What's more important - HP or Torque?

[sailorman]

I like the Nanni and they do a nice 62hp. The one I'm looking at though is a Mermaid MP58, it's £5k supplied and £1.5 fitting plus my new prop, whatever that turns out to be. The bruntons gets more complicated when you factor in £1.5k for the required shaft brake.
I'm guessing my gearbox ratio, it has 3.1 stamped in front of the serial number, but that could be something else, but it does make sense of the huge prop. Kiwi are now doing the K4 and that's a four blade prop which doesn't need a shaft break. All in all the Kiwi gives most of the benefits of the Bruntons and is £4.5k cheaper as a package. New engine, new feathering prop and all the benefits therein for £8k is a highly attractive proposition if, and only if, the 58hp is big enough. Some of the panel's views though cast doubt. I suspect that the engine I have has masses of torque in the low revs but the proposed new one will not deliver its big dollop of torque until much later in the revs.

Fit a PRM box then no brake is req
http://boatdiesel.com/Forums/index.cfm?CFAPP=9&Forum_ID=159


http://boatdiesel.com/Engines/Thornycroft/Thornycroft.cfm

 

[Tranona]

Not sure the Bruntons requires a shaft brake. It is self pitching which will suit your motorsailing very well. Mate had one on a similar size engine in a similar boat (Nauticat) and it worked very well with no shaft brake. The Kiwi is not the same as a Bruntons. If you just want a feathering prop there are many others on the market, but they are mostly (like the Kiwi) fixed pitch, although some are adjustable.

Would be more concerned about your choice of engine. Not sure about buying an unknown quantity when there are several well established alternatives, but appreciate the price issue.

 

[geem]

Trying to arrange a new engine installation for next year, looking at a range of engine sizes, one is the largest version in the mid range with 58hp and 152 torques, the next is the smallest in the maxi range from the same manufacturer, it's only 12hp bigger but has 226 torques. I don't know what torques are, are they really important? It's £2190 extra and I need to know if it's worth it! The designer specified 35hp to 100hp for the CW 34, currently we have a 1977 75hp Thornycroft that in the foulest weather only occasionally goes to 70% throttle and cruises nicely at 50% throttle. How many HP's has it lost in 45 years? And will it have lots of torques.

I don't know much about Diesel engines but I'm trying to learn quickly!

we have a perkins M92B. This is 86hp and has an impressive peak torque of 320Nm at approx 1300rpm. it has over 300Nm across its working rev range from 1000rpm to approximately 1900rpm. Our boat is about 17 ton and we cruise at about 1200-1400rpm. This is using the maximum torque of the engine and is ecenomical at these revs. This is a modern diesel of 4.4ltr displacement but weighs in at a hefty 500kg including gearbox.
worth thinking about although it may be too large for your application. This engine feels very much on top of the job in our boat!

 

[ianj99]

I agree, but the yard who worked on it said 31" and it has that stamped on it. It is huge. Once I'd discounted Bruntons because it cost the same as a new engine, I looked into Kiwi and they make a four blade equivalent. They didn't seem surprised by the size of the present one, said it matched the type of engine we have. Still, 31" is 2.5 feet!

I would definitely look at getting the 20.50" 4blade 65-100bhp rated Kiwi Prop.

I have a 3blade 17" Kiwi on my 10t 37' long keel steel ketch with a 30yr old Perkins 4108 and with a PRM 150 2:1 'box and new R&D Supermounts, the result is smooth, effortless performance with much better retardation by going astern. The downside is more prop walk than the fixed 17" prop.
I adjusted the pitch (easily done with a 4mm hex key) for 6kts crusing at 2000rpm and 2400rpm max at the hull speed of about 7kts.

It looks puny but works!.
Ian

 

[Rivers & creeks]

Fit a PRM box then no brake is req
http://boatdiesel.com/Forums/index.cfm?CFAPP=9&Forum_ID=159


http://boatdiesel.com/Engines/Thornycroft/Thornycroft.cfm

Bruntons say shaft brake needed with PRM. Kiwi is variable pitch prop so would suit this engine and when we change next year.

That Perkins sounds great. I'm interested by the HP, at 86hp it's only 11hp more than our present Thornycroft but you're 5 tonnes heavier, so we may be OK with 58hp. I know there's lots of science here but it's also a dollop of art isn't it, an experienced mechanic looking at the boat and saying it'll do or not. GL Watson knew a thing or two about yacht design and specified 30hp to 100hp, that's a heck if a range, I suspect the 100hp was for the non sailing versions.

We're in the process of a major redesign of the engine room. Removal of the HUGE 400L thick steel tanks will loose us 0.7 of a tonne and a modern engine will loose the rest of the tonne in weight compared to the Thorny lump. That's 8% off our displacement, what's the figure - 10% increase in weight needs 20% more power?

 

[sailorman]

Bruntons say shaft brake needed with PRM. Kiwi is variable pitch prop so would suit this engine and when we change next year.

That Perkins sounds great. I'm interested by the HP, at 86hp it's only 11hp more than our present Thornycroft but you're 5 tonnes heavier, so we may be OK with 58hp. I know there's lots of science here but it's also a dollop of art isn't it, an experienced mechanic looking at the boat and saying it'll do or not. GL Watson knew a thing or two about yacht design and specified 30hp to 100hp, that's a heck if a range, I suspect the 100hp was for the non sailing versions.

We're in the process of a major redesign of the engine room. Removal of the HUGE 400L thick steel tanks will loose us 0.7 of a tonne and a modern engine will loose the rest of the tonne in weight compared to the Thorny lump. That's 8% off our displacement, what's the figure - 10% increase in weight needs 20% more power?

Are you sure re the Kiwi, i think it feathers with an adjustable,not variable pitch.
Have you looked at the 3 blade MaxProp

 

[ianj99]

Bruntons say shaft brake needed with PRM. Kiwi is variable pitch prop so would suit this engine and when we change next year.

That Perkins sounds great. I'm interested by the HP, at 86hp it's only 11hp more than our present Thornycroft but you're 5 tonnes heavier, so we may be OK with 58hp. I know there's lots of science here but it's also a dollop of art isn't it, an experienced mechanic looking at the boat and saying it'll do or not. GL Watson knew a thing or two about yacht design and specified 30hp to 100hp, that's a heck if a range, I suspect the 100hp was for the non sailing versions.

We're in the process of a major redesign of the engine room. Removal of the HUGE 400L thick steel tanks will loose us 0.7 of a tonne and a modern engine will loose the rest of the tonne in weight compared to the Thorny lump. That's 8% off our displacement, what's the figure - 10% increase in weight needs 20% more power?

When motorsailing Kiwi Props states:-

"With the Kiwiprops™ blades free to rotate on their individual shafts, when incremental speed is obtained from the sails when motor sailing, the blades will " see " what is effectivley an increase in pitch as speed builds above what you would obtain under motoring alone and automatically adopt a higher pitch position.
Thus when motor sailing - the pitch automatically increases to compensate for any additional boat speed.
Sailing speed can thus be augmented with relativly low engine rpm removing the need to increase engine rpm to generate thrust as vessel speed increases."

Ian