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Old 12-13-2020, 07:20 PM   #1
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Power/Propulsion Problem - or Maybe Not?

Do we have a propulsion problem we should be searching for - or perhaps the speeds we achieve is just the characteristic of our trawler?

Two summers now with our 1989 36ft Heritage East sundeck trawler, pair of turbo 150HP Volvo diesels (TMD41A), and we haven't hit theoretical hull speed yet...

With a water line of about 33 feet, theoretical hull speed is a little more than 7.5 knots.

Reading various posts I am curious and concerned than many suggest they achieve hull speed with as little as 2000 RPM, plus or minus.

With our two summers of experience, we achieve about 5 knots at 1600-1800 RPM, 6 knots in the 2200RPM area, and need 2800RPM or more for 7 knots. Wide Open Throttle is 3800RPM according to Volvo (we have yet to push above 3000RPM).

On the other hand, cruising almost always in the 5-6 knot range, we have averaged 1 gallon per hour per engine; so approximately 1/2 gallon per NM which appears respectable with respect to other posts.

A found internet quote: "For displacement hulls of reasonable shape (not a square box, which would require more power, and not a rowing shell used for competition, using less power), about 1 horsepower per long ton (2200lb) of displacement will get the boat to hull speed in calm water."

Assuming our trawler is 30,000 lb (14 Long Tons max), we should be seeing hull speed at 14HP, or somewhat more for inefficiencies. Volvo's TMD41A engine curves state 50HP crankshaft power should be produced at 1500 RPM, 75 crankshaft HP at 2000 RPM, and 120 crankshaft HP at 2500 RPM. If the HP per Long Ton is even roughly accurate, we should be hitting hull speed well before 2000RPM?

Cognizant of the large variety of (full displacement) trawlers and engines in this Forum, is your experience that theoretical hull speed should be achieved at something significantly below Wide Open Throttle?

(1) When we purchased the boat the mechanical inspection recommended overhaul of the 6 Injectors in each engine (exhaust colour may be indicating incomplete burn).

(2) I bought a Tach Reader and will check the accuracy of the RPM gauges in the spring.

(3) Given an average fuel burn of 2 gallons/hour is respectable, this isn't necessarily an issue I want to go to great mechanical expense to investigate (such as experimenting with propellers), but with a plan to do the Great Loop in 2 years it would be nice to know we can easily achieve hull speed when a situation requires.
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Old 12-13-2020, 07:50 PM   #2
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OK, a few comments:

First the rule of thumb I have seen and have confirmed with several full displacement trawler hulls is 1.5 hp per thousand lbs of displacement to achieve hull speed. But, no matter.

Secondly, am I reading you right that the engine is rated for 3,800 rpm but you can only achieve 3,000 rpm at wot? If so the engine isn't producing the power it should or the engine/boat is grossly over propped. Since you can't get to hull speed at even 3,000 rpm I am thinking the former.

Fueling could be the reason for not producing enough power. Does the engine produce black smoke, particularly at higher rpms? If not then perhaps the engines aren't getting enough fuel, maybe due to a bad injection pump but since both seem to be doing the same, maybe the PO replaced the injection pumps with ones for a lower displacement or non turbo charged engine.

If you were grossly under propped that could cause your speed issues. But then you should easily hit wot and then some if they are under propped.

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Old 12-13-2020, 08:41 PM   #3
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Sounds like you should be cruising at 2800rpm. That’s 1000 rpm down from rated. You like many other trawler skippers need to stop thinking the engine is going to self-destruct.

Diesel engines are noisy.
Diesel engines are noisy.

If your engines are in good working order and propped correctly you should be able to run at 3000 to 3,200rpm all day long. But check w your engine people (Yanmar I assume) ... but first you need to get 3800rpm loaded w full fuel ect at WOT.
Once you achieve that consult Yanmar (or whomever) to learn what the maximum continuous power rpm is. Then given good maintenance you only need to not exceed that.
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Old 12-13-2020, 08:56 PM   #4
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Quote:
Originally Posted by CPD View Post
Do we have a propulsion problem we should be searching for - or perhaps the speeds we achieve is just the characteristic of our trawler?







With our two summers of experience, we achieve about 5 knots at 1600-1800 RPM, 6 knots in the 2200RPM area, and need 2800RPM or more for 7 knots. Wide Open Throttle is 3800RPM according to Volvo (we have yet to push above 3000RPM).



