Engine - Hp - Turbo RPM - Fuel consumption - Gear ratio - Propshaft RPM

[alaskanviking]

Hi,

I have a knowledge based problem I'm trying to figure out. I think i have the answer, but its a complicated enough topic and i cant completely get my head around things for 100% confidence I'm on a good path. first there's some context involved

i have a 42ft Sailboat, has a pilothouse, and powered by a Perkins 4-108. was 30000lbs roughly when i had it shipped cross country. engine is ok, but i never had much doubt that its essentially enough power to get off the dock and out enough for the sails to take over. its "50hp" rating is really essentially 35hp. it does push the boat along at 7-8 knots on flat water. i have run in rougher water and with some current that bleeds off quick to 2-3knots. there is no extra power if you need it, even running at full throttle. for a big cruising boat that's kinda a bad thing

the next issue with the setup is an uncommon V-Drive transmission. that does give me a extra large engine compartment which is a big plus. there really isn't any way to switch the engine around for an inline setup. thats not a big deal but it does limit the gear boxes that are available. it has a ZF/Hurth 15IV transmission. the problems lie with that. it is a splash lubricated small transmission, same thing as a 15m but in vdrive form. 15 degree angle. there is a seawater cooling component but as there isn't an oil pump inside the gear, its essentially a heatsink with seawater running thru bolted to the side. problem as efficient and as effective as that sounds. equally disturbing to me is that also means there isn't an oil filter either. I have seen that this has caused problems and can damages the gear. the rating for the gear case is max 73hp at "recreational" power loads, for a "continuous" rating it was roughly 30hp. i have a brand new replacement for it, however the engine needs to be replaced with something with more oomph when its needed, and that's pretty much going to be to much for the little transmission.

in my mind, this will simply be swap in a different model of gear with more capacity, bolt on a larger engine, bolt it u to the shaft coupling and bobs your uncle problem solved! eh no, not really. for drive's all but the velvet drive v-drive have 12 degree shaft, and the velvet drive or a multiple shaft style vdrive gear will push the engine to far aft for the engine compartment. I've been thinking about this for a while.

so looking on marketplace a few weeks ago, I stumble onto a used Yanmar 4lh. originally listed for 5k, he had dropped it to $1500. checked measurements and such for comparison and since everything looks good i picked it up. older but rather nice engine, in good shape.

its a Yanmar 4lh-DTE turbo diesel with seawater intercooler. Yanmar rated it at 270hp, 150 continuous. SB Marine has a nice article about sizing engines for durability, has some good info on the differing manufactures definition of essentially where surprise Yanmar is a little bit exaggerated for "continuous duty", which is roughly running the engine constantly like a motor sailing all night kind of scenario. pretty much most cruisers. article essentially states that for long term durability, sizing for 20-35hp per liter, and 2gph of fuel at cruising speed 30-70% max rpm. not to many surprises here, and matches what I've seen working on Cummins jet drives. for the 4lh-dte that puts it at 2300-2700 85-110hp at the engine and with 3.45 liters roughly 120hp continuous. put 2 big alternators on and that drop 20-30hp so roughly 90-100hp continuous. which matches the 80-100hp in my head for an engine with enough horsepower for currents and winds that are in Alaska and Washington. the 170hp listed does kind of freak me out as it would be replacing a Perkins 4-108 but math appears to support it. size wise the engine is 4" wider, and 3 inches longer than a perkins 4-108 so space and mounting consideration there isn't a problem

pretty much only available transmissions are the ZF63IV and Yanmar KMH50A. length not much different than the 15iv,but taller and has both filtration and real water cooling. continuous rating of 150hp.

knowns are - I would have to replace the transmission with a bigger unit anyway. which also means i have to change the shaft angle, and move the shaft log forward 6-10 inches maybe. I also need to replace the shaft strut anyway as i seen some pitting. looks like i can fit or adapt the 63IV to most engines in the 75-100hp range.

so here is the problems I'm having trouble squaring in my head:

1.) gear ratio - higher ratio's = slower rpms of the shaft = better, but increases torque. currently have a 2:1 and research suggests this would be good here. prop size might be able to go slightly bigger with angle and shaft strut change, but realistically were limited here. indicates the current prop actually might be a good start off point. another point of consideration however is the RPM range of the turbo. low engine RPMs on a regular basis will coke up the turbo, and there's an efficiency range where if your always at low rpms the turbo isn't doing anything for you. it needs to spool up some. my thoughts are to get a 2.5:1 gear which seems to be max for that transmission. that will reduce the shaft rpm's and should keep the turbo spooled up higher as engine will need higher rpm for the same shaft speed as i have now with the perkins. however higher engine rpms should also = more fuel usage and will probably need more prop blades or more pitch to load the engine correctly.

2.) fuel consumption in general. i understand more rpm and more load = more fuel. but for 170hp engine and limited prop size i think loading it down fully may be difficult. the torque should be there. it is also known that pushing a displacement vessel at a cruising seed of 2000-2400 rpm, isn't going to load the engine as much, without the load your not using as much fuel.

so lets say I'm cruising, motor sailing or just motoring on the autopilot at night. 2000-2400 rpms with lightly spooled turbo. the engine isn't struggling, its not flat out like a power boat so were not really burning much fuel in theory how about charging the batteries off the engine, running t at 1500 or so to charge.? hwat are we really looking at here with a 2.5:1 or a 2.5:1 as far fuel consumption goes? how is accessories like alternators affecting fuel consumption in this case? is it worth going to 2.5:1 reduced maintenance from better running turbo.?

in my head I'm envisioning a high pitch prop shooting water out the back of the boat not really catching the water, or if its aggressively pitched the boat shoots forward in idle. would the slower prop speed be better for the bot

would the 2.5:1 with less load save fuel more, and will that be 2gm or less . is my thinking on this sound? im having a hard time thinking higher Eng. rpms no matter the load are going to use more fuel unless loads increase. i haven't worked with governors much and guessing that it would meter down the fuel if its not needed?

are my rough calculations good for this engine to replace the 4-108? i had been thinking about a cummins 4bta ~ 120-150hp, as i know cummins well. but it was too tall

please if you can send some insite here. in theory this all seems to work out but its hard to work around the marketing figures designed for a different type of boat. trawler forums suggest this is a good path. 4lh-dte running about 2000-2700 on 2.5:1 with the lowest fuel consumption it can.

another option would be to change injectors, pump and injectors to detune into a 4LH-HTE which is about 120hp,

i know long winded, thanks for any information
Robert

[alaskanviking]

for info purposes



Perkins 4-108 in torque band from 2000-2400rpm.

2000 - 29 engine hp, 7hp to prop
2200 - 33 engine hp, 10hp to prop
2400 - 35 engine hp, 12hp to prop

max 3000 - 42 engine hp, 21 to prop



Yanmar 4lh-dte 170hp
peak torque band at 2700-2900
fuel consumption band 2300-2800 (lowest)
2300 - 118 eng hp, 55hp at prop
2400 - 130 eng hp, 60hp at prop
2500 - 140 eng hp, 70hp at prop
2600 - 145 eng hp, 80hp at prop
2700 - 150 eng hp, 90hp at prop
2800 - 155 eng hp, 100hp at prop

max 3200 165engine hp, 165hp at the prop

[donradcliffe]

Too much engine. You would be happier with 75 maxhp.

[Dockhead]

Way too much engine for that boat. You are proposing to swap an 1800cc engine for a 3500cc one with nearly 4x the hp. You think the engine will fit your space, but how about the drive which suits that power and torque? You would need one of those big power boat ones. And even if you could make it fit, it wouldn't be happy driving a 30,000 pound sailboat. You couldn't fit a prop of appropriate size.


