Proper engine height

[diaric]

I just installed a CMC tilt trim on my old runabout with my 1966 merc 500. The motor sits back an extra 6 inchs now and by the instructions the motor should be mounted from 1 to 3 inchs higher than stock. I have it about 2 inchs higher. The boat doesn't get up and go as well now, it takes longer to get up on plane and when you give it throttle it doesn't rev up immeidatly like there is more water resistance against the prop. Top speed hasn't changed, still does 30 mph with 11 pitch prop. Is the motor still alittle low and how do u figure the best height?

 

[Texasmark]

Re: Proper engine height

Your trim position will make a tremendous difference in your hole shot. You'd have to ask Walleyehed about what the push back does for hole shot.

Trim in to get the bow down fast on exiting the hole and push out for reduced wetted area and more speed once you are out.

Your variable is prop slip which seems to be less now than it was. If the engine is sitting at the wrong attitude, as mentioned above, it could put more load on the engine and seriously affect the slip.

Mark

 

[steelespike]

Re: Proper engine height

I would raise the motor until you have ventilation problems,just be sure it is pumping water properly.
If your transom is very perpindicular you may have to add wedges in order to get more tuck under(down trim) to improve your hole shot.

 

[diaric]

Re: Proper engine height

The transom isn't perpindicular, tilts the motor in a bit. If i trim up just a little too much as its getting to speed seems to rev up like its geting some air.Don't seem to have a heck of a lot of trim adjustment and was wondering if i should shim it to tilt it under a bit more

 

[diaric]

Re: Proper engine height

forgot to mention, that it has a hydrofoil on it as well

 

[steelespike]

Re: Proper engine height

By the way I have a 65 tiller Merc 500.Mine is on a 17ft.56 Crestliner utility.Any way back to your boat.
The hydrofoils can cause problems.I would remove it.you may find no need for extra trim without the foil and you will probably find additional speed. If you feel you still need help after careful adjustments and trimming I would reccomend something like smart tabs. Being able to tuck under more should help your hole shot.I would give it a good test after removing the foil before making any decisions on modifications.I wouldn't trim out until you see how the boat settles at speed then trim a little at a time and note the response before further trimming.It would seem that if it is starting to blow out(ventilate) without too much out trim you may be close to as high as you can go.However the condition and style of your prop can have a big affect on blowout.
Not an expert but from your prop pitch it would seem your boat may be heavy.Or you ski alot or haul some weight.
If you describe your boat and its approximate weight that may help some too.

 

[Texasmark]

Re: Proper engine height

Hole shooting with the trim out can/will cause ventilation. Does on mine.

But if it locks in (a few hundred rpm's above no slip rpm's, not runaway blowout) it's good as it helps you develop your hp and that spins your prop and pumps you outta da hole.

If I want a fast hole I always trim out. I know you are supposed to trim in to get the bow down, and once out push out, but on my boat I can punch out faster by trimming out and getting the engine rpm's up.......besides, the engine roars, the prop screams, and the water goes up in the air and it's fun. 8)

Mark

 

[roscoe]

Re: Proper engine height

Start by removing the hydrofoil.
Then get things set up as best you can.

Only put the foil back on if it is really needed.

 

[Crownie2]

Re: Proper engine height

The setback will tend to get the bow up more and therefore the needed trim in to counteract it. It's just a small negative we put up with to get a good running attitude to go fast!!

I agree with the others - you will probably need more tuck in to improve the hole shot - I know both my boats did. I agree with the prop comment also.

At this point the hydrofoil may not be bad for you - but I guess you will not know if you don't remove it and try without.

Bob

 

[diaric]

Re: Proper engine height

 

[diaric]

Re: Proper engine height

ok, removed the hydrofoil. It ran like crap. steering was erratic and the boat wanted to raise the bow way to much at any throttle. motor seemed to be bogging out and struggling never got to WOT, it felt uncontrollable. I don't know what it weighs, its a 14" runabout, very back heavy and bow light. prop is for 1000-1800 lbs and 26-30 mph, i'll try to upload an image. i wondered if this was enough prop but was what came with the boat. sorry about photo size,, haven't figured out resizing on photobucket yet. BTW. the back seats are not mounted where u see. they were just sitting there and have been mounted directly behind the front seats facing back

 

[diaric]

Re: Proper engine height

 

[diaric]

Re: Proper engine height

 

[Crownie2]

Re: Proper engine height

Any way to get any weight transfered to the front?

This may be a case where the medicine is stronger than what the patient needed.

A stern shot may have been more helpful...

Bob

 

[diaric]

Re: Proper engine height

There us nothing that i can move up front. I was thinking of putting a livewell/cooler up there. figured the 100 lbs plus of water would make a difference. I will take a stern shot and post later

 

[Texasmark]

Re: Proper engine height

I wouldn't do (with trim) what I do with that rig. Do like the others suggest and tuck her in.

What's on the bottom of your boat? It looks very rough as compared to the sides. A rough bottom causes excessive drag, sucking up hp and causing speed loss. May be part of your problem.

Mark

 

[diaric]

Re: Proper engine height

ok, here is the motor trimmed in all the way

 

[diaric]

Re: Proper engine height

I don't know why there is a wavy texture to the bottom. I got it that way, it is smooth in finish, but not a smooth bottom. That might be an issue in general, but not this one. I had great hole shot before the tilt trim unit and i still hit 30 mph, which is pretty much max for this prop. weight transfer- I have 2 batteries, 1 for boat, other for trolling motor, I could rewire stuff and move them forward, other than that is just a fuel tank. with the hydrofoil on it was planing great just lost the hole shot. with off, can't keep bow down, motor bogging and if trimmed up a little sounds like cavitation. as well steers horrible, like when motor trimmed up all the way in shallow water, bow keeps wandering side to side. thanks for advice and suggestions. what is the best way to shim the motor in more with this setup?

 

[xtraham]

Re: Proper engine height

trim unit is so low it's digging in from the looks of the pics, when the bow comes up it's probaly acting like a sea anchor

 

[Ron G]

Re: Proper engine height

Thats a good possibility they make a back splash plate to go on the bottom of them now.