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Old 12-26-2014, 03:20 PM   #1
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Confirmation of prop/gear thinking

Greetings Forum

Curious mind is asking: What is the explanation for the different WOT RPMs for different applications? Recently a shop manual for the Perkins 4-236 engine purchased as a replacement for our Perkins 4-154.
In reading the manual there is a section giving the WOT hp rating. They are as follows

Pleasure-(High Speed) 84 bhp at 2800 RPM (This is the more common settings that I have read and I used to determine the recommended correct wheel measurements)
Pleasure- 76 bhp at 2500 RPM
Commercial- 65 bhp at 2250 RPM
Commercial (Heavy Duty) 54 bhp at 1800 RPM

The reason for the inquiry relates to the subject of “Over Propping”. There are existing forum subjects related to the good and evil of over propping with the majority stating that using the recommended setting of the manufacture is the better choice. As these four are manufactures recommendations it would seem the subject is open for discussion as to what determines each rating. There is none referenced in the manual

Having stated this information an assumption is being made as to the use of our current wheel on the Perkins 4-154 with a 3:1 gear which is 22 X 16. This is very close to the Vicprop formula of 21.5” X 15” for this engine/gear ratio.
The replacement engine the Perkins 4-236 with a 2:1 gear using the Vicprop formula calls for a 20” X 14” wheel. Both of these results are using the Pleasure (high speed) formula. As the boat is far from a high speed hull (6.9 knt hull speed by formula) the thought goes towards the “ Pleasure” of 2500 RPM. with the thinking that the difference in the two wheels recommended may account as compensation for using the current larger (Over Propped) sized wheel which is ou wheel 22” X 16”. in use.

The point of the engine swap is not to go faster but to accomplish the current results at a reduced RPM (Quite) with less fuss. A by product is anticipated in reduced fuel burn. With the current engine we turn 2400 RPM to obtain hull speed.(7 knt +-_ the shaft is turning 800 RPM with a fuel burn of close to 2 gallon per hour . With the replacement engine the same RPM at the shaft is obtained at 1600 RPM. the fuel burn is anticipated at 1 gallon +- per hour based on antidotal information from harbor mates running the same engine/gear ratio at a higher 1800 RPM settings.

A secondary reason is having two wheels on hand over replacing with a new or used smaller wheel.

A third reason is minimal fuel tankage of 74 gallons. Adding additional tankage is out of the question unless above deck mounted tank is considered.

Perhaps this will entice a discussion that will be enlightening to all.

Regards,
Al-Ketchikan (Bridge to Nowhere) Alaska
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Old 12-26-2014, 03:43 PM   #2
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Commercial rating may be M1, which means wot for 24 hours a day, such as used in a fish boat or similar. The injection pump is governed to the rated rpm, the wheel is then adjusted to be most efficient at that rpm, wot.

The pleasure use is rated at something like one hour at wot out of 8, or similar. I'm sure that someone will quickly supply the exact ratings. My engine is rated at 250 hp at 2100 for 24 hours a day. My friend has the same engines, rated at 420 horsepower for one out of 8 hours. My engine is governed and has the appropriate injectors for that rpm. It also has a 36" wheel, almost double the size of the 420s. I have no idea if mine were propped the same as his, if it would approach 420 horsepower if it was governed and fuelled for more...

You didn't say what your old engine was rated at but there is an almost direct relationship between horsepower and fuel burn. I would guess you need about 55 horsepower to get hull speed and I will further guess that the fuel burn between the two engines at 55 hp will be almost identical, if your old engine is properly propped and running well.
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Old 12-26-2014, 03:47 PM   #3
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There should be a plate on the engine stating max HP and RPM. The governor on the fuel pump is set to limit RPM. The ratings are for different use and are decided by Perkins. The pleasure craft rating allows so many minutes per hour of wide open (2800 RPM) throttle, the continuous rating allows just that, continuous 24/7/365 use at 1800 rpm. That's typically used in generators or tugs. The other ratings are steps in between, every manufacturer has their own.....
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Old 12-26-2014, 05:10 PM   #4
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Yes I believe the specs are for the maximum amount of fuel injected at the rated rpm is sufficient to produce maximum power and the amount of fuel is not excessive. So if one props to the rated rpm the right amount of fuel will be injected for full power and less for all engine speeds less than rated. And if one is over propped there will be an rpm that has the right amount of fuel injected but it will be lower than the rated rpm. Also all engine speeds above that will result in over fueling.

