Underpowered. A matter of RPMs???

[Warren Kaplan]

I started thinking about being "underpowered" at 2 AM. One of those things that just pops into your mind in the middle of the night that you just can't get rid of. I'm not a mechanic by trade so maybe some of you can clarify a question for me that seems elementary to you.
The CD27 came with many versions of engines. Originally 7.5 or 8 hp Yanmars, then 12 HP. Some have repowered with 18 hp, if I remember correctly. But let me see if I understand this. The CD27 no matter what year or engine still has a 1 inch prop shaft and still has a 12 inch diameter propeller. Lets say for arguement's sake that everyone either has a 2 bladed or a 3 bladed prop since we are talking strictly about engines. The cutout for the prop will not allow a larger prop. So, no matter what size engine you have, the prop spins "x" number of rpms on a 1 inch shaft and assuming the same pitch in the prop, moves the same amount of water no matter what engine you have. Now lets take "bathtub" conditions i.e. no current or waves, not even a tide to deal with. The CD27's hull speed is 6 knots. Both an 8 hp engine and an 18 hp engine should have no trouble getting the boat to hull speed and I would imagine that the rpm's would be the same because its an easy load for both engines. Right or wrong? If we pushed both trottles on both engines up to full, would the rpms on the 18 hp engine be greater than the rpms on the 8 hp engine under these "minimum load" conditions?? If now we add a 4 knot current against our progress I take it that it is "harder" for the engine to generate rpms against the current. Does the small 8 hp engine just simply reach its limit (i.e.full trottle) before it can generate the needed rpms to move the boat at hull speed while the 18 hp engine still has reserve power to spin the prop faster? In other words, one revolution of similar props will move a boat the same distance no matter what engine its attached to. The only reason for difference in speed, it seems to me, is due to the fact that the small engine can't turn the prop fast enough against resistance of fast current before it runs out of power. I read an article that said the proper size of an engine is roughly 2hp for every 1000 lbs dispalcement. The CD27 displaces 7500 lbs. Which means it should have a minimum of 15 hp for the iron genny.

Thanks for the input,

Warren Kaplan
Sine Qua Non ( with a YSM8)
CD27



Setsail728@aol.com

[Mark Yashinsky]

In most cases (someone messed up), you would NOT have the same prop. Either more blades (2->3) or more pitch to use the extra hp. You would turn the prop the same speed but the prop can now move more water (and have more drag at rest). Things as extra weight (tanks, supplies, people) add mass that the engine has to get moving (or stopped) as you jockey in and out of that slip. What about that blow that comes up as you are motoring along. Does it ever happen and what are your options when it does. If you have none, then a generously size engine might be your only option.

[Bill Goldsmith]

Warren,

Just to emphasize what Mark wrote, the prop is not a constant. A YSM8 spinning a 12 in prop would require a lower pitch than, let's say, a 2GM20 with a 12 inch prop. The bigger engine can handle more water per revolution while under way. Any good repower specialist could run the numbers and come up with the right pitch.

On Second Chance, a prior owner repowered with the 2GM20(F). Nice engine. It has a two blade prop. I have never checked whether it was repitched, or is a new prop that was installed contemporaneously with the new engine. However, I develop hull speed at about 2800 rpm which is about right so I am not complaining.

Interestingly, last July 4 we had a raftup about 3 miles from my YC to watch fireworks. One of my friends couldn't get his 1969 Contest 25 started for the trip home and there was no wind, bathtub conditions. So I towed him home. On the trip home, we were doing 5.5 knots at 2700 rpm!! I attribute the negligible loss in speed to the extra hp the engine brought to the task, and the fact that my prop appears to be pitched correctly.

I have heard that with a 3 blade I could get hull speed at a lower rpm, but I don't see the need. The Yanmar manual says go ahead and cruise all day at 3000 rpm, and even go up to 3200 for one hour or less if needed.

I am no mechanic and would welcome additional insight on these issues. Just some thoughts to get you thinking.....hopefully not all night!!


Bill Goldsmith
Second Chance
Cd27 # 173
1980


. Therefore, for : I started thinking about being "underpowered" at 2 AM. One of those things that just pops into your mind in the middle of the night that you just can't get rid of. I'm not a mechanic by trade so maybe some of you can clarify a question for me that seems elementary to you.

