Help with intentionally over-propping a sailboat for fuel efficency?

[calderp]

Hi there.
Background:
The boat:
Cape Dory 28, full-keel, heavy displacement
Displacement: 9,000lbs (maybe 10,500 loaded? Or is it better to run prop calcs off the listed 9,000?)
Hull Speed is approx. 6kts

The Engine:
Yanmar 1GM 7.5HP / 3600 RPM
Gear ratio: 2.62
Current prop is a 13 x 9 2-blade and appears to be severely under-propped. I want to go to 3-blade.

I have taken my calculations from Nigel Calder's 'Marine Diesel Engines'. My goal here is to fit a 3-blade prop which will maximize my fuel efficiency. I rarely run the engine and never try to run it at hull speed. If I'm running it at all I aim for a cruising speed of 4 - 4.5kts, being more inclined to fuel efficiency than speed. With this in mind my plan was to slightly overprop the engine while being careful not to run it so hard as to damage anything.

Note: I did all this math using a mistaken displacement of 11,000 so I'll need to go back and do it again but they should work for now as my question is mainly about the idea of over-propping and the technique for calculating a reasonable way to do so.

PROP PITCH:
Shaft speed at full power: 1300RPM
Typical shaft speed for prop calcs would be 1300 x .95 but I used 1300 x .85, the idea being to over-prop by 10%. This gives a shaft speed of of 1105RPM.
**Can anyone tell me is this is a reasonable way to calculate a 10% over-propping?**

With a shaft speed of 1105 the 'Perfect world' Pitch (ie. no slip) = 6.6"

I guessed my slip to be 40% for a 28' full-keel heavy displacement vessel. I have no idea how accurate this is, or how to properly calculate this number. Can anyone give some guidance here?

Assuming a slip of 40% the real-world pitch would be 6.6 x .6 = 11 inches

PROP DIAMETER:
The ideal diameter based on the nomograph taken from Dave Gerr's Prop book is 14". I need to dive down and double check but I think my aperture will only take a 12" prop, possibly 13" so this is the size I'm going off of. At this size (12") blade loading vs. blade area checks out fine (44in2 vs. 50in2)

My questions are these:
*Is my idea of over-propping and simply being careful not to run the engine too hard a bad one? Am I going to do damage if I accidentally accelerate too much once or twice, or am I in danger of damaging the engine by severely under-working it?

*Is calculating a 10% over-propping as simple as doing the usual calculations minus 10% of the shaft RPM or am I totally wrong there?

*Otherwise, can anyone contribute personal experience, or does anyone see major flaws in my calculations?

Any response helps, I'm pretty new to this!

[Lake-Effect]

Lower RPM but a higher load for a given speed does not necessarily translate to fuel savings. Also, a bigger prop is more stress on the transmission and shaft.

I would look at what RPM is your engine most efficient at, and what speed you want to most often use, and consider a prop that will facilitate that.

I'm no expert at engines, just trying to apply some physics.

[calderp]

Right. The engine will probably be more efficient, or at least 'happier' running at higher RPM's but what I'm going on here is the idea that motoring at or near hull speed is vastly less efficient than motoring at a knot or two less. Thus the efficiency curve of the engine (which I have no idea where to find anyway) is less relevant than the relationship of boat speed vs. the energy at the prop required for that speed. Most props are sized for the boat's hull speed, even though this causes them to be less efficient when run at the lower speeds that we typical motor. The idea is that if you size the prop so that it is optimized at less than hull speed then you are going to overwork your engine if you insist on running at hull speed, as most boatowners occasionally do. I am willing to sacrifice the ability to motor at hull speed in order to gain efficiency at the speeds that I actually motor at. What I'm trying to do is figure out how to make this calculation.
That's a very good point about increased stress on the transmission. Being also not an expert I really don't know how to consider that but I'm hoping this 10% number I've rather arbitrarily chosen will be low enough, assuming that I'm doing te calculations the right way. I very well might not be...

[Pete7]

I would be astonished if you can get that boat to 6.6 knots with a 7.5hp engine. We a 31ft hull (5100 kg) with twin fin keels and 28hp volvo. Its too big for the boat and could achieve 7 knots (3300 revs) but its sounds harsh at high revs. So the max we run at is 2600 at 6.5 knots, that is probably about 20hp. Prop is 2 blade 16 x 14" for reference.

If you put a 3 bladed prop on you will increase the water resistance and slow her down under sail. Since you rarely motor I would leave it alone and sail with the engine in neutral so the prop is free to turn, though you might find that marking the prop shaft when the blades are hidden behind the dead wood shows a performance increase under sail. Worth experimenting with.

