Adjusting prop pitch .. Doesit make a difference?

[Nostrodamus]

When buying a adjustable pitch prop the supplier will usually pre adjust it to what they think is right for your boat.
Do they usually get it right or have you adusted it afterwards and did it make a difference?
I feel I have not got enough umph when going forward and that the prop should dig in more which means increasing the pitch. This will give me a loss of 200 rpm on the top end but it it rare that you go anywhere full throttle anyway (unless you are French).
Will it make any other difference and how will over all speed be affected?
The boat will get it's anual haul out soon and I dont want to change the pitch only to find it is worse and then have to find a way of changing it back.
Any suggestions?

 

[sailorman]

When buying a adjustable pitch prop the supplier will usually pre adjust it to what they think is right for your boat.
Do they usually get it right or have you adusted it afterwards and did it make a difference?
I feel I have not got enough umph when going forward and that the prop should dig in more which means increasing the pitch. This will give me a loss of 200 rpm on the top end but it it rare that you go anywhere full throttle anyway (unless you are French).
Will it make any other difference and how will over all speed be affected?
The boat will get it's anual haul out soon and I dont want to change the pitch only to find it is worse and then have to find a way of changing it back.
Any suggestions?

the prop should be pitched to give your boat max lwl spd @ max revs or just under max rpm.
we have 2900 rpm max rpm engine rated at 3000

 

[stav]

Fiddled with the pitch on my last boat and ended up with 3000rpm max speed from a 3600rpm 27hp engine. This seemed to give me an economical cruise speed of 6 knts at 2500rpm (ish) but enough drive to punch in to a solid F5, but used a lot more desiel at higher revs. I think I adjusted the prop three times, but then I am on a drying mooring. It still took time though as I needed to test the engine in a wide range of conditions before adjusting again. I think in the end I adjusted by 1/2 to 3/4".

 

[Ex-SolentBoy]

the prop should be pitched to give your boat max lwl spd @ max revs or just under max rpm.
we have 2900 rpm max rpm engine rated at 3000

Spot on. As long as you get max speed a max rpm in flat water you have the best performance available.

If you don't set it up for that you probably would be enter off with a different prop.

 

[charles_reed]

At best, any fixed pitch will be a compromise, if you achieve max rpm at max hull speed in still water, the prop will be too coarse when bucking wind and tide.
For that reason adjustable-pitch props are an expensive way of arriving at the desired compromise when most prop manufacturers will produce a fixed prop at half the price and already optimised for the boat.
Far better to have a self-pitching prop which adjusts its pitch to the load - there is only one on the market and hoi polloi frequently lump it together with cheap ersatz substitutes like the Kiwi and Max props.
A separate consideration is the reduction in drag with a feathering prop.

 

[Robin]

Unless your engine reaches very close to it's max specified revs in gear under load it can not develop maximum horsepower and the prop should be set to provide that. We had a Brunton Autoprop which has the advantage that it self pitches to suit the load, highly recommended.

About the same time, we had friends who chose a Maxprop, because the Brunton type wouldn't fit in the aperture on their boat a Victoria 34. It took two years and I think at least 4 boat lifts to adjust that to get the best results, during which time they were not best pleased.

On the boat before last we inherited a prop that was over pitched (a Westerly habit) and had it altered, actually by a diameter reduction and a repitch to get all three blades the same which they were not originally! We then re-engined that boat and had to buy a new prop, I did endless calculations from various books by experts, had two different engine makers computer predictions (the same engine was offered by both Perkins and Volvo), went for my own and got it right first time but perhaps more luck than skill. Getting the prop right is very important in my book.

 

[ianj99]

Prop pitch tosh

If you NEED all the available engine power to achieve the speed you are expecting, then, yes the pitch needs to be adjusted to let the engine reach the rpm where max power is produced.
BUT, in many cases with displacement hulls, the engine may techically be larger than is required. In which you can overpitch the prop to the benefit of reduced revs and greater engine efficiency when cruising.

The Kiwi prop on my 36' 10ton heavy displacement steel ketch is according to 'standard practice' way to large because the Perkins 4108 will not exceed 2200rpm.
However in practice this equates to 6.8kts (about hull speed) and about 32hp. Crusing speed is 6kts at a nice smooth quiet 1800rpm.
I may yet compromise and reduce the pitch to allow 2500rpm, but this will mean cruisng rpm of about 2000prm for the same 6ts But adjusting it for 3000rpm would mean a very revvy and noisy cruising rpm of getting on for 2600rpm - neither acceptable nor necessary.

So my advice is to ignore any suggestion about adjusting it to achieve max bhp rpm unless you don't have any power to spare.