The CD27 came with many versions of engines. Originally 7.5 or 8 hp Yanmars, then 12 HP. Some have repowered with 18 hp, if I remember correctly. But let me see if I understand this. The CD27 no matter what year or engine still has a 1 inch prop shaft and still has a 12 inch diameter propeller. Lets say for arguement's sake that everyone either has a 2 bladed or a 3 bladed prop since we are talking strictly about engines. The cutout for the prop will not allow a larger prop. So, no matter what size engine you have, the prop spins "x" number of rpms on a 1 inch shaft and assuming the same pitch in the prop, moves the same amount of water no matter what engine you have. Now lets take "bathtub" conditions i.e. no current or waves, not even a tide to deal with. The CD27's hull speed is 6 knots. Both an 8 hp engine and an 18 hp engine should have no trouble getting the boat to hull speed and I would imagine that the rpm's would be the same because its an easy load for both engines. Right or wrong? If we pushed both trottles on both engines up to full, would the rpms on the 18 hp engine be greater than the rpms on the 8 hp engine under these "minimum load" conditions?? If now we add a 4 knot current against our progress I take it that it is "harder" for the engine to generate rpms against the current. Does the small 8 hp engine just simply reach its limit (i.e.full trottle) before it can generate the needed rpms to move the boat at hull speed while the 18 hp engine still has reserve power to spin the prop faster? In other words, one revolution of similar props will move a boat the same distance no matter what engine its attached to. The only reason for difference in speed, it seems to me, is due to the fact that the small engine can't turn the prop fast enough against resistance of fast current before it runs out of power. I read an article that said the proper size of an engine is roughly 2hp for every 1000 lbs dispalcement. The CD27 displaces 7500 lbs. Which means it should have a minimum of 15 hp for the iron genny.

Thanks for the input,

Warren Kaplan
Sine Qua Non ( with a YSM8)
CD27

goldy@bestweb.net

[matt cawthorne]

Warren,
A diesel engine has a governor. When you set the throttle you are essentially setting a target rpm for the governor. Up to the engines potential, the governor will put in enough fuel to attain that rpm. The potential has to do with the amount of oxygen in the combustion chamber and rpm. So, if you are moving along at three knots in calm conditions the potential for your engine at that rpm might be 4 hp. The propeller may only require 2 hp to push your boat at that speed, so instead of adding a full dose of fuel for each stroke, the governor adds a 1/2 dose and all is well. If you then try to go 4.5 knots the propeller demands 6.75 hp and your engine potential at the higher speed might be 7 hp. The governor is now adding 96% of a full dose of fuel per stroke and all is still well. If you add some barnacles to the prop and it demands 7.5 hp to reach that rpm you will not get there. What you will get is extra unburned fuel because there is not enough oxygen to burn it. It comes out the back in the form of black smoke. If you had a bigger engine with more potential it would have the extra air in the combustion chamber and could just add more fuel.

A propeller is a screw. If you have a 10 pitch propeller it will it will move 10 inches through the water for each revolution PROVIDED IT IS NOT PRODUCING THRUST. For it to produce thrust it must have some angle between the blades and the oncoming water. This is sometimes called slip. If it has 10% slip it will only be moving 9 inches through the water for each revolution. It will require a certain power to operate at 10% slip and a given rpm and will generate a certain amount of thrust. If waves and wind slow the boat down and you maintain the same rpm the amount of slip will go up. The amount of thrust that the propeller generates and the power will both go up. The engine will add fuel and all will be well. If the waves slow the boat enough the power required goes beyond the engine's capability and you get smoke. The engine will now operate at an rpm where the prop is just drawing the amount of power that the engine can supply. Since the engine power drops (sort of) proportionally to RPM and the propeller draws power proportionally to RPM cubed there is always an rpm where it will operate.

In general, if the prop pitch were to be dropped, the power to drive it at a given rpm and the force that it generates drop as well. In an ideal world you would be able to drop pitch when the engine gets overloaded and still work at an rpm which is getting the maximum potential out of the engine. Since you can't you end up operating at a lower rpm and just when you need the most power, you can not get it. The solution is to use a prop with less pitch all the time, so that when things get bad you can get the full rpm's out of your engine. The down side is that you have to run the engine at a higher speed to maintain the same cruising speed.

