Propeller position for optimal water flow

[Maxence]

Hello everyone,

I'm currently in engineering school and we have to do a boat all by ourselves (a small one of course) and my group came to the discussion about propeller. Where do we place it to optimize water flow rate ?
Our boat is 600mm in length, 97mm in height and the transom (back of the boat) is curved outward.
Our shaft will be horizontal. The center of the propeller will be 13.7mm above the bottom of transom and is 45mm diameter. My question is how far behind trnasom should it be ? We think of 22.5mm to 45 but we are not sure and I cant find any good reliable sources because google or chat gpt seems to misunderstand me.
Please help.
Ty.

 

[portacruise]

Sounds like a model boat if it's only 600 mm long.

My experience with one man boats is that the pointy shape on both ends, like a kayak is the most efficient, compared to Square or curved outward at the back, if highest speed is important. Also that an unsupported below surface flexible shaft version of a long tail motor, with a prop at the end, will continuously adjust to the horizontal and align with the flow. A version of the flex shaft was used to set the 24-hour world record for one man human powered boats, several years back. Consider searching the archives of this list which may have some more useful info.

Ps. Another unconventional design not using props, which is theoretically very efficient is a powered floating cylindrical shape, like a hamster water wheel. Water skin drag is used to an advantage, rather than something that has to be minimized. Let us know how you do with these or any other ideas.

 

[messabout]

Why would you locate the prop center above the bottom of the transom? The prop needs as nearly undisturbed water as can be made practical. Placing it above the transom bottom will put the prop in turbulent water when the boat is going slowly. If you improbably get the boat into planing mode, the prop will be ventilated and will cease to provide sufficient thrust. Put the prop under the boat near the transom but not in the same vertical plane as the transom. The engine or motor will be inside the boat and that means that the shaft angle will come into play. Keep the angle as low as practical but do not choke up on that part of your design. You can keep the angle lower by placing the engine farther forward. Be mindful of the weight distribution when you move the motor forward. Ideally you boat should float slightly bow up. The lift vector from the angled prop shaft will tend to raise the aft end when underway.

Designing the boat will be an appropriate exercise for engineering students. There will be a suitable amount of math and physics involved, statics and dynamics too. It is absolutely essential that you write a statement of requirements (SOR) for your boat. That is the first thing to do before you even consider designing a boat. Is the boat to be a fast boat , a slow but more economical to operate boat, a cargo carrier, a racing boat, a boat designed for warfare, spying or dope running, a cruising boat with accommodations for crew and passengers, a fishing boat designed to endure rough weather, a fishing boat designed for small lakes and ponds. those things are required to know before you begin to design a boat. They all have some differences in design and purpose.

There are boats that use surfacing props, but that is high tech stuff. They are ultra high speed boats, very expensive ones. That is not something that you should contemplate at this time.

Go for it Maxence

 

[philSweet]

Under the boat at about 98% of the waterline is best for displacement hulls.

 

[Maxence]

I didn't know you all would reply this fast, thanks for the answers.
Sorry to come this late I had other courses to prepare/study.

Trying to design a boat is hard, And we need to maximise 'space' to fit a lot of component so a kayak would be hard to do since we have a limited length.
Our current shape should not change much tho (in fact, we inherited another group project to 'teach us that in company you will inherit other peoples project'), we consider changing only height because there is not enough space and the professor allowed us.
Our overall shape is round on the front and back a bit like a mirror yacht (take the front of a yacht, we have this for the other side too).

with an horizontal shaft, what would be it's optimal lenghth ? And how flexible should it be ? If you need I can sens a screen of the inherited hull shape we got.

Thanks again for the replies.

 

[BlueBell]

Ty, welcome to the Forum.

For "optimal water flow"?
Midships as deep as possible.
But this is not practical.

A horizontal shaft running aft to a horizontal cutlass bearing in the stut would work depending on the hull profile / design / shape.
How many RPM, speed targets?

