Prop size and engine revs ?

[tosher]

I have a 57 ft semi trad (21" draft) fitted with a 42 hp Izusu engine/ PRM 150 gearbox driving through a python drive to a 17" x 12" prop. (total 400 hrs on the clock). With full throttle applied in neutral, full rated engine revs of 2800 is quickly achieved but when full throttle is applied in forward gear 2100 rpm is quickly achieved but this is the max obtainable. I also get some cavitation noise over about 1200 rpm. I suspected that the prop was oversized but speaking to people with similar sized boats 17" x 12" seems to be about right. looking at the power curve for the engine, 2100 rpm equates to about 32 hp so does this mean that I can't utilise the full 42hp of the engine ?? I would be very grateful for feed back regarding max engine revs achievable by people with similar set ups to mine to identify if I have a problem and if so suggestions on how to correct it. Many thanks

[dor]

Interesting this, as I have just had a 57' boat, with as 42HP Vetus going through a 17 x 12 prop. I also find revs limited in gear, to about 2200, whereas the engine will rev to 3400 in neutral.

 

The engine is clearly well loaded, but the Vetus main agent has assured me that this is the right prop for the boat and engine and the engines are better for being well loaded.

 

My normal cruising rpm is about 1400, which equated to a bit over 3mph. I was concerned that the alternator (95amp) wouldn't charge adequately at this speed, but it does seem to put out plenty of amps.

 

More of a concern to me is the speed at tickover, which is about 2 mph. I would prefer something a little slower as I can see lots of in and out of gear at low manouvering speeds, which is not beneficial to the Technodrive gearbox.

 

How do others get on with 57' / 42hp / 17x12?

[Timleech]

I have a 57 ft semi trad (21" draft) fitted with a 42 hp Izusu engine/ PRM 150 gearbox driving through a python drive to a 17" x 12" prop. (total 400 hrs on the clock). With full throttle applied in neutral, full rated engine revs of 2800 is quickly achieved but when full throttle is applied in forward gear 2100 rpm is quickly achieved but this is the max obtainable. I also get some cavitation noise over about 1200 rpm. I suspected that the prop was oversized but speaking to people with similar sized boats 17" x 12" seems to be about right. looking at the power curve for the engine, 2100 rpm equates to about 32 hp so does this mean that I can't utilise the full 42hp of the engine ?? I would be very grateful for feed back regarding max engine revs achievable by people with similar set ups to mine to identify if I have a problem and if so suggestions on how to correct it. Many thanks

 

Yes it does mean you're not able to use the full 42hp, but don't let that worry you.

If the prop is a bigger diameter than that recommended for 42hp, it will be to your advantage for starting, stopping, manoevring etc., also you may well find that general cruising is done at a more comfortable/relaxing rpm than with the smaller prop.

If your test was done in shallow water, perhaps tied to the bank, you'll probably find max rpm will run a little higher with the boat moving freely in deep water such as a river.

32hp should be more than enough for a 57' nb in the real world anyway

Some engine marinisers are wise to the nb situation and recommend a slightly oversize propellor.

If in doubt, compare notes with other owners of similar hulls with the same engine & see what they have & how well it works.

The main thing is, if it does the job for you don't lose any sleep over it.

 

Tim

Edited July 23, 2007 by Timleech

[stuart]

I was concerned that the alternator (95amp) wouldn't charge adequately at this speed, but it does seem to put out plenty of amps.

 

I've got an Isuzu engine and the 2x alternators have different output characteristics - the domestic one I think is geared differently to the starter alt. I wouldnt worry about it.

[John Orentas]

I have a 57 ft semi trad (21" draft) fitted with a 42 hp Izusu engine/ PRM 150 gearbox driving through a python drive to a 17" x 12" prop. (total 400 hrs on the clock). With full throttle applied in neutral, full rated engine revs of 2800 is quickly achieved but when full throttle is applied in forward gear 2100 rpm is quickly achieved but this is the max obtainable. I also get some cavitation noise over about 1200 rpm. I suspected that the prop was oversized but speaking to people with similar sized boats 17" x 12" seems to be about right. looking at the power curve for the engine, 2100 rpm equates to about 32 hp so does this mean that I can't utilise the full 42hp of the engine ?? I would be very grateful for feed back regarding max engine revs achievable by people with similar set ups to mine to identify if I have a problem and if so suggestions on how to correct it. Many thanks

 

 

Yes I agree with most of what has been said, you should get a 15 - 20% reduction in revs when the engine is loaded and as Tim says the revs will naturally increase with boat speed. There is not a direct relationship between revs and power so you will probably be close to maximum torque when 2500 or so is reached and as has been said being a bit over-propped makes for more relaxed cruising.