The best way is to use an online bhp caclulator to see what power your hull needs to achieve hull speed and then use Castle Marine's prop size program to get an idea of size and pitch.

There's not a problem with overloading the engine if you over prop, provided you simply back off the throttle so the rpm drops back by about 100rpm on those occasions when you need the maximum available power.

ian

 

[NormanS]

Or get a Bruntons Autoprop, and it does it all for you.

 

[Robin]

The boat I mentioned in my first reply where I had the prop adjusted and then later replaced when we re-engined was a Westerly 33 ketch, displacement around 8 tons and with originally a Mercedes OM636 42hp, replaced by a Volvo MD22L 50hp.

When we resized the original prop, actually by reducing the diameter from 17" to 16" as well as correcting the pitch to the one stated on all blades as every one was different, the max revs under load in gear went up close to the design hp max. The engine was livelier, prop kick astern was reduced, the cruising speed was the same but at 200rpm higher and yet it used less fuel. Max speed was 8kts on a 28'10" waterline, cruise speed was 6kts. With the replacement 50hp engine and a new prop matched to it, we still cruised at 6kts, still had 8kts max and yet the consumption actually dropped.

I don't believe in over propping, any more than I would favour driving everywhere in 5th gear in the car regardless of being on a 1 in 5 hill. Why have 50hp available and yet fit such a dragging prop that only 25hp is in fact usable? IMO I would rather have the reserve capacity to plug into a bad head sea and head wind, or even to tow in another boat if needs be than to simply throw away the hp that was there to be used.

 

[theoldsalt]

Nostrodamus,

The maximum rpm of the engine is largely irrelavant as this is set by the governor. Max power/torque do not occur at max rpm that is why max rpm is largely irrelavant. If you engine reaches max rpm on load it is almost certainly under propped. It is a common mistake to assumption that an engine that fails to reach max rpm on load is over propped.

You need to look at the power and torque curves for your engine.

The prop fitted/adjusted should enable the engine to achieve the rpm for either max torque or max power at full throttle (depending which characteristic you wish to achieve as they are not the same). This rpm is often 10to 20% below the max rpm.

 

[CelebrityScandel]

There's not a problem with overloading the engine if you over prop, provided you simply back off the throttle so the rpm drops back by about 100rpm on those occasions when you need the maximum available power.

There is a potential overload problem if you overprop even if you never use full revs. For example, driving into seas or strong winds even at normal cruise RPM may overload the engine and also engine load increases with hull fouling and the load on the boat - for these reasons new boats are normally slightly underpropped they being clean bottomed and lightly loaded with equipment, stores, etc.

I would never recommend anyone overprop a pleasure boat engine and in the case of a new boat or engine one may lose the engine warranty if one does. Overpropping a pleasure boat is a silly practice with no basis whatsoever.

 

[theoldsalt]

CelebrityScandel,

How do you define "overprop"

 

[CelebrityScandel]

CelebrityScandel,

How do you define "overprop"

Anytime the power absorbed by the prop and drive chain exceeds the manufacturers rated power output of the engine at the particular engine RPM's, which I think, is pretty much what you have said. So, the following by way of explanation, not as teaching egg sucking - and I am taking of pleasure boats here, not tugs, trawlers, etc.

With modern marine diesels, including small ones, the engine crankshaft power curve will increase as RPMs increase until it becomes pretty much flat at maximum rated horsepower for 300 or more RPM's below maximum engine RPM's. The engine manufacturer's crankshaft power vs RPM curve will likely also show a propeller load curve and almost always this curve assumes that power absorbed by the propeller follows the cubic pump curve - which except in some special instances it will not and will deviate greatly from it (an example exception is waterjet drives do follow the cubic curve closely as they behave as a pump and they do so are dislocated from external influences such as the effects of the displacement of the boat, sea state, wind, etc). One can see the curves, for example, on the Volvo Penta site in their engine brochures e.g. the D2 series.

For a propeller driven boat the prop curve will not be a nice never changing cubic curve but will depend on the boat hydrodynamics and boat speed (an extreme example of that is a planing boat going through the hump or increasing RPM at above displacement speed in a non planing boat), plus the curve is not a given as it rises and changes shape as the loading needed to maintain cruise speed (for example) against particular sea states, wind and the changing weight of the boat.

How do you tell?

