Changing props to go slower

[Ad Hoc]

... Please see my posts here for the perceived problem/issue.

You mean this:

....What I'm looking for is a book which would describe in simple terms, given hull length/weight/engine power/revs, what to expect from different prop sizes/pitches. ...

Is this correct?

 

[kapnD]

I've looked at the manual for these Cummins - they are called CPL 1322 as there are many variations of the 6BTA's - they say "Idle Speed Rating RPM 650 -850", "High Idle Speed Range RPM 2770 - 2880" and "Rated engine speed Range RPM 2600". The boat is out of the water now but from memory she was idling at 900 (so quite a bit beyond the 650 -850) and I can't remember what WOT was, but guessing was about 2800 (we tried it a lot faster with the previous owner prior to purchase than we did cruising as owners last year), but not sure at all about cruising RPM. The gearboxes are Hurth HSW 800A2 (this is the same as a ZF but not sure of number - 80?) and noted as 2:1 reduction. I'm really asking is there any downside to reducing the pitch to simply lower the cruising and "slow" speed, accepting the fuel economy might be worse?

This is the absolute simplest solution, and you may well find that fuel economy improves when operating underloaded in spite of operating at higher rpms.
You should be able to idle at 650 in or out of gear, regardless of engine or prop mods.

 

[HelmutSheina]

I've looked at the manual for these Cummins - they are called CPL 1322 as there are many variations of the 6BTA's - they say "Idle Speed Rating RPM 650 -850", "High Idle Speed Range RPM 2770 - 2880" and "Rated engine speed Range RPM 2600". The boat is out of the water now but from memory she was idling at 900 (so quite a bit beyond the 650 -850) and I can't remember what WOT was, but guessing was about 2800 (we tried it a lot faster with the previous owner prior to purchase than we did cruising as owners last year), but not sure at all about cruising RPM. The gearboxes are Hurth HSW 800A2 (this is the same as a ZF but not sure of number - 80?) and noted as 2:1 reduction. I'm really asking is there any downside to reducing the pitch to simply lower the cruising and "slow" speed, accepting the fuel economy might be worse?

Have you spoken to an expert about the idle RPM? It may not fix your issue, but will be heading that way if you can reduce it by 200 or so.

 

[ianc1200]

You mean this:


Is this correct?

Yes, very much so.

 

[ianc1200]

Have you spoken to an expert about the idle RPM? It may not fix your issue, but will be heading that way if you can reduce it by 200 or so.

Another good point, I've asked Tony at Seaboard Marine (via his forum) about this.

 

[ianc1200]

This is the absolute simplest solution, and you may well find that fuel economy improves when operating underloaded in spite of operating at higher rpms.
You should be able to idle at 650 in or out of gear, regardless of engine or prop mods.

Thanks. Need to check this when we launch (protracted launch this year, bow and stern thrusters 80% done, but will be copper coated when warm dry/less windy weather arrives here on UK east coast).

 

[Ad Hoc]

Yes, very much so.

Ok..so it is not a problem with the boat per se, but your understanding of what is going on in terms of general hydrodynamics?
Is that correct?

 

[fallguy]

Ian

Please post the following per engine:

RPMs at idle in neutral
RPMs at forward engagement
RPMs at various cruising speeds say every 5 kts.

You probably have something wrong with the engines or drive system. Yes, the pitch might be high, but the tossing you over is probably another problem.

If your engines got turned up to deal with another problem; the idea they don’t ‘like’ to idle low is probably root cause unless you mean it as a character to the engines.

What we want to see is rpm drop on engagement. If it doesn’t you are addressing an engine problem with the props.

 

[HelmutSheina]

Another good point, I've asked Tony at Seaboard Marine (via his forum) about this.

It would at least give a better chance of modifying current propellers to achieve requirements.

