Advice on new boat buy please/ underpropped?

[jointcustody]

Hey there,

I currently own a 34 chb. 30ft waterline. 11'10'' beam. single m135 2003 perkins. 2.0:1 borg warner. Around 20,000 lbs. I get 2600? or 2800rpm wide open (havent in awhile, but i know for a fact its propped correctly because i have checked this before) it cruises great and i get around 6 knots at 1400, 7.5 at 1800, 8.5 at 2200rpm sort of thing. Very happy with the setup on this boat.

Today I sea trialed a 41 ft trawler. 37 ft waterline. 13'10'' beam. twin 135 6.354 perkins. survey claims 2.5:1 borg warners. between 30 and 35,000lbs. survey says prop is 24x17 3 blades (x2) The boat seemed very slow- noticeably slower than my boat, yet with a much longer waterline. The owner ran the the boat at 2200 rpm- and it was seeing like 7.0 knots. I verified the speed with my phone secretly lol.

I ran my numbers for my boat in the vic prop calculator and the prop was correct per my survey so i know im doing the numbers right. When I input the numbers for the sea trialed 41 I get 24x19.5- 3 blade as the suggested size, yet hes got 24x17's.

Both boats have recently cleaned and painted bottoms.

Does that sound like enough of a pitch difference to cause it to be that slow? I would think its at least 4 degrees or more out.
Do you see any bad effects on that engine running that fast but with a light load on it? owner claimed 2.5gph total at 7 knots.
What are your thoughts? Thanks.

 

[AusCan]

Your CHB 34 has a semi-displacement hull. It will lift up out of the water to a partial plane allowing increased speed.
It sounds like the 41 ft trawler has a full displacement hull. The hull shape is not designed to lift the bow. Maximum speed on it would be about 8 knots regardless of engine size, revs, or prop pitch. An FD hull travels through the water, not over the water. Stability in rough seas is usually better, but speed is limited by the hull shape.

 

[Flatswing]

Did the Perkins reach spec rpm @ WOT during the sea trial? If so, we’re the rpm’s confirmed with an optical sensor?

 

[ranger58sb]

Today I sea trialed a 41 ft trawler. 37 ft waterline. 13'10'' beam. twin 135 6.354 perkins. survey claims 2.5:1 borg warners. between 30 and 35,000lbs. survey says prop is 24x17 3 blades (x2) The boat seemed very slow- noticeably slower than my boat, yet with a much longer waterline. The owner ran the the boat at 2200 rpm- and it was seeing like 7.0 knots. I verified the speed with my phone secretly lol.

Ignore boat speed for a bit. What is rated WOT for those engine? What WOT did they reach on the sea trial?

-Chris

 

[O C Diver]

It's unclear from your original post as to whether you want more speed or 7 knots at a lower RPM.

I don't think you're going to get much more speed efficiently as has been mentioned above, as it's likely more of a displacement hull with a lot more weight.

It may turn rated RPM and only gain slightly more speed. In essence, the propellers may be cavitating quite a bit at the higher RPM. This is essentially what my boat bid. The last 500 RPM above 2,000 made no speed difference as the stern dug a hole trying to climb over the bow wave.

If it were me, I would seek an understanding from the owner as to whether it will reach rated RPM and if he knows why it's pitched the way it is.

If you choose to buy the boat and want to cruise it at 7 knots, I would see if a torque curve is available for that model of engine. I would prefer to operate the vessel at peak torque (maybe 1,800 RPM) at normal cruise. While you may see some minor consumption improvement, noise level and wear and tear would be my reason for doing it.

Ted

 

[jointcustody]

I reached out to a friend who has a 38 Ocean Alexander with twin 135 lehmans. He is running 24x20 props and gets excellent numbers at 1400 and 1800rpms. Those were the same props the vic prop calculator recommended.

I am definitely leaning towards it being propped incorrectly, as the hull speed is about 8.25 knots. im not even sure from what i saw yesterday that the boat could reach hull speed, Let alone faster than hull speed at WOT.

