Reduction ratio

[dvm004]

Hello, new member with a question about choosing a reduction ratio for a zf 220a for a cummins 6bt. My son and I are repowering a 66’ webber cove 34’ lobster boat that weighs about 16k lbs. She cracked a head on the old volvo penta tamd40 and we pulled it and junked it. We took our time and built a 6bt, with mild p-pump changes. mild 6bt, no aftercooler, 230/240hp engine.

The 220a down angle matches perfectly to our 10 degree shaft currently in the boat. I realize this trans is on larger size for this engine setup but we will fish her about 2500 hours per year.

The shaft will be replaced with the current propeller. I am limited to a 1.5” shaft and currently spinning a 19 x 15 , 4 blade prop. I will be limited to about a 20” max prop due to the hull. If I use the nomogram and run my line from 240hp through 20” prop my propeller shaft rpms are about 2500. With this number divided by 2500rpms for the 6bt turning 240hp I am thinking a 1:1 reduction ratio. Does this make sense to others more experienced than myself and my son?

Thanks for the great site, we will keep reading and learning. Mounting the trans first and the engine second is our gameplan as suggested on the site and I will need to order the 220a mounts next. Appreciate the help.

Eric

Topic

[dvm004]

Hi Rob, yes the shaft slid back from the original location. The nice wrap of braided line, looks like 50lb jbraid, is closer to where shaft sat. I will watch my clearances for the exhaust, the front of the volvo sat significantly higher than the trans due to the in line install. I am hoping to remedy this with the offset zf. This is a pic with volvo/velvet drive gone.

[Jamie flood]

Prop calculation

Hi, I’m also a new member came across your question, to my knowledge when changing the output ratio of any gearbox weather marine, car, rig even motorcycle you are putting engines performance and reliability at risk by it over revving due to not having enough load decreasing fuel economy and wasting horsepower or the opposite effect of having to much load resulting in engine working unnecessarily hard increasing risk of over heating and over fuelling resulting in exhaust fumes being smokie and in time coking up injector tips stopping fuel being atomised properly increasing temperature fuel burns when ignited this can burn a hole in your piston. The prop is extremely important and must be correct if not it’s pointless doing all the work as you will not get the optimum performance from your engine!!

Once you know the ratio output to shaft that along with boats weight, hull type, engine horsepower will need to be entered into a calculation software that works out what size prop and pitch you need for performance and economy, don’t guess not worth the risk even if you end up paying more for props than you’ve allowed for you’ll save money in the long run in fuel and reliability

[Rob Schepis]

That’s quite the overhang there —- based on the clean spot forward on the shaft and the line wrapped around it I have to assume that the shaft was disconnected and slid back in that pic?

As to the deck height – do not cut yourself short as to the needed engine room height for a safe exhaust that will need to “ride” above the height of the engine. Plenty of exhaust design articles on the website here for you to read through. Especially important for an offshore fishing boat.

[dvm004]

Thank you for the reply.

1. Understood, this is why we are asking the question. We are trying to understand the relationship between reduction ratios and desired rpms vs prop size and how they all interact. We will continue reading and researching.

2. Our 6bt model number is 44661056 and was out of a 91 diesel pusher. The long block has been rebuilt stock and we have yet to purchase our exhaust manifold, turbo or accessories. The p-pump has been rebuilt to stock as well with a slight air fuel shift for the additional hp changes. This can easily put back to the 190hp that is stock on this setup. The p-pump numbering is 391-9090.

3. The prop that is currently on the boat is a 19 x 15 prop and it is being replaced with the current shaft. The webber cove boats were always limited with the prop size and a 20” would be close to the largest that could fit. This boat is a 1966 and most sections are being completely replaced. The boat yard that is taking the boat this winter is replacing the decks, cabin, windows, and doing the shaft work. They asked if we could pull the old engine and place the new so they have the finished height of decking to build hatches for. The volvo was standard inline drive and rose up into the cabin and covered by an engine box. With using a down angle zf transmission we are hopeful to have the 6bt back to deck height.

The enclosed pictures are from our boat before we pulled the volvo short block. There are 50 gallon fuel tanks port and starboard of the volvo. The second photo is of the current prop and 1.5” shaft we have disconnected the shaft from the old velvet drive before removal but the shaft is still in the boat.

Appreciate the assistance.

[Tony Athens]

#1——————For 100% sure you do not want anything close to a 1:1 ratio.. Don’t know where you are receiving guidance or input up to now on that, but it its totally bogus and coming from someone with no clue.. Let’s get past that.

#2———————Let’s get the exact model of the exact 6BT you have.. Best to post a serial number, Then tell me the history of the engine—New, rebuilt, RTO ( from a truck) you bought from ???,, etc etc—need it all..

Explain all of this & be verbose and to how this came to be ( kinda souds like a truck engine)?? :

We took our time and built a 6bt, with mild p-pump changes. mild 6bt, no aftercooler, 230/240hp engine.

#3——– You said you had a 19 x 15 4B prop.. You also said you have room for a 20″ prop………….So, are you willing to buy a new prop or ? Have you had the current prop scanned as to quality/condition, etc? Without that, you really have no clue other than maybe it’s no better that a “flat tire”…

#4———————————–You came here for answers—I will give them to you and not try to sell you anything—You help me and I’ll help you .. Good pics of what you have as to your boat and your “finished engine” would help me immensely..

Tony

[Author]

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