Over propped? B28

[scenarioL113]

Hi guys

This is a few videos of my B28 with Twin Cummins 4BT 150 HP. ZF45a gears 1.5:1. 18x18 3 blade.

I believe I am slightly overpropped. Tony Athans wants me to get 2900 rpm or so out of my baby 4BT's. I am struggling to get passed 2600 rpm. I did hit 2700 rpm today but I just cant get to where we want to be. Mark @ LI propeller was the last one to balance them and he played with them a little bit but I still think we are over propped.

I want to give them back to him in the fall but just want you guys to take a peek at these videos and see if you agree.

Tony at SBR marine services suggested to take some off the pitch. (maybe go 18X17)????

Bob Lico????
Anyone


Thanks a ton guys



https://www.youtube.com/watch?v=r6AYfA9p9gg


https://www.youtube.com/watch?v=7SgWfbDqkq4

[MarkD]

Frank:

I think you said 17 knots on the second video. What do you typically get for cruise at typical cruise and WOT - also ,what is the fuel burn.

Thanks!

[Navatech]

Tony Athans wants me to get 2900 rpm or so out of my baby 4BT's.

Tony Athens has a very good reputation... I'm assuming the 2900 RPM comes out of the manual?!...

I am struggling to get passed 2600 rpm. I did hit 2700 rpm today but I just cant get to where we want to be. Mark @ LI propeller was the last one to balance them and he played with them a little bit but I still think we are over propped.

If Tony's number is correct then you ARE over propped... Assuming you have a clean bottom and you're not overloaded with people and/or stuff...

Tony at SBR marine services suggested to take some off the pitch.

Sounds like a plan... Assuming the number is out of the manual and the boat has a clean bottom and is loaded normally...

[Tony Meola]

Try reducing pitch but you may need to add cup to offset some of the loss of pitch. Taking off some pitch will cause your speed to drop. The cup will bring the speed back up. You may need to go to 16 inch pitch. See if you can borrow a pair of 18 x 16.

[scenarioL113]

Frank:

I think you said 17 knots on the second video. What do you typically get for cruise at typical cruise and WOT - also ,what is the fuel burn.

Thanks!

Well my calculations are approximate but I have traveled about 120 miles since last fillup and I have used about 60 gallons.
That is a mix of all speeds but mostly 2400rpms and above. from lots of troubleshooting. I had a few gremlins in the fuel system and I was running the boat on a lot of test runs. Ended up being a vacuum side air leak.

That is pretty close to what the specs from Cummins say she should run. Lets say 10GPH @ 17.5 knots (20MPH) "TOTAL" Consumption.

I cruise between 2200 and 2400 depending if I am in a rush, hehe, 2200 gets me like 14 knots and 2400 gets me about 17.5 knots.

WOT I get just about 23 knots (just a touch under) @ 2650-2700RPM I am not getting 2800 like I should....actually should be getting 2900.

[Yannis]

I noticed black smoke in the first video while accelerating. Could it be that your injectors need some sort of servicing?

Also, I always close ALL windows and most importantly the front hatch while traveling. You never know how quickly the conditions change and suddenly you're in trouble.

I liked that women behave the same in all countries while men "work" at the helm !

Enjoy !

[Preston Burrows]

You don't mention trim tabs, hull bottom condition or hull loading/weights..........nor transmission gear ratio.

Video shows pax all astern and wake suggests not being trimmed out..........from my experience hump 'speed' for our B28's is at around 23-24 knots at which they level out moreso.

At typical hull planing / on step speed,around 14 knots or so, to there it's a question of ensuring clean bottom, tabs and weight distribution is such that you are not digging in astern...........the 'hyena' effect shall we call it..........

Which is a very roundabout way of saying make sure you have a clean bottom and that your weight and balance is 'good'..........or at least such that you are not stern heavy.

An immediately easy comparison would to be testing your hole shot again with all pax on the bow and see how that affects your current numbers.