The sea trial was a sort of handshake ride, as there is a current "handshake to buy" deal with another couple that expires in two days, but it appears they won't be buying it. Some things the owner stated "I have never revved the engine wide open" "I would think it might be around 2900 rpms" All things that suggest its underpropped. I do really wish I would have pushed him to open it up to WOT. By the tone of his voice though that would have been a very taboo thing to do lol.

Does anyone have a torque curve chart for this engine? I can't find it anywhere. I have read conflicting information regarding what proper rpm it should reach. my m135 perkins i believe is 2600 rpm, its supposedly the new version of a 6.354 but I have seen forums where people state the 6.354 is 2800

Ted, Ideally I am looking for 1800rpm around 8 knots. I actually would prefer if they were slightly overpropped- say 50rpm or so, as they will never be pushed beyond hull speed anyhow- and so kept lightly loaded. And yes, in the salon at the current cruise speed, it was louder than it needed to be for sure.

Thanks

 

[sunchaser]

Two things. Firstly, use a photo tach as too many engines do not have reliable tachs unless they’re newer digitals. Secondly, pay little heed to waterline length guesses and the fallible 1.4 multiplier X sq rootWL. I’d guess the 41 footer to have no more than a 1.2 multiplier if it indeed is FD.

As previously suggested insure full RPM test is done to assess current propping and engine health. Then, once the correct props are in place you can do a fuel burn vs RPM vs boat speed to establish your desired cruising regimen.

 

[DavidM]

Notwithstanding your sellers squeamishness about running at wot, and for whatever it is worth running wide open for a few minutes won’t hurt a thing, it gives two important pieces of info in buying a boat:

One, it tells you if the cooling system is compromised. If while running at 2000 rpm or so with the temp running 180-200, you open it up to wot and the temp rapidly rises to 220+, you know you have restricted r/w flow, fouling or maybe a problem with the coolant system.

WOT rpm’s also tell you if you are overpropped or under. But you first have to do a quick check of wot at the dock, to make sure the throttle stop is correct and not restricting rpm’s.

I prefer to prop to 50-100 rpm over rated to have some margin for prop or bottom fouling.

But it sounds like your seller is nervous about running at wot, so it is probably best to do it with a surveyor on board who can assure the seller it is ok.

David

 

[O C Diver]

Ted, Ideally I am looking for 1800rpm around 8 knots. I actually would prefer if they were slightly overpropped- say 50rpm or so, as they will never be pushed beyond hull speed anyhow- and so kept lightly loaded. And yes, in the salon at the current cruise speed, it was louder than it needed to be for sure.

Thanks

I don't think you're going to get there. You're certainly not going to get there and turn rated RPM.

I can also see the current props being correct to turn rated RPM. I can also see it not making 9 knots with rated RPM and the correct pitch.

Ted

 

[jointcustody]

Two things. Firstly, use a photo tach as too many engines do not have reliable tachs unless they’re newer digitals. Secondly, pay little heed to waterline length guesses and the fallible 1.4 multiplier X sq rootWL. I’d guess the 41 footer to have no more than a 1.2 multiplier if it indeed is FD.

As previously suggested insure full RPM test is done to assess current propping and engine health. Then, once the correct props are in place you can do a fuel burn vs RPM vs boat speed to establish your desired cruising regimen.

Just purchased one on amazon. Thanks

 

[jointcustody]

Notwithstanding your sellers squeamishness about running at wot, and for whatever it is worth running wide open for a few minutes won’t hurt a thing, it gives two important pieces of info in buying a boat:

One, it tells you if the cooling system is compromised. If while running at 2000 rpm or so with the temp running 180-200, you open it up to wot and the temp rapidly rises to 220+, you know you have restricted r/w flow, fouling or maybe a problem with the coolant system.

WOT rpm’s also tell you if you are overpropped or under. But you first have to do a quick check of wot at the dock, to make sure the throttle stop is correct and not restricting rpm’s.

I prefer to prop to 50-100 rpm over rated to have some margin for prop or bottom fouling.

But it sounds like your seller is nervous about running at wot, so it is probably best to do it with a surveyor on board who can assure the seller it is ok.