Pete

[Pete7]

Originally Posted by calderp

Is my idea of over-propping and simply being careful not to run the engine too hard a bad one? Am I going to do damage if I accidentally accelerate too much once or twice, or am I in danger of damaging the engine by severely under-working it?

*Is calculating a 10% over-propping as simple as doing the usual calculations minus 10% of the shaft RPM or am I totally wrong there?

*Otherwise, can anyone contribute personal experience, or does anyone see major flaws in my calculations?

Any response helps, I'm pretty new to this!

Your sentences in blue are contradictory. Over propping an engine is not a good idea, normally shows up with black smoke from the exhaust meaning fuel isn't being burnt properly. Nor is running a diesel engine very lightly particularly for a short time as the bores glaze up. You need the graphs for the engine which show fuel used v revs and hp generated, this will be the start point for fuel efficiency.

Pete

[calderp]

Pete - Good point, I'm never going to get the boat to hull speed with my tiny engine, the only reason I mention hull speed is that is the number typically used for prop calculations. Maybe I should be using a lower number anyway, which I thought is what I was do with my "over-propping". I suppose I should be saying 'oversize prop' rather than over-propping as it's true that this is usually used in the context of blue smoke / an overworked engine.
Let me try and make myself more clear.

As I understand it, most props are sized for an engine being run at near-full throttle, (say 3500RPM) but in practice the engine is usually being run at much lower speeds, (say 2700RPM). Doing this under-loads the engine, making it less efficient, but this sacrifice is made because if the prop is sized to be run at the average cruising speed of 2700RPM then when occasionally pushed to 3500 it is going to over-load the engine, producing black smoke, (the engine will be 'over-propped'). I never intend to run my engine at full throttle, and in practice I usually run it lower than most people because I am more concerned with fuel efficiency than speed. This means that if I sized my prop normally (ie. for my engine being run at near-max RPMs) I will consistently be running at low power, which is both inefficient and bad for the engine. If, instead, I over-size my prop then I will be spinning a larger prop at slower speeds, creating a higher load on the engine at a given RPM. Since I typically run my engine at low throttle anyway (I think) this higher load will actually be better for the engine, and more efficient. It will mean that I risk being 'over-propped' (ie overloading the engine) at high-RPMs but since I simply won't run the engine at full speed this ought not to be an issue. Bear in mind that I'm not talking about a huge increase in size here, I'm shooting for a modest 10% in hopes that I will have some gains without the risk of really screwing something up. What I'm trying to determine is ow to rework the 'typical' calculations to come up with a prop which is actually oversized for my boat/engine but not so much so that I overwork the engine at cruising speed, or cause it to stall out if I occasionally push it to higher RPMs.

As for 2-blade vs 3-blade a big part of why I want to reprop is for increased acceleration and, most importantly, more power in reverse. I almost never run my engine to cover miles but I often use it when I run aground and for this purpose my low-acceleration, useless-in-reverse 2-blade prop doesn't do much good. For this a 3-blade is vastly superior. My 10,000lb full-keeled boat is already a bit of a pig and I find it pretty hard to believe that an extra 10-15 square inches of prop is going to make any difference. I can just about tow a 5-gallon bucket without any appreciable loss in speed!

What I'm aiming for is a prop that is oversized such that it is more efficient at the speeds that I plan to run it. This may mean that it will be 'overpropped' if I try to run the engine at full throttle but I don't intend to do this. The theory that I'm going on is that a larger, slower-moving prop is by definition more efficient than a smaller, faster-moving one. The danger is in overcalculating and ending up overloading/stalling the engine. As for running it very lightly I don't think we're getting into an area where there is short-term danger. Actually, most engines are consistently run 'too lightly' because the prop has been sized for an engine being run at full throttle but most of the time it is being run at much less.

[calderp]

Originally Posted by Pete7

You need the graphs for the engine which show fuel used v revs and hp generated, this will be the start point for fuel efficiency.

Pete

Would love to have these, but no idea where to get them for a 25-year old engine that no one gives a damn about anymore. Regardless I'm just aiming for a ballpark which will then be fudged a bit more depending on what I can find used!

[CPseudonym]

For ungrounding yourself you need a depth finder not another blade on your prop Is my smart arse reply.

Honest answer is the most "efficient" prop has only one blade. It would however be a bitch to balance. 2 blade is the most efficient "practical" choice.

Call a good local prop shop and have them run the numbers for you. They will also "measure" your existing prop rather than rely on the stamped numbers, sometimes numbers lie. They may just change the pitch of your existing prop for far less money than a decent used wheel.

