Propeller sizing advice....

[yourmomm]

Hi. Ok I've got a craftsman cf16 (16hp 2 cylinder) engine, running through 2.72 reduction ratio gearbox, and a 15 x 10 three blade prop, pushing around between 4000 and 4500kgs of long keel, fully laden 25ft boat.

I'm motoring at 3knots, when this boat should be 5. Sure I can get 5knots, but only at revs I'd rather not sustain. The props the weak spot.

Any idea what sized prop i actually need? (sizing formulae I've found on the internet dont seem to work for my setup).

 

[savageseadog]

Propeller suppliers will do the calcs for you

 

[lw395]

Hi. Ok I've got a craftsman cf16 (16hp 2 cylinder) engine, running through 2.72 reduction ratio gearbox, and a 15 x 10 three blade prop, pushing around between 4000 and 4500kgs of long keel, fully laden 25ft boat.

I'm motoring at 3knots, when this boat should be 5. Sure I can get 5knots, but only at revs I'd rather not sustain. The props the weak spot.

Any idea what sized prop i actually need? (sizing formulae I've found on the internet dont seem to work for my setup).

What RPM are you alluding to?
I'm not familiar with that particular engine, but I'd guess it's only 16HP at max power RPM. It's not going to do 5 knots in a short heavy boat at a relaxed RPM.
What shape is the hull? Overhangs? LWL?

The sizing formulae are generally not badly wrong.
You could buy a copy of Gerr's book on props, it's cheaper than buying the wrong prop.
Do check you gearbox really is what you think it is, you wouldn't be the first.
Also the rev counter.

 

[neilf39]

Using Propcalc (which worked well for my boat) with the figures you gave it comes up with a 15" x 12" or 14" x 15" for a 3 blade prop. That is for a top speed of 7.5kn (max practical speed for 25ft lwl) at max revs which I assumed for your engine is 3600 (figure for a cm16). So on that basis your prop is a bit low on pitch. If your engine max specified revs are lower then the prop size/pitch goes up, and down if higher. If your lwl is actually say 23ft prop size above is still ok but max speed is 7.1kn at max revs.

 

[yourmomm]

Thanks all. Sorry should have said. LWL is only 20.67ft (6.3m). I just looked at propcalc:

http://www.rbbi.com/folders/prop/propcalc.htm

and couldn't get that one to work either? How did you get those figures @neilf39?

My engine instruments are very limited (the Craftsman cheaper option seems to only have warning lights for when things go wrong, no other instrument counters or measures!). So I dont actually know the revs I'm pushing (or theoretical max, for that matter)...I just go on the sound the engine makes. Admittedly I treat diesel boat engines very conservatively: I dont like to hear high revs, so wouldn't ever get near max revs, or beyond "relaxed" range.

I guess my question is based on my lack of understanding about props, but I'd rather get easy to understand advice here, than read a whole book about them! I understand the principal of the diameter x pitch measurement; but I dont understand the relative pros and cons of larger diameter props with lower pitch, vs smaller diameter props, with larger pitch...neilf39 quotes two options, above (although these.may be different for my lower LWL?), but why choose one, over the other? And would I get noticeable speed/power improvements to justify the cost of replacement (at relaxed revs only!), over my current 15 x 10 prop?

Thanks again

 

[earlybird]

Consider buying a cheap hand-held optical tacho, £12 on Ebay. You can then test what rpm your engine reaches at full throttle in flattish water.
This figure should be compared with the engine manufacturer's rpm for max power output. Don't know the engine but probably between 3K and 3.6K.
If your installation reaches close to that rpm then your prop. is a good match, if it reaches max rpm at less than full throttle, then the prop. is a bit undersized and you'd benefit from a small increase in size.
My guess is that 5kts from 16hp is fairly near the top end of what you can expect from a heavy 25 foot long-keeler.

 

[yourmomm]

My guess is that 5kts from 16hp is fairly near the top end of what you can expect from a heavy 25 foot long-keeler.

Frankly, I'd be happy with 4

Yeah I'd forgotten about this method to size props....did it with an outboard once, but never with a diesel inboard...but then outboard dont mind being revved hard...every time I hear a diesel inboard being driven too hard, it pains me.