Just remember, the boat, engine, transmission and prop are a system. If you change the pitch on the prop you might need a bigger engine or a higher gear ratio on the transmission to get the same performance. If the system is set up to give full potential in high wind and waves and you increase the pitch of the prop to make it cruise at lower rpm's you will loose when the waves arrive.

matt





mcawthor@bellatlantic.net

[Bob Malinka]

Warren:

I must say you must not have gotten much sleep last night judging by your quest for desideratum. But first you must remember that horsepower is measured by the ability to do work. The higher the engine horsepower the more work it is capable of doing. Which brings us to the propeller. The more pitch on a propeller the more water you move. Higher horsepower translates to higher pitch propellers that can do decidedly more work for you by pushing more water, ergo a faster boat with more umph!

Capt Bob
Ranger #144
CD25D '84

I started thinking about being "underpowered" at 2 AM. One of those things that just pops into your mind in the middle of the night that you just can't get rid of. I'm not a mechanic by trade so maybe some of you can clarify a question for me that seems elementary to you.
The CD27 came with many versions of engines. Originally 7.5 or 8 hp Yanmars, then 12 HP. Some have repowered with 18 hp, if I remember correctly. But let me see if I understand this. The CD27 no matter what year or engine still has a 1 inch prop shaft and still has a 12 inch diameter propeller. Lets say for arguement's sake that everyone either has a 2 bladed or a 3 bladed prop since we are talking strictly about engines. The cutout for the prop will not allow a larger prop. So, no matter what size engine you have, the prop spins "x" number of rpms on a 1 inch shaft and assuming the same pitch in the prop, moves the same amount of water no matter what engine you have. Now lets take "bathtub" conditions i.e. no current or waves, not even a tide to deal with. The CD27's hull speed is 6 knots. Both an 8 hp engine and an 18 hp engine should have no trouble getting the boat to hull speed and I would imagine that the rpm's would be the same because its an easy load for both engines. Right or wrong? If we pushed both trottles on both engines up to full, would the rpms on the 18 hp engine be greater than the rpms on the 8 hp engine under these "minimum load" conditions?? If now we add a 4 knot current against our progress I take it that it is "harder" for the engine to generate rpms against the current. Does the small 8 hp engine just simply reach its limit (i.e.full trottle) before it can generate the needed rpms to move the boat at hull speed while the 18 hp engine still has reserve power to spin the prop faster? In other words, one revolution of similar props will move a boat the same distance no matter what engine its attached to. The only reason for difference in speed, it seems to me, is due to the fact that the small engine can't turn the prop fast enough against resistance of fast current before it runs out of power. I read an article that said the proper size of an engine is roughly 2hp for every 1000 lbs dispalcement. The CD27 displaces 7500 lbs. Which means it should have a minimum of 15 hp for the iron genny.

Thanks for the input,

Warren Kaplan
Sine Qua Non ( with a YSM8)
CD27

Ranger1442@hotmail.com

[Warren Kaplan]

Matt and everyone else who answered,

Thanks for all the information. I printed much of it out so I can "educate" myself about all this.
Matt, just a question to see if I understand all this. Not that anyone would ever do this but..if one was to put a supercharger on a little YSM8 (can you supercharge a diesel?), that would blow more air into the combustion chamber wouldn't it?? Would it take care of excess fuel and therefore eliminate black smoke up to a point? If so, what would that do to the HP of the engine and would that be a way to get more "work", rpm, and better performance out of that little engine?

Warren



Setsail728@aol.com

[Mario]

In like fashion, how fast do folks run their Volvo MD7A engines on a CD 30? I'm cautious and try to keep the RPMs down, which of course means I poke along at 4+ knots in smooth water.

What do others run their CD 30/MD7As at?

Mario



capedory252@aol.com

[matt cawthorne]

Warren,
It would most certainly do that. Diesels supercharge/turbocharge well. It might also blow it up if the engine were not designed for the higher loads. It might also melt it down if the engine had insufficient cooling for the higher heat loads.

It comes down to a matter of philosophy. Is it better to have a larger engine which probably weighs more or a smaller engine with more moving parts to be maintained. Larger engines might have more cylinders which would normally mean that it would have lower vibration. I used to sail my brothers Cal-2 27 with a 1 cylinder diesel. I would have to go to the dentist on Monday mornings to have my fillings re-installed. When I bought my CD-36 I was truely amazed at what the extra cylinders do in terms of reducing vibration.