 

As for the engine speed/ charge rate this hasn't come up lately. The boy on a bike with a dynamo principal, the faster you pedal the brighter your lights. As many people seem to have clamp meters now I wish a few would do the experiment, how much extra current can you get extra raising the revs from a fast tick-over under any conditions at all, with my boat the answer is 'none' but a few will think 'logically' and say that is rubbish. The alternator will spin 2 or 3 times faster than the engine anyway and remember there is a regulator in there.

[blackrose]

I have an Isuzu 55 with PRM 150 and a 19" x 13" prop. This is for a 57' x 12' widebeam. I suspect I'm slightly underpowered for the size of the boat but it pushes it along ok. I've been up and down the tidal Thames a couple of times including a Limehouse basin entrance where I had to use full power (about 2,700rpm?). I've never tried full revs in neutral.

[Theo]

Yes I agree with most of what has been said, you should get a 15 - 20% reduction in revs when the engine is loaded and as Tim says the revs will naturally increase with boat speed. There is not a direct relationship between revs and power so you will probably be close to maximum torque when 2500 or so is reached and as has been said being a bit over-propped makes for more relaxed cruising.

 

As for the engine speed/ charge rate this hasn't come up lately. The boy on a bike with a dynamo principal, the faster you pedal the brighter your lights. As many people seem to have clamp meters now I wish a few would do the experiment, how much extra current can you get extra raising the revs from a fast tick-over under any conditions at all, with my boat the answer is 'none' but a few will think 'logically' and say that is rubbish. The alternator will spin 2 or 3 times faster than the engine anyway and remember there is a regulator in there.

 

Ah, John...

 

You have indicated an answer to a question that I have been meaning to ask for a while, now. with a regulated alternator I would have thought that the actual revs of the alternator would not matter that much (within reasonable limits, that is) because the rotor current will be adjusted by the regulator to maintain the required output. what the reasonable limits are, I would not like to hazard a guess. I am encouraged to buy a clamp meter.

 

Sorry that this is going a bit

 

Nick

[John Orentas]

You have indicated an answer to a question that I have been meaning to ask for a while, now. with a regulated alternator I would have thought that the actual revs of the alternator would not matter that much (within reasonable limits, that is) because the rotor current will be adjusted by the regulator to maintain the required output. what the reasonable limits are, I would not like to hazard a guess. I am encouraged to buy a clamp meter.

Nick

 

 

I would recommend everyone to buy a clamp meter, as for the rest I would agree with you, but would not dare to make further comment. Private territory.

[bottle]

Beta 39

39

2,800

1647

45 - 60’

17 tons

18” x 10”

Beta 43

43

2,800

1857

50 – 70’

18 tons

18” x 12”

The above is from the Beta Marine website, engine,BHP,revs,size cc,boat length,weight,Prop guide.

[Sir Nibble]

I seem to remember from tech college, that an engine is developing it's maximum power when running at full fuel and niether gaining nor loosing speed.

with my boat the answer is 'none' but a few will think 'logically' and say that is rubbish.

And others may think logically and say that just because your car starts doesn't mean my battery isn't knackered.

what the reasonable limits are, I would not like to hazard a guess.

Whatever the engine supplier says is reasonable, unfortunately that same engine supplier may feel that running all day at under 2000 revs is not reasonable.

[dor]

OK, I've now had my prop changed. The boat supplier did this eventually after I gave him accurate (GPS derived) speeds at various revs.

 

My original prop on a 57ft narrowboat with Vetus 42HP engine & 2:1 gearbox was 17 x 12. Vetus insisted this was the right prop for my engine, but the boat was doing 2.9 mph at tickover. Most vetus engines are sold for lumpy water boats so the prop probably is right for the engine, but is clearly unsuitable for canal use.

 

With the 17 x 12, I was doing 2.9-3.0 mph at 900rpm (tickover), increasing fairly linearly to 4.6mph at 1400 rpm. Flat out in deep water (River Weaver) was 6.6mph at 2,200 rpm (engine revs to 3,500 rpm under no load).

 

The prop has now been changed to a 17 x 11 and the speeds are now 2.4-2.5mph at 900rpm, 3.8mph at 1400rpm and 4mph is about 1500 rpm. I haven't had the opportunity to try it on a river yet, but in a deep wide bit of canal it looks as though it will top out at 6.8 mph at 2,500 rpm.

