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Old 05-04-2011, 11:29   #1
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Engine Change

After a multitude of conflicting advice - this post outlines the things I have learnt should be considered if you are thinking of changing your engine.


Engine Horsepower
There are more myths around on the matching of horse power to boat than you can imagine. So if you hear them – stories like “one hp per foot” or “always better to have some power in reserve” – disregard them!
Boat speed (for displacement boats) is limited by waterline length regardless of horsepower. So you need to identify the maximum hull speed and then use one of the formulas on the web to determine the smallest engine you can use to reach that speed. Unless you frequently pull excess weight – either acting as a tug, or due to load of another sort.

Glazing
People will say that if you have a diesel engine that is over powered you can easily cause glazing (see below). Engine load is relative to throttle position, so provided the correct propeller is fitted for the engine it very unlikely that the engine will glaze when it is working. The risk is more of an issue when the engine is run purely to charge the batteries.

Shaft size
Using an engine with a shaft that is too thin will cause the shaft to distort and causes serious damage to its mountings, one of which is the exit tube and that will allow water into your hull!

Engine weight
The weight of the engine has to be within the design range for the boat. If overweight, the engine may cause the boat to sink a few centimetres resulting in unfavourable sailing performance, exhaust being more easily flooded with water (which can be pushed back through the exhaust valves into the cylinders). HOWEVER for my boat (34’) 750kg of additional weight will drop the boat about 3.5 cm (1.4”) in the water (350kg makes this 1.7cm) if the motor is centred – maybe 1.5 times that if not! I can’t see that this can be an issue! After all the diesel and water tanks would contribute over 400kg. The heavy engine I have been considering weighs 750kg, but the one that is in place must weigh half that.

Engine size
Remember that the engine must be moved into the engine space (and the old one removed) if it is too large (particularly if it has to be man handled down the companionway and into place) it can result in damage to you or the yacht finishes or even not fit!

Engine Bed
The fitting of the new engine needs to be appropriate for the engine bed. There are two major aspects. First, the engine must be aligned with the shaft, and there are two major possibilities - the shaft may extend horizontally out of the crank case, or it may go through a gearbox that effectively runs parallel to the crankcase at a lower level. Second, the engine mounts on the two rails in the engine bed, if these are not sufficiently far apart, or if there is insufficient room below the level of the mounts there will be a problem.
If your new engine differs from the old on either of these it may be necessary to rebuild the engine bed. Such a rebuild can be done, but adds to the expense and pain.

Exhaust
If the exhaust volume is significantly different (exhaust volume is dictated by a combination of RPM and engine capacity/displacement) to the old there is the possibility that the old exhaust system may not allow sufficient gaseous exit. The ideal exhaust is determined by a mixture of volume of gas exited and velocity of gas exited. Thinner pipes increase velocity - so thick pipes is not the easy answer! Building back pressure will be noticeably detrimental on performance. You may have to restructure your exhaust system - remember to protect the engine from a water influx via the exhaust!

Water Cooling
If the new engine and old engine are cooled differently - e.g. old is raw water cooled and new is clean water cooled then it may be necessary to upgrade the water inlet piping, stop cock and strainer. This is not a biggie - but again adds cost.

Propeller
It is quite likely if you change your engine for a new one, and the hp of the engines are mismatched that a new propeller may be required. Again - not prohibitive on its own, but can add to an already bulging price tag!

Conclusion
So after all that – what conclusion have I reached – well you won’t believe it but for every opinion I have others to contradict. Is it really a case of you can have any engine size you want, so long as you can deal with the consequences – as set out above?

Glazing
Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.
This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings, which causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly-sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating anenamel-like glaze which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface which are there to hold oil and return it to the crankcase via the scraper ring.
Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures.
Unburnt fuel then leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.
The problem is increased further with the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.
This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.
Under-loaded running inevitably causes not only white smoke from unburnt fuel but over time will be joined by blue smoke of burnt lubricating oil leaking past the damaged piston rings, and black smoke caused by damaged injectors. This pollution is unacceptable to the authorities and neighbours.
Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures allows the piston rings to scrape glaze off the bores and allows carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves, this will not have any effect.
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Old 05-04-2011, 11:34   #2
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Re: Engine Change

The above post was after some frustration at some of the posts on the thread: BMW Diesel Inboard
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Old 05-04-2011, 11:39   #3
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Re: Engine Change

Quote:
Originally Posted by Gooosberry View Post
Engine Horsepower
There are more myths around on the matching of horse power to boat than you can imagine. So if you hear them – stories like “one hp per foot” or “always better to have some power in reserve” – disregard them!
Oh? and why is that then? we have just under 1hp per foot and it's spot on.

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Old 05-04-2011, 12:51   #4
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Re: Engine Change

I am not saying that 1hp per foot wont work, I am trying to say that there are other ratios that may also work.
I guess (and I do not hold myself out to be any sort of expert) it would depend on things like displacement (heavy steel boat would use more power than light grp), maybe windage etc.

What do you think?
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Old 05-04-2011, 14:16   #5
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Re: Engine Change

I am just coming to the end of the repower project on my boat. Did all the work myself and it was a rewarding project. Sometimes we have a tendency to overthink things. A suggestion would be to look at the motor that has been in the boat and determine if that setup gave good reliable service and then duplicate that arrangement as close as possible with the new motor.
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Old 05-04-2011, 14:28   #6
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Re: Engine Change

Gooseberry... don't think I've heard a clearer more concise explaination of glazing and how to guard against it... great job... Capt Phil
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Old 05-04-2011, 14:39   #7
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Have to add quickly that the note on Glazing was taken off the web. Not my work!
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Old 05-04-2011, 15:10   #8
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Re: Engine Change

Nevertheless, great to have informative info on CF... thanks, Capt Phil
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Old 05-04-2011, 17:53   #9
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Re: Engine Change

This is an issue that I have to cover prior to every repower I do.

Engines that adverise themselves as direct replacements for XXXX usually mean they have the same "footprint" that's it.

I worked in a yard years ago......two of the "mechanics were repowering a vessel with a larger engine.....I told them that they needed to enlarge the exhaust hose......they told me they were just going to put in a reducer because the exhaust run was too much work....some woodworking was needed. Since I was low man on the totem pole (at that time) they didn't heed my input. It was a couple of months of smoking/poor performance and a lot of customer ranting....that they finally did the job right.
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Old 05-04-2011, 18:11   #10
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Just as an aside for EU boats subject to CE certification, repowering with a different engine for the original MAY require getting a " notified body" to redo the emissions testing and in some cases perform a sound by-pass test. Engine changes are regarded as a major craft modification and the onus is legally on the installer to ensure the CE certification is maintained.

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Old 05-04-2011, 18:55   #11
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Re: Engine Change

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Originally Posted by goboatingnow View Post
Just as an aside for EU boats subject to CE certification, repowering with a different engine for the original MAY require getting a " notified body" to redo the emissions testing and in some cases perform a sound by-pass test. Engine changes are regarded as a major craft modification and the onus is legally on the installer to ensure the CE certification is maintained.

Dave
What an absolutely terrible system... may we never give in to such garbage here!

I notice a certain tendency for folks to insist they need more hp also. The effect of the extra hp is often only felt in increased fuel use, and extra weight... I have seen a couple of re powers with smaller motors... those folks seemed more pleased with their decision then the others.
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Old 05-04-2011, 19:29   #12
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Re: Engine Change

Small motors in sailboats work good until you, for some reason need to bang into weather, then a bit more power is real handy.
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Old 06-04-2011, 11:31   #13
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Re: Engine Change

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Originally Posted by foamcore View Post
Small motors in sailboats work good until you, for some reason need to bang into weather, then a bit more power is real handy.

I'll vouch for that. I've been underpowered in a decent blow a few times. I used to have a Merc 9.9 on my Grampian. For reasons no longer remembered I decided to switch it to a Mariner 5 hp job. This arrangement worked very well under normal condition, and gave hull speed at a better fuel consumption. However, I got caught twice in one week where I was seriously under powered.


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Old 06-04-2011, 12:22   #14
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Re: Engine Change

I might add... IF you want better thrust in a strong head wind, have the room under the hull, and are willing to use a folding or feathering prop... A large prop turning slow will work much better than a small prop turning fast. (And a folding or feathering prop will keep you from loosing too much speed while sailing, from the substantial drag created by a standard prop. It's more than you'd think.) "Flex O Fold is primo"

If you get a Yanmar engine There are three different transmissions available to facilitate the size prop you choose. The prop & transmission should be such that when at your normal "ALL DAY motoring" hull speed, the engine RPMs are between 2,500 and 2,900 RPMs.

If you use an AquaDrive CV joint like we did, It greatly simplifies the installation and living with the engine in general. The double universal joint allows the engine to be several degrees out of alignment, and when it shakes, the shaft does not, it only spins. This makes dripless shaft seals, like our PSS, work much better. Since the thrust now goes ONLY to the engine beds and not on the engine itself, bearings & engine feet last longer. If you ever snag a crab pot at speed, it is less likely to ruin the transmission as well. Ours is 15 years old & counting... They're expensive, but money VERY well spent IMO. I have told many friends about this advantage, and all that have installed them, rave about how cool they are.

Best of luck, M.



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