David

Definitely will be doing this during a mechanical survey. Kicking myself I didn't do it already

 

[jointcustody]

I don't think you're going to get there. You're certainly not going to get there and turn rated RPM.

I can also see the current props being correct to turn rated RPM. I can also see it not making 9 knots with rated RPM and the correct pitch.

Ted

I carefully read and re-read everything you said here, and I think within the limits of what you said, I could still be happy. I do believe what you said to be true. I think if I could cruise north of 8 knots id be happy. I don't even think the boat would make 8 knots right now.

We have beat around the bush with this all day- and the final conclusion we realized is that at 2200 rpm, the boat was not at all lifting out of the water, making a big wake etc. It just didnt seem to be loaded up enough to warrant that being its "wall". We have all felt that feeling and know the noise, and it just didnt feel that way.

Also i know this is a completely different boat and in an entirely different league, but a 42 krogen with a single 135 lehman will do 8 knots quite easily. That is a full displacement boat. The specs are similar to the boat I am looking at- waterline especially but with ours having even less weight.
With twins it should only be easier.
WOT for this engine is 2800 rpms- i found that info

This is with the current prop pitch- showing 43.5% prop slippage. Note mph was only option so 7 knots= 8 mph
It says 5-20% slip is acceptable. So it appears these props are cavitating.
that ^ was with the app. Heres the web page with the tool if anyone is interested;

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

 

[sunchaser]

JC
You’re too early in the game to be talking prop numbers, especially when using theoretical prop curves. Once you’ve established full throttle accurate RPM then propping discussions can be entertained.

 

[O C Diver]

I carefully read and re-read everything you said here, and I think within the limits of what you said, I could still be happy. I do believe what you said to be true. I think if I could cruise north of 8 knots id be happy. I don't even think the boat would make 8 knots right now.

We have beat around the bush with this all day- and the final conclusion we realized is that at 2200 rpm, the boat was not at all lifting out of the water, making a big wake etc. It just didnt seem to be loaded up enough to warrant that being its "wall". We have all felt that feeling and know the noise, and it just didnt feel that way.

Also i know this is a completely different boat and in an entirely different league, but a 42 krogen with a single 135 lehman will do 8 knots quite easily. That is a full displacement boat. The specs are similar to the boat I am looking at- waterline especially but with ours having even less weight.
With twins it should only be easier.
WOT for this engine is 2800 rpms- i found that info
View attachment 156086

This is with the current prop pitch- showing 43.5% prop slippage. Note mph was only option so 7 knots= 8 mph
It says 5-20% slip is acceptable. So it appears these props are cavitating.
that ^ was with the app. Heres the web page with the tool if anyone is interested;

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

I'm sure you can reach 8 knots. I doubt you will reach 8 knots at 1,800 RPM. At what RPM (noise level) are you willing to cruise at 8 knots?

As mentioned by Sunchaser, prop calculators are good for making adjustments based on known parameters from the same boat, engine, load, and most importantly hull shape. They excel on planing hulls with the above parameters. They are not as good with displacement hulls. If you look at the calculator name that you're using, it says, "Mercury RACING propeller slip calculator ".

The other point that I would ask is how many engine hours and how old are the engines? I could see pushing newer engines to a higher percentage of maximum output. Are we talking about 10 year old engines with 2,000 hours or 25 year old engines with 5,000 hours?

Ted

 

[jointcustody]

I think if I could gain a knot (8 knots) as well as drop 200 rpms (@2000 rpm) I would be happy.

The engines are 40 years old with 2000 hours but in excellent condition. Do not smoke. Had injection pumps and injectors done a few years ago.

The mechanical trial being a couple weeks away- i do have pictures of the hull. Does this look semi or full disp to you?

 

[DavidM]

Definitely semi displacement.

 

[sean9c]

Here's something that I'm curious about. Aren't diesels governed? So how do you know if it's prop load that's limiting rpm or the governor?
Seems to me, and it's what I do, is have an EGT gauge. That way you know how hard your engine is working. If your engine spins right up to max rpm with low EGT it's not working very hard so you're likely not over propped. Also if your boat is heavily loaded and maybe the bottom is dirty 80% throttle might show a high EGT and you might want to back off a little.

