I recently repowered a 20' catboat with a new 3cyl 20 hp diesel. The boat originally had a single cyl 10 hp diesel that would push it at hull speed. Both use a 2-1 reduction. I went with the 3 cyl to get smoother performance, and the extra power. My question is about prop size.The new engine is rated at 3800 rpm which to me is very fast. Using a prop that allows the engine to top out at 3800 requires it to run in the high twenties to get hull speed. Using a much bigger prop, the boat can hit hull speed in the high teens, but the engine will only max out in the high twenties, and blue dark smoke if given full throttle. It runs smooth and clean cruising and burns a quart an hour. With the small prop, it's noisy and burns much more fuel. My question is, am I hurting my nice new engine running it at really a fast idle? The 20 hp obviously develops the needed 10 hp at half speed. It sounds like it's just loafing along. It's not over heating. but I only have an idiot buzzer, no gauge. I keep hearing that a prop MUST allow an engine to top out at "rated" rpms, but what if you have an oversized engine? It seems to make sense to me that running an engine at reduced rpms will prolong it? Good sailing Ron Brassord Lighthouse Point FL.
New diesel question
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engine speed
What I gather from mechanics on other lists and discussions is that it is indeed harmful to always run a diesel at lower rpm and loads than recommended.
What I gather from mechanics on other lists and discussions is that it is indeed harmful to always run a diesel at lower rpm and loads than recommended.
diesel engines
You made a mistake by putting in an engine that is to big for your application. Now you will not be able to run the engine the way it was suppose to be run.
You made a mistake by putting in an engine that is to big for your application. Now you will not be able to run the engine the way it was suppose to be run.
Will be fine
If you where to split the difference between your large prop and small prop, allowing the motor to reach the high twenties to low thirties, you will be fine. Don't run it too long under full throttle if it is smoking a lot, but do use it hard, diesels like a load. People re-power with larger engines all the time and they last forever with a proper maintanance schedule. Just get out there and have fun and congrats on your new kicker... Cheers, Dave
If you where to split the difference between your large prop and small prop, allowing the motor to reach the high twenties to low thirties, you will be fine. Don't run it too long under full throttle if it is smoking a lot, but do use it hard, diesels like a load. People re-power with larger engines all the time and they last forever with a proper maintanance schedule. Just get out there and have fun and congrats on your new kicker... Cheers, Dave
What is a reasonable load?
I have a three cylinder 30HP Yanmar. I think that max rpm is 3400. At 2400 it is pushing the boat at about 6 knots which feels pretty good to me. Hull speed for the boat is about 7.5 knots. At 2400rpm am I loading the engine enough to stay healthy? I have run it in strong wind and sea at 2800rpm for extended periods and it doesn't seem to mind, but it uses much more fuel than at 2400. Any advice??
I have a three cylinder 30HP Yanmar. I think that max rpm is 3400. At 2400 it is pushing the boat at about 6 knots which feels pretty good to me. Hull speed for the boat is about 7.5 knots. At 2400rpm am I loading the engine enough to stay healthy? I have run it in strong wind and sea at 2800rpm for extended periods and it doesn't seem to mind, but it uses much more fuel than at 2400. Any advice??
full RPM?
Hi...neighbor I have a 2gm20 I put in my H-28 in 98...had same questions about props...talking to desiel people they told me as long as the engine has a LOAD on it when run everything wil be OK..My engine will not run past 3300 as itis loaded up at the dock..no smoke but prop16x13 sure moves the water..go with the load
Hi...neighbor I have a 2gm20 I put in my H-28 in 98...had same questions about props...talking to desiel people they told me as long as the engine has a LOAD on it when run everything wil be OK..My engine will not run past 3300 as itis loaded up at the dock..no smoke but prop16x13 sure moves the water..go with the load
Yanmar
I have a 3 cyl, 27 hp Yanmar 3GM30F, the manual says to cruise at 80% WOT, which works out to 2,800 rpm.
I have a 3 cyl, 27 hp Yanmar 3GM30F, the manual says to cruise at 80% WOT, which works out to 2,800 rpm.
Bert
Bert, Have you put a lot of hours on your diesel at 2800? Does it seem to like it? What kind of boat? Thanks.
Bert, Have you put a lot of hours on your diesel at 2800? Does it seem to like it? What kind of boat? Thanks.
Max ratings is 3600 rpm on GM engines
I think you will find that the MAX rating is 3600 RPM. But you should review this website. This guy has a lot of info on the Yannies. He gives explanations too. http://yanmarhelp.com/
I think you will find that the MAX rating is 3600 RPM. But you should review this website. This guy has a lot of info on the Yannies. He gives explanations too. http://yanmarhelp.com/
It is really a lot better.....
It is really a lot better to get information first hand. Keep this link, it is a good reference.
