prop rpm

In a new build I would expect the builder to have specified one RH and one LH transmission. Not all gearboxes take kindly to being run in reverse continually, although some don't mind it (Frosty's is evidently the latter variety). I would definitely expect the same gear ratio on each side and would be quite skeptical of a builder who installed differing ratios at the factory. As a retrofit, sure it might work, but it still doesn't strike me as sound engineering.

 

The book that came with the ZF explain this quite clearly. Borg warners for instance do not run in reverse.

http://www.simplicity-marine.com/hurth630.htm

They don't refer to rotation as reverse or forward but selection A or B

 

Jeembo
You will be able to look at these things if you run JarvaProp.

If she does 8kts at 800rpm on the prop then each diesel should be only putting out around 40HP. The prop efficiency goes up to almost 60%. It should be economic speed. Cannot find data on the Lehman 160HP but I expect it will be well within its torque curve. Should not smoke and fuel consumption around 2gph for each engine - maybe 3gph for tired older engines.

To get 8kts using one engine, revs go up to 2060rpm, prop efficiency drops off below 50% and power climbs to 94HP compared with 80HP combined running both. There would be some saving by not running auxiliaries on the second engine but this is likely offset by spinning the unpowered prop plus constant helm offset. Without the engine data it is not certain but 14HP over 80HP is a lot to make up through gain in engine efficiency. (My preference would be to keep both engines ticking over anyhow.)

You can do some trials on your delivery trip. Love to hear how it goes. Hope weather is kind and wish I was able to come along but am on the wrong side of the globe.

Rick W.

 

gentlemen - we hauled, scraped, and powerwashed the hull and re-ran the seatrial. It was covered in barnacles, especially the props. Got 2500/2350 rpm and 10 knots. The props don't match, but the surveyor thinks they are fine. Good running at 8 knots. I feel much better now. JeemboNC

 

Sounds all is good to go. Will be interested in fuel rate on your trip.

Rick W.

 

Rick - will do. I got pics for you of the props - if you tell me your email I'll send to you. JeemboNC

 

Jeembo
You can attach pictures to a post you make on this forum. I am sure others would find the detail on the props and hull of interest.

If you cannot, or do not wish to, do that then my email is:
rickwill@bigpond.net.au

I will look forward to the photos one way or the other.

You can use this address if you want to do exchanges on learning to use JavaProp. Now that you have a good example it will reinforce what you can get from JavaProp.

Rick W.

 

Prop size

Dear Mister
Please write this hull parameters we are going to calculate to find propeller size.
Hull Offset Plans or
Lwl ----------waterline length
Bwl ...............Waterline beam
Twl ...............Waterline draft
Displacement LT
Cx ................ Midship
Wetted surface ...............

Engine .....How Many
.......brand Name
Engine Revolutions .......
Transmission Ratio

and Picture of Vesells Bord side and Front
Best Regards
Oktay Çemberci

 

let's see - here they are. see some MAJOR barnacles?! I'll do a bottom job this spring.

 

Bottom

Dear Mister

Do you have its hull offset plans.
Oktay Çemberci

 

sorry. no, I do not

 

Oie. If the running gear is that badly fouled, no math is going to mean a darn thing.
Curious to see how it runs once Barnacle City has been wiped off the hull....

 

Cemberci - I am sorry - I just now saw you other email. Here is the info:


Hull Offset Plans or
Lwl ----------waterline length ~38
Bwl ...............Waterline beam ~14
Twl ...............Waterline draft ~3.5
Displacement LT ~27000#
Cx ................ Midship
Wetted surface ............... ? - guess 600 sq ft

Engine .....How Many 2
.......brand Name Ford Lehman Turbo 160HP at 2500 RPM
Engine Revolutions ....... 2500, I like to cruise at 1600
Transmission Ratio 2:1 and 1.9:1 for contra=rotating engine (port)

and Picture of Vesells Bord side and Front
Best Regards
Oktay Çemberci

 

Marshmat
You have not read the entire story. See the first post. It did 8kts fouled and 10kts "clean" as predicted assumung the props were matched to the engines originally.

Rick W.

 

Marchmat - yep, she was awful! We got 10 kts at 2500/2350 rpm and cruise at 8 @ ~1600. One engine WFO would get her to 8 kts, as well. The boat will never be a speed demon, but I did want a leisurely 8kts and reasonable power/economy. I was told to expect 3 gph by the previous owner, but I think it will be more (hopefully no more than 4) To get that I expect a nice fresh bottom will be required. At least we got the crud off her.

Did you notice the props had different blade profile? Kinda bugs me - but at least I can't see them...