Propeller Shaft? I need help...

[Reid Crownover]

Ok, So I have had this project I have been working on for a few months (18 foot trawler style work of art) and I have run into a sort of fork in the road. I plan on using a small 23hp Volvo diesel engine as my source of power, however, I am not sure how to run my driveshaft from inside the boat to outside the boat without letting water flow into my hull. Nor am I absolutely sure what to use for my propeller shaft. I think it is worth mentioning that I am a broke 17yo high school kid and the boat has been assembled with primarily scrap wood people have given me from house build sights. That being said, I need to find a way to do this with little cost. Does anyone have any ideas on how I might be able to accomplish this with stuff I can get from the home depot? It needs to be salt water friendly and be able to stay in the water for long periods of time.

After a test run in Canyon lake the maiden voyage of my craft will be up the intercoastal from Matagorda bay all the way to New Orleans where I will be attending school at the University of New Orleans to study Naval Architecture and Marine Engineering. If anyone has any pointers or questions about the project or anything else, My email is Reidcrown@gmail.com

 

[gonzo]

What you need is a stuffing box or packing gland. For a small engine like that, a 3/4" shaft will be adequate. Try finding a used one. Do you already have a propeller for it?
Packing Boxes / Stuffing Boxes / Packing Glands https://www.go2marine.com/category/13869/packing-boxes-stuffing-boxes-packing-glands.html?view=item

 

[Reid Crownover]

I have not picked a propeller yet, probably going to end up finding one off and old sailboat or something.

 

[gonzo]

Propellers need to be sized to the application. Otherwise, that may end up being the equivalent of installing Ford Focus tires on a semi.

 

[Reid Crownover]

based off what I have read im going to need a pretty aggressive pitch on my propeller. Its an 18foot displacement hull.

 

[gonzo]

The pitch is calculated a function of power and speed. Your maximum power and shaft RPM are already determined. By calculating the resistance of the hull, or using tables, you can get the blade area and pitch necessary for optimum performance.

 

[Reid Crownover]

Yea, I have the equasions to calculate that in one of my textbooks I have picked up to read before I got to college. Also another question, how much void space am I going to need between the propeller and the sides of the hull, I plan on putting the propeller in the aft portion of the boat and cutting up into the hull to put the prop in a sort of canal. the only downside I see to that is that the propeller needs to be able to suck in sufficient water to create thrust. Is it ok to put the water/exhaust coming out of the engine behind the propeller so it feeds into the prop?

 

[gonzo]

If you put exhaust into the propeller it will ventilate. That is the equivalent to spreading ice in front of your tires (they will spin). This nomograph will give you an idea of your top speed. Based on that, you can calculate the approximate pitch and blade area.

 

[Blueknarr]

Is it ok to put the water/exhaust coming out of the engine behind the propeller so it feeds into the prop?

I prefer my wet exhaust to discharge above the water so I can verify cooling flow.

 

[Reid Crownover]

I prefer my wet exhaust to discharge above the water so I can verify cooling flow.

That's a good idea, I will have to incorporate that into my design.

If you put exhaust into the propeller it will ventilate. That is the equivalent to spreading ice in front of your tires (they will spin). This nomograph will give you an idea of your top speed. Based on that, you can calculate the approximate pitch and blade area.View attachment 139371

So, I calculated my speed at 14.5 knots which is kinda what I was expecting. Do you know of a place that makes propellers at reasonable prices? I was thinking of just going to the lake where I live and going through some of the wrecked boats to see if I can find the parts I need.

 

[gonzo]

That would be planing speed for the hull length. Since you have a displacement hull, the speed will be about 1.2 to 1.4 times the square root of the waterline length. At 16 feet of waterline length it comes to 4.8 to 5.6 knots. Unfortunately, those are the laws of physics.

 

[Reid Crownover]

That would be planing speed for the hull length. Since you have a displacement hull, the speed will be about 1.2 to 1.4 times the square root of the waterline length. At 16 feet of waterline length it comes to 4.8 to 5.6 knots. Unfortunately, those are the laws of physics.

I remember seeing that equation in one of the books I read, going to be a long voyage from south Texas to NOLA... Hull has the same form factor as a sailboat with some changes.