What speed should we make under motor ?

[dunedin]

I have a 36ft yacht which sails like a dream. We try to motor as little as possible (filled the tank when purchased in march, only used half a tank so far!!)

However, on the rare occasions we do use it it seems slow and rough compared with other boats have been on.
Have a Yanmar 30hp and Gori folding prop

Last weekend noted the speed made in calm water
5.3 knots @ 2,600 RPM
5.7 knots @ 3,000 RPM and getting vibration
Didn't push farther as mechanically too sensitive

I assumed a 36ft yacht would do nearer 7 knots all out (had been doing 7.9 knots under sail half an hour earlier)

Any comparative date and/or suggestions?

NB just cleaned log impeller last weekend and boat / GPS speed were very similar

 

[sailorman]

wot boat
wot LWL
assume engine rated 30 hp @ 3600 rpm

 

[jwilson]

With a clean hull/fixed 2 bladed prop my modern 35 ft AWB makes 6 knots at about 2,800 rpm, and about 7.5 flat out at 3,400 (slightly overpropped as Yanmar 27 hp engine rated to 3,600 max). Doesn't take a lot of fouling, particularly on the prop, to cut that back AND cause vibration.

 

[ChrisE]

Assumnig that your 36' yot has LWL of at least 29' then I'd reckon thay you should be getting about hull speed with your set up. My guess is either old wheezing engine or incorrectly specified Gori prop. The fact that you can do 7.9 knots under sail rules out underwater growth unless it is on the prop (in which case see above answer).

I speak as one with a 37.5' boat 29' LWL with 37.5 hp new engine plus Autoprop (took two gos to get the prop size right) who gets 6.5 knots at 1800 rpm, 8knot at 3000rpm

 

[reeac]

Eight knots? Your hull speed should be 7.2 knots.

 

[stephenmartin]

I agree...7.2 knots full tilt for a 29ft lwl hull

sqrt 29 = 5.385 x 1.35 =7.269 knots

 

[Robin]

Hull speed figures sound very precise at 1.34 x sq root LWL don't they! In practice a bit more is easily possible with sufficient additional power, sail or motor, up to say 1.5 x sq rt LWL. In our case with about 36ft LWL we reach 1.5 and occasionally even more under sail but not under power where 1.4 is the figure with our 44hp Yanmar/Brunton prop. We have a confirmed 16.7kts under sail and a 19.5kts only seen by me just before that, these were surfing/planing in a 30kt gust on a big wave, but that is a different scenario.

Our previous (W33) boat had a LWL of 28'10" and could do 8.5kts flat out under engine, but it did have a 50hp motor and 3 bladed fixed prop, the stern wave was half up the transom at that speed.

 

[Evadne]

Boats also sail faster when heeled, mine is supposed to increase her waterline length by about 4' (from 20 to 24) if you get the toerail under.

 

[Robin]

That is very true. Our waterline length is considerably longer under way than at rest too because the last 3ft of hull sits only a couple of inches clear of the water but upright or heeled it is all in play above about half a knot boatspeed.

 

[AlJones]

Our Sigma 38, with spotless hull, a two blade folding prop, a Volvo2030 28hp, does a nice 5.5-6knts. I'm happy with that.

Only use it to park really.

AJ

 

[Kilter]

As others have said fouling on the prop itself can make a heck of a difference to both speed and vibration, I'd start there.

 

[Mirelle]

I am in a different world...

At full throttle, clean bottom, flat calm, we get 3.25 knots.

15 hp, 10 ton boat.

five knots usual under sail, sometimes six.

 

[fireball]

Re: I am in a different world...

Hmm ... 30' 5.5 knots under motor - goes faster under sail ... I need to clean the bottom and prop! ...

 

[starboard]

[ QUOTE ]
Boats also sail faster when heeled, mine is supposed to increase her waterline length by about 4' (from 20 to 24) if you get the toerail under.

[/ QUOTE ]......Aye but how much leeway will you then make???

Paul.

 

[dunedin]

For info, it is a modern (2000) Yanmar and LWL of 30.5ft.

As I said, rarely use the engine in anger but the 5.3-5.7 knots does sound very low compared to other experience.
Will check the prop for fouling and/or mal-function when pull her out. Otherwise might investigate a kiwi prop (no way will I fit a fixed prop)

 

[Anonymous]

The fuel consumption will give you a good guide as to whether your enging is actually pushing out shaft horsepower or just making a noise. The specific fuel consumption will be around 170 grammes per horsepower-hour.

 

[homa]

I have an AWB which is 33 LOA, has a 3 blade (Volvo) folding prop and 23hp engine, will happily cruise at 6kts and will do just under 7kts flat out with clean hull.
I'd check bottom & prop for fouling and check cutless bearing for wear (vibration at high revs?).
This is my first folding prop and I'll never go back to fixed now, as I have seen the improved performance when sailing!

 

[DeeGee]

[ QUOTE ]
The fuel consumption will give you a good guide as to whether your enging is actually pushing out shaft horsepower or just making a noise. The specific fuel consumption will be around 170 grammes per horsepower-hour.

[/ QUOTE ]Would that not depend on what developed hp you have? Your formula is way off for me (using .88T/m**3, your formula gives nearly 5.5 litres/hr with my 28hp, which is more than double).

 

[DeeGee]

dunedin, you may have a fouling problem of some sort, but you may also have a mechanical problem - shaft alignment, bent p-bracket., worn cutlass bearing... best to get it checked out at some point, and in the meantime be satisfied with your 5 or so knots. If you push into the operating regime where it vibrates, you are quite likely to be making something worse. OTOH, keeping it smooth is unlikely to do much harm. Alos, you say you have used about half a tank since March. How much fuel is that? How many engine hours?

 

[Anonymous]

I doubt it - if you are correct then you are saying that there is a 100% discrepancy in efficiency between one engine and another. Not very probable, for modern engines! My figure is from the Yanmar 110hp motor which is turbocharged so might be a bit more efficient than yours, but not a huge amount.

Let's be clear, we are talking about the fuel actually burnt at a given power output and the output is shaft output from the engine/gearbox, not delivered by the prop. I can't follow your arithmetic. What is 0.88T ? To get the figure I gave to litres per hour you need to convert mass (grammes) to volume (litres) so you need the specific gravity of diesel at ambient. The other factor is 'horsepower-hours' which is the actual horsepower being used, not the rated hp of the engine, of course. Does that help?

edit:- ps if your calculation suggests double the fuel actually used then could it be that you normally run at 50% of rated output and you have used the rated figure of 28 when calculating horspower-hours?