75 ski barge

Alright, picked up this 75 ski barge for next to nothing and put my 85 horse on it and now I've got issues. First off, the motor on it was a short shaft and my motor is a long shaft so I had to build the bracket you see in the pics to raise my motor. Could have gotten a jack plate but did this for $25. Anyways, I put it in the water and it planes out around 15mph and cruises good up to 25 mph. Anything thing over 25mph causes the front of the boat to sit back in the water. I've tilt/trimmed it every which way and the same issue. The more throttle, the more the front sits. Any ideas what would cause this issue? Here's some pics of how my motor is sitting right now.

http://www.flickr.com/photos/48844415@N03/4734439388/
http://www.flickr.com/photos/48844415@N03/4734441260/
http://www.flickr.com/photos/48844415@N03/4733802033/
http://www.flickr.com/photos/48844415@N03/4734442056/

Re: 75 ski barge

Come on guys, surely I'm not the only one that has this issue. The only possible answer I can think of is I'm overpowered. The boat originally had a 70hp but would 15hp really cause this problem?

Re: 75 ski barge

Alright guys I'm pretty much stuck. I've taken a video of what the boat is doing. I'm running 22mph and it's planed out good. If I give it more throttle, you can see how the front starts to sit back in the water. I'm at 32mph at the end of the video with the front back in the water. The second link is of my prop. I just had it cupped yesterday in hopes of it helping stay on plane but it didn't seem to help much. It helped more with me getting on plane quicker but that's it. Any suggestions?

http://www.flickr.com/photos/48844415@N03/4744363616/
http://www.flickr.com/photos/48844415@N03/4743623751/

Re: 75 ski barge

First, I cant see the pics or vid, but if it has a whale tail take it off!

second, It has power trim? What happens if you just keep trimming it up? Sounds like the motor could be at a bad angle or something. Again, I cant see the pics or vids so im not sure.

Re: 75 ski barge

nmhellraiser said:

I've tilt/trimmed it every which way and the same issue. The more throttle, the more the front sits. Any ideas what would cause this issue?

Click to expand...

Makes me think you might have a deformed hull with some "hook and/or rocker". Check the aft half of the bottom with a straight-edge or string to see if it has any humps, bumbs or valleys. Although you don't state how big the boat is or it's maximum rated horsepower, I don't believe the problem you are experiencing is "over-powering". Too much power would probably makes it handle erratically and possibly chinewalk. Good luck!

Re: 75 ski barge

Willyclay said:

Makes me think you might have a deformed hull with some "hook and/or rocker". Check the aft half of the bottom with a straight-edge or string to see if it has any humps, bumbs or valleys. Although you don't state how big the boat is or it's maximum rated horsepower, I don't believe the problem you are experiencing is "over-powering". Too much power would probably makes it handle erratically and possibly chinewalk. Good luck!

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Sorry, it's a 16' ski barge. I don't know what it was rated at but had a 60 hp on it but I had a good 85 hp laying around so I put it on. I picked it up cheap and plan on using it strictly for the salt water. Would a prop with more rake help in this situation? I don't like the fact the motor is probably only running at 3500 rpm's to keep it planed out. I'd like to run it in the 5K range and not loose the steering. It's hard to steer this thing when the front sits back in the water.

Re: 75 ski barge

I gave you my best shot with "hull deformation". Another thought is excessive weight forward in the boat, i.e. water-saturated foam. If you have tried ALL trim positions without any improvement, then you need to do some detective work and let us know what you find.

Regarding overpowering your boat, you will have to decide that on your own. Based on info I found at NADA for a 1975 Ski Barge 16ft, the USCG formula for maximum horsepower results in 60HP if your transom heighth is less than 20inches and 115HP if the transom heighth is 20inches or greater. Here's a link to those regs if you want to run your own numbers. Be safe!

http://edocket.access.gpo.gov/cfr_2004/julqtr/pdf/33cfr183.53.pdf

Re: 75 ski barge

Can't really tell what I'm looking at with the pics and video. The vid looks like the wake may be moving toward the bow but all I really see is the side of the boat. Are there 2 different motors in the pics? One looks like it has a trim tab and whale tail and the other doesn't? Overall pics of the boat may also help and an explanation of the set-up (where are the fuel tanks, batteries, gear ect.

