Engine sizing

[casper ghost]

Yeah but then the cut would sound like it was full of Transit vans.

That's all I hear going passed my mooring, loads of Mitsubishi boats, hardly ever does a nice pop pop go passed. It's very disappointing while i'm sat out with my cider..

Casp'

[Onewheeler]

Another thing to consider is the hull design. Our Beta 38 achieves within a gnat's whisker of its rated full power RPM (3000) with a matched prop, but the boat is trying to plane at that speed (IIRC it's about 11 or 12 km/h), the wash is horrific and it drinks diesel like it's going out of fashion. Not much point in going too large. We cruise comfortably at 9 km / h at 2000 rpm with a bit in hand for when it's needed. (15.7 m boat with a fairly deep draft).

 

Propcalc.xls is a very illuminating spreadsheet to play with, daresay it's still around on t'interwibbly.

[Guest]

 

And unless you are propped so you can get the maximum revs (which you can not) you had rather less than 33 hp at your disposal.

Very true. TBH I have never needed much more than 1700 rpm in any conditions. Maybe 15-20 hp? The boat doesn't seem to make much more progress with any more revs, but digs its bum in, sounds louder and gives the illusion of going faster. That is if my phone app is to be believed.

 

 

 

The above a precisely the point.

A deep draughted boat with a large propeller plus a slow revving engine will most probably be using its 15 HP output

Not only is the egg whisk analogy apposite - all froth and no substance - but also the modern industrial engines used have their power quoted at 3,600 rpm for the smaller engines and 2,800 for the larger, neither of which you are likely to achieve in practice.

Never owned a large boat with a big prop, but can imagine that this is true. Torque and all that. Our previous boat had a tiny engine but a low box and (for its size) a larger prop than you might expect on a tiny narrowboat. It got along very nicely indeed.

[nicknorman]

I would look at the torque curve rather than the max power figure. The 38 is quite a bit less cc but makes the power up by being revvier. One time when this can be significant us if you have the usual modern powerful alternators well - pullied so they can deliver 175 A or whatever just above idle. And more so if you have a Travelpower. These devices can sap a lot of power - well a lot of power in relation to that available at low revs. So whilst the boat doesn't need 43 HP, the generating devices might need the extra torque that the 43 gives at idle.

 

Edit: I just looked. Beta 43 at 1000 rpm gives 110nm of torque. Beta 38 at 1000 rpm, about 62nm. Big difference!

Edited August 31, 2016 by nicknorman

[Neil Smith]

My 58 footer has a barrus shire 38 which in fact is 40hp the only difference between the 38 and 40 is the gearbox being a 120 on the 38 and a 150 on the 40, also the 38, 40, 45 and 50 are all based on the same yanmar 2190cc engine and get the higher powers from higher revs ie 2600rpm, 2800rpm and 3000rpm. so on advice from them I was advised to go for the 38 or 40.

 

Neil

[Murflynn]

The beta marine guidelines suggest -

Beta Boat length weight

38 40~50ft 14t

43 50~70ft 18t

50 60~70ft 18t

 

My boat is 56ft with a Beta 43. Even going up the River Severn against a couple of mph current I do not need more than about 1800rpm, so a fair bit in hand. Which would indicate a 38 would be ok, but I have not tried a Beta 38.

I guess it depends upon the price difference between the 2 engines and if there were a strong current would you be prepared to wait for it to slow down???

there is no reason why a 70ft boat weighing 18T should need more power than a 40ft boat weighing 14T, if the bottom of the boat is reasonably clean, if the criteria is reaching a certain cruising speed.

 

the advantage of more power is only apparent when accelerating (and decelerating, which is more important) the rate of which is inversely proportional to the weight of the boat, and the need for either depends largely on the early or late anticipation of events by the helmsman.

Edited August 31, 2016 by Murflynn

[Keeping Up]

I guess if they started fitting 3-1 gearboxes then this might help.

We have a 3:1 gearbox on our Beta 43 with an appropriately coarser prop and it works extremely well (and doesn't sound like a Transit van)

[frahkn]

there is no reason why a 70ft boat weighing 18T should need more power than a 40ft boat weighing 14T, if the bottom of the boat is reasonably clean, if the criteria is reaching a certain cruising speed.

 

the advantage of more power is only apparent when accelerating (and decelerating, which is more important) the rate of which is inversely proportional to the weight of the boat, and the need for either depends largely on the early or late anticipation of events by the helmsman.

 

I've got a Beta 43 in my 70' Orion which (according to Dominic's crane) weighed 23 ton before my wife added a ton or two of clothes etc. I don't know the gear box ratio or the prop size but the boat is 31 inches below the waterline so it may have a reasonably large prop.

 

I have never had it past 1500 rpm except in the occasional lock and perhaps when manoeuvring. The engine is really quiet, I got away with leaving it running in every Thames lock between Oxford and Teddington this summer.

 

So I think more depends on how the boat swims than the engine power.

 

Frank.

 

Edited to say it's a 21" prop (from the survey report).

Edited September 1, 2016 by frahkn

[Murflynn]

 

I've got a Beta 43 in my 70' Orion which (according to Dominic's crane) weighed 23 ton before my wife added a ton or two of clothes etc.

 

 

you'll be in trouble if she reads this ..............