On the other hand, cruising almost always in the 5-6 knot range, we have averaged 1 gallon per hour per engine; so approximately 1/2 gallon per NM which appears respectable with respect to other posts.

Given an average fuel burn of 2 gallons/hour is respectable, this isn't necessarily an issue I want to go to great mechanical expense to investigate (such as experimenting with propellers), but with a plan to do the Great Loop in 2 years it would be nice to know we can easily achieve hull speed when a situation requires.


Your engines appear to be reaching rpm levels that are reasonable, if there were fuel issues or injector problems you would either see smoke or you wouldn’t be able to achieve rpms. Your hull appears to have trim tabs installed, that with the twin 150 HP engines indicate it’s a semi displacement hull with flat section aft. Your props look substantial but you need to get the pitch and diameter specs to begin troubleshooting. Also important is the gear ratio of your transmissions. With that data you can run prop calculators to see if your readings match what you’re seeing. On a 30 year old boat it’s possible the wrong props or transmissions were installed by previous owners.

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Old 12-13-2020, 09:17 PM   #5
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A couple thoughts;
Your boat is a planing hull, not full displacement. It will take more HP to reach hull speed. How much more? Endless arguments.

Verify your tachometer is accurate at 2000 rpm. Use a photo-tach.

Will your boat reach rated WOT RPM as-is?
What RPM is 6.8 knots (7.8 MPH)? That's where I cruise with my 36 footer.
You have twins. The extra drag by the 2nd drive train costs at least 10% efficiency.
Your boat is heavier than mine. Weight needs HP to push.
2 MPG is doing ok in my book but you should be in the 6.5 kt range.
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Old 12-13-2020, 09:25 PM   #6
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Thanks for the inputs thus far; as Nomad Willy correctly assumed, WOT is 3800 RPM but we've been too amprehensive so far to run the engines above 3000 RPM.
No noticeable smoke once warm and underway, and never use the Trim Tabs as they likely do little at our normal 6 knots.
MS4A transmission provides 1.9:1 Forward gearing (2.6:1 Reverse).
Once we check out the accuracy of the Tachometers, guess we should just punch it over 3000 RPM and assess resultant speed change, though the Volvo engine curves show the crankshaft HP essentially flattening out at 145-150HP from 3000 to 3800 RPM.
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Old 12-14-2020, 08:27 AM   #7
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"Volvo's TMD41A engine curves state 50HP crankshaft power should be produced at 1500 RPM, 75 crankshaft HP at 2000 RPM, and 120 crankshaft HP at 2500 RPM. If the HP per Long Ton is even roughly accurate, we should be hitting hull speed well before 2000RPM?"

The power curves are for what the engine can produce , but weather the drive and props are putting that much power into the water is the variable .

"Given an average fuel burn of 2 gallons/hour is respectable,"

2 GPH at best is 40HP. To go faster will need to burn more fuel, use more HP.

Try running for 2 min at WOT ,speed , note RPM and weather black smoke pours out the exhaust .

Let us know WOT results .A graph of RPM vs boat speed would be best.

IF the boat was created to plane , and you only desire 7K, new props will be needed.
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Old 12-14-2020, 08:35 AM   #8
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3800 is a higher max RPM than many of the trawlers on here. And 150hp isn't that much more than the common 120hp Lehmans and such. So it's expected that you'd need higher RPM vs boat speed, but the RPM is likely a similar percentage of WOT RPM.
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Old 12-14-2020, 09:21 AM   #9
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I'm not familiar with the particular boat, but it must be some sort of planing or semi planing hull? Do you have any idea what the top speed is supposed to be?


With twin 150 hp engines, and the 1.9 gearing, it appears set up to be a planing boat that can go close to 18 kts. You aren't getting even half that, so I think something big is wrong. The fuel burn seems reasonable at your lower speeds, so my guess would be that you are massively under propped. Is there any chance you know what the prop diameter and pitch is?
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Old 12-14-2020, 09:44 AM   #10
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Thanks for the inputs thus far; as Nomad Willy correctly assumed, WOT is 3800 RPM but we've been too amprehensive so far to run the engines above 3000 RPM.
This is the start of your problem. There is no issue with running a diesel up to WOT RPM's. In fact, you should do it regularly as it is critically important to engine longevity that you can attain this RPM. If you can't then your engines are overloaded and perhaps over-propped. If you go well over WOT then you are under-propped. Checking WOT is one of the best tests of a properly functioning powertrain. So don't be apprehensive.
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Old 12-14-2020, 09:44 AM   #11
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In a 42 footer, 2x 150hp is likely not enough for it to plane. So I don't think it's powered for more than a few kts above hull speed. A quick google search showed only 1 Heritage East 42 for sale that mentioned speed. That one had 2x 200hp Volvos and listed 12 kts as the top speed, so I doubt this one will do any better.

