what's wrong part II

[camoto]

Background:
New to me 17' Checkmate Sportfire with a Merc 150 that has been worked over: 1:1 gears, tuner, nosecone, powerlift, punched out .30, lightweight rods and pistons.
(I just confirmed the background info on the motor last week after talking to the guy who built it up. I think the key piece of info here is the gears.)

After struggling getting the boat out of the hole, I realized that I was missing some spark on some cylinders. Cleaned all the connections, and installed new switch boxes, and had good healthy spark on all 6. Compression is 115-125 on all 6, fuel supply is good.

Today took the boat out again after doing the work. Originally had the Trophy (4-blade) 26p on there. It still bogs out of the hole. Changed to the aluminum (backup prop) 3-blade 21p and it's better but not good at all. Seeing that I just learned that this motor has 1:1 gears, I am starting to question the prop pitch and thing things may be way off here. I want to ski, and be able to move across the lake at more than 40mph. I found with the 21 on there I could slow down to almost a crawl and get on it and it would take right off, but if I stopped, it can't even get going without really playing with tilt and lift in a way that I shouldn't have to. Once on plane and accelerating, the thing really goes great. However, I am not even close to the right (from what I am told) RPMs.

Met a guy with another checkmate who had the same version motor as me except it was a mariner. He stated that his prop was either a 19 or a 17 (he wasnt' sure) but after hearing that I am thinking that he probably doesn't have 1:1 gears either.

Here are my stats for WOT:
50 mph @ 5400 RPM with the 21p

When I try to take off, it just bogs horribly. This is without a skier. I am thinking that I am going to need a MUCH lower pitch prop, but wanted to check with everyone else to see if it seems like I am on the right track. I have been over the motor a LOT and everything seems like it is running just fine and the only problem I have is holeshot.

Does this seem right? Am I on the right track? How low do I go (pitch)? I know I want to go with a 4-blade so I can yank a skier.

Thanks in advance for your expert advice!!

 

[Bob_VT]

Re: what's wrong part II

You are not going to ski with that gear ratio. Period.

That is built for speed only.

 

[twister1r]

Re: what's wrong part II

bob is correct too much gear to pull skiers.

 

[camoto]

Re: what's wrong part II

That's just what I was told it was, but something seems wrong with the numbers to me.

If you do the math on those numbers something is not right.

 

[studlymandingo]

Re: what's wrong part II

I wouldn't go to much lower in pitch, your top end is within specs. I would guess you could maybe go down to a 17 though. Venting the prop can help it spin up faster to get you out of the hole. As far as "playing with the tilt and lift" you do need to trim in all the way before you accelerate, that is what helps you get out of the hole faster. That is what the trim is for, to adjust the motor angle for the maximum performance, in when you are coming out of the hole, out as you increase speed to lift the bow.​

 

[camoto]

Re: what's wrong part II

The motor is not buried when I start, I know it is supposed to be tucked in some but not underwater. That's regardless because I have tried every single possible position imaginable to start off. Trust me, I am not THAT stupid.

Let me help everyone out. It's not the operator. I know it may be tempting to say I am doing something wrong with the motor position, but believe me when I say, this thing will NOT get out of its way.

So that leaves two possibilities as far as I am concerned: Setup or the engine.

I think I have ruled out the engine. I have checked everything on it and if you read my post above it says that this is only a problem from a dead stop. Meaning that in the VERY low RPMs it just doesn't seem to have the torque to get me out of the hole. Meaning, it feels like I need to go to a lower pitch prop.

Sounds like the gears are not 1:1, what are they? Who really knows at this point. However, seeing that this person even mentioned "gears" to me, I would think that they are not stock, hence if this thing is geared for top end, a 21 is not really a 21 now is it? So in my mind that would explain the fact that it cannot get out of the hole with a standard prop, because it's not really standard.

EVERYTHING else about this boat runs great. As long as it has about 5mph of forward motion, you would never think anything was wrong with it. In fact it runs great. Come to a complete stop and you would think that it was firing on one cylinder and 20 year old gas.

Hope this clears some of it up.

I was thinking of trading this lower end for a stock one. I really don't need this retarded setup. I just want to ski.

 

[j_martin]

Re: what's wrong part II

If you put it into gear with the engine off, and turn the flywheel while watching the prop, you should be able to figure out the gear ratio, for instance 9 flywheel turns to to 5 on the prop = 1.8 (1.78)

I would think that anything under 1.78:1 turning that prop would act like the anchor rope is wrapped in the prop for a hole shot.

hope it helps
John

 

[studlymandingo]

Re: what's wrong part II

When you said the motor shouldn't be underwater, do you mean that the cowl is near the water when it is trimmed in?

Where is your A/V plate in relationship to the bottom of the boat?

Is your prop vented?

Venting the prop can allow it to spin up more quickly which may get you over the hump.

The motor most likely needs to be trimmed all the way in when you are taking off; I can get my boat to plane without trimming all the way in, but it planes much faster when fully trimmed in.​

 

[Esox]

Re: what's wrong part II

That's just what I was told it was, but something seems wrong with the numbers to me.

If you do the math on those numbers something is not right.

You are correct about there being something wrong here. 50% slip? I would verify the gear ratio like John said. 15% slip with that setup comes out to 91Mph and 15% slip isn't all that good.​