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Old 30-07-2011, 15:44   #1
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Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?


I have a bit of a problem.

I've spent the last 6 years fixing up an old 1985 racing monohull - a 40 ft model designed by Ben Lexcen (designer of Australia II of America's Cup fame - so it shouldn't be an overly crap design). Over these years, a phenomenal amount of time and a lot amount of money has been spent on it - uncountable hours and about USD$160k - setting it up as a fast (!?) world cruiser. Totally gutted and redone from scratch basically - all except the hull.

Yes, I know I could have bought a new boat blah blah blah.. anyway, it's done, and - no - I don't want to buy something else..

During this time I haven't been able to use it at all and it is only recently that since she is able to move under her own power again. (Btw - I don't have a baseline speed from 6 years ago and the PO is uncontactable - although I know this boat was used in and did OK in some offshore sailing races).

The mast isn't back in for another month, but I just installed the GPS on the N2K bus the other day and decided to do a lap around the bay and measure top speed to see if I can get to hull speed under power (approx 8 knots).

So as you may have guessed - the result was a whopping 3-3.5 knots.

The engine works perfectly, a reconditioned Yanmar 3GM30 with only 40 hours on it, clean bottom, 3 blade feathering Kiwiprop, no wind, no chop. No other loads on the engine. Boat is 40ft LOA, Beam 10ft and approx 13,000 lbs displacement.

Engine gets to 4000 rpm or so out of gear and about 3600 rpm in gear with the latest pitch I've set it to. I have tried various prop pitches to vary the point at which point black smoke comes out to confirm correct pitch and regardless of what I do, I don't get over 3.5kn or so. So more/less pitch is not the answer.

Manufacturer says the prop diameter is correct (although perhaps I can squeeze in another 2" of prop diameter, based on clearance available).

I'm measuring speed via GPS (Maretron GPS200) - so SOG. Although in the morning with zero wind and zero current, in two directions and getting the same results.

(I am about to order a Garmin GPSMAP 78 handheld to confirm SOG).

Several questions:
  • Assuming I can't solve this problem somehow - is a cruising speed of 2.5 to 3 knots under power acceptable for a circumnavigation? (Obviously it is a sailboat, but anchoring etc will be under power, as well as some motorsailing etc at times) We're planning on living aboard and cruising slowly (ha!) wherever our fancy takes us. Comments on convenience, safety etc are appreciated.
  • What is a minimum distance to test SOG with? I have limited space where I am; in that we have an 8ft draft and we run out of depth unless we head out into a busy area - so I'm doing this over a 300m (1000 ft) distance each way - is this enough to get an accurate SOG? I accelerate steadily and the speed tops out quite quickly well before the end of the 300m/1000ft.
Thank you very much in advance, and any other constructive thoughts/ideas would be greatly appreciated.

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Old 30-07-2011, 15:58   #2
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

sounds odd... but the black smoke tells me you are getting close to (if not at ) max output from the engine/prop. Just for giggles, were it me, I’d see if I could find a fixed blade prop to try. Feathering props are always a compromise. I went through something similar with my CAL, but maxed at 4kts.... switched to a fixed blade and got over six. Lost half to 3/4kt under sail though.

I would think 1000 feet might be a bit short for accuracy, but if the GPS is not climbing anymore and holds steady, I guess you’re close enough.

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Old 30-07-2011, 16:11   #3
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

If you are not in a hurry I guess it would suffice but if you hit current going into or leaving port I would get a bit nervous

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Old 30-07-2011, 16:20   #4
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

G'Day Akio,

Something is surely awry! I'm not familiar with Yanmar nomenclature, but I suppose that your engine is rated at 30 HP from the number you quoted. That should be easily adequate to reach hull speed under calm conditions. Our previous boat was also an old IOR 36 foot one-tonner, but she displaced around 20K lbs in cruising trim. Her 35 HP BMW would drive her to hull speed (in flat water) with no worries, so I think you must have some problem besides the rated HP.

Some possibilities are:

Your GPS may be set to average speeds over a longish period, which means that it will take some time for the display to catch up with changes in boat speed. Many GPS units allow you to select the averaging time, so you could try checking that factor.

The bottom may be dirty,or you may have snagged something on that 8 foot keel. Is it a bulb type by any chance? They are bad about picking up stuff and not shedding it.

Could there be a restriction in the exhaust somewhere? Excessive back pressure can limit engine output to a surprising degree, and on otherwise nicely running engines. Can be carbon buildup in the mixing elbow, a collapsed or kinked hose, animal or insect intrusion (boat has been on the hard for a long time, I think) or just anything that restricts the cross sectional area of the passage somewhere along its length.

I wonder if the boat feels like it is going faster than indicated? You can estimate speed by the position of the bow wave along the hull, ie at hull speed it will have reached the aft end of the WL, or by the old "chip log" where you time the passage of a floating object from bow to stern and calculate the speed.

I'm pretty sure that you will be able to achieve much higher speeds under power, but to answer your question about the suitability of a low speed motor for extended cruising... well, folks manage to cruise with no motor at all... but I for one don't think that is a good plan, especially with a boat the size of yours. Simply being able to motor in and out of places where sailing is difficult or actually impossible (entering passes in the Tuomotos where there is no room to tack and the current flows out 24/7 come to mind) is a huge advantage IMO. And, while ~3 knots is better than nothing, being able to push through some chop and some adverse wind/current while making those dicey entrances might well require a bit more grunt. So, to me your current condition (if in fact the speeds recorded are correct) is not good.

So there's one cruiser's opinion, based on a lot of experience.

Let us know how the situation develops -- I'm certainly interested in what you find out!