I would sell it on and look for a 4JH3 E or TE, or a Beta engine, of appropriate size. As Don said, max 75hp. The 4JH3's would be a great choice as they are actually lighter than the Perkins. Don't be afraid of a turbo, which will be fine running at low power as long as you run it up periodically as the Yanmar manual says to do. Turbo is more efficient and makes the motor smoother and quieter and is dead simple with one moving part -- a good thing on a marine diesel engine.



Nothing wrong with your 15IV gearbox so long as you don't overload it. You'll save a lot of money and time by keeping that.

[NorthCoastJoe]

I replaced a Perkins 4108 with a Beta 50. No problems and very happy with it crossing Hawaii to San Francisco.

The motor had zero hours in Honolulu. About 180 hours of motoring in an 18 day trip. My boats loaded weight was about 30,000 pounds.

[skipperpete]

The most powerful version of the 4LHA is 169 hp continuous and if you prop your boat to absorb the torque from that engine, the vee drive will be toast before the sea trial is over. Too much engine??……. We have an Apreamare 22 with a displacement hull on our marina powered by twin 4LHA-STE’s, that’s a lot of power in a 22 footer. Nearby are two sailboats with 4LHA’s, one is 60’ the other is 75’.
Just forget about installing the 4LH-DTE

[Montanan]

Your starting remarks.

"i have a 42ft Sailboat, has a pilothouse, and powered by a Perkins 4-108. was 30000lb".

From then on you wonder off towards desiring to have a motorboat.

170 HP!!! Do you intend to plane with that displacement hull?

Please reconsider. It is a sailed boat, keep it that way. IMO, keep it simple and not overpowered and overthought.

[Lee Jerry]

Agree with what others have said - way, way, way too much engine.

However, I also noticed you said this:

Originally Posted by alaskanviking

so looking on marketplace a few weeks ago, I stumble onto a used Yanmar 4lh. originally listed for 5k, he had dropped it to $1500. checked measurements and such for comparison and since everything looks good i picked it up. older but rather nice engine, in good shape.

So it appears you already bought it. I think that was a mistake and you should consider just re-selling it. And then get a better sized engine, if actually required, probably in the 60-70-75 hp range. But since you have it, I'll give a few thoughts.

First, however, why do you say this w.r.t. the current Perkins:

Originally Posted by alaskanviking

its "50hp" rating is really essentially 35hp.

You go on to say:

Originally Posted by alaskanviking

it does push the boat along at 7-8 knots on flat water.

Which is it? There's a big difference between 7 kt and 8 kt in required power for a 42 ft displacement boat. 8 kt is probably about your hull speed, so if you can hit that then you're probably actually at reasonable power.

The $64,000 question is: What RPM does the current engine reach (actual at the engine, not just indicated on the tach unless "calibrated") at WOT (wide open throttle) in calm water with clean bottom and prop? And then what boat speed (through the water) is reached? Also, what size/type is your current prop?

Originally Posted by alaskanviking

i have run in rougher water and with some current that bleeds off quick to 2-3knots.

Current doesn't really factor into it; the boat/engine only knows speed through the water. Wind and waves do have to be overcome.




So on to the proposed new engine...

Originally Posted by alaskanviking

its a Yanmar 4lh-DTE turbo diesel with seawater intercooler. Yanmar rated it at 270hp 170hp, 150 continuous.

Again, way too much power. I'm going to jump to your last statement in the first post:

Originally Posted by alaskanviking

another option would be to change injectors, pump and injectors to detune into a 4LH-HTE which is about 120hp,

Yes, you would absolutely have to do this. You simply have too much power, so reducing it as much as possible is required. You may even want to talk to Yanmar about removing the turbo.

Originally Posted by alaskanviking

knowns are - I would have to replace the transmission with a bigger unit anyway. which also means i have to change the shaft angle, and move the shaft log forward 6-10 inches maybe. I also need to replace the shaft strut anyway as i seen some pitting. looks like i can fit or adapt the 63IV to most engines in the 75-100hp range.