 

[O C Diver]

I think if I could gain a knot (8 knots) as well as drop 200 rpms (@2000 rpm) I would be happy.

The engines are 40 years old with 2000 hours but in excellent condition. Do not smoke. Had injection pumps and injectors done a few years ago.

The mechanical trial being a couple weeks away- i do have pictures of the hull. Does this look semi or full disp to you?

This is my opinion, it's worth no more than you paid for it. So consider it one more waypoint on the chart plotter.

From your original post:
It's 41' LOA
It's 37' WLL
Beam is unknown
Displacement is 30K to 35K

IMO, that's a heavy boat with almost no flat surface in the forward 2/3 of the the hull. I don't think it will come close to getting on top with less than 500 HP of the low weight high HP type engines (I know your goal isn't to make it plane). As a result, past a certain speed and because of less buoyancy in the stern, the boat will dig a hole where the stern will settle as the boat lacks the HP to get over the bow wave and on top. So, I really don't think it matters whether you call it displacement or semi displacement.

My guess is yes to 8 knots and maybe at 2,000 RPM. But, where will you be on the HP curve? With more modern engines, one of the graphs that it available, is peak HP for each corresponding RPM. So from maybe 1,000 RPM to maximum RPM there's a peak value for HP. Lets say you need 100 HP to make the boat go 8 knots. Lets say that at 2,000 RPM each engine produces 60 HP and at 2,200 RPM each engine produces 75 HP. Pitching the propellers to push the boat at 8 knots could require 83% of maximum HP (at that RPM) to accomplish that. At 2,200 RPM the percentage drops to 67%. At what percentage do you want to load your 40 year old engines at?

I don't know if it's important to you, but if you pitch the props to do 8 knots at 2,000 RPM, I'd almost bet money it won't turn rated RPM (be over pitched). Have to be careful not to overload engines.

Ted

 

[jointcustody]

If you look at the calculator name that you're using, it says, "Mercury RACING propeller slip calculator ".

I thought that too at first but then realized its just a measurement of how a screw is turning through the water and how much slip there is on that screw based on gear ratios and speed. I doubt that would make a difference regardless what prop you were talking about -being a racing motor or a trawler- but I did think about the twin engines. Which actually proves the point that its underpropped even more- because both those engines would have to be slipping 43% to get those numbers at this speed.

My guess is yes to 8 knots and maybe at 2,000 RPM. But, where will you be on the HP curve? With more modern engines, one of the graphs that it available, is peak HP for each corresponding RPM. So from maybe 1,000 RPM to maximum RPM there's a peak value for HP. Lets say you need 100 HP to make the boat go 8 knots. Lets say that at 2,000 RPM each engine produces 60 HP and at 2,200 RPM each engine produces 75 HP. Pitching the propellers to push the boat at 8 knots could require 83% of maximum HP (at that RPM) to accomplish that. At 2,200 RPM the percentage drops to 67%. At what percentage do you want to load your 40 year old engines at?

The boat has a 13' 10'' beam

I just found the performance curve for this engine. 90hp at 1800rpm. Thats 180hp available.

The prop calculator from vic prop shows 66hp required to push the boat 8 knots. That is 33 hp per engine. That is 24% load per engine. Hell I would do it even if it was 50% loading- what am i missing there?
Thats with 25x20 3 blade props. current props are 24x17 3 blades. The extra inch of diameter its recommending, i wonder if thats a huge difference too. The owner actually made the comment when i read the survey that he thinks the surveyor was wrong and that those are 22x17 props which would be WAY worse.

This boat has to be underpropped right? The further I get into this the further im convinced its wrong. I really appreciate the insight. And I understand I wont know much more till sea trial.

One more question im hoping someone can clarify for me;
If we go to do the sea trial, and it is wayyy underpropped... will it just stop at the fuel injector pumps max rpm? I can check this by getting up to temp, then putting into neutral and throttling up to top speed?
So lets say thats 3000rpm. Then you put it in gear and slowly increase the throttle to max rpm. What if this reaches 3000 under load? my understanding is at that point, you really dont know how far past the rpm COULD rev? Is that correct? Its almost like having no oil on the dipstick and not really knowing how much is left? LOL.

