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Wckoek · 26-07-2018, 08:03

Hello, say I have a displacement motorboat with 3.5 feet of draft, and I intended to fit it with a 1500 rpm motor, and I have the choice of running it either with 3:1 or 2:1 reduction gear.

According to calculation with 2:1 gear I can run 23" 3 blade prop, with 3:1 I can run a 29" prop.
I understand that for displacement speed, the larger the prop the more efficient, for a draft of 3.5 feet, would a 29" prop be excessive?

Also hypothetically, another similar boat but with a draft of 2.25 feet running the same engine, would it be possible to run the engine without reduction gear at all to use a 15" propeller?

**atoll** · 26-07-2018, 08:20

Quote:

Originally Posted by Wckoek

Hello, say I have a displacement motorboat with 3.5 feet of draft, and I intended to fit it with a 1500 rpm motor, and I have the choice of running it either with 3:1 or 2:1 reduction gear.

According to calculation with 2:1 gear I can run 23" 3 blade prop, with 3:1 I can run a 29" prop.
I understand that for displacement speed, the larger the prop the more efficient, for a draft of 3.5 feet, would a 29" prop be excessive?

Also hypothetically, another similar boat but with a draft of 2.25 feet running the same engine, would it be possible to run the engine without reduction gear at all to use a 15" propeller?

you are missing some important information if you want an opinion.
IE
Displacement,waterline length,and horse power of engine

Wckoek · 26-07-2018, 09:12

40 feet and around 11k lbs, I got the calculation from post #1 on vic prop calculator site.

**atoll** · 26-07-2018, 09:28

Quote:

Originally Posted by Wckoek

40 feet and around 11k lbs, I got the calculation from post #1 on vic prop calculator site.

the vic prop calculator works well for displacement boats.
there are other calculators that you can add in more variables for planing vessels.

but basically the bigger the prop the better ,as there is less slip and probably better fuel consumption if correctly matched to a low revving engine.

Wckoek · 26-07-2018, 10:24

Yes, I mean it is for displacement boat.

Probably I should rephrase the question as:
1 Is it reasonable or practical to run a 2.4 feet prop on 3.5 feet of draft or 2 feet (23") is better instead?
2 In case of a shallow draft say 2.25 feet, would it be possible to run a marine diesel without gear just for the sake of smaller propeller?

**Jammer** · 26-07-2018, 13:06

Those are ponderously large props.

I don't know the answers to your questions, though.

gjordan · 26-07-2018, 13:14

Are you sure the engine you are talking about is a 1500 RPM motor? That is only common on very old designs or huge engines. A 3:1 reduction would put you shaft speed at 500 RPM. That would be very unusual. ___Grant.

**atoll** · 26-07-2018, 13:20

Quote:

Originally Posted by Wckoek

Yes, I mean it is for displacement boat.

Probably I should rephrase the question as:
1 Is it reasonable or practical to run a 2.4 feet prop on 3.5 feet of draft or 2 feet (23") is better instead?
2 In case of a shallow draft say 2.25 feet, would it be possible to run a marine diesel without gear just for the sake of smaller propeller?

you still have not said how much horse power.

but most 6000kg 40 ft displacement vessels with a 2:1 box and engine reving to 3500 rpm would be using an aprox 18-20 inch prop so in your case probably the 23 inch prop would be a better choice

for shallow draft you should probably be looking at a jet drive

a direct drive small prop may have cavitation issues

**a64pilot** · 26-07-2018, 14:02

There is of course a point of diminishing returns with bigger is better, then add in increased shaft size and mass to swing that heavy ole prop, and price of the prop itself, then of course how vulnerable is this prop to being run aground?

Wckoek · 26-07-2018, 15:16

Gjordan: The engine is Weichai 2100, 2015, or available in export in variation as WP2.1, but domestic ones are available from 26hp to 37hp, 220kg, not really that huge, but gearbox is not included though.

Atoll: it is a medium speed engine at 1500rpm, jet drive is not an option.
Would 15 inch have cavitation issue as well?
I'm not sure if running without reduction gear at all for 1500 rpm engine would work.

I found this engine priced at about $760 usd new (minus shipping), heavy/commercial duty too and like gjordan said it isn't common, the gear is not included but priced at $440 and weighted 120kg, figuring hypothetically what it can power

**Wotname** · 26-07-2018, 15:46

You need to consider the prop tip to hull clearances, IIRC this should be >10% of prop diameter.

Olddan1943 · 26-07-2018, 15:58

The following is just and example of my "thinking"
How much would it cost to get a qualified marine engineer involved?
Is your goal is fuel efficiency or speed?
Any decision might just might require changing the diameter of the shaft and and and.

**atoll** · 26-07-2018, 15:59

Quote:

Originally Posted by Wckoek

Gjordan: The engine is Weichai 2100, 2015, or available in export in variation as WP2.1, but domestic ones are available from 26hp to 37hp, 220kg, not really that huge, but gearbox is not included though.

Atoll: it is a medium speed engine at 1500rpm, jet drive is not an option.
Would 15 inch have cavitation issue as well?
I'm not sure if running without reduction gear at all for 1500 rpm engine would work.

I found this engine priced at about $760 usd new (minus shipping), heavy/commercial duty too and like gjordan said it isn't common, the gear is not included but priced at $440 and weighted 120kg, figuring hypothetically what it can power

this sounds like an engine that is designed for a large controlable variable pitched propellor,ideal for commercial fishing vessels operating in various sea conditions.
the engine maybe cheap but the stearn gear and gear box to drive the variable pitched prop will be expensive.

see link
http://www.kastenmarine.com/CPprops.htm