OMC COBRA HARD TO PLANE

[wheeler87]

Re: OMC COBRA HARD TO PLANE

no i dont know the gear ratio , how do i tell what it is ? do I have to take it apart to find out?

 

[jfcar]

Re: OMC COBRA HARD TO PLANE

Take off the top out drive cover and count the gear and pinion teeth should be 21/21

 

[dmarkvid2]

Re: OMC COBRA HARD TO PLANE

Don, Can you please explain more on the trim? I have an OMC King Cobra 460 which is in a 29' Bayliner. I also have trim tabs. I thought the whole idea was to get the bow down, and up on plane. If you trim the drive up, wouldn't that make the bow come up even higher? It seems to take forever to get on plane. What is the best, and fastest way to plane out using the trim, and the trim tabs. For me to get on plane, I need to be WOT at 4500 RPMs.

 

[Boatin Bob]

Re: OMC COBRA HARD TO PLANE

Your right, the idea is to get the bow down and you may need to have both your drive trimed down all the way plus your tabs extended down to do this in a reasonable time. Once the bow has come down and you are on plane you may find the bow starts to dig in which is not good and can affect steering. Now you want to raise your tabs and the drive to find the optimum running position that gives you the most efficient fuel use. You don't want the bow to raise so far that you fall off plane if it starts to you may have to increase RPM's or lower drive/tabs again. That is a big heavy wide beamed boat to only run with a single in it, you may have to run around 3500 RPM to stay on plane. Another factor could also be your prop, if the pitch is too much that can affect how quickly you get on plane. Do you know the current size and what the RPM's are at WOT?

 

[dmarkvid2]

Re: OMC COBRA HARD TO PLANE

Bob, The size of my prop is 15 something, I'm not sure of the other number. WOT is 45 to 4800 rpm. I've tried to get on plane at 3500 rpm, and noticed at 3500, with the tabs fully down, the RPMs "SLOWLY" creep up (by itself) to about 4200. Is there a number stamped on the prop? I ask that because I'll be taking my boat out of the water on Monday (bummer) for the winter, and I can check for the other number then. On a bigger and heavier boat like mine, do I want a prop for power or speed out of the hole? This is my first I/O boat, I've had inboards, and out boards, and I know I/Os seem to be more fussy. I just bought my boat this year in August, and I'm still trying to learn its little quirks. One other thing that I've noticed, is that if I have 3 or 4 other people in the boat, and they are standing toward the stern, its ALMOST impossible to get on plane. Should I have them move to a different spot in the boat when I get underway?

 

[Mystic Dragon]

Re: OMC COBRA HARD TO PLANE

Hey guys! I'm just wondering if his engine coupler could be slipping? I'm not familiar with this engine but if this happened, wouldn't it rev the engine higher and not convert the power to the prop?

Just an idea.

 

[Boatin Bob]

Re: OMC COBRA HARD TO PLANE

dmarkvid2....you probably should start your own post for this as we are kind of hijacking wheeler's....

The 15 you see is probably the diameter, prop should have a number like 15 x 17 ?? the second number is the pitch of the prop. Sounds like your RPM's are just about right bang on though, 460 should be 4400-4800 at WOT. The prop may also just have a PN on it, if you see it post it as there are many cross reference sources on the web.

 

[marunr]

Re: OMC COBRA HARD TO PLANE

As I said earlier, I have the same engine/drive combo on a 17 foot boat and have had it since it was new in 1988. I use a 15 prop because that is what came with the boat, With it, I can achieve 5400 rpms (WOT according to my owners manual is 5200 to 5600). With a 17 prop I can only achieve 4600 to 4800, and I have trouble trying to pull up a skier. I think you should change your prop.

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

gear and pinion teeth are 21/21

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

ok so should i go with a 15 pitch prop?

 

[QC]

Re: OMC COBRA HARD TO PLANE

I would say based on this quote from above "8 WOT 5500 rpm 26 to 28 mph" that something else is still screwy. Based on what I can find you have a final gear ratio of 2:1 (21:21 in the upper 13:26 in the lower). If this is accurate then your prop is at a 37% slip!! This should be closer to 10%. If you were getting even 15% slip then top speed should be around 37 MPH. I don't see it above, how are you getting your speed number?

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

my speed reading is coming from the in dash spedometer , unfortunatly on my boat i do not have gps or a fishfinder which tells me speed.

 

[QC]

Re: OMC COBRA HARD TO PLANE

Well I think you're going closer to 35 and probably faster. Conventional boat speedos are notoriously inaccurate.

I guess the next move depends on what your goal is, if you can live with less top speed and really need better hole shot, then go down to a 15. If you only need a small improvement in hole shot (could be substantial) you could stick with the existing 17 and add trim tabs. They will improve your time to plane and minimum planing speed. It is amazing how well they work and I will never own a V-hull without them . . .

What is the condition of the existing prop?

 

[marunr]

Re: OMC COBRA HARD TO PLANE

Properly trimmed I hit about 38 mph at wot with the 15. I don't have trim tabs, so maybe you could even do better, but for my purposes (water skiing) I don't need to go any faster. I've checked it with a GPS and it's almost right on with the speedo. I don't think you would be unhappy with the 15...it's not bad performance for such a small engine IMO.

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

thanks guys i will try trim tabs although with weather the way it is in nj who knows if ill go out anymore this year .....thanks again for youre help!!!

 

[QC]

Re: OMC COBRA HARD TO PLANE

wheeler87,

I found some additional info on these:

-The 2.3 is actually 128 HP.

-5500 RPM is not too high. That engine needs high RPM to perform.

-The engine has a very basic carb. If it is bogging, the accel pump is probably out.

- You also need to check the flyweights in the bottom of the distributor. If one is broken or missing, it will be a dog out of the hole. That's true with any engine though.