Re: Itchen: MaxProp Pitch vs RPM, Diesel mechanic rantings

The goal with any propeller used in conjunction with a Yanmar 3GM30F is to achieve max RPM 3,600 plus 150 RPM or 3,750 wide open throttle, no wind-waves-adverse tides, on flat water with an unladen boat. It doesn’t matter what propeller brand you use so long as you achieve these parameters.

We ran a propeller experiment with our 2 bladed fixed Michigan Wheel Sailor propeller, altering it from 16x11 to 16x12, which drop the RPMs down to 3,600 max and went from no smoke to black smoke at max RPM. We dialed it back to 16x11 and it has been smoke free since.

There are a number of people who subscribe to over proping and running at lower rpms, near the peak of the torque curve, believing max torque at lower RPM will make their engine last longer, not so; these little Yanmar engine are high RPM screamers, if you load them up at lower RPMS, they tend to load up with carbon, blown head gaskets are only one of the symptoms you will experience.

We service and repair a lot of these little Yanmars in the San Francisco Bay Area, we service and maintain our 3GM30F and never had a breakdown on the the water:

Before starting your engine, open the engine compartment and do your pre-run checks, check engine and transmission fluid levels, check all hoses and fitting, looks for fuel, oil, water leaks, fix before running.
Start your engine and idle it at 1,000 rpm until warm, prevents shaking your boat to death when it is cold, it’s easy on the mounts, propeller shaft and packing gland.
Look for the sweet spot when cruising under power, ours is from 2,750 to 2,900 RPM, it run free and smooth. If you listen to your engine it will tell you if it is working to hard or being under worked.
Operate your engine for 4 minutes at max RPM at least once a month to blow out the carbon, it won’t hurt it one bit and it will pay you back ten fold in longer engine life.
When you return to the dock, let your engine cool down for 5 minutes, then in neutral, raise the RPM to 1,700 for 4 seconds, burns off the carbon without re-heating the engine, then drop it back to idle and let the engine smooth out, then stop the engine.
Pet Yanmar problem areas:

The U-shaped cast iron exhaust mixing elbow: water and exhaust passages are small, prone to clogging. The water gallery will clog where the water enters the elbow, in early stages, you can remove the pipe fitting and dig around with a screw driver and clean it out. Annually remove your exhaust hose and look up into the outlet of the exhaust elbow with a bright flashlight and a mirror, check for restriction of both exhaust and water galleries, don’t attempt to clean, replace it; don’t expect more than 4 good years of service, expect less if you are consistently running at lower RPMS. Just because there is water coming out the transom thru-hull fitting, it doesn’t necessarily mean the elbow isn’t restricted, if you push the limit on time in service, eventually carbon or corroded metal will shed and block the outlet, engine will blow black smoke at lower RPMS, eventually it will not start. Run your engine until warm, place your bare hand on the top of the elbow, it should be cold to
the touch, if not the water gallery is clogged or restricted, replace it. Look for rust water stains or cracks, if so replace, remember for optimum performance, even if you think it still works, chuck it and buy new, your engine will thank you with many more years of good service.
Pickle your seawater cooling circuit with white vinegar every year to reduce mineral build up. Run your engine till warm, shut off the seawater seacock, disconnect the hose and put it in a bucket of white vinegar, restart your engine and pump in white vinegar, let it sit over night. The next day, reconnect the seawater inlet hose, open the seacock and run till warm, flush out the vinegar. There are other chemical that will do the job in less time, but white vinegar is non-toxic to you and your environment, besides, what ever you spill won’t eat thought your boat and it also kill mold.
Fuel system: All small Yanmar diesels tend to shake loose the banjo fuel fittings on both sides of the fuel lift pump and from the pump to the secondary fuel filter, annually tighten up these fittings, not to tight, we just don’t want them to leak. When you can’t snug them up, don’t over tighten, replace the crush washers, 2 per fitting, remember your tightening steel banjo bolts in aluminum body, it is easy to strip them out.
Fuel lift pumps, as noted earlier, below the large round section of the pump body (diaphragm & valve body), you will see a 1/4" diameter round circle of metal on both sides, this is the exposed ends of the pivot shaft, axle, that the pump lever pivots on, the shaft is staked into place with a punch at the factory, to keep it in place; as the pump wears, this axle will tend to slide out one side and drop out of position, causing the pump lever to cock in the pump and rupture the diaphragm, resulting in pump failure or diaphragm perforation and fuel in the block. Preventive maintenance is key to preventing failure on the water, we replace ours every 4 years and forgo the headache, its better to replace it while it’s working than to deal with the issue of worrying about it condition or worst yet dealing with a failure.
Black dust on the front of the engine: All rubber products biodegrade and wear out over time, even v-belts, ensure your spares are not getting to old, chalking, cracking. Renew your belts when they start to ride down into the pulleys, keep your belts properly tensioned, 3/8-1/2" deflection in the middle of the belt. Look for any signs of wear, cracks, shiny sides means it’s slipping, slipping belts cause rubber to shed, black dust, take a cheap chip paint brush and knock the dust loose while you suck it up with a vacuum, your air filter will love you for this.
Cycling your spares: Use your spare parts to service your engine, buy new spares to store for future servicing or emergency spares, rotate your spare stocks. If a spare rubber product is chalky chuck it in the trash, you’ll be glad you did, as old spares fail prematurely.
Renew your air filter every year when you do your annual service, impeller, oil & filter, transmission oil and fuel filters, depending on hours of operation, v-belts can last 2 years depending on hours and proper tension, do not exceed 150 hours between oil changes, never go more than a year between oil changes even if your hours are below 150 per year to prevent moisture and acid build up.
Look for leaks, sweating hose to barb junctions in the cooling circuit, drain systems, replace hoses, use good clamps ABA or AWAB non-perforated clamps, they don’t tear up the hoses, they cost more, but they last longer, careful, these clamps can be easily over tightened.
Seawater pumps: Yanmar seawater pumps are disposable assets, unless your cruising and you have no other choice, replace it, don’t rebuild it, when the shaft seals start to leak. Most seawater pump run a stainless steel shaft, the seal runs on this shaft, cutting a tiny grove in the shaft, a new seal will only last for a little while running in this groove before failing. By the time you have to replace the shaft, bearing are not far behind, the cost of repair is to close to the cost of a new pump, again unless your cruising and you have no other choice but to rebuild, just throw it away, you’ll be glad you did.
Anti-siphon valves: Always install a good quality anti-siphon valve in you seawater circuit, usually between the heat exchanger and the exhaust mixing elbow, height of the valve should be 8-12" above the waterline and on center-line with the boat. We use to use Vetus anti-siphon valves religiously for years, they had a 5/16 hose barb to attach a vinyl drain hose which we ran to the bilge in case it spit salt water as it opened and closed, lately they have doubled the price and we are seeing more failures, leakage while running and have stopped using them. Since the price of the Vetus is so close to the Groco, we have gone back to using the bronze Groco valves with the 1/8 NPT female threads in top of the valve, install a hose barb and run a 5/16 vinyl hose to the bilge; you can take these valves apart and soak them over night in white vinegar to clean them up and replace the rubber flapper valve for $15, darn things nearly last for ever. Remember to apply a little water proof
grease on the cap threads when installing, it makes it easy to remove the cap for future servicing.
Corrosion: get rig of any signs of corrosion, sand, wire brush, convert minor corrosion with a chemical to turn minor pit corrosion to iron phosphate, prime and paint, yes use Yanmar paint it looks better. Corrosion, frozen bolts can add 30% or more to the cost of repair. When your remove your seawater pump to replace the impeller, grease the bolts that hold the pump on to the engine, grease the full length of the bolts as these tend to weld them self’s to the timing cover, this repair is expensive, requiring drilling thought the timing cover from below, through the bolts to remove the cover, in most cases the engine has to be removed to to this.
Invest in drip-less packing material for your packing gland. The manufacturer suggests alternating synthetic rings with drip-less packing. As an experiment, we deleted the synthetic rings and use 3 rings of the drip-less packing (IE.green goo), the shaft runs cool, minimal seepage, almost none and our bilges stay dry, using a conventional packing gland. Another alternative is the PSS drip-less packing gland, note the term is “drip-less”, not drip free; Yanmars, especially when cold or at lower RPM tend to jump around the engine room, this upsets the seal in the PSS gland, making it drip, at higher RPMS it is virtually drip free. The only thing we don’t like about the PSS glands, is having align the engine, fighting to push the shaft back while taking measurements. If you have to remove the transmission, you will have to release the set screws in the SS rotor and slide the shaft back through the rotor to allow room to remove the transmission, you also need to tie back the coupling
to keep tension on the PSS to prevent the seal from unseating and pouring water into the boat. With the Spartan conventional packing gland, we just clean the shaft forward of where it exits the packing gland and slide the shaft back, simple. Use ABA or AWAB clamps on your packing gland, slight drips eat up perforated conventional SS clamps in short order.
Finally, install an R&D Flex coupling between your transmission and propeller shaft couplings, it reduces engine movement transmitted to the propeller shaft, reduced propeller shaft packing wear and leaks, reduces loading of the transmission bearings, reduces or eliminates damage to the transmission in the event you strike something with the propeller, if nothing else, with reduced vibration transmitted to the hull, you butt will thank you.
From max prop to total engine care, who would figure? Diesel mechanic rantings.

