Reduction gear, prop size and draft

[Wckoek]

Yanmar would be nice and light, but wasn't continuous duty.
Just figuring if the engine applicable to a simple chined or dory hull with keel design as some sort of displacement cruiser.

Glad to hear that direct drive would be alright.

[a64pilot]

A KBW-20, a real common transmission coupled to a Yanmar can have about a 2 to 1 reduction or usually about 2.6 to 1 I believe.
At an engine speed of 3600 at 2 to 1 then of course prop RPM is 1800or at the 2.6 to 1 it’s close to 1400.
So 1500 isn’t all that bad really.

[Wckoek]

So we can deduce at least its alright to run 1800 rpm to prop without reduction.
With such high rpm, probably a 2 blade can be efficient, but comes at the cost of draft as well.

[Wckoek]

Just learn that this is the common way to mount their engine without reduction gear, installation looks like a breeze, but not sure if there is any advantage, disadvantage to shaft.

[Jim Cate]

Originally Posted by Wckoek

Just learn that this is the common way to mount their engine without reduction gear, installation looks like a breeze, but not sure if there is any advantage, disadvantage to shaft.

Without wanting to sound snotty, the term "agricultural" comes to mind when viewing that setup. Probably won't see too many of them at the big boat shows, hanging from the stern of fancy yachts!

Lots of vulnerable iron mongery dangling from the engine and mounts.

not my idea of good practice, but ymmv.

Jim

[Wckoek]

I agree, probably bad idea going to sea with the "agricultural" setup, but I am thinking or messing around with the engine and see what I can do with it.
A proper shaft is the way to go to sea

[Wotname]

Am I missing something here???

Even without a reduction drive, you will still need some sort of transmission just to get fwd and astern - surely ????

[Sailmonkey]

Originally Posted by Wckoek

Just learn that this is the common way to mount their engine without reduction gear, installation looks like a breeze, but not sure if there is any advantage, disadvantage to shaft.

I would be VERY surprised if there was no reduction in all of those right angle drives.

[Olddan1943]

Originally Posted by Sailmonkey

I would be VERY surprised if there was no reduction in all of those right angle drives.

I "visited" am old boat with a one cylinder, air start motor. No reduction gear, no transmission. It had a manual variable pitch prop, controlled by a hand crank.
It had a mast for sailing, he removed it, put on a hollow tube to hold FW for heating.
Was interesting.

[atoll]

Originally Posted by Wotname

Am I missing something here???

Even without a reduction drive, you will still need some sort of transmission just to get fwd and astern - surely ????

in the old days they just stopped the engine and restarted it in the opposite direction ,quite easy to do with compressed air start

as technology moved on controllable variable pitch propellors became more common ,allowing the engine and prop shaft to run at set revs and the blades of the propellor rotated in the hub to go from forward thrust ,feathered to reverse thrust,this also allowed adjustment of pitch for differing sea conditions.
with exhaust temperature indicating the load on the engine.

[Wotname]

^^, yep I understand that but I didn't get the impression the OP going with a fully variable pitch prop but then again, maybe he has already considered these options and is way ahead of me

[atoll]

Originally Posted by Wotname

^^, yep I understand that but I didn't get the impression the OP going with a fully variable pitch prop but then again, maybe he has already considered these options and is way ahead of me

the op seems pretty keen on re-inventing the the wheel to save a few pennies

[Wckoek]

Yes, there is a transmission of some sort though the smallest one, I don't see any on it. They made it for as small as 6hp 4 stroke diesel, mostly used as outboard.
The good thing is the prop can be configured to be lifted up for beaching or shallow draft situation.

Not to pinch pennies or reinventing the wheel, just to see how I can play with it.

[ramblinrod]

Originally Posted by a64pilot

1500 direct drive would be fine at those HP levels and prop sizes.
Think about shaft RPM of a Yanmar turning 3600 RPM.

But then again, neutral and reverse is nice. ;-)

[Cadence]

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?

The largest and slowest prop. is my choice for a displacement boat. With a 3.5 draft can you swing a 29" wheel with the clearance needed. What is the engine.