RPMs and Two Strokes

I have discussed this topic more than once. I helped a guy with an OMC Sea Drive not too long ago that wouldn't respond to the throttle to get his boat up on plane....all he did was to allow the prop to spin up in the hole and the engine responded to the throttle. I suggested that he port his prop like the Merc. Laser II. As I recall that is what he did, either ported the one he had or bought one with ports. He left happy.

I have been running a 24P prop on my little tin lizzie with my 90 Merc. With the boat, hull, water conditions, altitude, weight and load, and the engine running properly, I could get up on plane pretty fast and move out. I could even leave the trim out, like it was set when I was running at WOT and get it to pop out of the hole as the engine would suck air (ventilate), unloading the prop, the engine would rev, the prop would bite, etc. etc. and in a flash you were up and away.

Today, in test driving the repairs to my engine I put a Ballistic 21P on the engine. In a straightaway this prop would let me run well over 6000 rpms, but I would immediately cut it back to around 5500 or a tad over.

So, just for fun, I would be idling along, trim still out in the WOT position (all the way out), give it some throttle to get up around 2500 and then smack it to the firewall. The boat would leap out of the water, would porpoise a couple of times while the engine at times would hit 7k ventilating but as soon as the prop caught (a second or two) the front half of the boat would literally leap out of the water; speed would pickup so fast that it would throw you against the seat. My sweetie just went nuts, hanging on to me.....YEAH!!!!!!! In no time, a couple of seconds, I was over 45 mph, loaded rpms well over 6k and at that I would cut the throttle back to get the rpms in range.

Point here is that a 2 stroke that isn't hindered by too much pitch for the work to be done can be an awesome power source.

So folks, prop them so that they can run where they love it and that is at the higher rpms. If you load them up, they bog down and die......but I'm not an expert, just commenting on the fun I have with my boat.

Surely a boo bird or two will jump in and poo poo on my thread. Well the lake is about 1200 acres, there were 3 boats on the lake, I avoided the other two who were fishing at the dam, we were both wearing our life vests, we were sitting on a vinyl covered and padded bench seat with back rest in normal bass boat fashion and the boat is BIA rated for 130 hp....I have a 90. Smile!

My 2c,
Mark

Re: RPMs and Two Strokes

It sounds great. Having the extra unused RPM at WOT would be gretly appeciated if running more weight or more passengers as well. Even if you decide in the furture to get a heavier boat this set up will work possibly. Just keep it from ventilating as this in time mixed with other issues like un known water intusion can cause severe damage to the crank and bearing wear with the lack of lubericaion (thined fuel by water). My first crank on my 40 HP as destroyed as a result of this. My procraft was a bit heavy for the engine and would ventilate when turning to sharp at WOT. After fixing the water damage and replacing the crank it was an OK motor and ran for the most part very well. Thats when i aquired the 70 HP powerhead (the rest of the engine was crap and turns out i had to rebuild the powerhead (Severe water damage) and am almost finnished. Started it yesterday after a swichbox replacement and Finally heard it rev up right with no backfiring. (I was concerend that I may have over honed it, but looking good at this point). I,m having issues getting the dang exhaust cover to seal. I have all three gaskets either here or on order at Tracker up the road. I know the feeling of thinking A motor might be better in the scrapyard and just to through your hands up and say ta hell with it!. I,m glad I stuck it out although these gaskets are driving me into the poor house. I think I can get it right this time if not it,s going to have to wait until sping. My Bday and re registration run up on 11/11 and I was hoping to get at least one run as becuase of the $$ Cha $$ ching spent on the motor parts has dried up the well here. Enjoy the boat and motor, and have many a safe fun trips.

Re: RPMs and Two Strokes

Maxz695 said:

It sounds great. Having the extra unused RPM at WOT would be gretly appeciated if running more weight or more passengers as well. Even if you decide in the furture to get a heavier boat this set up will work possibly. Just keep it from ventilating as this in time mixed with other issues like un known water intusion can cause severe damage to the crank and bearing wear with the lack of lubericaion (thined fuel by water). My first crank on my 40 HP as destroyed as a result of this. My procraft was a bit heavy for the engine and would ventilate when turning to sharp at WOT. After fixing the water damage and replacing the crank it was an OK motor and ran for the most part very well. Thats when i aquired the 70 HP powerhead (the rest of the engine was crap and turns out i had to rebuild the powerhead (Severe water damage) and am almost finnished. Started it yesterday after a swichbox replacement and Finally heard it rev up right with no backfiring. (I was concerend that I may have over honed it, but looking good at this point). I,m having issues getting the dang exhaust cover to seal. I have all three gaskets either here or on order at Tracker up the road. I know the feeling of thinking A motor might be better in the scrapyard and just to through your hands up and say ta hell with it!. I,m glad I stuck it out although these gaskets are driving me into the poor house. I think I can get it right this time if not it,s going to have to wait until sping. My Bday and re registration run up on 11/11 and I was hoping to get at least one run as becuase of the $$ Cha $$ ching spent on the motor parts has dried up the well here. Enjoy the boat and motor, and have many a safe fun trips.