Cheers,

Marty Chin, BCC Shamrock

1 Like

Thank you; most informative.

David Kent
Sea Star

-----Original Message-----
From: owner-bccforum@coledata.com [mailto:owner-bccforum@coledata.com] On
Behalf Of BCC Forums
Sent: Saturday, January 28, 2006 2:00 PM
To: bccforum@samlmorse.com
Subject: Re: Itchen: MaxProp Pitch vs RPM, Diesel mechanic rantings

Author: Shamrock
Username: Shamrock
Subject: Re: Itchen: MaxProp Pitch vs RPM, Diesel mechanic rantings
Forum: BCC Forum
Link: http://www.samlmorse.com/forum/read.php?5,4420,4420#msg-4420

The goal with any propeller used in conjunction with a Yanmar 3GM30F is to
achieve max RPM 3,600 plus 150 RPM or 3,750 wide open throttle, no
wind-waves-adverse tides, on flat water with an unladen boat. It doesn’t
matter what propeller brand you use so long as you achieve these parameters.

We ran a propeller experiment with our 2 bladed fixed Michigan Wheel
Sailor propeller, altering it from 16x11 to 16x12, which drop the RPMs down
to 3,600 max and went from no smoke to black smoke at max RPM. We dialed it
back to 16x11 and it has been smoke free since.

There are a number of people who subscribe to over proping and running at
lower rpms, near the peak of the torque curve, believing max torque at lower
RPM will make their engine last longer, not so; these little Yanmar engine
are high RPM screamers, if you load them up at lower RPMS, they tend to load
up with carbon, blown head gaskets are only one of the symptoms you will
experience.

We service and repair a lot of these little Yanmars in the San Francisco
Bay Area, we service and maintain our 3GM30F and never had a breakdown on
the the water:

Before starting your engine, open the engine compartment and do your
pre-run checks, check engine and transmission fluid levels, check all hoses
and fitting, looks for fuel, oil, water leaks, fix before running.
Start your engine and idle it at 1,000 rpm until warm, prevents shaking
your boat to death when it is cold, it’s easy on the mounts, propeller shaft
and packing gland.
Look for the sweet spot when cruising under power, ours is from 2,750 to
2,900 RPM, it run free and smooth. If you listen to your engine it will tell
you if it is working to hard or being under worked.
Operate your engine for 4 minutes at max RPM at least once a month to
blow out the carbon, it won’t hurt it one bit and it will pay you back ten
fold in longer engine life.
When you return to the dock, let your engine cool down for 5 minutes,
then in neutral, raise the RPM to 1,700 for 4 seconds, burns off the carbon
without re-heating the engine, then drop it back to idle and let the engine
smooth out, then stop the engine.
Pet Yanmar problem areas:

The U-shaped cast iron exhaust mixing elbow: water and exhaust passages
are small, prone to clogging. The water gallery will clog where the water
enters the elbow, in early stages, you can remove the pipe fitting and dig
around with a screw driver and clean it out. Annually remove your exhaust
hose and look up into the outlet of the exhaust elbow with a bright
flashlight and a mirror, check for restriction of both exhaust and water
galleries, don’t attempt to clean, replace it; don’t expect more than 4 good
years of service, expect less if you are consistently running at lower RPMS.
Just because there is water coming out the transom thru-hull fitting, it
doesn’t necessarily mean the elbow isn’t restricted, if you push the limit
on time in service, eventually carbon or corroded metal will shed and block
the outlet, engine will blow black smoke at lower RPMS, eventually it will
not start. Run your engine until warm, place your bare hand on the top of
the elbow, it should be!
cold to
the touch, if not the water gallery is clogged or restricted, replace it.
Look for rust water stains or cracks, if so replace, remember for optimum
performance, even if you think it still works, chuck it and buy new, your
engine will thank you with many more years of good service.
Pickle your seawater cooling circuit with white vinegar every year to
reduce mineral build up. Run your engine till warm, shut off the seawater
seacock, disconnect the hose and put it in a bucket of white vinegar,
restart your engine and pump in white vinegar, let it sit over night. The
next day, reconnect the seawater inlet hose, open the seacock and run till
warm, flush out the vinegar. There are other chemical that will do the job
in less time, but white vinegar is non-toxic to you and your environment,
besides, what ever you spill won’t eat thought your boat and it also kill
mold.
Fuel system: All small Yanmar diesels tend to shake loose the banjo fuel
fittings on both sides of the fuel lift pump and from the pump to the
secondary fuel filter, annually tighten up these fittings, not to tight, we
just don’t want them to leak. When you can’t snug them up, don’t over
tighten, replace the crush washers, 2 per fitting, remember your tightening
steel banjo bolts in aluminum body, it is easy to strip them out.
Fuel lift pumps, as noted earlier, below the large round section of the
pump body (diaphragm & valve body), you will see a 1/4" diameter round
circle of metal on both sides, this is the exposed ends of the pivot shaft,
axle, that the pump lever pivots on, the shaft is staked into place with a
punch at the factory, to keep it in place; as the pump wears, this axle will
tend to slide out one side and drop out of position, causing the pump lever
to cock in the pump and rupture the diaphragm, resulting in pump failure or
diaphragm perforation and fuel in the block. Preventive maintenance is key
to preventing failure on the water, we replace ours every 4 years and forgo
the headache, its better to replace it while it’s working than to deal with
the issue of worrying about it condition or worst yet dealing with a
failure.
Black dust on the front of the engine: All rubber products biodegrade and
wear out over time, even v-belts, ensure your spares are not getting to old,
chalking, cracking. Renew your belts when they start to ride down into the
pulleys, keep your belts properly tensioned, 3/8-1/2" deflection in the
middle of the belt. Look for any signs of wear, cracks, shiny sides means
it’s slipping, slipping belts cause rubber to shed, black dust, take a cheap
chip paint brush and knock the dust loose while you suck it up with a
vacuum, your air filter will love you for this.
Cycling your spares: Use your spare parts to service your engine, buy new
spares to store for future servicing or emergency spares, rotate your spare
stocks. If a spare rubber product is chalky chuck it in the trash, you’ll be
glad you did, as old spares fail prematurely.
Renew your air filter every year when you do your annual service,
impeller, oil & filter, transmission oil and fuel filters, depending on
hours of operation, v-belts can last 2 years depending on hours and proper
tension, do not exceed 150 hours between oil changes, never go more than a
year between oil changes even if your hours are below 150 per year to
prevent moisture and acid build up.
Look for leaks, sweating hose to barb junctions in the cooling circuit,
