Semi-displacement hull w/single 120hp Lehman. What Prop Size?

[RichF]

I have a 36' Island Gypsy Europa with a 120hp Lehman (2715e). It has a Warner gear box, 2:1. I have tried two different size props. Prop #1, a 24x17 four blade Michigan only allowed the boat to turn up to ~1900 RPM (8.5 knots) at wide open throttle (WOT). I switched it out that day in the yard to Prop #2, a 24x16 3 blade that I had on board as a spare. I gained another 100 RMP and about the same speed. At WOT with both props the exhaust was black, showing the engine is under heavy load. It isn't a governor issue as I tried this procedure with my hand on the actual engine throttle lever (just in case the cable had a limit nut). Does this prop seem too big? It is the only thing I can ascertain from this situation. I had two marine mechanics on board for this trial as well and the engine has a clean bill of health. ANY suggestions or advise would be great. I'm really interested in what other members have for their propellers.

 

[Vashon_Trawler]

Your WOT should be much higher. I believe your WOT under load should be around 2500 RPM as I recall. Four blade props turn less so I would have expected your drop in pitch by 1" and move toward a three blade prop to at least increase your RPM by around 200, but there is always trial and error from my experience (online calculators will get you close though).

 

[Nomad Willy]

Vashon_Trawler wrote

"Your WOT should be much higher"

Yes 2500 to 2550 would be ideal.

Sounds like your'e swing'in a prop suited for a 2.57-1 ratio on a 2-1 gear.

 

[Scary]

Check no load engine rpm

Your engine should run max rpm under no load. This your first check on governor setting. Boat bottom and prop must be clean. Both can hold down rpm. You may be able to reduce prop pitch by two inch's on existing props. I have seen boats over pitched by owners with intention of never running close hull speed with the interest of gaining range. I'm pretty sure 8.5 @knts is over hull speed for your boat. How does the engine run at 7knts.

 

[Ski in NC]

Make sure you really have a 2:1 gear. I've run around in circles before when actual reduction was different due to mechanic shennanigans. Get a phototach and shoot engine crank pulley and gear coupling while idling in gear. Divide and you get the ratio.

Also, if you intend to run just 7kts or so, you can leave it overpropped like that. Just recognize that anything above that speed-wise can cause harm to the engine. Not that there is much above that, anyway.

 

[FF]

Sounds like the PO was knowledgible about diesels and has on board two cruising props.

A 36 ft boat should be delightful normal cruising at 6K or perhaps 6.5K.

8.5K will probably be 3X the fuel burn regardless of weather at 1900 or 2400RPM

Try 1600 or so , should be quiet , efficient , good for the engine , and will give the boat her normal cruise speed.

 

[Vashon_Trawler]

This is treading on a very touchy subject concerning overloading marine diesels There is a lot of good debate/discussion on this forum, which I encourage you to read carefully.

I'm just offering my opinion here, but I have taken the stance that it is not healthy for most marine diesels to reduce RPM to compensate for an overloaded engine. I also like to have enough reserve HP. My diesel's factory rated WOT under load is 3200-3600, so I've installed a prop that keeps me on the very upper end of this range. My typical cruise RPM (2900 RPM) assures my Volvo Penta is a little over 50% load.

Opinions vary so do a search on the forum--great info here!

 

[psneeld]

Like many discussions there are extremes that people often refer to and make a big stink over....sure I have seen them too and agree that if you over or under prop and are in the few percent that blow up their engines prematurely surely should be used as examples.

But we have seen that people break the "rules" all the time with their engines...they idle them in the winter, they use more additives than an old persons home at pill time, they run'em hot, cold, use Walmart oil, change it every 50-200 hrs, etc...etc...

And they all claim long engine life and I have no reason to disbelieve them. I have seen the abuse an engine can take that makes many mechs scratch their heads.

A TFer here used to regularly post (and most believed his honesty) that Grand Banks overpropped ALL their engines for quite awhile...probably back before they got into the extremes....and possibly why they were grenading all those 475hp 3208 Cats.

So drawing between the line is usually the safest way to run your boat...but it's certainly NOT the only way.

AND like too many discussions here...where people want toenphasize "personal preferences" as absolutes.....