It is really a lot better to get information first hand. Keep this link, it is a good reference.
Yanmar
Liam, My WOT is 3,600, (I know, 80% of 3,600 is closer to 2,900, but Yanmar said to use 2,800), and I have not put that many hours on the motor. However, everybody I talk to and all the diesel people say to run them hard...they like it and in the end will last longer. The worst thing you can do is baby them, and mine seems to like the 2,800...it's in a Catalina 320, which weighs 12,000 empty. Yanmar says one HP for each 500 lbs of boat weight, so I'm just about right.
Liam, My WOT is 3,600, (I know, 80% of 3,600 is closer to 2,900, but Yanmar said to use 2,800), and I have not put that many hours on the motor. However, everybody I talk to and all the diesel people say to run them hard...they like it and in the end will last longer. The worst thing you can do is baby them, and mine seems to like the 2,800...it's in a Catalina 320, which weighs 12,000 empty. Yanmar says one HP for each 500 lbs of boat weight, so I'm just about right.
new diesel
My 2 cents... Yanmar builds high speed diesels not the old diesels you read about in the past that push a boat fast at 600 rpm. Diesels need a load so that they don't soot up their combustion chambers and exhaust systems. If you can install a prop that gives you hull speed in a moderate sea that also gives you a tack reading in the mid to high 20s you'll do fine. I prefer more pitch on a two blade prop rather than a bigger three blade prop...after all this is a sailboat.
My 2 cents... Yanmar builds high speed diesels not the old diesels you read about in the past that push a boat fast at 600 rpm. Diesels need a load so that they don't soot up their combustion chambers and exhaust systems. If you can install a prop that gives you hull speed in a moderate sea that also gives you a tack reading in the mid to high 20s you'll do fine. I prefer more pitch on a two blade prop rather than a bigger three blade prop...after all this is a sailboat.
Learned a lot
Thanks to everyone for all the great info. I have really learned a lot from this discussion.
Thanks to everyone for all the great info. I have really learned a lot from this discussion.
New Diesel - Alternator RPM
Ron Running engines at low speed is not good practise. Apart from being bad for the engine, the alternator can not deliver the full output. However if the manufacturer recommends 2800 for cruising, I guess he will have considered the alternator too. As an aside, I can recommend "Boatowners mechanical and electrical manual" Third edition by Nigel Calder. It covers just about everything and has already saved me twice its purchase price. (PS It doesnt address your specific question)
Ron Running engines at low speed is not good practise. Apart from being bad for the engine, the alternator can not deliver the full output. However if the manufacturer recommends 2800 for cruising, I guess he will have considered the alternator too. As an aside, I can recommend "Boatowners mechanical and electrical manual" Third edition by Nigel Calder. It covers just about everything and has already saved me twice its purchase price. (PS It doesnt address your specific question)
Prop vs engine Speed
The engine will never develop full power if it is not able to turn rated RPM and you may never need the full power. In 30-40 knots of wind on a lea shore, you will bless the day you went to the bigger engine because it will simply let you motor into seas that will stop people with small engines. The engine needs to run at about 60% rated RPM (or well into the torque curve- see your owners manual) and still have a load on the engine. If you don´t have a turbo on the engine, just about any speed, under a good load, will work--but-- you will loose all of the advantages you paid for if you can´t get the RPM´s into the troque band. If you are putting out black smoke, you are definitely over propped.
The engine will never develop full power if it is not able to turn rated RPM and you may never need the full power. In 30-40 knots of wind on a lea shore, you will bless the day you went to the bigger engine because it will simply let you motor into seas that will stop people with small engines. The engine needs to run at about 60% rated RPM (or well into the torque curve- see your owners manual) and still have a load on the engine. If you don´t have a turbo on the engine, just about any speed, under a good load, will work--but-- you will loose all of the advantages you paid for if you can´t get the RPM´s into the troque band. If you are putting out black smoke, you are definitely over propped.
so the proper answer is.
