Prop Ventilation Issues

[trdprotruck]

Is your centerboard all the way up?

Yep

If you are getting any splash into the motor well, then you plate is too low--it does not look like you are planing yet.

Definately not plaining yet, but I'm not sure if I am getting splashes into the motor well. The water doesn't seem to be exiting cleanly though from what I can tell.

-Randy

[trdprotruck]

I definatley have the mac bug and want to go faster. I think I'll be happy if I can get 16 or 17 mph out of her. So it seems most of the issues I had earier were prop related. From what I have been reading, the consensus is to get a prop with the largest diameter, largest surface area on the blade and possibly a 4 blade for maximum perfromance. It seems like most people are pitched between 8-13".

The temporary 12.5 x 13" SST prop I would think would be too much pitch for my little johnson 50, but I don't seem to be bogged down. I guess the current thinking would be to keep going higher in pitch (14?) until I start getting bogged down at 5500 WOT? I am guessing there must be a lot prop slip in the water, if I can only do 12.8 mph at 5500 RPM.

I really thought that the problem 4 bladed prop at 11.5 x 9 would be the ticket, but I'm not sure why it's not working.

I'm now thinking about possibly playing with the engine height since I also noticed what aya16 was saying (water not exiting cleanly) .

4 bladed 11.5 x 10 (Mi Wheel)

or

3 bladed 13 x 11 (Largest diameter prop I could find for my motor) Solas, OMC or Mi Wheel

I'm not sure which prop has more surface area 3 blade (large diamter) vs 4 blade (smaller diameter), but I am thinking of trying out the 13 x 11 since I had so many problems with the Mi Wheel 4x11.4x9.

I'm not sure whether I am making a whole lot of sense, but I love hearing what other people have to say on the subject.

Thanks,

Randy

I guess I didnt answer your question, It looks like the cavitation plate is a little low in the water and doesnt look like a clean break in the water. But I dont know if you are plaining or not,
at 12mph, I dont think so. run without ballast and see what you get. Then look at the plate.

Whats the working red line of your motor? if its 5500, you are already there, may need to have the 13 inch prop pitched some. Im not sure if your johnson could handle a 14 inch prop
you raise the motor, and dump the ballast, you might be spinning at 6000 rpm, if your motor is rated for 5500, that extra 500 rpm is not good if you have it pinned for long periods of time.

If you dump the ballast, that might raise the engine an inch or so, when you go full throttle.

keep us up on what you do
Mike

[aya16]

speed wise you will go slower with a 4 blade than a three, ( thats true on all boats) That 13 you are using is doing a great job, mostly because you have a lighter two stroke, and you are using a stainless prop
that bites better in the water. I don't think you can run a 14, but if you get someone to lend you one, its worth a try. You are sailing uncharted waters with the Johnson 2 stroke.
Most everyone giving you advice on props run a 4 stroke, what works for them may not work for you.

Try the 13 without ballast, see what you get , (rpm, speed). The engine may ride higher then too. The performance you get now, I don't think you will be able to improve much more
you are at the point of splitting hairs, (splitting hairs is expensive, and the gains are very little) What you can do, if the mac runs around 6000 rpm without ballast, is have the stainless prop
cupped some more, that will drop your rpm down and the prop will bite even better.

Once you get it dialed in, be very careful with that prop, it will be a one of a kind, hard to replace. come up to marina del ray some time and we will go sail....
Mike

[bscott]

4 bladed props create more resistance/drag. You need a pontoon boat prop to try--a good prop shop should lend you props until you find the correct design--stainless props are for high speed and big pockets.

Bob

[Crikey]

At this point I think you should go for the motor height trial, then go from there if still needed. RPM is your first concern - not speed. The four blade will lower the revolutions due to the extra blade's resistance as long as everything else is remains equal, but your engine position must first be correct for the water flow aft of the boat. That is determined by the hull profile and weight of your loaded Macgregor, in this instance. Because of the very rounded bottom with no strakes it is always possible to stall (cavitate) a prop during a hard turn at speed, unless the motor is set very deep. It's a compromise!
Stainless props have a much narrower blade profile and less flex than an aluminum wheel which results in less friction and more freedom to re-contour the surfaces when fine tuning cups and angles.

