Determining cruising revs

WindermereColvic

WindermereColvic

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Just a quickie to find out what rpm we should be cruising at. Engine is a Mitsubishi Sole mini 26(hp). Full chat is 3600rpm, so if i were to aim for 70% of that, i should be going round at roughly 2500rpm. Is that about right? Or should i be finding 70% of 3600-700(idle)? Currently getting a very reasonable 5.5kns at 2200rpm, and when we take the engine up to 2500, it just doesn’t sound quite right...
Hoping someone can confirm my reasoning, Matt
 
The Volvo Penta guidance plate on our boat says "Cruising revs 200rpm below max obtained speed".

That seems to make sense, so if you apply similar rules / logic to your vessel I'm sure it will work.
 
I don't know what the max revs are for my VP2003 (28hp) but cruising revs are 2000 at which level it is unobtrusive. Even an increase to 23000 brings a new level of noise and fuel consumption. I know all the stuff about killing deisels with kindest but revs at which it sounds right and not straining and pushes you along nicely does for me.
 
Had a look at the Mitsubishi website. Couldn't find anything on the Sole 26 hp, however available at http://www.solediesels.co.uk/Brochures/mini29brochure.pdf is the brochure for the Sole 29 hp. Examination of the engine torque graphs shows that torque is at a maximum between 2000 to 2500 rpm. Examination of the corresponding fuel consumption graph shows that fuel consumption increases rapidly from about 2200 rpm. I think, therefore, your most efficient rpm is about 2200 rpm as it gives the maximum torque for the least fuel consumption. At that rpm your engine is only developing about 19 hp. Given that there is no mention of a 26 hp engine, have you mistaken yours and in fact possess a 29 hp engine?
 
OFG, Thanks for takeing the time to look this up. We do have a Mini 26, but it is only listed in the 'out of production' section, here:
http://www.solediesels.co.uk/mini26.html
This shows the touque curve
http://www.solediesels.co.uk/Brochures/mini26brochure.pdf
but im not sure what to make of it, i must admit it looks like so many curves to me. Im asumeing the results will be simalar to the Mini 29, but it would be handy if you could take a look and see what you make of them.
It seems odd that they would take a Mini 26 (25hp) out of prouction and replace it with the mini 29(27.5hp)
Matt
 
Lower graph gives the quantity of fuel used to produce the amount of power in the middle graph per hour. Grams per KW per hour on the left and grams per CV (which is basically horse power as explained here http://en.wikipedia.org/wiki/Horsepower#CV_and_cv) per hour on the right.

Therefore you will understand that where the lower graph is lowest your maximum power per weight of fuel is achieved. This coincides with the maximum torque at 2500 which I'd assert is your best cruising revolutions. However, I am not a trained engineer and would defer to any who are willing to voice an opinion.
 
oldfatgit said:
Lower graph gives the quantity of fuel used to produce the amount of power in the middle graph per hour. Grams per KW per hour on the left and grams per CV (which is basically horse power as explained here http://en.wikipedia.org/wiki/Horsepower#CV_and_cv) per hour on the right.

Therefore you will understand that where the lower graph is lowest your maximum power per weight of fuel is achieved. This coincides with the maximum torque at 2500 which I'd assert is your best cruising revolutions. However, I am not a trained engineer and would defer to any who are willing to voice an opinion.
Click to expand...

2500 rpm looks pretty good to me from an engine performance point-of-view. Provided the prop uses that torque well at the boatspeed you are achieving, and provided that 2500 rpm doesn't set off any nasty resonances anywhere in the boat, then I would say 2500 is your cruising revs.
 
It's all about best hull speed.

I'd go for a trial sail. On smooth water and no wind you should gradually increase power and note the boat speed at each 500rpm. Also watch the wash. Take her up to max speed and note it. Now reduce power until the big wash disappears and the hull is slipping along at what you guage to be most efficient. Note the revs and boat speed.

So there you have your cruising revs in calm conditions. For all other conditions simply increase the revs to maintain that boat speed.

I suspect that some of the earlier recommendations will have you charging around Windermere creating walls of water. Ungentlemanly and inconsiderate of others.
 
WindermereColvic said:
Just a quickie to find out what rpm we should be cruising at. Engine is a Mitsubishi Sole mini 26(hp). Full chat is 3600rpm, so if i were to aim for 70% of that, i should be going round at roughly 2500rpm. Is that about right? Or should i be finding 70% of 3600-700(idle)? Currently getting a very reasonable 5.5kns at 2200rpm, and when we take the engine up to 2500, it just doesn’t sound quite right...
Hoping someone can confirm my reasoning, Matt
Click to expand...

For nearly all engines, and diesels especially, the most economical revs are thos at which the torque-curve is at it's maximum - on my Yanmar 3YM that is at 2400r pm, on a maximum power r pm of 3600.

Engines do vary, I'd suggest referring to the literature on the Mitsubishi and going from that. Being an industrial engine I'd suspect the gap between max torque and max power is probably smaller.
 
I was once told:

1. Choose a calm day.
2. Note speed in 100rpm increments.
3. Note rpm when speed stops increasing.
4. Cruise at 80% of that rpm in flat water.
5. Maintain that speed by increasing rpm when water disturbed.

It seems to work as a good rule of thumb.
 
Prop Pitch May be An Issue - Not Likely Though

My boat prop has a large pitch. A previous owner had the prop selected to speed the boat through doldrums at low engine speed. As a result at normal revolutions my prop cavitates very loudly. Of course a lot of this is to do with hull shape to. So if you get your cruising speed sorted and it doesn't match the expected engine revs, this could be your culprit.
 
Lots of good advice above, but just to add my 2p worth.

If you do, as suggested, a run on a calm day etc etc, please do it after you have had your bottom and prop scrubbed.

You need a good benchmark to start from.

Just had my botton and props cleaned, and what a difference.

Yanmar 3GM30 in a 33 footer Gibsea.

5.2 knts at 2000 rpm, 6 knts at 2500 rpm (light sea, but not dead calm). Before the scrub, lucky to get 4.5 at 2700 rpm.

I found that the boat "feels and sounds" just right at about 2200-2400.

I do not think that you can use a set formula, as people carry different amounts of kit on board, some have small water tanks, some cruisers have double storage tanks, extra weight etc. Just use the foumulae as guidelines and adjust as you feel fit.

As I said above, you will find a speed/rpm where it just feels right.
 
Cruising speed

It is all a matter of best or chosen speed. The hull as said should not be pushed too far beyond the hull max speed as the effort required starts to increase dramatically. The engine will have as suggested an ideal speed for torque and lowest fuel consumption. Hopefully these will be 2 factors will match depending on the prop pitch. Then as said you want a low engine speed for less noise and finally on the particular trip you may want to hurry or not hurry so cruising speed may vary.
Just keep track of all the factors and after a while you will come to a conclusion. good luck olewill
 
WindermereColvic said:
when we take the engine up to 2500, it just doesn’t sound quite right...
Hoping someone can confirm my reasoning,
Click to expand...
Absolutely. References to cruising speeds etc are artificial as they don't take into account prop pitch and efficiency etc.
The best speed for an engine is the one it runs happiest at. When the harmonics are all over the spot it's because the motor isn't running at it's sweet spot.
When it sounds right, it is right.
If that's too slow, change your prop.

Mechanical sympathy can save you lots of money in repairs.
 
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