Is that all correct? Anybody?
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Old 12-26-2014, 05:18 PM   #5
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I don't really know what Perkins means by the different load ratings at different rpms and hps. My guess is that the fuel injection pump is calibrated differently for each type of rating.

Another comment: I don't believe that if one engine takes 2 gph to reach 7 kts that another bigger engine, geared slower with maybe a heavier pitched prop can make the same rpm with half the fuel consumption.

There are two things at work here: prop efficiency and engine efficiency.

I don't know if your bigger, slower geared, fatter pitched prop is more or less efficient. That is something for a prop pro to comment on. But it can't be more than 20% difference either way.

Engine efficiency is another issue. Running the same hp at a slower rpm will usually result in lower fuel consumption- rotating all of that machinery faster takes more parasitic hp. But the larger displacement- 236 cu in vs 154 cu in will definitely eat up more fuel to turn over the bigger engine- more bearing area, more piston ring swept area, bigger cooling and lube oil pump, etc.

My guess is that the two effects will cancel each other out and it will be a wash- 2/3 of the rpm will offset 50% more displacement, so the fuel consumption will be approximately the same with either engine at 7 kts.

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Old 12-26-2014, 07:05 PM   #6
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The Perkins 4-107/8 has three ratings ...
50hp at 4000rpm
40hp at 3600rpm
36hp at 3000rpm

I assume they calibrate the FI pump for the best mixture at each rpm and the above power ratings result.

When you see boats w this engine most of the time they wrongfully claim 50 hp whereas it's really 36hp. And in the Willard boats it's actually about 33hp as they over propped.

I agree w you David but think the smaller engine will have the edge in efficiency due to the considerably less heat loss.
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Old 12-26-2014, 07:17 PM   #7
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Running that 154 at 2400 is probably not going to be super efficient. My bet is that the 236 at 1600 may have a slight edge. But that's a guess, and they will probably be close. I don't have data for these.

Diesels tend to be most efficient around peak torque rpm, which for the 236 is probably around 1600-1800. No idea where peak torque rpm is for the 154, it will be higher but not 2400.

The 236 will be quieter, and parts are easier to get. That may be good enough reason.
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Old 12-27-2014, 01:34 AM   #8
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Thanks forum members, response is well thought out by each of you.

One question asked was the hp of the current engine. It is 58 hp. When that engine is running at 2150 RPM and seems the best level of harmony, the hull is moving at about 5.8 to 6.3 Knts. Hull speed computes out to 6.9. So in effect the engine and hull are mated at that setting. However, as we all are aware, when elements come into play –tides and wind. the desire to offset the friction that causes less speed the frustration sets in. Like having to take stuff to the boat, it is always low tide. same with the elements come up is seems always to be adverse.

At 2150 RPM the fuel burn on the 4-154 is just a tad over one gallon an hour which speaks to the question of mating the RPM to the ultimate fuel burn. However when the RPM is increased to the 2400 RPM the burn does in fact almost double to near 2 gallon per hour. At this point one has to believe that many boaters “Fudge” on their fuel burn. As to the 4-236 being the bigger engine at 85 hp, it would be fair to suspect that the fuel burn of one gallon claimed by the two harbor mates having the same engine/gear running at 1800 RPM would fit that “Fudge” factor yet even if that is so, by my more honest known usage still seems to be a net gain in fuel burn. Make note, the fuel burn at either rate is not going to be a deal breaker more an interest.

The ability to turn the engine at 1800 RPM and obtain near 900 turns at the shaft knowing that 2400 on the 4-154 at 2400 RPM is turning at 800 RPM. Using the same wheel in both applications seemingly will provide ample “Snoose”when and if the need to exceed the achieved speed of close to 7 knots. 7 knots is the average with the current 4-154 at 2400 RPM.

TAD- There is not indication on the engine as to classification or setting on the injector pump. I have no argument over it being anything but a 2800 RPM rated engine as the engine was in a motor sailer craft not a commercial rig. All things being with in reason the comfort level being sought is running a 22 X 16 wheel in a recommended 20 X 14 formula application.
Using the simple formula of one inch of diameter equals 2 inches of pitch and that equals to 200 RPM drop then I have in theory, about 500 potential drop in RPM which means that with a 2800 RPM engine I will lose to 2300 maximum RPM. If one holds to the 3/4 or WOT power as the maximum safe and logical running RPM it works out to 1725 RPM or at the shaft 860 RPM. Having spoken to the achieved speed of 7 knts or hull speed at 800 RPM it would on the surface appear that all is well and good.
While running beyond that imposed max RPM would not be a normal application, that one could increase for the short time required still exist.