 

[Tranona]

First it is important to establish what revs you are actually achieving and real speeds at those revs, otherwise you are guessing about any changes.

Just to set some parameters. You are limited by your LWL to around 5.7knots with 16hp. You need 20hp with that displacement to get over 6knots. So you have to accept if you want to cruise near to 5 knots you are going to need to use the higher rev range. The other constraint you have is that the reduction ratio is wrong. It would be more normaal to use a 2:1 reduction meaning a higher shaft speed and a smaller faster turning propeller. It may not change the ultimate speed much but would improve performance in the mid range. You may also find that you large propeller gives clearance problems in the aperture and rudder leading to cavitation and loss of efficiency.

However, you have what you have and don't think a major change in prop is appropriate. To see if you can improve speed you need to carry out some accurate trials in flat water and plot speed against revs (using one of the tachos suggested above) from 2000 rpm up in 200 increments. If the prop is right you should be able to get up to at least 3400. I suspect with your current prop you should achieve at least that and get around 5.5 knots. If it goes easily to 3600 then it might be worth having the prop repitched to 11" (1" pitch changes revs by approx 300) which will bring engine speed down a bit.

However I think you have to accept that to achieve a decent cruising speed you need to use higher revs. Much as that offends the ears, the engine will bless you as that is what it is best at!

I had exactly the same issue with my boat which displaced 3500kgs, similar size to yours. With a 9hp (optimistic!) Yanmar 1GM we ended up with a compromise of 3.1:1 gearbox and a very flat pitch 15" feathering propeller and cruised at 5 knots at 3000rpm and on a good day got up to 5.5 flat out.

 

[omega2]

Paul Williams. www.clementsmarine.co.uk may be able to help?

 

[neilf39]

Thanks all. Sorry should have said. LWL is only 20.67ft (6.3m). I just looked at propcalc:

http://www.rbbi.com/folders/prop/propcalc.htm

and couldn't get that one to work either? How did you get those figures @neilf39?

My engine instruments are very limited (the Craftsman cheaper option seems to only have warning lights for when things go wrong, no other instrument counters or measures!). So I dont actually know the revs I'm pushing (or theoretical max, for that matter)...I just go on the sound the engine makes. Admittedly I treat diesel boat engines very conservatively: I dont like to hear high revs, so wouldn't ever get near max revs, or beyond "relaxed" range.

I guess my question is based on my lack of understanding about props, but I'd rather get easy to understand advice here, than read a whole book about them! I understand the principal of the diameter x pitch measurement; but I dont understand the relative pros and cons of larger diameter props with lower pitch, vs smaller diameter props, with larger pitch...neilf39 quotes two options, above (although these.may be different for my lower LWL?), but why choose one, over the other? And would I get noticeable speed/power improvements to justify the cost of replacement (at relaxed revs only!), over my current 15 x 10 prop?

Thanks again

Just plugged in the figures in the green boxes on each sheet. Using the new lwl figure. At that waterline length propcalc suggests a max hull speed of 6.8 knots, but assuming 6.5 then it suggests a 15" x 11". This is not far from what you have so you probably have the right prop for the engine/gearbox. Are the prop and hull clean? Tubeworm on the prop can steal between 1 and 2kn off your speed. Given the waterline length I think you may struggle to get 5kn without applying a lot of revs.

 

[yourmomm]

The other constraint you have is that the reduction ratio is wrong. It would be more normaal to use a 2:1 reduction meaning a higher shaft speed and a smaller faster turning propeller. It may not change the ultimate speed much but would improve performance in the mid range.

Thanks for this comprehensive response, I'll definitely do as you suggest with the tacho and 200rpm increments. My guess is that this prop will easily get way over 3600, but you're right, without a way of measuring revs this is all pure guesswork...

The gearbox (which i suspect is nigh on 50 years old) was originally fitted to a volvo MD 2A (12hp), which all of these little swedish storfidra boats originally came with. Previous owner had to re-engine, (virtually new), and simply kept the old gearbox, to keep costs down. I anticipate that the gearbox will need replacing at some point, since I intend to keep this boat until I die.

