In the latest edition of Model Boats magazine (April 2017) Dave Wooley is building an OSA 2 Fast Missile launch which has a three propeller set-up. The article's photographs show the three motor and prop installation. This model also has three rudders.

Ashley

Are we talking "prop walk" here? I didn't think it happened with multiple counter-rotating props i.e. >2 – which I thought was the whole purpose of fitting them. Maybe one of our resident naval architects can give chapter and verse.

Dave M

If it is prop walk that is the theoretical problem, the pair that are counter rotating will cancel each other which lets the unpaired one do its own thing, but that will be no more of a problem than gets experienced by a single prop boat anyway, so no real problem in practice.

The really important thing is nothing to do with handling. Having the rudders out of line with the prop shafts means that the shafts can be withdrawn easily, which is good for maintenance. It also means that the inboard end needs to be fixed so that it can't unfix itself. I've seen a complete shaft and prop lost that way. After it had finished unscrewing itself from the coupling, there was enough drag to pull it out.

Having three (or two) props all rotating the same way will induce 'prop walk' on model boats but oddly enough the larger the boat the less the phenomena is noticed.

For example 600mm long hull will need almost constant time/distance steering adjustment to maintain a straight course whereas a 1200mm long hull, where you might expect a 50% time/distance steering adjustment, actually requires only 20% time/distance steering adjustment.

When looking at full size craft the need for steering adjustment falls to 5%>.

Before people start pointing to the excellent photos provided by Bluebird that clearly show a R/R/L configuration and say that the phenomena must exist in full size boats I would ask them to consider those craft that have multi outboard motors all of which have props rotating in the same direction and offer the proposition that the Vosper designers were simply being cautious.

Paul

Edited By Paul T on 12/03/2017 17:07:25

Hi Ash

Proppery?

Ah……good term.

I agree that wind, wave and tidal force will have a far greater effect on a full size boats performance than a non contra-rotating prop.

Paul

Hi Ashley

You are quite correct, there were several MTBs and MGBs built during the War which had all the props turning in the same direction. One of the main reasons was a short supply of the reverse gear boxes. To cut a long story short, after the War, they did experiment with configuration of rotation of propellers – they found there was a slight improvement in speed with all the props turning in the same direction – but – only a slight improvement.

One of the most notorious vessels that Royal Air Force had built was the 68 ft RTTL It originally started off with 3 props but then was reduced and re-engine to have 2 props (all rotating in the same direction) and this vessel was renowned to have prop walk especially when diving off the top of a wave at speed. On numerous occasions, several of these vessels, split the hull at the bow. So, what Vosper tried to do for one of the fixes was add an extra layer of planking to the bottom and then they experimented with an aluminium hull.

If I can find the pics, they will follow

John

Edited By bluebird on 12/03/2017 19:01:10

Edited By bluebird on 12/03/2017 19:01:37

Edited By bluebird on 12/03/2017 19:02:04

Wise man. This article might be of use to you **LINK**

DM