 

As well as having a more acceptable speed at tickover, the engine is quieter as presumably it is not having to work as hard at low revs, which I never felt the engine was happy with. There is less shake on the tiller as well.

 

All in all, it is much improved. A further reduction to 17 x 10 would probably also be acceptable. Interestingly, although I felt I was going too fast before when passing moored boats, I was never shouted at. I presume this was because the boat was at tickover. I suspect the "going too fast" shouters respond more to engine revs than any actual movement of their boats!

[Billypownall]

I agree with all you are saying Dor as I have been through all this. We also have a 57' John White hull with a 42 h.p. Vetus.The boat was supplied as yours with a 17 x 12 prop. The figures you give are correct. The boat was going far too quickly on tickover and 1200 r.p.m. was a cruising speed. At this speed the 90 amp alternator was only giving out 25 amps. I was also told that modern diesel engines would be better for revving a bit faster, We went down to a 17" x 10" prop.and were delighted with the result. The boat in tickover goes quite fast enough and we find that about 1000 revs is just nice for passing moored boats. According to our GPS 1600 r.p.m. is 4 m p h and we run at about 1500 normally. Charging rate at 1500 r.p.m. is three times the rate at 1200 and 70 amps. The engine is very much smoother with no tiller shake and is a great deal quieter mainly because the higher revs eliminated a faint but annoying transmission whine.I got slanged off on this forum some time ago for suggesting that most modern engines were overpropped. Of course boats must vary but I still think this to be the case. Last but not least it was suggested that higher revs lead to higher fuel consumption. I unfortunately did not keep a record before we changed the prop but feel that there is not much difference and in any case fuel consumption is that small as not to be a consideration.

[PeterF]

I'll throw mine in as this seems slightly different. Boat is 55ft narrowboat with Barrus Shire 40hp engine, peak rpm = 3400, max power at 2,600rpm with 2:1 prm 150 gearbox. I have an 18" x 12" prop but have not done any speed measurements, so everything is subjective. The boat on tickover at 900rpm seems to be a good slow passing speed. On canal we often cruise at 1400rpm but this is in wider northerm canals. When on the northern rivers I usually run 1800-2000 rpm which is quite rapid. Have blatted it down the River Aire at one point at 2,400rpm and the engine handled this - it would rev a little higher so my guess is that for my engine when running in a river the rpm would match with the peak power point on the engine. If on the narrower Midland canals I agree I may be a little overpropped but I want to have a bit more oomph if I need to run against the Aire or Calder with some runoff. The manufacturers data says the calculated max speed is 8mph but no eveidence to support this.

 

Only issue is the engine is one of the older Barrus ones where the alternator was not sorted well and the ratio from crank pulley to the laternator pulley is not very high and I get very little amps when running at canal speed. I am going to put a larger seperate pulley for domestic alternator like the newer Shire engines over the winter.

[DHutch]

Sounds very much like what i would expect in a way, espcailly after seen the results!!

- As dor says, vetus will be basing most of there comment on the boats that there engines are more commonly fitted to.

- And these boata will, in the main, be far more streamlined boats, with far more water under/around them as well.

 

Dor's boat builder proberbly know all to much about it now, ahving jsut swapped his prop for him, but billypownall etc could do worse than tell theres about there findings, and maybe somone else (proberbly less perceptive than nessary to notice) will end up with a better prop'ed set up because of it.

 

No point in having a 350bhp lotus and sticking it on mini clubman wheels! And neather cropsprayer or monster truck rubber would be any better.

 

 

 

Daniel

[dor]

I have an 18" x 12" prop but have not done any speed measurements, so everything is subjective. The boat on tickover at 900rpm seems to be a good slow passing speed.

 

Whilst I don't want to dispute your observations, you do say your speed measurements are subjective. Before I checked with GPS (and I mean GPS, not a sat-nav) I thought I was doing 2 mph at tickover. As noted above, the reality was somewhat different. I also thought it wasn't too bad for passing moored boats, but I am happier passing them now than I was at 3 mph.

