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Old 15-07-2012, 06:26   #1
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Sailboat propulsion engines generally oversized?

Note the question mark in the title.
Am approaching this like the Doha debates on BBC, ie, this side argues that most sailboats are fitted with larger engines than necessary to move forward under extreme conditions, AND THEREFORE, much of the propellor size considersation discussion, based on attaining maximum RPM, is irrelevent.

A case scenario:

Boat 41ft LOD, 29ft LWL, 24000lbs, 6ft draft, full straight keel.

Engine Westerbeke W46 4 cyl, 127 cu ins.

32 hp @ 2000rpm
46 hp @ 3000rpm

Hurth HBW 150 2:1 red, turning 3 bladed 17 x 13.

Clean bottom, 7.2 knots at 2100 rpm, by GPS.
Quarterwave peeling off of tramsom/ bottom at this speed
Engine temp when seawater is 85F, is 200F.

At 1800 rpm, hp output is 29 cont, torque is 78 ft lbs.
At 2000 rpm, hp output is 32 cont, torque is 80 ft lbs.

Fuel consumption is .42 lbs/hp/hr.

Engine will NOT rev higher than 2100 rpm.

Before you yell OVERPROPPED, consider the following.

Normal cruise is 6-6.2 knots in 1-2 ft sea., @ 1800rpm = 29 hp
Bucking a headwind 30 knts, 3-4ft sea, back to 6 knots = 32hp, 2100 rpm.

If I were to prop this boat as generally advised, ie, attain 85% of 3000 rpm/46 hp, what would I gain other than increased engine wear and fuel costs.?

(You probably Guessed that this is my boat, convince me I need to change the prop)

When new, 1965, Philip Rhodes speced a 4-107, direct drive, 2 blade 17 x 11 IIRC.
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Old 15-07-2012, 07:50   #2
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Re: Sailboat propulsion engines generally oversized?

With the engine at idle speed, what speed is the boat running at? The ideal boat speed at idle is about a walking pace. This way you approach a mooring or marina berth under control. If you are doing 3 to 4 knots at idle, the constant in and out of gear will definately shorten the life of your transmission, never mind the occasional panic moment on the boat.

Because you have much more engine than your boat needs, the torque developed by the W46 is driving your boat well but as your propeller is oversized for the engine, you are limiting yourself to a 32HP engine although you paid for a 46 HP one. By holding your engine back as you are, you are building up carbon in the combustion chamber and exhaust manifold. If it were me, I would change the prop so that your engine can reach about 2800 in gear.

I suggest that you forget what the designer/builder fitted to the boat for power. When your boat was new there was a poor choice of engines available to the boatbuilder so they fitted what was cheap and available. Today there is a much greater choice of engines and transmissions from a number of manufacturers available so with the correct advice, it is easier to power a boat correctly.

Have fun, Stanley
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Old 15-07-2012, 08:28   #3
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Re: Sailboat propulsion engines generally oversized?

I maintain that much of the prop/hp/torque/gear ratio calcs are just a "more precise rule of thumb". I.e. a place to start, but better than starting with any old setup.

These days, there are additional factors that might need consideration. Parasitic loads come to mind (large alternator(s), PTOs etc).

Also, there is a wider range of engine selection all with different power/torque/rpm curves. Plus there are turbos, intercoolers, etc. all need consideration based on other requirements (wear, serviceability, etc).

So much so, in the end, and in the water, there is no guarantee these calcs will get you what you 'theoretically want'. A good place to start, but to fully optimize, things will still need dialing in after installation (which is why adjustable pitch props are nice).
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Old 15-07-2012, 08:54   #4
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Re: Sailboat propulsion engines generally oversized?

I think we are all on the same page.
At idle, 375 prop rpm, approach speed is about 2 knots.
On a mooring in front of the house, with lots of room, coast the last 4-5 boat lengths in nuetral, touch reverse, and parked.
Boat was in the day charter business for a while, licensed for 18, so the PO put a bigger engine in.
Pulled the head at 2100hrs after a leak down test showed no more the a 5% drop in one cylinder, no carbon, which as you say quite rightly is a concern.
No room for a V/P prop, which would you do for the extra recommended rpm, reduce pitch or diameter, how much each?

Thanks guys, for your opinions.
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Old 15-07-2012, 09:03   #5
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Re: Sailboat propulsion engines generally oversized?

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No room for a V/P prop, which would you do for the extra recommended rpm, reduce pitch or diameter, how much each?
If you have plenty of room at the prop tips then do whichever is easiest. Check the prop. calcs. pages on the internet that will help you decide how much you need to rework the prop. and then get the advice of the prop. shop that will be doing the work.
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Old 15-07-2012, 09:22   #6
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Re: Sailboat propulsion engines generally oversized?

Your data indicates that the max rated power of the engine is produced at 3,000 rpm but the way you are propped, you can only reach 2,100. RPM. That is way, way overpropped.

Low speed, normally aspirated engines can live with a few hundred rpms of overpropping, but not 900. When you run 2,100 at wot, you are putting stresses on the piston, rings, and bearings. Why not prop down for another 500 rpms and reduce those stresses?

That engine will last longer running at higher rpms with less torque than what you are doing now. And running an engine at high torque means that the prop is running with increased slip and is inefficient. Reduce its pitch snd speed it up and the increased efficiency will probably compensate for whatever engine efficiency you may lose.

And if your boat only requires an additional 3 hp to buck a 30 kt headwind with 3-4' seas, we need to put the designer up on a pedestal, because he acheived something no other boat can do.

David
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Old 15-07-2012, 09:30   #7
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Re: Sailboat propulsion engines generally oversized?

Overpowered sailboats, I have three of them. Oh Joy weighs 15K and has a Perkins 50 that will rev all the way to 4K RPMs. It will push the boat up on her bow wave, bury the transom and do 7.5 knots. Not much use for all of that power except to acelerate quickly. The Mariner 31 also has the same engine at 11K but only revs to 3K. Seasmoke weighs 15K and has a Nissan-Chrysler 75HP four cylinder. What were they smoking??? All of these boats would do just fine with a 30-35 HP engine.
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Old 15-07-2012, 09:41   #8
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Re: Sailboat propulsion engines generally oversized?

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Originally Posted by djmarchand View Post
Your data indicates that the max rated power of the engine is produced at 3,000 rpm but the way you are propped, you can only reach 2,100. RPM. That is way, way overpropped.

Low speed, normally aspirated engines can live with a few hundred rpms of overpropping, but not 900. When you run 2,100 at wot, you are putting stresses on the piston, rings, and bearings. Why not prop down for another 500 rpms and reduce those stresses?

That engine will last longer running at higher rpms with less torque than what you are doing now. And running an engine at high torque means that the prop is running with increased slip and is inefficient. Reduce its pitch snd speed it up and the increased efficiency will probably compensate for whatever engine efficiency you may lose.

And if your boat only requires an additional 3 hp to buck a 30 kt headwind with 3-4' seas, we need to put the designer up on a pedestal, because he acheived something no other boat can do.

David
This is my whole point, I am nowhere near WOT to attain 2100 rpm.
Engine, when head was lifted, showed no carbon, no ovality on the bores, doesn't consume lube oil.
As I conjectured, if the engine is happy and shows no signs of being stressed, all I will do is accelerate wear--and burn more fuel.

I put this question out there to motivate the intelligent feedback I have received to date.
Recently there have been a few threads about re-powering sailboats, and it seems we err on the side of bigger engines.

--and Phil Rhodes' pedestal was built a long time ago.
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Old 15-07-2012, 10:10   #9
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Re: Sailboat propulsion engines generally oversized?

Overpowered would be if the engine is capable of bringing a full displacement boat past hull speed. There is no harm in being slightly overpowered other than the additional expense and weight.

If an engine cannot reach its maximum RPM with a clean hull and a healthy engine then the propeller is overpitched/over diameter or both. If one is over propped, it is better to reduce pitch than diameter. For the same horsepower output of a prop at a given RPM, the larger diameter/smaller pitch prop is more efficient than a smaller diameter larger pitch prop. The downside to larger diameter prop is more drag when it is not being used.

If an engine cannot reach full RPM then what will happen is carbon will build up in the combustion chamber which will shorten the engines life. Diesels need to be ran at full speed periodically to allow the combustion chamber to get hot enough to blow out the accumulated carbon. You do not want to lug a Diesel by having an over sized prop. You will shorten its life.

Cummins will not warrant a new engine installation where the engine cannot reach full RPM for the reasons I described. Cummins also requires that you run the engine for a half-hour at full speed for 10% of the hours that you operate the engine. This does not put premature wear on the engine. It actually does the opposite, it prolongs the engines life. Unlike gasoline engines, Diesels last longer when run hard.

It is indeed less efficient on a miles per gallon basis to run a boat at higher speeds, but then that is the big compromise, wearing an engine out prematurely versus having to purchase more fuel to cover the same amount of water.

Given how expensive engines are and what a pain they are to change out, it's better to run a Diesel up to the speeds that will make it last longer.
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Old 15-07-2012, 10:38   #10
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Re: Sailboat propulsion engines generally oversized?

Quote:
Diesels last longer when run hard.
I would say that diesels last longer if run at the proper temps., oil temp and water temp. There is plenty of evidence of diesel engines being run mostly at low rpm for many thousands of hours with no carbon buildup and no damage to the engine. The biggest thing on a sailboat is running the engine long enough to reach proper operating temp. on a regular (and often) basis. If they are run at proper operating temps there should be no reason to run them at WOT.

Now, given that many sailboat engines are never run long enough to reach proper operating temp they may need to be run hard periodically to clean them out.
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Old 15-07-2012, 10:53   #11
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Re: Sailboat propulsion engines generally oversized?

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Originally Posted by DeepFrz View Post
I would say that diesels last longer if run at the proper temps., oil temp and water temp. There is plenty of evidence of diesel engines being run mostly at low rpm for many thousands of hours with no carbon buildup and no damage to the engine. The biggest thing on a sailboat is running the engine long enough to reach proper operating temp. on a regular (and often) basis. If they are run at proper operating temps there should be no reason to run them at WOT.

Now, given that many sailboat engines are never run long enough to reach proper operating temp they may need to be run hard periodically to clean them out.
Diesels being run hard are also at the proper temperature. This is why they have thermostats. I'm just passing along what a number of Diesel mechanics, research on Diesels and Cummins has told me about Diesels.

With all due respect, you are inaccurate about never needing to blow the carbon out of a Diesel if all you ever do is run it at easy speeds and up to temperature. This is not what a number of experts and a major manufacturer has told me.
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Old 15-07-2012, 10:59   #12
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Re: Sailboat propulsion engines generally oversized?

That depends on what you want the engine to do. Is the engine a primary propulsion means or is it an auxiliary to the sails?

If you want to be able to make progress under power in any wind- or sea-state then they aren't. To me that says a lot about people level of comfort with their sailing skills.

If you want to be more economical then that becomes a much more involved question.
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Old 15-07-2012, 13:21   #13
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Re: Sailboat propulsion engines generally oversized?

This is my very biased view gained from experience with various boats:

When it comes to motoring against wind/waves I found only two things count:

- engines raw horse power, continuous duty,
- the hull's hydrodynamics + the rig's drag.

I like to see 5, but ideally 6-7, h.p. per each 1000 kilograms of fully loaded displacement.

Your boat of 24000 lbs I would like an engine of minimum 55, optimum 75 h.p..

A smaller engine, any, will do. But if you want to bash, may horse power be with you!

barnie
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Old 15-07-2012, 13:44   #14
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Re: Sailboat propulsion engines generally oversized?

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Diesels being run hard are also at the proper temperature. This is why they have thermostats. I'm just passing along what a number of Diesel mechanics, research on Diesels and Cummins has told me about Diesels.
Have you ever looked at specs on an engine where they list the RPM max for light duty, medium duty and heavy duty usage. In every case that I have seen the RPM max for heavy duty usage is lower than for the light and medium ratings. The only difference (as I understand it, I may be wrong) is the setup of the governor.

Sorry, this is really off track for the thread..
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Old 15-07-2012, 13:50   #15
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Re: Sailboat propulsion engines generally oversized?

That's different from running the engine at max RPM for a half hour to blow the carbon out. The manufacturer is not saying to run it at maximum RPM all the time.

Cummins has some Diamond series engines with a higher RPM and higher horsepower which they said they would never sell to me because my boat is essentially a commercial application. The Diamond series engines have some things which my engine does not have, and not just a engine speed governor set at a lower speed. The Diamond engines were more for sport fishing type vessels, which the owners liked to make go fast which did not accumulate thousands of engine hours per year.
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