and more I think the less worried I am about downscaling to even 85%. :roll:
Layout and sole I am going to rework to suit in every case, low freeboard is ok for my purpose and would not be an issue to raise a little. She would become a little over-built but that would also improve the sought sea-keeping qualities. Fuel tank can be much smaller, say 90-120l, and transom weight with the Suzuki or Tohatsu 140hp should not be excessive.

But maybe I'll hope to get some comments regarding the weight & power for MG20 first and also comments regarding the scaling from designer.

TomW1
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Re: Boat Project II

Post by TomW1 »

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.

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

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BarraMan
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Re: Boat Project II

Post by BarraMan »

Tom, I know exactly what my boat weighs!

1900kg - with 300 litres of gas and 10 kg of ice. Its a heavy build, with 3 x external and 2 x internal layers of 12 oz biax. It also includes 112 lb Minn Kota and 5 x 120 Ah batteries (150 kg?).

I think you can probably reduce that 1900 kg by 30% for a 0.9 Mangusta build ie 1,300 kg ??

The 1,900 kg did not include people.

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Re: Boat Project II

Post by Fuzz »

I think too often folks forget about the people weight. We work to keep the boat weight down but forget how great the percentage of total weight is people. I know for me the normal people weight leaving the dock is around 1000 pounds. As the boat gets larger the lower the percentage of total weight is people. Might want to think about how much to down size the boat depending on how it will be used.

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