Originally Posted by malbert73
Sure. But remember engine
manufacturers assume boat
operators are dumb and will pin the throttle every chance they get. So they are making guidelines that protect their engine
from being quickly ruined via lugging. It’s the “decerebrate” approach and certainly safest.
Automobiles run with gears and no one runs at highway speed in second gear
. A64 makes good points about under loading but I think the intermittent high load of acceleration is what makes them not suffer effects of under loading.
But I’m no expert - don’t take my word for it. But I read and spoke to a lot of experts and didn’t rely on dogma in my setup.
Read Nigel Calder, John Harries (Morgan’s cloud), and Steve Dashew’s approach. All have done the same thing or are proponents of it. Andy Schell did same on his repower
and wrote about it.
I've already written that I believe you are overpropping the safe way with the installation
of an EGT. But I also believe that you over did it with your Beta 50. Here's why I say that.
In Dashew's case, as far as I can remember, his concern was about the high revving Yanmar
causing cavitation with his MaxProp when the shaft RPM
exceeded some value because of excess tip speed. Which it often did due to the Yanmar's power curve. Yanmar
was a real interrupter in the recreational marine
engine market due to their stated marketing
HP and small envelope. The stated gross HP was a marketing
angle, yes they'll produce mid 50 HP but at what RPM
, 3600 to 3800. Nobody runs there so it was foolish to size or pitch
your prop to that gross HP RPM. So what Dashew reckoned was to over pitch
his Yanmar down to the continuous HP rating and maybe a bit more to match the increments afforded by his MaxProp. No issue with this at all.
John Harries had the same cavitation issue with his MaxProp and during repower
he was having an issue finding a perfect an engine that met all his criteria, again it's been years since I read his account. But I believed he settled on a normally aspirated Perkins
(Sabre?) that had more HP than he needed or wanted so he over pitched his MaxProp to get the shaft RPM's down. He also installed an EGT to monitor
temps because he was aware of the issue. I also remember in Harrie's account that the yard in Maine
that did the repower was strongly against the overpropping. This is the same yard/diesel mechanics that Harrie states forgot more about diesel
engines than he'll ever know. Which is why he travelled down to Maine
from Lunenberg. In fact I sort of remember a throw away comment he made at the time for none of his readers to let the yard know he overpropped. But he didn't over prop by as big a margin as you seem to have.
Andy Schell, if I remember correctly, was all about matching your engine's output to the manufacturer's fuel
map to attain the highest efficiency for the HP put into the water
. He did that using a variable pitch Hundested propeller
. The wind
up of his article was to show how one could vary the pitch of the propeller
AND rpm to get the EGT into the fuel
maps sweet spot. You can't match that with a fixed pitch prop. You can match to one sweet spot but all the other RPM points are sub optimal. And the higher RPM's are now off limits to you.
I took the liberty of running the HP requirements for your Tartan 40, great boat
BTW, and the estimate that my re-powering software
provides is that your T40 (I assumed a laden displacement
of 19,000 lbs.) requires 36 HP to drive your hull
to 7.5 knots with a clean bottom and calm sea. From the Kubota power curve for your V2203, I see that engine produces 36HP at about 1900 RPM. So for you to reach hull speed
at 2200 you have in reality turned your 50 HP/ 2800 RPM engine into a 36 HP / 2200 RPM engine with a rich fuel mixture throughout the 2200 RPM range. The reason your EGT goes north when you cross 2200 is because the prop power curve has intersected and gone above the engine's power curve and serious overloading has begun. Sailors without an EGT don't know this because most can only monitor cooling water
temperature which doesn't measure engine load. It's not that engine manufacturers think boaters are ignorant, they know that most do not have the instrumentation necessary to run slightly overpropped.
Call me a heretic but I don't follow Nigel Calder.
Your boat your call, but I believe you have excessively overpropped and would be well served to dial it back to 2650-2700 RPM.