PDA

View Full Version : What speed do you do under power? Leopard 42



LifePart2
27th April 2013, 09:42 AM
I am wondering if we have the wrong prop dimensions.

We have two Yanmar 34's on our Leopard 42 built 2001. We have two fixed props - not sure of the dimensions, but since the boat was bought out of charter I am guessing that they are the original OEM props.

Under one engine at about 1800 rpm, we cruise at about 4kts.

With both engines at full revs - 2500rpm - we just about make it up to 6kts

That, to me, seems slow.

How does that compare with what you get under engine power, and what props do you have?

Thanks

Noel

dmmbruce
27th April 2013, 10:42 AM
If you were a monohull your boat speed would be approx sq root of 42 times 1.4. That is roughly 9 knots.

Your waterline beam should be a lot less than a monohull 1/3 water line length, so the constant above should be more than 1.4. There seem to be lots of theories about what it might be however.

Your engines should surely drive the boat at hull speed in reasonably calm waters.

At a guess, I would have expected more like 11 knots flat out.

So, is the bottom clean? Are you trailing a heavy dinghy? Are the engines working properly?

Mike

smj
27th April 2013, 12:56 PM
Sounds to me as your props are the wrong size or pitch. At full revs the Yanmars should run close to 3400-3600?

Karen
27th April 2013, 01:28 PM
A pal of ours who owned a Leopard was shocked by its weight - far in excess of that advertised by R&C. Could weight also be a factor giving you less bang for your Yanmar buck? Do you know the real-world cruising-mode weight?

Mike, 11 knots is overly optimistic if flat out with both engines. 8-9 would be a little more realistic.

LifePart2
27th April 2013, 01:51 PM
Yes, engines working fine, bottom clean. As for revs - well, ,the sticker next to the dial (put there by the charter company) says to not exceed 2500rpm. The manual says about 3700 as max revs. So we take all that to mean 2500rpm if we are in a serious hurry, otherwise we go at 1800-2000 which uses only about 1.25 ltr per hour per engine. I am reluctant to push the engine too hard - or am I just being overly cautious?

No idea on our real weight, of course. But we do not float low.

So people figure we should be getting more speed? What do you actually get for your engines in real life? I would be interested to read real life experiences, particularly, of course, in a Leopard 42.

And, if you are getting much better speeds than us with the same size boat and engines - what props are you using?

Noel

FSMike
28th April 2013, 12:53 AM
You're being too cautious. You should be able to attain max revs (or nearly so) under way. With our three cylinder Yanmars on the previous boat we could reach about 3300 rpm (if I recall correctly), and we cruised at 2500 - 2600 unless we were in a hurry, when we used 2800, more for short periods. Diesels like to be worked, you are not doing them any favors by babying them.
We were instructed to run them wide open for the final 5 minutes or so of a trip, then let them cool down while anchoring, docking, etc.
Great engines.
To determine if you have the right size prop first see if you can pull max rpm. If you can't then you have too much prop, size or pitch or whatever.

Peter
28th April 2013, 05:11 AM
I am wondering if we have the wrong prop dimensions.

We have two Yanmar 34's on our Leopard 42 built 2001. We have two fixed props - not sure of the dimensions, but since the boat was bought out of charter I am guessing that they are the original OEM props.

Under one engine at about 1800 rpm, we cruise at about 4kts.

With both engines at full revs - 2500rpm - we just about make it up to 6kts

That, to me, seems slow.

How does that compare with what you get under engine power, and what props do you have?

Thanks

Noel

Have look at the power curves for your engines. The maximum torque is usually achieved between 2400 - 2800 and for economy you should aim to cruise at this point on the curve. You should check that you are also be able to reach maximum RPM (typically 3400-3600) otherwise the boat is considered to be "overproped". This is unlikely on a (ex) charter boat as they are usually fitted with propellers that err on the small side.

It may not be possible to reach max revs on one engine alone, in the case of fixed props. Running one engine is less efficient than with folding or feathering props and is subjected to the additional drag of the non working prop. So you should try to reach max revs with both engines running.

When cruising at optimum revs with the correct propellers your speed will vary depending on wind, sea state, hull condition and weight. The heavier the boat or more fouled the hull the slower. Windage is a huge factor on catamarans and the less aerodynamic the profile the slower into the wind.

Make sure all tests are done using a through the water speed log, many charter boats relay only on GPS for speed and this will not show current variations.

What size pitch and number of blade are your propellers?

FWIW given the weight of your boat 6 knots with both engines running at 2500 rpm is about what you would expect.

You could improve your economy, by approx 50%, by replacing the props with feathering or folding and running only one engine but your speed would be less probably about 5 knots at 2500 rpm and you have to weigh up the cost of replacement with the fuel saved. Of course you would probably also sail better requiring less engine use in the first place.

