Running a diesel at high revs?

[Tranona]

We motor at a sedate 6kts (2000-2200 rpm) - never said anything about hull speed (which is approx a bit over 8kts)



Darglow Featherstream (see #17)

OK. The revs you are using sound about right for that speed. Misinterpreted your comment about not using higher revs because of the effect on steering. yes, the Featherstream can be adjusted in both forward or reverse, but if Darglow set it according to the boat data you gave them it should be right.

Out of interest did you have the same control issues with a fixed prop with this engine (assuming you have used one)?

 

[gandy]

You should be able to achieve 2800 and cruise at 2000+. If you are achieving hull speed at 2200 you are overpropped (and probably overpowered).

That begs a possibly unrelated question if the boat is in fact overpowered. For example can reach hull speed with 10hp but has a 15hp engine. In that case should it still be propped to reach maximum revs, or should it be propped to reach the most efficient rpm for that 10hp?

Referring back to Beta, our older Beta has a rated maximum power at 3600rpm, but also a slightly lower "continuous" rating at 3000rpm.

 

[dom]

Couple of questions from a tech muppet:

1. I, like many others, simply prefer the sound of a diesel engine at 2500rpm, as opposed to the rasping sound of 3500rpm, which whilst fast and powerful, sounds to the untrained ear as if the entire installation is about to explode!
2. When we buy a prop, a techy typically first whips out the engine's power curve, which represents the available power the engine can produce (hp or kW) as speed (rpm) increases. This is typically a sharply convex curve. The tech now selects a prop to match. Unfortunately the curve representing the amount of energy it takes to spin a propeller is concave. The two curves can consequently only be made to cross at one point so he selects that to be at max hp (i.e. about 3600 rpm for my revvy little Yanmar 75)
3. Because of the seriously different shape of the two curves, the more I throttle back, the curves separate. By 2500 rpm the engine can now produce way more power than my propeller can absorb.
4. Yet my boat (I think) can reach hull speed with about 40hp in calm conditions with fresh antifoul, which my engine could theoretically deliver at 2500rpm. However, the prop will only absorb c.25hp at that speed. Pushing into a good blow I need the full 70hp odd.

Have I just made a case for multiple gears, or do some of the better feathering props adequately get around this problem?

 

[ex-Gladys]

That begs a possibly unrelated question if the boat is in fact overpowered. For example can reach hull speed with 10hp but has a 15hp engine. In that case should it still be propped to reach maximum revs, or should it be propped to reach the most efficient rpm for that 10hp?

Referring back to Beta, our older Beta has a rated maximum power at 3600rpm, but also a slightly lower "continuous" rating at 3000rpm.

Don't forget that 10 hp may be ok in flat water and no wind, but punching into a dig sea and a gale, the extra oomph may be welcome....

 

[Koeketiene]

Out of interest did you have the same control issues with a fixed prop with this engine (assuming you have used one)?

We re-propped at the same time as we re-engined - so no way to tell

 

[25931]

We are now the proud owners of a Varne 27 with an almost new Beta 14 diesel.
I've always been a bit nervous about running diesels at higher revs but the other day we were stonking along at 6 knots, with the engine at 3,000 rpm and it made such a difference getting home when we were tired and just wanted a beer and a hot bath.
I actually phoned the tech dept. at Beta Marine the next day and asked whether they were OK with their engines running for hours at near max revs. They said yes, no problem, obviously they use quite a bit more fuel, but then they are so miserly anyway. They are tested at max and designed to do it.
Any thoughts/advice?

We have used Betas (different sizes in different boats) at full chat for many hours without the slightest problem having been advised, as you were, by Beta.
Times change - when I started to be involved with ic engines nearly seventy years ago they were a touch tender but today ....

 

[LittleSister]

We have had a Beta 14 for about 6 years. I would have no hesitation in using it at whatever speed I want to use. It has certainly run at about 3,000 or 3,100 revs for hours on end (on a particular trip where we needed the speed to clear a series of tidal gates). Have run for a continuous 21+ hours at probably varying between about 2,500 and 2,800 revs before now. Have used max revs (3,600) on a number of occasions, usually only very briefly, but once 20 mins or more as we fought a fierce spring tide running about the same as our max hull speed in the wrong direction.

