Testing Your Pulling Power. Model Boats July 2021. Phil Scalesi

[Chris EParticipant @chrise]

There is a very interesting article on tug bollard pull in the July 2021 Model Boats. The last page has a potentially very valuable guide to bollard pull from various propellers. Unfortunately it doesn't include details of the motor used, the rpm of the propeller nor the amps/watts consumed etc.

I don't know if Phil Scales (the magazine contributor) is on the forum but if he is then is there any chance of getting the extra information? If anybody is a fellow club member of Phil perhaps they would be kind enough to pass on the message.

This is potentially a very good extension of the Colin Bishop articles of a couple of months ago.

Edited By Chris E on 29/06/2021 10:42:46

Edited By Chris E on 29/06/2021 11:11:15

[Chris EParticipant @chrise]

[ashley needhamParticipant @ashleyneedham69188]

Good point Chris. The bollard pull and prop size/type verses the amps used would be useful to know.

Apart from a propeller enclosure (Kort, say) the shape of hull will or will not have a meaningful effect on the pull figure?.

Ashley

[Richard SimpsonModerator @richardsimpson88330]

Ashley, hull design plays a huge part in the flow through a propeller and hence its pushing power. This is precisely why a lot of ships now use pods because the propeller can sit in a much less restricted water flow.

Another popular recent development is to have little clearance between the propeller boss and the leading edge of the rudder and incorporate a fairing into the rudder to promote better flow through the propeller.

Ship design at the back end is always a compromise between wanting the squarest shape for the purposes of available internal space and the finest shape to enable the minimum restriction to water flow through the propellers.

Then you can start looking at the different efficiencies of Voith Schneider versus propellers and it soon becomes obvious that the measurement of amps against bollard pull can be misleading if you do not take all the other variables into account as well, which can soon get very complicated.

[Chris EParticipant @chrise]

How many of us use Voith Schneider drives to get confused? For the rest of us the data is largely transerable between models when going forward.

Going astern is a whole different ball game.

[Richard SimpsonModerator @richardsimpson88330]

Another interesting aspect of tug design is the fact that the designers are happy to sacrifice propeller efficiency for the sake of manoeuvrability. Most deep sea propellers are far more efficient than a tug propeller as fuel efficiency is of much greater concern over a long passage, which is why in the 70s and 80s controllable pitch propellers found favour with ferries and coastal vessels while deep sea ships tended to use solid propellers.

[Colin BishopModerator @colinbishop34627]

Presumably because deep sea vessels spend day after day just plugging along at a constant speed and their props (and machinery setup) can be optimised for this. Ferries and workboats need to be much more agile with constant changes of speed and direction. Horses for courses.

Colin

[Richard SimpsonModerator @richardsimpson88330]

Exactly Colin and controllable pitch propellers were really inefficient but extremely manoeuvrable. Having said that the first ships I sailed on in the 70s were on the Manchester to Montreal run and they were fitted with CPPs, although at that point in time, before the first major oil crisis fuel costs were not as big a concern.

Also to what you said above a propeller can only be made to its maximum efficiency at a specific number of revolutions.  Hence a deep sea propeller will be designed to be at its maximum at the normal speed of the ship, usually around 85 to 90% of full engine load.  As you move away from these revs the efficiency drops off.  Tug revs of course are continuously varying so another reason for inefficiency.

Edited By Richard Simpson on 29/06/2021 13:09:14

[Richard SimpsonModerator @richardsimpson88330]

Posted by Chris E on 29/06/2021 12:18:05:

How many of us use Voith Schneider drives to get confused? For the rest of us the data is largely transerable between models when going forward.

Going astern is a whole different ball game.

 

Well I know of two of them for a start at Kirklees and I'm sure some of the lads at Balne Moor use them as well so perhaps they are only rare in your area.  And, as mentioned above, hull design plays a big part as well.

As an example look at Model Boats Mag Winter Special 2019.  The free plan with that magazine is of a pusher tug by Steve Whitehead.  When he first built the model the performance was almost non existent and the boat hardly moved through the water.  Redesigning the back end by tapering it to allow flow over the propeller made a huge difference.

Edited By Richard Simpson on 29/06/2021 13:16:40

Edited By Richard Simpson on 29/06/2021 13:23:43

[Chris EParticipant @chrise]

Ok I know people with Voith Schneider drives and for that matter water jets. Neither invalidate the basic value of data from a given motor/ battery / prop combination and the vast majority of models I see use traditional drive systems..

The article referred to in the winter 2019 issue – Boston Lock – states that the "………the model performed well going ahead………". Whilst the design might not be optimal for propeller efficiency it is unlikely to be that much of a problem going ahead. As I said before going astern is a whole different subject. The design modifications were to address problems going astern.

Edited By Chris E on 29/06/2021 13:51:16

[Richard SimpsonModerator @richardsimpson88330]

Posted by Chris E on 29/06/2021 13:31:47:

Ok I know people with Voith Schneider drives and for that matter water jets. Neither invalidate the basic value of data from a given motor/ battery / prop combination and the vast majority of models I see use traditional drive systems..

The article referred to in the winter 2019 issue – Boston Lock – states that the "………the model performed well going ahead………". Whilst the design might not be optimal for propeller efficiency it is unlikely to be that much of a problem going ahead. As I said before going astern is a whole different subject. The design modifications were to address problems going astern.

Edited By Chris E on 29/06/2021 13:51:16

I wasn't quoting the article, I had a conversation with Steve when I bought the model off him and he told me the story.

It doesn't alter the fact that hull shape plays a big part in propeller efficiency. If you choose to ignore that then I'm sure you will be more than happy with the figures for amps.

[ashley needhamParticipant @ashleyneedham69188]

I always like to know the amps as it tells you how long the battery will last, and that you are not over-loading the motor.

For a bollard pull test, having more-or-less the same pull with two different setups BUT having wildly different amp consumption would be handy in choosing the less ampy combination for a very minor difference in pulling power.

We have some underwater motored craft here and when it’s nice I might try a bollard test with different props just to see the difference.

Ashley

[Richard SimpsonModerator @richardsimpson88330]

Completely agree Ashley, measuring on load amps is an excellent way of determining the effectiveness of a single change to a model such as propeller, motor, or even coupling.

Never heard of the term "Ampy" before, is that a new S.I. unit?

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