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Thanks.

I thought about having reserve rpms also. I like the idea besides this makes for a stellar hole shot and my sweetie likes to feel the effect of putting along at midrange and hammering down on the throttle....well, I like it too. After all we go out for an hour or two's fun. That's what it's all about. Grin

Mark

Re: RPMs and Two Strokes

Texasmark have you done a port job on a prop without them ,and if so what is the best way to do it? I have a Merc 70hp 1987 that I run a 13x19p on and it is not ported and I would like to see if it would help this prop work better on the performance of it. Got several props and just never thought about trying one that is ported just to see what happens.
Just love to try things out.
Oldman570

Re: RPMs and Two Strokes

If I'm understanding this correctly, you had a 24p on then you fixed the motor and put a 21p on? Seems backwards to me.

Re: RPMs and Two Strokes

Propping to the high side of your max rated RPM is good. Propping several hundred to a thousand RPM beyond that is bad. Once you get out of the recommended RPM range you are likely not making any more horsepower. My old Johnson crossflow V4 90hp was originally rated to run 4500-5500 RPM. I've propped mine to peak at 5800 RPM with a light load and it is very happy. Unless I had a very light hull (I don't) and was seeking max top end (I'm not) running beyond 6000 would burn more fuel and would probably be slower in the end... although acceleration would probably be DYNAMITE!

Re: RPMs and Two Strokes

less pitch gives less stress turns less water faster and creates more RPM

Re: RPMs and Two Strokes

oldman570 said:

Texasmark have you done a port job on a prop without them ,and if so what is the best way to do it? I have a Merc 70hp 1987 that I run a 13x19p on and it is not ported and I would like to see if it would help this prop work better on the performance of it. Got several props and just never thought about trying one that is ported just to see what happens.
Just love to try things out.
Oldman570

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Somewhere on this site is a diagram of an OMC SS prop that someone ported with the dimensions specified. If you title a thread as such someone may produce it for you. Some props come ported from the factory and they are for sale on here.

I had the 24P Ballistic which ran my boat at 48+ GPS but to get that top speed, the hole shot was not a burden, but I thought I could improve upon it....no need to labor the engine if you didn't have to and as you said I just wanted to do it since I had a Laser prop years ago that was ported and really enjoyed the way it performed.

I bored 1/4" holes where the OMC diagram indicated they should be and in doing so had to dodge the ribs of the prop that tie the outer diameter to the inner diameter. I had a tungsten carbide tipped drill bit and a drill press, but even at that, SS is a tough prop. But I got her done and the ports did help the hole shot.

On changing props during my repair these last few days, not knowing that I had found the problems, I wanted to help the engine as best I could and I had the prop handy. Additionally, when I bought the 24 I boated alone and now have a sweetie who loves to go out with me so I figured I'd lighten it up by going back to the 21. I did not port the 21 because the pitch is low enough that it really is less pitch than this boat can handle so I just let it be.

That's about it.

Mark

Re: RPMs and Two Strokes

Dave1027 said:

If I'm understanding this correctly, you had a 24p on then you fixed the motor and put a 21p on? Seems backwards to me.

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As I said in another reply, I opted to install the 21 I had purchased years ago before I bought the 24 to help the engine get the rpms up just in case I didn't find the smoking gun......the crud blocking the high speed jet in carb #3 which, as expected was just that.

Other thing is that we cruise a lot now and I don't need to prove to myself how fast this boat goes any more. My sweetie likes motorcycles and since I outgrew that years ago, this is our motorcycle. So she likes to hear the rpms and likes to feel me smack the throttle and force her against the seat back. As I said, I think it's fun too. The 21 does a better job of it than the 24 as the engine is considerably unloaded and takes the throttle with a jolt!

Mark

Re: RPMs and Two Strokes

excuse my curiosity, but when you say "port the prop", what does that refer to? many years ago i used to run rc hydros, and would spend hours grinding and balancing props, and even found info on drilling tiny holes ( tiny props!) in the trailing edge to get more "traction" without loosing much top end performance. ( this was before the internet!) . just curious

Re: RPMs and Two Strokes

Chinewalker said:

Propping to the high side of your max rated RPM is good. Propping several hundred to a thousand RPM beyond that is bad. Once you get out of the recommended RPM range you are likely not making any more horsepower. My old Johnson crossflow V4 90hp was originally rated to run 4500-5500 RPM. I've propped mine to peak at 5800 RPM with a light load and it is very happy. Unless I had a very light hull (I don't) and was seeking max top end (I'm not) running beyond 6000 would burn more fuel and would probably be slower in the end... although acceleration would probably be DYNAMITE!