As all of the mumbo jumbo by myself is all without engineering basis and seat of the pants, the conversation with the suggestions is really a treat to receive and digest. Thanks again to the forum to this point on the subject and to additional comments or suggested criticisms to come, Confirmation or correction it is all good.

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Al
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Old 12-27-2014, 01:40 AM   #9
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A slight counter diction in the above post. Earlier on it is noted running at 1800 RPM with the Perkins 4-236. However later in my post I am speaking to the increased diameter and pitch formula that restricts those numbers to 1725 RPM. There is room for correction in these two statements. The answer may be left till the exchange is completed and the boat is in actual running status.

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Old 12-27-2014, 07:10 AM   #10
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The pleasure boat rating is used if you want to tow water skiers, or cruise on the plane.

The commercial HD rating would allow normal displacement speeds .

I would run the boat and see if at 1800 rpm , you can push the throttle and see ,perhaps 200 rpm (10% ) more RPM.

If so at 1800 you will not be overloaded and the fuel burn should be under 3GPH.

How far under is determined by LWL, weight, bottom condition .

If you dont have a ski boat , there is no reason to prop as a ski boat.
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Old 12-27-2014, 07:36 AM   #11
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Where are you getting the fuel numbers?
Actual results?
Max hp fuel curve?
Prop fuel curve??

It will take about the same fuel( hp) to move the boat at the same speed. Youcan waste fuel with too much propslip but the hp delivered to the water will be similar
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Old 12-27-2014, 11:45 AM   #12
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bayview,
I suspect more trawlers suffer more from blade drag than blade slip.
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Old 12-28-2014, 07:49 AM   #13
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Most would seem to suffer from being operated out of the most efficient BMEP range because the engine is underloaded at cruise .

A 120 hp car rated engine might really be 65 realistic hp so operating at 2 gph , 35 hp is not grand , but changing and doinf 20% better would not save the cost of a prop in 5 years for most.

A real 300HP industrial engine would pay for the improved setup (to operate at 2 gph) far more quickly.
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Old 12-28-2014, 11:57 AM   #14
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Bayview-

If you are asking me, it is from actual use on our current boat and the numbers developed on our prior boat that reflected similar characteristics in the area of engine performance to wheel used.

FF's responses are seemingly more in line with my thoughts. Our last boat was rated at 2800 RPM (WOT). The best obtained was 2400 RPM. After that all you obtained was black exhaust. We ran the boat at 1650 RPM for a 7 knot average and 1.5 gallon fuel burn.
At that setting all systems and sound levels were smooth all in harmony.

Our current boat with the anticipated engine change to 85 hp from 58 hp when compared to the 60/65 hp engine/gear system would reflect similar characteristics of being able to throw the larger wheel within the preferred limits RPM wise.
Even more so as the need to exceed the 1600 RPM area with the 2:1 gear has been witnessed running the current engine with a 3:1 gear to the equivalent shaft RPM using the same wheel.

The conversation has brought a level of comfort to the project going forward.

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Old 12-28-2014, 05:23 PM   #15
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That smoke indicated that there was more power to give if the prop were less.


That apparently was not important to you but the potential was there to use in accelerating the boat when hitting frequent head seas. The boat would not go any faster that how you used it but faster acceleration can be useful in head seas when the boat is about stopped by each wave.
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Old 12-28-2014, 08:59 PM   #16
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Bayview- Correct, you confirm the over propping. Not that it is not important as it can be were one to ignore the issue and the solution a smaller wheel.

Knowing being over propped keeps me alert to retaining throttle settings that will not exceed the limits of common sense (black smoke!).

7 knots is the hull speed. Pushing the hull has not proven to be advantageous in terms or returns for fuel and noise invested. We obtain this hull speed on our current 58 hp engine at the 2400 RPM setting with a full 600 RPM reserve. All that happens when we increase throttle is more hull fuss (No smoke as the correct wheel is in play).

As to being helpful in a head sea, on paper maybe so however in my life I tend to slow down in heavy head seas or quartering when even that becomes a nuisance with steering.