Some people MIGHT prefer lower RPM, quieter cruising, longer life on other components and ACCEPT 5000 vs 10,000 engine hours of total life or life before an overhaul...

But the best part is that if you do one or two of the "dangerous" things to your engine....for every "expert" that says you shouldn't do those things...there's no comprehensive "proof" to back up his/her claims...just experience that many times is limited in scope for the most part.

 

[Nomad Willy]

VT,
No reason to hide as there will are others w many varying opinions on this issue but I think the bottom line is what engine manufacturers recommend.

I prop mine the same way you do yours. About (exactly in my case) 700rpm down from Max hp engine speed. Many manufacturers recommend a "range" of engine speeds and I'm curious where exactly the max power point lies (rpm wise) when a range of engine speeds is given. I suspect as you seem to that it's at the upper end of the "range".

I think my Willy is propped about exactly as your VT is and runs at very close to 50% load. Seven hundred rpm down in my case. If the OP read too much of the archives he may be a little frustrated by all the opinions.

 

[Nomad Willy]

Like many discussions there are extremes that people often refer to and make a big stink over....sure I have seen them too and agree that if you over or under prop and are in the few percent that blow up their engines prematurely surely should be used as examples.

But we have seen that people break the "rules" all the time with their engines...they idle them in the winter, they use more additives than an old persons home at pill time, they run'em hot, cold, use Walmart oil, change it every 50-200 hrs, etc...etc...

And they all claim long engine life and I have no reason to disbelieve them. I have seen the abuse an engine can take that makes many mechs scratch their heads.

A TFer here used to regularly post (and most believed his honesty) that Grand Banks overpropped ALL their engines for quite awhile...probably back before they got into the extremes....and possibly why they were grenading all those 475hp 3208 Cats.

So drawing between the line is usually the safest way to run your boat...but it's certainly NOT the only way.

AND like too many discussions here...where people want toenphasize "personal preferences" as absolutes.....

Some people MIGHT prefer lower RPM, quieter cruising, longer life on other components and ACCEPT 5000 vs 10,000 engine hours of total life or life before an overhaul...

But the best part is that if you do one or two of the "dangerous" things to your engine....for every "expert" that says you shouldn't do those things...there's no comprehensive "proof" to back up his/her claims...just experience that many times is limited in scope for the most part.

You make a lot of good points today psneeld. Good enough to merit re-posting.

It seems GB did overprop and Willard did too. And I'm sure others did also.

There's a lot of variation of opinion on how long an engine lasts at various engine loads. Marin used to say it was dependent on how many times the piston went up and down. In my own mind a 50% load and a 75% load seems excellent but there are so many variables.

psneeld wrote;
"there's no comprehensive "proof" to back up his/her claims". There is in my case (but only to a degree) in that I've always recommended or inferred that what the engineers and engine manufacturers recommend in their manuals should be taken seriously. However that is (to a significant extent) just another opinion. I've never heard of a study where engines were tested to destruction over a wide range of conditions so all we can do is pick the best opinion. And sometimes engine manufacturers recommend what is best relative to laws and the court or even convenience of the operator in addition to other variables that may include public opinion. There's that "opinion" word again.

So I think your point that there is no proof is true but not very usable information in that we all come to a conclusion on engine loading and frequently it's just a matter of what sounds good and feels good.

I still think the engine manufacturers and their engineers have the best information. The smallest amount of opinion and the greatest degree of objectivity. I wonder if GB and Willard owners in the 70s wondered why their engines wouldn't develop maximum power as installed in their boats. There must have been some interesting conversations. Or perhaps engine manufacturers did'nt recommend power loading in the 70s. They probably just left it up to boat manufacturers at that time.

 

[psneeld]

I have limited experience in the following areas....

Engineering
Sales
Management
Operations

All can prove or disprove each others claims to product life, efficiency, desired use and MOST importantly how long to WARRANTY an item.

In business....that may be more of what you are reading and learning about a product as much as it is reality.

As we have heard...Lehmans were total messes in trucks but when overpropped and run at low RPMs outlast almost every owner. What did all the parts of the FORD group with Lemans think about that?????

