New engine size? - 4bhp per ton or larger??

[swanny]

Hi again. Doing the works this year! New Beta Engine but can't decide on size. Taking out old 35hp MD17c considering Beta 38 or 43 hp replacement. Boat is 8.2tons dry but probably close to 10t full (big tanks). 36ft but 28.5 waterline and a ketch to boot so some windage up there. 38hp would do it, especially with new Kiwi prop but in larger seas, against the wind think I am better off with 43hp. Not much more expensive, few hundred quid. Fuel economy with larger engine would be good but would it 'underwork'? Engineer tells me not sooo important with modern engines these days. What do people think?

 

[No Regrets]

Go bigger.

 

[BoyBlue49]

BIGGER and you won't underwork it. 5 HP per ton is nearer.

 

[Amari]

Yes, bigger! Last year we changed from Volvo 28 hp to 40, thus bigger prop. Wauquiez 35 abt. 7k kg.Overall engine dimensions old/new similar.
Now we can bash against Force 6 headwind at 4 rather than 2 knots. We rarely need more than 2,000 rpm. Reassuring to have power in reserve. I read in one of the mags that higher engine revs better for long engine life but an engineer told me that might be true of petrol engine but diesels designed to run on lower revs. Anybody got a view on this?

 

[PeterGibbs]

"underwork" is a consideration, but like all good diesel owners you will give it a full-on sesion now and then, to keep it in trim. Won't you. Well, you should (after the new one is run in, of course!)

I read the same is true for petrol engines too - perhaps once a month, it's an art, as you appreciate!

Be sure of your choice of props - the more complete the disc, the more thrust you will deliver. For this you can go for 5hp per ton - especially when there's a windage consideration, as you describe. The idea that you can achieve max output for minimum blade size is simply not factual. Min blade size is good for lower drag in light winds (virtually no effect in stronger breezes) of course, but when it all gets rather serious, you'll need the full output of the new engine - at the end where it counts!

PWG

 

[peter60]

our friend that advises 5hp per ton is right it is better to have the little extra when you need it than to have the bare a. we always have enough powermount required. we have a CW 32 which all up for cruising is 12 tons and we have a 65hp

 

[rwoofer]

Intuitively it feels right that as you go larger the ratio should start to drop, because of the increased momentum you get with increased mass. My old 12 tonne (all in) Moody 44 never had a problem with 50hp in some quite difficult conditions. Would a 100 tonne boat need a 500hp auxillary engine?

 

[jrt]

Between 5 & 6 hp per ton (or tonne) is the normally accepted figure.

8 ton boat, engine between 40hp and 50hp.

If you are changing the engine size check the gearbox and prop. You may need tpo alter either or both to cope with the higher power.

 

[roly_voya]

Presumming you mean the 37.5 which is the smallest of the lower speed engins they do. I fitted one this year, brilliant, but I am a bit smaller, only 7.5 ton and sloop rig with low freeboard. This engine gives me the power to motor to windward in heavy weather which is a very useful ability and has proved very ecconomical. Given your extra wt and windage I would expect you to need the bigger size if you want to be able to make progeress to windward above F5 but if you want it as a true'auxillary' I sure the smaller on would do it. Can also recomment the 100a alternator and toothed belt + the remote oil filter option (so you can put it where you can get at it and get a tray under it!) both well worth the extra. Prop choice though might be the key, you have to get the power into the water. Beta reffered me to someone on the south coast for advice on drive train who was exellent (cant remember the name!) and it was them that recommended the 37.5, Beta suggestion was the 28 or 32 but the other guy said "not if you want to make progress in a seaway"

 

[swanny]

Well thanks for all that guys, its the 43hp for me! It will fit no problem and even with the Hydraulic gearbox it is lighter than the old Volvo. I will be going for the 100amp alternator and the remote oil filter which will be on the other side of the engine without it. All will come new, propshaft and propeller. I want to go with the Kiwi prop, heard good things about it and friends have it on their boats in NZ - full pitch in reverse and 'true' feathering with independant blades a bonus. Goodbye to the smoke screen!!!