Of course, this is from a guy that hasn't done one on his own and basically hasn't (yet?) jumped due to the premium price asked for these baby's. For what it's worth - you can crash one and repair it a lot easier that an aluminum spinner.
Sounds like you're very close at this point......

Ross

[aya16]

I'm not so sure I buy into the pontoon boat theory, that's been pushed since mac's were hollowed out trees. What we have here is a large boat being pushed by
a small engine, a pontoon boat is heavy and lots of wetted surface, that really isn't aerodynamic, on the other hand the macs are light and have good clean surface on the bottom.
That's why we can get what we get in speed now with a 50hp.

getting 12-15 mph on a 26 foot boat is really doing pretty good with a 50hp, although its fun to some, to try and drag out as much performance from the little 50, there really is no substitute for hp.
But reality is as we add weight to our boats, start towing dinks, add people the little 50 is going to go slower and slower, on any boat.

whats going on in this thread, we started with a 50hp engine, that was way wrongly propped, not so much ventilation as the prop seemed to not be able to bite the water very well and slipped.
A new prop was introduced that seemed to do the job well, with 5500 rpm and not much slippage. MPH numbers in line with the pitch, and load of the boat (with ballast). Before moving the engine up,
I would run the boat without ballast to see what happens, if the engine is moved up, and the prop starts to slip, we are back to square one again.

right now with the numbers (mph, hp) with a full ballast seem to be right, and with an empty ballast, the boat jumps to 15-16mph with except-able rpm, that's about the best she will do.

If we really want this boat to perform, then a good used 2 stroke Johnson, same year, in the 70-90hp range will bolt right up, and the same cables, controller and wires can be used, Then no matter
how much weight is added (within reason) the boat will perform better.

I'm not sure about adding 90hp to an X, don't know if anyone has done it, and if the boat gets knocked over, I'm not sure it will recover when running without ballast.
Its certainly something to think about.
Mike

[trdprotruck]

Hmm I will try again with the same prop unballasted to make sure the plate height is still looking good. I'm not trying to argue, but I'm just trying to make sense of things in my head with respect of the prop selection.

- 13" Pitch prop should travel 13 inches for every revolution.
- Max RPM at WOT is 5500
- Gear ratio is 2.23:1

13 x (5500/2.23) = 32063 inches/minute = 30.36 mph (theoretical)

My actual recorded speed was 12.8 mph

[30.36 (theoretical speed) - 12.8 (actual)] / 30.36 (theoretical) = 58% prop slip.

This amount of prop slip seems high to me and I would think that the engine would have bogged down before slipping this much? Is there a problem causing the prop not to bite very well and bog down the engine? Am I missing something or is this normal?

[mastreb]

That would be the zero-load efficiency of the prop. You'd get somewhat close to that with your motor and an unloaded skiff. Getting 42% prop efficiency (the inverse of drag) is pretty normal depending on how loaded your boat is. Is the bottom clean?

[trdprotruck]

That would be the zero-load efficiency of the prop. You'd get somewhat close to that with your motor and an unloaded skiff. Getting 42% prop efficiency (the inverse of drag) is pretty normal depending on how loaded your boat is. Is the bottom clean?

The bottom is clean, but I do have bottom paint. The boat isn't loaded too heavy. Just have two people, mast, boom and sails. I'm curious if all the props I can put on my motor (small gearcase) just don't have enough surface area to push the boat and lug the engine.

I'm wondering if going to a prop like this

http://shop.evinrude.com/product/448779 ... _1975-2005

would help.

-Randy

[Crikey]

This isn't a jello ocean, on the planet Uranus you're calculating for ...... you can't do math like that with real-time situations. This is why a significant part of the energy in a rotating screw ends up as turbulence and heat.
Similarly, a painted bottom will cost you a significant portion of your hypothetical speed as well as the other mentioned factors like weight and trim.

Go through the props - report back, on the double!

R

[trdprotruck]

This isn't a jello ocean, on the planet Uranus you're calculating for ...... you can't do math like that with real-time situations. This is why a significant part of the energy in a rotating screw ends up as turbulence and heat.
Similarly, a painted bottom will cost you a significant portion of your hypothetical speed as well as the other mentioned factors like weight and trim.

Go through the props - report back, on the double!

R

LOL!!! I wasn't saying that I should be getting 30mph like my theoretical calculations, but I have been doing research and expect it to be at least lower than 25%. I'm wondering if something is wrong if I am over double at 58% prop slip of a heavy workboat.

"Slip is never a negative number, and will range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent."
http://www.go-fast.com/prop_slip.htm

-randy

[bscott]

trdprotruck--I ran a Hydrus 14 x 9 on my E-tec 60 with good success--16 knts at 8,150' It had excellent reverse thrust and a good hole shot. I just switched to a std BRP 3-14 x 9 as an experiment, then added a Power Thruster--same top end but the boat planed out faster and has less bow rise, better fuel economy in mid range and quieter exhaust. Reverse thrust and slow speed maneuverability is improved. The PT at $90 it is a better choice than the Hydrus plus you can return it if you are dissatified.

Bob

[aya16]

Yes in theory, your boat will move 13 inches, with every revolution of the prop.
slippage/ drag will knock that down some.

But real life seat of the pants stuff, My example: 50 hp 11 inch prop turning 6000 rpm, mac boat speed 12-14 mph unballasted.
same engine on a Boston whaler that's 13 feet long, that prop will push the whaler at 22mph at 6000 rpm, but lots of throttle left (will hit rev limiter)
change prop to a 15 inch and at 5900 rpm boat hits speeds of 34 mph.

That prop you linked to, looks good except its for an evinrude, may not fit your engine, the other thing is, if it doesn't slip like your prop now
your rpm's may be dragged way down, so reality you might have to get the same prop in a 10 or 11 inch pitch, if they make them (and it fits)
Now you are back to slower speeds again.

I really think there is a good possibility the prop you have now (the stainless) could get you the 16mph numbers with out ballast.
but you have to keep an eye on the rpm, if it goes much higher than 5500, either a new prop or have the one you have cupped more.

the problem your facing is getting the boat to plane, if your boat gets up on plane, and stays there, on an M that planing speed is around 17mph, not sure about the X.
The only small engine I know that will plane an M and keep it there is the etec 60, your in that same class, with a Johnson 2 stroke 50. you might be able to make it happen
with the right prop and sea conditions.

I'm not trying to argue

no this is fun, I know what you are after, and I don't advise spending lots of money with the current engine to get it. I want you to succeed, just be mind full that a good used 70 Johnson, around the same year
will cost about a grand or less. That I know will work to what you want. Heres the other thing, the older 2 strokes like yours suck gas at an alarming rate at speed.
Mike

[aya16]

The ballast makes a huge diff, in getting and keeping your boat on plane, without ballast, if the engine can get it there, maintaining speed is easier than when you try and plane with ballast
The ballast will require lots of throttle to maintain speed, without ballast the mac is like any other boat on plane, once you get there, you can back off on the throttle to maintain it.

I really don't think its a reach, that you will be able to pick up 4mph without ballast with the current prop.
lets us know.
Mike

[trdprotruck]

Well I finally got the chance to test the top speed un ballasted. The last couple of outings were with several people in the boat and I didn't have the opportunity to let it out without ballast. This past weekend on a run from Alamitos harbor to Catalina unballasted, I was able to get to a top speed of 14 mph. A little better, but still felt like I was plowing slowly through the water. On the way back home I decided to duckin behind the breakwall and I was able to attain 16.5 mph at 5500 rpm. Much closer to what I was expecting in performance and I almost felt like I was on plane, but not quite. I guess this probably the best that I can squeeze out of this motor, but I'd really like more power and speed in rough water so that I can at least match the speed of following seas and ride on the back of waves.

Thanks everyone for all of your help.

-Randy