Catalina36
N/A
4hp per ton is the standard. Overpower it and the engine will never work hard enough, it will soot up and and the bore will glaze.
New engine for 28ft yacht - power or size/weight?
- Thread starter Captain Crisp
- Start date
billcole
Well-Known Member
Whatever you go for, do do some research to find out what propellor you'll need to transfer the power to the sea.
On my boat I went from 18 to 25hp and had to go from 2 blade to 3 blade, which was not a problem, but a 2 blade with adequate blade area would have been too big to clear the bottom of the hull.
If there's a choice of gearbox for your chosen engine then the reduction ratio will of course affect the prop calculations.
On my boat I went from 18 to 25hp and had to go from 2 blade to 3 blade, which was not a problem, but a 2 blade with adequate blade area would have been too big to clear the bottom of the hull.
If there's a choice of gearbox for your chosen engine then the reduction ratio will of course affect the prop calculations.
Captain Crisp
Well-Known Member
Many thanks.
CAPTAIN FANTASTIC
Well-Known Member
If the 10hp engine is the same base engine as the 14hp and 16hp, it will be better to go for the 16hp; there will be times that you will need the extra power.
Last edited:
Bobc
Well-Known Member
If they are the same engine, take the 10. On the basis that your cruising revs will be 2000rpm, unless you want a screaming engine, the bigger engine will be a waste of money, and running an engine at half revs or less will do it more harm than good.
Birdseye
Well-Known Member
Even in a sailing boat there are times that you need a decent engine with sufficient power. The question therefore was whether the 10hp did a decent job of driving your boat. Did it get you up to 6 knots or thereabouts. Could you punch into a stiff headwind? The extra weight of a bigger engine u=is unlikely to be more than 20kg or thereabouts. In the context of a 5 tonne yacht, thats a trivial penalty that likely could be cancelled out by getting shut of some of the junk that you like the rest of us will have on board.
I once had a Prout cat which at 34 ft was powered by a 20bhp diesel on a 3 blade prop. In 20 kn on the nose it might get to 2kn. It was an infuriating problem. Never again
fredrussell
Well-Known Member
Is that true? I’m guessing all modern boat manufacturers would disagree. Your average 5 tonne 30ft JenBenBav will come with, what, 20hp minimum I would have thought? My 1991 31ft 3 tonne boat has It’s original 27hp Yanmar that runs beautifully 29 years on. 10hp on a 5 tonne boat doesn’t seem much to me.
bluerm166
Well-Known Member
Have a look at the Beta engine selection bank Yachts with Displacement Hulls Installation Bank - Marine Propulsion Engines Beta Marine which tells you what power has been employed on a good variety of makes including two examples of the Twister.
doug748
Well-Known Member
A number of new boats come, as standard, with quite small engines, the 30ft Oceanis with 14hp for example. No doubt to keep the bottom line cheap but there we are.
There has been marked inflation in the size of boat engines and it does not always make sense esp on an easily driven, low windage hull like a Twister - With a prop running in an aperture, You very likely end up with either a large diameter creating more noise than thrust, a three bladed effort blunting your sailing performance or something that, at great expense, may or may not feather or fold.
.
PaulRainbow
Well-Known Member
Let's say your 2000 RPM figure is correct and that most of the time you run the 14hp engine at 2000 RPM, how is that any different to running the 10 hp at 2000 RPM, if they are the same engine ? At least with the 14hp engine you can open it up some more if you need to.
Buck Turgidson
Well-Known Member
Prop size.
if you use exactly the same prop on the two installations you are correct. However the 14 would be under propped. If you prop for the 14 the 10 would be over propped and wouldn't make max RPM.
michael_w
Well-Known Member
I would be hesitant to get the 3 cylinder Beta 20. I think it vibrates way more than a simple 2 cylinder one.
