Prop choice

[NewEngland1]

hi all,

I am converting an old 18ft catboat into a faux tug/rec trawler which should be a cool little weekender...

I have a 50hp kubota diesel and a 2:1 twin disc TMC60A gear

I looked at beta marine web site to see what they use and the 43 uses a 18x12 and the 50 uses a 19x12

I can ONLY swing a 12 diameter, so I will have to make it up with pitch.

has anyone here run a 12" prop on a 45-50hp diesel and what pitch did u run?

 

[Nomad Willy]

The solution to your problem is the 4 blade prop or a three blade w sufficient blade area. It’s about a ratio of pitch and diameter usually expressed as blade loading on a per inch basis.

Too much blade area and you’re just dragging blades through the water … think of an oar going through the water w the blades flat. No noticeable thrust at all. Then think of an oversized blade (area) whereas it would be too hard to pull on the oar.

So if (like you) you’ve got limited clearance it’s more blade area or more pitch. Blade area can be had by profile design (shape) or just size. Many skippers with rec trawlers gravitate to four blade props and suffer from too little pitch and the prop being great for speeds slower than even a trawler but running out of thrust as speed/rpm increases really fast. Good for tug boats but not “faster boats”. Too much pitch and blade area and one suffers from all the over prop problems .. that have been discussed extensively previously on this forum.

The three blade prop is usually ideal but four can be used to load the prop correctly. But only for that reason.

Oh, I should mention you need a much smaller engine. Think about 5hp.

 

[NewEngland1]

Hi Eric, thanks. I was told the engine thing but I would like this faux tug to have some "pullin power" even if its not ever used. Plus it will be nice to cruise at a very low rpm, just putt along. The 4 cylinder will be way smoother than a 1 or 2 or even 3 cyl (I would think).

Anyway props are expensive and even used they not free so I'm looking for experiences of people who like me had to go smaller diameter.

Not only that but I took 500lbs of ballast from the bilge and the kubota weighs 400 dry plus the gearbox and prop shaft and log.

One big negative is the size which will be right in the cabin. Well, I can always make a tabletop for the engine cover lol

 

[ofer]

sizing your prop has more to do with your vessel than your engine and even gear ratio. there are prop size calculators you may try. you are limited to 12 inch diameter and that narrows your possibilities.

you can go with something like a 12x15 and would be fine for your intended use.

 

[NewEngland1]

I just bought a 12x15 (used) will start with that!

sizing your prop has more to do with your vessel than your engine and even gear ratio. there are prop size calculators you may try. you are limited to 12 inch diameter and that narrows your possibilities.

you can go with something like a 12x15 and would be fine for your intended use.

 

[Fish53]

You've got way more engine than you need and with your restriction on propeller diameter the whole setup will be very inefficient. You only need about one horsepower per 500 pounds of boat. You have a catboat so it'll never plane and your hull speed will only be around five knots.

 

[NewEngland1]

You've got way more engine than you need and with your restriction on propeller diameter the whole setup will be very inefficient. You only need about one horsepower per 500 pounds of boat. You have a catboat so it'll never plane and your hull speed will only be around five knots.

thats what I was trying to find out. Im not sure it would be "very inefficient" though. a 12x15 prop would be found on a 300hp ski boat for example. Im thinking it would be slightly less efficient that 16x12 prop. As to the power, yes I know but thats the engine thats available and running it just over idle should be very quiet and sip diesel at about .5 gph or so...

I looked at the fuel consumption of smaller and then this diesel (kubota) and its very close at the HP used.

But we'll see I guess...

 

[Fish53]

thats what I was trying to find out. Im not sure it would be "very inefficient" though. a 12x15 prop would be found on a 300hp ski boat for example. Im thinking it would be slightly less efficient that 16x12 prop. As to the power, yes I know but thats the engine thats available and running it just over idle should be very quiet and sip diesel at about .5 gph or so...

I looked at the fuel consumption of smaller and then this diesel (kubota) and its very close at the HP used.

But we'll see I guess...

