Cruising RPM

[CalicoJack]

We have a 10m Neptunian ketch fitted with a Perkins 4108 driving a Kiwi Prop through a TMP gearbox. For years we have run it at 2200 rpm when cruising and had an additional 300rpm in reserve if we ever needed to go flat out.

Kiwi serviced the prop and have adjusted the pitch , ie made it coarser, so that now it will only reach a maximum of 2350rpm flat out. What would be a sensible setting for cruising rpm? I tried it at 2000 rpm yesterday which gave us very nearly 7kts and everything seemed very happy . The temperature remained constant and much the same as it normally does as did the oil pressuse.

 

[Supertramp]

We have a 10m Neptunian ketch fitted with a Perkins 4108 driving a Kiwi Prop through a TMP gearbox. For years we have run it at 2200 rpm when cruising and had an additional 300rpm in reserve if we ever needed to go flat out.

Kiwi serviced the prop and have adjusted the pitch , ie made it coarser, so that now it will only reach a maximum of 2350rpm flat out. What would be a sensible setting for cruising rpm? I tried it at 2000 rpm yesterday which gave us very nearly 7kts and everything seemed very happy . The temperature remained constant and much the same as it normally does as did the oil pressuse.

Much of the decision about cruising speed is personal choice. I have a similar size boat and a 60hp engine. I cruise at about 1500rpm vs a max (for a short period) of 2750rpm. But my 1500 only gives 6 knts in flat water. I choose it because its just short of hull speed, feels right and gives good economy. My fuel economy drops at hull speed and above not surprisingly. I don't subscribe to the "run it flat out" school of diesel running in boats or cars but more to the "make sure the oil pressure and temperature are in the right place and its running smoothly". And have plenty in reserve for sea conditions.

If the engine is healthy and you are running at 60%+ of sustainable max revs, I think the choice is yours. Perhaps check the max revs specified for your engine type in case your prop adjustment is constraining the max revs significantly.

 

[dunedin]

We have a 10m Neptunian ketch fitted with a Perkins 4108 driving a Kiwi Prop through a TMP gearbox. For years we have run it at 2200 rpm when cruising and had an additional 300rpm in reserve if we ever needed to go flat out.

Kiwi serviced the prop and have adjusted the pitch , ie made it coarser, so that now it will only reach a maximum of 2350rpm flat out. What would be a sensible setting for cruising rpm? I tried it at 2000 rpm yesterday which gave us very nearly 7kts and everything seemed very happy . The temperature remained constant and much the same as it normally does as did the oil pressuse.

Driving a 10m boat, with perhaps a 7-8m LWL at 7 knots must be close to hull speed and needing a lot of power. Motoring at 5 knots probably would reduce the drag by at least 50%, possibly two thirds.

 

[CalicoJack]

Sorry, I’m more concerned with what’s best for the engine. Theoretically the Perkins could rev to over 3500 rpm, but the noise would drive you mad. One of the reasons why the boat is a bit over propped is to keep the revs down and consequently the noise level. It only burns 0.9 gallons an hour, allegedly and we carry plenty of fuel.

 

[scottie]

Sorry, I’m more concerned with what’s best for the engine. Theoretically the Perkins could rev to over 3500 rpm, but the noise would drive you mad. One of the reasons why the boat is a bit over propped is to keep the revs down and consequently the noise level. It only burns 0.9 gallons an hour, allegedly and we carry plenty of fuel.

By the rule of thumb current at time of 4/108 that would suggest about 18hp/hour then

 

[Minerva]

cruising RPM for us is when there is a totally flat wake (well ripples)

When we push it, we start to drag along a series of 1ft waves - at that point it's just a waste of diesel.

 

[Tranona]

Sorry, I’m more concerned with what’s best for the engine. Theoretically the Perkins could rev to over 3500 rpm, but the noise would drive you mad. One of the reasons why the boat is a bit over propped is to keep the revs down and consequently the noise level. It only burns 0.9 gallons an hour, allegedly and we carry plenty of fuel.

4l an hour sounds heavy consumption, but when I see 7 knots it seems OK. You could halve that consumption at 5.5 knots. However if you are happy with that consumption sounds a reasonable use of the engine. TBH the actual revs used has minimal effect on what is "good" for the engine as it is being well loaded, maybe to the point of being overloaded with the steeper pitch. You have excess potential power for the boat so sensible to keep revs down by propping such that you can't achieve maximum rated engine speed. If you go to far symptoms would be knocking from the engine and lots of smoke as the engine gets overfuelled trying to get the revs up to meet the power the prop demands. What diesels don't like is prolonged periods of light loading such as battery charging or motorsailing when sails are providing good motive power.

In essence, choose a cruising speed you are happy with. One rule of thumb which I find common in the way that many boats are powered is to run at revs that equate to a minimum of half maximum power which is what your 2-2200 rpm is (based on 3500 maximum). Personally I would prefer to prop it to run a bit higher than 2000. I would guess they increased the pitch by around 1" which changes engine revs for a given speed by about 250rpm.