The Right Prop.....

[Troubadour]

Help!!!!

Gentlemen,

Newbie Trawler guy here..... we have a purchase agreement for a 42' 1986 CHB Sundeck Trawler. Surveys and sea trial were performed yesterday. Main survey came back fairly clean....typical stuff for an older boat. The engine survey (twin Perkins 6354T's 200HP turning 27x22 props) came back fairly clean also except for props need replacing due to Galvanic Corrosion. PO has replaced combo heat exchangers and exhaust systems. The "fly in the ointment" is that the engines at FT will not spin up to the rated 2800rpms. They would only muster 1800 (both engines). The mechanic (which came highly recommended by multiple sources) suggests the props are causing the lugging and that replacement with the correct size prop would bring the rpms back into spec. He advised that some owners feel that by increasing the size of the prop and running at lower rpm's one would see an increase in fuel efficiency. He says this is a fallacy and that lugging the engines will actually increase fuel consumption and could cause damage to the engines in the long run. What would be your recommendations? Sounds like a lot of rpms to gain off just changing props. I'm a lifelong blowboater trying to convert to a Trawler so my knowledge of large systems like this is limited. Thanks

 

[Brooksie]

Unless there are other factors like severe bottom or prop fouling, gross tach error/s, plugged intakes, and so on; I think your mechanic is correct. Further, if these engines were run by the PO at WOT under these conditions there may be some damage as a result. If I ran your specs through my prop calculator or if someone with a similar vessel & power gave his prop/gear ratio specs, then you would know how far off these props are or if your problems are elsewhere.
Lugged down to 1800 @ WOT, you should have seen a lot of black smoke, did you? Do you know the weight and transmission ratios?

 

[sunchaser]

Smart surveyor Id say.

The curse of over propping is the next buyer - in this case you - is in a quandry as to what to do. Yes the boat needs to be able to demonstrate full RPM under load (verified by phototac) without an overheat or lots of smoke. It is the responsibity of the seller to make the props right since they are shot, but with what size?

The simplest thing to do is request the owner replace the props and insure the engines can operate as designed at full load RPM. If he is unwilling to do it then assume the owner knows of a latent engine or exhaust system problem and walk.

BTW, do these engines have manicoolers and if so how old? If originals watch out!

For lots of good feedback on same question post on boatdiesel.com. Be aware, you will have to pay $25 to join.

 

[Troubadour]

Bottom and drive train were power washed and scraped at haul out. Sea trial followed bottom inspection so we had a clean bottom for the sea trial. I do not know the trans ratio but do know they are velvet drives. Displacement is ~34Klbs dry...unfortunately I was below with the mechanic and did not see if there was any black smoke. Should have reports in hand later today. I called Admiral Propeller in St Pete and they were asking the same type questions. They also inquired about the props being cupped.... Said they would need to do a field visit to the boat to be able to give me a recommendation and quote.

 

[Marin]

There is so much theory and opinion floating around about props, the only way I would go after credible information about them is to go to a reputable prop shop and ask there. That's what we did when we were told our props were "worn out" and needed replacement.

After listening and reading a lot of amateur theories and opinions, much of it directly contradictory, we asked people we know in the marine industry here who they thought was the best prop shop in the area. When we kept getting the same answer, we talked to that shop and the end result was an outstanding, no-nonsense solution for us which has proven to work exactly as advertised.

So my firm opinion on the subject of props is to go straight to the pros.

 

[Troubadour]

Sunchaser.....the manicoolers were replaced a few years ago. The mechanic was impressed that both manicoolers and full exhaust sytems were replaced as well as other visible replacements and upgrades. Engines cranked over with no hesitation and ran fine until we went to WOT.

 

[Underway]

If you have the specs on the boat, engines and transmissions, this calculator will give you an idea of what should be on the boat. It's spot on for our OA. I'd also try to speak with the owner to see if he understands the concept of overpropping and the reduced power lever settings that go with it. If he does, and he was the guy who overpropped it in the first place, then you can make a better determination.

Vicprop - Propeller Calculator

 

[Troubadour]

Marin....Roger that! The props are visibly deteriorated...edges sooo thin you could shave with them. Surveyor also said they did not "ring" correctly when tapped with hammer which would indicate Galvanic Corrosion.

 

[Troubadour]

Thanks skidgear..will check it out...I would love to speak with the PO but he has passed away and the Estate Agent (BANK) and broker that i'm dealing with have no info on the boat.

