Please help me find my revs

[hundee r1]

Guys, please help!!

I currently live in Australia and need help with my Force 90hp.

If I haven’t made you close this window yet, this is my dilemma:

I have a 1992 90 HP force on a 17ft Fishing boat.

Boat is a deep 19 degree V, weighs around the 2400-2500LB ( attached a pic of the same hull )

I have rebuilt the motor, run it in and seems to be in very good tune of now. Everything on it has been cross checked and runs fine all the way to redline.

Problem is redline is 4000rpm and max's out around 50km/hr.
had a shipwright look at the mounts and the motor is perfect height on the transom.


I’m at a stage where the only thing left to explore is a prop swap.

Atm its running a 13.25 x 17P aluminium 3 blade standard Merc thru the hub prop.

Any ideas where my 1000rpm can be found?

edit ; Revs have been checked with snap on tool, and speed is GPS verified

 

[smokeonthewater]

Re: Please help me find my revs

you say you rebuilt the motor but have you done a compression test to make SURE all is well?

Also have you been reading the plugs to make sure it's jetted right... IE hold it wot for 10-20 seconds and then turn the key off and pop it in neutral.. then pull all plugs and look at em... black or wet = rich and white = lean

Have you done a link n synch, and verified wot timing?


What's a kilometer... J/K... getting 30 mph for the metric challenged LOL

 

[hundee r1]

Re: Please help me find my revs

Yep, compresion is great at 145-150 on all 3 cylinders
yep been lyncd and synced, timing has been verified at both WOT and Idle
50km/hr: 31 MPH: 27 Knots

Plugs will be checked next time out on the water, been running 25;1 oil due to run in, but the above figures are no diffrent to before I rebuilt the motor.

I bought the boat, started investigating why it wouldnt rev past 4000rpm. Thats when I discovered broken rings amnd low compression on 2 cylinders.
Rebuilt the motor, replaced fuel pump,tank pick up, stator, reg/rec , non of it has made a difference.

Im thinking wrong prop for the motor had the previous owner labouring the engine and thus killing the rings.

 

[hwsiii]

Re: Please help me find my revs

hundee, I show your Gear ratio to be 2.31, if this is true then my numbers are correct, so please check this and verify by turning the flywheel 1 complete revolution and counting the revolutions of the prop at that time. I see where you say you have double checked the RPM's and GPS speed, if these are correct then you either have a problem with the gear ratio on your motor or the pitch in your prop. It is impossible for you to be getting 31 MPH with the 2.31 Gear ratio with the 4,000 RPM's that you are showing and a 17" pitch prop. I have run a prop analysis to show you why I say that.



Real Speed 1 shows your 31 MPH to be faster than the theoretical top speed of 28 MPH, with a Prop slip of - 11%, and that is impossible.

2 shows your 31 MPH to be a realistic number Only if your motor is turning 5,000 RPM's, it also shows a more realistic slip of + 11%.

3 shows your 31 MPH to be another unrealistic number even if your prop is a 19" pitch, and 0% slip is also unrealistic.

4 shows your 31 MPH to be realistic only if your prop has a pitch of 21".

So, there is definitely a major problem with one or more of the numbers you have given us to work with.

As a normal course of events prop slip on a boat like yours should run between 6 % to 14%, I believe about 8% slip is veryyy good, and at 15% you should change props to one that is better suited for your boat and your uses of it.

I almost always run a speed analysis for any threads I look at on Iboats, exactly for these types of problems.

As people who help solve problems with speed and RPM's we almost always assume that the gear ratio we have for that motor is correct, but in some cases people have changed the gear ratios for various reasons and then the advice we give them is incorrect and makes it veryyyyy hard to help them pick a better prop.

We also assume that the pitch stamped on the prop is correct as well, but we also know that people also send their props to a prop repair shops to have the prop blades repitched or even add or delete cupping to the prop. When this is done if you run a prop analysis you will know there is a problem.

So I recommend you check the gear ratio, your tach and bring the prop to a prop shop to check the pitch on it. because GPS's are usually extremely close. And if you are only turning 4,000 RPM's it is not good for the motor and you need to change props.