As a (relevant) aside, how do you determine ideal reduction ratio, given various parameters (eg engine power, boat displacement, etc)? It sounds like this should occur, before prop size is calculated...

This is all new to me. Mebbe I SHOULD read that propeller book, mentioned above.

 

[Tranona]

As a (relevant) aside, how do you determine ideal reduction ratio, given various parameters (eg engine power, boat displacement, etc)? It sounds like this should occur, before prop size is calculated...

This is all new to me. Mebbe I SHOULD read that propeller book, mentioned above.

It actually works the other way round. You determine the prop size first and then work back to shaft speed and then engine power. Shaft speed is a function of engine speed and reduction ratio. This is explained in the books such as Geer if you ever get round to reading them.

Your original gearbox was attached to an engine that was slow revving - think around 2600 so the shaft speed would have been around 1000 whereas now your shaft speed is 300 rpm higher. Hence my comment about the ratio being wrong.

However in practice with today's high speed diesels the tendency has been to use higher shaft speeds and smaller propellers, primarily because small modern boats are much lighter and often cannot swing large propellers. A modern 25' boat will be typically half the weight of yours and will have no problem reaching hull speed with a 2:1 ratio and a 12" prop.

The challenges with powering short heavy boats like yours are many including getting the prop to run efficiently behind a full keel in a restricted aperture rather than the mostly clear water you get in modern hull forms. One approach, as used in your boat is to fit as large a diameter prop as you can with the biggest reduction ratio and flat pitch to enable the engine to get into its working range for cruising. That range is usually around 2400-3000 rpm. On balance this is perhaps the best compromise, but will only power the boat as fast as the hp allows, which in your case is less than hull speed. Going up 25% in hp will get you an extra half knot but at great expense and you probably can't fit the bigger engine easily anyway.


Not sure there is much to be gained from reading the books unless you like the intellectual challenge. The relatively simple on line calculators like Propcalc are fairly accurate if your basic data is correct and pretty sure you will find you get the speeds predicted once you get a chance to conduct trials. You have to accept though that speed under power will always be problematic, not only because your boat is a poor power boat with limited power, but is also at the mercy of wind and waves.

 

[Sandy]

When was the hull and prop last cleaned?

Have you ever managed to get 5 knots?

 

[yourmomm]

When was the hull and prop last cleaned?

Have you ever managed to get 5 knots?

Of course! With a 30knot wind, and the tide ripping behind me, it was easy....but when I turned around, and suddenly dropped to 0.8knots, at (what I would normally consider) excessive revs, I became a little concerned....

 

[yourmomm]

It actually works the other way round. You determine the prop size first and then work back to shaft speed and then engine power. Shaft speed is a function of engine speed and reduction ratio. This is explained in the books such as Geer if you ever get round to reading them.

Your original gearbox was attached to an engine that was slow revving - think around 2600 so the shaft speed would have been around 1000 whereas now your shaft speed is 300 rpm higher. Hence my comment about the ratio being wrong.

However in practice with today's high speed diesels the tendency has been to use higher shaft speeds and smaller propellers, primarily because small modern boats are much lighter and often cannot swing large propellers. A modern 25' boat will be typically half the weight of yours and will have no problem reaching hull speed with a 2:1 ratio and a 12" prop.

The challenges with powering short heavy boats like yours are many including getting the prop to run efficiently behind a full keel in a restricted aperture rather than the mostly clear water you get in modern hull forms. One approach, as used in your boat is to fit as large a diameter prop as you can with the biggest reduction ratio and flat pitch to enable the engine to get into its working range for cruising. That range is usually around 2400-3000 rpm. On balance this is perhaps the best compromise, but will only power the boat as fast as the hp allows, which in your case is less than hull speed. Going up 25% in hp will get you an extra half knot but at great expense and you probably can't fit the bigger engine easily anyway.


Not sure there is much to be gained from reading the books unless you like the intellectual challenge. The relatively simple on line calculators like Propcalc are fairly accurate if your basic data is correct and pretty sure you will find you get the speeds predicted once you get a chance to conduct trials. You have to accept though that speed under power will always be problematic, not only because your boat is a poor power boat with limited power, but is also at the mercy of wind and waves.

