RPMs and Two Strokes

[Faztbullet]

Re: RPMs and Two Strokes

Here ya go...Merc has a bulletin also. Also the newer props (bigger motors) have the vent holes in them from factory and you can fine tune them by changing the vent plug hole size. The Trophy and Tempest from Merc the vent holes are also 3/4 diameter!!!

 

[Texasmark]

Re: RPMs and Two Strokes

Here ya go...Merc has a bulletin also. Also the newer props (bigger motors) have the vent holes in them from factory and you can fine tune them by changing the vent plug hole size. The Trophy and Tempest from Merc the vent holes are also 3/4 diameter!!!

View attachment 174405

Mercs Laser II is part of that too as I recall and their holes now are square; maybe makes it easier to use the plugs.

Are the 3/4" holes for the 14+ diameter props run on the 150 hp up engines? I know the ones on the Laser I saw were huge. I think on my small gearbox 13+ diameter wheel, I would really have to plug it down for my engine to keep from having overkill if the holes were that large on that prop series.

For those following this and may not be in the know, the setup of the boat and the hole size/prop combination, in MY opinion would be such that when you hammer down in the hole, the engine would rev 1500 rpms give or take over what it would do without ports. The over rev condition would continue after the boat is on plane and then soon after squelch; i.e. holes cover, engine rpms drop to where they were before porting and boat speed increases as a result.

I had a Ranger pad hull (BB) and a Merc Tower with a Laser back in '89. What I said in the hole would happen, making for a very rapid shot and once up (on plane) the prop would lock up and rpms back to normal and it was just like back in the '50's when you would put your stick shift car in third gear with engine running some kind of rpm, let off the gas feel the OD unit kick in, and touch the gas again with the rpms somewhat lower depending on the OD gear ratio and the car accelerating. It's really neat and I used to love to do it....both the car and the boat.

Mark

 

[Maxz695]

Re: RPMs and Two Strokes

This diagram specifies 7/32 holes for all 50 thru 235. This is where I would start and then up the hole size to tweak it?

 

[Chris1956]

Re: RPMs and Two Strokes

Let's not overstate the advantages of ventilated props. We still need to prop the boat for the middle of the max RPM range at WOT, with a standard load. I do not think that venting a prop will allow you to add pitch, that you couldn't otherwise.

Ventilating the prop can (can) add some holeshot. Too large vents cause venitlation and loss of prop bite. I will bet that there are a fair number of boats who will not benefit from ventilated props.

 

[Maxz695]

Re: RPMs and Two Strokes

View attachment 174477View attachment 174478View attachment 174479 I,m not looking for pitch advantage it was stated that heavier bass boats like mine pictured will get an advantage out of the hole by this proceedure. I just thought that drilling the recommended 7/32 hole would be a good thing to try. I can see drilling a 3/4 hole and then adding a plug to adjust the amount of ventilation created for the weight and size of a boat. It would also seem less stressful getting up on plane forthe engine by doing this as well.

 

[Texasmark]

Re: RPMs and Two Strokes

View attachment 174477View attachment 174478View attachment 174479 I,m not looking for pitch advantage it was stated that heavier bass boats like mine pictured will get an advantage out of the hole by this proceedure. I just thought that drilling the recommended 7/32 hole would be a good thing to try. I can see drilling a 3/4 hole and then adding a plug to adjust the amount of ventilation created for the weight and size of a boat. It would also seem less stressful getting up on plane forthe engine by doing this as well.

Relieving the stress is what is is all about, in addition to getting the boat on the water. I don't know about your hole diameter; I think you are light. As I said I drilled a 1/4" in mine and It helped the hole but it was not so large as to get the rpm increase that I was looking for; I couldn't even tell you that it did, but the hole time told me it was doing some good. But, that is behind me now so it matters not.

What I would suggest is to slowly increase the diameter till you get what you want. Plugs are probably prop unique and you may find that if you over do it you can't find plugs to suit you.....but I don't know.

Mark

 

[Texasmark]

Re: RPMs and Two Strokes

Let's not overstate the advantages of ventilated props. We still need to prop the boat for the middle of the max RPM range at WOT, with a standard load. I do not think that venting a prop will allow you to add pitch, that you couldn't otherwise.

Ventilating the prop can (can) add some holeshot. Too large vents cause venitlation and loss of prop bite. I will bet that there are a fair number of boats who will not benefit from ventilated props.

Well Chris that is exactly the point. You prop for WOT performance, be it mid range rpms or in my case stretching the upper limit where my engines for as long as I can remember seem to be happiest, and then if you have too much prop to get your lead sled on the water, you port it to suit.

I never ported an I/O as I only had one and it was probably before it became something to do. Maybe on them trim tabs work best. One of the reasons I say that is that most I/O's in my vocabulary, stay in the water, unlike a BB which gets up on the pad, minimizing water resistance (drag) and allows your 60-65 mph with a 150-175 engine. Check out Forrest Wood's pics/videos of his Ranger BBs and look at how much of his boats ARE NOT in the water when on the pad at WOT.

That rig I bought new in '89 did 55 the first time out and it was only a 115 on a Ranger pad with a Laser prop. I say first time out as yeah verily it hit the number, but was over propped and rpms were on the low end of the range. I promptly returned it for a prop with about 3" less pitch which I ran for 7 years and BIL is still running it. On an I/O, I had a friend with an 18' Searay with the 165 inline 6 and 45 was tops with 2 on board and best trim.

Mark

 

[Maxz695]

Re: RPMs and Two Strokes

Agreed and being my bottom is like a piece of plywood (Long and flat) as well as wide without trim tilt I see no reason not to try the modification esspecially when it was introduced i the add above by mercury. I have a SS prop that works well as so far I can tell. (havn,t had much water time due to rebuilding water damaged internals) and would hate to mess it up. So I will after maiden run consider the recommended 7/32 and your suggestion of 1/4 if it shows promise after that.

 

[Faztbullet]

Re: RPMs and Two Strokes

Mercs Laser II is part of that too as I recall and their holes now are square; maybe makes it easier to use the plugs.

The early style Laser2 used the square hole and all the new ones have round as they use the PVS plugs. The square port was a better prop out the box for performance but are harder to find as these were aggressive progressive pitch props that once reworked on a pitch block is no better that a regular SS prop..

Are the 3/4" holes for the 14+ diameter props run on the 150 hp up engines?

They are 3/4 on the 13" and 14" Lasers for the Xr6 series gearcase.....
I guess I was under propped big time as I turned 6000-6500rpm on a 14.5X 28 pitch chopper

 

[Texasmark]

Re: RPMs and Two Strokes

Let's not overstate the advantages of ventilated props. We still need to prop the boat for the middle of the max RPM range at WOT, with a standard load. I do not think that venting a prop will allow you to add pitch, that you couldn't otherwise.

Ventilating the prop can (can) add some holeshot. Too large vents cause venitlation and loss of prop bite. I will bet that there are a fair number of boats who will not benefit from ventilated props.

I don't know about others but I'm not overstating anything. It has worked for me. My point is that once you propped to suit your WOT requirement, middle for you charlie, upper end for me, if your hole shot is sloppy, port it. Other thing that I have mentioned and experienced is the loading of the 2 cycle engine and the fact that if you can keep the load light performance is definitely enhanced. Works for me.

On the "there are a fair number of boats who will not benefit from ventilated props" I totally agree. I report on things that effect me and I am one application in millions of boats/boaters out there.

HTH,
Mark