Yes, please send a screen shot, with many viewing angles.
Please show the prop and assignment as well.

 

[Maxence]

It's only the beginning of the year for us we are not that deep in the project, but here are some screenshot of our current boat (big image trigger warning). The small rectangle you see top left of the side view is where the rudder will later be added, and the little tube is the shaft but we will change it horizontally. Idk if this many view is enough, tell me.

 

[portacruise]

Here's the general idea, but it is more efficient without the encased cover on a flexible trailing shaft, and the bare shaft will curve while exerting propelling Force so that the prop aligns to the horizontal constantly to the flow of water, once a certain minimum of force is applied.

1/16 inch spring steel bare for the 600mm boat length shaft, Maybe 75% the boat length, or longer, depending upon how much force is being exerted by the motor. The flexible shaft remains straight below some minimum force but Bends to the horizontal once over that minimum force. The design avoids the friction and leaks associated with a below surface water bearing, leaves more room inside, reduces or avoids the spinning prop operating in Disturbed water and most appendage blockage directly in front of the prop from the hull Easy and fast to change out, replace or repair compared to through the hull designs. Just my humble experience. The shaft can be easily tuned or replaced with thinner or thicker diameter to get the required bend to the horizontal at an extended cruising range , easily trimmed to the most efficient length for the particular boat and Power parameters, or be made telescopic or two-piece., if required. Also can be run in very shallow water,flexes to avoid prop damage if hitting bottom or obstacles, can be run in crude surface piercing mode. It's a good testbed setup for many ideas..

 

[BlueBell]

No target speeds?

Looks like two propellers are planned.
Why two? What kind of props?

Hard to comment further without knowing the objective of the assignment.

 

[messabout]

Make the transom of the boat straight up and down, not curved. The small rudder mounting pad is not big enough and it is in the wrong place if you mean to have twin propellers. A single rudder would work alright at speed but not effectively at low speed. Put the rudder behind the propeller, or in this case, two rudders each behind and in line with its propeller. You have drawn the bottom as a straight line from bow to stern. Improved design will use less curve in the stem and add a little bit of bottom rocker near the bow. The bottom width is not nearly wide enough for the width of the sheer (top of the sides). There is no reason for having the sides sloped as much as you have shown. Fifteen degrees would be considered excessive. Ten degree is adequate for most applications. Twin propellers offer almost no advantage and make the power transmission more complicated than needed.

You have designed something more like a Grand Banks dory than a simple little inboard runabout. Try not to stray very far from designs that have proven their worth over hundreds of years. I suspect that your professor is testing your ability to do the required math, along with some basic physics, and apply it to your design. You will have to do some careful weight estimates along with placement of the motors, batteries and other objects that have weight. You must get the fore and aft balance somewhere near the right place.

 

[gonzo]

Our shaft will be horizontal.

Your drawing shows a shaft that is not horizonta. However, if it is horizontal, the only propeller that would work would be surface piercing. It is an inefficient shape, unless there is a specific use that requires it. Also, you are specifying the dimension of the propeller before finalizing the rest of the design. It should be one of the last things you do.

 

[BlueBell]

" Also, you are specifying the dimension of the propeller before finalizing the rest of the design. It should be one of the last things you do."

I believe a 45mm prop was a given in the assignment.
Shape, pitch are unknown.
No pictures.

We need to see the entire assignment.

Now the OP's gone.

 

[gonzo]

A horizontal shaft with the center 13.7 mm above the bottom would have to be tiny, since the thickness of the hull has to be deducted from that dimension. It would also have no space for a gear or motor.

 

[messabout]

Requirements specified by the professor are suspect. Is the prof familiar with practical boat design? LOA at 24 inches, with a 1.71 inch prop is satisfactory except that the model does not need a prop that big..........unless the boat is to be a tug boat.

 

[messabout]

the original post says that is to be 45 mm.