The keel shown in the pictures is also pretty deep for a planing hull, so I'd figure this is meant more like an older Grand Banks in terms of speed.
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Old 12-14-2020, 10:33 AM   #12
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In a 42 footer, 2x 150hp is likely not enough for it to plane. So I don't think it's powered for more than a few kts above hull speed. A quick google search showed only 1 Heritage East 42 for sale that mentioned speed. That one had 2x 200hp Volvos and listed 12 kts as the top speed, so I doubt this one will do any better.

The keel shown in the pictures is also pretty deep for a planing hull, so I'd figure this is meant more like an older Grand Banks in terms of speed.

I'm going based on the BoatDiesel calculator. I agree that top speed will depend greatly on the intents of the hull design. What I found interesting is that the only hull forms that utilize the full 300hp are planing hulls reaching 18 kts. All the other variations of semi planing hulls yielded lower speeds, typically up to about 13 kts, so very much like you suggest. But those utilized only a small fraction of the HP and/or called for a much higher gear reduction. So there seems to be a big mismatch here somewhere, either in our understanding of the hull's intents, or in the selection of machinery that has been installed.


So far his fuel burn at various speeds suggests the engine is working fine, and that this is a "gearing" problem. And when I say "gearing", I'm including everything that reduces the engine shaft speed to resulting water speed, so the reduction ratio in combination with prop pitch.


It seems very unlikely that the gearboxes have been changed to a different ration, though not impossible. But it is very possible that the props have been changed. So I think that's where I'd be looking at the moment.


But I agree the first step is to run at WOT and see what the RPMs, boat speed, and fuel burn are. My guess is that he will exceed 3800 RPM with no trouble, yet have a fuel burn well below the 150hp capability of the engines. That would be consistent with his current performance, and with the power curve of the engines.
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Old 12-14-2020, 10:37 AM   #13
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Our Heritage East trawler is a full displacement (not planing) hull.

Measured the prop's last year but can't find where I wrote down the diameters - memory is in the 20-22 inch diameter area, and I've no idea of the pitch (don't know how to measure pitch, and haven't inspected if the Props are stamped with the pitch). Can sense what feels like a little cavitation for a few seconds when accelerating from a steady 2200-2400 to a higher RPM, but then it soon smooths out.

Given DavidM metric, 1.5 hp per thousand lbs of displacement to achieve hull speed, we would be up to 7 knots or so with 30 x 1.5 = 45HP. FF's input is that our average 2 gallons/hour should represent about 40HP on average. Yet the TMD41A Engine Curves show 50 crankshaft HP at 1500 RPM and we're only at 4+ knots. In the absence of any smoke or other obvious indications of a loss of power transfer, thinking this is just a characteristic of the Heritage East hull design.

We are all wrapped up for the Canadian winter now - but after spring launch will check tachometer accuracies, then go for WOT for the first time and observe. Given your experience and advice, we will be less apprehensive of sustained cruising at the 75-80% RPM range (~3000 RPM) with the occasional clean-out bursts to a higher RPM.
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Old 12-14-2020, 11:01 AM   #14
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As mentioned by other posters validate the RPM and WOT. But keep on the forefront of your mind the 100% Tony Athens mantra. "Props move boats."

Generally speaking your boat, props and installed engines should reach hull speed at around 40% fuel load, meaning about mid 2xxxish RPM.

With winter now set in, go to Tony's website, Seaboard Marine, and read all about props and boats. Tis' great stuff.

BTW, the engine HP data is not necessarily too accurate, see if you can find a prop loading curve. As TT mentioned use a prop calculator, boatdiesel.com has a good one.
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Old 12-14-2020, 01:27 PM   #15
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I will strongly second [or 3'rd or ??? ] Sunchasers suggestions.
As for the "tach reader" does that mean a digital photo tachometer?
Volvo did publish prop curves which will highlight the HP needed vs prop load. You may have to bug Volvo for them or peruse the website. I have not kept up on what they put online but they USED to put a lot online.

Props do not demand much HP or should not untill VERY close to the maximum rated engine rpm.