Jim and Ann
s/v Insatiable back in MBTBC marina, waiting for next eye jobs to be done
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Old 30-07-2011, 16:36   #5
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

Don't know anything about your local bay but there are several places I know of where you can do a lap around the bay and be going against significant current the entire way.
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Old 30-07-2011, 17:01   #6
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

Guess you don't have a knotmeter?? Comparing that against the GPS would take care of an adverse current. The 3GM30 produces something under 25 hp which is not a lot for you boats length. It still shoud get your boat up to 6kn or so without a problem in light wind and wave condtions. From the RPM you've been able to get, would seem that you have the proper pitch and diameter on the prop. I'd look at getting a calibrated knotmeter and trying another GPS. Something ain't right.

Just out of curiosity, have you cleaned the bottom and prop recently. Even a little growth on a prop greatly affects it's thrust.

3.5kn will not win you any power boat races. Its suitability really depends on your mindset, however. If you think of the engine as an auxillary to get you in and out of the harbor and cover some distance when the wind don't blow, it's acceptable, though barely. Personally, I look for a minimum of 5kn under power as my cut off point.
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Old 30-07-2011, 17:15   #7
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

3.5kts is not right for that setup. Something is wrong. The boat weighs much more than you think, the prop is out of whack,...., the speed measurements are wrong, something.. Firstly I'd make sure you have a good through the water speed measurement. You can measure off 1/2 mile, a mile is better, on a chart with land marks you can see - or drive along a parallel road and measure off he distance. Then time your passage, in both directions and average your speed. This will take care of any current issues and only requires a digital watch. Then see if you are still down at the 3.5 range. If so, then something is wrong with the setup - 30hp should push the boat fast enough in normal conditions. A boat that can only do 3.5kts in light condition, will barely move when loaded for cruising and headed into a mean chop and 30 kts of approaching squal -- like maybe 1 kts.
Good luck
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Old 30-07-2011, 17:32   #8

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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

Racing boats often do not have real auxiliary engines, they only have 'docking' engines, intentionally underpowered to stay lightweight, and just used when they absolutely have to be used. So not making speed under power wouldn't surprise me.

But an engine that big, on a boat that size, and presumably light displacement because it is a racer, ought to have more umph than that. If the bottom is clean, and the prop is clean (a fouled prop can cut your speed to nothing) and you're sure about the actual speed through the water, not just speed over bottom, then I'd suspect the engine is tired out.

Maybe low compression, try a cylinder leakdown test, I think you can buy a kit for under $50 if you don't want to bring in a mechanic. Could be the injectors, could be the whole usual raft of diesel problems but if the engine is simply OLD and tired...that would explain a lot. If you can get a referral to a competent mechanic (I know, ask at the local unicorn ranch) they should be able to spot something within an hour and not need to rack up a huge bill on diagnosis.
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Old 30-07-2011, 17:33   #9
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

try this

Hull Speed

if you are waterline 40 feet than 8.47 is hull speed

i am guessing that you are about 35 waterline that should be about 7.9 knots

something is wrong here
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Old 30-07-2011, 17:43   #10
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

Not enough horse power?You said it was an ex racer and I am assuming ocean racing. Don't these racers need some serious winds to power up the hull? The eight foot draft adds a lot of parasitic drag. I'll bet she is a lot faster on her side than standing straight up in a good blow. See if you can find out what the prismatic coefficient of the hull is.

From what I understand it is meant to determine how easily the hull drives through the water and thus the horse power required to achieve theoretical hull speed.
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Old 30-07-2011, 17:52   #11
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

I, too, think its the prop. A nice 3 blade fixed prop is a wonder for a world cruise. Folding props can get barnicles etc and make them difficult to flick out(of course the folding prop mob will jump on me and say bulltwaddle... but it aint)

Being a cheap-skate this is what I would do:
Put on a mask and snorkle and jump in (pitty its winter ) and check the hull, shaft and prop for problems.

Go for a long run right up the harbour and back... a good 10 miles, and a bit of circle work.

If the hull is clear and you are still getting those low speeds at those high revs in all driections to discount current then I would, as capngeo says, change the prop and see what happens.

3.5 knots is a problem you need to solve.

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Old 30-07-2011, 18:03   #12
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Tight engine...

Your engine could be somewhat tight from the rebuild.

Have you discussed this with the rebuilder, checked what sort of tolerances they used. At 40 hours it may not be anything like broken in.

Your rebuilder should have filled the engine with the right oil and advised you on correct breaking in procedures. This could be a good time to change both the engine and gearbox oils.

Most of what I would be looking at has been discussed already, but a few more:- Is there any chance the prop shaft could be binding (turn over by hand with engine out of gear)? Engine out of alignment (Aligned out of water)? Air inlet restriction?

I had an old beat up Yanmar in my 7 tonne Hartley ferro too many rears ago and the old VDO (pre GPS) showed 7 knots at times and we'd happily go at 4-5 kts all day.

3kts is what I'd expect from a 2.2 hp outboard!
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Old 30-07-2011, 18:33   #13
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

Just throwing out ideas here but Prop slip?It sounds like your in the equivalent of first gear.
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Old 30-07-2011, 20:39   #14
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

You sure you have the correct prop and that it is installed correctly. Could be something crazy like a RH prop and a LH drive? Prop not fully unfeathering. I have an 18,000lb (31' DWL) boat a Nanni 40hp diesel and an Autoprop and easily do 7.5 knots through the water (3400 rpms) in just about anything but a real blow on the nose.
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Old 30-07-2011, 21:01   #15
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Re: Six Year Refit . . . Is 3.5 Knots Under Power Too Slow ?

My 43' 23,000 lb boat with a Perkins 4-108 50 HP at 2,000 RPM's and a 17x14" prop will push me right at 6 Kts.
IMO somethings not right.

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