[charles123]

I agree with all you are saying Dor as I have been through all this. We also have a 57' John White hull with a 42 h.p. Vetus.The boat was supplied as yours with a 17 x 12 prop. The figures you give are correct. The boat was going far too quickly on tickover and 1200 r.p.m. was a cruising speed. At this speed the 90 amp alternator was only giving out 25 amps. I was also told that modern diesel engines would be better for revving a bit faster, We went down to a 17" x 10" prop.and were delighted with the result. The boat in tickover goes quite fast enough and we find that about 1000 revs is just nice for passing moored boats. According to our GPS 1600 r.p.m. is 4 m p h and we run at about 1500 normally. Charging rate at 1500 r.p.m. is three times the rate at 1200 and 70 amps. The engine is very much smoother with no tiller shake and is a great deal quieter mainly because the higher revs eliminated a faint but annoying transmission whine.I got slanged off on this forum some time ago for suggesting that most modern engines were overpropped. Of course boats must vary but I still think this to be the case. Last but not least it was suggested that higher revs lead to higher fuel consumption. I unfortunately did not keep a record before we changed the prop but feel that there is not much difference and in any case fuel consumption is that small as not to be a consideration.

 

My Vetus 42 hp has a 18 x 10 prop on a 57ft boat, I have not measured the speed at tickover with a GPS but will do that next time its out. At tickover to 1000 revs it feels to be the right speed to be passing moored boats and 1200 to 1500 for steady cruising, on rivers I have been up to 2000 but thats about it, higher than that the engine seems to be labouring the stern is pulled well down and the boat does not seem to want to go any faster.

 

Billys comments on the revs to get the best charge are interesting and I charge at 1500 revs, I dont have an ameter so its a guess what the most efficint speed is to maximise the alternator output on the 95 amp fitted, Billy do you have any amperage output figures at say 1800 RPM?

 

Charles

[Billypownall]

My Vetus 42 hp has a 18 x 10 prop on a 57ft boat, I have not measured the speed at tickover with a GPS but will do that next time its out. At tickover to 1000 revs it feels to be the right speed to be passing moored boats and 1200 to 1500 for steady cruising, on rivers I have been up to 2000 but thats about it, higher than that the engine seems to be labouring the stern is pulled well down and the boat does not seem to want to go any faster.

 

Billys comments on the revs to get the best charge are interesting and I charge at 1500 revs, I dont have an ameter so its a guess what the most efficint speed is to maximise the alternator output on the 95 amp fitted, Billy do you have any amperage output figures at say 1800 RPM?

 

Charles

Charles I suspect that in our case the alternator is putting out about it's maximum at about 1500 r.p.m. Its just that according to my ammeter the charge quickly falls off below 1500.

[PeterF]

Whilst I don't want to dispute your observations, you do say your speed measurements are subjective. Before I checked with GPS (and I mean GPS, not a sat-nav) I thought I was doing 2 mph at tickover. As noted above, the reality was somewhat different. I also thought it wasn't too bad for passing moored boats, but I am happier passing them now than I was at 3 mph.

Trouble with subjectivity, I did all my early boating in the Midlands but now live in Yorkshire and speeds may be higher than they appear on the larger aterways. Also, I am assuming the previous owner has not changed the prop, will check it when I have it out for blacking next year. Lacking a GPS I will do a trial run this weekend over a known length of water and take the distance off an OS map and hope that can tell within say +/- 0.25mph accuracy. It will be interesting to try this as an experiment just to obtain the info.

Charles I suspect that in our case the alternator is putting out about it's maximum at about 1500 r.p.m. Its just that according to my ammeter the charge quickly falls off below 1500.

I have been looking at this as I do not seem to get very good charging and the graphs from alternator suppliers show a marked drop of in output below about 4000rpm (alternator speed) and nothing below 1500rpm. Above 4000rpm there is some but not much increase all the way to 6,000+rpm. I know on an old post Gibbo saying that the pulleys should be set to give 4,000rpm when boat is cruising which means 3.33:1 pulley or thereabouts. I really notice this because my pulley is only 2:1 so when I am normally cruising at 1200-1400rpm engine, the alternator is only 2500rpm and not giving a good output. When I rev up I get noticable increase on charging voltage (during period when voltage is below 13.7V) and more amps on the clampmeter. I need 2000rpm on engine to get good charging. Engine will get a new pulley for Christmas as I have said in a post above.The link here will show you some data Balmar alternator data which shows in my case that 2500rpm give about 50% of the rated capacity, but moving up to 4,000rpm would give about 90% of rated capacity.

[tosher]

OK, I've now had my prop changed. The boat supplier did this eventually after I gave him accurate (GPS derived) speeds at various revs.