drain systems, replace hoses, use good clamps ABA or AWAB non-perforated
clamps, they don’t tear up the hoses, they cost more, but they last longer,
careful, these clamps can be easily over tightened.
Seawater pumps: Yanmar seawater pumps are disposable assets, unless your
cruising and you have no other choice, replace it, don’t rebuild it, when
the shaft seals start to leak. Most seawater pump run a stainless steel
shaft, the seal runs on this shaft, cutting a tiny grove in the shaft, a new
seal will only last for a little while running in this groove before
failing. By the time you have to replace the shaft, bearing are not far
behind, the cost of repair is to close to the cost of a new pump, again
unless your cruising and you have no other choice but to rebuild, just throw
it away, you’ll be glad you did.
Anti-siphon valves: Always install a good quality anti-siphon valve in
you seawater circuit, usually between the heat exchanger and the exhaust
mixing elbow, height of the valve should be 8-12" above the waterline and on
center-line with the boat. We use to use Vetus anti-siphon valves
religiously for years, they had a 5/16 hose barb to attach a vinyl drain
hose which we ran to the bilge in case it spit salt water as it opened and
closed, lately they have doubled the price and we are seeing more failures,
leakage while running and have stopped using them. Since the price of the
Vetus is so close to the Groco, we have gone back to using the bronze Groco
valves with the 1/8 NPT female threads in top of the valve, install a hose
barb and run a 5/16 vinyl hose to the bilge; you can take these valves apart
and soak them over night in white vinegar to clean them up and replace the
rubber flapper valve for $15, darn things nearly last for ever. Remember to
apply a little water p!
roof
grease on the cap threads when installing, it makes it easy to remove the
cap for future servicing.
Corrosion: get rig of any signs of corrosion, sand, wire brush, convert
minor corrosion with a chemical to turn minor pit corrosion to iron
phosphate, prime and paint, yes use Yanmar paint it looks better. Corrosion,
frozen bolts can add 30% or more to the cost of repair. When your remove
your seawater pump to replace the impeller, grease the bolts that hold the
pump on to the engine, grease the full length of the bolts as these tend to
weld them self’s to the timing cover, this repair is expensive, requiring
drilling thought the timing cover from below, through the bolts to remove
the cover, in most cases the engine has to be removed to to this.
Invest in drip-less packing material for your packing gland. The
manufacturer suggests alternating synthetic rings with drip-less packing. As
an experiment, we deleted the synthetic rings and use 3 rings of the
drip-less packing (IE.green goo), the shaft runs cool, minimal seepage,
almost none and our bilges stay dry, using a conventional packing gland.
Another alternative is the PSS drip-less packing gland, note the term is
“drip-less”, not drip free; Yanmars, especially when cold or at lower RPM
tend to jump around the engine room, this upsets the seal in the PSS gland,
making it drip, at higher RPMS it is virtually drip free. The only thing we
don’t like about the PSS glands, is having align the engine, fighting to
push the shaft back while taking measurements. If you have to remove the
transmission, you will have to release the set screws in the SS rotor and
slide the shaft back through the rotor to allow room to remove the
transmission, you also need to tie back the !
coupling
to keep tension on the PSS to prevent the seal from unseating and pouring
water into the boat. With the Spartan conventional packing gland, we just
clean the shaft forward of where it exits the packing gland and slide the
shaft back, simple. Use ABA or AWAB clamps on your packing gland, slight
drips eat up perforated conventional SS clamps in short order.
Finally, install an R&D Flex coupling between your transmission and
propeller shaft couplings, it reduces engine movement transmitted to the
propeller shaft, reduced propeller shaft packing wear and leaks, reduces
loading of the transmission bearings, reduces or eliminates damage to the
transmission in the event you strike something with the propeller, if
nothing else, with reduced vibration transmitted to the hull, you butt will
thank you.
From max prop to total engine care, who would figure? Diesel mechanic
rantings.