H

 

[steelespike]

Re: Please help me find my revs

H really knows his stuff.
Just to confirm something is up with the numbers my prop calculations come up with the same numbers.

 

[hundee r1]

Re: Please help me find my revs

Wow, what can I say!!! you guys seem to know your stuff!! at last some real numbers and formulas to work with.

I just had a troll thru my manuals for this motor, I dont have the factory one, so I cant seem to find the final ratio anywhere, but what I will do is take the cowling off tonite and just do a simple count of revolutions. No guessing then.

A local shop has offered to lend me a 15P prop aswell.

So armed with this spare prop, I will head down to the local river on saturday morning and run all the numbers again.

I have 4 days to find a new tach that I can retro fit to cross check the one I have.

Thanx a heap for your time guys, Il be back with at least a gear ratio confirmation 2 morro.

 

[hundee r1]

Re: Please help me find my revs

Can I add,
assume my tacho is right, the gear ratio is 2.31 and the motor is only going to 4000rpm on the 17P prop, what speed should I be getting?

 

[steelespike]

Re: Please help me find my revs

If we manage 12% slip ,works out to about 24 mph.
You may be able to borrow or rent a shop tach.
An induction tach works off one plug wire.

 

[hundee r1]

Re: Please help me find my revs

Ok, so last night I braved one of our spring storms and went out side.

Put the motor in gear at the powerhead(so cant remember if it was reverse or forwards but shouldnt matter right? ).
Then marked the prop with the motor at TDC and the slack taken off the prop, then
turned the flywheel clockwise.
The flywheel was 6 teeth short from doing 2 full rotations after the prop had done 1 full rotation.
I counted the teeth on the flywheel and came up with 98.

Do I indeed have a gear ratio of 2.31?
I have found confirmation that the equivalent Merc gearbox is indeed a 2.31, but I cant find any info on the Force one.

Im about to ask the question of on that part of the forum

Sorry for my late postings, but for some reason I can only access this forum from my work PC, the site seems to be blocked from my home PC?

 

[smokeonthewater]

Re: Please help me find my revs

no, that would be something like a 1.93 (ish) ratio

that would explain why you are going faster than expected for the prop

 

[hundee r1]

Re: Please help me find my revs

Ahh, the plot thickens then, maybe my numbers are clsoe to reality then?

 

[hundee r1]

Re: Please help me find my revs

can comfirm my lower unit looks like the "Type C" on the attachment, ie 1.93 ratio

 

[smokeonthewater]

Re: Please help me find my revs

DAYUMMM I'm good LOL

 

[smokeonthewater]

Re: Please help me find my revs

I'm thinkin 13-14 pitch

 

[steelespike]

Re: Please help me find my revs

I'm thinking the 92 is the original Force gear case I think around 95 they went to a Merc gear case thus our error thinking 2.31.
Good job Hundee.

 

[Fed]

Re: Please help me find my revs

Long shaft motor short shaft transom? Sort of hard to see.

 

[hundee r1]

Re: Please help me find my revs

20" leg, motor has been mounted high to try and get the cav plate level with the bottom of the V.

had a shipwright look at this and he believes its at the correct height.
But I do intend on getting a video at full noise and posting it up

 

[Fed]

Re: Please help me find my revs

Hundee you'd laugh if you could see me here with a ruler on my screen.
I measured from the steering tube to the projected front of the AV plate @ 9 centimeters then from the steering tube to the bottom of the transom @ 7 centimeters.

You can see it in real life so if the AV plate is level with the bottom then that's the end of it.
AV plate being the first one above the prop.

 

[hundee r1]

Re: Please help me find my revs

ahahaha, I will have another look on the weekend to double check.

But the pix are hard to make out, the transom actually has a rake back into the hull at the bottom which makes it confusing.

 

[Moggy23]

Re: Please help me find my revs

I'd be checking your tune of the engine first, use a timing light to make sure you are getting 30 deg under load at max. advance which is easy to see under the flywheel, make sure your getting max throttle, then go for a smaller pitch if your not getting 5,000 rpm.
Good luck.