 

[ianc1200]

For general reference re the Aquastar 38, here are a couple for sale in England.
Aquastar 38 boats for sale UK, used Aquastar boats, new Aquastar boat sales, free photo ads - Apollo Duck https://www.apolloduck.co.uk/boats/aquastar/38

Re propellers, the Propeller Handbook by Dave Gerr is generally regarded as a good bible for all things to do with props -
Amazon.com https://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/087742988X/ref=tmm_hrd_swatch_0

Do you know what size / type of propellers you have currently?
You would go slower for the same revs if you reduce the pitch of the propellers. You do not want to be too drastic here though, otherwise you might end up with egg beaters thrashing away and the boat going very slowly - and then she might not even make 12 - 13 knots.

Hi Bajansailor - just wanted to say thanks for this recommendation. It looks exactly what I need. It will need quite a bit of effort on my part to read & try to understand the concepts but it's going to be very worthwhile I believe. thanks again.

 

[ianc1200]

Ian

Please post the following per engine:

RPMs at idle in neutral
RPMs at forward engagement
RPMs at various cruising speeds say every 5 kts.

You probably have something wrong with the engines or drive system. Yes, the pitch might be high, but the tossing you over is probably another problem.

If your engines got turned up to deal with another problem; the idea they don’t ‘like’ to idle low is probably root cause unless you mean it as a character to the engines.

What we want to see is rpm drop on engagement. If it doesn’t you are addressing an engine problem with the props.

Thanks for the reply, but the boat isn't in the water at present & I can only repeat the information in the manual. However I do believe the system is working properly, it's just this boat and it's engines are set up to go faster than we want. I had a mechanical survey, the engineer doing doing this did sea trials & also both engines had oil analysis (engines and gearboxes). From the expert Tony at Seaboard Marine he says the fuel pumps, Nippon Denso EP9's, are very reliable, but industrial/agricultural, and don't idle easily. It really is my fault for buying the boat but I'm just looking at my alternatives now.

Edit - Tony Athens at Seaboard Marine has sent me details about how to adjust the idle on the pumps (but I don't think this is a great idea as they really sound unhappy at the current idle and would be worried going for a lower idle) but said reducing prop pitch by 3 - 4" is good. So I guess that's what I'll do eventually.

 

[kapnD]

To do any mods to props or motors, you first need to be sure that the motors are performing to spec.
I would put some energy into the rough idle problem first.
You are glossing over the fact that the idle is significantly higher than spec, and that is obviously affecting the ability to go slow.
Do some easy testing and observations.
Injector break test, WOT in and out of gear, smoke color and quantity?
Fuel delivery system, vacuum readings?
Air intake side restriction?
Chart performance over entire rpm range?

 

[philSweet]

Given the age, there is no telling what parts the engines actually have on them. I would attack the idle issue first. Get your CPL number and parts list for those engines. Pull some fuel injectors, figure out if it's all Bosch or has Cummins nozzles on it, inspect it, and get a fuel pressure reading on the engine to compare to specs. That motor was pushed by Cummins to over 315hp using non-Bosch injector nozzles (later versions went to 370hp). Getting CPL-correct or derated nozzles and decent fuel pressure at idle is relatively cheap and easy, and there isn't a lot of mystery to it, just a PITA to find the parts for a derated 6BTA. May require the Bosch <edit - Denso> pump to be recalibrated - nontrivial, but it's kinda important. You can't do much about the boat's performance until the engines are working properly. If it has been sitting, there are probably fuel supply issues to deal with. Replace everything rubber that you can access and replace. Remove and clean fuel pipes. You'll probably find all manner of of stuff that isn't good while you are doing this.

 

[fallguy]

I cannot offer 1% of what Phil just did, but you don’t correct a clear engine issue with props. Take his list and chase it.

If the boat still throws you on gear engagement, verify rpm drop or you also may have a governor issue. It seems to me the engines have been over tinkered with…

 

[ianc1200]

Thanks for the replies fallguy/philsweet/kapnD but just to re-iterate these engines do not like to idle - this is what I've seen on the water and the opinion of the expert for these engines (and in particular these fuelpumps) says. The engine is set up to run at a certain revs and not below. This is (for me) a downside of these fuel pumps, Nippon Denso EP9's, for others an advantage.