prop RPM question Black Diamond Mercury

[CWKboat]

Hi,

I have a perplexing issue with props/set up. The boat is 1978 Checkmate Tri-mate 17 footer with a 1987 Mercury 115 with a recently rebuilt powerhead. The engine came with a Stainless 21 P prop, and when I got the engine set up on the boat, the 21 P would turn about 4700 RPMs WOT at 47 MPH. Knowing that I needed to have this engine turn higher RPMs at WOT, I then purchased a 19 P Mercury Black Diamond aluminum prop fron iboats and tried it out yesterday. To my surprise, the engine would not go over 4000 RPMs with the 19 P.

I have not had a chance to verify that compression is good, but asuming that is so, is there any conceivable reason why a 19 P prop would turn less RPM than the 21 P at WOT? All the plugs are firing at idle at least, I did verify that. Could I have lost a cylinder or two to account for the RPM discrepancy, or is there anything peculiar to the Black Diamond Flo-Torque II prop itself that might account for the RPM loss from the 21 P - I'm totally baffled.

The midsection is a 20" and is set one hole higher, FYI.

Thanks for the replies or suggestions!.

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Black Diamond Prop in question is a 13X19 Aluminum..

 

[Ski'nBlind]

Re: prop RPM question Black Diamond Mercury

I have a similar setup, an 84 Checkmate Enticer and 83 115hp "tower". I also have a 21p stainless Merc prop. With a full tank of fuel and 2 in the boat I can trim it to 54mph (guage) and 5700 rpm. I would venture to say there is nothing wrong with the boat or the props. The first thing that comes to mind with yours is....are you sure your tach is accurate?????? If you've been around boats for a while you would know if your tach is off and the sound of the motor "screaming" at 6500 rpm when it shows 4000rpm on the guage. If you are pretty sure your tach is accurate the prop is not the issue on your motor. How is the holeshot? Does it seems to power out quickly then sorta fall flat on performance or does it stumble and fight itself to get on a plane?

These motors have a high speed and low speed stator winding, also seperate windings for charging the battery. If a motors idles great but wont pull rpm it could be the high speed stator windings. You will have spark at idle like you have checked. Do a search on this forum it is discussed so much you will fall asleep reading. Other things to check and eliminate as possible causes:

- Fuel pump, fuel hose, primer bulb, does it make any difference to squeeze the bulb at said 4000 rpm. Is the bulb hard while running the boat?

-Have you checked the max timing, performed a linc and sync...just to make sure its set right.

-Read the spark plugs. Are they black, wet, sooty, or god forbid clean!

- A compression test is probaby the first place I would start to make sure you have a "good" engine to start spending time and money on.

I haven't played around with setup on my boat (yet) as it's plenty fast for fun with the kids. No jackplate, no setback like lots of guys do to find more speed. The centerline of my prop is 3-3/4" below the bottom of the pad of the boat as measured with a 4 foot level along the bottom of the boat. I don't have any cavitation or prop blowout issues with mine, and that's fine for right now. Measure yours from the bottom of the boat (with a straight edge placed along the bottom of the boat) to the "point" of the lower unit ahead of the prop.

I think you'll get lots of responses if you post some more detailed info.

 

[Chris1956]

Re: prop RPM question Black Diamond Mercury

OK, A damaged prop can turn higher revs than a good one. However, my experience with 21" pitch props indicates your is acting normal, as RPM and MPH as where I expect them to be.

Swap props and see if the RPMs return with the 21. If not, you may have a bad cylinder.

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Ski'nBlind and Chris,

Thanks so much for the detailed reply and suggestions, I will put the 21 back on and see if RPMs return to baseline. Will also verify/check compression and work from there. It might be the weekend before I can check back, but I will be sure to do so.

Thanks!

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Ski,

To answer a couple of your questions, I'm fairly certain the tach is accurate as 4000 RPMs definitely sounds like 4000 and the motor is not screaming like my other inline 1500 does at 5500, but more like cruising. The hole shot seems OK and it jumps on plane quickly, but then just tops out at 4000 with the 19P. I really hope it wouldn't be compression related, but will verify that and also link and sync as well. I'm wondering now if the high speed windings could be the culprit, but I will start eliminating things this weekend hopefully and report back..

 

[Ski'nBlind]

Re: prop RPM question Black Diamond Mercury

Cool....no worries.

I never really believe anything is rebuilt unless someones got specs, receipts, or both....and that's maybe. You might get lucky and find out that your carbs aren't opening all the way and that there really isn't even anything that needs repaired. Just need to check as much as you can, one step at a time.

