Changing props to go slower

I'm after advice - this is a bit similar to the "downsizing engines" topic. Here in the UK I bought a 38' boat, an Aquastar 38, which satisfies our needs re size/airdraft etc but is a semi displacement boat. Top speed with a clean hull I can get 23.6 knots. It has twin Cummins 250hp 6BTA's which are the original engines from 1994. It has fuel pumps - Nippon Denso EP9's - which do not like to idle. I have considered various options - selling boat and getting different/re-engining/trolling valves/3rd small engine/electric pods - but changing the props seems to be least disruptive option. 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. Or alternatively a profesional I could pay to design the best prop option for me. The boat will idle at 8.5k on two engines, 4.5k on one engine, but the engines are not happy idling for long periods. We live on the NE Essex Coast, so 40 miles to the Broads, 70 miles to the Thames, or 130 miles to Holland. So I'm looking to achieve a cruising speed at sea of 12 - 13k (we're too old to be thrashing about any faster), but an inland waters speed of 4k. Any suggestions?

 

I’d work with a reputable local prop shop to take some pitch out of the existing props.

 

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.

 

Thanks for the replies. Current props are 22 x 28. When you engage in gear, at idle, you almost have to grab on to something to stop yourself flying off.

This is the boat in an advert before we bought it.

 

Maybe it might be easiest and simplest in the long run to take out the big Cummins engines, and install a new pair of engines that are perhaps half the power, if that will still give you 12 knots?
Although this would probably involve cutting a large aperture in the roof of the wheelhouse - or does it have a 'soft patch' area built in, in anticipation of having to take the engines out at some stage?

 

I've considered re-engining, which would involve creation of a hatch in the saloon roof to get the old out and new in. For a pair of 150hp Iveco or Vetus Deutz the engine costs plus installation costs could quite likely be 75k (English pounds) and almost exactly what I paid for the boat. New props would be about 7k, and so much simplier.

 

I think I would start with a Cummins dealer, and ask about better fuel pumps.

Certainly reducing pitch will give higher RPM for a given speed. But also maybe more fuel consumption.
What are the rpms for idle, cruise, and WOT?
-You may very well be substantially overpropped, in which case your props would be overloading the engines at idle. A condition they do not like at all.
Also, whatis the reduction in your gearbox? If you can't make close to rated max rpm for your engines, then you are definitely overpropped.

 

No history on the boat I take it?

Have you considered selling one engine and converting the other to a centre drive single?

OR

Removing one prop and running on just the other?
You could alternate engines between haulouts.


Reducing pitch and possibly even diameter if necessary would be a good place to start.

 

I have to assume the props were originally matched to the engines, unless a previous owner changed them.
Can you get full RPM or close to it?
If so you're not over propped, if you reach max RPM and still have throttle travel left you're under propped.
When you reach a point where the engine won't gain RPM and you still have throttle travel left you're over propped.
Trolling valves are for what the name suggests, trolling for game fish or moving thru a harbor.
Actually, for general "easy cruising" some over propping is ok, it's like having overdrive in a vehicle when you don't need to climb a hill, knowing that you don't have full rated HP available.
Then we have to ask ourselves a question; "How much time do I spend at full throttle"?
Under propping is like driving around in an indirect gear, higher RPM for less speed, nobody wants that.
Fuel burn is related to how much load the engine has.
It seems the boat has more power & speed than you want/need.
If so, changing props is a Band-Aid not a cure.
The 4BTA engines probably would have been a better fit.

 

It may be possible to detune your motors, I’ve seen 6B’s rated at 150 hp.
It would probably entail at least changing to a CAV pump and changing the injectors and lines, and a smaller or no turbo, and re engineering the propeller specs.
Tony Athens at Seaboard Marine would be the guy to talk to.
Or you could just spec the props to achieve the low speeds you target, and install a throttle stop at an appropriate rpm to prevent excessive eggbeater syndrome.
.

 

^That might prove to be an optimum solution.
It reminded me of the original 2 valve head 6-71 GMC engine, it started out with ~160 HP give-or-take, but when they stuck in big injectors, turbos, intercoolers, aftercoolers and changed the heads to 4 valve and the cam profile they ~doubled the power.
Of course, the de-rated engine would want smaller props, but that's to be expected.

 

It is not clear, what your "problem" or issue(s) is...?

 

Talking to Tony at Seaboard Marine is a great suggestion - he has been very helpful in the past (& ships super quick California to UK).

 

I really don't understand your comment. Please see my posts here for the perceived problem/issue.

 

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?