Yes, you'll need a new transmission, but also a new (larger) prop shaft. Your current shaft will not be able to handle the torque of the new engine (and prop, see below). So new engine beds, new transmission and mounts, new shaft log and location (and fill the old), new strut, new shaft - that sounds like a ridiculous amount of work to put into a 40 year old boat. Maybe if you're doing it yourself and enjoy that type of work...

Originally Posted by alaskanviking

1.) gear ratio - higher ratio's = slower rpms of the shaft = better, but increases torque. currently have a 2:1 and research suggests this would be good here. prop size might be able to go slightly bigger with angle and shaft strut change, but realistically were limited here. indicates the current prop actually might be a good start off point.

The new engine will require a larger prop (diameter and pitch). The prop has to be able to absorb the engine power and efficiently transfer it into the water. Without the proper prop, the power of the engine is nearly meaningless. There is no way your current prop, presumably sized to absorb the current ~47hp delivered to it, will be able to absorb ~120+hp. I would ask what is the max diameter prop you can fit, but you'll be re-arranging the running gear anyway, so maybe not germane.

Originally Posted by alaskanviking

another point of consideration however is the RPM range of the turbo. low engine RPMs on a regular basis will coke up the turbo, and there's an efficiency range where if your always at low rpms the turbo isn't doing anything for you. it needs to spool up some. my thoughts are to get a 2.5:1 gear which seems to be max for that transmission. that will reduce the shaft rpm's and should keep the turbo spooled up higher as engine will need higher rpm for the same shaft speed as i have now with the perkins. however higher engine rpms should also = more fuel usage and will probably need more prop blades or more pitch to load the engine correctly.

The gear ratio will need to be determined with the prop sizing. If it can even be made to work properly...

Originally Posted by alaskanviking

2.) fuel consumption in general.
{snip}

Immaterial at this point.

Originally Posted by alaskanviking

are my rough calculations good for this engine to replace the 4-108?

No. Not even close. Sorry.

Originally Posted by alaskanviking

for info purposes
Perkins 4-108 in torque band from 2000-2400rpm.

2000 - 29 engine hp, 7hp to prop
2200 - 33 engine hp, 10hp to prop
2400 - 35 engine hp, 12hp to prop

max 3000 - 42 engine hp, 21 to prop

There is no "eng hp." At any particular RPM, the engine can produce up to the max output line. The actual power produced is determined by the demand, in this case, that of the propeller (plus losses and connected loads). So if the Perkins was operating at 3000 RPM (steady state) along that prop curve then the engine would be producing around 23-25 hp (~21 to prop, ~1 to losses, ~2 to connected loads). (Or perhaps the engine is producing 21 hp with ~18 going to prop, ~1 to losses...depends on how the calcs are done, but the concept is the same.)



If you actually plan to attempt this engine swap, I recommend you seek some professional assistance, at least for the planning/engineering phase.

[SailingHarmonie]

270 hp? for a 42 foot sailboat? No. Just NO. Others have had the patience to explain why, but I will add my voice to telling you this is an incredibly bad idea.

[MicHughV]

All that aside, going for larger horsepower also means a larger prop, larger shaft, etc

All of the above needs to be taken into account and it's no simple matter. diameter, pitch, number of blades, etc on a prop, prop clearance, and many other things, too many to list here, all need to be addressed.

I'm going to refer you to " The Propellor Handbook" authored by Dave Gerr. In it you will find the answers to all your questions. It's a complicated and very detailed analysis to answer all your questions.

While nobody ever complained about having " too much horsepower", 270 hp is a tad too much for your boat for many, many reasons, some of which I outlined above.

Lastly, diesel engines are very unlike their gasoline counterparts, and can get quite temperamental if not run at the right load and rpm.

Do yourself and get the book !!

[Lee Jerry]

Originally Posted by SailingHarmonie

270 hp? for a 42 foot sailboat? No. Just NO. Others have had the patience to explain why, but I will add my voice to telling you this is an incredibly bad idea.

That's a typo. It's only 170 hp. So...you know...perfectly reasonable.

