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Old 10-12-2015, 11:37 AM   #41
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Ski,
I don't have that on paper but it's probably on the Mitsubishi web site that I have bookmarked on my crashed i-mac. Miss that thing.

OK so this sweet spot is a junction where power and torque curves meet. Kind of a best of both worlds thing? Looks like Mark's running his Deere at a little lower load than me. It would be interesting to see all our boats cruise rpm as a percentage of max power rpm. It would not however indicate a percentage of engine load. I often have viewed it as X # of rpm down from max rated. But that's not apples and apples unless all engines considered have the same max power rpm.
Eric - My Vetus (Mitsubishi) 42 hp has peak torque at 1750 rpm, so I would expect yours is close to the same. At that speed the engine is developing 25hp, which is enough to drive my boat at very close to hull speed in calm water.

I've still got the old prop on which is way over pitched and 1750 rpm is all I get because the torque is dropping off above that. I can still do 7 knots at 1750 rpm as it is. I'll be changing the prop next month, but will leave it slightly over pitched, to stay in the high torque range while cruising.
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Old 10-12-2015, 12:05 PM   #42
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Have to say it again . The toque peak is at max load, far different from the prop load. Boats never use peak torque except perhaps when accelerating.


The makers say nothing about how torque varies along the prop curve.


If you want to measure the efficiency of your boat set up the only way is to measure fuel use vs distance. Just like a car MPG vs speed.
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Old 10-12-2015, 12:37 PM   #43
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Beat me to it bayview.

But in the bottom of the paragraph below the graphics TDunn points out that full bore and cruise are different. But then omits it later.

TDunn's post is very good and he makes a seldom understood adequately power and fuel burn function quite clear. Many fail to notice most of the curves are for maximum WOT operation. Usually leads to a lot of confusion.
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Old 10-12-2015, 01:11 PM   #44
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Have to say it again . The toque peak is at max load, far different from the prop load. Boats never use peak torque except perhaps when accelerating.
Since I am currently way over propped, and I am only getting 1750 rpm at full throttle, wouldn't max load and prop load be the same in this particular case?
or am I not grasping this?
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Old 10-12-2015, 02:56 PM   #45
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TDunn:

Are you sure that the formula isn't:

Gallons per hour = 0.05963*hp+0.054815
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Old 10-12-2015, 03:06 PM   #46
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Oh.....uhm.....thanks TD.... I see it, now....What he said!
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Old 10-12-2015, 03:26 PM   #47
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AusCan,
Yup .. Re #44 .. exactly.
And indeed you are WAY over propped. My max while trying a 5 blade prop was 2000rpm. Jerked the boat right out and swaped props.
I have a 2.57-1 BW gear and a 18 X 13 prop w a little too much blade area. The boat seems fine at cruise rpm (2300) and below but feels like it's a truck going up a hill at higher revs especially near a max ov 2900. The last 200 rpm comes slowly. Was not like that w the old prop. Went right up to 3000 w no struggling.

Other than a small bit of extra economy and a little less noise what's the advantage? The advantage to running at max torque seems fly stuff if meaningful at all to me.
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Old 10-13-2015, 01:00 AM   #48
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Eric - Yes - The prop shop is recommending changing from 17x15 LH to 16 x10 RH. My new gearbox ratio is 1.94:1 which replaced the 2.34:1.
I was just getting used to operating the throttle control in reverse, so I'll have to re-learn it again.
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Old 10-13-2015, 06:57 AM   #49
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Beta Marine 85 HP 2,800 rpm or the optional Yanmar 110hp 3,200 rpm.

Great but at a fantastically optimistic 20hp/gal The beta will be drinking 4+gph and the Yanmar 5.5.

As you will probably be at 6 -7 mph 6K-7K for 99% of the boats cruising , the question of how to do that with efficiency remains

For most its moot as a 15% better fuel burn at 1 1/2 or 2 gpg doesn't amount to a big deal , and most tiny motors do not suffer from underloading as a more industrial engine would.
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Old 10-13-2015, 09:32 AM   #50
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Auscan as manyboats said yes you are running at the max tated curve. Looking at the curve at that RPM will show what power and fuel you should be using.


The reason that is your max RPM is that the engine lacks extra torque to spin faster. The overload concerns become high cylinder temps due to low air flow. and possibly high exhaust temps.


Any black smoke??
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Old 10-13-2015, 10:13 AM   #51
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AusCan,
One situation where a correctly propped engine/boat combination shines is in reversing. Willy weighs 8 tons and to stop her in reverse w only a knot or two of weigh takes a bit of time w considerable throttle/rpm.

Too bad you don't have a deeper reduction. When I repowered Willy I thought of changing the gear ratio to 3-1. I have plenty of prop clearence. But w bigger props and deeper reductions have a problem related to the essence of this thread. The prop load decreases as the rpm decreases faster w a big prop so the big prop's thrust advantage is lost at lower rpm. I think that happens because of the much higher blade area of the big prop. With a small prop you have less thrust at high speeds/rpm but loose less thrust as the rpm decreases. And I suppose at some point the thrust is the same (big prop/little prop) and below that point perhaps the little prop has more thrust.

This is just my own theory and may not hold water (couldn't resist that) but a well known example of this being wide spread is outboard engines .. especially small ones. Nobody at the factory knows what boat these engines will be used on so trying to guess what prop (dia/pitch) should be employed reveals the necessity of using a prop that will present enough load to prevent the engine from over reving. The small prop becomes the answer as the load changes less up and down the rpm scale. But a big hit is taken in thrust and efficiency. But it's acceptable as the engines are light and fuel consumption small even w large inefficiency.

So w your smallish prop far less thrust is lost at cruising speed (about 50% load) than w a bigger prop at 50% load. So the inefficiency you experience is considerably less than it would seem.