LifePart2
28th April 2013, 06:25 AM
Thanks, Peter, that was a very helpful post.

We will do some tests over the next few days running at higher revs and see what we get and report back.

multihullsailor6
28th April 2013, 08:25 PM
Hi Noel,

On the FP Belize equipped with twin 27 hp engines we today motored in flat seas on one engine running at 2.8000 rpm and achievd a GPS derived COG of 5.6 kts.

Catch you soon
Roger

gramos
28th April 2013, 09:25 PM
with one engine I do 5 knots at 2500 rpm and a max of 7.5 with both engines on at 3000 rpm ,

the engines ?

tiddly VP md 2010 ,s

all at under 1 litre an hour per engine , giving around 400 miles
from full tanks , quite happy with that ,:D

Tropic Cat
29th April 2013, 01:17 AM
I agree with the OP that both engines at 2500 RPM resulting in 6 knots does seem on the low side. If wide open throttle is 3400 to 3600 RPM then I'd say your boat has the correct propellers and needs more horse power. If on the other hand the max RPM achieved isn't equal to max engine speed, then your propellers have to be looked at.

When publishing your speed info here it's helpful if you note seas state (Flat seas) where the wind is coming from and if there is a current running. Over the years I've had both currents and winds push me backward when at full speed.

I went through the same gyrations with my little cat and now cruise at 5.5 knots on one diesel engine, and at 6.75 knots with both engines at full speed. Last time I calculated fuel consumption it was in excess of 40 MPG.

On Yanmars the biggest bang for the buck is adjusting the valves. The manual specifies this should be done periodically and with little diesel engines .... misadjusted valves are the number 1 cause of missing horse power. If the engines have more than 4000 hours a valve job should be considered. If you are handy with a wrench, a valve job (merely reseating valves) costs in the neighborhood of $150 per engine.

steve sharp
30th April 2013, 04:23 AM
You do not mention what model and HP your engines are so it is a bit hard to say what speeds you might be able to do.

A litre of diesel fuel will produce a bit over 5 HP in a modern diesel engine. If you are burning 1.25 litres of fuel an hour you are getting less than 7 HP out of the engine. I expect your engines are rated for a lot more than that.

I have two Yanmar 3JHC engines rated at 40 HP at 3800 RPM and 36 HP continuous duty at 3600 RPM. One engine running at about 2,800 RPM puts out almost 17 HP and moves my 46-foot 14 tonne catamaran at 6.2 knots. (Remember that the horsepower produced by the propeller is proportional to the cube of the RPM.) The engine burns 3 litres per hour so I am getting just under 2 miles per litre. I use Volvo 3-bladed folding props.

Two engines running at 2,800 RPM get me 7.5 to 8 knots in calm water with a clean bottom and the fuel consumption drops to about 3 litres per hour per engine.

Flat out at 3,600 RPM on both engines I can get over 9 knots but I am sucking up 14 litres per hour, 7 litres a side, so I only do this occasionally to blow out the engines and load them up after idling for a long time. I can get 15 to 17 knots under sail in a good breeze so the limitation under power is not the maximum hull speed but horsepower.

I assume that your engines are rated at 27 HP at maximum RPM and about 24 HP at continuous duty revs. You should have no problem getting up to 5.5 or six knots on one engine and 7 to 7.5 on two engines assuming that your props and bottoms are clean.

I hope these figures are useful to you. I certainly agree with others that, if you decide to invest in new props you should get folding or feathering ones. You will see a very nice improvement in sailing speeds, especially in lighter airs and performance while motoring on one engine should be much better.

Good luck!

Steve

LifePart2
7th May 2013, 08:27 AM
Today I did some tests. Flat water, almost flat calm, no current. Here is what I got:

1 engine @ 2500 rpm -> 5.4 kt
1 engine @ 3500 rpm -> 6.9 kt (full throttle)
2 engines@ 2500 rpm -> 6.5 kt
2 engines@ 3500 rpm -> 7.9 kt (full throttle)

So:

1) Does that sound like we have properly matched props, ie are we getting the speeds we should be getting?

2) Adding in the second engine adds very little extra speed for double the fuel consumption. What I don't know is the fuel consumption at full throttle. If we are in a hurry, is it better for the engine, and more economical, to run one engine at, say, 3300 rpm than two engines at 2500rpm?

Yanmar 34hp 3JH3E with fixed three blade props

Noel

Tropic Cat
7th May 2013, 10:46 AM
....
1) Does that sound like we have properly matched props, ie are we getting the speeds we should be getting?

2) Adding in the second engine adds very little extra speed for double the fuel consumption. What I don't know is the fuel consumption at full throttle. If we are in a hurry, is it better for the engine, and more economical, to run one engine at, say, 3300 rpm than two engines at 2500rpm?