 

My original prop on a 57ft narrowboat with Vetus 42HP engine & 2:1 gearbox was 17 x 12. Vetus insisted this was the right prop for my engine, but the boat was doing 2.9 mph at tickover. Most vetus engines are sold for lumpy water boats so the prop probably is right for the engine, but is clearly unsuitable for canal use.

 

With the 17 x 12, I was doing 2.9-3.0 mph at 900rpm (tickover), increasing fairly linearly to 4.6mph at 1400 rpm. Flat out in deep water (River Weaver) was 6.6mph at 2,200 rpm (engine revs to 3,500 rpm under no load).

 

The prop has now been changed to a 17 x 11 and the speeds are now 2.4-2.5mph at 900rpm, 3.8mph at 1400rpm and 4mph is about 1500 rpm. I haven't had the opportunity to try it on a river yet, but in a deep wide bit of canal it looks as though it will top out at 6.8 mph at 2,500 rpm.

 

As well as having a more acceptable speed at tickover, the engine is quieter as presumably it is not having to work as hard at low revs, which I never felt the engine was happy with. There is less shake on the tiller as well.

 

All in all, it is much improved. A further reduction to 17 x 10 would probably also be acceptable. Interestingly, although I felt I was going too fast before when passing moored boats, I was never shouted at. I presume this was because the boat was at tickover. I suspect the "going too fast" shouters respond more to engine revs than any actual movement of their boats!

 

Thanks dor for your update, really pleased it worked ok for you. I will try and do some accurate speed checks next time I go out on the boat and see how they compere with yours (and some of the other posters). Thanks again - tosher

[chris w]

I have a Vetus 17"x11" prop on an Isuzu 35 with a 50' boat. At first notch (ie: gearbox just engaged) the revs are about 800 rpm and is an ideal speed for passing moored boats. Leisurely cruising is at about 1000rpm with "fast" (although not over 4mph as measured on a GPS) cruising at about 1200rpm. Stacks of reserve in hand for any tricky situations or rivers.

 

Chris

[Billypownall]

I have a Vetus 17"x11" prop on an Isuzu 35 with a 50' boat. At first notch (ie: gearbox just engaged) the revs are about 800 rpm and is an ideal speed for passing moored boats. Leisurely cruising is at about 1000rpm with "fast" (although not over 4mph as measured on a GPS) cruising at about 1200rpm. Stacks of reserve in hand for any tricky situations or rivers.

 

Chris

 

Your figures concur more or less to what Dor and myself were faced with on our boats. I stand to be shot down again but I still think your engine is not revving fast enough. I am sure it would be better for it to be running faster. Is your situation like driving your car up hill in too high a gear? Further of course is the battery charging sitiuation. Assuming a 2-1 ratio on your pulleys I dont think you will be getting much charge from an alternator running at 2400 r.p.m. Lastly I do think it to be a false assumption that an extra 2-300 r.p.m. makes an engine a noisy beast. As I've said before ours seems much smoother and if anything quieter.

[chris w]

Your figures concur more or less to what Dor and myself were faced with on our boats. I stand to be shot down again but I still think your engine is not revving fast enough. I am sure it would be better for it to be running faster. Is your situation like driving your car up hill in too high a gear? Further of course is the battery charging sitiuation. Assuming a 2-1 ratio on your pulleys I dont think you will be getting much charge from an alternator running at 2400 r.p.m. Lastly I do think it to be a false assumption that an extra 2-300 r.p.m. makes an engine a noisy beast. As I've said before ours seems much smoother and if anything quieter.

 

I can't say whether it would be better at higher revs but it doesn't sound at all laboured and chugs along nicely and quietly all day - it's the standard prop recommended by Isuzu for that size of boat (50') and for that engine (35HP). Fuel consumption is no more than 1 litre per hour. I recently measured this again after having done 18 engine hours on a trip and several hours Webasto use (which is about 0.25l/hr) and the total consumption was only 20 litres.

 

With regard to the battery charging, you're correct in that I have a 2:1 pulley ratio (16cm and 8cm pulleys) and, after a night or two's mooring, when the batteries are down to 50%, the charge current is about 30A at idle (750rpm), about 45A at 1000rpm and about 55A at 1200rpm. The alternator on the domestics is an 80A Lucas/Prestolite A127.

 

I do have an alternator controller fitted but of course that will be contributing zippo until the battery voltage exceeds the alternator's internal regulator voltage at 14.2v so the figures above are just the alternator on its own.

 

Chris

PS: edited to say gearbox is a Newage 120 (ie: mechanical not hydraulic) with a 2:1 ratio

Edited October 18, 2007 by chris w