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Ski,

It's definitely rebuilt by a reputable rebuilder as I bought it with a bad #6 from water intrusion and had it professionally rebuilt with one new piston and the rest of the holes re-ringed and honed. I did verify that the carbs were all opening all the way. Like you said, I'll do one thing at a time, it can just get so annoying as well all know, LOL. Thanks again..

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Hi, to update, I did confirm that the compression is 120 lbs in all 6. I then removed the idle stablizer, replaced the rectifier that tested bad, and also replaced the ignition switch which was acting up. This enabled me to recover the missing RPMS and exceed 4000. I put the 21 P back on and the boat would run about 50 MPH trimmed out at about 4900 RMP. Then I put the Mercury 19 P Aluminum back on and it would go about 46-47 at about 5100 RPMs. This was with the motor set at the second hole from the lowest.

So, yesterday I raised the motor another hole and now the 19 P gets about 48 MPH at 5200 RPMs. I do have a hydrofoil stablilizer on the lower unit, so I may try removing that and see if I can get a further RPM increase in the event that it is causing drag. I'm also thinking about trying to raise the engine another hole and see if I can get 300 more RPMs out of it at WOT to put it at the top end of the WOT RPM range. The hole shot is excellent at the current setting.

Thanks for the help and replies, I will update further when I have time to take it back out hopefully later in the week.

 

[Chris1956]

Re: prop RPM question Black Diamond Mercury

You can't have a whale tail on a speedboat! it ain't natural. You want to carry the bow as much as possible. Ditch the trim tabs as well.

Look for a PTT unit for that motor. On my '74 16 Avenger (similar to Checkmate), with my Merc 1500 TOP, trimmed down max speed was 45MPH. Trimmed to max was 60+MPH.

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Chris, I'll take the whaletail off this afternoon and then plan to raise the motor another hole and see if in a best case scenario I can run the 21 P prop maybe. The whaletail came with the 87 115 that I mounted on the Checkmate and I just didn't think at first to take it off. Come to think of it it does throw up an awful lot of water to the right and left when trimmed up at speed. I figured I should be getting more out of this combo and hopefully will. Thanks for the reply!

 

[j_martin]

Re: prop RPM question Black Diamond Mercury

A whale tail functions to get the stern up at slow speeds, and is helpful with an underpowered boat having trouble getting onto plane.

If I want to kill the performance of my high performance bass boat and drive slow, I just put 150 - 200 lbs of sandbags in the bow. ( I do that when I take the 80 year old preacher fishing)

my 02
John

 

[Ski'nBlind]

Re: prop RPM question Black Diamond Mercury

CW, you do have power trim...yes? It sounds like you are slowly finding speed and more importantly rpm for that motor. I am still confused about how deep you are running that lower in the water and hope you post how deep the centerline of the prop is off the bottom of the boat. Lots of guys at the Checkmate forum are looking for max speed and are running 2" or less centerline to the bottom. I use my boat for pulling the kids and top speed isn't really an issue. I am surprized by your numbers cause they just don't make sense on the prop slip calculator. 6-8% slip is pretty darn good for a speed boat running out of the water but you have negative numbers. I wouldn't think running the lower deep in the water could mess up slip numbers that much, but I just don't know. I know you say you keep raising the motor and your getting rpm which is good but at some point the prop will "blowout" on turns or trimming too much if you go too high.

Calcutator http://www.mercuryracing.com/propellers/propslipcalculator.php

My boat porpoises badly at tubing or skiing speed, even with 2 in the bow. I put a stingray on and it helped tremendously at those speeds. If it took away a few mph on the top end I'm ok with that. I don't have allot of spray or a big rooster tail when trimmed out. I do have some chine walk issues over 50mph which may be compounded by the stingray but haven't tested that yet. But the speed and rpm I mentioned with my setup is with the stingray on.

What is Checkmates listed weight for the Tri-mate? I thought it would be close to mine of 750lbs.

Did you check (and set) your max timing after removing the idle stabilizer?

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Ski,

I did check/verify the max timing and it's 20 at cranking speed. Not sure about the weight of the boat, but will check and post hopefully tomorrow. It seems pretty light though.