 

[DavidM]

Well, this is a debate we have from time to time on this forum, so let's separate opinion from fact:

My opinion is that it won't hurt anything to overprop a Lehman as long as you cruise it at several hundred rpm below its new wot rpm, say 1,600 in the OP's case and never try cruising at wot.

Why? Well the Lehman is a 5 liter or so engine and even if severely overpropped this way it is probably only producing 40 hp at 1,600 rpm. There are many modern 5 liter engines that produce 50 hp at 1,600 rpm on their prop curve. So its unlikely that doing the same to the old Lehman is going to hurt it, although there are design features of modern diesels that help- better cooling system, oil squirting under the piston, etc.

But lets move to facts. Overpropping is done to ostensibly save fuel. If you overprop an engine then it will cruise the same speed at lower rpm. That should save fuel as you don't have to expend fuel to rev the machinery to a higher rpm.

But it isn't much. Lehman hp and fuel consumption curves are barely readable. So you have to look at a modern engine's curve to analyze this situation and I acknowledge that modern engines do better.

For example, my 5.3 liter Yanmar 6LY makes a little over 19 hp per gph at 2,300 rpm and drops to a little under 19 hp per gph at 1,800 rpm. As best I can read the curve there is less than a .5 gph difference. That is less than a 10% difference for a 500 rpm drop in rpm.

So I have to ask, if the fuel savings are not significant, why take the risk as well as not have the opportunity to use all of your hp in a tough situation?

David

 

[RichF]

Rec to reducte prop size

I have been told by several mechanics over the years that RPM's will not hurt your engine as long as they fall within the safe operable limits recommended by the manufacturer. LOAD is what can reduce the life of an engine (gas or diesel). Overloading and engine so it can turn up is bad for them. For example, MY IG may actually not have much life left in its engine because for the past 3,400 hours, it has been loaded up and unable to cruise at its recommended RMP (and still have the ~500-600 RPM buffer to the max rated load RMP of ~2,500 recommended by Lehman). I'm sure the previous owners cruised her at ~1,800 RMP thinking it was right when they may not have realized that they only had another 100 RMP to go before maxing the engine out.

Well, because I have a spare, I think it will be worth having maybe the four blade turned down a bit and see what it gets me. Speed isn't my ultimate goal although I know as all of us trawler owners do, we count the .1, .2 knots we gain (for current, wind, swell, whatever it may be) and a little more is always nice. I mainly want to make sure I'm not stressing the engine.

This weekend I will take her our and put a mechanical tach on the flywheel and post some accurate readings for everyone to see.

PS, the bottom is very clean and so was each prop. See the attached photo (taken a few days ago).

 

[RichF]

Your engine should run max rpm under no load. This your first check on governor setting. Boat bottom and prop must be clean. Both can hold down rpm. You may be able to reduce prop pitch by two inch's on existing props. I have seen boats over pitched by owners with intention of never running close hull speed with the interest of gaining range. I'm pretty sure 8.5 @knts is over hull speed for your boat. How does the engine run at 7knts.

Everything was clean, and the twin engine IG's, same hull with twin 275hp Volvo's in them can reach a max speed of 16 knots. Not at all what I'm looking for but because it is semi-displacement it is capable of planning given enough hp. What is strange is that the three blade that I mentioned in the OP is the one that the engineers and builders designed for the boat, its original. There is a good chance that IG, like the old GB's did, just propped it when they were built.

 

[sunchaser]

Rather than intrude on the same old blather, I can say I am not in the minority when it comes to this simple fact:

Unless buying a beater boat for major redo, if the vessel you are looking at cannot achieve full rated RPM without overheating, walk away. It may be propped wrong or a multitude of even worse things.

 

[RichF]

Well its done...


The IG is mine now, so I hope that its a simple solution, otherwise I will just run it at 1600 to keep the load down.

Also, not that is does much for information, but here is a video of that day, of the boat and engine room. I just happened to have them so I thought I'd share them with you guys.

http://youtu.be/t1MkMp0m7vE

 

[Bluto]

Wouldn't an EGT gauge be good to have? It would let you know how much pressure and heat you're inflicting on the engine at all times.
It would also show why you can't run at idle or wide open throttle all day. There are surprisingly few gauges to monitor the engines on a lot of older boats. If I had a $20K engine I'd like lots of sensors.