Without actually doing the WOT test with a decent digital phototach [not expensive though] you are guessing.

If indeed you are under propped, and it sounds like you are , there is nothing wrong with being underpropped but to get close[r] to the expected boat speed the pitch may have to be increased. That will increase fuel use somewhat. The pitch increase will increase the boat speed at what ever rpm you run the engines so you can keep the rpms at or closer to the ones you run at now.

But you do need to find out how the boat is propped NOW before making any changes.
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Old 12-14-2020, 01:55 PM   #16
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Yet the TMD41A Engine Curves show 50 crankshaft HP at 1500 RPM and we're only at 4+ knots.
The engines can only produce the power required by the load that is put on it. Yes the manual says it can make 50 HP at 1500 RPM, but if the load does not require 50HP, it won't be making that power. If you put the transmission in Neutral and run it up to 1500 RPM, is it making 50 HP? No, it does not have a load on it, it's only making some tiny amount of HP to spin the internals of the engine at 1500 RPM.

RPM alone is not an indicator of power output. The performance curves assume a particular load (prop) driven at a particular gear ratio (transmission). If either of these items is different than what was used to produce the curve then the RPM numbers won't match up with your installation.

A fuel flow meter can give a good estimate of how much power you are actually producing in any given situation.

If your prop is under-pitched you could be making significantly less power at 1500 RPM than the manual says. If your WOT RPM maxes out right at redline, that helps qualify that your particular installation is fairly close to the curve depicted in the charts. The variables include prop size, trans ratio, hull style, hull cleanliness, driveline cleanliness, etc...

If you can't keep it at WOT without over-speed on the engines then your prop isn't matching up with the curve and you are 'under' propped. If you can't reach redline at all, you are 'over propped'.
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Old 12-14-2020, 04:38 PM   #17
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I didn’t see it mentioned here...might have missed it...
Make doubly sure your engine room has an adequate air supply from the outside...and that your engines are not air-starved. This, for sure, would prevent them from reaching their full potential. In some cases it can be easy to check for this by simply opening an engineroom hatch while running at a fairly high speed. Any change in RPM’s? In sound??
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Old 12-15-2020, 08:26 AM   #18
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After you do some run testing contemplate that,

every 1 inch of pitch in the prop will result in 1K of boat speed at 1,000RPM shaft speed ,
IF there was NO slip.

A well built boat may see 60%+ of shaft theoretical speed , tho many will be in the 50% slip area.
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Old 12-15-2020, 10:09 AM   #19
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Go to full throttle and report the speed and RPM on both engines. If there is no black smoke then the engines are of probably OK. insufficient air would show as smoke. Dont speculate on all sorts of stuff until yo do the WOT test.
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Old 12-29-2020, 02:50 PM   #20
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A couple of comments re: your engines and gear boxes.....

The engine power curves that are published on the VP product bulletins always show BHP, (brake horsepower), that is the power that the engine can produce at a given RPM at FULL THROTTLE on an engine brake or dynamometer. This condition will never occur on a correctly propped boat except at full throttle.

Some VP documents will show a “prop load curve” which is quite lower than the full throttle curve, and this is the approximate power absorbed by the props at some partial RPM below max throttle. The prop load curve and the full power curve come together at full RPM if the vessel is propped correctly. If you are cruising at some RPM below wide open throttle, you are actually using significantly less power than that shown on the full power curve.

Your transmission, the MS4, was available in 1.9:1 and 2.6:1 ratios ( and maybe 1.5:1 ..don’t recall),, but whatever ratio it has it carries the same ratio in both forward and reverse. The gear box is run in “forward” gear for one rotation prop and “reverse” gear for the other rotation prop. The gearbox doesn’t care which way it is run and reduction ratio is the same in both directions.

I took a quick look at my prop program for a vessel using the parameters which you gave: 30,000 pounds, 33’ LWL, guessed at a beam of 12’....., 150 h.p. X 2, and the best I see is a max speed of about 9.5 kts. at full throttle. With 2.6:1 reduction a prop of about 22”x 14” x4 or with a 1.9:1 reduction a 20 x 10 x4 or maybe 19 x 11 x 4.

Even though the hull shape looks like it might plane, it’s too heavy, especially with only 300 hp.

As some have said earlier, warm the engines thoroughly and take it for a full throttle run to get a good performance baseline. Cruising the engines at 20% off the top end shouldn’t be a problem.
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