If the engine is computer managed then at commissioning on anything other than a tiddly little engine the engine manufacturer will monitor fuel burn against revs (which for diesel engines is convertible direct to Power off the specific fuel consumption curve) for various boat speeds at the current displacement of the boat (which will be estimated including tanks state, persons on board, etc, etc) and the sea and wind conditions on the day of the trial and produce power demand curves for that case against RPM. Being a new boat, tanks likely not full, lack of stores on board and usually a calmish day the engine manufacturer will usually require the power demand on the engine not exceed rated power at any RPM and often will require an overrun allowance in that the engine be able to exceed max rated revs by some small percentage. The overrun allowance on a new boat giving headroom for latter fouling of the hull, fuller tanks and stores (and stores, equipment, etc usually increase in mass during the life of the boat), and environmental. In that case one is set to go.

Now that is not much help for an existing engine or the ordinary bloke buying an existing boat, but the engine should be able to go through all RPMs up to maximum rated RPM without any signs of overload such as black smoke (and that includes in a wet exhaust no smoke but soot in the cooling water coming out the exhaust) and that in any likely state of the boat insofar as hull cleanliness, boat displacement, and sea state/wind environment. For a displacement boat these signs will normally only appear once a certain RPM is reached and will appear from that on up to the governed RPM.

As diesels are governed and so fuel is increased to maintain constant RPM as load increases if the engine starts bogging down losing RPM at the same throttle setting then it is also likely going into overload (but don't be confused by a change in engine note, as the engine is loaded up it's note will change, its RPM should not).

There are some small allowances though, for example, proceeding in steep seas one is likely not wanting to proceed at or above normal cruising speed so one may allow the potential of overloading if one happened to drive the boat at displacement speed against them. Another possible exception, not relevant to sailing boats, is planing boats when going through the hump where power demand climbs rapidly but in which state one would not normally drive the boat.

That all said, these things are considerably more important on modern engines which are able to put out much more power for smaller engine mass than the old luggers. The best example of that is possibly engines which are computer managed, that management allowing the engines to be rated considerably higher (power at RPM) than non electronic engines typically otherwise would have been.

 

[CelebrityScandel]

...(and that includes in a wet exhaust no smoke but soot in the cooling water coming out the exhaust)...

Should read ...(and that includes in a wet exhaust no smoke nor any soot in the cooling water coming out the exhaust)...

Apologies for any other typos yet to be found. I'll write a paper next time .

 

[Nostrodamus]

I do thank you all for your replies. It is a very foggy subject and you need a degree in witchcraft to be able to understand it.
My problem is that I have a 14 year old Volvo MD22 59 hp engine in a heavy displacement boat. The old volvo 120 saildrive leg had to be replaced due to corrosion. They dont make that leg any more so I had to fit their replacement which is the 130 I believe. This leg is made really for the newer Volvo engine which are lower reving and therefor it has a different gear ratio to spin the prop at the equivelent speed. With the 130 coupled to the old engine it effectivly is spinning too fast. I tried several manufacturers who were not happy and the only one to really try and help were Bruntons. The auto prop could not be used as ther would be cavitation. We tried the vari prop but the boat just would not go backwards at all. In the end I ended up with the Vari profile which is similar to a Max prop but the pitch in forward and reverse can be ajusted seperatley.
In reverse the prop really digs in and works great. In forwad though there is no real bite and I have to be very careful with manouvers when going astern to know she will stop and start going forward again.
I need a little more bite in forward so if I pitch the prop up a tad I may get that but will loose 200RPM from the top end and I don't want to overload the engine or punch through rough seas.
Is it worth trying to pitchit a little more or is it something I just have to live with. Will increasing the pitch make any other differnce?
Thanks

 

[Tranona]

You need to look at the power and torque curves for your engine.

The prop fitted/adjusted should enable the engine to achieve the rpm for either max torque or max power at full throttle (depending which characteristic you wish to achieve as they are not the same). This rpm is often 10to 20% below the max rpm.

That is just not the case. On every single Beta engine up to 38hp (for example) maximum power is at maximum governed revs. In other words if you removed the governer the engine would produce more power at higher revs. The 10 and 14, and the 35/38 are in fact the same engines - the difference in rated power is mainly achieved by running at higher revs.

With regard to torque, maximum is not anywhere near max revs - typically around 2400-2600 on an engine designed for 3600. This is around 70-75% of both revs and power. It is also usually the point of minimum specific fuel consumption.

Therefore to achieve the best from the engine (if it is sized correctly for the boat), it should be propped to achieve displacement hull speed at max revs. This gives a comfortable cruising speed at around 70% power (ie max torque and minimum specific fuel consumption). Restricting maximum revs means you never use maximum power. If the boat does not need the maximum power, fit a lower powered engine.