prop so the engine can run max rpm. that will ensure you have the power when you need it. and you will get the max in mileage and efficency. if you dont you wont be as happy as you could be. and you may have a much less reliable system. Dont forget you need to keep the alternator running too! two blade vs. three in more interesting. two blade = less drag while sailing and less efficency as you want to increase power in roughy conditions. and Way less reverse thrust. Three blades + more drag, better range of power and better reverse. three blades will also usually induce a bit more reverse prop walk, but lots more power when you want it. Dont be afraid to run your yanmar, just change the oil and give it clean air and fuel and it will be there for years. mine has a couple thousand hours and is perfect/
prop so the engine can run max rpm. that will ensure you have the power when you need it. and you will get the max in mileage and efficency. if you dont you wont be as happy as you could be. and you may have a much less reliable system. Dont forget you need to keep the alternator running too! two blade vs. three in more interesting. two blade = less drag while sailing and less efficency as you want to increase power in roughy conditions. and Way less reverse thrust. Three blades + more drag, better range of power and better reverse. three blades will also usually induce a bit more reverse prop walk, but lots more power when you want it. Dont be afraid to run your yanmar, just change the oil and give it clean air and fuel and it will be there for years. mine has a couple thousand hours and is perfect/
Sincere thanks
I have studied all the great responses, and learned much. I appreciate the advise, and I think I am going to be OK with the setup I have running.I am over powered I agree, and will probably never need or can use the full 20 hp. I also agree that if I can get 70 percent rpm at WOT, and will cruise at 1800+/- the engine will function fine. None of my old fashioned diesels ever turned anywhere near the 3800 rpm rated on this new Kubota 3 cyl, but I'm sure it's up to it. I just don't like hearing an engine winding up.If I had bought a 10 hp engine, I would have no choice but run it at a very high rpm. I do like the four hours I get on a gallon too. Time will tell. Time to go sailing Thanks again Ron Brassord Lighthouse Point Fl.
I have studied all the great responses, and learned much. I appreciate the advise, and I think I am going to be OK with the setup I have running.I am over powered I agree, and will probably never need or can use the full 20 hp. I also agree that if I can get 70 percent rpm at WOT, and will cruise at 1800+/- the engine will function fine. None of my old fashioned diesels ever turned anywhere near the 3800 rpm rated on this new Kubota 3 cyl, but I'm sure it's up to it. I just don't like hearing an engine winding up.If I had bought a 10 hp engine, I would have no choice but run it at a very high rpm. I do like the four hours I get on a gallon too. Time will tell. Time to go sailing Thanks again Ron Brassord Lighthouse Point Fl.
Prop for more rpm
As a diesel engine mechanic in past years I think you were correct to go with a 3 cylinder engine as they are inherrently smoother and easier on engine mounts because of it. I do however disagree with over propping as it will lead to premature engine failure. While you don't know it now because the problem will only manifiest itself later with shorter engine life for some of the following. 1)Running it even if it doesn't make smoke would be like taking your car up a hill in a gear to high. You may yet get to the top with the throttle floored but use much more fuel and put undo stress on the rings which can break them or carbon up the combustion chamber and ring lands. 2)With an over propped engine you WILL be dumping more fuel into the cylinder and not get complete combustion. What happens to the excess fuel? Some will go out the exhaust and some will likely get past the overworked rings and go into the oil diluting it. This will cause thinning of the oil (increased consumption) and a high potential to wash out the main and rod bearings. By the time you notice rod knock it will be to late. 3)If and engine can't accelerate cleanly it is being lugged which will cause the previous mentioned problems along with shorter life span. I've preferred changing the oil at least every 50 hours if not sooner. Do NOT use regular car oil but oil specifically for diesel. I've preferred the delo 400 by Chevron but others will work. BTW I don't and have never worked for Chevron. I've seen many engines fail from lack of maintenance and in trucking, drivers that abused their engines. Diesels like to be run at 70-90 percent of max rpm usually where the horsepower and torque curves meet. This typically gives the lowest fuel consumption for hp generated. If your engine can't run at that rpm + 300 rpm you are way over propped and will do damage. Its only a matter of time.
As a diesel engine mechanic in past years I think you were correct to go with a 3 cylinder engine as they are inherrently smoother and easier on engine mounts because of it. I do however disagree with over propping as it will lead to premature engine failure. While you don't know it now because the problem will only manifiest itself later with shorter engine life for some of the following. 1)Running it even if it doesn't make smoke would be like taking your car up a hill in a gear to high. You may yet get to the top with the throttle floored but use much more fuel and put undo stress on the rings which can break them or carbon up the combustion chamber and ring lands. 2)With an over propped engine you WILL be dumping more fuel into the cylinder and not get complete combustion. What happens to the excess fuel? Some will go out the exhaust and some will likely get past the overworked rings and go into the oil diluting it. This will cause thinning of the oil (increased consumption) and a high potential to wash out the main and rod bearings. By the time you notice rod knock it will be to late. 3)If and engine can't accelerate cleanly it is being lugged which will cause the previous mentioned problems along with shorter life span. I've preferred changing the oil at least every 50 hours if not sooner. Do NOT use regular car oil but oil specifically for diesel. I've preferred the delo 400 by Chevron but others will work. BTW I don't and have never worked for Chevron. I've seen many engines fail from lack of maintenance and in trucking, drivers that abused their engines. Diesels like to be run at 70-90 percent of max rpm usually where the horsepower and torque curves meet. This typically gives the lowest fuel consumption for hp generated. If your engine can't run at that rpm + 300 rpm you are way over propped and will do damage. Its only a matter of time.
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