Thanks again

 

[C lectric]

Go to Trans Atlantic Diesel and ask them if they can help with an engine loading vs rpm sheet. They are on the US east coast, MAYBE near you.
You will need the engine serial number, model and likely build year or at least the best you can find. Expect a charge though as they may have to do some digging.

A question. Does the owner have that info because without it we can yammer all day and come to poor conclusions unless some one here has that info and posts it.

The suggestion has been made to get a handheld digital phototach available from Ebay or Amazon for $20 - $30 and use it. Lay a strip of the reflective tape on the dampers so the strip can be easily seen and measure the rpms. Depending upon typical dash tachs is OK ONLY after the digital has checked the dash tach accuracy.
Other wise you are working to estimate if you have prop problem handicapped big time. To do the comparisons you will need to write the info down. I suggest 100 rpm differences.

The checks can be done dockside out of gear.
Although the worst, I have seen dash tachs discrepancies as much as 150-200 RPM.

The digital tachs should be used on ANY boat you are considering unless the tachs are already AETNAs, a known digital unit with known high accuracy.
For now the handhelds are excellent for low cost and portability.

Good luck but if you don't try this you are handicapped.

 

[O C Diver]

I thought that too at first but then realized its just a measurement of how a screw is turning through the water and how much slip there is on that screw based on gear ratios and speed. I doubt that would make a difference regardless what prop you were talking about -being a racing motor or a trawler- but I did think about the twin engines. Which actually proves the point that its underpropped even more- because both those engines would have to be slipping 43% to get those numbers at this speed.

The boat has a 13' 10'' beam

I have had a hard time finding a performance curve for this engine- but my newer m135 perkins has almost 100hp at 1800rpm. So lets get crazy and say this 6.354 135 only makes 80hp at 1800 rpm. Thats 160hp available.

The prop calculator from vic prop shows 66hp required to push the boat 8 knots. That is 33 hp per engine. That is 1/5 load per engine. Hell I would do it even if it was 50% loading- what am i missing there?
Thats with 25x20 3 blade props. current props are 24x17 3 blades. The extra inch of diameter its recommending, i wonder if thats a huge difference too. The owner actually made the comment when i read the survey that he thinks the surveyor was wrong and that those are 22x17 props which would be WAY worse.

This boat has to be underpropped right? The further I get into this the further im convinced its wrong. I really appreciate the insight. And I understand I wont know much more till sea trial.

One more question im hoping someone can clarify for me;
If we go to do the sea trial, and it is wayyy underpropped... will it just stop at the fuel injector pumps max rpm? I can check this by getting up to temp, then putting into neutral and throttling up to top speed?
So lets say thats 3000rpm. Then you put it in gear and slowly increase the throttle to max rpm. What if this reaches 3000 under load? my understanding is at that point, you really dont know how far past the rpm COULD rev? Is that correct? Its almost like having no oil on the dipstick and not really knowing how much is left? LOL.

Thanks again

A pretty good rule of thumb is around 20 HP per gallon of fuel burned. In your original post the quote was 2.5 gallons per hour at 7 knots. That would roughly equate to 50 HP to push the boat at 7 knots. My guess would be 90 to 100 HP (not 66 HP per the prop calculator) to go 8 knots. It could be higher than that.

You keep coming back to the prop calculator expecting correct answers that it just can't give you. If you take the parameters of this boat, you can have numerous hull configuration for the same values, that are going to require different HP values to push the boat at 8 knots. As an example, take draft. Do you think the keel on your perspective boat has a drag value as opposed to one that doesn't have a keel? Do you think that a boat with more weight in the bow as opposed to the stern will make a difference? Do you think a deadrise bow with a deep foot is more or less efficient than a very tapered bow with that is shallow? As you increase speed and the bow starts to rise, which bow design requires less HP?

The engines should have governors that limit maximum RPM to maybe 200 RPM over.