Re: 75 ski barge

Willyclay said:

I gave you my best shot with "hull deformation". Another thought is excessive weight forward in the boat, i.e. water-saturated foam. If you have tried ALL trim positions without any improvement, then you need to do some detective work and let us know what you find.

Regarding overpowering your boat, you will have to decide that on your own. Based on info I found at NADA for a 1975 Ski Barge 16ft, the USCG formula for maximum horsepower results in 60HP if your transom heighth is less than 20inches and 115HP if the transom heighth is 20inches or greater. Here's a link to those regs if you want to run your own numbers. Be safe!

http://edocket.access.gpo.gov/cfr_2004/julqtr/pdf/33cfr183.53.pdf

Click to expand...

The transom height is greater than 20" now. I think the 85 pushes it very well but just sits the front back in the water. I'm happy where it runs but just wish it was at more RPM's.

Re: 75 ski barge

Lion hunter said:

Can't really tell what I'm looking at with the pics and video. The vid looks like the wake may be moving toward the bow but all I really see is the side of the boat. Are there 2 different motors in the pics? One looks like it has a trim tab and whale tail and the other doesn't? Overall pics of the boat may also help and an explanation of the set-up (where are the fuel tanks, batteries, gear ect.

Click to expand...

That's exactly what the boat is doing. I'm on plane but as I throttle up, the boat sits back in the water. It's the same motor I've just put a whale's tail on since my original post to see if it had an effect. I've also cupped the prop since then also. It was a 17pitch prop that I cupped. It's a 16' ski barge with a 85hp evenrude on the back. I built a braket to raise the motor up b/c the original 60hp was a short shaft and this 85hp is a long shaft that I've had for a while and I know how it runs.

Re: 75 ski barge

I suspect what is going on hear is the long shaft outboard. In this case you've had to raise the height of the transom to ensure the cavitation plate lines with the keel. This is raising the point where the motor exerts force on the transom.

I liken this to trying to push a tall dresser across the room. Push toward the bottom and the dresser moves across the floor. Push at the top and it tips over. I would bet if you placed the proper sized motor on teh boat and didn't rasise the transom, you wouldn't have this issue.

Re: 75 ski barge

I agree with Shrew, I think that it has to do with the motor height. Regardless of the trim, the motor is connected to the boat at a particular height.

I included a crude picture of what I think is going on. The arrows in the picture show the direction and location of the force.

In example "A", the motor is lower and it pushes more directly in-line with the hull.

In example "B", the motor is raised on the jack-plate, then the force is applied above the hull and would tend to push it down in front. The faster you go, the more force is applied above the hull and thus, the more the bow dives.

In example "C" the motor is raised and you are trying to overcome by triming the motor up, but you are asking the motor to try to overcome its own force of pushing the bow down. Also by triming the motor up, you are asking the prop to basically lift the entire weight of the front of the boat.

Re: 75 ski barge

I never thought of it that way. I suspect y'all are probably right. So in other words I should be happy with the speed I'm getting then. Any way other than swapping motors to solve this issue? I would go back to the original motor but its an electric shift motor and I don't want to put any money into it.

Re: 75 ski barge

Or see if the possibility exists to get a short shaft lower unit and drive shaft.

Re: 75 ski barge

Unless something is flexing the long shaft with a higher mounting point vs. the short shaft with a lower mounting point will not make any difference. In either case, as long the position of the prop with respect to the bottom of the transom is the same, then the boat should behave the same. However, the long shaft will put more torque on the transom and will therefore be more likely to flex the transom and change the angle of thrust on the boat. Specifically, if your transom is flexing, then the motor will tend to tuck under and will raise the stern and drive the bow down. This is exactly what you are describing as happening. Check and see if the new added on transom is flexing. I will bet it is doing so. As a side effect, the lower unit will flex closer to the bottom of the transom. I am not sure what that might do, but it could also have some negative effect. Check the new transom bracket for flex under power and I bet you find that you have something like that happening. Have someone else drive the boat to that power level and I bet you can feel the motor tilting under the boat. That is why a jack plate is so much more than the $25 that your bracket costed you.

Re: 75 ski barge

I can't see the transom flexing under load but it could be. I've watched the motor and it doesn't budge. To me a jack plate seems like it would do the same it not worse. It would have the motor about 8 inches higher off the transom. I will fabricate some brackets to tie the top of my bracket to the floor. This should solve any flex I can't see. If its flexing, it should pull against the bottom of the floor then. I know its solid, I just put it in. Once alex passes, I'll give it a spin.