However if it were my boat I'd get a lower gear and bigger prop. That said I assume you have room to swing a bigger prop w/o too little clearence to the hull. But as you point out the boat works fine. But your idle speed is probably too fast so you go in and out of gear in Harbour quite a bit and reversing probably requires more time and advanced planing.
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Old 10-13-2015, 10:47 AM   #52
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The manufacturer is likely just offering you the option of what brand of engine you want. I personally do not see much difference in 85hp and 110hp. I do not know Beta Marine engines well. I do know Yanmars so I would choose the 110hp engine....strictly because it is a Yanmar...not because of the power it is making.
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Old 10-14-2015, 06:48 AM   #53
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"I do not know Beta Marine engines well. I do know Yanmars so I would choose the 110hp engine....strictly because it is a Yanmar...not because of the power it is making."

You might consider a call to the parts dept to price different repair items.

The tractor engine will be far cheaper for parts than the Yanmar.
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Old 10-14-2015, 10:04 AM   #54
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One in the Willard group and I think one or two here has Beta engines. From all the first hand stuff I hear they seem top quality. Didn't know about them when I repowered.
I had a Yanmar in my Albin and it was fine except for a sea water leak on the exhaust manifold that dripped down on the throttle shaft and I noticed a sticky throttle as corrosion set in. That experience turned me off of aluminum on the engine in contact w the coolant. That w the memories of all the corrosion on aluminum outboards.
The Yanmar was a good engine other than that little event. Started well w/o glow plugs, even in cold weather, made good power and was light. Made more noise than my Mitsu and I had some alignment issues that was probably related to the soft engine mounts and ironically had some harmonic vibration issues.
The only downside to the Beta that I can see is that it comes from the UK and dealers aren't very numerous. Both engines are what I'd call expensive.

As to what engine for the SeaPiper and the OP it should have little to do w engine brand and mostly or all to do w power. I don't know what the design speed range is for the SeaPiper. Could be that the Beta is at the lower end and the Yanmar is capable to run the boat at the high end. Or neither one has enough power or they both are too much power. In my opinion the hull design needs to be matched to the power first. Then personal preferences and cost ect can be factored in. Seems to me the boat was closer to a planing hull but I think I saw an image of some kind that showed the hull more like a slower speed SD hull. If the 85hp engine can push the boat easily throught it's design speed range I'd say go for the 85. Not many boat manufacturers offer underpowered boats so I'd bet the 85 is plenty of power and I'd recomend it.
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Old 10-15-2015, 06:22 AM   #55
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"The only downside to the Beta that I can see is that it comes from the UK"

The ones I have seen were Kubota powered with Beta just providing marinization parts.

So replacement or repair parts for the base motor are cheap at the local lawn tractor or truck reefer shop/
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Old 10-15-2015, 11:17 AM   #56
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I agree FF but at times getting on the phone and calling the people you bought the engine from for advice or parts that may need to be flown in on a small plane can be of great value. And of course over time modifications and changes are made to engines and the farm tractor store may not be able to relate completely. But the low cost route to parts can very well be to the farm tractor store. But if I'm buying anything but a crankshaft I'm going to where I bought the engine. But in defense of your post if the tractor store is close oil filters and thermostats could be bought there but you never know when there's going to be a difference in the parts.
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Old 10-15-2015, 12:43 PM   #57
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Baker wrote;
"I personally do not see much difference in 85hp and 110hp"

That's almost 25%. Add 25% power to my boat and it's in the "reject the engine for too much power" catergory. The engine I really wanted for Willy was a 54hp Isuzu. If you added 25% more power to a GB36 w twins it would have 300hp.

True though if you added 25% power to a typical SD trawler it wouldn't go much faster. And if you added 25% power to a FD boat most of the difference would be in it's wake.
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Old 10-15-2015, 01:01 PM   #58
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"I'm going to where I bought the engine."

Don't bother if its a Volvo. Even if its only a year or two old.

The use of a common engine is great as any changes start with a certain number , worldwide , and in many cases tractor parts are expected to be available 30-40 years later .

This can not be claimed for most marine engines.

The Ford Econo power is one exception as its one fellows business.

Try Westerbeke for 30myear old conversion parts.
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Old 10-15-2015, 01:53 PM   #59
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Quote:
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The makers say nothing about how torque varies along the prop curve.
They don't have to. You can compute torque, given horsepower and RPM. TDunn gave the formula a few posts ago.
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Old 10-15-2015, 03:33 PM   #60
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Ski,
"Cruise at max torque" ...
Is that for max efficiency or is there more to it?
I've heard that many times so there must be something to it. However I would think loading would be more important than small differences in efficiency. Most of the engines we're running were designed for duties other than powering boats. As I recall someone said a long time ago that most gen sets run at 1800 and that's why so many have max torque at 1800. What do you know about that? I think my engine is in that catergory but I run it at 2300. My 2300 is a result of choosing the right amount of power, propping the engine for rated rpm (3000) and then driving the hull at about one knot below hull speed. Fuel burn wasn't considered important enough to put on the chart. But my fuel burn isn't important enough to consider 10% this way or that. To me that seems more like a Passagemaker issue.
Part of the reason for either 1500 or 1800 RPM on some gensets is 1500 = 50hz and 1800 = 60 hz on the same windings of the generator.

You'll notice that faster turning engines have "over square" dimensions on their pistons, and need higher RPM to develop their peak power. Any engine with piston width greater than stroke is "over square" and typically a faster running engine. All the older, long stroke, under 2500 RPM engines seem to run forever if you keep them lubed.

It is my theory that the maximum efficiency "cruise" on most FD boats will be at the peak torque of the engines. Turbos tend to sway that to the upper end of the peak torque, but NA engines are most efficient at their peak torque.
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