[skipperpete]

Regarding the Cummins 4BT that you suggested, its even heavier than the Yanmar 4LH and almost exactly the same height. A bobtail 4BT is apparently 499kg and 31” tall. The 4LH is around 31” tall and 400 kg.
This raises the question of how much extra weight your sailboat can carry ( and of course the extra fuel you’ll need )
Here in Langkawi I once did some engine work on a Caterpillar 3208T in a 40’ Hartley ferrocement sailboat so other folks have a similar soft spot for vastly overpowering cruising yachts.

[SailingHarmonie]

Originally Posted by Lee Jerry

That's a typo. It's only 170 hp. So...you know...perfectly reasonable.

Oh, yeah, My bad.... Sure go right ahead.

[Dockhead]

Originally Posted by Lee Jerry

Agree with what others have said - way, way, way too much engine.

However, I also noticed you said this:

So it appears you already bought it. I think that was a mistake and you should consider just re-selling it. And then get a better sized engine, if actually required, probably in the 60-70-75 hp range. But since you have it, I'll give a few thoughts.

First, however, why do you say this w.r.t. the current Perkins:

You go on to say:

Which is it? There's a big difference between 7 kt and 8 kt in required power for a 42 ft displacement boat. 8 kt is probably about your hull speed, so if you can hit that then you're probably actually at reasonable power.

The $64,000 question is: What RPM does the current engine reach (actual at the engine, not just indicated on the tach unless "calibrated") at WOT (wide open throttle) in calm water with clean bottom and prop? And then what boat speed (through the water) is reached? Also, what size/type is your current prop?


Current doesn't really factor into it; the boat/engine only knows speed through the water. Wind and waves do have to be overcome.

So on to the proposed new engine...

Again, way too much power. I'm going to jump to your last statement in the first post:

Yes, you would absolutely have to do this. You simply have too much power, so reducing it as much as possible is required. You may even want to talk to Yanmar about removing the turbo.

Yes, you'll need a new transmission, but also a new (larger) prop shaft. Your current shaft will not be able to handle the torque of the new engine (and prop, see below). So new engine beds, new transmission and mounts, new shaft log and location (and fill the old), new strut, new shaft - that sounds like a ridiculous amount of work to put into a 40 year old boat. Maybe if you're doing it yourself and enjoy that type of work...

The new engine will require a larger prop (diameter and pitch). The prop has to be able to absorb the engine power and efficiently transfer it into the water. Without the proper prop, the power of the engine is nearly meaningless. There is no way your current prop, presumably sized to absorb the current ~47hp delivered to it, will be able to absorb ~120+hp. I would ask what is the max diameter prop you can fit, but you'll be re-arranging the running gear anyway, so maybe not germane.

The gear ratio will need to be determined with the prop sizing. If it can even be made to work properly...

Immaterial at this point.

No. Not even close. Sorry.

There is no "eng hp." At any particular RPM, the engine can produce up to the max output line. The actual power produced is determined by the demand, in this case, that of the propeller (plus losses and connected loads). So if the Perkins was operating at 3000 RPM (steady state) along that prop curve then the engine would be producing around 23-25 hp (~21 to prop, ~1 to losses, ~2 to connected loads). (Or perhaps the engine is producing 21 hp with ~18 going to prop, ~1 to losses...depends on how the calcs are done, but the concept is the same.)

If you actually plan to attempt this engine swap, I recommend you seek some professional assistance, at least for the planning/engineering phase.

Wow, masterful post. I hope the OP pays attention -- this answers virtually every question. And answers correctly

[FPNC]

A 30,000 pilot house vessel could use 60 to 70 hp. Unfortunately, Passport never left too much room for a large diameter propeller. Even 50 hp starts to overload a 18" diameter, 3 blade prop. Which your Passport may be limited to.

Figure out the largest propeller that will fit and the number of blades that you want to use. With that data, figure maximum HP and proper gear reduction to get the power to the water in an efficient manner. That's your best engine/transmission option. Basically, you start at the prop and figure forward.