Engines, props, and boatspeed

[lw395]

...
http://yaffa-cdn.s3.amazonaws.com/yaffadsp/files/dmfile/foldingprops.pdf

Pretty coloured graphs in the link showing all the props come out quite similar in most ways.
The dominant effect is that it takes a lot more power to do ever incremental half a knot once you get above 6 knots.
The test boat seems to have ample power in the engine, so can drive firmly into the diminishing returns zone known as 'hull speed', regardless of minor differences in the prop.
The gearbox ratio will influence the pitch required. Article says 2.74:1 'which is higher than usual'

In the real world what you might want to know is how the thing will go into the teeth of a gale and Solent chop. If you worry more about that than optimum flat water performance, it's going to move you to a smaller prop. If you want something in hand for when your engine is old and down on power, smaller prop.

 

[Bobc]

Yes, it's starting to appear obvious now that there is a balance in prop size, from what might produce the ideal in calm weather and what will allow you to punch into the chop or into a head-wind.

So much seems to come down to engine torque, the rpm which that torque is achieved at, and the volume of water that the engine is able to shift.

My current engine is a Perkins-Prima based unit with a turbo, which has close to F**k-all low-down power and needs to be revved in order to get any grunt out of it (looks to me like it is more suited to a small motorboat than a large yacht). The engine that I'm looking to replace with has a lower ultimate hp rating, but at cruising revs, the current engine is only producing 30% of the rated hp and not a lot of torque at all, whereas the engine that I am looking to put in has similar hp and more torque at its cruising revs, even though that lower than the cruising rpm of the current engine.

There's more to specifying engines than simply "how much hp it has", that is clear now. The same appears to be the case with prop size.

 

[westernman]

c) the new engine rotates the other way.

Some gearboxes are bidirectional. I.e. can be run indefinitely in either direction.

So you just need to change the linkage to the lever in the cockpit to work the other way around if you can't get your head around moving the lever forwards to go backwards and vice versa.

 

[Daydream believer]

I have, but:-

1./ They have all given me different answers
2./ I don't just want to accept what I'm told without understanding why, as buying the wrong size of prop is an expensive mistake to make.

Has it occurred to you that different blade shapes may require different pitch/diameters?
Bruntons alter the effective "working "pitch of their autoprop, just by grinding a small bevel on the outer end of the prop blade
So working on that basis it may be that different blade designs need different pitches & diameters which is why the manufacturers are giving you different figures. They may also be applying different requirements into the calculations, such as fuel economy, or maximum thrust at different speeds. Have you specified what you want. Ie speed through chop ( an important factor in my view as my boat stops dead in a bit of chop) or max speed in flat water (great if you only go out in calm weather) or fuel economy ( if you are a tight wad) . I would rather have a boat that went 1/2 a knot slower at top speed but lost less in a chop than do 7knots in a canal & 3 knots in Fishermans Gat in F6.
There are so many factors that you need to question each potential supplier. Or so I would have thought.

 

[KellysEye]

If you haven't made a decision about the engine I asked a number of marine engine repair shops in the Caribbean which engine they had to least repair the consensus was Yanmar.

 

[Bobc]

If you haven't made a decision about the engine I asked a number of marine engine repair shops in the Caribbean which engine they had to least repair the consensus was Yanmar.

I'm a big fan of Yanmar. Had one in my old Bene. Unfortunately it won't fit the current boat. I'm pretty much settled on Beta, as it's a Kubota engine.

 

[wombat88]

Just at thought, in my experience, which isn't world class, an overpropped boat has another big advantage if you aren't so good at judging things...stopping. Just pop it into reverse and give it plenty of throttle. Almost looks like you know what you are doing.

The above may, or may not be of interest to the OP.

I put a second hand Beta 20 in my previous boat. I don't know how old it was but it just went about its business without fault for the 10 years I had it. Belts and filters cheap from local agricultural machinery people. Apart from oil annually, nothing else required.

 

[lw395]

Just at thought, in my experience, which isn't world class, an overpropped boat has another big advantage if you aren't so good at judging things...stopping. Just pop it into reverse and give it plenty of throttle. Almost looks like you know what you are doing.

The above may, or may not be of interest to the OP.

.....

For best stopping power, would you not want a large diameter, finer pitch, rather than coarse pitch, moderate diameter?
Wouldn't what many people call 'overpropped' be prone to stalling and not giving much thrust astern?
I'm used to folding props, where you need some revs in reverse to open the blades.

 

[Bobc]

I put a second hand Beta 20 in my previous boat. I don't know how old it was but it just went about its business without fault for the 10 years I had it. Belts and filters cheap from local agricultural machinery people. Apart from oil annually, nothing else required.

That's kinda where my head is at. I've fallen out of love with VP because of the cost of parts, and also their propensity to destroy the expensive bits (like the heat exchanger casings and the EVC systems) that you can only buy from VP.

 

[Bobc]

For best stopping power, would you not want a large diameter, finer pitch, rather than coarse pitch, moderate diameter?
Wouldn't what many people call 'overpropped' be prone to stalling and not giving much thrust astern?
I'm used to folding props, where you need some revs in reverse to open the blades.

That's where I've got to. So I've decided to go for the Kiwiprop and get one size bigger than recommended on the basis that I can always knock the pitch down a bit if needs be, but I'll get less slippage and more grunt for manoeuvring.

 

[earlybird]

The YM prop tests a few years back showed the 3B Kiwi as bottom of the heap for forward speed and fairly low for stopping power.

 

[Bobc]

The YM prop tests a few years back showed the 3B Kiwi as bottom of the heap for forward speed and fairly low for stopping power.

I hear different views from people who have them fitted on their boats.

 

[Bobc]

All ordered up now.

Beta engine with Featherstream prop.

Phew...

 

[Leighb]

You should be happy. We had that combination on our last boat and had no problems and excellent performance.

 

[Bobc]

You should be happy. We had that combination on our last boat and had no problems and excellent performance.

That's good to know.