The larger the diameter of a propeller the more efficient it is. Diesel engines need to be put under enough load that they reach and maintain a proper operating temperature. Running a 50hp diesel at low RPM and only extracting 5hp is simply going to cause problems like glazed cylinders and severe carbon fouling. Most engine manufacturers give a lowest continuous RPM, on my 25hp Kubota it's 1600RPM, way above 5hp.

 

[rslifkin]

The larger the diameter of a propeller the more efficient it is. Diesel engines need to be put under enough load that they reach and maintain a proper operating temperature. Running a 50hp diesel at low RPM and only extracting 5hp is simply going to cause problems like glazed cylinders and severe carbon fouling. Most engine manufacturers give a lowest continuous RPM, on my 25hp Kubota it's 1600RPM, way above 5hp.

More blade area can make up for lacking diameter, at least up to a point. The required diameter, blade area and pitch for a given boat / engine combo can be adjusted somewhat based on choice of reduction gearing.


I do agree that 50hp is a lot for that boat unless it's going to be towing stuff or otherwise loaded much of the time.

 

[Fish53]

More blade area can make up for lacking diameter, at least up to a point. The required diameter, blade area and pitch for a given boat / engine combo can be adjusted somewhat based on choice of reduction gearing.


I do agree that 50hp is a lot for that boat unless it's going to be towing stuff or otherwise loaded much of the time.

It's going to be pretty hard to squeeze the blade area required in this situation into a 12" propeller. Regardless, for full displacement speeds larger diameter equals greater efficiency. Certainly the op can buy a different ratio gear or even a new gear and diesel engine, indeed a whole new boat but that exceeds the scope of the question.

 

[Nomad Willy]

New England 1,

Four cylinder engines vary a lot re how much vibration they generate.
I had a light 3 cyl. Yanmar in oy previous boat (Albin 25) and don’t recall any annoying vibrations at all. Other 4 cyl engines shake profoundly. I had a 3 cyl BMW that had extensive anti-vibration built it like it was geared such that the engine/flywheel rotated in opposite directions. One of the smoothest engines w less than 8 cylinders I’ve known.

But the Kabota is a very good engine. May not have any anti-vibration tech in it at all. A very heavy flywheel will help a lot though.

I agree w almost all the above comments. Especially Fish53 and rslifkin.

 

[DavidM]

A three cylinder is often smoother than a 4. If you haven't committed to the engine, look at something like the Beta 20 if you want smooth.

David

 

[NewEngland1]

The larger the diameter of a propeller the more efficient it is. Diesel engines need to be put under enough load that they reach and maintain a proper operating temperature. Running a 50hp diesel at low RPM and only extracting 5hp is simply going to cause problems like glazed cylinders and severe carbon fouling. Most engine manufacturers give a lowest continuous RPM, on my 25hp Kubota it's 1600RPM, way above 5hp.

Just an update, I found a 25hp Kubota and will most likely be going with that. Cute compact little engine and should be sufficient for this boat.

 

[BruceK]

For anyone wondering, like me, "What`s a catboat?" I searched it: https://www.catboats.org/page/About

 

[NewEngland1]

Hi Bruce, yup you got it! Very pretty design dating back to the 19th century. I have a hull that was salvaged and had been wanting to build a trailerable pocket trawler that has some tugboat inspirations when this opportunity came up. The one I have is a very solidly built hull and I think will look nice to extend the cabin straight up a couple feet so there is a walkable cabin space.

Mine is 18ft long. Cats are at half as wide as they are long, mine being 8.5ft, which is the very maximum width that can be towed on the road without a permit here.

 

[NewEngland1]

A three cylinder is often smoother than a 4. If you haven't committed to the engine, look at something like the Beta 20 if you want smooth.

David

Hi Dave, I heard that. I have gone with a 3cyl Kubota D1105 now, and it is rated at anywhere from 21 - 30hp. Most likely 24hp (there were all kinds of HP ratings for these depending on the application)... already have the manifold-heat exchanger and bellhousing and trans, so pretty committed now.

An online prop calculator says 5.5hp will get the boat to hull speed, but then there's going to be windage and current to fight sometimes. I think this motor will do everything needed. (I hope anyway!)