Max RPM Universal M35BC / Prop replacement

[Bill W]

I am new to the group and my 2002 C34 MKII and am looking for feedback on if anyone with the same boat and engine has experienced reduced speed over ground at what seems to be very high RPM's for the M35. 

At 3,200 RPM with a clean bottom and calm seas I barley reach 5.1 knots.  For short periods I have accelerated RPM's (per the tachometer) to 4,300-  at this level the engine still seems to be operating smoothly- no smoke or excessive vibrations but speed will not get over 6 knots.

I am surprised it will go to that high RPMs and suspicious that the tachometer is off.

Any insights on this would be appreciated.  Also considering a new folding 3 blade prop and would like to know what has worked for other C34's.

Thanks

Bill

[skyward]

Hi Bill, those are some extreme RPMs you are posting. I can get over 5 knots with no current effect on my 2-blade. I don't know if Stu is monitoring this thread, but I assume my prop is standard.

[Noah]

First thing to do IMO is buy a digital handheld tach gun (cheap on Amazon) and double-check your Tach.  I do not have your engine, but max RPMs should be no where as high as you reported. More like 3,200 wide open throttle, I would guess. Once that issue is sorted, then move on to the next—speed through the water.

[Jim Hardesty]

Bill,
Welcome to the group.  I believe your your question has been discussed here before at length.  I think you will find a lot of information if you do a search. Shamrock, similar to your boat but with a 3 blade Maxi prop, gets 6 to 7 knots at cruising RPM, 2000-2200.  Max RPM is 2800.  In addition to checking your tack, check knot meter, may be reading low.  If you don't have a chart plotter, try some of the phone apps for speed over ground.
On another turn, you will get better answers if you add your boat information and local to your signature.
Hope you are enjoying your C-34 as much as I enjoy Shamrock,
Jim

[Dave DeAre]

My boat is a 2002 with a 3 blade Flex-o-fold 15x9 RH prop. The m-35 is governed at 3,000 rpm. My boat will approach 7 knots at 2,800 and more important to me, move less than 2 knots at idle for docking, minimum prop walk. The Flex-o-fold is a good prop, major upgrade over the factory 15R9 prop installed at the factory. I have had the Flex-o-fold on my boat for 14 years with no problems and good performance. I believe your tach is off, maybe an alternator upgrade?

[Geoffreykwright]

Sounds like your tach is off.  My tach is off...as other have suggested, I used a laser hand-held tach.  When my tach reads 1100, the laser read just about 1600.  I've got a Gori 15x9 RH folding prop which has an over-drive feature.  When over-drive is engaged, I can do close to 7 knots at about 2000 RPM (again - tach is off, so this is an estimate).  If you're thinking about a prop replacement, I'd recommend the Gori.

Hope this helps.

[Geoffreykwright]

...sorry - I should have mentioned that I also have a M-35B.

[scgunner]

Bill,

I'm not sure exactly what the problem is but my boat is a Mk1 with the smaller 21hp M25 and I have no problem making 6 knots in conditions you describe at around 2400 rpm. While your tach is probably malfunctioning that's not the reason you can't make more than 5 knots. You can hookup any auto analyzer and get the correct tach reading. As far as your hull speed is concerned, I'd check the prop first, a dinged up prop can seriously degrade performance.

[Colonel Butler]

I have a 2006 MkII with the M35B and 3 blade Varifold adjustable prop. I purchased the boat a year ago so this season has been an exercise in resolving a bunch of minor issues, one of which was an erratic tachometer. Sometimes it would read a reasonable idle speed (900 RPM) and then the next time it would read 4000 RPM!

The first thing I discovered was a loose connection to the alternator which I rectified. Worked good for a while then erratic again. I downloaded the troubleshooting guide for the instruments from the link provided in the Wiki here. I opened uu the instrument pod to get to the back of the tach to check connections etc. and all were good. There is a range adjusting screw on the back of the instrument. The mark aligns with a number of letters A - E I think. Mine was set on C with a dab of paint to seal it in place. I read somewhere that the internal contacts for that switch may be corroded/dirty causing erratic operation. After noting the original position, I used a screwdriver to gently rotate it back and forth several times to to wipe the contacts in the switch and hopefully clean them. Then I started the engine and with the engine at idle, gently turned the screw back and forth at the C position until I got a steady RPM reading of idle speed. I did that at the beginning of the season and the tach has not been erratic all summer.

Good idea to check it with a handheld after you get it behaving and providing consistent readings.

My idle is 900 RPM and max RPM is 3000 as per the Universal manual. Flat water, no wind I can get 7+ its at max RPM.