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.