[scenarioL113]

I did mention the gear ratio but will again it is 1.5:1 reduction.

My hull bottom is good. I have no difference in performance now than when first put in the water.

I do have trim tabs and I have not seen much difference in performance using them. If I trim them down too much I am plowing and lose about 1/2 knot @ cruise (about 17 knots). I actually saw best performance with the tabs slightly trimmed down and I mean slightly and saw about .2 increase in knots.

I may have to tinker a bit more with trimming but I just play with the tabs till I get the best speed out of said RPM. I am no expert Captain but that has seemed to work.

I am not sure how the wake should look if you think I am not trimmed out enough. I would take any suggestions.

@yannis I think the black smoke on the holeshot is normal on these old school non-emission B-series. I could be wrong though, it is the only pair I have owned. The smoke clears within seconds and nice and clear when cruising, just the normal ever so slight steam coming up from the wake.

You are right about front hatch. I dont leave like that off shore. I am operating in south bay of Long Island and dead calm.

Yes the women are the same....lol

[Navatech]

I noticed black smoke in the first video while accelerating. Could it be that your injectors need some sort of servicing?

Yannis, it take time for the turbos to spin up and start pushing the air... It even has a name - "turbo lag"... So some smoke for a second or two after throttling up somewhat aggressively is only natural...

I liked that women behave the same in all countries while men "work" at the helm !

There's not that many women I'd trust on my helm... Not that many guys either...

[Yannis]

Ha.
In the open sea, when its calm, I like to hand over the helm for a while and...feel like an invitee!
I'll get to check this and that, fix a drink, wonder around as if it's somebody elses boat.
But the conditions have to be right and the occasional captain be sober.
With a woman I haven't tried it yet. I mean let her drive!

[Carl]

Provided motors are running as they should...your throttle cable goes to max pull, and bottom is clean...yes you are over propped as your not hitting WOT by 200+.


Remove an inch should get you to 2900.
Removing 2" and add or increase cup will more then likely get you to 2900+. Cup should give you a bit of top speed back.
But that is rules of thumb...

Personally, in situations like this, I like working with knowledgeable prop shops that take all your info...see the wheels your running and make suggestions based on that information...not just rules of thumb.

If you have an option of borrowing a set of props to save coin on prop work, its not a bad idea. Although different brands and styles of wheels can have very different performances.

[scenarioL113]

Thanks to all for great advice and opinion.

I owned my B28 since 2000 and I set this boat up with the Diesel repower in 2009 and after 2011 she has sat in dry dock. I ran the engines every spring and fall and made sure they were fogged every winter. June 2016 is first time back in water and I have had my share of cobwebs in need of clearing to get her back in mid season form.

Back in 2010 I knew that I was a bit overpropped bc I was getting similar results as I am this year. When I had the props redone after the 2010 season it was a trial experiement with what was done. I just dont remember exactly what was done, maybe a little cup and some off but the end result is basically the same and actually a possible loss of 75 - 100 rpms unfortunately. The props sat in my garage until June of 2016 bc the boats been dry since then. Thats why my memory is foggy.

Now, 2016, I had been thinking the prob way laying elsewhere like in the fuel systems possibly bc she was sitting, i.e. old fuel? but I have put some hours on her and have gone thru the fuel system and all is good as far as I am concerned. I am happy with the engines performance with exception of the rpm and I just have to concede that the prob lays with overpropping. It is also possible the prop guy misunderstood what I needed on the props. I trust my prop guy and he is reputable.

The boat is not heavy and bottom is clean.

Both engines are running spot on identical and my gut tells me they are fine.

Back to the prop shop next month

Frank V

[Carl]

LOL...think you answered your own question.

I thought I had understood a diesel has a no load test to verify motor cranked up to a specified rpm as part of the propping process.


LI Prop is used by a few of my customers and they have been very happy with them....and I should mention very picky customers.