Cheers,

Marty Chin, BCC Shamrock

Thanks Marty, more than I asked for, but all interesting and useful.

But . . . . here is the difficult question for you or anyone else who
has wrestled with BCC Yanmar - MaxProp pitch settings. There is quite
a bit in the archives from past discussions over the years and I have
seen nothing that indicated that anyone was using pitch settings low
enough to allow a BCC-mounted Maxprop to reach 3600 RPM – or,
yikes(!) 3750 allowing for the reported underreading of the Yanmar
instrument panel tachometer! Owners and Sam L. Morse Co. have
reported settings in the range of 16 to 22 degrees pitch andI believe
I recall a consensus that 16 is what most finally have chosen. No way
will that give an RPM of 36600 - 3700 on Itchen. At aprox 300 RPM
gain per degree of pitch reduction we would be lucky to reach 3000 RPM
at max throttle.

So what gives? Are many of us over-pitching our Yanmar 3GM30’s and
doing them damage over the long run? Shuld we be running them at a
normal just-under-hull-speed RPM of 2800 - 3000 (or even more) instead
of the 2200 - 2400 that many report?

The www.yanmarhelp.com advisor recommends even higher cruising speed
RPM’s than that. Quoting YanmarHelp, “Scott you were massively
over-propped you should be able to reach 3600rpm If your are using the
Yanmar tacho that’s about 3700 as they under read by about 100rpm at
the top. probably why you’ve experienced the head gasket failure. You
definitely need to decrease pitch further. Not sure how much
adjustment you have but you need to pick up another 800-900rpm!”

So, what am I missing? Perhaps others can reach 3600 or 3750 RPM wide
open and my engine will soon be able to, after we put the rebuilt head
back on? If not, I’ll be hauling next week and can easily change the
pitch at that time.
TIA, Scott

<SNIP

Fly in the ointment, ever vessel is different, every BCC ever produced is slightly different from the other, it’s the nature of boat building.

Other than a clean propeller and clean hull, the engine RPMS have virtually nothing to do with the vessel, each boat owner will have to tweak their propeller to achieve the engine performance design specifications of 3,750 RPM WOT.

The 3,750 target RPM came directly from Yanmar school we attended years ago, thus far it has worked to our customers advantage, we have very little breakage or at least nothing that can be attributed to over-proping.

Marty

BCC Forums bccforums@samlmorse.com wrote:
Author: Itchen
Username: Itchen
Subject: Re: Itchen: MaxProp Pitch vs RPM, Diesel mechanic rantings
Forum: BCC Forum
Link: http://www.samlmorse.com/forum/read.php?5,4422,4422#msg-4422

Thanks Marty, more than I asked for, but all interesting and useful.

But . . . . here is the difficult question for you or anyone else who
has wrestled with BCC Yanmar - MaxProp pitch settings. There is quite
a bit in the archives from past discussions over the years and I have
seen nothing that indicated that anyone was using pitch settings low
enough to allow a BCC-mounted Maxprop to reach 3600 RPM – or,
yikes(!) 3750 allowing for the reported underreading of the Yanmar
instrument panel tachometer! Owners and Sam L. Morse Co. have
reported settings in the range of 16 to 22 degrees pitch andI believe
I recall a consensus that 16 is what most finally have chosen. No way
will that give an RPM of 36600 - 3700 on Itchen. At aprox 300 RPM
gain per degree of pitch reduction we would be lucky to reach 3000 RPM
at max throttle.

So what gives? Are many of us over-pitching our Yanmar 3GM30’s and
doing them damage over the long run? Shuld we be running them at a
normal just-under-hull-speed RPM of 2800 - 3000 (or even more) instead
of the 2200 - 2400 that many report?

The www.yanmarhelp.com advisor recommends even higher cruising speed
RPM’s than that. Quoting YanmarHelp, “Scott you were massively
over-propped you should be able to reach 3600rpm If your are using the
Yanmar tacho that’s about 3700 as they under read by about 100rpm at
the top. probably why you’ve experienced the head gasket failure. You
definitely need to decrease pitch further. Not sure how much
adjustment you have but you need to pick up another 800-900rpm!”

