Propeller position for optimal water flow

[BlueBell]

The center of the propeller will be 13.7mm above the bottom of transom and is 45mm diameter

My apologies. I saw 75mm but now can't find it.

My mistake.

 

[messabout]

Not a mistake Bluebell. The number 75 did appear elsewhere in the replies.

Prop center 13.7 mm above the bottom is a puzzling requirement. horizontal Shaft below bottom.. prop center above bottom............none of it makes sense. Someone is wasting the time of well meaning forum members, I suspect.

 

[Maxence]

I'm not gone,

I'm in an engineering school with every kind of fields so I'm doing this boat while doing ton of other things
But thank you all, we will take all your replies into account (I don't know if this expression work in english) and we will change some thing.

Also the prof did'nt give special requirement, except under 60cm, can go back at it's first position etc... What I was talking about is that our class for this boat is divided in differents group and for 'realism' we just exchanged design and have to work with another groups design.

Anyway, thank you for the suggestions !

 

[gonzo]

If your only constraint is the length, then make a spreadsheet with requirements and constraints. A requirement defines what a system must do, while a constraint is a limitation on how it can be built. You have a single constraint, which gives a lot of flexibility. Start a Statement of Requirements (SOR) with the constraint as the only limit. In engineering a decision matrix is usually used to help organize requirement by order of importance. Specifications, like the shape of the hull or the diameter of the shaft are the end result. If you include them as constraints, the job is much harder.

 

[Maxence]

Well, it's pretty simple we're in second years over 5 ; the condition for our boat to be validated are : it shall not sink, be 60cm in lenght max, not twin hull (with hollow in center), come back at initial position and last 30 minutes + a water canon to shoot in every direction + a camera. It should also be openable so we can access any component at any time, dismount hull in less than 2 minutes for a good repairability (that's why theres a removable plate on top and we're considering carvel planking).

 

[gonzo]

The first is really easy; keep the center of gravity (GG) low. A narrow hull will recover from a capsize faster. If the ratio of width to depth is too high, the boat will float upside down and not recover.
Resisting the water cannon is a function of making it waterproof. There should be not openings that allow water to enter.
Is the dismounting requirement meaning it should be taken apart completely or just the access? If it is only the access, a lid with locks and a rubber seal will let you open it in seconds.
Carvel planking is complicated and requires high skill. At that size it would also be more difficult. Thin plywood will make it a lot easier. If there is no requirement for minimum speed, weight, or others that may limit shape, a punt type is the easiest.
Punt (boat) - Wikipedia https://en.wikipedia.org/wiki/Punt_(boat)

 

[Maxence]

Yea the 'keep boat afloat' is trivial ; canon will be tricky for waterproofing cuz it have to take water from environement.
For the repairability, the mandatory thing is easy access but it is better if we can repairs planks. There's no requirement for weight, speed or performances. If Carvel planking is this hard what can be good ? Using glue seems a bit hard to then remove planks, we didn't decide on the method but we hesitate between carvel, Lapstrake, strip planking, batten seam and some glue methods.

Unfortunately I doubt the professor will let us use this shape :')
We can only change hull if there is a valid physic/performance reason (performance still not the first think evaluated but the more performance the better the boat)

 

[gonzo]

So the requirement is that you disassemble the whole hull in 2 minutes? Can you use power tools?

 

[Maxence]

Sorry for long reply, My group were discussing new shapes possibilities and methods.

No power tool, by hand/screwdriver, etc... Manual tools.

 

[gonzo]

Do you have a reason to believe your professor won't accept a punt?

 

[BlueBell]

Maxence,

You need to post the entire assignment.
We won't do the work for you, but we can help.
Lets see the assignment as you received it please.

 

[Maxence]

Im going to reply both comment :3

For the punt, it's my group who don't want it's too simplistic and we want to build some sort of "impressive" boat for our level.
The whole assigment (if i understand assigment as directive) is :
- Has to be monohull
- 60cm maximum in length
- 30min autonomy
- Brushless motor
- Can come back at it's designed position
- Remote controle via wifi and the boat has to host a server for a website
- Has to float and not sink
- Can shoot in every direction with a canon (the pump take water in environment)

- We get another group work that we have to finish to "simulate the project you get when you enter a company" they say, the work we inherit was done last year by member of my class so assume they are bad and pretty much everything can be optimized by a lot.

Aside these we are totally free.

 

[Skip Johnson]

Most "inpressive" solution is the simplest one that does the job.

 

[gonzo]

For the punt, it's my group who don't want it's too simplistic and we want to build some sort of "impressive" boat for our level.

That is a typical amateur attitude. Building something at your skill level is going to be of better quality than something that is beyond your skill. For example, you mention carvel planking which is extremely difficult at that size. Since the edges are too narrow for caulking, it would have to be fitted with tight planking. Further it would take days to disassemble without destroying it.

 

[BlueBell]

Maxence, you have much to learn, it's good you're at school.

Simplicity is at the heart of sound engineering.