Measuring Engine “Thrust” or Pressure

[Ray WatsonParticipant @raywatson57390]

I am having fun setting up a scale model, so I can extrapolate a full size vessel performance.

http://schoolroad.weebly.com/rpgmodel_11.html

I wonder if anyone else has tried any experiments along those lines ?

I have canvassed a few ideas here

http://www.sciencechatforum.com/viewtopic.php?f=2&t=27715

[Ray WatsonParticipant @raywatson57390]

Measuring motor power used

[ashley needhamParticipant @ashleyneedham69188]

No, is the likely answer. Have seen various attempts at trying to get some sort of table together but there are so many different factors and variables involved we all just argue…..

Measuring thrust is tricky, as static "bollard" thrust (ie a boat attached to a spring balance and put on full throttle to see how much pull there is stationary) is different to top speed thrust..as this has other factors working on it. Water flow around the hull and of course actual water flow around the prop once moving. Kort nozzles would give much more thrust at low speed than they would at high speed, and surface peircing designs do not work at all until the boat is going very quickly (but then generate massive thrust).

The navy of course had Mr Freud to thank for this sort of thing, having made founsands of scale models and wornot, apparently it was almost a matter of deciding length and breadth of boat, installed power and displacement and then looking all this up in the tables and simply choosing the best hull shape to suit!

I know this is not helpful, but…

Ashley

[Colin BishopModerator @colinbishop34627]

Bit of a Froudeian slip there Ashley….

[ashley needhamParticipant @ashleyneedham69188]

Whoops!!

[Gareth JonesParticipant @garethjones79649]

Ray,

I have measured static propellor thrust or bollard pull as it is sometimes known, using a digital fishermans scale. The scale I use is a Berkley unit that measures up to 15 lb/7kg and typical thrust for a range of models is from 500 g up to about 3 kg.

Normally I tether the boat to the scale using about 1 metre of line, usually with the scale attached to a suitable point on the pond bank with the hook overhanging the edge as shown in the picture below.

As Ashley says, static thrust does not necessarily give the whole story on propulsion performance but it is a repeatable, measurable number that helps build up understanding of what is going on. For a tug, where low speed pulling perormance is important, the static bollard pull is a very good indication of the model's performance. For a high speed planing model or racing boat it's less easy to relate static thrust to the real running cases (believe me I have been there, debating the installed, in flight performance of an aircraft engine compared with the static thrust on the test bed). However it is a useful parameter to measure and it helps to understand the effect of changing propeller type or size and trying to estimate the optimum propulsion system performance and efficiency. How you then relate that to a full size boat I have no idea and have never tried to do it, but give it a go and let us know how you get on.

Gareth

 

Edited By Gareth Jones on 17/09/2014 12:22:51

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