two bladed prop ??

[hathanger]

We just returned from the Florida Keys and we were passed up by everybody
while motoring. Our boat is an 35 mklll 1984 with a Universal 21 hp diesel. The motor runs great and even at 2400 rpm's and a clean bottom, we still only move at 4.5 knots sog. Will changing to a three bladed prop HELP???? Or there something else taht will give us another couple of knots? When under sail and no motor we pass everbody!!!!! Need for speed. HELP

 

[Loren Beach]

Prop Questions

I would guess that you should be doing 6.5 kts with a clean bottom on a quiet day. Maybe 6.8 or 7.0 in a light load conditon... all shined up, just out of the boat yard...


Can you easily turn up the revs to 2900 or 3000 rpm (about tops for that model, maybe)?
If so, what speed are you going at WOT? And when at that speed, is the exhaust showing any carbon smoke?
Suspicion here is that prop is not pitched correctly. A friend of mine with the same engine model, M25, in his E-33RH keeps up with me at these speeds easily, and he has a two blade feathering Maxprop.

All this speculation assumes that your engine is running up to par, also.
Also, do not count on SOG for this info -- what is needed is accurate speed thru the water. Is your KM calibrated?

Regards,

Loren
Olson 34, Universal M25XP (23 hp)

 

[hathanger]

two bladed prop??

Loren, Thanks for the reply!! There is no sign of black exhaust or smoke when doing 2400-2500 rpm's and I was told that is probably the max on the engine??? So, I have never taken it any higher then 2500. My prop is a two bladed Michigan with a 1" shaft. I think it's a 15" Hope this helps!! Chip

 

[Emerald]

Hi,

is this your engine, an M-25?

http://www.marinedieseldirect.com/universal/200157/universal-owners-manual-m25-specifications.html

If so, it is listing your top RPM at 3200. If you can't achieve this RPM on a clean bottom, smooth water, and no tide or wind etc fighting you, then it would make me say you are over pitched with your current prop. It has too much bite and you never spin it up fast enough to get good performance. The ideal would be knowing the specification of the same brand prop you want that someone else has done the homework on for you.

As I recently went through researching fixed two versus three blade props, it seems that where the 3 blade will pay off is sustaining punch in wind waves etc. If you are just trying to get out to race around the bouys, the 2 blade will be your choice. If you are long range cruising, like it sounds you are, then I'd look hard at the three blade. Both Michigan Wheel and Campbell offer "Sailor" varieties of their three blade props. These are supposed to reduce drag over the standard three blade yet maintain a lot of the three blade performance.

Of note, I just have a good generic three blade I got from my local marina out of their used prop pile. I repitched it based on the homework Glyn (also on this board) had done on the Independence 31. It was great to have Glyn's knowledge of what worked on our boats with different motor configurations. So, who's got a 35 mkIII with an Universal M25 and a three blade that revs up to 3200 RPM, and what's your diameter and pitch? and then same question again but for a two blade fixed?

.

 

[Loren Beach]

Kinda hard to trouble shoot from so far away... but if you are actually going 4.5 kts at 2500 rpm, I would say that you have too little pitch, i.e. you are underpitched.


If you were overpitched, you would be going at 6 kts at, say, 2000 rpm in smooth water. I know this because our boat came to us with an over pitched prop from the first owner. Heck, for the first year, I thought that 1800 rpm was a normal cruising rpm...


Of course the engine started blowing carbon when I would try to rev it up to overcome a chop in a narrow channel. Cut me some slack, here -- it was our first-ever inboard diesel boat.


Loren

 

[Emerald]

If underpitched, though, shouldn't he spin up to his 3200 RPM?

How about underpitched with a governor issue, or something else limiting throttle? Throttle cable slip and not having full travel?

Now you've got me confused...


.

 

[hathanger]

Yes indeed I do have a M-25 and the tach goes to 3500. I have no doubt that I can get 3000 -3200 rpm's but have never gone their { yet } !! Maybe the thing to do first is try more pitch before going to the 3 blade prop. I,m going to do my homework on the Michigan and the Campbell props.What degree pitch wuld be most likely needed for more performance ? Thanks, Chip

 

[Emerald]

Yes indeed I do have a M-25 and the tach goes to 3500. I have no doubt that I can get 3000 -3200 rpm's but have never gone their { yet } !! Maybe the thing to do first is try more pitch before going to the 3 blade prop. I,m going to do my homework on the Michigan and the Campbell props.What degree pitch wuld be most likely needed for more performance ? Thanks, Chip

I think the next step is to do a flat water test. Get the engine warmed up, pick a day without wind/wave/tide action to distort your readings, and see 1) what is your flat out max RPM you can go, and 2) what speed you achieved. From here, you'll have a base line to see if you have a pitch problem or not. Of note, most diesels like to get run at the upper RPM range. It may turn out that you are pitched fine, but just need to get use to cranking her up to 2800+ RPM for cruising speed. I believe the theory behind this is you achieve max theoretical speed at the same point you hit max RPM e.g. if you have a hull speed of 6.5 knots, you ought to be able to achieve it at about the point you are hitting your upper RPM range.

Hope this helps.

 

[Loren Beach]

Our "XP" version of the M25 engine is similar enough that our experiences might be helpful.
After getting the prop pitch ironed out several years ago, we now cruise at 6 kts easily at about 2500 rpm. The Universal manual sez to cruise a 2400 to 2600 rpm. With a clean bottom, and in light load condition, the boat will sustain 7.1 kts at about 2950 to 3000 rpm. That's max for our engine.

Variables here would include: 1) tach readout errors, 2) coming up against the governor limit on revs, 3) being still slightly over pitched and loading up the engine at those atttempted WOT revs. Note that the exhaust is putting out some carbon-ish smoke at max revs (or WOT as the powerboat drivers like to call it).

Sidebar:
One advantage we do have is the ability to change our prop pitch with a screwdriver and an end wrench (Martec Autostream featherer). Although we still have to pay to lift the boat clear of the water. A friend of ours at the YC has adjusted his Martec underwater wearing a mask and using a hookah.

I sure hope that Chip gets this figured out; 4.5 kts is a mighty slow way to get anywhere in a 35 foot boat!

Cheers,
Loren