 

[Colvic Watson]

Thornycroft up rated a number of engines when they were marinised but stated very clearly that the top 10% of revs must only be used for 5% of the duty cycle. Ours is a 74hp as marketed but I think it's only a 65hp base engine. Hull speed in flat water at about 2/3 revs which means all the rest is for wind and waves and we make good progress through that. You see some Colvic Watson 34's advertised with 100hp engines which must be sweet at 1/3 power but by golly I bet they never rev very high! And at full throttle it must be a magnificent sight. The designer is no help, he recommended 35hp to 100hp. If ever we re-engine 65hp will be adequate.

 

[ex-Gladys]

Gladys will not top out, and as you start to get to the top end, the black smoke starts (engine can't overcome the load) and the temperature will rise inexorably, which is a show stopper....

 

[theoldsalt]

Gladys will not top out, and as you start to get to the top end, the black smoke starts (engine can't overcome the load) and the temperature will rise inexorably, which is a show stopper....

Black smoke may be caused by being over propped, and overheating caused by lack of coolant which may be a coked-up exhaust water injection point or some other flow restriction.

 

[ex-Gladys]

Autoprop... Coolant fine, Exhaust elbow replaced 2 months ago, exchanger checked out this winter ... I used to have a Merc 813 horsebox, and just like the Ford in Gladys, would run hot on extended higher speed running...

 

[nigelmercier]

... one should change oil probably as a function of revolutions turned rather than mere time running.

Now that sounds like an ideal project for a YAPP: a cumulate rev counter.

In fact I can easily incorporate one into my Mk 2 fuel gauge, which is next but one on the list.

 

[LadyInBed]

turbo waste gate - not something I need worry about as non turbo unit
As much as a diesel likes being run at high revs, I feel that the bits strapped to it prefer lower revs ie: alternator, belts, water pump, shaft bearings, 'course I may be wrong.
But the fuel economy at lower revs does it for me.

 

[Ric]

I read this thread this morning, so decided to run my MD2020 flat out for 30 mins this afternoon. Engine was fine - but the temp of the Calorifier must have exceeded the max temp of the PRV because it dumped most of my freshwater supply into the bilge. I'll stick to 2500rpm from now on!

 

[gandy]

3. it puts an equally high demand on oil: one should change oil probably as a function of revolutions turned rather than mere time running.

The old mechanical rev counter / hour meters used to work like that, only read actual "real time" hours at rated rpm.

 

[theoldsalt]

I read this thread this morning, so decided to run my MD2020 flat out for 30 mins this afternoon. Engine was fine - but the temp of the Calorifier must have exceeded the max temp of the PRV because it dumped most of my freshwater supply into the bilge. I'll stick to 2500rpm from now on!

PRV operates on pressure not temperature. By "freshwater" I take it you mean coolant then I suggest you investigate the problem. One day you may need to run your engine "flat out" perhaps in an emergency.

 

[theoldsalt]

Autoprop... Coolant fine, Exhaust elbow replaced 2 months ago, exchanger checked out this winter ... I used to have a Merc 813 horsebox, and just like the Ford in Gladys, would run hot on extended higher speed running...

If you are getting black smoke out of exhaust then the fuel burn is incomplete. Either the engine is overloaded or it is being starved of air or your injectors need servicing.

 

[Colvic Watson]

PRV operates on pressure not temperature. By "freshwater" I take it you mean coolant then I suggest you investigate the problem. One day you may need to run your engine "flat out" perhaps in an emergency.

I suspect he does mean the freshwater. If the temp in the calorifyer got up to 85/90 degrees or so then the pressure in the calorifyer would increase substantially and blow the PRV. Thread on this earlier in the year with exactly this result.

 

[Nordild]

turbo waste gate - not something I need worry about as non turbo unit
As much as a diesel likes being run at high revs, I feel that the bits strapped to it prefer lower revs ie: alternator, belts, water pump, shaft bearings, 'course I may be wrong.
But the fuel economy at lower revs does it for me.

I think that it is a very good point

 

[cryan]

You should always run as much load as possible on small diesels. Not only will you coke up the top of the engine with slow running but you'll also glaze the cylinders and that will lead to burning oil.
Your engine will very likely have had its mechanical throttle stops adjusted to somewhere between the normal continous rating and the maximum continous rating. The factory should have adjusted the stops to make sure you're unable to reach the Very Likely to Explode rating and so you will be perfectly safe to run at full speed.