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I don't know what the torque curve is on this engine. When I run up the high rpms, I am in the hole, motoring up to almost a plane at around 2500, trim is still out where it was when I ran before I shut her down to hole shoot again. I firewall the throttle and she ventilates shooting water and running up the rpms, the prop grabs, then vents again, and grabs again essentially launching the boat like a pogo stick. This lasts for about 3-4 seconds whereas the boat has jumped onto the surface and is hauling ..... As soon as it does, I cut it back to a reasonable 5500 to 6000 and after a couple of laps of that on this little lake, I cut her back to just over planing and we do some sight seeing for awhile and repeat the process. Loads of fun. Obviously wind and waves dictate conditions as this little boat is not a heavy deep V and can beat you to death, but on this trip out the weather was absolutely perfect.

On fuel economy, I had about 5 gallons in the tank and added 5 on the way. After a couple of hours of fooling around, I still had what appeared to be the 5 gal. I had before I started. This Merc looper, on this light hull is a real economical engine and behind my Silverado 4.8 costs me less than a gallon of mpg.

I realize that what I do and what I do it with is unique and doesn't apply to the majority of rigs and boating experiences. Just so happens I wanted a light boat for economy reasons, easy to tow and adapted to shallow, small lakes, with all the bass boat amenities, and 8 years ago I found this rig that was 2 years old in pristeen shape for $6k. Have been happy ever since.

Have fun.
Mark

Re: RPMs and Two Strokes

Mark, Can you advise on the year of the motor and the size and type of the boat? I would also like to know what the gear ratio is. In my experience, a 24" pitch prop is too much for an IL6 on a boat that is rated for the motor. Drilling 5/16" vent holes helps a bit.

FWIW, I have had the best performance with a 21"P prop with a Merc 1500, on a 16' bassboat-style vee hull (rated for 85HP). A 23"P prop performs OK, suffers in hole shot, and has no real speed advantage, due to RPM limit.

Re: RPMs and Two Strokes

If you have a engine rated at 150 HP at 5500 RPM and you run it up to 6500 that does not mean its "not making any more HP " . Maybe its making 130 HP at 6500 RPM but it might only take 130 HP to run the speed your'e running at. You could prop it so that it is going as fast as possible at the max rated HP but what are you proving by doing that? You end up with a slower accelerating boat due to the loss of mechanical advantage that a lower pitch prop gives you and you end up with a prop that slips more in the water as its harder for a higher pitch prop to move water efficiently. Propping to max speed at max HP is a terrible way to rig a boat unless you are building a top speed only boat.

Re: RPMs and Two Strokes

Bump Answer has been relayed to the new thread

Re: RPMs and Two Strokes

wired247 said:

If you have a engine rated at 150 HP at 5500 RPM and you run it up to 6500 that does not mean its "not making any more HP " . Maybe its making 130 HP at 6500 RPM but it might only take 130 HP to run the speed your'e running at. You could prop it so that it is going as fast as possible at the max rated HP but what are you proving by doing that? You end up with a slower accelerating boat due to the loss of mechanical advantage that a lower pitch prop gives you and you end up with a prop that slips more in the water as its harder for a higher pitch prop to move water efficiently. Propping to max speed at max HP is a terrible way to rig a boat unless you are building a top speed only boat.

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Totally agree. Since HP is rpm x torque x a constant of proportionality, one needs to know the torque available at the rpm to be able to determine the available HP which you never get. Then one needs to know the ability of the propeller to transmit that HP into usable thrust. So we have speed props and power props. Then there is the bass boat prop with ports which give you the best of both worlds....you prop it for WOT and port it for the hole shot. Yes!!!!!

Mark

Re: RPMs and Two Strokes

Thanks mark so on my 17.5 procraft bass boat running my 70 HP with the apearent stock SS 10 5/8 X 14 P can be ported as I last ran about 5200 RPM at WOT? Engine at present under last stage of rebuild. Only had it out twice before discovering major internal damage. I will in the future be wanting to investigate the porting and proceedure for it

Re: RPMs and Two Strokes

Chris1956 said:

Mark, Can you advise on the year of the motor and the size and type of the boat? I would also like to know what the gear ratio is. In my experience, a 24" pitch prop is too much for an IL6 on a boat that is rated for the motor. Drilling 5/16" vent holes helps a bit.

FWIW, I have had the best performance with a 21"P prop with a Merc 1500, on a 16' bassboat-style vee hull (rated for 85HP). A 23"P prop performs OK, suffers in hole shot, and has no real speed advantage, due to RPM limit.