Yeah this makes a bit more sense to me, thanks, although I suspect the high reduction ratio is more a case of needs must when the devil drives, than careful planning by the previous owner. I guess he should have gone the next engine size up, (27hp) ideally, when he re-engined in 2015, but I think size and cost was a consideration, and it seems odd when the original he was replacing was only 12hp...

So, if the gearbox ever needs replacing, something with a reduction ration of 2.0 would be better, then a 15" prop with a larger pitch? Or did I misunderstand this?

 

[lw395]

A slightly different way of looking at it.
Your hull will need a certain amount of thrust to drive it at a speed you choose. That translates to power.
You look at graphs of propellor efficiency and find something that has good efficiency at the speed and thrust in question. Then see if the reduction ratio is available.
Then look at the efficiency at other speeds.

Ideally you want to use the full power of the engine flat out against a headwind, have good fuel efficiency at cruising speed amd plenty of thrust when you're aground. And negligible drag when sailing.
There is no right answer, more like worse and not-so-bad compromises.
Maybe your best value for money would be improved soundproofing so using the engine at full chat was less of a pain? It worked (a bit) for me, but small boats against the tide under power is always tedious.

 

[scottie]

Can you confirm that the gearbox is a Volvo RB manual (ie long gear lever) with drop down reduction gear as this whilst being fitted to MD2A would not be the easiest box to adapt to a non Volvo due to lack of oil supply?

 

[Pye_End]

Yeah this makes a bit more sense to me, thanks, although I suspect the high reduction ratio is more a case of needs must when the devil drives, than careful planning by the previous owner. I guess he should have gone the next engine size up, (27hp) ideally, when he re-engined in 2015, but I think size and cost was a consideration, and it seems odd when the original he was replacing was only 12hp...

So, if the gearbox ever needs replacing, something with a reduction ration of 2.0 would be better, then a 15" prop with a larger pitch? Or did I misunderstand this?

It would be a mistake to think that your woes would necessarily be improved by a 27hp engine for a 25foot boat. As Tranona says, work from the prop, and the engine sizing then becomes apparent.

Even an oversized engine trying to work a boat into adverse conditions can really struggle.

You should be able to get decent performance with the engine/gearbox combination you have. Many gearboxes in the past were higher ratio than 2:1 (perhaps changes in hull design has led to a higher prop speed?).

The comment over how clean your prop and hull is when appraising your current setup cannot be over emphasised.

Regarding engine revs - you say that you are conservative over engine revs - until you are able to see what revs you are using it is difficult to comment on your overall set-up - it could be that what you consider over powering your engine others might consider cruising revs. Hard data is key.

Regarding a slower running prop (ie high gearbox ratio): Depending on hull shape - a larger prop will have more surface area and perform better in adverse conditions (eg motoring into a 30kn wind and seas to boot). However, a smaller prop will give the boat a better sailing sailing performance. Without knowing your underwater profile both options might be suitable for you.

Choice of prop might be considered a compromise - ie what is most important to you? Motoring into a big sea or sailing well or motoring with least revs on a flat calm day.

 

[Balticfly]

try Mark Cooper Tech sales
01394 388 770
www.propospeed.co.uk

 

[lw395]

Hi. Ok I've got a craftsman cf16 (16hp 2 cylinder) engine, running through 2.72 reduction ratio gearbox, and a 15 x 10 three blade prop, pushing around between 4000 and 4500kgs of long keel, fully laden 25ft boat.

I'm motoring at 3knots, when this boat should be 5. Sure I can get 5knots, but only at revs I'd rather not sustain. The props the weak spot.

Any idea what sized prop i actually need? (sizing formulae I've found on the internet dont seem to work for my setup).

Have a play with this calculator:
https://www.vicprop.com/displacement_size.php
Your existing prop will be about optimum, if you can get the weight down to about 5000lb.
My guesses for beam and draft might be wrong, to be fair.
But the weight seems to be the killer in your case.