I'd disagree, Admittedly, I have no personal experience of the Beta 20, but most three cylinder engines are very smooth, like their cousins the 6 cylinders, It's the 120 degree crank angles.
billcole
Well-Known Member
My 3 cylinder Beta 25 is definitely smoother than the old Volvo 2002 twin was (although to fair they were separated by 30 years in age) .
Any straight twin cylinder 4-stroke will have both pistons moving up and down at the same time won't it ?
JumbleDuck
Well-Known Member
I went up from 10hp (1GM10) to 14hp (Nanni N2.14) and the extra power is very nice, although the new engine had a lot of teething problems. The boat is 26' LOA, long keeled, 4 tons; with the Yanmar she'd do 4.5kt flat out and 2kt into a 20kt headwind while with the Nanni she'll do 6kt and 4.5kt into the headwind.
In retrospect I wish I had bought another 1GM10 and saved myself time and a lot of money, but since like-for-like replacement isn't an option for you, I'd recommend going up a size.
alahol2
Well-Known Member
I re-engined our Stag 28 (from Volvo) with a Beta 13.5. Much more windage but maybe a less compromised prop than a Twister but we've never felt the need for more power.
It is worth pointing out that Beta can (and frequently do) engineer special feet to assist with re-engining. In our case we only had to raise the beds by about 20mm, easily done with a couple of oak planks.
It is worth pointing out that Beta can (and frequently do) engineer special feet to assist with re-engining. In our case we only had to raise the beds by about 20mm, easily done with a couple of oak planks.
TernVI
Well-Known Member
Not all twin cylinder 4 strokes are 360 degree cranks, but it may be the case for diesel boat engines.
Yes, triples are usually smooth, unless it's an early Laverda.
But it seems to me that even a 1GM can be much smoother and quiter in some boats than others. I've owned one, it was the kind of thing that encourages putting the sails up ASAP. I suspect the subtleties of the installation are a big subject, like 'NVH' (noise vibration harshness) in cars.
I have known a few people go against conventional advice and fit bigger engines than normal, all have been very pleased with the outcome. I'm not sure if the issue of a prop in a restricted aperture applied to any of them though.
Talking to owners of the same or very similar hull is the best way.
LONG_KEELER
Well-Known Member
I find folkboat type derivatives motorsail very well as you have no doubt experienced.
Particularly to windward , as there is far less slamming than a modern hull and higher pointing. Motorsailers being the leading examples.
I worked out that in weekend yottie mode, most of my motorsailing was to windward , with the tide , and lumpy . I
think I mentioned this to Norris Marine at the time and they came up with a slightly different prop calc to take this into account. Might be worth considering if it doesn't compromise other priorities.
Particularly to windward , as there is far less slamming than a modern hull and higher pointing. Motorsailers being the leading examples.
I worked out that in weekend yottie mode, most of my motorsailing was to windward , with the tide , and lumpy . I
think I mentioned this to Norris Marine at the time and they came up with a slightly different prop calc to take this into account. Might be worth considering if it doesn't compromise other priorities.
ashtead
Well-Known Member
Remember all those years ago centaurs had a fairly large Volvo for their size. We always thought our 34 foot Bav was underpowered with the standard 19hp. 29hp is the one to go for if you don’t mind paying out for a little more derv.
JumbleDuck
Well-Known Member
I kept the same prop in the hope that going from 2.4:1 to 2:1 would spin it fast enough to absorb the extra power. On the whole it seems to have worked, though I may have a tad knocked off the pitch some time.
langstonelayabout
Well-Known Member
I really-engined my previous Catalina 270 from a Westerbeke 20 to a Beta 20 with a 2 blade folding prop. I never needed to use full throttle on this engine installation.
All worked well after being professionally fitted by a co. that I’ll never deal with again but in hindsight I would have gone for the Beta 16 with folding prop.
All worked well after being professionally fitted by a co. that I’ll never deal with again but in hindsight I would have gone for the Beta 16 with folding prop.