Ted

 

[Turtle Blues]

Questioning if 2200 was the top rpm or just what owner was willing to do?
Believe WOT should be nearer 2800?
Thinking there might have been a little in reserve?
It just seems that might have been a comfortable demonstration not a throttles down full out measurement.
As a questionable comparison my Golden Star weighs in at a hefty 31klbs / 42'loa / 14' beam / 24x19 props. I have turbo version so additional 100hp per engine will put this whale on plane at 22 knots. I turn white when it's done.

 

[DavidM]

Yes, marine diesels are governed, but… there is a lot of slop in the mechanism.

Say that engine has a rated maximum hp rpm at wot of 2,800. But if you open the throttle wide open with the engine in neutral, it is likely to hit 3,100 rpm or more. But if you then start putting

 

[DavidM]

My post above got truncated somehow.

So if the engine will hit 3,000 rpm or so at wot with no load, then measure the loaded rpm at wot in gear. 2,800-2,900 is about right. 3,000+ is underpropped. Less than 2,800 is over propped.

David

 

[jointcustody]

Questioning if 2200 was the top rpm or just what owner was willing to do?
Believe WOT should be nearer 2800?
Thinking there might have been a little in reserve?
It just seems that might have been a comfortable demonstration not a throttles down full out measurement.
As a questionable comparison my Golden Star weighs in at a hefty 31klbs / 42'loa / 14' beam / 24x19 props. I have turbo version so additional 100hp per engine will put this whale on plane at 22 knots. I turn white when it's done.

What numbers do you get a trawling speeds? 6,7,8,9 knots how many rpm are you pushing?

Questioning if 2200 was the top rpm or just what owner was willing to do?

Owner stated that was their normal cruise rpm- which seemed high to me right when he said it. Then was super disappointing when the boat would barely make 7. And I mean like 7.0 7.1 sort of thing. Engines sounded pretty wound out

 

[BruceK]

As there seems to be some uncertainty about prop size as fitted is, might be good if the mechanical surveyor sees it out of the water. If you combine surveys it should be possible, if not the general surveyor should be able to ascertain.
Leaving the over/under propping to experts contributing, on a different issue I did notice some blisters, and ? some blister repairs. Unsurprising on a 40 yo boat, but worth keeping in mind.

 

[jointcustody]

Leaving the over/under propping to experts contributing, on a different issue I did notice some blisters, and ? some blister repairs. Unsurprising on a 40 yo boat, but worth keeping in mind.

Thank you for pointing that out actually- I did not notice that. I can see the blisters I believe, but can you point out any of the repair spots by chance?

 

[BruceK]

Thank you for pointing that out actually- I did not notice that. I can see the blisters I believe, but can you point out any of the repair spots by chance?

Pic 3 has some uneven areas which could be old repairs, but could be other things, like where the old a/f lost adhesion and came away, brush marks, etc. Pic 4 shows several odd indents in the transom which look like unrepaired conical grinds. I can`t enlarge the pics much, more enlargement might help. Best time to identify blisters is just after haulout while still wet. I`d be surprised if there were not some, it`s just something to be aware of, unless it turns out to be extensive.

 

[Turtle Blues]

What numbers do you get a trawling speeds? 6,7,8,9 knots how many rpm are you pushing?

Varies greatly by how clean the hull is, 6kts requires about 1000rpm, 8/1200 12/1600 22/2400
I checked the old Perkins rating chart in my manual, it seems the turbo version of that engine is only 50hp higher than the NA version.
Rating was at 2800 for NA and 2400 TC.
I will say that 1600 is the high end of the comfort range for cruising and the 1200 is a lot more pleasant for long days. Nobody wants to be wearing hearing protection.
My personal thought is there was a good bit of power in reserve that could be utilized at a lower rpm and the boat may be under propped but I would want to be talking to an prop pro with more experience and computer savy than me.

 

[ranger58sb]

I checked the old Perkins rating chart in my manual, it seems the turbo version of that engine is only 50hp higher than the NA version.
Rating was at 2800 for NA and 2400 TC.

???

Natural WOT higher than turbo?

-Chris