Matt



mcawthor@bellatlantic.net

[Ben Thomas]

In like fashion, how fast do folks run their Volvo MD7A engines on a CD 30? I'm cautious and try to keep the RPMs down, which of course means I poke along at 4+ knots in smooth water.

What do others run their CD 30/MD7As at?

Mario

Mario, These threads are sounding all to familiar. I have a cd30 (1982) #248 with the volvo md7a. I have been concerned about being underpowered for a while now. And short of replacing engine altogeather this is what I have done. I check the air cleaner after every 10hours of run time. It clogs easily and chokes the engine in no time.
I discovered that the throttle linkage on engine was not adjusted properly and the pin holding linkage was rubbing up against water muffler, added two washers which lifted pin and now it clears bracket allowing it to fully open. which added 200 rpms to throttle/engine.
I have gone around and around with props, (no Pun) ha. orginal was two blade 13x13, surpriseingly engine struggled to reach 2000 rpms at 4 knots. I changed this to a three bladed 13x10, way overpitched, this got repitched to a 13x8 and I arrived at what I thought was a range I could live with. top rpm is 2360 at 5.5 knots.(top rpm under no load is 2900 rpms)you don't want to be there very long. this was averaged over a one mile course upstream, across current, down with current, across current again then up stream, down diagonally and around up diagonally, whew! The average speed was 5.5 knots. the problem is tides/currents
the mouth of Columbia river is hairy at times and I have battled both to make barely 2 knots speed over ground. at least I'am in the higher power range with the engine. good luck, Keep us posted Ben

[sloopjohnl]

diesels no matter the size like to work. they don't want to be over revved for long periods, but neither do they like lugging. under a load they are better off working hard than taking it easy. an auxillary diesel is happier day in and day out working at a high rpm pushing a boat at 5 knots than it is lugging along at a moderate rpm pushing the boat at 3 knots.

Warren,

Just to emphasize what Mark wrote, the prop is not a constant. A YSM8 spinning a 12 in prop would require a lower pitch than, let's say, a 2GM20 with a 12 inch prop. The bigger engine can handle more water per revolution while under way. Any good repower specialist could run the numbers and come up with the right pitch.

On Second Chance, a prior owner repowered with the 2GM20(F). Nice engine. It has a two blade prop. I have never checked whether it was repitched, or is a new prop that was installed contemporaneously with the new engine. However, I develop hull speed at about 2800 rpm which is about right so I am not complaining.

Interestingly, last July 4 we had a raftup about 3 miles from my YC to watch fireworks. One of my friends couldn't get his 1969 Contest 25 started for the trip home and there was no wind, bathtub conditions. So I towed him home. On the trip home, we were doing 5.5 knots at 2700 rpm!! I attribute the negligible loss in speed to the extra hp the engine brought to the task, and the fact that my prop appears to be pitched correctly.

I have heard that with a 3 blade I could get hull speed at a lower rpm, but I don't see the need. The Yanmar manual says go ahead and cruise all day at 3000 rpm, and even go up to 3200 for one hour or less if needed.

I am no mechanic and would welcome additional insight on these issues. Just some thoughts to get you thinking.....hopefully not all night!!


Bill Goldsmith
Second Chance
Cd27 # 173
1980


. Therefore, for : I started thinking about being "underpowered" at 2 AM. One of those things that just pops into your mind in the middle of the night that you just can't get rid of. I'm not a mechanic by trade so maybe some of you can clarify a question for me that seems elementary to you.

The CD27 came with many versions of engines. Originally 7.5 or 8 hp Yanmars, then 12 HP. Some have repowered with 18 hp, if I remember correctly. But let me see if I understand this. The CD27 no matter what year or engine still has a 1 inch prop shaft and still has a 12 inch diameter propeller. Lets say for arguement's sake that everyone either has a 2 bladed or a 3 bladed prop since we are talking strictly about engines. The cutout for the prop will not allow a larger prop. So, no matter what size engine you have, the prop spins "x" number of rpms on a 1 inch shaft and assuming the same pitch in the prop, moves the same amount of water no matter what engine you have. Now lets take "bathtub" conditions i.e. no current or waves, not even a tide to deal with. The CD27's hull speed is 6 knots. Both an 8 hp engine and an 18 hp engine should have no trouble getting the boat to hull speed and I would imagine that the rpm's would be the same because its an easy load for both engines. Right or wrong? If we pushed both trottles on both engines up to full, would the rpms on the 18 hp engine be greater than the rpms on the 8 hp engine under these "minimum load" conditions?? If now we add a 4 knot current against our progress I take it that it is "harder" for the engine to generate rpms against the current. Does the small 8 hp engine just simply reach its limit (i.e.full trottle) before it can generate the needed rpms to move the boat at hull speed while the 18 hp engine still has reserve power to spin the prop faster? In other words, one revolution of similar props will move a boat the same distance no matter what engine its attached to. The only reason for difference in speed, it seems to me, is due to the fact that the small engine can't turn the prop fast enough against resistance of fast current before it runs out of power. I read an article that said the proper size of an engine is roughly 2hp for every 1000 lbs dispalcement. The CD27 displaces 7500 lbs. Which means it should have a minimum of 15 hp for the iron genny.