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Engine is '02 90 triple looper; gearbox is 2.33. Boat is a bass boat built by an Arkansas co. name of Rivertrail and sold under the name "Travis Edition" and sold by Travis Marine in Lewisville, TX. I have posted picks several times on here. Even though it's 17.5' and has a very wide beam, made of alum. and weight of around 700 empty, it has a river bow (long shallow slope) which means that for the first 3' of the boat there isn't much boat. There is no setback but the transom has a high angle such that at max trim I can't blow the prop out. The engine is up one notch which sets the AV plate 1 1/2" above an extended straight edge from the bottom of the boat.

It has a stepped hull like a FG pad boat but being alum the "pad" is formed in 3 distinct steps so that as a function of speed the boat lifts out of the water. My Avatar is a pic of it at WOT and you can see by the wake there isn't much boat in contact with the water and this was at a GPS (Magellan Explorist 200) of 48.5 mph with the 24P prop and 5600 rpm. I boated alone with 2 batteries, usually 5-10 gal of fuel, 300# engine, tackle and 250# of me. I boat on a 1200 acre lake that is rarely rough. Usually like in the Avatar, nice and smooth.

I bought the boat used and it had a 17P Ballistic. First time out I almost got whiplash. I couldn't believe the PO had this thing like this for 2 years. I immediately went to iboats and bought a 21 guessing from running some numbers this would work. Well, it did a lot better, but I still had a lot of rpms left over before the throttle was firewalled. So I bought another Ballistic in 24P. This put my rpms up at 56-5800 at WOT which was great. Hole shot was a little slow, but not much; like about 4-5 seconds.

After several years of running that, I was fooling around one day and decided to port my 24. I got the dimensions off here of an OMC SS porting which seemed ok and drilled my 3 ea 1/4" ports. I selected that number as I could always up the diameter if this wasn't enough. This helped the hole but they were not so large as to cause detectable rpm surging as was the case with a Laser I ran on another boat with factory porting.

Next thing you know my wife died, decided I didn't want to be alone and found this fabulous girl friend. She and I have a great time and as I said yesterday this is our "motorcycle". With her added weight in the boat the 24 did seem a bit much so when I was working on my recent problem, I just decided to put the 21 back on that surely wouldn't hurt. It didn't.

So, that's it from here.

Mark

Re: RPMs and Two Strokes

The boat my original 40 2 stroke came off was an aluminum 17.5 bass boat that the guy scrapped. I got the engine with bad electrical for $50.00 I searched and found a fiberglass bass boat with a junk trailer for 500.00. I could imagine it,s much heavier than the aluminum was. The front of the aluminum boat was not a V hull like my fiberglass it had angles that came off the bow somewhat then started to round outwards from there. It needed alot of work and I was short on cash so I had to let it go.

Re: RPMs and Two Strokes

nwcove said:

excuse my curiosity, but when you say "port the prop", what does that refer to? many years ago i used to run rc hydros, and would spend hours grinding and balancing props, and even found info on drilling tiny holes ( tiny props!) in the trailing edge to get more "traction" without loosing much top end performance. ( this was before the internet!) . just curious

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Porting is the term I use and maybe industry does too whereby you drill holes in the side of a thru hub prop about an inch behind the leading edge of the blade and half an inch below (on 13-14" diameter props for that approx dimension).

What it does is it allows exhaust gasses to pass over the blades when there is none to slight forward water over the blade like would be experienced in a "hole shot" (getting on plane) and works wonders on heavy stern bass boats that are propped for WOT, but have all this baggage that they have to get going from an idle throttle.

The exhaust gasses reduce the volume/density of water the prop has to push against thus reducing the HP required to twist it, reducing HP reduces the drive requirements and since rpms are a part of the HP equation, it allows the rpms to build rapidly even though you have a high pitched prop.

Once up and running, the water pressure created by blow by water from forward travel of the boat seals off the holes and the prop only sees solid water. It locks in and away you go.

Chinewalker has a 20H Merc for an Avatar with a Quicksilver lower unit. The exhaust on that comes out aft of the prop and if you could, porting would have no effect on it's performance. When I was a young lad, that and the 4 cylinder big brothers is what I experienced (as a spectator, friend of the owner) as racing engines down in Houston, TX.

HTH,
Mark

Re: RPMs and Two Strokes

Maxz695 said:

Thanks mark so on my 17.5 procraft bass boat running my 70 HP with the apearent stock SS 10 5/8 X 14 P can be ported as I last ran about 5200 RPM at WOT? Engine at present under last stage of rebuild. Only had it out twice before discovering major internal damage. I will in the future be wanting to investigate the porting and proceedure for it

Click to expand...

Porting is for the hole shot, has no effect on top end other than possibly some ventilating at high trim angles in rough water or hard turns.

Mark