With the anticipated engine and gear change, 2:1 vs 3:1, these same characteristics will develop at a lot lower RPM setting of 1600 vs: (2400) with a remaining antidotal estimate of 600 RPM reserve in both cases to WOT.

Now to confuse the discussion (It may have been mentioned earlier on in this forum) the Perkins Marine Power booklet for the 4-236 specifically uses 2500 RPM in the performance graph as WOT. The booklet specifically mentions intermittent shaft horsepower at 77 hp. This being stated all of the discussion really will have to await a sea trial to ascertain actual WOT using the aforementioned over sized propeller currently being used with the 3:1 gear.

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Al.
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Old 12-29-2014, 06:47 AM   #17
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AN EGT gauge is about $100 and will keep you out of trouble if you attempt very high power operation for some reason. Glowshift gauges , inside placement only.

Combination digital and analog.

Most folks re propping believe the motto, 3rd time lucky.
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Old 12-29-2014, 12:40 PM   #18
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I have twin 4.236s and have the 2:1 reduction swinging 19x17.7 props. I know they're 17.7 due to my PropScan report.

I used to think I was running my engines at 2000 RPM because my more stable and consistent stbd alternator-driven tachometer always read 2000 RPM at cruise while the port tach jumped from 1800 +/_ 200 RPM. Then I replaced my stbd alternator with a Balmar and both tachs settled down at 1800 RPM at the same throttle position, noise level and speed. I recently bought a digital laser photo tach to confirm my actual RPM, but haven't had the opportunity to use it yet. So let's assume my RPM is actually 1800 at cruise.

At 1800 RPM cruise, I consistently burn 3.0 GPH total, 1.5 GPH per side at about 7.4 kts. I measure this every time I top off my fuel tanks after 30+ hours of operation and this has not varied by more than .25 GPH in for the years I've been running at this RPM. I attribute this variance to more or less slow speed due to speed zones or idling along looking for fish on my fishfinder.

I used to run at 2200 RPM (which I used to think was 2350) and saw a consistent 4 GPH total fuel burn and 8 kts but with much more noise. WOT RPM is 2600 (which I used to think was 2800) at about 10 kts. The plates on the fuel pumps read 2650 RPM.

These are actual fuel burn numbers with no 'fudging'.
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Old 12-29-2014, 04:28 PM   #19
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I had low powered sail boat and can assure you that rounding some points on the CA coast was impossible because the boat was stalled by waves. Just about the time it regained forward speed another wave hit. It wasn't particularly rough just frequent waves stopping the boat. Better acceleration would have allowed forward progress. I would always want full power to be available for those circumstances. I don't see any negative to propping to allow full power YMMV.
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Old 12-29-2014, 11:52 PM   #20
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Bayview-

Now you are close to what is the case here. With 58 hp we too have low power.
However, were there to be seas that you describe here in the inside passage of Southeast Alaska as a routine expectation I would have a different boat! You are visiting with a senior boater who is weather feather wise. Often I have to make a weather judgement from Wrangell Alaska regarding water that is 60 mile away and considered "Open" by our local standards (Clarence Straits). This is a 9 or 10 hour call. Given a forecast that read as you describe which if I am to make a judgement, waves exceeding 3 feet with likely an above 20 Knt wind. I would not break loose from Wrangell!!
If caught out in a situation with deteriorating weather, this little boat and boy head for a comfortable cove, hot coffee a good book and wait it out. (Retired did I mention?)

Yes I have had the incidents of making later thought, bad boating judgement with prodding ahead thinking the weather would moderate. As I age and retain memories, those incidents are becoming distant (I hope).

Secondary, Al (flywright) is reflecting the anticipated results. 1.5 Gal per hour @ 1800 RPM sounds reasonable. One should be able to anticipate a more close to 1 gallon at 1600 RPM which with the 2;1 gear give me the same turns and 7 knots as the 58 hp engine with a 3;1 gear making that same 7 knots at 2 gallon per hour. Like Al this is actual fuel stop to fuel stop by nautical miles run.and clock/engine hours time.
Our 22x16 vs: his two 19x18 wheels seems reasonable.
If I am using 1600 RPM as my sea speed travel RPM then in effect I have several hundred RPM in reserve to off set any wind/tide/waves if the choice is required to meet the conditions you speak too. However I repeat, caught in those conditions I slow down not speed up. if slowing down required increase RPM it remains available.

Thank you for the clarification. The situations reflect differences explained.
Al
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