Thanks for the input,

Warren Kaplan
Sine Qua Non ( with a YSM8)
CD27

[Ken Cave]

This discussion on props came up a few months ago, and I mentioned that I had my two bladed prop switched to a three bladed prop that was re-worked and installed by Cap Sante Marine in Anacortes. (Apparently some of the Cape Dory owners have called the folks at Cape Sante Marine for their new props, as it has done wonders in boosting the speed on a Cape Dory 28, yet keeping the engine revs around 2000 rpm)

Now, in calm waters, I usually travel at around 6 knots instead of 4 knots at the same rpms with the same Volvo MD7A!

Ken Cave
"Dragon Tale"




bcave@whidbey.net

[Jerry J Commisso]

According to the Volvo manual for the MD7A MAXIMUM engine speed should be 2600 RPM. The book reads as follows. In order to utilize the maximum performance of the engine, an engine speed as high as possible should be chosen but not, however, higher than 2600 RPM. I own a CD30 with the MD7A engine and a two blade prop, which I run between 2200 and 2400 RPM. This will give me a boat speed of about 6 knots depending on the conditions.

Jerry J Commisso
CD30c CHELSEA ROSE

In like fashion, how fast do folks run their Volvo MD7A engines on a CD 30? I'm cautious and try to keep the RPMs down, which of course means I poke along at 4+ knots in smooth water.

What do others run their CD 30/MD7As at?

Mario

liasboat@aol.com

[Larry DeMers]

Ken,

Ok, that is the 'Ying', now how bad is the'Yang'..in this case, the loss of sailing speed, since the boat is presumed to sail more than it motors? Do you notice much degradation in top end speeds? How bad?
With the best-part of two blades hanging out there in the slip stream, there has to be a penalty in sailing performance.
Wish we could talk PYI or MAX Prop into making a 12 or 13 in. 3 bladed self-feathering prop for us with Cd30's and smaller.

Larry DeMers
s/v DeLaMer
Cape Dory 30

Cheers,

Larry DeMers

This discussion on props came up a few months ago, and I mentioned that I had my two bladed prop switched to a three bladed prop that was re-worked and installed by Cap Sante Marine in Anacortes. (Apparently some of the Cape Dory owners have called the folks at Cape Sante Marine for their new props, as it has done wonders in boosting the speed on a Cape Dory 28, yet keeping the engine revs around 2000 rpm)

Now, in calm waters, I usually travel at around 6 knots instead of 4 knots at the same rpms with the same Volvo MD7A!

Ken Cave
"Dragon Tale"

demers@sgi.com

[Mark Yashinsky]

Mark the prop shaft when the prop lines up w/ the keel (2 marks). When sailing, lock the prop shaft (using your favorite method) when your marks line up. Viola, minimal drag, as the prop should be lined up w/ the keel.

[Ken Cave]

Larry:

Sad to say, but up here in the beautiful northwest, the months of July and August are usually motoring weather. In fact, the first time we were actually sailing for most of the day was last Sunday when we were returning from Friday Harbor in the San Juans to Anacortes!

Do I notice any difference? On this trip, we were traveling around 6 knots with a 130 genoa only and pulling a six foot inflatable besides!

Since I don't race, I really feel the three bladed prop is the way to go, as I can usually do 6 knots at about 2000 rpm with my MD7a!

Sure wish we had more days like this past sunday-instead of always trying to sail with the wind on the nose-if there is any wind at all!

Ken Cave



bcave@whidbey.net