Yanmar 34hp 3JH3E with fixed three blade props

Noel

If the waterline length of your boat is around 38 ft, I would expect your max speed to be in the neighborhood of 8.25 knots. Based on that, you're not that far off. The fact that you can spin the engines up to 3500 RPM without hitting hull speed, suggest the props might be 1 size too small or if your LWL is a bit shorter, you'd be dead on hull speed calculations.

If you read my earlier post, adding the 2nd engine also adds little speed with my boat. I would imagine this is common as the boat approaches theoretical hull speed, a bow wave develops which needs more HP to overcome.

As for fuel economy ... I'm not certain as I've never run a comparison. Yet my gut tells me that shutting one engine completely off, must be the most economical configuration.

steve sharp
7th May 2013, 01:52 PM
Noel,

You appear to be getting close. If your bottoms and props are clean you might still get an extra knot when using both engines. Here is how to find out if that extra bit of speed is potentially there.

When you run one one engine do you generate full rated RPMs? My guess is that you come pretty close to it. If that is the case your props are well tuned to getting maximum performance when using one engine but you could go faster using two engines if you added a bit of pitch although that would very likely slow you down a bit when running on one engine.

As far as fuel consumption goes, as long as you are running at speeds lower than the maximum you can get out of one engine (6.9 knots) you are only burning a little more fuel per hour when running two engines than you are when running one. Basically, it takes very nearly the same horsepower to move the boat at the same speed whether you are using one engine or two. When using one engine you need a bit of extra power to compensate for the drag of the idle prop and the slight extra drag of the rudders to keep you on a straight course. When running two engines you are spending extra power running pumps, the alternator and overcoming engine friction. The difference is too small to matter much in practice.

When you work out the cost of depreciation, oil and filter changes and maintenance, you will find that about half the cost of running the engine at cruising speed (80% of rated continuous duty RPMs) is fuel (the exact percentage depends on the cost of fuel in your area) the rest is the other factors. At less than cruising speed the fuel cost is much less than half. If you can accept a speed of 6.2 knots when cruising for any length of time then use one engine and save some money.

I hope this is useful.

Steve

searenitysail
7th May 2013, 02:58 PM
Just to throw a little more input into the mix---

I don't know how similar the hull designs of the Leopard 42 and Leopard 43 are, but we just chartered a 2008 Leopard 43 from the Moorings the last week in March. Bottoms were pretty clean.

We were able to do 8.0 kts at 2800-2900 RPM. We did notice the sterns "squat" at that speed and actually have water over the bottom step.

I think the engines in the 43 might be larger than the 42, but I am not sure.

Marshall

LifePart2
7th May 2013, 08:39 PM
Steve, we are used to traveling slow (we are not in a hurry!), so we will continue with just the one engine routine.

The manual says:

1 hour rating: 3650 rpm

continuous rating: 3800 rpm (so presumably continuous means less than an hour?)

At full throttle I was getting 3500 rpm on both engines, so not quite the full advertised rpm. A bit of a noise change on one engine, sounded a bit like the prop was cavitating perhaps. They both have about 6,000 hours on them.

I also noticed that the oil pressure on one of the engines is just below the bottom end of the green area on the dial. Has been like that since we bought her 4 years ago, so don't know that it is significant?

Marshall, interesting that you got to that speed. There seem to be different engines in the various 43's, but I think you are right that it probably had a slightly larger one.

Conclusion so far seems to be that we are probably about right with the props, but could probably do with a valve adjustment. Not quite sure what that entails? Are we talking of lapping with some carborundum paste?

Any other comments or suggestions?

Thanks,

Tropic Cat
7th May 2013, 09:01 PM
A valve adjustment is simply a regap the valves exercise. I believe all Yanmars are .0008 inches, but your manual will tell you the required gap. Remove the valve cover, rotate engine to TDC and adjust.

Lapping is part of a valve 'job', and would require the engine be disassembled a bit. The head would be removed.

If you don't have one, a copy of the Yanmar service manual can be found as a link on this page. (http://www.catamaransite.com/engine.html)

steve sharp
7th May 2013, 10:32 PM
Noel,

It sounds as though your props are not far off the proper pitch. If and when you have to replace them I would suggest going with 1 inch less pitch so that you can reach full RPM when using both engines. You will actually go faster. Until then, when running on one engine run her up to the maximum RPM you can get. It will take minute or two to get there. Then select 300 RPM less than that as your maximum cruising RPM. If you can tolerate the speed you get at about 2700 RPM use that most of the time and I expect you will be pretty pleased with your mileage.

It appears that there is a typo in your owner's manual. The continuous duty rating is always equal to or less than the one-hour rating.

Have a great summer cruising.

Steve

LifePart2
8th May 2013, 08:27 AM
Thank you all for such very helpful replies!

Noel