So, what am I missing? Perhaps others can reach 3600 or 3750 RPM wide
open and my engine will soon be able to, after we put the rebuilt head
back on? If not, I’ll be hauling next week and can easily change the
pitch at that time.
TIA, Scott

Other than a clean propeller and clean hull, the
engine RPMS have virtually nothing to do with the
vessel, each boat owner will have to tweak their
propeller to achieve the engine performance design
specifications of 3,750 RPM WOT.

Is everybody using the same transmission gear-ratio?
My Clymer “Yanmar” manual shows two possible Yanmar
gear boxes and three possible gear ratios for the
3GM30F engine. Different prop speeds will need different
prop pitches even for the same engine rpm.

The 3,750 target RPM came directly from Yanmar
school we attended years ago, thus far it has worked
to our customers advantage, we have very little breakage
or at least nothing that can be attributed to over-proping.

I am not the expert here, but here are the numbers I have
found for this engine.

The Clymer manual shows the full throttle no-load rpm
rating for the 3GM30F is 3825 rpm. For other engines
listed there it is between 3600 and 3825 rpm.
There are governor adjustments for this, but changing
those will void your warranty, since it could damage
your engine. The same manual shows the rated HP/rpm
for the 3GM30F is 24 hp @ 3400 rpm.

I found a Yanmar web page and it gives the same ratings
for the 3GM30F:
Continuous 24 hp @ 3400 rpm
Max (one hour?) 27 hp @ 3600 rpm
max idle 3825 rpm plus/minus 25

Nigel Calder’s Cruising Handbook has a section on
propeller sizing. “If a propeller has a power curve that
crosses the engine curve below full engine speed, the
propeller will fully load the engine at this speed and
overload it at higher speeds. The engine will “bog down”
and fail to reach its maximum speed. If, on the other
hand, a propeller has a power curve that never crosses
the engine curve the engine will go to full rpm, but its
full power will never be used and boat performance will
be commensurably reduced.” He goes into considerably
more detail.

  • Norris (Bristol Cream)

My take on engine vs prop pitch is oversimplified but has always worked. The engine must be able to reach its designed rpm or you are lugging it down…at all speeds. And it is not only the pitch of the blade you have to compare but the diameter and number of blades. (and let’s not forget the different transmissions as already mentioned).

Every boat, whether the same design or not will power differently…they are usually ‘overloaded’ but in any case will be loaded differently because of the gear aboard and the BCC sinks about one-inch per 850 lbs aboard…thus a boat equiped to make a weekend or day sail is way lighter than one loaded for cruising or a long passage.

Everyone would like to say they can motor at 6 knots at 1800 or 2200 rpm but most can’t and shouldn’t. Yanmar and almost all other diesel engines get their horsepower claims from high rpm’s and geared up transmissions…to lug them down with large, high pitched props just isn’t good for them and will tell in the long run.

After doing Itchen’s recent engine work - planed the head, ground the
valves,new head gasket – she now starts almost instantly even when
cold(!), a big and welcome change, and runs smoother and uses less
oil. Presumably some more power is available but that is not
particularly apparent one way or the other. As previously mentioned in
an earlier post we also checked the Yanmar tachometer with the
mechanic’s digital tachometer and the Yanmar tach was reading waaay
low. Depending on wind & wave she now reaches hull speed at around
2200 - 2400 indicated, which is 2900 - 3200 “actual”, with her 15"
MaxProp set at 18 degrees and a 2.61 gear ratio transmission (if
memory serves). I just bought a new tachometer to replace the very
rusty-cased and “sticky needled” old one and will install it when we
return to the boat in a couple of weeks. WIll be interesting to see
what sort of readings we get after that. One has to take the
precision and accuracy of these tachometer readings with a big grain
of salt as Marty pointed out previously, but the trend seems clear and
I am surmising that the current prop pitch and load is in the correct
ballpark and not vastly overloaded as indicated by the old tachometer
readings. To be continued . . . .

Scott

On 2/14/06, BCC Forums bccforums@samlmorse.com wrote: