Prop venting with the 3.0 merc.

[Suprathepeg]

OK so which of the solas should I get the Rubex or Amita?

 

[Suprathepeg]

Took the boat for a river run the other day. It runs so smooth now. Only issue we ran into is the float on the carb it a little high for slow speed running. We will adjust the float and try again. Also I'm picking up a new Solas 21P this weekend when the old man and I do the dirt track tour, might grab a 23P today as well. I'll resume the venting tests next week.

 

[Suprathepeg]

So I grabbed a Solas 21p as recommended it ran ok but tons of slip. Max speed was 34mph at 4800rpm which is the same as with the old worn 19p.

One catch is that it wouldn't clear the standard trim tab so I received the tap to give 1/4" of clearance. The wake seemed pretty violent with a lot of spray. I don't know it it's a trim tab issue or not, today I picked up the proper high rake trim tab, plan to test the boat again tomorrow.

I haven't drilled any vents yet, hopefully start messing with vent holes tomorrow.

 

[bruceb58]

Only issue we ran into is the float on the carb it a little high for slow speed running.

As long as the level isn't so high that it is overfilling, how do you think that actually makes a difference?

 

[Suprathepeg]

We had a bogging issue when warm thought it might not be holding enough fuel so we raised the float and it fixed the issue but we were a little high so we backed off on the float. No more big and no spilling fuel.

 

[steelespike]

So I grabbed a Solas 21p as recommended it ran ok but tons of slip. Max speed was 34mph at 4800rpm which is the same as with the old worn 19p.

One catch is that it wouldn't clear the standard trim tab so I received the tap to give 1/4" of clearance. The wake seemed pretty violent with a lot of spray. I don't know it it's a trim tab issue or not, today I picked up the proper high rake trim tab, plan to test the boat again tomorrow.
I haven't drilled any vents yet, hopefully start messing with vent holes tomorrow.

Have we confirmed the tach is accurate. Rpm the same for a worn 19" prop and a new 21" prop.doesn't add up.
Is the speed by gps?

 

[Suprathepeg]

Im still waiting on the tach I ordered to arrive. This one may not be dead on but it isn't totally off. The prop wasn't grabbing water very clean, lots of spray. Like I said I think the trim tab was too close to the prop.

Wasn't able to get out today, I'll try out the new tab this week.

 

[Suprathepeg]

All speeds are GPS yes.

 

[Suprathepeg]

So a bit of an update.

1. The new prop with a 5/32 vent holes really gets out of the water nice. If I punch it the boat gets on plane as fast as I can manipulate the trim with about 420lbs of people in the boat. easily as good as the old 19p. If I slowly bring on the power it comes out of the water nicely. The vent holes on the 3.0 are certainly effective. There is no question I would recommend venting with the 3.0 Mercruiser.

2. The accelerator pump jets can and should be larger on the 2BBl mercarb. My setup has the older straight jets so I can't comment on the newer 90* style but for the older mercarbs more fuel is a huge benefit. I probably shouldn't have been combining the prop venting and carb tuning in the same process/thread but summer is short and I had to tune the carb anyhow to deal with a bogging issue.

Next time out I'm going to remove the whale tale and see how the boat handles. I'd like to get smart tabs but I'm not sure I can justify the cost, maybe next year.

Next time out I'm going to enlarge the vent holes and see if that makes a difference.

 

[Suprathepeg]

The 21 is working really well, I'm thinking of trying a step up in pitch. So here is a question for you prop guys. Looks like the next step up is a 14.25x23p 3 blade. What should I expect if I try a 4 blade? Standard thinking is I should be running a 22p in a 4 blade. What difference can I expect with the 4 blade?

 

[Watermann]

Personally I think you'll start going downhill with the pitch increase to 23 with the 3.0. You're already 2" of pitch beyond what most run, I don't have a 4 blade but most people loose speed and gain bottom end with planing at lower speeds if they drop 1" of pitch from the 3 blade. I'm thinking your comment sums it up pretty well "The 21 is working really well" :thumb:

I agree with taking off the fin thing from your LU in favor of smart tabs. I run the Nauticus tabs (factory blemish) from here at iboats on my Chief that's a bit stern heavy and really think they're a great investment. Oh yeah and I run a QS Black Diamond 14 x 23p 3 blade but it's hooked up to a bit different motor and drive ratio. :happy:

 

[Suprathepeg]

Took the foil off today. No difference noticeable in time to plane, I'm sure with more than 2 people in the boat it would make a difference. Lot less spray and cleaner wake. Maybe gained 1mph. I had higher hopes for running without the foil.

I'm wondering if a 4 blade will bite better and have less slip.

 

[steelespike]

Your slip with the 21 is 28% If your tach is accurate. I think your vents are too big. Normal speed with a healthy motor and 12% slip
is 42 mph.If we assume your tach is in error and we use your numbers and solve for actual rpm we get about 3,900.
If you go to a 4 blade you won't need to vent. A 4 should give you strong hole shot, stay on plane easier at reduced throttle
all around better grip,and respond better around the dock.You will lose some top end.
I wouldn't make any prop changes until we get an accurate rpm.

 

[Suprathepeg]

Well one of the problems is that the hull is a true full V right to the to the transom, so there is no flat portion at the rear to get any real efficiency. The tach may be off, but we're talking maybe a couple hundred RPM not a significant amount, not enough to get the 28% slip down to 12%. Yesterday at about 5k we were bouncing around 39mph. Here is the interesting thing, the larger we made the vent holes the better our top speed got. In other words the bigger the holes the less slip. All the testing started with props that had no holes.

 

[Chris1956]

Supra, Some experiences I have had.

I found that 5/16" prop vents were optimum, on my outboard. Any larger, and it would ventilate.

I plugged the vents on my sterndrive prop, as I felt I did not need any slip to get to the powerband.

A straight Vee hull can be hard to trim up, which can cost you some speed. Are you able to get the nose up at all?

My '88 SeaRay 18 footer with a 4.3LX motor/Alpha Drive would hit about 42-43 MPH. If you get 40MPH out of that 3.0, I think you have done well.

 

[Suprathepeg]

The vent holes have definitely been a plus.

The nose trims up but not a lot, with the foil off I can definitely get more trim out of the leg but I don't think the boat is any more clear of the water.

Yea this hull isn't built for top end speed. I guess I gotta accept that.

I'm gonna keep my nose out for a good used 4blade 21p and see how that works. Worst case I just flip it back to the sea.

 

[Suprathepeg]

Hah picked up a 14x20p mercury 4 blade for $70 today. Looks like its in pretty decent shape. Hopefully try it out this week.

 

[StingRay_90V4]

Where exactly are you drilling holes in the prop ? Pictures of the prop with the wholes drilled in it ? Im totally curiuos on the placement of the wholes. I have a 90 hp v4 outboard that i wonder if drilling wholes in it would help it out for top speed and the whole shot at all ??

 

[Suprathepeg]

I don't have a pic right now. Basically I went off of this drawings.

http://www.drolsum.net/forum_images/Prop Ventilation Holes Info.jpeg

My holes are 1" from the boat side of the propeller (Y) and 5/8"-3/4" from the root of the blade (X)

 

[StingRay_90V4]

Any idea if a Outboard Evinrude V4 90 hp would benifit from drilling wholes in the prop as well ?