Universal M35 engine

[captaingary]

I am curious to find out what RPM everyone has been able to motor with the M35 engine. I installed a Martec 15X12 folding prop and have been unable to reach hull speed under power. The Martec people say that I should be able to peak out around 3600 rpm then back off about 20% for cruising speed. My governor is set for a max rpm of 2800 but anything above 2600 causes the engine to start overheating. I am having Martec up the pitch to 14 but they think that will cause more overloading to the engine.

[msenko]

Gary,

I believe Martec is correct. The greater pitch will create higher loads for the engine, causing it to lug, coke up, and overheat at lower rpm's.

I just test sailed a 1987 C34 with the 21hp engine. Its prop is a 15X10 fixed 2 blade and reached 3000 rpms at WOT and hit hull speed at 7.3kn. Should I end up buying the boat, I too want to change the prop to a Martec folding prop. I queried the list about this recently and had a response from one gentleman that the prop he was using was a 15X13 Martec folding prop (not feathering!) and was satisfied. He peaked out at 2880 rpm's and hull speed which is about perfect. Now I am confused as to what would be optimum if your 15X12 on a higher hp engine is having trouble coming up to speed. Other boats I have owned peaked out at around 2800 rpm's and cruise was 2000. Both had a good turn of speed and still were easy to dock. You see, if you go to the higher pitched props, at low engine rpm's the boat will go faster necessitating shifting in and out of gear to keep the speed down while docking. I have seen the extreme of this where with a highly pitched prop, the engine will only max out at 1800 rpm's. When at idle, the boat is going 3 to 4 knots which IMHO is too fast to dock a boat, especially that one which weighs 27,000 lbs (Westsail 42)!

Another way to look at this is from the engine standpoint. If the prop is too highly pitched, the engine will not get up to proper operating rpm's and you end up lugging the motor. When your diesel is overloaded (lugging) it produces prodigous amounts of soot which is deposited on your nice white transom. Not all of the soot makes it out of the motor, however, the rest being deposited inside the motor which cokes it up.

So where you really want to operate the engine is out on the end of the torque curve which is probably around 2100 rpm's or so. Universal states this at 80% throttle (it is on the web site for the M25's) or from 2100 to 2400 rpm's.

So I would not go to a pitch of 14, that will only exacerbate your problem. I would go with 10 or 11.

Incidentally, the best way to improve prop performance would be to go to a larger diameter prop and spin it slower through a higher reducion transmission. This is hard to do for the C34's because the 15 inch props are already encroaching on the 20% guideline for tip clearance. I measured and found it to be 2 3/8 inches, so even though a 16 will fit, it ends up too close to the hull. When the prop is too close to the hull, it tends to cavitate, causing tip erosion.

Lastly, there are some good books on this subject that can go into a lot more depth than this forum allows.

Hope this helps,

Mike Senko
Currently between boats.
Seattle

[PLKennedy]

After we adjusted the tach to correspond to the actual measured max and min rpm's, we found that our Flex-o-fold three blader wouldn't turn more than 2200 rpm.  It was a 15" with an 11 pitch.  We replaced the blades with a 10 pitch and now go up to 2800 rpm.  We usually cruise at 2000-2200 rpm which gives us approximately 6.5 knots.  We have no complaints at all with this performance.

Peter