Of course you cannot get exactly the right match of power/prop because engines usually go up in increments and boat weight (in particular) is variable from boat to boat, so it is sensible to have a small power reserve. However, modern engine ranges such as the Kubota based Beta and Nanni have much closer increments than older ranges where there was often a jump from 10 to 20 to 30hp, so it is much easier to get an appropriate powered engine.

 

[theoldsalt]

CelebrityScandel,

I would put it much more simply. An engine is overpropped if it fails to achieve the max power/torque rpm at full throttle.


Nostrodamus,

Quote "I need a little more bite in forward so if I pitch the prop up a tad I may get that but will loose 200RPM from the top end and I don't want to overload the engine....."

As previously suggested pitch the prop to achieve max power/torque rpm at full throttle. I cannot give you precise figures as I do not know the details of your setup.

Losing 200rpm if you "pitch the prop up a tad" will not matter if you still achieve max power/torque rpm at full throttle.

 

[theoldsalt]

Tranona,

I don't think we disagree. If a manufacturer chooses to govern an engine below it's max achievable power and market it so, then obviously the max rpm coincides with max power and that could be the criteria for choosing a prop size. As I said, "depending which characteristic you wish to achieve", if fuel consumption is most important than other factors then the setup would be different.

Quote "Restricting maximum revs means you never use maximum power. If the boat does not need the maximum power, fit a lower powered engine." But you have already given an instance when the manufacturer does this.

Choosing an engine capable of achieving max hull speed in calm water at max power does not leave a margin of power for punching into head seas and wind.

Personally I am usually not interested in conserving fuel. If the engine is on (I do have a yacht not a motor boat) it is to get me places to make a tide and I want power in hand so that I can do this in adverse conditions. I can achieve max hull speed at approx threequarters throttle and less than max power in calm water. But I can maintain max hull speed in adverse conditions by opening up to full throttle and max power.

If I want to save fuel I hoist the flappy bits !

 

[Tranona]

Tranona,

I don't think we disagree. If a manufacturer chooses to govern an engine below it's max achievable power and market it so, then obviously the max rpm coincides with max power and that could be the criteria for choosing a prop size. As I said, "depending which characteristic you wish to achieve", if fuel consumption is most important than other factors then the setup would be different.

Fuel consumption is proportional to the amount of power used. More power, more fuel.

Running the engine at its most efficient point (ie when the specific fuel consumption is at its lowest) gives you the optimum speed consumption point.

If you are powered and propped correctly you will still have reserves to deal with adverse conditions as you are only using 70% of available power for cruising speed.

 

[CelebrityScandel]

That is just not the case. On every single Beta engine up to 38hp (for example) maximum power is at maximum governed revs. In other words if you removed the governer the engine would produce more power at higher revs. The 10 and 14, and the 35/38 are in fact the same engines - the difference in rated power is mainly achieved by running at higher revs....

Agree completely with that. Diesel marine engine manufacturers often produce an engine and present it with a range of duties ranging from higher power output and revs down to lower output and revs and all that is done for part of the range is change the fueling so the higher output engine is able to be fueled at a higher rate.

For the original poster you say you have a 59hp MD22 which for that base engine is from memory the mid powered one, the MD22L being 50hp (through reduced fueling alone) and the turbo charged higher (I think) powered one. The MD22 is a robust engine (we had a 22L in one of our previous boats). In the manual it gives a range of maximum RPM's - for the 22L I believe it was 2,700-3,000, and for the higher rated non turbo versions 3,500-4,000 (yours I think), and for the turbo versions a bigger range again from 3,500.

If you are saying that your engine's recommendation is 3,500-4,000 and you want to operate it at 3,800RPM then you don't have a problem. If you are saying you want to prop it so it only reaches 3,300RPM then you are saying that you would like to prop it so at only a little over cruising revs (I think VP recommend cruise at 300-500RPM below maximum) the engine is overloading (i.e. it won't rev higher) then my opinion is that you are opening yourself up for downstream problems with the engine and (as Tanona has also alluded to) you will have in effect also likely reduced the engine's maximum output.

My background is oriented to general performance of marine engines and their duties (while my business is far from marine engines my marine experience means that I help out insurers as an expert witness in defended claims related to marine engine abuse e.g. from overload, duty) rather than hands on experience with particular models. But there is another on this forum, Skipper_Stu, who seems to have quite alot of experience with the MD22 and who generally makes sense to me in his posts regarding those. Perhaps you could send him a message asking his opinion regarding overloading the MD22 engine through overpropping.

If his recommendation is not to do it then you have a prop selection problem to work through and I am surprised that no good prop builder cannot solve that for you.