design - engine placement - ELI5

[Seapig]

I apologize in advance if this is the wrong place for this post but I cannot find a more appropriate place.

ELI5 - comes from reddit and means explain like I am five.


Why do boat shafts point to the stern? In other words why does the engine have to be ahead of the propeller?


I have seen pictures of a few boats where the engine is astern, the shaft goes forward and the propeller can be thought of as a puller.


The only downside I can think of is that this would put the engine too far back from a balance standpoint but in many boats this is the whole point of a saildrive. I personally have no love of saildrives. Having the drive shaft go forward would achieve the same thing as a saildrive without the complications.
Thank you in advance for sharing your insights.
SP

[sailormed]

I am aware of V-drive systems where the engine output/gearbox is towards the bow of the boat. The shaft is then taking a nearly 180° bend towards the backend of the boat in the V geardrive under the engine. The propshaft is exiting the boat in the normal way and PUSHING the boat.

[Chotu]

My personal theory is it’s mostly to protect the prop. If a big floating log is coming along, it’s going to bounce off the hull and the prop will be OK. If the prop is out to the front, that might be the first thing that hits it.

There’s also possibly a little bit more chance of getting air to the prop toward the front of a boat. The water at the stern is fairly calm and has been settled by the hull going through it already. Up by the bow, as you notice, sometimes it just leaves the water. And the prop may leave the water with it.

[malbert73]

Also, given the slope underwater toward stern a forward shaft would be a very steep and inefficient angle to get prop clearance from hull, unless you are talking about a prop forward of keel which would be weird.

[Seapig]

This is the sort of thing I am talking about





On my boat there is enough room astern of the engine to host a barn dance yet the motor is tucked up under the bed which makes working on it less fun than paying taxes. Also the area is nearly flat so the difference in angle between what it is now and what would be the case were things reversed is negligible.

[Searles]

Not common ,fwd facing props known as tractors drives ,on a yacht ,not verry practical ,a need for prop wash over the rudder for slow speed manovering can be vitally important .⛵️⚓️

[vpbarkley]

There are pods (which are basically steerable saildrives) that are pullers.

Pusher props are afforded some protection.

The photo shows a steeply inclined prop shaft which is inefficient.

A central engine location gives the least inclined prop shaft.

[tkeithlu]

The only additional reason I can think of, of the many listed above, for running the shaft aft is weight distribution. In my boat, the very heavy engine is very close to the C.G. That means less weight at the ends.

[Cadence]

You don't want the prop wash hitting the hull, you might as well push the boat backwards, but going across the rudder as someone mentioned. If you want the engine weight aft a V-drive would be the way to go also mentioned.. Someone mentioned a tractor drive. Good if you have a docking tug boat.

[boatpoker]

Originally Posted by Seapig

I have seen pictures of a few boats where the engine is astern, the shaft goes forward and the propeller can be thought of as a puller.

Please provide links to those pictures. I've done thousands of surveys and never seen such an animal other than Volvo pod drives on power boats.

[Seapig]

As to rudder placement, both the Lagoon catamarans and the Albin Vega have the rudder ahead of the propeller. With the cat the majority of the turning in close quarters is with engines so that may be why. I have never seen an Albin Vega it just came up on a google search.
I agree having the motor over the keel would be optimum but on the condo-cats they have moved the engines as far back as they could and then stuck a sail drive on them. While I don't like saildrives I do like being able to get at the engine. Working on my engine causes me to break the second commandment.
As to the picture a buddy sent it to me when we started discussing this point. I will see if I can find out where he found it.

[boatpoker]

Originally Posted by Seapig

As to rudder placement, both the Lagoon catamarans and the Albin Vega have the rudder ahead of the propeller.

I've surveyed about 30 Lagoon cats (power & sail) of various sizes and never seen one with the rudder forward of the propeller. To which models do you refer ?

[Searles]

Some of the Duncanson designed and built yachts of the early 1970s had the prop aft of the rudder ,absolute pigs at slow ahead ,even worse astern , I later years this design type of instillation was fazed out

[zstine]

1st the bow of boats pitch over waves. So to keep the prop underwater, it needs to be near the stern of the boat... especially in planing hulls.

2nd, Efficiency. Prop accelerates water. If the shaft and support strut were behind the prop, it would be in a higher flow location and cause more drag.

3rd, protection. When in front of the prop, the shaft and support structure can push debris like a log out of the way.


4th mounting angles. The hull curves up in the stern. So a shaft mounted in front of the prop can be much closer to level than a shaft going forward to a prop.

[Cheechako]

I've not seen a boat with a "pullpellor". Normally the prop pushes the shaft hard against the transmission and the tranny is built for that. One reason is just that; it's built to take the thrust. The other reason to not have the prop in front is it will hit things and catch things...