 

[Troubadour]

I used the VIC Prop calculator and it suggests we look at 22x17 4-blade props. It asked for gear ratio and I entered 2:1...hope that is right. Seems like a big difference from 27x22 4-blades.

 

[sunchaser]

Troubador

It appears the vessel was propped intentionally to achieve no more than 35% or so power. The remaining 65% when called for by moving the throttles up is where the problems, if any, will show up. If you are willing to pay for the right props, do a sea trial and something then goes amiss, who pays to fix the amiss? The amiss may not be chump change.

Not to forget, a survey is done to see if the problems that arise are quantifiable so buyer and seller can move forward. It is far more than a mere re-propping IMHO, it is a business decision via risk assessment.

Good luck, you sound like a smart guy and will figure it out.

 

[Underway]

I used the VIC Prop calculator and it suggests we look at 22x17 4-blade props. It asked for gear ratio and I entered 2:1...hope that is right. Seems like a big difference from 27x22 4-blades.

Sounds ballpark...our 30,000 pound OA with twin 250s, 3000 redline, and 2:1 ratio has 24x20's and it makes max rpm plus a tad @ 18 knots. (three blade props). The survey should show the trans reduction ratio....

 

[Nomad Willy]

Not many trawlers need the blade area of a 4 blade prop.

Usually just those that have limited prop clearance.

 

[ianonhercules]

I believe you need to know the reduction ratio of the transmissions before you can enter the data into the calculator. Guessing is not good. I have experience with the previous generation of engines and their WOT RPM is 2400. The 2800 is the no load RPM, I believe. With the original engine, with a 1.91:1 reduction the prop was a 26 X 17 - 3 blade, slightly over the ideal pitch. The replacement engine had a reduction of 1:2.54:1 and the prop was 26 X 22 - 4 blade. It too was slightly overpitched. However, you can see the significant difference that the reduction ratio makes. (The reason I did nothing about the pitch was because I am a cheap Scotsman and I believed that the difference was insignificant.) I may be wrong on the max RPM as your engines are 200 HP and mine were 160 HP but since the Range 4 is essentially the same engine with a different turbo and injectors and a different intercooler and exhaust/manifold arrangement I be very surprised if the WOT RPM were different.

Ian
Seattle

 

[sunchaser]

I believe you need to know the reduction ratio of the transmissions before you can enter the data into the calculator. Guessing is not good. I be very surprised if the WOT RPM were different. Ian Seattle

I have a Perkins Sabre book in front of me. For these engines the M225/215 are rated at 2500 RPM and the M185 at 2100 RPM. So do use the book to insure you are using the correct full load RPM for the specific engine model in question

 

[Troubadour]

Thanks for all the info.....I believe I'm going to pitch the problem back into their court and wait to see if they bite....if not, there are plenty more out there to consider. LOL

 

[Mule]

Not many trawlers need the blade area of a 4 blade prop.

Usually just those that have limited prop clearance.

The way it was explained to me was for every blade added the propulsion got smoother. Other than that, a 3 blade would perform as efficiently as a 4. I also understand size and pitch are a factor. A big slow moving wheel is more efficient??? 3 or 4 to 1 with 30 inch with less pitch vs 24in with more pitch?? ??? So many questions about this witch craft.

 

[FF]

Other than that, a 3 blade would perform as efficiently as a 4.

Not really the 3 blade will have less induced drag than the 4 blade , a 2 blade would be even better IF it had the blade area to match the thrust required.

A big slow moving wheel is more efficient?

Yup, most ships operate at 100 or so engine and shaft RPM..

For small boats much below 500 shaft RPM does not help efficiency very much.

The cost of large diameter and slow RPM is frequently too much draft for the boat .

 

[Mule]

And perhaps too much draft to the bank account

 

[Mule]

I have a 38 Present, 135 FordL full disp. Tranney 2.57 to 1 waterline 36' (guess), 12 ft beam. 2650 out of gear max rpm. 2500 max loaded. (Book numbers). Displacement 26,000 dry likely 30,000 fully loaded or more. I have not scoped RPM and accurized my tachs I have a spare prop from my other boat that is 21x21 4 blade that will need a rebore from 1.5 to 1.75. Would like to have a good spare. Any thought on size and pitch? I do not know current prop and pitch of 4 blade. PO does not know and I have done all but dive, pull the prop or put er on the hard. Thoughts?