 

[Wxx3]

I have been told by several mechanics over the years that RPM's will not hurt your engine as long as they fall within the safe operable limits recommended by the manufacturer. LOAD is what can reduce the life of an engine (gas or diesel). Overloading and engine so it can turn up is bad for them. For example, MY IG may actually not have much life left in its engine because for the past 3,400 hours, it has been loaded up and unable to cruise at its recommended RMP (and still have the ~500-600 RPM buffer to the max rated load RMP of ~2,500 recommended by Lehman). I'm sure the previous owners cruised her at ~1,800 RMP thinking it was right when they may not have realized that they only had another 100 RMP to go before maxing the engine out.

Well, because I have a spare, I think it will be worth having maybe the four blade turned down a bit and see what it gets me. Speed isn't my ultimate goal although I know as all of us trawler owners do, we count the .1, .2 knots we gain (for current, wind, swell, whatever it may be) and a little more is always nice. I mainly want to make sure I'm not stressing the engine.

This weekend I will take her our and put a mechanical tach on the flywheel and post some accurate readings for everyone to see.

PS, the bottom is very clean and so was each prop. See the attached photo (taken a few days ago).

Ok.

Maybe I am slower today then usual, or maybe I'm just slow, period.

I've read this thread twice now and it's still not clear to me what your objective is.
I'm worried that it is me, as a lot of seemingly good advice had been given, but I'm still don't understand the question.

A lot of assumptions have been made, but sometimes I find it best to solve a problem, not to make assumptions.

 

[RT Firefly]

Greetings,
As an aside... Whether correctly propped or over propped the original performance spec's could be off a fair amount as from new. Keep in mind the upgraded battery banks, possible generator addition, full tanks, provisions, spares and tools etc. will add, in some cases, significant tonnage and depending on hull shape a degradation of "performance". This does not appear to be the OP's situation.

 

[FF]

Everything was clean, and the twin engine IG's, same hull with twin 275hp Volvo's in them can reach a max speed of 16 knots. Not at all what I'm looking for but because it is semi-displacement it is capable of planning given enough hp.


A planning hull gets up on top of the water and can run efficiently at higher speeds.

A Semi planning , even with triple+ the hp 2x275 vs 120 never gets a clean wake.

The question of power and overloading gets confusing to folks because the engine is able to create safe long term power at every RPM (above some minimum) with out overloading. Power of course varies with the RPM .

Engines are only efficient at small areas inside their fuel map , and the goal (unless its a ski boat or sport fish WOT lifestyle ) is to match the load with the boats power requirement for the desired cruise speed.

Sure 100% rated RPM is useful IF you usually travel at or near 100%.

Few cruisers do , because the advertising dept and engine cost works in favor of BIG , far more powerful than required engine selection.

The simple rules of thumb about power loading MUST be used with the HP guide from the engine builder.

An engine that can make 50 hp at 1600rpm is far better in terms of engine life operating there with a 40 hp prop load than a prop that can give 2500 rpm at WOT and then pulling back to whatever it is that gives 40 hp , and pushes the boat at the same speed.

The fuel saving may only be 10% to perhaps 30% , minor in boats that use under 3GPH , but the engine life , the numbers of hours it will run is increased by the proper load.

Again perhaps minor as an 8000 hour engine reduced to 4000 hours by underloading might take too many decades to notice , as the engine will be scrapped by lack of Da Book maint and parts availability.

Overloading is bad for any engine but a look at any prop curve will show a 10% RPM reduction from OBSERVED WOT RPM solves the problem.

Yes an EGT gauge (under $100) is worthwhile , but for most a painted red line 300rpm below the highest observed rpm will do the trick.

 

[Nomad Willy]

Rich F wrote:
"I have been told by several mechanics over the years that RPM's will not hurt your engine as long as they fall within the safe operable limits recommended by the manufacturer."

