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Propellers

WOT revs slightly lower than expected

by sprocker » 26 Oct 2015, 11:03

Whilst out for a run on Sunday we had the opportunity to run the boat flat out (Not many of those opportunities have presented themselves this year....).

So running flat out (WOT) gave me 5400RPM, and around 25 knots. (I was expecting around 28/29 knots, but end of season dirty bum could possibly/probably account for lower speed than expected).

Checking the specs for our engine (2014 Suzuki DF115) gives us a 'recommended operating range' of 5000-6000RPM.

The prop is aluminium, although I don't know what size it is, and can check this next weekend, and has been on the boat since new in 2014.

We are out and out 'cruisers' and are not too bothered about high end speed, but being an engineer I like to meddle and make sure things are 'as should be'.

Do the numbers sound OK to you for a boat that is purely used for cruising? Or would you think it needs further investigation and maybe some fine tuning in the prop department? Could a different prop give us better fuel economy for our usual planing speed (16-18 knots @ 4000-4200 RPM)?
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by Ianfs » 26 Oct 2015, 23:06

When you say you were expecting 28knts flat out, what is that based on?

I've said this before, but propellers are still a black art. Although in recent years single prop outdrives and outboards have taken a different slant, with 4 & 5 blades coming on the market, with others having different rakes and cupping.

You can get good recommendations from people like Steel Developments, Streamlined Propellers or even specialists like Propeller Solutions. However unless someone will actually loan you several different types of propeller to try, it really becomes a suck it and see type of circumstance based on an informed decision.

I would say that if your working range for that engine is 5 - 6K revs that's quite a difference but I think there are others with the same broad rev range like Honda. At 5,400 rpm your engine should return about 7 galls per hour, if you increase revs to max at 6k consumption goes up to 8.75 gals per hour.

I think you are right about the dirty bottom but overall it sounds like your boat is performing OK. :)
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by sprocker » 27 Oct 2015, 07:38

Ianfs wrote:When you say you were expecting 28knts flat out, what is that based on?


Hi Ian,

I seem to remember getting around 28 knots on a very short burst during our initial trip when we brought her home from Plymouth to Torquay. Thinking about it she was newly antifouled at that point so I would expect better performance than now.

I agree that she seems to be running OK with the current setup, I guess its just the engineer in me looking for a winter project........
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by _Ed_ » 27 Oct 2015, 11:12

sprocker wrote: I guess its just the engineer in me looking for a winter project........


That will only end in trouble!!!
www.aboardmyboat.co.uk - boat projects and stuff!
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