[Jim Cate]

To quote another CF member, "I ain't no expert", but I think that you are making a big mistake in your plans. First, it takes a fixed amount of delivered power to drive your boat at any given speed. To a first approximation, diesel engines all consume a similar amount of fuel per hp-hour... doesn't matter very much what part of the RPM curve you are in. There is a small improvement at the "sweet spot" that you mention, but in general it is not a very big improvement that you are courting.

Second, by most standards your boat as loaded is woefully underpowered. Selecting a prop that will not allow the engine to reach max rpms means not only will she not reach hull speed (or likely anything near it) but will severely limit your ability to power into chop or adverse winds... at any speed! While I too avoid motoring as much as possible, there are times when being able to make headway into adverse conditions when sailing is not possible (narrow channel, blown sails, parted halyard or even entering the odd marina or harbour where sailing is forbidden) is essential.

Third, don't fool yourself about the deleterious effects of a big prop on your sailing ability... it will make a difference.

To me, thus handicapping your boat for a (possibly imaginary) tiny gain in fuel economy is a foolish decision. Do check it out more carefully, mate.

Cheers,

Jim

[SkiprJohn]

Just another sideline comment here. One common cause of engine overheating is someone putting a larger prop on their boat for just the reasons you stated. Your engine has a specific RPM that is most efficient for it to run. That's your goal. Revving too high burns more fuel and doesn't get you much faster. Turning too low RPMs doesn't give the engine the workout it needs to stay healthy.
I would venture a guess that your engine's sweet spot is about 2100 rpm or just a bit one side or the other.
Some full displacement full keel boats just don't back down worth a dang no matter what size 3 or 4 blade prop they use.
kind regards,

[charliehows]

intentionally overpropping - why dont you try to get the elusive best match of prop, engine and hull? On a 1gm fuel efficiency is a ridiculous concept - what do you do with your car, pull a couple of sparkplug leads off to get better efficiency?

[daddle]

Overpropping cannot be the right answer. For all the reasons above it is a bad idea.

You are actually saying you want to operate your engine at a lower more efficient power setting than full RPM. This would imply *less* prop power, not more.

Do the calcs with the actual numbers you have. Forget trying to outsmart the engineers. Or contact a prop shop. To save fuel use the correct prop and simply slow down.

Engine damage will cost more than the tiny bit of fuel saved.

[roverhi]

We had an MD2B Volvo in our Westsail 32. Engine was supposed to put out 25hp for the 20,000 pounds plus displacement of the boat. We overpropped the the boat with a 2 blade. It pushed the boat at 5k in calm conditions without straining the engine but was down about 300 rpm at full power. The combination worked fine for lowspeed maneuvering in the marina giving more than adequate acceleration and stopping performance.

Wanted a two blade prop for sailing performance. Sailed with it locked vertically behind the deadwood. It worked fine for us because I hate engines and only want it for getting in and out of the slip and powering in flat conditions. Even if you discount the advertised horsepower of the Volvo, we were still way overpowered in relation to your boat.

I locked the prop with a pair of vice grips. Would grab hold of the shaft and set the prop vertical by feel. It was surprizing how much force the water pressure put on the prop even at relatively low speeds. Personally, since sailing is apparently your priority, I wouldn't sacrifice that to get some illusory power from the engine. Also, believe you'd be way better off with a two blade prop optimized for your boats displacement and engine hp rather than going for a 3 blade that your engine won't be able to swing at some less than optimum rpm. Spinning your engine at max rpm is going to be way more effective than a bigger prop at some lesser hp.

If it was me, would go with a 2 blade prop of largest diameter commensurate with pitch that would allow you to get the engine's maximum rpm. You will get good fuel economy, have excellent sailing performance as long as you lock the prop behind the deadwood, and the best engine performance when/if you need it. After all, how much fuel does your dinky engine burn at 4k boat speed. Probably so low a fuel flow that spiders making webs in your fuel lines will be the biggest issue.

[goboatingnow]

You cannot cheat physics. , running slower with a larger prop , will just consume the same power as running faster with a smaller prop , with the added inefficiency that the torque of a diesel is anything but linear , so the slower engine will be outside its optimum running regs

All in all a really bad idea

Dave

[eyschulman]

Try going to Seaboard marine site and read Tony's tips article on props. Actually from your post you might do well to read all of Tony's tips same info on Boatdiesel.com. This is directed at original poster I believe the help you need is a better understanding of how props and motors actually work. I think Tony Athens explains it better than I can thus the referral.