I've been under that assumption all along. But/and "hurt" is a relative word. To properly pickel your engine for 10years will "hurt" it less than to run it in every way conceivable way. I have a VW w a 1.8L engine that turns about 1800rpm at 70mph and I downshift it to 4th gear on hills thinking it's causing less wear and reducing the potential for damage. With the car and w the boats it's a similar consideration.

But the question that many are still asking is "will it hurt to overprop my diesel boat engine"? And I think it's been answered many times here recently and in the past. But I do believe RichF's mechanic is correct.

 

[Nomad Willy]

Fred wrote;
"An engine that can make 50 hp at 1600rpm is far better in terms of engine life operating there with a 40 hp prop load than a prop that can give 2500 rpm at WOT and then pulling back to whatever it is that gives 40 hp , and pushes the boat at the same speed."

That is indeed the question but that raises other important questions. It's (usually here) a 120hp engine so what happens when one wants 80hp? What happens when you sell the boat? Or someone else operates your boat for whatever reason?

But the answer to Fred's question as stated w obvious assumptions is probably to possibly yes. Do you know? Not sure really. Does the boat make less noise? Probably. Burn less fuel? Yes. Lot's of variables and many unknowns.

But as psneeld puts it there's not only one way to do it. But there's a best way and there's the safest way and that is to prop to rated rpm. That's why manufacturers recommend it.

Overpropping actually comes under the heading of modifying your boat. On that thread I said it was fine as it's your boat. That applies here too. But suffering the consequences does also.

 

[FF]

I have a VW w a 1.8L engine that turns about 1800rpm at 70mph and I downshift it to 4th gear on hills thinking it's causing less wear and reducing the potential for damage.

For cars and trucks the usual system is to only downshift IF the engine can not hold the speed on the grade.

Good precaution if your car is towing a heavy trailer .

For folks that worry about a next owner not being briefed/instructed on the use of the engine as fitted with a cruising prop, the simple solution is to reinstall the stock screamer, and let the next guy cruise at 2300 till he wises up.

Lots of noise and high rpm make the boat sound faster so he might be happier than with as near the sound of silence one can cruise at.

For someone borrowing the boat ,> observe the RED line < is hardly too much to ask.

 

[JDCAVE]

As the question pertains to Lehman's, I'd ask Bob Smith at American diesel. He spoke of prop sizing in his 2 day course. Bob will be able to comment on all aspects that relate to the situation about your engine performance.

Jim, Sent from my iPad using Trawler

 

[sunchaser]

Being over propped is like being overweight, it shortens your lifespan and causes you to over heat when put under stress. But if one is not concerned about the last 1/3 of your life, just loaf along and enjoy a full course meal 3X per day.

 

[Vashon_Trawler]

Being over propped is like being overweight, it shortens your lifespan and causes you to over heat when put under stress. But if one is not concerned about the last 1/3 of your life, just loaf along and enjoy a full course meal 3X per day.

Great analogy!

 

[Nomad Willy]

FF says;
"For cars and trucks the usual system is to only downshift IF the engine can not hold the speed on the grade."


Going full throttle up a grade should NEVER be done in a car unless it's for 10 or 20 seconds unless you're at the upper end of engine speed and then only for a minute or less. Just downshift.

 

[psneeld]

Great analogy!

Actually not..being overweight isn't necessarily bad...obese is...research has shown some weight is beneficial in recovery from illness, etc. It's just a matter of how much.

Besides unless you talk specifics on overpropping.... it's all just talk..and speculation of wear, tear, longevity etc...all you know is that you aren't following some arbitrary number.

 

[hollywood8118]

Actually not..being overweight isn't necessarily bad...obese is...research has shown some weight is beneficial in recovery from illness, etc. It's just a matter of how much.

Wow... I am going to pitch this one to the Admiral every time she yells at me to put down my beer and get off the settee!

Although she is a touch obsessive.. 26 marathons in the last 8 years and she is out cycling today as she is in training for a Ironman

HOLLYWOOD

 

[psneeld]

Wow... I am going to pitch this one to the Admiral every time she yells at me to put down my beer and get off the settee!

Although she is a touch obsessive.. 26 marathons in the last 8 years and she is out cycling today as she is in training for a Ironman

HOLLYWOOD

Cruise in the Arctic or Antarctic and you can justify the love handles also as survival gear...