When you say center line of the prop, do you mean the hub itself? On mine, with raising it to the 3rd hole, the cavitation plate on the lower unit now looks to be an inch or two above the bottom of the transom with the eyeball test. I'm getting a lot of spray presumably from the foil. I also have a one of those swim platform type ladders on the back on the port side and who knows, that could be plowing a little too. It will run 50 MPH with the 21 P, but only gets to about 4900 RPMs - but I have not tried the 21 on the third hole yet, or the fourth if I raise it again. The boat really has a great hole shot with the 19P, and I'm just hoping that I can find 300-400 more RPMs somewhere.

PS. I do have integral PTT.

Will check on the weight of the boat and post again..thanks for the reply.

 

[Ski'nBlind]

Re: prop RPM question Black Diamond Mercury

Centerline of the propeller shaft (right thru the hub). The "point" of the lower unit towards the transom is the centerline, I reference that cause it's easiest with a 4 foot level stuck to the bottom of the boat and a tape measure. My cavitation plate is at least a couple inches up from the bottom of the boat but most people use prop centerline as a comparison. Lots of Checkmates run between 2" and 4" below the bottom of the boat. I have the same ladder on the port side and it throws up lots of water at slower speeds from dragging.

At first I was thinking it sounded like you might of had an ignition issue not getting rpm but seems like you are heading in the right direction. From your first post with the engine all the way down to where you have it set last with the 19P prop you have gained 1200rpm...that's allot! So it may just be in the set-up. When I had my 19P prop on earlier in the year I had to back down the throttle to keep the rpm from going over 6000 and your boat should do the same. Again, my prop centerline is 3-3/4" below the

Regarding the ignition....about 2 weeks ago I fried one of my high speed stator windings on my (new) month old stator. I didn't know for sure out on the water when it happened but tested it when I got back home. Anyways, the motor was still running on 3 cyl's and I cruised it back to the launch under power. The guages showed 38mph and 4400 rpm...kinda similar to what you were experiencing from your first post. Have you already tested the ignition components according to factory or CDI specs? If not, check every wire, check ohms and volts on every component just to eliminate the possibility it as bad.

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

Ski,

I went home yesterday and raised the motor to the 4th hole (only one left so I assume I will stop there). I also took the whale tail off. I took it out last night and the 19P, with the old, original speedometer that flutters somewhat, was just hitting 49-50 MPH and would make 5400 RPMs. I trimmed up to a point that I could audibly hear the motor get louder, but it did not cavitate. I stopped because I was afraid of losing water pressure. I'll have to get a level and check the measurement from prop hub to bottom of transom, but it looks like it couldn't be any more than 2-3."

I didn't have a DVA meter to check output of the ignition system at higher RPMs. I can look at that again and check the ohms of the stator. Also, re the size of the boat, it is a 17 footer and I couldn't tell from the plate how much it weighs, but I think it said 790 lbs for max occupancy w persons and 1240 for persons, gear, and engine. Not sure if that helps or not.

If this is all I can get out of it, it still seems pretty fast and at least now I'm in the ball park as far as RPMs. I do have some bow rise with the hole shot and will have to see how it does with others in the boat or with pulling a wake boarder. I will also throw the 21 P stainless back on there and see what it does at the new height setting with the thinking that maybe I can trim it out further with the increased bite. Thanks for your help and suggestions!

 

[CWKboat]

Re: prop RPM question Black Diamond Mercury

To update, last night I put the 21 P on and tried it out. Bear in mind that the motor is now raised to the 4th hole. My speedometer is not the greatest and flutters when it gets past about 45 MPH. With the 21P, I was able to to trim out to 5400 RPMs and the speedometer needle was bouncing somewhat, but it looked like I was getting low 50's or thereabout. I can probably trim a little more, but I got antsy with the motor setting so high and I was running out of straightaway as I had to go to a smaller part of the lake with less chop. But looks like I can just make 5500 on a good day with the 21 P, so I guess I'm going to enjoy it for now and maybe try a manual jack plate next year and see how that does with some setback. It seems plenty fast though at this point and this has all been a primer course for me in how engine set up works. Thanks for the help and replies!

 

[Chris1956]

Re: prop RPM question Black Diamond Mercury

Sounds like you are dialed in, or real close. One more factor. As you raise the motor up on the trailer, it can lose some of it's ability to lift the bow, when trimmed. So you will gain speed due to less drag of the gearcase, but lose speed since you cannot lift the bow out of the water to reduce wetted area. Obviously a compromise is best. Use a GPS to measure speed, and keep notes on performance at various settings.

In additon, if you plan to pull skiers, I would test your settings. With the motor raised up, sometimes the motor will ventilate when trying to pull a skier.